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 Front Cover
 Florida State Board of Conserv...
 Transmittal letter
 Table of contents
 Abstract
 Introduction
 Geology and drainage
 Methods of investigation
 Long-term position of salt water...
 Short-term position of salt...
 Manipulation of the control...
 Summary
 Conclusions
 References


FGS








STATE OF FLORIDA
STATE BOARD OF CONSERVATION
DIVISION OF GEOLOGY



FLORIDA GEOLOGICAL SURVEY
Robert O. Vernon, Director





REPORT OF INVESTIGATIONS NO. 24
PART IV







SALT-WATER MOVEMENT CAUSED BY
CONTROL-DAM OPERATION IN THE
SNAKE CREEK CANAL,
MIAMI, FLORIDA

By
F. A. Kohout and S. D. Leach
U. S. Geological Survey









Prepared by the
UNITED STATES GEOLOGICAL SURVEY
in cooperation with the
CENTRAL AND SOUTHERN FLORIDA FLOOD CONTROL DISTRICT

S.Tallahassee
1964









FLORIDA STATE BOARD

OF

CONSERVATION




FARRIS BRYANT
Governor


TOM ADAMS
Secretary of State




J. EDWIN LARSON
Treasurer



THOMAS D. BAILEY
Superintendent of Public Instruction


RICHARD ERWIN
Attorney General




RAY E. GREEN
Comptroller



DOYLE CONNER
Commissioner of Agriculture


W. RANDOLPH HODGES
Director






LETTER OF TRANSMITTAL


iorida geological Survey

Callahassee

October 10, 1963

Honorable Farris Bryant, Chairman
Florida State Board of Conservation
Tallahassee, Florida

Dear Governor Bryant:

The Florida Geological Survey has published four short papers
as Parts 1, 2, 3, and 4 of Report of Investigations No. 24. This
publication, Part 4, is the last of these brief papers and discusses
the "Salt-Water Movement Caused by Control-Dam Operation in
the Snake Creek Canal, Miami, Florida." The report was prepared
by F. A. Kohout and S. D. Leach, hydrologists for the United
States Geological Survey.

The principles of the relationships of fresh and salt water in
the Biscayne aquifer and the movement of the salt-water, fresh-
water contact along the canals and in the aquifer, with use, with
development of the control stations in the Central and Southern
Florida Flood Control District, and in response to cyclic changes
in the weather, are of great importance in understanding similar
occurrences. These four reports present interesting details of
these relationships.

Respectfully yours,
Robert O. Vernon
Director and State Geologist





















































Completed manuscript received
August 16, 1963
Published for the Florida Geological Survey
By E. O. Painter Printing Co.
DeLand, Florida

iv









CONTENTS


Abstract _---- -... .___.__.__ ____...... -......._..._.................. 1
Introduction __ _____ __.._______... 2
History ________ 2
Purpose of investigation ----------.. ........._...._----------............-......... 5
Location and extent of area ____ _---....___ .._--.......-.....-....... 6
Climate .-- ---__-..._-.. .___-._ -- -- -- -------.--..___ .___ -----_.----.-__ 7
Previous investigations __ _--.._.. ___-__.. ..-....-................._ 7
Personnel and acknowledgments ___----------._______ ..--__ .__ 7
Geology and drainage -______________ 8
Methods of investigation ____.... ___......__ -- ......------------- 9
Long-term position of salt water -.----- -. .. .....__________ .............._ 10
In the canal _------ __--._- ..... .... .......... .... ----------- 10
In the aquifer ____. ...___._....... ___...... .......... 12
Short-term position of salt water ___. ..___...... .._ 16
Salt-water movement in the canal .. ...-........_ ......____ _.. 17
Natural tidal oscillation __.._................_______ 17
Trapping salt water _20
Flushing of salt water from the canal 24
Ultimate loss and flushing of the remaining trapped salt water 26
Salt-water movement in the aquifer 29
Natural tidal oscillation -----------_______ ----.. -_....... 29
Recharging the aquifer with trapped salt water _-____ 36
Ultimate loss and flushing of salt water in the aquifer --__ 38
Manipulation of the control dam for maximum benefit ._ ----__ ----------__ 39
Factors affecting inland movement of salt water ___ 40
Discharge related to gate opening 41
Summary _____._ 45
Conclusions as to operating criteria __ 47
References ---_ ______ 48


ILLUSTRATIONS

Figure Page
1 The Miami area, Florida, showing the approximate landward
extent of salt water at the base of the Biscayne aquifer in 1961 3
2 Photographs of the salinity-control dam (S-29) in the Snake
Creek Canal: (a) with right-hand gate opened 1 foot; (b)
with one gate fully open 4
3 Map of the northern part of the Miami area showing obser-
vation points along the Snake Creek Canal 6
4 Section along the south bank of the Snake Creek Canal
showing generalized lithologic characteristics of the Biscayne
aquifer 8
5 Graphs comparing rainfall, gate opening, and fluctuations






of water level and chloride content along the Snake Creek
Canal, 1960-61 _______- 11
6 Graphs comparing fluctuations of discharge, water level, and
chloride content in the Snake Creek Canal during the special
test of March 27-31, 1961 ______ ____--- -__ 12
7 Sections along the south bank of the Snake Creek Canal
showing the zone of diffusion in the Biscayne aquifer, 1960-62 --. 14 & 15
8A Sections 1, 2, 3, and 4 through the Snake Creek Canal
showing movement of salt water during the rising tide, March
29, 1961 -___ ______________ 18
8B Sections 5, 6, 7, and 8 through the Snake Creek Canal showing
movement of salt water during the falling tide, March
29, 1961 _______ ___----..-- __- 19
9 Sections 9, 10, and 11 through the Snake Creek Canal showing
movement of salt water after closing of the control dam at
high tide, March 30, 1961 ___--_-____- ... .. ...___- 21
10 Photograph of rock pit (at left) looking in the downstream
direction of the Snake Creek Canal (at right) _---__..-----__..._- 22
11 Rock pit showing approximate bottom topography ..-----_------ 23
12 Sections 12, 13, 14, and 15 through the Snake Creek Canal
showing movement of salt water during and after flushing,
March 31 to April 1, 1961 ______ _25
13 Sections 16, 17, and 18 through the Snake Creek Canal
showing changes in isochlor position, April 3-11, 1961 ..--- 27
14 Sections 19, 20, and 21 through the Snake Creek Canal
showing changes in isochlor positions, April 18 to May 16, 1961 .---. 28
15 Sections 22, 23, and 24 through the Snake Creek Canal
showing changes in isochlor positions May 29 to July 10, 1961 .---- 30
16 Graph showing fluctuations of chloride content at well
stations 1, 2, and 3, 1960-61 ._--- ---_ .. ------.--. 31
17 Graph showing fluctuations of chloride content at well
station 4, 1960-61 ___ .... ._ ...__....---------... 32
18 Graph showing fluctuations of chloride content at well
station 5, 1959-61 ____ ----- 33
19 Graph showing fluctuations of chloride content at well
station 6, 1960-61 .__ ... ...-__---_...._ ...._.. 34
20 Graph showing fluctuations of chloride content at well
stations 7, 8, and 9, 1960-61 _-_ __-_ -- _- 35
21 Schematic flow diagrams: (a) with gate partially submerged,
(b) with gate fully open -----___-- 42
22 Graph showing relations of head differential and discharge
for various openings of one gate of the control dam in the
Snake Creek Canal __ 43







SALT-WATER MOVEMENT CAUSED BY
CONTROL-DAM OPERATION IN THE
SNAKE CREEK CANAL,
MIAMI, FLORIDA
By
F. A. Kohout and S. D. Leach

ABSTRACT
Movement of salt water in the Biscayne aquifer and the Snake
Creek Canal was investigated to establish criteria for operation
of the salinity-control dam in the canal. All four gates of the dam
were opened for 3 days during a special test in March 1961.
Although salt water oscillated landward and seaward a distance of
2 miles from the control in response to tide, the salinity of the
ground water near the canal did not change greatly during this
time. Salt water trapped in the canal by closing the dam at high
tide moved landward as a density current at a rate of about 900
feet per hour and began to flow into a connected deep rock pit. A
flushing operation removed salt water from the canal but the salt
water in the rock pit remained. During subsequent months, wind
and tide action caused upward dispersion of salt into the upper-
most water of the rock pit, and the trapped salt water was
gradually removed by seaward discharge through the canal.
Some of the salt water trapped in the canal moved into the
aquifer. This salty water was traced by sampling fully cased wells
of different depths and distances from the canal. Some of the
salty water was retrieved by drainage from the aquifer when
the gates were partially opened 2 months after entrapment. Be-
cause of its relatively great density some of the salt water moved
downward in the aquifer and was not retrieved.
The data show the negative effects of trapping salt water, and
also that the control can be closed gradually without trapping salt
water. If the total opening of the control is distributed uniformly
among the four partially closed gates, the increased velocity of
the fresh water as it flows under the submerged sluice gates, will
impede upstream movement of salt water at the bottom of the
control. Calculations indicate that salt water probably will not
move upstream in the canal if a head of 0.3 foot is maintained
between upstream and downstream sides of the control at the time
of minimum head differential, about 2 hours before high tide.





FLORIDA GEOLOGICAL SURVEY


INTRODUCTION

This report gives detailed attention to the movement of salt
water in and adjacent to the Snake Creek Canal in the northern
part of the Miami area, Florida (fig. 1). The movement results
primarily from operations of a salinity-control dam located near
the mouth of the canal.
From 1909 to 1930, canals were constructed westward from
the coast through the Miami area into the Everglades to drain low-
lying land for urban and agricultural use. The uncontrolled canals
caused overdrainage. The water table was lowered nearly to sea
level in some areas and this caused salt water to encroach into
the Biscayne aquifer by two processes (Parker, 1951, p. 826):
(1) Where the water table was lowered greatly, salt water
moved horizontally landward from the ocean through the highly
permeable solution-riddled limestone of the Biscayne aquifer to
replace the drained fresh water.
(2) Salt water moved directly inland in the canals where, be-
cause of its greater density, it tended to leak downward and
laterally to contaminate the underlying fresh ground water.
Although the end result of these separate processes is the
same, they attack the fresh-water resources from different
directions and occur at different rates. The general aim of this
report is to establish the conditions under which each process
provides the greater danger of salt-water contamination.

HISTORY

In 1945, salinity-control dams were installed in most of the
canals of the Miami area as barriers against further encroachment
of salt water. These were emergency dams constructed by driving
sheet-steel piles into the limestone. The dams were opened and
closed by removing some or all of the piles during the rainy
season and replacing them during the dry season. This was an
inefficient procedure which frequently resulted in the dams being
left open much longer than necessary.
In recent years, the emergency dams have been replaced by
permanent concrete structures with submerged sluice-gate open-
ings. The control structure S-29 (Snake Creek Canal) is repre-
sentative of some of the more recent structures. Each of the four
gates, figure 2, is connected through a gear box to rollers which
are driven by the rear-wheel drive of an anchored automobile.
In the most recently constructed controls, in the Little River and







REPORT OF INVESTIGATIONS No. 24 3







I.c.r .






I-.-..
S .BIACAYN C A

0v









































SCALE IN MILES


Figure 1. Miami area, Florida, showing the approximate landward extent of
salt water at the base of the Biscayne aquifer in 1961.






FLORIDA GEOLOGICAL SURVEY


I I


Figure 2. Photographs of the salinity-control dam (S-29) in the Snake Creek
Canal: (a) with right-handgate opened 1 foot; (b) with one gate fully open.


~L
Ih`
a


v r






REPORT OF INVESTIGATIONS NO. 24


Biscayne Canals, electric motors raise and lower the gates
automatically by sensing the water levels upstream and down-
stream from the control dam. These automatically operated dams
permit delicate control of water levels. However, the relation of
salt-water movement to head differential needs further study
before fresh-water resources can be effectively entrusted to
automatic head-activated devices. As will be shown later, the
relations of head and salt-water movement are very complicated
and transient, and much more study and experience should be
accumulated before true automatic control is attempted.




PURPOSE OF INVESTIGATION

Frequently, after heavy rainfall, headwater areas of the Snake
Creek Canal remain flooded at the same time that fresh-water
discharge at the control dam is not sufficient, with fully open
gates, to keep the canal flushed of salt water. Under these circum-
stances salt water moves upstream in the lower reaches of the
canal and contaminates the fresh ground water.
Questions had arisen as to the magnitude of this encroach-
ment, the extent to which the encroachment might be tolerated
during gate openings, and the feasibility of flushing the salt
water out later when the gates were closed after flooding subsided.
Accordingly, goals of the investigation were to establish the time-
rate of movement of the salt water for various circumstances
of dam operation and, if possible, to develop principles that might
have practical application not only in the operation of the salinity-
control dam in the Snake Creek Canal but also in other canals of
the area.
The investigation was made in cooperation with the Central
and Southern Florida Flood Control District (C&SFFCD). Most
of the data were collected between October 1960 and January 1962.
The investigation was under the general supervision of M. I.
Rorabaugh and, subsequently, C. S. Conover, Tallahassee, A. O.
Patterson and K. A. Mac Kichan, Ocala, district engineers
respectively of the Ground Water, Surface Water and Quality of
Water Branches, U.S. Geological Survey. The investigation was
under the immediate supervision of Howard Klein, geologist-in-
charge, Ground Water Branch, and J. H. Hartwell, engineer-in-
charge, Surface Water Branch, Miami.





FLORIDA GEOLOGICAL SURVEY


LOCATION AND EXTENT OF AREA

The Snake Creek Canal extends about 18 miles inland to the
vicinity of Levee 33, on the east side of Conservation Area 3
(Inset map, fig. 3). Under the plans of the C&SFFCD (the operating
agency) and the U. S. Corps of Engineers (the constructing
agency), Conservation Area 3 will impound water pumped west-
ward from urbanized areas during the rainy season. During the
dry season, water stored in the conservation area will be delivered
coastward to maintain water levels, thus preventing salt-water
encroachment and providing replenishment water for municipal
and private well systems.
This report covers the seaward reach of the Snake Creek Canal
(Canal C-9) extending about 15,000 feet westward from the
salinity-control dam (structure S-29). The width of the area
extends 200 feet on either side of the centerline of the canal, but


Figure 3. Northern part of the Miami area showing observation points along
Snake Creek Canal.






REPORT OF INVESTIGATIONS NO. 24


as one side should be the mirror image of the other, work was
confined to the south bank of the canal. The canal widens into a
lake whose dimensions are about 3,500 by 1,700 feet at the western
end of the area of investigation, between State Highway 9 and
Miami Gardens Drive (fig. 3). The lake, formed by removal of
rock for concrete aggregate and other building materials, has a
maximum depth of 40 feet below msl (mean sea level) and was
included in the investigation because of its important influence on
salt-water contamination.

CLIMATE

The climate of the Miami area is subtropical. The average
temperature for January is about 68F and for July about 820F.
Rainfall averages about 60 inches per year, three-quarters of
which falls in the May to November rainy season.
In relation to this investigation, 1960 was unusually wet with
rainfall of about 75 inches, 25 inches falling in September when
hurricane Donna and tropical storm Florence passed nearby. The
year of 1961 was unusually dry; the rainfall amounted to about
42 inches.

PREVIOUS INVESTIGATIONS

Some phases of the present study coincided with, and were
contemporaneous with, a study of the general hydrology of the
Snake Creek Canal area by Leach and Sherwood (1963). Where
data and graphs were applicable to the present report, they were
used freely without acknowledgment.
Reports by Parker (1951), Parker, Ferguson, and Love (1955),
and Kohout (1961) give much background information on the
influence of drainage canals on salt-water encroachment in the
Miami area. Many other reports cover the hydrology of the area
and may be found in the "List of Publications" of the Florida
Geological Survey in Tallahassee.

PERSONNEL AND ACKNOWLEDGMENTS

Thanks are extended to the Dade County Office of Water
Control, especially to John Wilhelm, for furnishing rainfall and
gate-opening data. Robert L. Taylor of the C&SFFCD aided by
making available the facilities and personnel of his organization
during the special test period of March 1961.






FLORIDA GEOLOGICAL SURVEY


Electrical-conductivity measurements of water in the canal
during the test period of March 1961 were made under the direc-
tion of R. N. Cherry of the U. S. Geological Survey, Ocala. Other
Geological Survey personnel who contributed materially were B.
F. Joyner of Ocala and C. B. Sherwood of Miami.

GEOLOGY AND DRAINAGE

The Biscayne aquifer of the Miami area is a highly permeable'
water-table aquifer consisting of solution-riddled limestone and
calcareous sandstone with fairly frequent layers of unconsolidated
sand. The generalized lithologic characteristics in wells drilled
during the investigation are shown in figure 4.
Except for the peat and muck deposits at the surface, the
sediments were deposited during the Pleistocene glacial epoch.
Accumulation of snow and ice during glacial advances in the
northern latitudes removed water from the oceans and lowered sea
leveL Alternately, melting of the glaciers returned water to the
oceans and caused sea level to rise. The alternation of limestone
and sandstone beds in figure 4 relates to these fluctuations of sea
level-estimated by Cooke (1939, p. 34) to have ranged from about
270 feet above to 300 feet below present sea level.



DISTANCE FROM CONTROL DAM.IN FEET
2 39_G 700 OCT 0 _00C 9000 8000 7000
LAND SURFACE-
S/ BOTTOM OF CANAL





0'STANCE FROM CONTROL DAM,IN FEET
503 50co 4CCO 3000 2000 I000 0
-LAND SURFACE n .
BOTTOM .F CANAL /' _f T 1' l g -- -- -


-- STONE *MITE d r ~ ~

-120


Figure 4. Section along the south bank of the Snake Creek Canal showing
generalized lithologic characteristics of the Biscayne aquifer.





REPORT OF INVESTIGATIONS No. 24


The peat and muck deposits were formed by the accumulation
of partially decayed vegetation in swamps after the last glacial
recession, about 8,000 years ago.
The Snake Creek Canal traverses a natural overflow channel
from the Everglades. These numerous overflow channels are wide,
topographically low areas (4 to 8 feet above sea. level) that cut
through relatively high land (8 to 15 feet) near the coast; they
are called transverse or finger glades because of the manner in
which they cut through the coastal ridge.
Prior to construction of drainage canals (fig. 1), the water
level in the Everglades was high enough to cause discharge through
the finger glades by poorly channelized sheet flow. In the deeper
pockets, aquatic plants grew lushly and, after dying, settled to the
bottoms of sloughs. In these locations, the organic material was
protected from oxidation and accumulated to form the peat and
black muck deposits.
In the higher parts of the finger glades, where water levels
were below ground surface during the dry season, rapid oxidation
prevented heavy accumulation of organic material. .In these areas,
the remnant plant materials were mixed with particles of limestone
and sand, eroded from the adjacent higher limestone hills, to form
a deposit called marl (fig. 4).
Since construction of drainage canals, the finger glades are
flooded only during extremely heavy rainfall, and the rich organic
soils are cultivated- to provide table vegetables for northern
markets in the winter season. With continuing improvements in
the canal system and improved efficiency of water control, the
agricultural activities are gradually giving way to urbanization.



METHODS OF INVESTIGATION

The investigation followed two main courses: (1) Long-term
observation of the position and rate of movement of salt water
under the past method of operating the control dam (S-29). The
dam has been operated on an arbitrary water-level basis with no
consideration being given to the movement or the position of salt
water; (2) detailed inventory of the position and movement of
salt water under special short-term test conditions. The short-term
test served as an index of typical conditions for analysis of the
long-term data. These phases required continuous inventory of
changes, both in the canal and in the aquifer.





FLORIDA GEOLOGICAL SURVEY


The long-term study of salt-water occurrence in the canal was
made primarily from fluctuations of chloride content as determined
by three electrical-conductivity recorders located at S-29, at West
Dixie Highway, and at the footbridge (fig. 3). The electrodes of
these recorders were positioned about 1.5 feet above the bottom
of the canal, and the landward movement of salt water was sensed
by a sharp rise in the electrical conductance of the water. During
and after the short-term test of March 27-31, 1961, the position of
salt water in the canal was determined by traversing the canal-
water vertically with the cell of a portable conductivity instrument.
The measurements were made from a boat that moved to specific
stations along the canal.
The long-term study of salt-water occurrence in the aquifer was
made from fluctuations of chloride content in samples of ground
water from nine stations of wells drilled along the south bank of
the canal (fig. 3). The general scheme was to drill individual
wells to different depths at distances of about 5 and 60 feet from
the canal. Thus, a typical station might consist of wells 10, 20,
50, and 100 feet deep near the canal and wells 10 and 20 feet deep
at a distance of 60 feet from the canal. Although drilling operations
did not always fulfill the preferred condition of paired wells
terminating at the same altitude, the rectangular coordinate
positioning of the wells relative to the canal was virtually fulfilled.
Each well was cased so that the water which it yielded was repre-
sentative of that particular point in the aquifer. Details of the
position and depth of each well terminus relative to the canal are
shown in the map and section views of figures 16 to 20.
An attempt has been made to simplify the illustrations as much
as possible, but as this three-dimensional problem can be shown
only in two dimensions, much must be left to the imagination of
the reader to keep himself oriented in three-dimensional space.
Herein lies the understanding of the physical parameters being
discussed.

LONG-TERM POSITION OF SALT WATER
IN THE CANAL
The long-term position of salt water in the canal can be inferred
by comparing figures 5 and 6. Figure 5 shows the peak daily
chloride content, as indicated by the conductivity recorders at S-29,
at West Dixie Highway, and at the footbridge. Also shown are the
water level in the canal at West Dixie, the gate opening at S-29,
and rainfall at the Opalocka gage. The maximum vertical opening








REPORT OF INVESTIGATIONS NO. 24


1960
QRV Ania SEPtg OCT


1961
NOV ODEM Ei N. FEB. MAR. APR MAY JUNE ALN AUG. SEPT OT. NOV. DEC.


I11f02 --- -- -- -- -I- -- ------- -- --- -- ---- ----- ---- ;0


- L 19-
S| 100




.1 -- ---- J[- ---- ----- ----- ----- -----.

Zloci
. W




j II ...


, I CON
o- I .I .. .
w sow





w I CONTROL DPM S-29





OPA-LOCA GdGE


li0 I
u300
I/W ---1 -----":^ : -- --------- -- --


E.
z
z
)Z
z
0
We
0
cc
0
_3
Xo
0
LL
4
9
X
CL
CL
-1


Figure 5. Graphs comparing rainfall, gate opening, and fluctuations of water
level and chloride content along the Snake Creek Canal, 1960-61.

of each of the four gates is 15 feet (fig. 2); therefore, if all gates
are fully open, the total gate opening is 60 feet. As the gates of
S-29 are of equal width, the amount of gate opening in figure 5
represents the discharge area.
All gates were fully open from early September to late October
1960, after the heavy rains accompanying hurricane Donna and
tropical storm Florence produced widespread flooding (fig. 5). Only
during the period September 10 to 14, 1960, was the discharge from
the canal great enough to keep the canal completely flushed of
salt water (chloride-content graph at S-29, fig. 5). At all other
times full opening of the dam permitted salt water to move
upstream on a rising tide. During the special short-term test of
March 27-31, 1961 (fig. 5), salt water invaded the canal inland
as far as the footbridge. The time scale for this period is expanded
in figure 6 to show the detailed relations of intrusion, water-level
fluctuation, and discharge. Salt water intrudes on the rising tide,
first appearing at S-29, then at West Dixie Highway 2 to 3 hours





FLORIDA GEOLOGICAL SURVEY


MARMII AT M11ARCH1to3 isAR1HER MARC"3 30 ARCUII APRIL
AVAq I I CI M i
Ar um lowr

---- 1A I -'---,- I '-- .-



- I T i i .pr i .FS CLOSED-. i-

i irNA v s 1 1 E
S" \ ..-: ..- ........ ...



Sla, N Pr: I
I mI I v Iv L

I I "I I I 7


: -. I / I / Ui i iI lf_ II' l I I I S'.-A--- 'LJ-

Figure 6. Graphs comparing fluctuations of discharge, water level, and chloride
content in Snake Creek Canal during the special test of March 27-31, 1961.

later, and finally at the footbridge during the subsequent falling
tide, about 8 hours after first appearing at S-29. Complicated
relations are involved in this time lag and they will be discussed
later, but it should be noted that the length of time of salt-water
stay decreases landward and that the maximum concentration also
decreases landward. For example, salt water did not reach the
footbridge at all on March 29, just before noon (fig. 6). In light of
the movement of salt water shown in figure 6, the long-term position
of salt water in the canal can be inferred from figure 5 by com-
paring the relative fluctuations of conductivity at the three
recording stations.
IN THE AQUIFER
The interface between fresh water and salt water in the Bis-
cayne aquifer is not sharp; a zone of diffusion is invariably present.
The chloride content in the zone of diffusion ranges from about 16
ppm (parts per million), the concentration of fresh water in Miami,
to about 19,000 ppm, the concentration of sea water. The concen-
tration of chloride ion is commonly used to represent the






REPORT OF INVESTIGATIONS NO. 24


distribution of salt water, and isochlor lines are drawn through
points of equal chloride content as determined by analyses of water
from wells.
Cross sections of the zone of diffusion along the south bank of
the Snake Creek Canal are shown in figure 7. The black dots
represent the termini of individual fully cased wells located about
5 feet from the canal; the open circles represent the termini of
wells located about 60 feet from the canal. In intercanal areas the
the isochlor lines turn sharply downward at the inland extremity
of the salt water, and the zone of diffusion takes the shape of a
blunt-nosed wedge of salty water (Kohout, 1961, fig. 3, 4). Along
the Snake Creek Canal (and presumably other canals) the blunted
shape, although still present, tends to be modified by large changes
in gradient and head near the canal.
When the dam is open, the lowest level of the water table
coincides with the water level in the canal, and ground water
flows laterally toward the canal from either side. Also, at such
times, ground water beneath the canal tends to flow upward
through the canal bottom as it follows the part of least resistance
to the ocean.
When the control gates are closed, the water level in the canal
becomes the highest level of the water table because water flows
quickly downstream from inland points of higher head. Under this
condition the exchange of water between the canal and the aquifer
is reversed, and water, whether it be fresh or salty, moves laterally
and vertically from the canal to the aquifer.
In figure 7, the general processes of salt-water encroachment
in the Biscayne aquifer are recognizable:

(1) The deep salt water, below and seaward of the 1,000-ppm
isochlor, moves landward from the ocean in an essentially
horizontal direction. This type of encroachment is rela-
tively slow and insidious. It results from heads that are
too low to prevent intrusion, and can be stabilized or
corrected only by raising the heads.

(2) The lenses of salt water that appear near the bottom of
the canal and then drift progressively downward with time
originate as a result of inland movement of salt water
directly in the canal. This type of encroachment occurs
very rapidly and is the consequence of inadequate
knowledge of the position of salt water in the canal during
control-dam operations.











DISTANCE FROM CONTROL DAM,IN FEET
14nnh 1 in..n an. .
LAND *UR FAf r-
----------------------- o----o7--,--a-- u ,- a--
.--,--50--, .-;-... 0- do---
EXPLANATION Or C. A*--A *Lo

-0 ISOCHLOR LINE IN PPM DECEMBER 8, 1960 / *-
WELLS 5 FET FROM CANAL

..to _
WELLS 60 FEET PROM CANAL

IAND SURFAIF-
TM OT CANAL.-

a JANUARY 3,1961 //d 00





40 -0-- -

. -o .MARCH 24, 1961 soo' /''p"
IDz A *R































LAND SURFACE-i
--Jy ------ -- "sOTTOM OF CNALN' '" '""

-80 JANUARY 24, 1962 p ..' -



Figure 7. Sections along the south bank of the Snake Creek Canal showing
the zone of diffusion in the Biscayne aquifer, 1960-62.






FLORIDA GEOLOGICAL SURVEY


Although the isochlor pattern in figure 7 appears to remain
practically stable from December 1960 to January 1962, careful
comparison of data shows that significant changes occurred at
certain wells. The chloride content in the deepest well at station
4 decreased from 11,850 to 535 ppm, indicating seaward expulsion
of salt water from the aquifer. During the same time, the chloride
content in the deepest well at station 6 decreased from 1,220 to
490 ppm.
The reason for the expulsion of salt water may be understood
by following the events shown in figure 5. From early September
to late October 1960 the control dam was fully open, because of
flood conditions. During this time the heads in the aquifer in the
vicinity of the canal were below normal and salt water moved
horizontally landward and mounded beneath the canal. The chlo-
ride content in well G 420A (the only long-term data covering the
period, fig. 18) increased from 6,800 ppm in June 1960 to 9,200 ppm
on October 19, 1960. The year 1960, then, was essentially a year
of encroachment.
The year 1961 was abnormally dry, and even during the
normally rainy months of September and October the dam remained
practically closed. The infrequent opening of the control dam
caused heads to remain sufficiently high so that expulsion of salt
water from the aquifer occurred throughout 1961 (fig. 7). The
fluctuation graphs in figures 16 to 20 confirm the seaward move-
ment of salt water; chloride contents in most of the wells decreased
generally during 1961.
It is indeed a contradictory situation that the greatest inland
movement of salt water took place in a wet year and the greatest
expulsion of salt water took place during a dry year. Nevertheless,
this fact indicates that the means are available for very effective
control of the movement of salt water in the Biscayne aquifer.


SHORT-TERM POSITION OF SALT WATER

While it is generally not feasible to observe continuously the
complete distribution of salt water, short-term tests, keyed to
recording instruments, provide much information with which to
interpret the long-term data.
A short-term test was performed March 27-31, 1961. The
control gates were completely closed for 6 weeks prior to the test,
and conditions were almost stable despite rainfall of 2 inches that
occurred 3 days before the test (fig. 5).






REPORT OF INVESTIGATIONS No. 24


The sequence of control-dam operations is shown in the water
level graph of figure 6. All four gates were opened fully at 10:00
a.m., March 27, just after high tide. The canal was in natural
tidal oscillation until 9:15 a.m., March 30, when S-29 was completely
closed just before high tide. The intention was to trap salt water
in the canal, as might happen during normal control operations,
and then trace its movement both in the canal and in the aquifer.
The trapped salt water moved inland in the canal much more
rapidly than expected, and at 1:30 p.m. on March 31, at high
tide, two gates were opened for a flushing operation which con-
tinued until low tide, at about 5:45 p.m. The dam was then closed
and remained so until May 26, 1961.

SALT-WATER MOVEMENT IN THE CANAL

The tidal movement of salt water in a canal is rapid and
transient. In figures 8-9 and 12-15 this movement is depicted by
a time-lapse sequence of canal sections showing the approximate
position of isochlor lines along the longitudinal axis of the canal.

NATURAL TIDAL OSCILLATION

The movements of salt water under unimpeded tidal oscillation
on March 29 are shown in figures 8A and 8B; figure 8A shows the
inland extension on the rising tide, and figure 8B shows the sea-
ward movement on the falling tide.
The stage of the tide cycle represented by each isochlor section
is shown by an arrow on the water-level fluctuation graph (inset).
The measured velocity profile for the middle vertical in the canal
at West Dixie Highway is shown at the lower right side of each
section. The discharge is divided into landward and seaward com-
ponents as shown by the figures near the darkened arrows.
Comparison of sections 1, 2, and 3 (fig. 8A) indicates that the
inland advance of salt water was slow during the early part of
the rising tide. Significant landward advance of salt water did not
take place until all flow became negative (landward) in section 3.
This indicates that seaward-flowing water in the upper part of
the canal removes the landward-flowing salt water in the bottom
of the canal by a combined dispersion and interface erosion process.
Maximum inland velocity occurred at 7:37 a.m., about 11/
hours before high tide, section 4; the unshaded discharge arrows
and dashed velocity curve show this condition. The solid velocity
curve at 8:44 a.m., which corresponds in time with the isochlor








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FLORIDA GEOLOGICAL SURVEY


section, indicates that relatively fresh water near the surface
began to flow seaward at least 1 hour before high tide. The high-
salinity water at the inland toe of the wedge, however, continued
to move landward until well after high tide. This time lag in
landward movement of salt water was shown previously in the
chloride-fluctuation graph of figure 6, where salt water did not
appear at the footbridge until about 2 hours after high tide.
These out-of-phase flow characteristics in the upper and lower
parts of the canal result from the density contrast between fresh
water and salt water. At, and for some time after, high tide the
fresh-water head at inland points is too small to counterbalance
landward movement of salt water and the inland toe of the wedge
moves landward as a density current. As the tide begins to fall
more rapidly after high tide, the landward hydraulic gradient
associated with the density current is overcome by seaward
hydraulic gradient from falling ocean level. The salt water at the
bottom of the canal, then, reverses direction and flows seaward.
The salinity distribution in the canal reflects the out-of-phase
flow pattern. The upper part of the canal is rapidly freshened by
large positive (seaward) velocities in the fresh water as the
tide falls (fig. 8B), section 6. However, even after the reversal of
landward movement of salt water, seaward velocities in the salt
water near the bottom of the canal are small compared to those
of the overlying fresh water (velocity curves, sections 7, 8).
This velocity differential causes a lens or slug of salt water to lag
behind in that part of the canal westward from West Dixie High-
way. Apparently, downward-directed velocity components (possibly
related to turbulence near bridge pilings) develop in the fresh
water near the outlet. Salt water in this part of the canal is eroded
and flushed seaward before all the salt water that moved land-
ward during the rising tide can retrace its path to the outlet. The
phenomena occurred repeatedly when the control dam was fully
open. This indicates that the control should not be operated ex-
clusively on the basis of water-level measurements, for slugs of salt
water could be inadvertently trapped. The position of salt water
should be determined directly by conductivity measurements at the
bottom of the canal.

TRAPPING SALT WATER

This section will show the dangers of inadvertently trapping
salt water in a canal by shutting the gate on it. On March 30 at
9:15 ajn., just before high tide, all four gates of S-29 were closed.








REPORT OF INVESTIGATIONS NO. 24


The salinity distribution just prior to the time of closing is shown
in figure 9, section 9.
A stable density stratification exists when fresh water overlies
salt water with the isochlor lines horizontal. Sloping isochlor lines
in stagnant water may be generally interpreted as an indication of
hydraulic instability related to a nonstable density stratification.
After the gates were closed, the water (for purposes of this dis-
cussion) may be considered to have been virtually stagnant, though


I2J
13 83
13
9:
9: 13
iz:
13 ~
13 A.
83


k t
13 4:

A3r 13


9: l.


DISTANCE FROM CONTROL DAM,IN FEET
14000 12.000 10000 8000 6000 4000 2000 0
^ 0 : -- ----- ----- h ----- I -----i-i ,
4 (9) MARCH 30,1961 (8:50A.M.)
Q WATER SURFACE
: ISOCHLORS,IN PARTS PER MILLION


S -i MAR.27 MAR.28 MAR29 MAR.I MAR.31 APR.I CONTROL
SA 5 SP Ii t A, 1 2 I? G G8P 12 GA1 S PR; 1 2 t2 S- P OPEN
S-20 OPEW



J 9 ; -l-i-
-4(] ''''''I

(10) M RCH 30 1961 (11:30A.M.)


ISOGHLORS.IN PARTS PER MILLION


I g 9 .eto MAR.27, M iAR.2I M A19 lMAR30 MAR.31 APR. I I CONTETPP COTwRO





"11 1 1.6 1 .
(II) MARCH 30,1961 (2:40P.M.)

S ISOCHLORSIN PARTS PER MILLION _
(11) MARCH 30,1961( 2:40P.AP






.,- --
-i ">'SOC, --, .WI 'P 'A '-"S' 'PE 'LLI
9:




yi 5" "-,--':. i -{ ,a,,,,,-i -


Figure 9. Sections 9, 10, and .11 through the Snake Creek Canal showing
movement of salt water after closing of the control dam at high tide, ;1
March 30, 1961.





22 FLORIDA GEOLOGICAL SURVEY

some net inflow of fresh water entered the canal as the stage rose;
the sloping isochlor lines in section 9, then, indicate instability.
As expected, a density current developed when the gates were
closed and salt water in the seaward reach of the canal moved
landward. To maintain continuity between the individual water
masses, fresh water from inland points moved seaward through
the upper part of the canal to replace the salt water which moved
landward through the lower part.
These movements were traced primarily by making vertical
conductivity traverses of the canal water from the boat. Also,
vertical conductivity traverses were made continuously at the
footbridge and a series of these profiles is shown at the lower
right side of section 10. The leading nose of the salt-water wedge
passed the footbridge at 10:44 a.m., it was about 5 feet thick and
had a maximum chloride content of about 1,500 ppm.
By tracking the intruding nose of the salt-water wedge for a
period of time it was determined that the rate of landward move-
ment was about 15 feet per minute, or about 900 feet per hour. The
serene water surface completely belied the rapid encroachment
taking place at the bottom of the canal.



if -. ?-"



















direction of the Snake Creek Canal (at right).





REPORT OF INVESTIGATIONS No. 24


The salt water moved nearly 1 mile in the 41/4 hours between
9:15 a.m., the time of closing the gates, and 1:30 p.m. Careful
inspection of the discharge graph at Miami Gardens Drive (fig. 6)
shows a gentle increase in discharge at about 1:30 p.m. This
increase was sensed by a small velocity vane positioned in the
fresh water above the intruding salt-water wedge, and reflects the
change in velocity in the seaward flow section as the wedge passed
Miami Gardens Drive.
At about 2:00 p.m. salt water began to flow into the rock pit,
in a manner similar to water passing over a waterfall (fig. 3, 9, 10,
11).
By late afternoon the hydrologic significance of the rock pit
became clear. The rock pit was a hydraulic sink in which the salt
water would temporarily collect. The salt water would resume its
upstream movement only after it filled the rock pit above the level
of the bottom of the canal.
Thorough investigation of the rock pit began the following
morning, March 31. As shown by figure 12, section 12, a con-
siderable quantity of salt water had flowed into the pit. The
chloride content in the rock pit was low compared to that of water
in the canal because, as the salt water flowed downward into the


Figure 11. Rock pit showing approximate bottom topography.





FLORIDA GEOLOGICAL SURVEY


lake, heavy mixing and dispersion of salt reduced the maximum
concentration to less than 2,000 ppm chloride, though water with
a concentration of 5,000 ppm was observed flowing into the pit.
Soundings, made in conjunction with numerous samplings and
conductivity traverses during subsequent months, were used to
develop a rough contour map of the bottom of the rock pit (fig. 11).
Two deep holes nearly 40 feet below sea level are separated by a
ridge having an altitude of about 25 feet below msl. In these holes
most of the salt water samples were collected at depths greater
than 20 feet below msl. The profile of the pit bottom in the
canal sections (fig. 8, 9, 12-15) is generalized and represents no
particular section line.

FLUSHING OF SALT WATER FROM THE CANAL

Once having entered deep holes such as those in the rock pit,
the heavy salt water cannot easily be retrieved and flushed. As
most of the salt water remaining in the canal would eventually
flow into the rock pit, the canal was flushed on March 31, the
day after closing of the control gates. At high tide, at 1:30 p.m.,
two gates of S-29 were opened fully. The distribution of salt water
before flushing is shown in figure 12, section 12, and the progress of
flushing is shown in sections 13 and 14.
The principle of flushing is based on the tidal fluctuation of the
ocean. In Miami, the range of fluctuation from high tide to low
tide amounts to about 2 feet. Assuming that the water level up-
stream from the control dam is at least as high as the level of the
ocean at high tide, the dam can be opened at high tide or shortly
thereafter. The falling tide level continually creates a head
differential between inland parts of the canal and the ocean; thus,
for a period of about 6 hours, from high tide to low tide, a steep
seaward hydraulic gradient produces a high rate of discharge from
the canal. The discharge reaches a maximum slightly before low
tide (fig. 6). Referring to the velocity curve in figure 8B, section 7,
the seaward velocity of the fresh water just below low tide can be
estimated at about 2 fps (feet per second) while that of the salt
water can be estimated at about 0.7 fps. At this velocity, the salt
water will move seaward at a rate of at least half a mile per hour.
During the 6 hours of falling tide, flushing of 2 to 3 miles of the
downstream reach of a canal is feasible. Turbulence disperses large
quantities of salt water upward into the high-velocity fresh water
and the flushing is probably more effective than the calculation
indicates.







REPORT OF INVESTIGATIONS No. 24


C 3


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6, IA,
0 25


DISTANCE FROM CONTROL DAM;IN FEET
S i.oo -10o.o : 0o Q. 6 4000 2_
0(2) MARCH 31,1961 (B:50A.M.)
WATR Fi)VFArcF-


so ,-.'. _----- 7---- -


MA 9? MA L2 MA 2L MR AR. AP1LI

62 P 1 G'A SP &A 2 12 SA I G ., P .





(13) MARCH 31,1961 (2:40P.M.)
WATER SURFACE"A
ISOCHLORSIN PARTS PER MILLION TE


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10-- -----------". ~cK--------*-!--F- ----.- --,::---,'----- I----
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(14) MARCH 31,1961(440 P.M.)
WATER SURFACE--


S (15) APRIL

ij ISOCHLORSIN F
t
-to
9too
to ec. ^-a
s- 06


Figure 12. Sections 12, 13, 14, and 15 through the Snake Creek Canal showing
movement of salt water during and after flushing, March 31 to April 1, 1961.


ISOCHLORS,IN PARTS PER MILLION 1

-I l_
MAR.? MA28 MAR29 MAR30 MAR.31 APR I CONTROL
_GA I P I 6AI_6PIA1SP16 *PI2A SP 12 GAIIAP 12 GA I POPEN


WE. Ai l I N


-40
_,< = <


I





FLORIDA GEOLOGICAL SURVEY


During the dry spring of 1962, high easterly winds on March
8 and 9 coincided with high average tides and the wind-driven
waters of Biscayne Bay overtopped many control dams in the
Miami area at high tide. Salt water invaded the canals at a time
when dams throughout the area were held completely closed to
maintain heads and to conserve fresh water. In spite of the entrap-
ment of salt water in the canals there was reluctance to flush the
canals, as it was felt that the consequent loss of fresh water could
not be tolerated.
The series of isochlor sections for unimpeded tidal oscillation
(fig. 8A, 8B) show that it is not necessary to have a high fresh-
water head behind the dam in order to perform a flushing operation.
The principle of flushing depends almost entirely upon the falling
stage in the ocean itself. The primary requisites are: (1) the
canal should be long enough to tap a reservoir of fresh water at
inland points, and (2) the control dam should be of a type that
permits rapid operation so that opening and closing can be
accomplished readily at opportune moments.
Flushing of salt water from a canal entails some loss of fresh
water. However, wasting a small portion of the vast reservoir of
fresh ground water tapped by a canal is preferable to allowing a
small volume of salt water to move inland considerable distances
where it may contaminate much larger volumes of fresh water.
ULTIMATE LOSS AND FLUSHING OF THE REMAINING
TRAPPED SALT WATER
In the 41/-hour flushing operation of March 31 not all salt
water was removed. As shown by figure 12, section 15, a lens of salt
water remained in the downstream reach of the Snake Creek Canal.
Also, most of the relatively heavy salt water in the rock pit
remained. These separated masses of salty water were traced
until July 1961 (fig. 13-15).
The downstream lens in the canal tended to spread toward the
rock pit, but a large part of the salt water in the lens disappeared
between April 1 and 3 (compare sec. 15, 16). The disappearance
coincided with an increase of chloride content in observation wells
adjacent to the canal and will be considered in a later section.
The salt water in the rock pit appeared to remain practically
stable from April 1 to 18 (sec. 15-19). The primary change
during this period was a dispersion-caused widening of the contact
between the 1,000- and 200-ppm isochlors.
Between April 18 and May 3, the isochlor positions changed
significantly in the rock pit and also in the canal (compare sec. 19,






REPORT OF INVESTIGATIONS NO. 24


. k I tI
I 5 | a I I
ICe F O M EET
DISTANCE FROM CONTROL DAM,IN FEET


Figure 13. Sections 16, 17, and 18 through the Snake Creek
changes in isochlor positions, April 3-11, 1961.


Canal showing


20). An increase in chloride content was sensed by the conductivity
recorder at the footbridge between April 22 and 26 (fig. 5) and,
therefore, this was probably the period when the distribution of
chloride content in the rock pit changed.
In the bar graph in figure 14, lower part of section 19, the wind
movement exceeds 100 miles per day for 3 consecutive days, April
21-23; this period nearly coincides with the period of increase in
chloride content at the footbridge. Apparently, the 3 days of strong
wind mixed the water in the upper part of the rock pit. The


61:
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S, v 000 6000 4000 2000 0
(16) APRIL 3, 1961
WATrR SURFACE-'
0 ISOCHLORS, IN PARTS PER MILLION



CONTROL
2 CLOSED


6 1 (17)

Cj isoc



-IC




$-4
IC
(18)



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FLORIDA GEOLOGICAL SURVEY


~-IC
I~ ~
Ifl


'U
h *0
<6 4 O .
I D l i;4


Figure 14. Sections 19, 20, and 21 through the Snake Creek Canal showing
changes in isochlor positions, April 18 to May 16, 1961.

downward movement of isochlors in section 20 represents dispersion
and upward movement of salt ions into the uppermost water of the
rock pit. Small hydraulic gradients, associated with the wind
and with ground-water discharge in the downstream reach of the
canal, then carried the salty water downstream where it was sensed
by the conductivity recorder at the footbridge.
Wave height on a given lake depends on the length of time the
wind blows as well as on the wind speed. Therefore, the 3 consecu-
tive days of high wind, April 21-23, were far more effective in





REPORT OF INVESTIGATIONS NO. 24


stirring the waters of the rock pit than were the single days of
similar wind speed in early April. The downward movement of
isochlors between May 3 and 16 (sec. 20-21) undoubtedly relates
to the relatively high sustained winds of May 6 to 9. It is
significant that wind and waves serve as agents for removal of
trapped salt water.
On May 26, after heavy rainfall, the dam was opened (fig. 5).
The resulting tidal oscillation and discharge produced small currents
in the bottom of the rock pit. This agitation reduced the chloride
content and density more in the eastern hole (fig. 11), by upward
dispersion, and the salt was gradually flushed toward the partly
open control dam. Although residual traces of the original con-
tamination remained, the partial gate openings that started March
26 were effective in reducing salinity even at the very bottom of
the rock pit (fig. 15). The decrease in chloride content at the
footbridge (fig. 5) at the same time indicates that the tidal im-
pulse of the ocean can be utilized .to remove salt water that has
become trapped in deep pits or lakes along the course of a canal.

SALT-WATER MOVEMENT IN THE AQUIFER

The short-term movements of salt water in the aquifer are
closely influenced by happenings in the canal. Fluctuations of
chloride content in wells along the south bank of the canal during
1960 and 1961 are shown in figures 16-20. During the special test
of March 27-31, water samples were collected as often as four
times daily and these are plotted on an expanded time scale.

NATURAL TIDAL OSCILLATION

Under unimpeded tidal oscillation, water moves from the aquifer
into the canal on a falling tide and from the canal into the aquifer
on a rising tide. This to-and-fro motion prevailed when the gates
were open March 27 to 30 and is indicated by the two-headed arrow
in the March 30 section of the sequence of insets in figure 19.
During this period the chloride content in the shallow wells
fluctuated only slightly at stations 1 to 5 (fig. 16-18), and practically
none at the more landward stations (fig. 19-20). Although high-
salinity water oscillated landward beyond station 8 in the canal
(fig. 8), salt water did not appear in wells located as little as 5
feet from the canal. This is not surprising. The net flow of fresh
ground water in the upper part of the aquifer is toward the
canal. Any small amount of salt water that penetrates the aquifer






30 FLORIDA GEOLOGICAL SURVEY



it i Z @

3 Q 1ti 3 1 n
DISTANCE FROM CONTROL DAM,IN FEET
14000 12000 10000 000 6000 4000 000 n n


(22) MAY 29.1961

ISIOGMLORS.IN PANTS PER MILLION

N .'- -- -o p- a g- -. u z -
-20





(23) JUNE 13. 1961
WATER SURFACE
sOCLLONRS. IN PAR' PER MILLION


-. ..- .., .PARLrLOPEN


R -340


(24) ULr 10I.1961
I WATER SURFACE-a
t" ISOCM4LORS.m PARTS PER MILLION









Figure 15. Sections 22, 23, and 24 through the Snake Creek Canal showing
changes in isochlor positions May 29 to July 10, 1961.

on a rising tide is flushed back to the canal on the subsequent
falling tide. Thus, the average flow of fresh ground water toward
the canal opposes and greatly minimizes movement of salt water
from the canal to the aquifer during unimpeded tidal oscillation.
A different situation exists in the deep part of the aquifer. The
average head in the aquifer is lowered when the gates are open.
This causes salt water, deep in the aquifer, to move landward. The
CtII




















movement is slow but relentless. An example is the increasehowin
chloride content from 6,800 to 9,200 ppm in well G 420A between














,a.
z 1,000




0: t00
0e
-l:
u


I
.
0




I
N
0O
Fj


Figure 16. Graph showing fluctuations of chloride content at well stations
1, 2, and 3, 1960-61.























* 1000


Figure 17. Graph showing fluctuations of chloride content at well station
4, 1960-61.
































Figure 18. Graph showing fluctuations of chloride content at well station
5, 1959-61.
















MAP VIEW


IAIi
sm~QDau


Figure 19. Graph showing fluctuations of chloride content at well station
6, 1960-61.















2 200 I I i 1 II11 11,- I I I I I I I I
L STATION 8 G G1152



TMESCALE -- ,;.
- 100 Wo .- 1;---', -50






8 II I 11 I I I I I 5I I0 I
200j--,---I---,--- --| II|1||H|--I ------ I---I ------ r-'--I---1--
S STATION 7.. 1146 1148



SG 1147. .- MP VIEW
TIME SCALE 0MI i, \

NOV. D.EC. JAN. FEB. MAR. ____ e APRH MAY JUIdE JUlY AUG. | SEPT. OCT. 1 NOV. J DEC.


7, 8, and 9, 1960-61.
NOV. EC. JAN 20 B.Grap MARshow I a tionsR. oA ch ide te at w l t


7, 8, and 9, 1960-61.





FLORIDA GEOLOGICAL SURVEY


June and October 1960, when the dam was open for hurricane
Donna floods (fig. 18).

RECHARGING THE AQUIFER WITH TRAPPED SALT WATER

The canal is quickly converted from a drainage canal to a
recharge canal by closing the control. The water which happens
to be in the canal at this time, either fresh or salty, is driven from
the canal under hydraulic gradient and appears in wells adjacent to
the canal.
All well stations (fig. 16-20) showed an increase of chloride
content on the morning of March 31, the day after salt water was
trapped. As will be noted by comparing the graphs, the degree of
reaction to the recharge of salt water varied. A partial list of
the factors involved is as follows: (1) The distance of the well
from the canal; (2) the altitude of the terminus of the well in
relation to the altitude of salt water in the canal; (3) the concen-
tration of salt water, which varies both vertically and longitudinally
along the canal; (4) the amount of dispersion of salt as the water
passes through the rock; and (5) the local permeability of the
aquifer near the well station.
The relations of salt-water movement at station 6 are shown in
the section views of figure 19. Section a for March 30 shows that
salt water had not invaded the aquifer during unimpeded tidal
oscillation.
In section b (10:00 a.m., Mar. 31), approximately 24 hours
after closing the control gates, the chloride content in well G 1138,
5 feet from the canal, had increased from 60 to 13,700 ppm while
that in well G 1140, 65 feet from the canal, increased from 60 to
288 ppm. The sharp increase of chloride content in these wells, in
contrast to the decrease in well G 1136 (about 22 feet deeper) from
125 to 102 ppm, indicates that the salt water moved nearly hori-
zontally. This is a reasonable observation because the horizontal
permeability is generally much greater than the vertical
permeability in stratified aquifers.
Section c (4:00 p.m., Mar. 31) shows the flow direction while
the canal was being flushed. The sharp decrease of chloride content
in wells G 1138 and G 1140 in the fluctuation graph at this time
indicates that part of the salt water, that had previously invaded
the aquifer, was flushed back toward the canal. After the flushing
operation was completed and the dam was closed, the unflushed
salt water in the aquifer resumed a flow direction away from the
canal (sec. d).






REPORT OF INVESTIGATIONS No. 24


In addition, salt water from the salt-water lens, which remained
in the downstream reach of the canal after flushing (fig. 12, sec.
15, and fig. 13, sec. 16),moved into the aquifer. The disappearance
of the salt-water lens from the canal by April 3 coincided with the
sharp decrease in chloride content in well G 1138 on that date; the
recharge water in the canal had become relatively fresh but the
increase of chloride content in well G 1140 between April 1 and 3
showed movement of the salt-water slug farther from the canal.
The low maximum chloride content in well G 1140 as compared to
that of well G 1138 indicates dilution of the salt water during
movement between the wells.
It is interesting to note that the chloride-content curve for well
G 1138 crosses over and becomes lower than that for well G 1140
after April 5 (fluctuation graph, fig. 19). As previously shown,
water in the canal continued to freshen with time. Thus, as the
slug of salt water passed farther into the aquifer it was followed
by relatively fresh water which appeared first in well G 1138, near
the canal bank, and then in well G 1140. The reversal of the
chloride-content curves shows the cause-effect relation of salinity
in the canal on salinity in the aquifer.
The chloride content in well G 1137 reached a maximum on
April 7, more than a week after the control was closed (fluctuation
graph, fig. 19). During this week, some of the salt water that
remained at the bottom of the canal was dispersed upward in the
canal by wind agitation (fig. 13, sec. 16, 17). As the shallow wells
were recharged by water from the upper part of the canal, the
time lag of salinity fluctuation in the shallow wells reflects the
changes of salinity in the upper part of the canal. The lower peak
chloride content in well G 1139, as compared to well G 1137 again
demonstrates the reduction of salinity by dispersion of salt as
ground water moves through the pore spaces of the aquifer.
Well G 1136 is 22 feet deeper than well G 1138 so that the
vertical component of salt-water movement is identified by the
relative fluctuations of chloride content in the two wells. The
chloride content in well G 1136 did not reach its maximum until
early May, about 1 month after the short-term test, whereas that
of well G 1138 reached its maximum 1 day after the control was
closed (fig. 19). This great difference in time results from several
factors. An important driving force in the vertical direction is the
difference in density between the overlying heavy salt water and
the underlying light fresh water. In open water, an unstable density
condition of this type is corrected rapidly by a massive discrete






FLORIDA GEOLOGICAL SURVEY


overturn of the two fluids. The settling of salt water in the rock
pit is an example. However, in an aquifer the pores of the rock are
small and the fluid movement is split into similarly small flow
filaments. The fresh water rises at the same time that the salt
water sinks. The overturn is reduced to slow infiltration wherein
dispersion of salt plays an important part in modification of the
original concentrations. The effect of trapping salt water in March
1961 is recognizable until October 1961 in the fluctuation curve for
well G 1136 (fig. 19).
In the longitudinal sections of figure 7, the downward movement
of salt water is depicted as a lens of salt water that appears near
the bottom of the canal and progressively drifts downward with
time (sections of Mar. 31 to May 16, 1961). At the same time there
is a horizontal component of flow perpendicular to and away from
the canal; this movement is shown by the arrows in section e of
figure 19.


ULTIMATE LOSS AND FLUSHING OF SALT WATER IN THE AQUIFER

Comparison of the fluctuations of chloride content at all stations
(fig. 16-20) shows that the deep wells were little affected by the
short-term test but that the shallow wells were affected in a
characteristic pattern. This pattern is responsive to the operations
of the control dam.
The control gates were closed on March 31, and they remained
closed until May 26 (fig. 5). Initially the chloride content in the
shallow wells rose sharply and then tended to decrease slightly. At
the downstream stations the decrease was somewhat larger because
ground-water movement around the control dam and toward the
ocean was more rapid there.
At the more inland stations (fig. 20), the aquifer is recharged
rapidly until canal water refills that part of the aquifer that is
unwatered during drainage. After this, the gradient away from
the canal becomes comparatively small as the ground-water flow
system approaches a steady state consistent with the boundaries
of the aquifer. As the rate of recharge is proportional to the
gradient, salt water at first moves rapidly into the aquifer, and
subsequently at a much slower pace. The salt water, therefore,
remains within reasonable proximity of the canal and is subject
to retrieval.
The chloride content in most of the shallow wells decreased
sharply after the control dam was opened at the end of May. The





REPORT OF INVESTIGATIONS NO. 24


fluctuations at station 7 (fig. 20) are especially interesting. The
chloride content in wells G 1145 and G 1146 near the canal de-
creased sharply, whereas that in G 1147 and G 1148, 60 feet from
the canal, remained unchanged. Figure 5 shows that the total
gate opening was decreased from 30 feet May 26 to 31 to about 5
feet thereafter. At the 30-foot gate opening, the canal caused
drainage and some of the salt water flowed out of the aquifer
into the canal. Water levels recovered somewhat after closing to
the 5-foot opening (fig. 5), and fresh water from the canal re-
charged the aquifer. The movement of fresh water into the aquifer
was sufficient to freshen the water in wells close to the canal
(G 1145-G 1146), although water in the wells farther from the
canal (G 1147-G 1148) remained relatively salty. Clearly, part of
the salt water trapped in March had been removed but a part still
remained as an isolated mass. During the next gate openings on
August 20 .the remaining salt water was completely flushed from
the aquifer and the chloride contents of all four wells became nearly
the same. Thus, the low-salinity water trapped in March was re-
covered from the shallow part of the aquifer by August 1961.
The recovery was possible because the density of this low-
salinity water (less than 200 ppm) was not great enough to cause
a large downward movement. The dense, high-salinity water that
percolated downward at station 6 (shown by the graph of well
G 1136, fig. 19) obviously was not recovered. It continued to move
downward until the density contrast was eliminated by confluence
with water of the same density in the deep part of the aquifer.

MANIPULATION OF THE CONTROL DAM FOR
MAXIMUM BENEFIT

The primary function of a canal in the Miami area is to remove
surplus floodwaters. As floods in Miami result from heavy rainfall,
it is obvious that the floodwaters must always be fresh. If salt
water penetrates landward in a canal, the drainage function of the
canal is impaired because floodwaters cannot flow seaward through
that part of the canal in which salt water flows landward; therefore,
the optimum removal of floodwaters occurs when no salt water
invades the canal. Whether the flow condition is steady state or
transient due to tide, it should be recognized that intrusion of salt
water in a canal is an obvious sign of reduced efficiency in the
discharge of fresh floodwaters.
All four gates of S-29 were fully open for nearly 2 months
after hurricane Donna passed on September 10, 1960 (fig. 3). Only






FLORIDA GEOLOGICAL SURVEY


during September 10 to 14 was the discharge great enough to keep
the canal flushed of salt water. At all other times during the period
of record shown in figure 5, full openings of the gates permitted
salt water to move upstream on a rising tide. This suggests that
the control was operated at full-open position much longer than
necessary.

FACTORS AFFECTING INLAND MOVEMENT OF
SALT WATER

The manner in which the total gate opening is manipulated has
a bearing on the landward extension of salt water in the canal.
For example, a gate opening of 30 feet may be represented by a
7.5-foot opening for each of the 4 gates or a 15-foot opening for
each of only two gates (the other 2 remaining closed).' The
schematic flow diagrams of figure 21 illustrate the hydraulic
advantage that attaches to uniform distribution of the gate
opening. In figure 21a fresh water is forced to flow downward
around the obstruction of the gate. This downward displacement
of the fresh-water discharge section impedes the upstream intrusion
of salt water. Uniform opening and submersion of the four sluice
gates (in contrast to fully open gates) can be used as a device
to distort the flow pattern deliberately in order to prevent salt-
water intrusion.
Questions arise as to whether the movement of salt water into
a canal is ever justified on the basis that widest opening of the
control gates will cause greatest discharge of fresh floodwater.
The critical point of entrance of salt water into a canal is at the
control dam, for here is the only place where intrusion can be
controlled. The problem involves the total head at the dam, the
amount of discharge at various gate openings under these various
heads, and the influence of fresh-water head in preventing upstream
movement of salt water beyond the control dam.
Using the principle of static equilibrium for fluids of different
density, salt water will not move upstream as long as the head
differential between the fresh-water and salt-water sides of the
control gate is greater than 0.3 foot (for a canal whose bottom is
12 feet below sea level). This value is obtained by balancing the

'The values used here only demonstrate a principle; the discharge area
for 2 gates opened 15 feet is not equal to the discharge area for 4 gates
opened 7.5 feet because the gates at wide-open position may be above the
actual water surface by as much as 3 feet.






REPORT OF INVESTIGATIONS NO. 24


pressure exerted by fresh water against the opposing pressure
exerted by sea water at the bottom of-the canal.

Pf = pfg lf P, pa g ls
Pt g 1 = ps g i,
letting if = h + 1, (See fig. 21a):
pf (h + 1I) = ps ,
h = 1, (p -pf)
Pf
h = 12 (1.025 1.000) = 0.3 ft.
1.000
Pf = pressure exerted by fresh water
Ps = pressure exerted by sea water
g = acceleration of gravity
pf = 1.000, density of fresh water
pS = 1.025, density of sea water
1f = length of fresh-water column, in feet
1i = 12 feet, length of sea-water column
h = head differential between fresh-water and sea-water sides of control
dam, in feet.

Referring to the schematic flow diagram of figure 21b, when
the gates are fully open, the head loss through the control structure
itself is very small. The flood conditions would have to be very
severe in order to maintain 0.3 foot of fresh-water head at the
control dam near high tide, when the greatest possibility of inland
movement of salt water exists. The reason for upstream penetra-
tion of salt water on September 15, 1960 (fig. 5), only 5 days after
the passage of hurricane Donna, is evident. With fully open gates,
the head differential and discharge decrease very rapidly until
salt water is able to flow upstream during the rising tide. Sub-
mergence of the gates, however, tends to focus all the head loss at
the control (fig. 21a). This is very desirable and if the four gates
of S-29 are uniformly adjusted to maintain a head differential of
0.3 foot during the rising tide, no salt water will invade the canal.

DISCHARGE RELATED TO GATE OPENING

The quantity of water discharged under a sluice gate depends
on the head differential across the gate and on the cross-sectional
area through which the discharge occurs.
Figure 22 shows the approximate discharge related to head
differential for various openings of one gate of the control dam in
the Snake Creek Canal. The plotted points represent the mean
daily discharge at West Dixie Highway and the head differential,






FLORIDA GEOLOGICAL SURVEY


(b)

I




WATER SURFACE

FRESH WATER

INTERFACE
W SUSALT WATER
Figure 21. Schematic flow diagrams: a, with gate partially submerged; b, with
gate fully open.
as obtained from the difference in mean daily gage height at West
Dixie Highway and Biscayne Bay at NE 84th Street. Comparison
of past records indicated that the mean daily gage heights of
Biscayne Bay at NE 84th Street and the now discontinued gaging
station just below the control dam in Snake Creek Canal were com-
parable when the control dam was closed or the discharge was
within the range shown in figure 22.
The curves were calculated by using the formula Q = ACV2 gh
where Q is the discharge, A is the cross-sectional area, C is. the
coefficient of friction, g is the acceleration of gravity, and h is the







REPORT OF INVESTIGATIONS NO. 24


S200


DISCHARGE,
400 600


IN
800


0I


3 .
o
Z o



z 0


00 0
oo
0 o

I Ij 0
0 0
LL 0 -




0 o.8 0
1 0


LI.//7 / o /

4.y


CFS
1000


1200 1400


Figure 22. Graph showing relations of head differential and discharge for
various openings of one gate of the control dam in the Snake Creek Canal.



head differential through the gate. As Q, A, and H were known
for each of the points in figure 22, the coefficient of friction (C)
was evaluated for the different gate openings. Then, using the
average value of C and holding the area (A) constant for the
individual gate openings, the curves were constructed through the
mean value of the plotted points by solving the formula for Q while
letting the head differential (h) vary.
The curves may be used to estimate low discharge for the
control structure provided the upstream tidal range is less than
0.5 foot. The accuracy decreases when the tidal range of the up-
stream stage recorder exceeds 0.5 foot during a given day.
The following table shows the recession of discharge following
the passage of hurricane Donna on September 10, 1960.


'.7


I-- I I I
.--HEAD DIFFERENTIAL THAT WILL PREVENT
UPSTREAM MOVEMENT OF SALT WATER
AT BOTTOM OF CONTROL DAM


sc~






FLORIDA GEOLOGICAL SURVEY


Date Mean daily head Mean daily
(September) differential (feet) discharge (cfs)

11 0.48 2,430
12 .38 2,180
13 .31 1,800
14 .28 1,660
15 .23 1,510
16 .17 1,290
17 .19 1,240
18 .22 943


Salt water invaded the canal on a rising tide on September 15.
A comparison of the measured discharges in the table (all gates
fully open) with the plotted points in figure 22 (only one gate
partially open) indicates that the four gates can be partly closed
within a week after a flood peak has occurred to maintain a head
differential of at least 0.3 foot. This would keep the salt water out
of the canal. The small increase in head required to maintain the
same discharge would be insignificant from the standpoint of flood
height above the dam.
There is a distinct difference between holding salt water back,
as shown in the schematic flow diagram of figure 21a, and allowing
salt water to invade the canal, as in figure 21b. Inland penetration
of salt water does not contribute to seaward discharge because, as
previously defined, true removal of floodwaters must only mean
removal of fresh floodwaters. Salt water which penetrates land-
ward in the canal merely replaces fresh water; once it has gained
entry, energy must be expended to remove it. In the velocity pro-
files of figures 8A and 8B, the velocities near the bottom of the
canal are landward during the rising tide, and are small relative
to the seaward velocities in the upper part of the canal during the
falling tide. These velocities reflect two things: (1) Sea water
provides back pressure which is greatest at the bottom of the
canal. (2) Energy is expended in driving the intruded salt water
back to the ocean on the falling tide. As the noncontributing
landward flows and the reduced seaward flows of salt water must
be integrated with the positive flow of fresh water to yield the
mean daily discharge, the mere presence of salt water in the canal,
in effect, reduces the mean daily discharge of true (fresh) flood-
waters.
In some respects, salt water acts as a movable dam in the
bottom of the canal; it effectively reduces the depth of the canal






REPORT OF INVESTIGATIONS NO. 24


through which fresh water can flow seaward. For example, if salt
water occupies the lower 6 feet of the canal when the control is
fully open (fig. 8), the effective flow section for fresh water is
reduced to a thickness of about 6 feet (total depth of canal is 12
feet). If the gates are lowered to an opening of, say, 7 feet, the
fresh-water discharge will be impeded and this will cause the fresh-
water head to rise until salt water is driven out of the lower part
of the canal. Conceivably, the head might build up sufficiently so
that the fresh water would flow through the full 7-foot section
below the sluice gate. However, there is no assurance that this
will occur-fresh water may discharge through only the upper
part of the 7-foot thick section while salt water continues to flow
landward through the lower part.
This leads to an important point. The effect upon head and
discharge caused by constriction from lowering the gates, as
opposed to constriction and back-water effect from salt water in
the bottom of the canal, is indeterminate without controlled tests.
The optimum gate opening probably occurs when the salt-water
wedge is held just downstream from the control and only fresh
water is flowing beneath the sluice gates. This gate opening will
discharge the maximum amount of water without permitting
salt water to invade the canal and aquifer above the control.
Future studies should be directed toward evaluation of the
relations of fresh-water to salt-water discharge with various gate
openings, in the immediate vicinity of the control dam.

SUMMARY

The Snake Creek Canal cuts through highly permeable lime-
stone of the Biscayne aquifer of southeastern Florida. The canal
extends inland about 18 miles from the salinity-control dam located
near the shoreline. The canal is somewhat representative of a
number of canals in the area and information learned during-the
study is generally applicable to other canals.
Salt water was traced in the Snake Creek Canal under various
circumstances of control-dam operation during a special test in
March 1961. In the first phase of this test all four gates of the
control dam were opened wide. Salt water moved inland about 2
miles during the rising tide and seaward during the falling tide.
Salt water penetration into the aquifer was negligible during the
3 days the gates were fully open.
In the second phase of the special test the control was completely
closed at high tide, trapping salt water above the dam. A density






FLORIDA GEOLOGICAL SURVEY


current developed in the trapped salt water, which caused it to
flow inland along the bottom of the canal at a rate of about 900
feet per hour until it reached a deep rock pit which acted as a
hydraulic sink. A flushing operation removed salt water from the
canal, but not that in the rock pit. Measurements of electrical
conductivity during the subsequent months in the rock pit showed
that the salt dispersed upward by a combination of wind, tidal
mixing, and molecular diffusion. After reduction of salinity by
this upward dispersion, the salt water in the rock pit was flushed
seaward through the canal.
When salt water is trapped in a canal by control-dam operation
or by overtopping the dam during abnormally high tides, it is not
necessary to wait for nor to have high fresh-water head behind
the dam in order to perform a flushing operation. The falling tide
(level) of the ocean creates a seaward hydraulic gradient and a
condition favorable for a high rate of discharge from the canal.
Calculations and observations show that flushing of 2 to 3 miles
of the downstream reach of a canal is feasible during one falling
tide. The canal should tap a reasonably large reservoir of fresh
water and the control dam should be the type that permits rapid
operation so that the gates can be opened just after high tide and
closed at low tide. Although some fresh water is lost during flushing,
good water management is better served by wasting a small portion
of a vast reservoir of fresh water than by allowing a small volume
of salt water to move inland to contaminate a much larger volume of
fresh water.
The movement of salt water in the aquifer adjacent to a canal
is closely related to happenings in the canal itself. When the gates
are closed, the downstream controlled reach of the canal is
immediately converted from a condition of drainage to a condition
of recharge. Salt water trapped behind the dam during the test
entered the aquifer and quickly appeared in wells adjacent to the
canal. Sections perpendicular to the canal showed that the salt-
water outseepage from the canal moved much more rapidly hori-
zontally than it did vertically. The residual effect of downward
movement of salt water in the aquifer was recognizable for 6
months at one of the observation stations. The fluctuations of
chloride content in shallow wells indicate that some of the low-
salinity water in the upper part of the aquifer was eventually
recovered and flushed seaward through the canal during subsequent
gate openings.






REPORT OF INVESTIGATIONS No. 24


The movement of high salinity water deep in the aquifer
responds to head changes related to long-term operations of the
control dam. The chloride content in deep wells increased during
the fall of 1960 when the control remained open because of flood
conditions. The year of 1961 was abnormally dry; infrequent
opening of the control caused heads to remain sufficiently high so
that general expulsion of salt water from the aquifer occurred
throughout 1961. It is a contradictory situation that the greatest
inland movement of salt water took place in a wet year and the
greatest expulsion of salt water took place during a dry year.
Nevertheless, this fact indicates that the means are available for
very effective control of the movement of salt water in the
Biscayne aquifer.

CONCLUSIONS AS TO OPERATING CRITERIA

The data in this report indicate that salt water need not and
should never be trapped in a canal. As the position of salt water
in a canal has only a tenuous relation to head, the following
conclusions are made with regard to the operation of the gates:
(1) Control gates, which operate automatically by sensing the
head differential between the upstream and downstream
sides of the dam, cannot be operated effectively on an
automatic basis until the relations of salt-water movement
and head are properly evaluated. It is entirely possible for
the automatic operations to trap salt water inadvertently.
(2) In.addition to the upstream and downstream stage recorders
which are being installed at the new automatically
controlled dams, a conductivity recorder should be installed
in each canal slightly upstream from the control dam. Data
from these three sources will permit future evaluation of
the relation of salt-water movement to total head in the
canal and to head differential through the control dam.
(3) Persons responsible for gate operation should be furnished
a portable conductivity instrument to determine the salinity
at the bottom of the canal at different points. Knowledge
of the presence and movement of salt water in the canal
will aid greatly in proper operation of the control.
The following suggestions are made for operation after heavy
rainfall:
(1) After the first inland penetration of salt water during a
rising tide (which may occur within a week after a flood






FLORIDA GEOLOGICAL SURVEY


peak), the gates should be partly closed as conditions
permit.
(2) The gates should always be opened uniformly so that
maximum fresh-water pressure is applied against the in-
truding salt water by increased velocities of the fresh water
as it flows under the submerged sluice gates.
(3) The instantaneous dynamic head that will prevent inland
movement of salt water cannot be determined without
controlled tests. However, the calculated hydrostatic head
that will prevent intrusion at the bottom of S-29 is 0.3
foot. Therefore, salt water will not enter the bottom of
the canal above the control structure if an instantaneous
head of 0.3 foot is maintained between upstream and down-
stream sides of the dam at the time of minimum head
differential, about 2 hours before high tide.
Some experimentation will be necessary to establish specific
operating criteria. Closing the control gates so that 0.3 foot of head
will be maintained during the rising tide will tend to increase the
stage in the upstream part of the canal by an amount not greater
that 0.3 foot. This is a tolerable increase in stage even during
floods.







REPORT OF INVESTIGATIONS NO. 24


REFERENCES

Cooke, C. W.
1939 Scenery of Florida interpreted by a geologist: Florida Geol.
Survey Bull. 17.
Ferguson, G. E. (see Parker, G. G.)
Kohout, F. A.
1960 Cyclic flow of salt water in the Biscayne aquifer of southeastern
Florida: Geophys. Research Jour., v. 65, no. 7, p. 2133-2141.
1961 Case history of salt water encroachment caused by a storm
sewer in Miami: Am. Water Works Assoc. Jour. v. 53, no. 11, p.
1406-1416.


Leach, S. D.
1963


(and Sherwood, C. B.) Hydrologic studies in the Snake Creek
Canal area, Dade County, Florida: Florida Geol. Survey Rept.
Inv. 24, pt. 3.


Love, S. K. (see Parker, G. G.)
Parker, G. G.
1951 Geologic and hydrologic factors in the perennial yield of the
Biscayne aquifer: Am. Water Works Assoc. Jour., v. 43, no. 10,
p. 817-835.
1955 (Ferguson, G. E., Love, S. K., and others) Water resources of
southeastern Florida, with special reference to the geology and
ground water of the Miami area: U. S. Geol. Survey Water-
Supply Paper 1255.
Sherwood, C. B. (see Leach, S. D.)




Salt-water movement caused by control-dam operation in the Snake creek Canal, Miami, Florida ( FGS: Report of investigat...
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 Material Information
Title: Salt-water movement caused by control-dam operation in the Snake creek Canal, Miami, Florida ( FGS: Report of investigations 24, pt.4 )
Series Title: ( FGS: Report of investigations 24, pt.4 )
Physical Description: vi, 49 p. : illus., map, diagrs. ; 23 cm.
Language: English
Creator: Kohout, Francis Anthony, 1924-
Leach, Stanley D. ( joint author )
Geological Survey (U.S.)
Publisher: s.n.
Place of Publication: Tallahassee
Publication Date: 1964
 Subjects
Subjects / Keywords: Water-supply -- Florida -- Miami Metropolitan Area   ( lcsh )
Salinity -- Florida -- Miami Metropolitan Area   ( lcsh )
Snake Creek Canal   ( lcsh )
Genre: non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by F. A. Kohout and S. D. Leach.
General Note: "Prepared by the United States Geological Survey in cooperation with the Central and Southern Florida Flood Control District."
General Note: "References": p. 49
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The author dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law and all related or neighboring legal rights he or she had in the work, to the extent allowable by law.
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Table of Contents
    Front Cover
        Page i
    Florida State Board of Conservation
        Page ii
    Transmittal letter
        Page iii
        Page iv
    Table of contents
        Page v
        Page vi
    Abstract
        Page 1
    Introduction
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
    Geology and drainage
        Page 9
        Page 8
    Methods of investigation
        Page 9
        Page 10
    Long-term position of salt water in the canal
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
    Short-term position of salt water
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 16
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
    Manipulation of the control dam...
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 39
    Summary
        Page 46
        Page 47
        Page 45
    Conclusions
        Page 48
        Page 47
    References
        Page 49
        Copyright
            Copyright
Full Text


STATE OF FLORIDA
STATE BOARD OF CONSERVATION
DIVISION OF GEOLOGY



FLORIDA GEOLOGICAL SURVEY
Robert 0. Vernon, Director





REPORT OF INVESTIGATIONS NO. 24
PART IV







SALT-WATER MOVEMENT CAUSED BY
CONTROL-DAM OPERATION IN THE
SNAKE CREEK CANAL,
MIAMI, FLORIDA

By
F. A. Kohout and S. D. Leach
U. S. Geological Survey









Prepared by the
UNITED STATES GEOLOGICAL SURVEY
in cooperation with the
CENTRAL AND SOUTHERN FLORIDA FLOOD CONTROL DISTRICT

.'Tallahassee
1964









FLORIDA STATE BOARD

OF

CONSERVATION




FARRIS BRYANT
Governor


TOM ADAMS
Secretary of State



J. EDWIN LARSON
Treasurer



THOMAS D. BAILEY
Superintendent of Public Instruction


RICHARD ERWIN
Attorney General



RAY E. GREEN
Comptroller



DOYLE CONNER
Commissioner of Agriculture


W. RANDOLPH HODGES
Director






LETTER OF TRANSMITTAL


iorida geoloical Survey

Callakassee

October 10, 1963

Honorable Farris Bryant, Chairman
Florida State Board of Conservation
Tallahassee, Florida

Dear Governor Bryant:

The Florida Geological Survey has published four short papers
as Parts 1, 2, 3, and 4 of Report of Investigations No. 24. This
publication, Part 4, is the last of these brief papers and discusses
the "Salt-Water Movement Caused by Control-Dam Operation in
the Snake Creek Canal, Miami, Florida." The report was prepared
by F. A. Kohout and S. D. Leach, hydrologists for the United
States Geological Survey.

The principles of the relationships of fresh and salt water in
the Biscayne aquifer and the movement of the salt-water, fresh-
water contact along the canals and in the aquifer, with use, with
development of the control stations in the Central and Southern
Florida Flood Control District, and in response to cyclic changes
in the weather, are of great importance in understanding similar
occurrences. These four reports present interesting details of
these relationships.

Respectfully yours,
Robert 0. Vernon
Director and State Geologist





















































Completed manuscript received
August 16, 1963
Published for the Florida Geological Survey
By E. 0. Painter Printing Co.
DeLand, Florida

iv









CONTENTS


Abstract 1--. 1.. ... .... ...... ........... ...... .... ........ 1
Introduction __..... .... 2
History ___ 2
Purpose of investigation ..--............... ....... ...--------------- -------.............. 5
Location and extent of area ------.........-.......-................ 6
Climate -----....... ...-- --- ---....- --------...--. 7
Previous investigations -......-..-.. .-..................... 7
Personnel and acknowledgments -.-------.------- -------.. 7
Geology and drainage 8
Methods of investigation ___.._ .....--- ..........---------------- 9
Long-term position of salt water --....... ... --... .............. 10
In the canal -----_.._. .. ...... ... ... .............. ..... ------------ 10
In the aquifer ... ....... .................... 12
Short-term position of salt water ........ ..... 16
Salt-water movement in the canal .- .. ........... ........... 17
Natural tidal oscillation __ ..-................ 17
Trapping salt water 20
Flushing of salt water from the canal 24
Ultimate loss and flushing of the remaining trapped salt water 26
Salt-water movement in the aquifer 29
Natural tidal oscillation -......... .__ .. .. .... 29
Recharging the aquifer with trapped salt water 36
Ultimate loss and flushing of salt water in the aquifer 38
Manipulation of the control dam for maximum benefit .--- -------.. 39
Factors affecting inland movement of salt water 40
Discharge related to gate opening 41
Summary ._ 45
Conclusions as to operating criteria 47
References ---___ 48


ILLUSTRATIONS

Figure Page
1 The Miami area, Florida, showing the approximate landward
extent of salt water at the base of the Biscayne aquifer in 1961 3
2 Photographs of the salinity-control dam (S-29) in the Snake
Creek Canal: (a) with right-hand gate opened 1 foot; (b)
with one gate fully open 4
3 Map of the northern part of the Miami area showing obser-
vation points along the Snake Creek Canal 6
4 Section along the south bank of the Snake Creek Canal
showing generalized lithologic characteristics of the Biscayne
aquifer 8
5 Graphs comparing rainfall, gate opening, and fluctuations






of water level and chloride content along the Snake Creek
Canal, 1960-61 _._----_. 11
6 Graphs comparing fluctuations of discharge, water level, and
chloride content in the Snake Creek Canal during the special
test of March 27-31, 1961 __-- -______ 12
7 Sections along the south bank of the Snake Creek Canal
showing the zone of diffusion in the Biscayne aquifer, 1960-62 ... 14 & 15
8A Sections 1, 2, 3, and 4 through the Snake Creek Canal
showing movement of salt water during the rising tide, March
29, 1961 ___ ----_ -..-.- 18
8B Sections 5, 6, 7, and 8 through the Snake Creek Canal showing
movement of salt water during the falling tide, March
29, 1961 _____-. ..------ 19
9 Sections 9, 10, and 11 through the Snake Creek Canal showing
movement of salt water after closing of the control dam at
high tide, March 30, 1961 -___ ___ ..... ...------ 21
10 Photograph of rock pit (at left) looking in the downstream
direction of the Snake Creek Canal (at right) ....-- -------- ..- 22
11 Rock pit showing approximate bottom topography ..---------- 23
12 Sections 12, 13, 14, and 15 through the Snake Creek Canal
showing movement of salt water during and after flushing,
March 31 to April 1, 1961 25
13 Sections 16, 17, and 18 through the Snake Creek Canal
showing changes in isochlor position, April 3-11, 1961 _..... 27
14 Sections 19, 20, and 21 through the Snake Creek Canal
showing changes in isochlor positions, April 18 to May 16, 1961 .--. 28
15 Sections 22, 23, and 24 through the Snake Creek Canal
showing changes in isochlor positions May 29 to July 10, 1961 ---- 30
16 Graph showing fluctuations of chloride content at well
stations 1, 2, and 3, 1960-61 ---- --.....----...----------- -- 31
17 Graph showing fluctuations of chloride content at well
station 4, 1960-61 ____ ... .........---------..... 32
18 Graph showing fluctuations of chloride content at well
station 5, 1959-61 ___- ---- 33
19 Graph showing fluctuations of chloride content at well
station 6, 1960-61 .-....... .... ...... .... 34
20 Graph showing fluctuations of chloride content at well
stations 7, 8, and 9, 1960-61 _---- 35
21 Schematic flow diagrams: (a) with gate partially submerged,
(b) with gate fully open -_____ 42
22 Graph showing relations of head differential and discharge
for various openings of one gate of the control dam in the
Snake Creek Canal 43







SALT-WATER MOVEMENT CAUSED BY
CONTROL-DAM OPERATION IN THE
SNAKE CREEK CANAL,
MIAMI, FLORIDA
By
F. A. Kohout and S. D. Leach

ABSTRACT
Movement of salt water in the Biscayne aquifer and the Snake
Creek Canal was investigated to establish criteria for operation
of the salinity-control dam in the canal. All four gates of the dam
were opened for 3 days during a special test in March 1961.
Although salt water oscillated landward and seaward a distance of
2 miles from the control in response to tide, the salinity of the
ground water near the canal did not change greatly during this
time. Salt water trapped in the canal by closing the dam at high
tide moved landward as a density current at a rate of about 900
feet per hour and began to flow into a connected deep rock pit. A
flushing operation removed salt water from the canal but the salt
water in the rock pit remained. During subsequent months, wind
and tide action caused upward dispersion of salt into the upper-
most water of the rock pit, and the trapped salt water was
gradually removed by seaward discharge through the canal.
Some of the salt water trapped in the canal moved into the
aquifer. This salty water was traced by sampling fully cased wells
of different depths and distances from the canal. Some of the
salty water was retrieved by drainage from the aquifer when
the gates were partially opened 2 months after entrapment. Be-
cause of its relatively great density some of the salt water moved
downward in the aquifer and was not retrieved.
The data show the negative effects of trapping salt water, and
also that the control can be closed gradually without trapping salt
water. If the total opening of the control is distributed uniformly
among the four partially closed gates, the increased velocity of
the fresh water as it flows under the submerged sluice gates, will
impede upstream movement of salt water at the bottom of the
control. Calculations indicate that salt water probably will not
move upstream in the canal if a head of 0.3 foot is maintained
between upstream and downstream sides of the control at the time
of minimum head differential, about 2 hours before high tide.





FLORIDA GEOLOGICAL SURVEY


INTRODUCTION

This report gives detailed attention to the movement of salt
water in and adjacent to the Snake Creek Canal in the northern
part of the Miami area, Florida (fig. 1). The movement results
primarily from operations of a salinity-control dam located near
the mouth of the canal.
From 1909 to 1930, canals were constructed westward from
the coast through the Miami area into the Everglades to drain low-
lying land for urban and agricultural use. The uncontrolled canals
caused overdrainage. The water table was lowered nearly to sea
level in some areas and this caused salt water to encroach into
the Biscayne aquifer by two processes (Parker, 1951, p. 826):
(1) Where the water table was lowered greatly, salt water
moved horizontally landward from the ocean through the highly
permeable solution-riddled limestone of the Biscayne aquifer to
replace the drained fresh water.
(2) Salt water moved directly inland in the canals where, be-
cause of its greater density, it tended to leak downward and
laterally to contaminate the underlying fresh ground water.
Although the end result of these separate processes is the
same, they attack the fresh-water resources from different
directions and occur at different rates. The general aim of this
report is to establish the conditions under which each process
provides the greater danger of salt-water contamination.

HISTORY

In 1945, salinity-control dams were installed in most of the
canals of the Miami area as barriers against further encroachment
of salt water. These were emergency dams constructed by driving
sheet-steel piles into the limestone. The dams were opened and
closed by removing some or all of the piles during the rainy
season and replacing them during the dry season. This was an
inefficient procedure which frequently resulted in the dams- being
left open much longer than necessary.
In recent years, the emergency dams have been replaced by
permanent concrete structures with submerged sluice-gate open-
ings. The control structure S-29 (Snake Creek Canal) is repre-
sentative of some of the more recent structures. Each of the four
gates, figure 2, is connected through a gear box to rollers which
are driven by the rear-wheel drive of an anchored automobile.
In the most recently constructed controls, in the Little River and







REPORT OF INVESTIGATIONS No. 24 3



























i~ a
I ,




































SCALE IN MILES


Figure 1. Miami area, Florida, showing the approximate landward extent of
salt water at the base of the Biscayne aquifer in 1961.





FLORIDA GEOLOGICAL SURVEY


I I


Figure 2. Photographs of the salinity-control dam (S-29) in the Snake Creek
Canal: (a) with right-handgate opened 1 foot; (b) with one gate fully open.


v r






REPORT OF INVESTIGATIONS NO. 24


Biscayne Canals, electric motors raise and lower the gates
automatically by sensing the water levels upstream and down-
stream from the control dam. These automatically operated dams
permit delicate control of water levels. However, the relation of
salt-water movement to head differential needs further study
before fresh-water resources can be effectively entrusted to
automatic head-activated devices. As will be shown later, the
relations of head and salt-water movement are very complicated
and transient, and much more study and experience should be
accumulated before true automatic control is attempted.




PURPOSE OF INVESTIGATION

Frequently, after heavy rainfall, headwater areas of the Snake
Creek Canal remain flooded at the same time that fresh-water
discharge at the control dam is not sufficient, with fully open
gates, to keep the canal flushed of salt water. Under these circum-
stances salt water moves upstream in the lower reaches of the
canal and contaminates the fresh ground water.
Questions had arisen as to the magnitude of this encroach-
ment, the extent to which the encroachment might be tolerated
during gate openings, and the feasibility of flushing the salt
water out later when the gates were closed after flooding subsided.
Accordingly, goals of the investigation were to establish the time-
rate of movement of the salt water for various circumstances
of dam operation and, if possible, to develop principles that might
have practical application not only in the operation of the salinity-
control dam in the Snake Creek Canal but also in other canals of
the area.
The investigation was made in cooperation with the Central
and Southern Florida Flood Control District (C&SFFCD). Most
of the data were collected between October 1960 and January 1962.
The investigation was under the general supervision of M. I.
Rorabaugh and, subsequently, C. S. Conover, Tallahassee, A. 0.
Patterson and K. A. Mac Kichan, Ocala, district engineers
respectively of the Ground Water, Surface Water and Quality of
Water Branches, U.S. Geological Survey. The investigation was
under the immediate supervision of Howard Klein, geologist-in-
charge, Ground Water Branch, and J. H. Hartwell, engineer-in-
charge, Surface Water Branch, Miami.





FLORIDA GEOLOGICAL SURVEY


LOCATION AND EXTENT OF AREA

The Snake Creek Canal extends about 18 miles inland to the
vicinity of Levee 33, on the east side of Conservation Area 3
(Inset map, fig. 3). Under the plans of the C&SFFCD (the operating
agency) and the U. S. Corps of Engineers (the constructing
agency), Conservation Area 3 will impound water pumped west-
ward from urbanized areas during the rainy season. During the
dry season, water stored in the conservation area will be delivered
coastward to maintain water levels, thus preventing salt-water
encroachment and providing replenishment water for municipal
and private well systems.
This report covers the seaward reach of the Snake Creek Canal
(Canal C-9) extending about 15,000 feet westward from the
salinity-control dam (structure S-29). The width of the area
extends 200 feet on either side of the centerline of the canal, but


Figure 3. Northern part of the Miami area showing observation points along
Snake Creek Canal.






REPORT OF INVESTIGATIONS NO. 24


as one side should be the mirror image of the other, work was
confined to the south bank of the canal. The canal widens into a
lake whose dimensions are about 3,500 by 1,700 feet at the western
end of the area of investigation, between State Highway 9 and
Miami Gardens Drive (fig. 3). The lake, formed by removal of
rock for concrete aggregate and other building materials, has a
maximum depth of 40 feet below msl (mean sea level) and was
included in the investigation because of its important influence on
salt-water contamination.

CLIMATE

The climate of the Miami area is subtropical. The average
temperature for January is about 68F and for July about 820F.
Rainfall averages about 60 inches per year, three-quarters of
which falls in the May to November rainy season.
In relation to this investigation, 1960 was unusually wet with
rainfall of about 75 inches, 25 inches falling in September when
hurricane Donna and tropical storm Florence passed nearby. The
year of 1961 was unusually dry; the rainfall amounted to about
42 inches.

PREVIOUS INVESTIGATIONS

Some phases of 'the present study coincided with, and were
contemporaneous with, a study of the general hydrology of the
Snake Creek Canal area by Leach and Sherwood (1963). Where
data and graphs were applicable to the present report, they were
used freely without acknowledgment.
Reports by Parker (1951), Parker, Ferguson, and Love (1955),
and Kohout (1961) give much background information on the
influence of drainage canals on salt-water encroachment in the
Miami area. Many other reports cover the hydrology of the area
and may be found in the "List of Publications" of the Florida
Geological Survey in Tallahassee.

PERSONNEL AND ACKNOWLEDGMENTS

Thanks are extended to the Dade County Office of Water
Control, especially to John Wilhelm, for furnishing rainfall and
gate-opening data. Robert L. Taylor of the C&SFFCD aided by
making available the facilities and personnel of his organization
during the special test period of March 1961.






FLORIDA GEOLOGICAL SURVEY


Electrical-conductivity measurements of water in the canal
during the test period of March 1961 were made under the direc-
tion of R. N. Cherry of the U. S. Geological Survey, Ocala. Other
Geological Survey personnel who contributed materially were B.
F. Joyner of Ocala and C. B. Sherwood of Miami.

GEOLOGY AND DRAINAGE

The Biscayne aquifer of the Miami area is a highly permeable'
water-table aquifer consisting of solution-riddled limestone and
calcareous sandstone with fairly frequent layers of unconsolidated
sand. The generalized lithologic characteristics in wells drilled
during the investigation are shown in figure 4.
Except for the peat and muck deposits at the surface, the
sediments were deposited during the Pleistocene glacial epoch.
Accumulation of snow and ice during glacial advances in the
northern latitudes removed water from the oceans and lowered sea
leveL Alternately, melting of the glaciers returned water to the
oceans and caused sea level to rise. The alternation of limestone
and sandstone beds in figure 4 relates to these fluctuations of sea
level-estimated by Cooke (1939, p. 34) to have ranged from about
270 feet above to 300 feet below present sea level.



DISTANCE FROM CONTROL ODAM.IN FEET
2 39_G 700 OCT 0 _00C 9000 8000 7000
LAND SURFACE--
S/ BOTTOM OF CANAL





0'STANCE FROM CONTROL DAM,IN FEET
503 50co 4CCO 3000 2000 I000 0
LAND SURFACE van c.r
BOTTOM .F CANAL /' _f T 1' l g -- -- -



-120


Figure 4. Section along the south bank of the Snake Creek Canal showing
generalized lithologic characteristics of the Biscayne aquifer.





REPORT OF INVESTIGATIONS No. 24


The peat and muck deposits were formed by the accumulation
of partially decayed vegetation in swamps after the last glacial
recession, about 8,000 years ago.
The Snake Creek Canal traverses a natural overflow channel
from the Everglades. These numerous overflow channels are wide,
topographically low areas (4 to 8 feet above sea. level) that cut
through relatively high land (8 to 15 feet) near the coast; they
are called transverse or finger glades because of the manner in
which they cut through the coastal ridge.
Prior to construction of drainage canals (fig. 1), the water
level in the Everglades was high enough to cause discharge through
the finger glades by poorly channelized sheet flow. In the deeper
pockets, aquatic plants grew lushly and, after dying, settled to the
bottoms of sloughs. In these locations, the organic material was
protected from oxidation and accumulated to form the peat and
black muck deposits.
In the higher parts of the finger glades, where water levels
were below ground surface during the dry season, rapid oxidation
prevented heavy accumulation of organic material. .In these areas,
the remnant plant materials were mixed with particles of limestone
and sand, eroded from the adjacent higher limestone hills, to form
a deposit called marl (fig. 4).
Since construction of drainage canals, the finger glades are
flooded only during extremely heavy rainfall, and the rich organic
soils are cultivated- to provide table vegetables for northern
markets in the winter season. With continuing improvements in
the canal system and improved efficiency of water control, the
agricultural activities are gradually giving way to urbanization.



METHODS OF INVESTIGATION

The investigation followed two main courses: (1) Long-term
observation of the position and rate of movement of salt water
under the past method of operating the control dam (S-29). The
dam has been operated on an arbitrary water-level basis with no
consideration being given to the movement or the position of salt
water; (2) detailed inventory of the position and movement of
salt water under special short-term test conditions. The short-term
test served as an index of typical conditions for analysis of the
long-term data. These phases required continuous inventory of
changes, both in the canal and in the aquifer.






FLORIDA GEOLOGICAL SURVEY


Electrical-conductivity measurements of water in the canal
during the test period of March 1961 were made under the direc-
tion of R. N. Cherry of the U. S. Geological Survey, Ocala. Other
Geological Survey personnel who contributed materially were B.
F. Joyner of Ocala and C. B. Sherwood of Miami.

GEOLOGY AND DRAINAGE

The Biscayne aquifer of the Miami area is a highly permeable'
water-table aquifer consisting of solution-riddled limestone and
calcareous sandstone with fairly frequent layers of unconsolidated
sand. The generalized lithologic characteristics in wells drilled
during the investigation are shown in figure 4.
Except for the peat and muck deposits at the surface, the
sediments were deposited during the Pleistocene glacial epoch.
Accumulation of snow and ice during glacial advances in the
northern latitudes removed water from the oceans and lowered sea
leveL Alternately, melting of the glaciers returned water to the
oceans and caused sea level to rise. The alternation of limestone
and sandstone beds in figure 4 relates to these fluctuations of sea
level-estimated by Cooke (1939, p. 34) to have ranged from about
270 feet above to 300 feet below present sea level.



DISTANCE FROM CONTROL ODAM.IN FEET
2 39_G 700 OCT 0 _00C 9000 8000 7000
LAND SURFACE--
S/ BOTTOM OF CANAL





0'STANCE FROM CONTROL DAM,IN FEET
503 50co 4CCO 3000 2000 I000 0
LAND SURFACE van c.r
BOTTOM .F CANAL /' _f T 1' l g -- -- -



-120


Figure 4. Section along the south bank of the Snake Creek Canal showing
generalized lithologic characteristics of the Biscayne aquifer.





REPORT OF INVESTIGATIONS No. 24


The peat and muck deposits were formed by the accumulation
of partially decayed vegetation in swamps after the last glacial
recession, about 8,000 years ago.
The Snake Creek Canal traverses a natural overflow channel
from the Everglades. These numerous overflow channels are wide,
topographically low areas (4 to 8 feet above sea. level) that cut
through relatively high land (8 to 15 feet) near the coast; they
are called transverse or finger glades because of the manner in
which they cut through the coastal ridge.
Prior to construction of drainage canals (fig. 1), the water
level in the Everglades was high enough to cause discharge through
the finger glades by poorly channelized sheet flow. In the deeper
pockets, aquatic plants grew lushly and, after dying, settled to the
bottoms of sloughs. In these locations, the organic material was
protected from oxidation and accumulated to form the peat and
black muck deposits.
In the higher parts of the finger glades, where water levels
were below ground surface during the dry season, rapid oxidation
prevented heavy accumulation of organic material. .In these areas,
the remnant plant materials were mixed with particles of limestone
and sand, eroded from the adjacent higher limestone hills, to form
a deposit called marl (fig. 4).
Since construction of drainage canals, the finger glades are
flooded only during extremely heavy rainfall, and the rich organic
soils are cultivated- to provide table vegetables for northern
markets in the winter season. With continuing improvements in
the canal system and improved efficiency of water control, the
agricultural activities are gradually giving way to urbanization.



METHODS OF INVESTIGATION

The investigation followed two main courses: (1) Long-term
observation of the position and rate of movement of salt water
under the past method of operating the control dam (S-29). The
dam has been operated on an arbitrary water-level basis with no
consideration being given to the movement or the position of salt
water; (2) detailed inventory of the position and movement of
salt water under special short-term test conditions. The short-term
test served as an index of typical conditions for analysis of the
long-term data. These phases required continuous inventory of
changes, both in the canal and in the aquifer.





FLORIDA GEOLOGICAL SURVEY


The long-term study of salt-water occurrence in the canal was
made primarily from fluctuations of chloride content as determined
by three electrical-conductivity recorders located at S-29, at West
Dixie Highway, and at the footbridge (fig. 3). The electrodes of
these recorders were positioned about 1.5 feet above the bottom
of the canal, and the landward movement of salt water was sensed
by a sharp rise in the electrical conductance of the water. During
and after the short-term test of March 27-31, 1961, the position of
salt water in the canal was determined by traversing the canal -
water vertically with the cell of a portable conductivity instrument.
The measurements were made from a boat that moved to specific
stations along the canal.
The long-term study of salt-water occurrence in the aquifer was
made from fluctuations of chloride content in samples of ground
water from nine stations of wells drilled along the south bank of
the canal (fig. 3). The general scheme was to drill individual
wells to different depths at distances of about 5 and 60 feet from
the canal. Thus, a typical station might consist of wells 10, 20,
50, and 100 feet deep near the canal and wells 10 and 20 feet deep
at a distance of 60 feet from the canal. Although drilling operations
did not always fulfill the preferred condition of paired wells
terminating at the same altitude, the rectangular coordinate
positioning of the wells relative to the canal was virtually fulfilled.
Each well was cased so that the water which it yielded was repre-
sentative of that particular point in the aquifer. Details of the
position and depth of each well terminus relative to the canal are
shown in the map and section views of figures 16 to 20.
An attempt has been made to simplify the illustrations as much
as possible, but as this three-dimensional problem can be shown
only in two dimensions, much must be left to the imagination of
the reader to keep himself oriented in three-dimensional space.
Herein lies the understanding of the physical parameters being
discussed.

LONG-TERM POSITION OF SALT WATER
IN THE CANAL
The long-term position of salt water in the canal can be inferred
by comparing figures 5 and 6. Figure 5 shows the peak daily
chloride content, as indicated by the conductivity recorders at S-29,
at West Dixie Highway, and at the footbridge. Also shown are the
water level in the canal at West Dixie, the gate opening at S-29,
and rainfall at the Opalocka gage. The maximum vertical opening





FLORIDA GEOLOGICAL SURVEY


The long-term study of salt-water occurrence in the canal was
made primarily from fluctuations of chloride content as determined
by three electrical-conductivity recorders located at S-29, at West
Dixie Highway, and at the footbridge (fig. 3). The electrodes of
these recorders were positioned about 1.5 feet above the bottom
of the canal, and the landward movement of salt water was sensed
by a sharp rise in the electrical conductance of the water. During
and after the short-term test of March 27-31, 1961, the position of
salt water in the canal was determined by traversing the canal -
water vertically with the cell of a portable conductivity instrument.
The measurements were made from a boat that moved to specific
stations along the canal.
The long-term study of salt-water occurrence in the aquifer was
made from fluctuations of chloride content in samples of ground
water from nine stations of wells drilled along the south bank of
the canal (fig. 3). The general scheme was to drill individual
wells to different depths at distances of about 5 and 60 feet from
the canal. Thus, a typical station might consist of wells 10, 20,
50, and 100 feet deep near the canal and wells 10 and 20 feet deep
at a distance of 60 feet from the canal. Although drilling operations
did not always fulfill the preferred condition of paired wells
terminating at the same altitude, the rectangular coordinate
positioning of the wells relative to the canal was virtually fulfilled.
Each well was cased so that the water which it yielded was repre-
sentative of that particular point in the aquifer. Details of the
position and depth of each well terminus relative to the canal are
shown in the map and section views of figures 16 to 20.
An attempt has been made to simplify the illustrations as much
as possible, but as this three-dimensional problem can be shown
only in two dimensions, much must be left to the imagination of
the reader to keep himself oriented in three-dimensional space.
Herein lies the understanding of the physical parameters being
discussed.

LONG-TERM POSITION OF SALT WATER
IN THE CANAL
The long-term position of salt water in the canal can be inferred
by comparing figures 5 and 6. Figure 5 shows the peak daily
chloride content, as indicated by the conductivity recorders at S-29,
at West Dixie Highway, and at the footbridge. Also shown are the
water level in the canal at West Dixie, the gate opening at S-29,
and rainfall at the Opalocka gage. The maximum vertical opening







REPORT OF INVESTIGATIONS NO. 24


1960
QRV ALna SEPt OCTO


1961
SNOV OE& i N.. FEB. MAR. APR MAY JUNE ALN AUG SEPT OCT. OV. DEC.


02 10 0 o 0 1 2 to m\ 0 D a
I11f02 -- -~- -- -- -- -- --- -- --- --- --- -- ---------- -~---- --- ;0

S| 100


. 1HYl



- -=- ^- = = = =


L,-') III .... ,,., K
, I .14
5 I CON P S.. .
oW sowI









*W l l 1 IL!I I I I I I I FRO DA 5-29l i
OP-LOCA CA oGE

Z I
_)'l900
It ,1- -" ": :": :":" 1:-300-
> 11003J^ ^ i ^-^ j~u j I [ J-


E.
z

12



z


Figure 5. Graphs comparing rainfall, gate opening, and fluctuations of water
level and chloride content along the Snake Creek Canal, 1960-61.

of each of the four gates is 15 feet (fig. 2); therefore, if all gates
are fully open, the total gate opening is 60 feet. As the gates of
S-29 are of equal width, the amount of gate opening in figure 5
represents the discharge area.
All gates were fully open from early September to late October
1960, after the heavy rains accompanying hurricane Donna and
tropical storm Florence produced widespread flooding (fig. 5). Only
during the period September 10 to 14, 1960, was the discharge from
the canal great enough to keep the canal completely flushed of
salt water (chloride-content graph at S-29, fig. 5). At all other
times full opening of the dam permitted salt water to move
upstream on a rising tide. During the special short-term test of
March 27-31, 1961 (fig. 5), salt water invaded the canal inland
as far as the footbridge. The time scale for this period is expanded
in figure 6 to show the detailed relations of intrusion, water-level
fluctuation, and discharge. Salt water intrudes on the rising tide,
first appearing at S-29, then at West Dixie Highway 2 to 3 hours





FLORIDA GEOLOGICAL SURVEY


A VAs s r -MIN C
Ar gWff* oW








1, 1E
S I I I I
a. a .in,,ep, .. CL-SED-U
, "fA ". ".. "e f 'AL '"






" 1 _- I % !+ __.


: -. I / I / Ui i iI lf_ II' l I I I S'.-A--- 'LJ-

Figure 6. Graphs comparing fluctuations of discharge, water level, and chloride
content in Snake Creek Canal during the special test of March 27-31, 1961.

later, and finally at the footbridge during the subsequent falling
tide, about 8 hours after first appearing at S-29. Complicated
relations are involved in this time lag and they will be discussed
later, but it should be noted that the length of time of salt-water
stay decreases landward and that the maximum concentration also
decreases landward. For example, salt water did not reach the
footbridge at all on March 29, just before noon (fig. 6). In light of
the movement of salt water shown in figure 6, the long-term position
of salt water in the canal can be inferred from figure 5 by com-
paring the relative fluctuations of conductivity at the three
recording stations.

IN THE AQUIFER
The interface between fresh water and salt water in the Bis-
cayne aquifer is not sharp; a zone of diffusion is invariably present.
The chloride content in the zone of diffusion ranges from about 16
ppm (parts per million), the concentration of fresh water in Miami,
to about 19,000 ppm, the concentration of sea water. The concen-
tration of chloride ion is commonly used to represent the






REPORT OF INVESTIGATIONS NO. 24


distribution of salt water, and isochlor lines are drawn through
points of equal chloride content as determined by analyses of water
from wells.
Cross sections of the zone of diffusion along the south bank of
the Snake Creek Canal are shown in figure 7. The black dots
represent the termini of individual fully cased wells located about
5 feet from the canal; the open circles represent the termini of
wells located about 60 feet from the canal. In intercanal areas the
the isochlor lines turn sharply downward at the inland extremity
of the salt water, and the zone of diffusion takes the shape of a
blunt-nosed wedge of salty water (Kohout, 1961, fig. 3, 4). Along
the Snake Creek Canal (and presumably other canals) the blunted
shape, although still present, tends to be modified by large changes
in gradient and head near the canal.
When the dam is open, the lowest level of the water table
coincides with the water level in the canal, and ground water
flows laterally toward the canal from either side. Also, at such
times, ground water beneath the canal tends to flow upward
through the canal bottom as it follows the part of least resistance
to the ocean.
When the control gates are closed, the water level in the canal
becomes the highest level of the water table because water flows
quickly downstream from inland points of higher head. Under this
condition the exchange of water between the canal and the aquifer
is reversed, and water, whether it be fresh or salty, moves laterally
and vertically from the canal to the aquifer.
In figure 7, the general processes of salt-water encroachment
in the Biscayne aquifer are recognizable:

(1) The deep salt water, below and seaward of the 1,000-ppm
isochlor, moves landward from the ocean in an essentially
horizontal direction. This type of encroachment is rela-
tively slow and insidious. It results from heads that are
too low to prevent intrusion, and can be stabilized or
corrected only by raising the heads.

(2) The lenses of salt water that appear near the bottom of
the canal and then drift progressively downward with time
originate as a result of inland movement of salt water
directly in the canal. This type of encroachment occurs
very rapidly and is the consequence of inadequate
knowledge of the position of salt water in the canal during
control-dam operations.











DISTANCE FROM CONTROL DAM,IN FEET
14nnh 1 in..n an. .
LAND *UR FAf r-
------------------------ o---o7--,--a-- u ,- a--
.--,--5,0-. ^-. X- ;- 0 d-- d 0
EXPLANATION Or N--A *L
ISOCHLOR LINE IN PPM DECEMBER 8 1960 **
WELLS 5 FEET FROM CANAL .
WELLS 60 FEET FROM CANAL

IAND SURFAIF-
TM OT CANAL.-

a JANUARY 3,1961 // 0





40 -,0---
. -eo MARCH 24, 1961 Soo ./''p
-IDA *RC

































LAND SURFACE-.
SSee 0, ..... ,. _
--Jy ------ -- "sOTTOM OF CNALN' '" '""

-so JANUARY 24, 1962 .' -0




Figure 7. Sections along the south bank of the Snake Creek Canal showing
the zone of diffusion in the Biscayne aquifer, 1960-62.






FLORIDA GEOLOGICAL SURVEY


Although the isochlor pattern in figure 7 appears to remain
practically stable from December 1960 to January 1962, careful
comparison of data shows that significant changes occurred at
certain wells. The chloride content in the deepest well at station
4 decreased from 11,850 to 535 ppm, indicating seaward expulsion
of salt water from the aquifer. During the same time, the chloride
content in the deepest well at station 6 decreased from 1,220 to
490 ppm.
The reason for the expulsion of salt water may be understood
by following the events shown in figure 5. From early September
to late October 1960 the control dam was fully open, because of
flood conditions. During this time the heads in the aquifer in the
vicinity of the canal were below normal and salt water moved
horizontally landward and mounded beneath the canal. The chlo-
ride content in well G 420A (the only long-term data covering the
period, fig. 18) increased from 6,800 ppm in June 1960 to 9,200 ppm
on October 19, 1960. The year 1960, then, was essentially a year
of encroachment.
The year 1961 was abnormally dry, and even during the
normally rainy months of September and October the dam remained
practically closed. The infrequent opening of the control dam
caused heads to remain sufficiently high so that expulsion of salt
water from the aquifer occurred throughout 1961 (fig. 7). The
fluctuation graphs in figures 16 to 20 confirm the seaward move-
ment of salt water; chloride contents in most of the wells decreased
generally during 1961.
It is indeed a contradictory situation that the greatest inland
movement of salt water took place in a wet year and the greatest
expulsion of salt water took place during a dry year. Nevertheless,
this fact indicates that the means are available for very effective
control of the movement of salt water in the Biscayne aquifer.


SHORT-TERM POSITION OF SALT WATER

While it is generally not feasible to observe continuously the
complete distribution of salt water, short-term tests, keyed to
recording instruments, provide much information with which to
interpret the long-term data.
A short-term test was performed March 27-31, 1961. The
control gates were completely closed for 6 weeks prior to the test,
and conditions were almost stable despite rainfall of 2 inches that
occurred 3 days before the test (fig. 5).






REPORT OF INVESTIGATIONS No. 24


The sequence of control-dam operations is shown in the water
level graph of figure 6. All four gates were opened fully at 10:00
a.m., March 27, just after high tide. The canal was in natural
tidal oscillation until 9:15 a.m., March 30, when S-29 was completely
closed just before high tide. The intention was to trap salt water
in the canal, as might happen during normal control operations,
and then trace its movement both in the canal and in the aquifer.
The trapped salt water moved inland in the canal much more
rapidly than expected, and at 1:30 p.m. on March 31, at high
tide, two gates were opened for a flushing operation which con-
tinued until low tide, at about 5:45 p.m. The dam was then closed
and remained so until May 26, 1961.

SALT-WATER MOVEMENT IN THE CANAL

The tidal movement of salt water in a canal is rapid and
transient. In figures 8-9 and 12-15 this movement is depicted by
a time-lapse sequence of canal sections showing the approximate
position of isochlor lines along the longitudinal axis of the canal.

NATURAL TIDAL OSCILLATION

The movements of salt water under unimpeded tidal oscillation
on March 29 are shown in figures 8A and 8B; figure 8A shows the
inland extension on the rising tide, and figure 8B shows the sea-
ward movement on the falling tide.
The stage of the tide cycle represented by each isochlor section
is shown by an arrow on the water-level fluctuation graph (inset).
The measured velocity profile for the middle vertical in the canal
at West Dixie Highway is shown at the lower right side of each
section. The discharge is divided into landward and seaward com-
ponents as shown by the figures near the darkened arrows.
Comparison of sections 1, 2, and 3 (fig. 8A) indicates that the
inland advance of salt water was slow during the early part of
the rising tide. Significant landward advance of salt water did not
take place until all flow became negative (landward) in section 3.
This indicates that seaward-flowing water in the upper part of
the canal removes the landward-flowing salt water in the bottom
of the canal by a combined dispersion and interface erosion process.
Maximum inland velocity occurred at 7:37 a.m., about 11/2
hours before high tide, section 4; the unshaded discharge arrows
and dashed velocity curve show this condition. The solid velocity
curve at 8:44 a.m., which corresponds in time with the isochlor









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FLORIDA GEOLOGICAL SURVEY


section, indicates that relatively fresh water near the surface
began to flow seaward at least 1 hour before high tide. The high-
salinity water at the inland toe of the wedge, however, continued
to move landward until well after high tide. This time lag in
landward movement of salt water was shown previously in the
chloride-fluctuation graph of figure 6, where salt water did not
appear at the footbridge until about 2 hours after high tide.
These out-of-phase flow characteristics in the upper and lower
parts of the canal result from the density contrast between fresh
water and salt water. At, and for some time after, high tide the
fresh-water head at inland points is too small to counterbalance
landward movement of salt water and the inland toe of the wedge
moves landward as a density current. As the tide begins to fall
more rapidly after high tide, the landward hydraulic gradient
associated with the density current is overcome by seaward
hydraulic gradient from falling ocean level. The salt water at the
bottom of the canal, then, reverses direction and flows seaward.
The salinity distribution in the canal reflects the out-of-phase
flow pattern. The upper part of the canal is rapidly freshened by
large positive (seaward) velocities in the fresh water as the
tide falls (fig. 8B), section 6. However, even after the reversal of
landward movement of salt water, seaward velocities in the salt
water near the bottom of the canal are small compared to those
of the overlying fresh water (velocity curves, sections 7, 8).
This velocity differential causes a lens or slug of salt water to lag
behind in that part of the canal westward from West Dixie High-
way. Apparently, downward-directed velocity components (possibly
related to turbulence near bridge pilings) develop in the fresh
water near the outlet. Salt water in this part of the canal is eroded
and flushed seaward before all the salt water that moved land-
ward during the rising tide can retrace its path to the outlet. The
phenomena occurred repeatedly when the control dam was fully
open. This indicates that the control should not be operated ex-
clusively on the basis of water-level measurements, for slugs of salt
water could be inadvertently trapped. The position of salt water
should be determined directly by conductivity measurements at the
bottom of the canal.

TRAPPING SALT WATER

This section will show the dangers of inadvertently trapping
salt water in a canal by shutting the gate on it. On March 30 at
9:15 anm., just before high tide, all four gates of S-29 were closed.








REPORT OF INVESTIGATIONS NO. 24


The salinity distribution just prior to the time of closing is shown
in figure 9, section 9.
A stable density stratification exists when fresh water overlies
salt water with the isochlor lines horizontal. Sloping isochlor lines
in stagnant water may be generally interpreted as an indication of
hydraulic instability related to a nonstable density stratification.
After the gates were closed, the water (for purposes of this dis-
cussion) may be considered to have been virtually stagnant, though


I2J
13 83
13
9:
9: 13
iz:
13 ~
13 A.
83


k to
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9: l.J


DISTANCE FROM CONTROL DAM,IN FEET
14,000 120000 10.000 8000 6000 4000 2000 0
^ 0 : -- ----- ----- h ----- I -----i-i ,
4 (9) MARCH 30,1961 (8:50A.M.)
Q _WATER SURFACE-
: ISOCHLORSIN PARTS PER MILLION


S -i MAR.27 MAR.28 MAR.29 MAR. MAR.31 APR. I CONTROL
SA 5 SP Ii t A, 1 2 I? G G8P 12 GA1 S PR; 1 2 t2 S- P OPEN


-, ; 6, I L i, j -. ,

0 ATW DIXIE_ mil
-401

An (10) MARCH 30,1961 (11:30A.M.)
4 WATERR SURFACE-
ISOCHLORS.IN PARTS PER MILLION- 0.c0 '



SMAR.27 AR.2 M 9 R30 MAR.31 APR.I Cr CRRR! A COTROL














March 30, 1961.
-45'4d(A t'iI44 Z{2fiprj 7. S?

(11) MARCH 30,M961(2:40P.M.)
















March 30, 1961.






FLORIDA GEOLOGICAL SURVEY


Although the isochlor pattern in figure 7 appears to remain
practically stable from December 1960 to January 1962, careful
comparison of data shows that significant changes occurred at
certain wells. The chloride content in the deepest well at station
4 decreased from 11,850 to 535 ppm, indicating seaward expulsion
of salt water from the aquifer. During the same time, the chloride
content in the deepest well at station 6 decreased from 1,220 to
490 ppm.
The reason for the expulsion of salt water may be understood
by following the events shown in figure 5. From early September
to late October 1960 the control dam was fully open, because of
flood conditions. During this time the heads in the aquifer in the
vicinity of the canal were below normal and salt water moved
horizontally landward and mounded beneath the canal. The chlo-
ride content in well G 420A (the only long-term data covering the
period, fig. 18) increased from 6,800 ppm in June 1960 to 9,200 ppm
on October 19, 1960. The year 1960, then, was essentially a year
of encroachment.
The year 1961 was abnormally dry, and even during the
normally rainy months of September and October the dam remained
practically closed. The infrequent opening of the control dam
caused heads to remain sufficiently high so that expulsion of salt
water from the aquifer occurred throughout 1961 (fig. 7). The
fluctuation graphs in figures 16 to 20 confirm the seaward move-
ment of salt water; chloride contents in most of the wells decreased
generally during 1961.
It is indeed a contradictory situation that the greatest inland
movement of salt water took place in a wet year and the greatest
expulsion of salt water took place during a dry year. Nevertheless,
this fact indicates that the means are available for very effective
control of the movement of salt water in the Biscayne aquifer.


SHORT-TERM POSITION OF SALT WATER

While it is generally not feasible to observe continuously the
complete distribution of salt water, short-term tests, keyed to
recording instruments, provide much information with which to
interpret the long-term data.
A short-term test was performed March 27-31, 1961. The
control gates were completely closed for 6 weeks prior to the test,
and conditions were almost stable despite rainfall of 2 inches that
occurred 3 days before the test (fig. 5).






22 FLORIDA GEOLOGICAL SURVEY

some net inflow of fresh water entered the canal as the stage rose;
the sloping isochlor lines in section 9, then, indicate instability.
As expected, a density current developed when the gates were
closed and salt water in the seaward reach of the canal moved
landward. To maintain continuity between the individual water
masses, fresh water from inland points moved seaward through
the upper part of the canal to replace the salt water which moved
landward through the lower part.
These movements were traced primarily by making vertical
conductivity traverses of the canal water from the boat. Also,
vertical conductivity traverses were made continuously at the
footbridge and a series of these profiles is shown at the lower
right side of section 10. The leading nose of the salt-water wedge
passed the footbridge at 10:44 a.m., it was about 5 feet thick and
had a maximum chloride content of about 1,500 ppm.
By tracking the intruding nose of the salt-water wedge for a
period of time it was determined that the rate of landward move-
ment was about 15 feet per minute, or about 900 feet per hour. The
serene water surface completely belied the rapid encroachment
taking place at the bottom of the canal.



if -. ?-"
-...... .. .

















Figure 10. Photograph of rock pit (at left) looking in the downstream
direction of the Snake Creek Canal (at right).





REPORT OF INVESTIGATIONS No. 24


The salt water moved nearly 1 mile in the 41/4 hours between
9:15 a.m., the time of closing the gates, and 1:30 p.m. Careful
inspection of the discharge graph at Miami Gardens Drive (fig. 6)
shows a gentle increase in discharge at about 1:30 p.m. This
increase was sensed by a small velocity vane positioned in the
fresh water above the intruding salt-water wedge, and reflects the
change in velocity in the seaward flow section as the wedge passed
Miami Gardens Drive.
At about 2:00 p.m. salt water began to flow into the rock pit,
in a manner similar to water passing over a waterfall (fig. 3, 9, 10,
11).
By late afternoon the hydrologic significance of the rock pit
became clear. The rock pit was a hydraulic sink in which the salt
water would temporarily collect. The salt water would resume its
upstream movement only after it filled the rock pit above the level
of the bottom of the canal.
Thorough investigation of the rock pit began the following
morning, March 31. As shown by figure 12, section 12, a con-
siderable quantity of salt water had flowed into the pit. The
chloride content in the rock pit was low compared to that of water
in the canal because, as the salt water flowed downward into the


Figure 11. Rock pit showing approximate bottom topography.





FLORIDA GEOLOGICAL SURVEY


lake, heavy mixing and dispersion of salt reduced the maximum
concentration to less than 2,000 ppm chloride, though water with
a concentration of 5,000 ppm was observed flowing into the pit.
Soundings, made in conjunction with numerous samplings and
conductivity traverses during subsequent months, were used to
develop a rough contour map of the bottom of the rock pit (fig. 11).
Two deep holes nearly 40 feet below sea level are separated by a
ridge having an altitude of about 25 feet below msl. In these holes
most of the salt water samples were collected at depths greater
than 20 feet below msl. The profile of the pit bottom in the
canal sections (fig. 8, 9, 12-15) is generalized and represents no
particular section line.

FLUSHING OF SALT WATER FROM THE CANAL

Once having entered deep holes such as those in the rock pit,
the heavy salt water cannot easily be retrieved and flushed. As
most of the salt water remaining in the canal would eventually
flow into the rock pit, the canal was flushed on March 31, the
day after closing of the control gates. At high tide, at 1:30 p.m.,
two gates of S-29 were opened fully. The distribution of salt water
before flushing is shown in figure 12, section 12, and the progress of
flushing is shown in sections 13 and 14.
The principle of flushing is based on the tidal fluctuation of the
ocean. In Miami, the range of fluctuation from high tide to low
tide amounts to about 2 feet. Assuming that the water level up-
stream from the control dam is at least as high as the level of the
ocean at high tide, the dam can be opened at high tide or shortly
thereafter. The falling tide level continually creates a head
differential between inland parts of the canal and the ocean; thus,
for a period of about 6 hours, from high tide to low tide, a steep
seaward hydraulic gradient produces a high rate of discharge from
the canal. The discharge reaches a maximum slightly before low
tide (fig. 6). Referring to the velocity curve in figure 8B, section 7,
the seaward velocity of the fresh water just below low tide can be
estimated at about 2 fps (feet per second) while that of the salt
water can be estimated at about 0.7 fps. At this velocity, the salt
water will move seaward at a rate of at least half a mile per hour.
During the 6 hours of falling tide, flushing of 2 to 3 miles of the
downstream reach of a canal is feasible. Turbulence disperses large
quantities of salt water upward into the high-velocity fresh water
and the flushing is probably more effective than the calculation
indicates.








REPORT OF INVESTIGATIONS No. 24


C3


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19 19


DISTANCE FROM CONTROL DAM;IN FEET
S i.oo -10o.o : 0o Q. 6 4000 2_
00(2) MARCH 31,1961 (B:50A.M.)
t- ir~~re/f s ri)/fc -^


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62 P 12 GAPt It la SPN L &A 12 P 1 2 SI G P






00
(13) MARCH 31,1961 (2:40 P.M.)


ISOCHLORSIN PARTS PER MILLION TE SUR

-10

MAR.F7 MAR.28 MAR.29 MAR.30 MAR.31 APR.I CONTROL
2 i- 12r GP 1 &A I 6P 12 A i2 6P? GA IZ6P I A 1 6P2 6A 1 GP OPEN



10-- ------------- f c --,'' --*!--F- ----.- --,::---,'----- I---
-20. ," T r, I






(14) MARCH 31,1961 (440 P.M.)
WATER SURFACE--,


" (15) APRIL


-i IS0CHLORS IN F

-t
9too
to ec. ^-a
s- 06


Figure 12. Sections 12, 13, 14, and 15 through the Snake Creek Canal showing
movement of salt water during and after flushing, March 31 to April 1, 1961.


ISOCHLORS,IN PARTS PER MILLION 1








-4
_,< A A3 t" C


I





FLORIDA GEOLOGICAL SURVEY


During the dry spring of 1962, high easterly winds on March
8 and 9 coincided with high average tides and the wind-driven
waters of Biscayne Bay overtopped many control dams in the
Miami area at high tide. Salt water invaded the canals at a time
when dams throughout the area were held completely closed to
maintain heads and to conserve fresh water. In spite of the entrap-
ment of salt water in the canals there was reluctance to flush the
canals, as it was felt that the consequent loss of fresh water could
not be tolerated.
The series of isochlor sections for unimpeded tidal oscillation
(fig. 8A, 8B) show that it is not necessary to have a high fresh-
water head behind the dam in order to perform a flushing operation.
The principle of flushing depends almost entirely upon the falling
stage in the ocean itself. The primary requisites are: (1) the
canal should be long enough to tap a reservoir of fresh water at
inland points, and (2) the control dam should be of a type that
permits rapid operation so that opening and closing can be
accomplished readily at opportune moments.
Flushing of salt water from a canal entails some loss of fresh
water. However, wasting a small portion of the vast reservoir of
fresh ground water tapped by a canal is preferable to allowing a
small volume of salt water to move inland considerable distances
where it may contaminate much larger volumes of fresh water.
ULTIMATE LOSS AND FLUSHING OF THE REMAINING
TRAPPED SALT WATER
In the 41/2-hour flushing operation of March 31 not all salt
water was removed. As shown by figure 12, section 15, a lens of salt
water remained in the downstream reach of the Snake Creek Canal.
Also, most of the relatively heavy salt water in the rock pit
remained. These separated masses of salty water were traced
until July 1961 (fig. 13-15).
The downstream lens in the canal tended to spread toward the
rock pit, but a large part of the salt water in the lens disappeared
between April 1 and 3 (compare sec. 15, 16). The disappearance
coincided with an increase of chloride content in observation wells
adjacent to the canal and will be considered in a later section.
The salt water in the rock pit appeared to remain practically
stable from April 1 to 18 (sec. 15-19). The primary change
during this period was a dispersion-caused widening of the contact
between the 1,000- and 200-ppm isochlors.
Between April 18 and May 3, the isochlor positions changed
significantly in the rock pit and also in the canal (compare sec. 19,






REPORT OF INVESTIGATIONS NO. 24


s k I tI
I 5 | a I I
DISTANCE FROM CONTROL DAM,IN FEET


Figure 13. Sections 16, 17, and 18 through the Snake Creek
changes in isochlor positions, April 3-11, 1961.


Canal showing


20). An increase in chloride content was sensed by the conductivity
recorder at the footbridge between April 22 and 26 (fig. 5) and,
therefore, this was probably the period when the distribution of
chloride content in the rock pit changed.
In the bar graph in figure 14, lower part of section 19, the wind
movement exceeds 100 miles per day for 3 consecutive days, April
21-23; this period nearly coincides with the period of increase in
chloride content at the footbridge. Apparently, the 3 days of strong
wind mixed the water in the upper part of the rock pit. The


61:
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6: ^ 6
6


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6:

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61
6

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S, v 000 6000 4000 2000 0
(16) APRIL 3, 1961
WATrR SURFACE-'
0 ISOCHLORS, IN PARTS PER MILLION




000
*2C CONTROL
2 CLOSED


6 1 (17)

isoc



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-4
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FLORIDA GEOLOGICAL SURVEY


h- I


'6 4
I *4


Figure 14. Sections 19, 20, and 21 through the Snake Creek Canal showing
changes in isochlor positions, April 18 to May 16, 1961.

downward movement of isochlors in section 20 represents dispersion
and upward movement of salt ions into the uppermost water of the
rock pit. Small hydraulic gradients, associated with the wind
and with ground-water discharge in the downstream reach of the
canal, then carried the salty water downstream where it was sensed
by the conductivity recorder at the footbridge.
Wave height on a given lake depends on the length of time the
wind blows as well as on the wind speed. Therefore, the 3 consecu-
tive days of high wind, April 21-23, were far more effective in





REPORT OF INVESTIGATIONS NO. 24


stirring the waters of the rock pit than were the single days of
similar wind speed in early April. The downward movement of
isochlors between May 3 and 16 (sec. 20-21) undoubtedly relates
to the relatively high sustained winds of May 6 to 9. It is
significant that wind and waves serve as agents for removal of
trapped salt water.
On May 26, after heavy rainfall, the dam was opened (fig. 5).
The resulting tidal oscillation and discharge produced small currents
in the bottom of the rock pit. This agitation reduced the chloride
content and density more in the eastern hole (fig. 11), by upward
dispersion, and the salt was gradually flushed toward the partly
open control dam. Although residual traces of the original con-
tamination remained, the partial gate openings that started March
26 were effective in reducing salinity even at the very bottom of
the rock pit (fig. 15). The decrease in chloride content at the
footbridge (fig. 5) at the same time indicates that the tidal im-
pulse of the ocean can be utilized .to remove salt water that has
become trapped in deep pits or lakes along the course of a canal.

SALT-WATER MOVEMENT IN THE AQUIFER

The short-term movements of salt water in the aquifer are
closely influenced by happenings in the canal. Fluctuations of
chloride content in wells along the south bank of the canal during
1960 and 1961 are shown in figures 16-20. During the special test
of March 27-31, water samples were collected as often as four
times daily and these are plotted on an expanded time scale.

NATURAL TIDAL OSCILLATION

Under unimpeded tidal oscillation, water moves from the aquifer
into the canal on a falling tide and from the canal into the aquifer
on a rising tide. This to-and-fro motion prevailed when the gates
were open March 27 to 30 and is indicated by the two-headed arrow
in the March 30 section of the sequence of insets in figure 19.
During this period the chloride content in the shallow wells
fluctuated only slightly at stations 1 to 5 (fig. 16-18), and practically
none at the more landward stations (fig. 19-20). Although high-
salinity water oscillated landward beyond station 8 in the canal
(fig. 8), salt water did not appear in wells located as little as 5
feet from the canal. This is not surprising. The net flow of fresh
ground water in the upper part of the aquifer is toward the
canal. Any small amount of salt water that penetrates the aquifer






30 FLORIDA GEOLOGICAL SURVEY




lit i Z @
30 Q &Z ti Qt

DISTANCE FROM CONTROL DAM,IN FEET
14000 12000 10000 8000 6000 4000 0n000 9


(22) MAY 29.1961
WATER SURFACE-.

ISCOGLORS.IN PARTS PE MILLION

N --~ .--- -. u z u -
-20






(23) JUNE 3. 1961
__WATER SURFACE
ISOCMILOLRS. IN Pr'S PE MILLION


-. ..- .., .PARLrLOPEN






(24) JULY 10I. 961
I WATER SURFACE-a
t .I eSOC,4LOSIv P ARTS PER MILLION


-IC














the canal opposes and greatly minimizes movement of salt water
from the canal to the aquifer during unimpeded tidal oscillation.
A different situation exists in the deep part of the aquifer. The
average head in the aquifer is lowered when the gates are open.
This causes salt water, deep in the aquifer, to move landward. The
i...





















movement is slow but relentless. An example is the increasehowin
chloride content from 6,800 to 9,200 ppm in well G 420A between
















Uo.






g 100
0


0

0
I-

N


0
Fj2
z
0
L~3


Figure 16. Graph showing fluctuations of chloride content at well stations
1, 2, and 3, 1960-61.























* 1000


Figure 17. Graph showing fluctuations of chloride content at well station
4, 1960-61.
































Figure 18. Graph showing fluctuations of chloride content at well station
5, 1959-61.















MAP VIEW
10,000 y

O ^OIio


Figure 19. Graph showing fluctuations of chloride content at well station
6, 1960-61.















2 200 I I i 1 II11 11,- I I I I I I I I


G
L STATION 8I Gll---.., SC.G
- 100 Wo ..- ... 1;1.. ---',, -650
G 1149 7 .,. G-4---.- 6. 4
G 111- G 141149-6*
D r-_ %G 1152 XPAN E I
NU V TIME SCALEr M Jd J Y U -

a WU'.8.V 1 I 3-4 5 6 0 so,*0 50 ft"
200j -- --- I ---,--- -- | II| 1||H |-- I ------ I --- I ------ r-'-- I --- 1 --
5" STATION 7 11.. .-., 146 1148
*00 -.-.__ -,,,--- ........".,... ./1,,^-- -- ---,,- -- --
8, and 9, 19G 1146 60-61.
.aS t s 1' 1 '- 1148 ...-,...,*^ ..' .......
,/ y "-<--G1145
G 114 MP vIEW SECTION VIEW j -
...EXPANOED_ sc,,. _

0 -- --- -- --- -- U J '''''.f 1-- --- -- --- 46-- g S 100 Fo moan-- "
NOV. DEOeC. JAN. FEB. MAR.7019 [ __ I AP S 4 R. MAY JUNE AG 9 SP W

Figure 20. Graph showing fluctuations of chloride content at well stations
7, 8, and 9, 1960-61.





FLORIDA GEOLOGICAL SURVEY


June and October 1960, when the dam was open for hurricane
Donna floods (fig. 18).

RECHARGING THE AQUIFER WITH TRAPPED SALT WATER

The canal is quickly converted from a drainage canal to a
recharge canal by closing the control. The water which happens
to be in the canal at this time, either fresh or salty, is driven from
the canal under hydraulic gradient and appears in wells adjacent to
the canal.
All well stations (fig. 16-20) showed an increase of chloride
content on the morning of March 31, the day after salt water was
trapped. As will be noted by comparing the graphs, the degree of
reaction to the recharge of salt water varied. A partial list of
the factors involved is as follows: (1) The distance of the well
from the canal; (2) the altitude of the terminus of the well in
relation to the altitude of salt water in the canal; (3) the concen-
tration of salt water, which varies both vertically and longitudinally
along the canal; (4) the amount of dispersion of salt as the water
passes through the rock; and (5) the local permeability of the
aquifer near the well station.
The relations of salt-water movement at station 6 are shown in
the section views of figure 19. Section a for March 30 shows that
salt water had not invaded the aquifer during unimpeded tidal
oscillation.
In section b (10:00 a.m., Mar. 31), approximately 24 hours
after closing the control gates, the chloride content in well G 1138,
5 feet from the canal, had increased from 60 to 13,700 ppm while
that in well G 1140, 65 feet from the canal, increased from 60 to
288 ppm. The sharp increase of chloride content in these wells, in
contrast to the decrease in well G 1136 (about 22 feet deeper) from
125 to 102 ppm, indicates that the salt water moved nearly hori-
zontally. This is a reasonable observation because the horizontal
permeability is generally much greater than the vertical
permeability in stratified aquifers.
Section c (4:00 p.m., Mar. 31) shows the flow direction while
the canal was being flushed. The sharp decrease of chloride content
in wells G 1138 and G 1140 in the fluctuation graph at this time
indicates that part of the salt water, that had previously invaded
the aquifer, was flushed back toward the canal. After the flushing
operation was completed and the dam was closed, the unflushed
salt water in the aquifer resumed a flow direction away from the
canal (sec. d).






REPORT OF INVESTIGATIONS No. 24


In addition, salt water from the salt-water lens, which remained
in the downstream reach of the canal after flushing (fig. 12, sec.
15, and fig. 13, sec. 16),moved into the aquifer. The disappearance
of the salt-water lens from the canal by April 3 coincided with the
sharp decrease in chloride content in well G 1138 on that date; the
recharge water in the canal had become relatively fresh but the
increase of chloride content in well G 1140 between April 1 and 3
showed movement of the salt-water slug farther from the canal.
The low maximum chloride content in well G 1140 as compared to
that of well G 1138 indicates dilution of the salt water during
movement between the wells.
It is interesting to note that the chloride-content curve for well
G 1138 crosses over and becomes lower than that for well G 1140
after April 5 (fluctuation graph, fig. 19). As previously shown,
water in the canal continued to freshen with time. Thus, as the
slug of salt water passed farther into the aquifer it was followed
by relatively fresh water which appeared first in well G 1138, near
the canal bank, and then in well G 1140. The reversal of the
chloride-content curves shows the cause-effect relation of salinity
in the canal on salinity in the aquifer.
The chloride content in well G 1137 reached a maximum on
April 7, more than a week after the control was closed (fluctuation
graph, fig. 19). During this week, some of the salt water that
remained at the bottom of the canal was dispersed upward in the
canal by wind agitation (fig. 13, sec. 16, 17). As the shallow wells
were recharged by water from the upper part of the canal, the
time lag of salinity fluctuation in the shallow wells reflects the
changes of salinity in the upper part of the canal. The lower peak
chloride content in well G 1139, as compared to well G 1137 again
demonstrates the reduction of salinity by dispersion of salt as
ground water moves through the pore spaces of the aquifer.
Well G 1136 is 22 feet deeper than well G 1138 so that the
vertical component of salt-water movement is identified by the
relative fluctuations of chloride content in the two wells. The
chloride content in well G 1136 did not reach its maximum until
early May, about 1 month after the short-term test, whereas that
of well G 1138 reached its maximum 1 day after the control was
closed (fig. 19). This great difference in time results from several
factors. An important driving force in the vertical direction is the
difference in density between the overlying heavy salt water and
the underlying light fresh water. In open water, an unstable density
condition of this type is corrected rapidly by a massive discrete





FLORIDA GEOLOGICAL SURVEY


overturn of the two fluids. The settling of salt water in the rock
pit is an example. However, in an aquifer the pores of the rock are
small and the fluid movement is split into similarly small flow
filaments. The fresh water rises at the same time that the salt
water sinks. The overturn is reduced to slow infiltration wherein
dispersion of salt plays an important part in modification of the
original concentrations. The effect of trapping salt water in March
1961 is recognizable until October 1961 in the fluctuation curve for
well G 1136 (fig. 19).
In the longitudinal sections of figure 7, the downward movement
of salt water is depicted as a lens of salt water that appears near
the bottom of the canal and progressively drifts downward with
time (sections of Mar. 31 to May 16, 1961). At the same time there
is a horizontal component of flow perpendicular to and away from
the canal; this movement is shown by the arrows in section e of
figure 19.


ULTIMATE LOSS AND FLUSHING OF SALT WATER IN THE AQUIFER

Comparison of the fluctuations of chloride content at all stations
(fig. 16-20) shows that the deep wells were little affected by the
short-term test but that the shallow wells were affected in a
characteristic pattern. This pattern is responsive to the operations
of the control dam.
The control gates were closed on March 31, and they remained
closed until May 26 (fig. 5). Initially the chloride content in the
shallow wells rose sharply and then tended to decrease slightly. At
the downstream stations the decrease was somewhat larger because
ground-water movement around the control dam and toward the
ocean was more rapid there.
At the more inland stations (fig. 20), the aquifer is recharged
rapidly until canal water refills that part of the aquifer that is
unwatered during drainage. After this, the gradient away from
the canal becomes comparatively small as the ground-water flow
system approaches a steady state consistent with the boundaries
of the aquifer. As the rate of recharge is proportional to the
gradient, salt water at first moves rapidly into the aquifer, and
subsequently at a much slower pace. The salt water, therefore,
remains within reasonable proximity of the canal and is subject
to retrieval.
The chloride content in most of the shallow wells decreased
sharply after the control dam was opened at the end of May. The





REPORT OF INVESTIGATIONS NO. 24


fluctuations at station 7 (fig. 20) are especially interesting. The
chloride content in wells G 1145 and G 1146 near the canal de-
creased sharply, whereas that in G 1147 and G 1148, 60 feet from
the canal, remained unchanged. Figure 5 shows that the total
gate opening was decreased from 30 feet May 26 to 31 to about 5
feet thereafter. At the 30-foot gate opening, the canal caused
drainage and some of the salt water flowed out of the aquifer
into the canal. Water levels recovered somewhat after closing to
the 5-foot opening (fig. 5), and fresh water from the canal re-
charged the aquifer. The movement of fresh water into the aquifer
was sufficient to freshen the water in wells close to the canal
(G 1145-G 1146), although water in the wells farther from the
canal (G 1147-G 1148) remained relatively salty. Clearly, part of
the salt water trapped in March had been removed but a part still
remained as an isolated mass. During the next gate openings on
August 20 .the remaining salt water was completely flushed from
the aquifer and the chloride contents of all four wells became nearly
the same. Thus, the low-salinity water trapped in March was re-
covered from the shallow part of the aquifer by August 1961.
The recovery was possible because the density of this low-
salinity water (less than 200 ppm) was not great enough to cause
a large downward movement. The dense, high-salinity water that
percolated downward at station 6 (shown by the graph of well
G 1136, fig. 19) obviously was not recovered. It continued to move
downward until the density contrast was eliminated by confluence
with water of the same density in the deep part of the aquifer.

MANIPULATION OF THE CONTROL DAM FOR
MAXIMUM BENEFIT

The primary function of a canal in the Miami area is to remove
surplus floodwaters. As floods in Miami result from heavy rainfall,
it is obvious that the floodwaters must always be fresh. If salt
water penetrates landward in a canal, the drainage function of the
canal is impaired because floodwaters cannot flow seaward through
that part of the canal in which salt water flows landward; therefore,
the optimum removal of floodwaters occurs when no salt water
invades the canal. Whether the flow condition is steady state or
transient due to tide, it should be recognized that intrusion of salt
water in a canal is an obvious sign of reduced efficiency in the
discharge of fresh floodwaters.
All four gates of S-29 were fully open for nearly 2 months
after hurricane Donna passed on September 10, 1960 (fig. 3). Only






FLORIDA GEOLOGICAL SURVEY


during September 10 to 14 was the discharge great enough to keep
the canal flushed of salt water. At all other times during the period
of record shown in figure 5, full openings of the gates permitted
salt water to move upstream on a rising tide. This suggests that
the control was operated at full-open position much longer than
necessary.

FACTORS AFFECTING INLAND MOVEMENT OF
SALT WATER

The manner in which the total gate opening is manipulated has
a bearing on the landward extension of salt water in the canal.
For example, a gate opening of 30 feet may be represented by a
7.5-foot opening for each of the 4 gates or a 15-foot opening for
each of only two gates (the other 2 remaining closed).' The
schematic flow diagrams of figure 21 illustrate the hydraulic
advantage that attaches to uniform distribution of the gate
opening. In figure 21a fresh water is forced to flow downward
around the obstruction of the gate. This downward displacement
of the fresh-water discharge section impedes the upstream intrusion
of salt water. Uniform opening and submersion of the four sluice
gates (in contrast to fully open gates) can be used as a device
to distort the flow pattern deliberately in order to prevent salt-
water intrusion.
Questions arise as to whether the movement of salt water into
a canal is ever justified on the basis that widest opening of the
control gates will cause greatest discharge of fresh floodwater.
The critical point of entrance of salt water into a canal is at the
control dam, for here is the only place where intrusion can be
controlled. The problem involves the total head at the dam, the
amount of discharge at various gate openings under these various
heads, and the influence of fresh-water head in preventing upstream
movement of salt water beyond the control dam.
Using the principle of static equilibrium for fluids of different
density, salt water will not move upstream as long as the head
differential between the fresh-water and salt-water sides of the
control gate is greater than 0.3 foot (for a canal whose bottom is
12 feet below sea level). This value is obtained by balancing the

'The values used here only demonstrate a principle; the discharge area
for 2 gates opened 15 feet is not equal to the discharge area for 4 gates
opened 7.5 feet because the gates at wide-open position may be above the
actual water surface by as much as 3 feet.






REPORT OF INVESTIGATIONS NO. 24


pressure exerted by fresh water against the opposing pressure
exerted by sea water at the bottom of -the canal.

Pf = pf g lf Ps pa g ls
Pt g 1f = ps g Is
letting 1f = h + 1. (See fig. 21a):
pf (h + 1) = p.s l
h = 1. (ps pf)
Pf
h = 12 (1.025 1.000) = 0.3 ft.
1.000
Pf = pressure exerted by fresh water
Ps = pressure exerted by sea water
g = acceleration of gravity
pf = 1.000, density of fresh water
pS = 1.025, density of sea water
1f = length of fresh-water column, in feet
1i = 12 feet, length of sea-water column
h = head differential between fresh-water and sea-water sides of control
dam, in feet.

Referring to the schematic flow diagram of figure 21b, when
the gates are fully open, the head loss through the control structure
itself is very small. The flood conditions would have to be very
severe in order to maintain 0.3 foot of fresh-water head at the
control dam near high tide, when the greatest possibility of inland
movement of salt water exists. The reason for upstream penetra-
tion of salt water on September 15, 1960 (fig. 5), only 5 days after
the passage of hurricane Donna, is evident. With fully open gates,
the head differential and discharge decrease very rapidly until
salt water is able to flow upstream during the rising tide. Sub-
mergence of the gates, however, tends to focus all the head loss at
the control (fig. 21a). This is very desirable and if the four gates
of S-29 are uniformly adjusted to maintain a head differential of
0.3 foot during the rising tide, no salt water will invade the canal.

DISCHARGE RELATED TO GATE OPENING

The quantity of water discharged under a sluice gate depends
on the head differential across the gate and on the cross-sectional
area through which the discharge occurs.
Figure 22 shows the approximate discharge related to head
differential for various openings of one gate of the control dam in
the -Snake Creek Canal. The plotted points represent the mean
daily discharge at West Dixie Highway and the head differential,






FLORIDA GEOLOGICAL SURVEY


(b)

I




WATER SURFACE

FRESH WATER

INTERFACE --
SA LT WA TER

Figure 21. Schematic flow diagrams: a, with gate partially submerged; b, with
gate fully open.
as obtained from the difference in mean daily gage height at West
Dixie Highway and Biscayne Bay at NE 84th Street. Comparison
of past records indicated that the mean daily gage heights of
Biscayne Bay at NE 84th Street and the now discontinued gaging
station just below the control dam in Snake Creek Canal were com-
parable when the control dam was closed or the discharge was
within the range shown in figure 22.
The curves were calculated by using the formula Q = ACV2 gh
where Q is the discharge, A is the cross-sectional area, C is. the
coefficient of friction, g is the acceleration of gravity, and h is the







REPORT OF INVESTIGATIONS NO. 24


S200


DISCHARGE,
400 600


IN
800


o.
I- )o
Z 0





0 0
o 0
/


LL0 0


0 0


CFS
1000


1200 1400


Figure 22. Graph showing relations of head differential and discharge for
various openings of one gate of the control dam in the Snake Creek Canal.



head differential through the gate. As Q, A, and H were known
for each of the points in figure 22, the coefficient of friction (C)
was evaluated for the different gate openings. Then, using the
average value of C and holding the area (A) constant for the
individual gate openings, the curves were constructed through the
mean value of the plotted points by solving the formula for Q while
letting the head differential (h) vary.
The curves may be used to estimate low discharge for the
control structure provided the upstream tidal range is less than
0.5 foot. The accuracy decreases when the tidal range of the up-
stream stage recorder exceeds 0.5 foot during a given day.
The following table shows the recession' of discharge following
the passage of hurricane Donna on September 10, 1960.


'.7


*.--HEAD DIFFERENTIAL THAT WILL PREVENT
UPSTREAM MOVEMENT OF SALT WATER
AT BOTTOM OF CONTROL DAM


"I----~- '






FLORIDA GEOLOGICAL SURVEY


Date Mean daily head Mean daily
(September) differential (feet) discharge (cfs)

11 0.48 2,430
12 .38 2,180
13 .31 1,800
14 .28 1,660
15 .23 1,510
16 .17 1,290
17 .19 1,240
18 .22 943


Salt water invaded the canal on a rising tide on September 15.
A comparison of the measured discharges in the table (all gates
fully open) with the plotted points in figure 22 (only one gate
partially open) indicates that the four gates can be partly closed
within a week after a flood peak has occurred to maintain a head
differential of at least 0.3 foot. This would keep the salt water out
of the canal. The small increase in head required to maintain the
same discharge would be insignificant from the standpoint of flood
height above the dam.
There is a distinct difference between holding salt water back,
as shown in the schematic flow diagram of figure 21a, and allowing
salt water to invade the canal, as in figure 21b. Inland penetration
of salt water does not contribute to seaward discharge because, as
previously defined, true removal of floodwaters must only mean
removal of fresh floodwaters. Salt water which penetrates land-
ward in the canal merely replaces fresh water; once it has gained
entry, energy must be expended to remove it. In the velocity pro-
files of figures 8A and 8B, the velocities near the bottom of the
canal are landward during the rising tide, and are small relative
to the seaward velocities in the upper part of the canal during the
falling tide. These velocities reflect two things: (1) Sea water
provides back pressure which is greatest at the bottom of the
canal. (2) Energy is expended in driving the intruded salt water
back to the ocean on the falling tide. As the noncontributing
landward flows and the reduced seaward flows of salt water must
be integrated with the positive flow of fresh water to yield the
mean daily discharge, the mere presence of salt water in the canal,
in effect, reduces the mean daily discharge of true (fresh) flood-
waters.
In some respects, salt water acts as a movable dam in the
bottom of the canal; it effectively reduces the depth of the canal






REPORT OF INVESTIGATIONS NO. 24


through which fresh water can flow seaward. For example, if salt
water occupies the lower 6 feet of the canal when the control is
fully open (fig. 8), the effective flow section for fresh water is
reduced to a thickness of about 6 feet (total depth of canal is 12
feet). If the gates are lowered to an opening of, say, 7 feet, the
fresh-water discharge will be impeded and this will cause the fresh-
water head to rise until salt water is driven out of the lower part
of the canal. Conceivably, the head might build up sufficiently so
that the fresh water would flow through the full 7-foot section
below the sluice gate. However, there is no assurance that this
will occur-fresh water may discharge through only the upper
part of the 7-foot thick section while salt water continues to flow
landward through the lower part.
This leads to an important point. The effect upon head and
discharge caused by constriction from lowering the gates, as
opposed to constriction and back-water effect from salt water in
the bottom of the canal, is indeterminate without controlled tests.
The optimum gate opening probably occurs when the salt-water
wedge is held just downstream from the control and only fresh
water is flowing beneath the sluice gates. This gate opening will
discharge the maximum amount of water without permitting
salt water to invade the canal and aquifer above the control.
Future studies should be directed toward evaluation of the
relations of fresh-water to salt-water discharge with various gate
openings, in the immediate vicinity of the control dam.

SUMMARY

The Snake Creek Canal cuts through highly permeable lime-
stone of the Biscayne aquifer of southeastern Florida. The canal
extends inland about 18 miles from the salinity-control dam located
near the shoreline. The canal is somewhat representative of a
number of canals in the area and information learned during-the
study is generally applicable to other canals.
Salt water was traced in the Snake Creek Canal under various
circumstances of control-dam operation during a special test in
March 1961. In the first phase of this test all four gates of the
control dam were opened wide. Salt water moved inland about 2
miles during the rising tide and seaward during the falling tide.
Salt water penetration into the aquifer was negligible during the
3 days the gates were fully open.
In the second phase of the special test the control was completely
closed at high tide, trapping salt water above the dam. A density





REPORT OF INVESTIGATIONS NO. 24


fluctuations at station 7 (fig. 20) are especially interesting. The
chloride content in wells G 1145 and G 1146 near the canal de-
creased sharply, whereas that in G 1147 and G 1148, 60 feet from
the canal, remained unchanged. Figure 5 shows that the total
gate opening was decreased from 30 feet May 26 to 31 to about 5
feet thereafter. At the 30-foot gate opening, the canal caused
drainage and some of the salt water flowed out of the aquifer
into the canal. Water levels recovered somewhat after closing to
the 5-foot opening (fig. 5), and fresh water from the canal re-
charged the aquifer. The movement of fresh water into the aquifer
was sufficient to freshen the water in wells close to the canal
(G 1145-G 1146), although water in the wells farther from the
canal (G 1147-G 1148) remained relatively salty. Clearly, part of
the salt water trapped in March had been removed but a part still
remained as an isolated mass. During the next gate openings on
August 20 .the remaining salt water was completely flushed from
the aquifer and the chloride contents of all four wells became nearly
the same. Thus, the low-salinity water trapped in March was re-
covered from the shallow part of the aquifer by August 1961.
The recovery was possible because the density of this low-
salinity water (less than 200 ppm) was not great enough to cause
a large downward movement. The dense, high-salinity water that
percolated downward at station 6 (shown by the graph of well
G 1136, fig. 19) obviously was not recovered. It continued to move
downward until the density contrast was eliminated by confluence
with water of the same density in the deep part of the aquifer.

MANIPULATION OF THE CONTROL DAM FOR
MAXIMUM BENEFIT

The primary function of a canal in the Miami area is to remove
surplus floodwaters. As floods in Miami result from heavy rainfall,
it is obvious that the floodwaters must always be fresh. If salt
water penetrates landward in a canal, the drainage function of the
canal is impaired because floodwaters cannot flow seaward through
that part of the canal in which salt water flows landward; therefore,
the optimum removal of floodwaters occurs when no salt water
invades the canal. Whether the flow condition is steady state or
transient due to tide, it should be recognized that intrusion of salt
water in a canal is an obvious sign of reduced efficiency in the
discharge of fresh floodwaters.
All four gates of S-29 were fully open for nearly 2 months
after hurricane Donna passed on September 10, 1960 (fig. 3). Only






FLORIDA GEOLOGICAL SURVEY


current developed in the trapped salt water, which caused it to
flow inland along the bottom of the canal at a rate of about 900
feet per hour until it reached a deep rock pit which acted as a
hydraulic sink. A flushing operation removed salt water from the
canal, but not that in the rock pit. Measurements of electrical
conductivity during the subsequent months in the rock pit showed
that the salt dispersed upward by a combination of wind, tidal
mixing, and molecular diffusion. After reduction of salinity by
this upward dispersion, the salt water in the rock pit was flushed
seaward through the canal.
When salt water is trapped in a canal by control-dam operation
or by overtopping the dam during abnormally high tides, it is not
necessary to wait for nor to have high fresh-water head behind
the dam in order to perform a flushing operation. The falling tide
(level) of the ocean creates a seaward hydraulic gradient and a
condition favorable for a high rate of discharge from the canal.
Calculations and observations show that flushing of 2 to 3 miles
of the downstream reach of a canal is feasible during one falling
tide. The canal should tap a reasonably large reservoir of fresh
water and the control dam should be the type that permits rapid
operation so that the gates can be opened just after high tide and
closed at low tide. Although some fresh water is lost during flushing,
good water management is better served by wasting a small portion
of a vast reservoir of fresh water than by allowing a small volume
of salt water to move inland to contaminate a much larger volume of
fresh water.
The movement of salt water in the aquifer adjacent to a canal
is closely related to happenings in the canal itself. When the gates
are closed, the downstream controlled reach of the canal is
immediately converted from a condition of drainage to a condition
of recharge. Salt water trapped behind the dam during the test
entered the aquifer and quickly appeared in wells adjacent to the
canal. Sections perpendicular to the canal showed that the salt-
water outseepage from the canal moved much more rapidly hori-
zontally than it did vertically. The residual effect of downward
movement of salt water in the aquifer was recognizable for 6
months at one of the observation stations. The fluctuations of
chloride content in shallow wells indicate that some of the low-
salinity water in the upper part of the aquifer was eventually
recovered and flushed seaward through the canal during subsequent
gate openings.






REPORT OF INVESTIGATIONS No. 24


The movement of high salinity water deep in the aquifer
responds to head changes related to long-term operations of the
control dam. The chloride content in deep wells increased during
the fall of 1960 when the control remained open because of flood
conditions. The year of 1961 was abnormally dry; infrequent
opening of the control caused heads to remain sufficiently high so
that general expulsion of salt water from the aquifer occurred
throughout 1961. It is a contradictory situation that the greatest
inland movement of salt water took place in a wet year and the
greatest expulsion of salt water took place during a dry year.
Nevertheless, this fact indicates that the means are available for
very effective control of the movement of salt water in the
Biscayne aquifer.

CONCLUSIONS AS TO OPERATING CRITERIA
The data in this report indicate that salt water need not and
should never be trapped in a canal. As the position of salt water
in a canal has only a tenuous relation to head, the following
conclusions are made with regard to the operation of the gates:
(1) Control gates, which operate automatically by sensing the
head differential between the upstream and downstream
sides of the dam, cannot be operated effectively on an
automatic basis until the relations of salt-water movement
and head are properly evaluated. It is entirely possible for
the automatic operations to trap salt water inadvertently.
(2) In.addition to the upstream and downstream stage recorders
which are being installed at the new automatically
controlled dams, a conductivity recorder should be installed
in each canal slightly upstream from the control dam. Data
from these three sources will permit future evaluation of
the relation of salt-water movement to total head in the
canal and to head differential through the control dam.
(3) Persons responsible for gate operation should be furnished
a portable conductivity instrument to determine the salinity
at the bottom of the canal at different points. Knowledge
of the presence and movement of salt water in the canal
will aid greatly in proper operation of the control.
The following suggestions are made for operation after heavy
rainfall:
(1) After the first inland penetration of salt water during a
rising tide (which may occur within a week after a flood






REPORT OF INVESTIGATIONS NO. 24


through which fresh water can flow seaward. For example, if salt
water occupies the lower 6 feet of the canal when the control is
fully open (fig. 8), the effective flow section for fresh water is
reduced to a thickness of about 6 feet (total depth of canal is 12
feet). If the gates are lowered to an opening of, say, 7 feet, the
fresh-water discharge will be impeded and this will cause the fresh-
water head to rise until salt water is driven out of the lower part
of the canal. Conceivably, the head might build up sufficiently so
that the fresh water would flow through the full 7-foot section
below the sluice gate. However, there is no assurance that this
will occur-fresh water may discharge through only the upper
part of the 7-foot thick section while salt water continues to flow
landward through the lower part.
This leads to an important point. The effect upon head and
discharge caused by constriction from lowering the gates, as
opposed to constriction and back-water effect from salt water in
the bottom of the canal, is indeterminate without controlled tests.
The optimum gate opening probably occurs when the salt-water
wedge is held just downstream from the control and only fresh
water is flowing beneath the sluice gates. This gate opening will
discharge the maximum amount of water without permitting
salt water to invade the canal and aquifer above the control.
Future studies should be directed toward evaluation of the
relations of fresh-water to salt-water discharge with various gate
openings, in the immediate vicinity of the control dam.

SUMMARY

The Snake Creek Canal cuts through highly permeable lime-
stone of the Biscayne aquifer of southeastern Florida. The canal
extends inland about 18 miles from the salinity-control dam located
near the shoreline. The canal is somewhat representative of a
number of canals in the area and information learned during-the
study is generally applicable to other canals.
Salt water was traced in the Snake Creek Canal under various
circumstances of control-dam operation during a special test in
March 1961. In the first phase of this test all four gates of the
control dam were opened wide. Salt water moved inland about 2
miles during the rising tide and seaward during the falling tide.
Salt water penetration into the aquifer was negligible during the
3 days the gates were fully open.
In the second phase of the special test the control was completely
closed at high tide, trapping salt water above the dam. A density






FLORIDA GEOLOGICAL SURVEY


peak), the gates should be partly closed as conditions
permit.
(2) The gates should always be opened uniformly so that
maximum fresh-water pressure is applied against the in-
truding salt water by increased velocities of the fresh water
as it flows under the submerged sluice gates.
(3) The instantaneous dynamic head that will prevent inland
movement of salt water cannot be determined without
controlled tests. However, the calculated hydrostatic head
that will prevent intrusion at the bottom of S-29 is 0.3
foot. Therefore, salt water will not enter the bottom of
the canal above the control structure if an instantaneous
head of 0.3 foot is maintained between upstream and down-
stream sides of the dam at the time of minimum head
differential, about 2 hours before high tide.
Some experimentation will be necessary to establish specific
operating criteria. Closing the control gates so that 0.3 foot of head
will be maintained during the rising tide will tend to increase the
stage in the upstream part of the canal by an amount not greater
that 0.3 foot. This is a tolerable increase in stage even during
floods.






REPORT OF INVESTIGATIONS No. 24


The movement of high salinity water deep in the aquifer
responds to head changes related to long-term operations of the
control dam. The chloride content in deep wells increased during
the fall of 1960 when the control remained open because of flood
conditions. The year of 1961 was abnormally dry; infrequent
opening of the control caused heads to remain sufficiently high so
that general expulsion of salt water from the aquifer occurred
throughout 1961. It is a contradictory situation that the greatest
inland movement of salt water took place in a wet year and the
greatest expulsion of salt water took place during a dry year.
Nevertheless, this fact indicates that the means are available for
very effective control of the movement of salt water in the
Biscayne aquifer.

CONCLUSIONS AS TO OPERATING CRITERIA
The data in this report indicate that salt water need not and
should never be trapped in a canal. As the position of salt water
in a canal has only a tenuous relation to head, the following
conclusions are made with regard to the operation of the gates:
(1) Control gates, which operate automatically by sensing the
head differential between the upstream and downstream
sides of the dam, cannot be operated effectively on an
automatic basis until the relations of salt-water movement
and head are properly evaluated. It is entirely possible for
the automatic operations to trap salt water inadvertently.
(2) In.addition to the upstream and downstream stage recorders
which are being installed at the new automatically
controlled dams, a conductivity recorder should be installed
in each canal slightly upstream from the control dam. Data
from these three sources will permit future evaluation of
the relation of salt-water movement to total head in the
canal and to head differential through the control dam.
(3) Persons responsible for gate operation should be furnished
a portable conductivity instrument to determine the salinity
at the bottom of the canal at different points. Knowledge
of the presence and movement of salt water in the canal
will aid greatly in proper operation of the control.
The following suggestions are made for operation after heavy
rainfall:
(1) After the first inland penetration of salt water during a
rising tide (which may occur within a week after a flood







REPORT OF INVESTIGATIONS NO. 24


REFERENCES

Cooke, C. W.
1939 Scenery of Florida interpreted by a geologist: Florida Geol.
Survey Bull. 17.
Ferguson, G. E. (see Parker, G. G.)
Kohout, F. A.
1960 Cyclic flow of salt water in the Biscayne aquifer of southeastern
Florida: Geophys. Research Jour., v. 65, no. 7, p. 2133-2141.
1961 Case history of salt water encroachment caused by a storm
sewer in Miami: Am. Water Works Assoc. Jour. v. 53, no. 11, p.
1406-1416.


Leach, S. D.
1963


(and Sherwood, C. B.) Hydrologic studies in the Snake Creek
Canal area, Dade County, Florida: Florida Geol. Survey Rept.
Inv. 24, pt. 3.


Love, S. K. (see Parker, G. G.)
Parker, G. G.
1951 Geologic and hydrologic factors in the perennial yield of the
Biscayne aquifer: Am. Water Works Assoc. Jour., v. 43, no. 10,
p. 817-835.
1955 (Ferguson, G. E., Love, S. K., and others) Water resources of
southeastern Florida, with special reference to the geology and
ground water of the Miami area: U. S. Geol. Survey Water-
Supply Paper 1255.
Sherwood, C. B. (see Leach, S. D.)










FLRD GEOLIOWC( ICA SURflViEWY~


COPYRIGHT NOTICE
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Under the Statutes of the State of Florida (FS 257.05; 257.105, and
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