|
UFL/COEL 89/002
SHORT COURSE
ON
PRINCIPLES AND APPLICATIONS
OF
BEACH NOURISHMENT
February 21, 1989
* Instructors *
Thomas Campbell
Robert G. Dean
Ashish J. Mehta
Hsiang Wang
* Organized by *
FLORIDA SHORE AND BEACH PRESERVATION ASSOCIATION
DEPARTMENT OF COASTAL AND OCEANOGRAPHIC ENGINEERING.
UNIVERSITY OF FLORIDA
Short Course
on
Principles and Applications
of
Beach Nourishment
February 21, 1989
Organized by
Florida Shore and Beach Preservation Association
Department of Coastal and Oceanographic
Engineering, University of Forida
Instructors
Thomas Campbell
Robert G. Dean
Ashish J. Mehta
Hsiang Wang
Chapter 4
SEDIMENT STORAGE AT TIDAL INLETS
Ashish J. Mehta
Coastal and Oceanographic Engineering Department
University of Florida, Gainesville
INTRODUCTION
Accumulation of sediment around tidal inlets has become a matter of
renewed interest mainly for three reasons. The first of these is the need to
estimate the shoal volumes, particularly in the ebb shoal, as a potential
source of sediment for beach nourishment. Portions of the ebb shoal can be
transferred to the beach provided there are no measurable adverse effects on
navigation, or on the stability of the shoreline near the inlet. Such an
operation, for example, has been carried out successfully at Redfish Pass, on
the Gulf of Mexico coast of Florida (Olsen, 1979). A schematic example of a
potential site for ebb shoal excavation and sand transfer to the downdrift
beach is shown in Fig. 4.1.
The second reason is the need to assess the role of the inlet in
influencing the rate of erosion of downdrift shoreline, as a result of
interruption or deflection of the littoral drift. For example, the effect of
construction of Port Canaveral Entrance channel, Florida, on the downdrift
beach is shown in Fig. 4.2 (Dean, 1987). The beach shoreline eroded at a
comparatively rapid rate over a ~ 5 km stretch immediately south of the inlet,
and a beach nourishment project was consequently carried out in 1974.
Finally, an evaluation of inlet sediment accumulation is essential to
account for the long term sedimentary budget of shorelines interrupted by
inlets, as schematized in Fig. 4.3. The budget in this case is for the "box"
volume enclosed by shore-parallel and shore-normal boundaries. Q1 through Q8
Figure 4.1.
5
w
z
LU
z
=
w
n:
0 -5
=
Cl)
Figure 4.2.
A Schematic Example of a Potential Site (Area Enclosed by Dashed
Lines) for Ebb Shoal Excavation and Sand Transfer. Depth Contours
are Hypothetical.
20 40 60
DISTANCE SOUTH FROM PORT CANAVERAL
ENTRANCE (km)
Effect of Construction of Port Canaveral Entrance, on the Atlantic
Coast of Florida, on Downdrift (Southward) Shoreline (After Dean 1987).
I
. \ ea
Q3. ."
Bay
Q Q7
Q.
Figure 4.3. Box Volume Approach for Sediment Budget near a Tidal Inlet.
are volumetric rates of sediment transport across these boundaries. The
algebraic sum of all the Q's equals the time-rate of change of sediment volume
within the box. For an illustrative example see Jones and Mehta (1978).
In reference to these issues, quantities of particular interest are the
volume of sediment presently stored in the ebb shoal, and the volume of
material trapped, either as a result of training works such as jetties, or as
a consequence of the opening of an artificial inlet and the growth of
associated shoals. There is also the related question of volumetric erosion
of the downdrift shoreline. These issues will be examined with specific
reference to major inlets on the east coast of Florida together with
additional examples from Georgia inlets, following some comments concerning
natural and artificial sediment bypassing at sandy inlets.
SEDIMENT BYPASSING
Natural Bypassing
It has been well established that waves striking obliquely along the
coastline cause a significant transport of sediment along the coastline in
what has been called "the littoral drift system." An inlet located along such
a coastline represents a discontinuity to the littoral drift system, and
although the exact processes of the interaction between the inlet and the
littoral drift system are not fully understood, the gross effects are; namely,
accumulation of sediment in the ebb and flood shoals and a considerable
sediment exchange between the inlet channel and the shoal complexes (Dean and
Walton, 1975, Byrne et al., 1974, and FitzGerald, et al., 1976).
A schematic representation of the sediment transport processes at an
inlet has been given by Bruun et al., (1978) and is shown in Fig. 4.4 (Winton
and Mehta, 1981). Sediment moving down the coast in the littoral drift system
iFLOODOR' \ EBB
\BAY SHOAL : MAIN EBB CHANNEL ,SHOAL
S :.(BED AND SUSPENDED LOAD)
-- MARGINAL FLOOD CHANNEL .
; (BED AND SUSPENDED LOAD)'--
BARRIER :" DRIFT
Schematic Diagram Showing Inlet Ebb/Flood
Transport (After Winton and Mehta, 1981).
Sho
SEA
als and Sediment
^-.> ^- BARRIER ISLAND
FLOOD SHOALS/ :: THROATiii
:. L i // -BAR BYPASSING
::::::: ::: p -
TIDAL
BYPASSING
BARI.iER I.SLAND-.. \ / EB H A
SEBB SHOAL
Figure 4.5a. Bar Bypassing and Tidal Flow Bypassing at an Inlet.
B
Figure 4.4.
enters the inlet mouth mainly through the swash or marginal flood channels,
and to a lesser extent over the ebb shoals. When the flow through the inlet
is in the flood stage, some of the littoral drift will be carried through the
inlet to the bay or flood tidal shoals, some still in suspended form and the
rest as bed load. When the flow through the channel is in the ebb stage, some
of the material which was transported through the channel to the bay shoals
may be transported back through the channel to the ebb shoals and beyond, and
"new" material (i.e. deposited from the littoral drift) will also be
transported out onto the ebb shoals and/or beyond. For inlets with certain
morphologic characteristics and strong ebb flow, some littoral material would
be transported as bed load in deep water past the inlet, while for inlets with
relatively small ebb flows subject to strong wave action at low tide, part of
the material in the littoral drift may effectively bypass the inlet and not
pass back and forth through the main channel.
Thus as noted by Bruun et al. (1978), there are essentially two ways by
which sediment (sand) is bypassed naturally around an inlet (Fig. 4.5a).
These two modes are referred to as bar bypassing in which sand is
predominantly transported from updrift to downdrift beach via the ebb shoal,
and tidal flow bypassing in which the material enters the channel and, under
the combined action of tidal currents and cross-flow (alongshore current), is
eventually transported downdrift. However, during this process of natural
transfer, particularly in the case of tidal flow bypassing, a certain fraction
of the sediment mass transported per unit time may end up in the interior,
bayward of the throat section, thus forming flood shoals.
Under natural conditions, inlets generally differ in character from those
modified, as for example in the case of Florida's east coast inlets prior to
their modification. These natural entrances and their associated shoals
are volumetric rates of sediment transport across these boundaries. The
algebraic sum of all the Q's equals the time-rate of change of sediment volume
within the box. For an illustrative example see Jones and Mehta (1978).
In reference to these issues, quantities of particular interest are the
volume of sediment presently stored in the ebb shoal, and the volume of
material trapped, either as a result of training works such as jetties, or as
a consequence of the opening of an artificial inlet and the growth of
associated shoals. There is also the related question of volumetric erosion
of the downdrift shoreline. These issues will be examined with specific
reference to major inlets on the east coast of Florida together with
additional examples from Georgia inlets, following some comments concerning
natural and artificial sediment bypassing at sandy inlets.
SEDIMENT BYPASSING
Natural Bypassing
It has been well established that waves striking obliquely along the
coastline cause a significant transport of sediment along the coastline in
what has been called "the littoral drift system." An inlet located along such
a coastline represents a discontinuity to the littoral drift system, and
although the exact processes of the interaction between the inlet and the
littoral drift system are not fully understood, the gross effects are; namely,
accumulation of sediment in the ebb and flood shoals and a considerable
sediment exchange between the inlet channel and the shoal complexes (Dean and
Walton, 1975, Byrne et al., 1974, and FitzGerald, et al., 1976).
A schematic representation of the sediment transport processes at an
inlet has been given by Bruun et al., (1978) and is shown in Fig. 4.4 (Winton
and Mehta, 1981). Sediment moving down the coast in the littoral drift system
approached long term equilibrium with the sand transport processes under
prevailing wave and tide environment. Due to the predominant northeast
direction of wave approach, the net longshore transport of sand along the
shoreline is from north to south. Typically, as demonstrated by Fineren
(1938), the characteristics of these inlets included a broad shallow ebb shoal
or ocean bar; perhaps with a channel incised through the bar. Table 4.1
demonstrates that the bar depth was typically 1 to 2 m, much too shallow for
navigational purposes. Although the channels through the bar were
considerably deeper, most of them were still too shallow for modern commercial
purposes. Additional serious navigational disadvantages of these natural
channels were their tortuous alignments and migrational tendencies.
Table 4.1. Natural Depths in Channels and on Ebb Shoals of Florida's East
Coast Inletsa
Inlet Depth on Bar Channel Depth
(m) (m)
Nassau Sound 1.2 6.4-8.2
Fort George 1.2 3.4-7.9
St. Augustine 1.8 3.1-9.1
Matanzas Nearly blocked 3.7-5.5
Mosquito Nearly blocked 2.7-7.9
Canaveral Bight 1.8 to 5.5 9.1-12.2
Indian River Blocked 2.1-2.4
St. Lucie 1.2 2.4-3.7
Jupiter Blocked 0.9-1.5
Lake Worth 0.9 0.9-2.7
New River 2.4 3.1-4.6
Hillsboro 0.8 0.9-1.2
Norris Cut Not affected by sand Shoal
Bear Cut 1.2 2.1-5.2
Cape Florida Channel Not affected by sand Coral reefs
aSource: Fineren (1938)
When an inlet of natural origin is trained by jetties, the associated
sedimentary volumes change until the bottom topography reaches a new con-
figuration, which can be considered to be approximately in equilibrium with
the prevelant currents and wave climate (Dean and Walton, 1975). Often, the
net accretion in the updrift beach fillet is of the same order of magnitude as
the corresponding erosion downdrift. The flood shoal may experience only
minor change in shoal volume. The most dramatic effect occurs at the ebb
shoal, which contains most of the stored material (Marino and Mehta, 1986).
Jetties, possibly coupled with a dredged channel, concentrate the ebb
flow and cause the shoal to move seaward into deeper waters (Fig. 4.5b).
Furthermore over the long term, a secular rise in mean sea level will cause
the nearshore waters to become deeper. The contribution to shoal volume, if
any, from sea level rise along Florida's east coast cannot be evaluated
easily; however, at all the jettied inlets, training is likely to be the
dominant factor. Given the same tide and offshore wave conditions, the
seaward shoal at a trained inlet can store a larger quantity of impounded
sediment (Figs. 4.6a,b) than prior to training. Indeed, in many cases, the
impounded volume associated with the ebb shoal due to training is the only
significant trapped quantity of practical significance (Marino and Mehta,
1986).
Artificial Bypassing
Sediment transfer systems are oftentimes necessary components in an inlet
improvement system for two reasons. First, the ability of a tidal inlet to
naturally flush material from its channel may not be adequate to meet
navigation requirements. Second, the improvement of a tidal inlet may
interfere with the inlet's ability to naturally bypass materials from one side
FLOOD SHOALS:
B.AY
':BAY
BAY
Figure 4.5b.
IEL
R SHIFT
iEW EB SEA
NEW EBB SHOAL
Plan View Changes at an Inlet Due to Jetties and Dredged Channel:
Shoreline Response to Modification.
Bay
Barrier
Figure 4.6.
Ebb Shoal Elevation View: a) Natural Inlet, b) Trained Inlet (After
Marino and Mehta, 1988).
to the other; hence, shoreline erosion is frequently intensified in the
vicinity of the inlet. Figure 4.7 and Table 4.2 illustrate the locations and
types of sand transfer systems in Florida. Note that more than one transfer
system may be employed at an entrance; the principal method is listed first
and the others are enclosed in parentheses. At Canaveral Harbor Entrance a
moveable sand transfer plant designed two decades ago has not been built.
Perdido Pass, Alabama has been included since it can be physiographically
considered to be part of the Florida panhandle.
The transfer systems have been divided into six types as follows (Jones
and Mehta, 1977):
Type I: Hydraulic dredging from the inlet, navigation channel, shoal areas
or sand trap (excluding weir jetty systems).
Type II: Hydraulic dredging in the entrance vicinity from an impoundment
basin adjacent to a weir jetty.
Table 4.2. Sand Transfer Systems in Floridaa
Entrance Transfer
System
Ponce de Leon II (I)
Canaveral Harbor IV
Sebastian I
Jupiter I
Lake Worth III (I)
S. Lake Worth III (I)
Boca Raton I
Hillsboro II (I)
Mexico Beach VI (V)
East Pass II (I)
Perdido Pass II (I)
aSource: Jones and Mehta (1977)
Ki
Perdido East "
Pass Pass
Mexico Beach N
Inlet
0%
Figure 4.7.
/~-'---
-C'
S--Ponce de Leon 9V
0- Inlet
Canaveral Entrance
Harbor
Sebastian Inlet
S___; .' Jupiter Inlet
--: 7 __ Lake Worth
Inletn
South Lake
--- Worth Inlet
Boca Raton
S Inlet
\. -Hillsboro
Inlet
.4-- ,,
Locations of Several Sand Transfer Systems in Florida (After Jones
and Mehta, 1977).
General Direction
of Net Drift
Bay
Ocean
Figure 4.8. Sediment Volumes near an Inlet (After Marino and Mehta, 1988).
Type III: Fixed bypassing plant.
Type IV: Moveable bypassing plant.
Type V: Land-based transfer by dragline, truck, etc.
Type VI: Jet pump system.
The stability of a tidal entrance is generally thought to depend upon the
balance between the littoral movement of sediment which tends to close the
entrance, and the ability of the entrance to scour the sediment that has been
deposited in the channel. If an entrance cannot maintain a stable navigation
channel by its own flushing capability, then this must be supplemented by
artificial means. However, merely improving an entrance and undertaking an
artificial sand transfer program does not guarantee that navigable depths will
always occur through the entrance. Nor is there any guarantee that beach
erosion conditions on the downdrift side of the entrance will be measurably
improved. These depend upon the stability of the entrance, the manner in
which it naturally bypasses materials from the updrift to the downdrift side,
the method of artificial transfer, and the geomorphic characteristics of the
entrance. The direction of wave approach during the period when sand
deposition on the downdrift beach is carried out is quite important. If this
direction is such as to result in an updrift sand transport along the
shoreline, then a portion of the transferred material may be transported into
the channel, thus clogging it.
Observed entrance stability, based primarily on historic information for
tidal entrances shown in Fig. 4.7, is cited in Table 4.3. Also included in
this table are descriptions of the natural bypassing tendencies (tidal flow,
intermediate between tidal flow and bar, and bar) of the entrances as
determined by a method developed by Bruun (1966) using the ratio of the net
Table 4.3. Florida Inlet Stability and Bypassing Tendencya
Observed Bypassing
Entrance Stability Tendency
Ponce de Leon Fair Intermediate
Sebastian Good Tidal Flow
Jupiter Poor Bar
Lake Worth Fair Intermediate
S. Lake Worth Poor Bar
Boca Raton Poor Bar
Hillsboro Fair Bar
Mexico Beach Poor Bar
East Pass Good Intermediate
Perdido Pass Fair Intermediate
aSource: Jones and Mehta (1977)
Table 4.4. Florida Inlet Bypassing Effectivenessa
Entrance Naviagation Beach
Erosion
Ponce de Leon Fair Fair
Sebastian Good Good
Jupiter Fair Fair
Lake Worth Fair Fair
S. Lake Worth Fair Fair-Poor
Boca Raton Fair Fair-Poor
Hillsboro Good Fair-Poor
Mexico Beach Poor Fair
East Pass Good Fair
Perdido Pass Fair Fair
aSource: Jones and Mehta (1977)
annual littoral drift at an entrance to the maximum discharge through the
inlet during spring tide conditions. At entrances where the numerator
predominates, the offshore bar plays a major role in bypassing material. At
entrances where the denominator predominates, tidal flow bypassing occurs.
The ebb shoal or offshore bar in this latter case is usually limited in size
and volume.
Table 4.4 gives an evaluation of the bypassing effectiveness of each
entrance and its associated transfer system, as related to their combined
ability, i.e. natural and artificial, to aid in maintaining navigable depths
and in retarding downdrift erosion. These estimates are based upon shoreline
changes on both sides of the entrance, dredging data, and discussions with
individuals having local knowledge of the entrance behavior and shoreline
history. As a result, these estimates are essentially subjective, but are
believed to be fairly representative of actual performance. Unacceptable
bypassing effectiveness for prevention of beach erosion at several inlets in
Florida has led to strong recommendations for enhancing sand transfer
capabilities at these inlets (Dean and O'Brien, 1987a,b).
SEDIMENT VOLUMES NEAR AN INLET
Figure 4.8 shows a schematic of an inlet through a land barrier. This
description applies, for instance, to Florida's east coast inlet down to
Government Cut except Nassau Sound and Matanzas, which have no jetties or
dredged channel. Significant features are the sea or ebb shoal, A; bay or
flood shoal, B; updrift and downdrift beach fillets, C and D; and navigation
channel, E. For convenience in describing Florida's east coast inlets, the
updrift beach may be considered to be north and downdrift beach south of the
inlet. Among these features, the flood shoal is typically the most poorly
described area at most inlets, because it occurs in confined waters where
limited bathymetric information exists. Additionally, the history of dredging
or spoil deposition from the internal waterways is not documented well. The
beach fillets, which define alongshore distances corresponding to the updrift
and downdrift influences (up to points 1 and 2, respectively), of the inlet
are difficult to identify unambiguously. The dashed line between points 1 and
2 indicates shoreline position in the absence of the inlet. Point 2 is
particularly difficult to locate, with consequent limitation for the accuracy
of estimates of downdrift loss of sediment over the selected time interval.
At some inlets the ebb shoal distribution varies widely and shoal contours are
not defined clearly.
EVOLUTION OF EBB AND FLOOD SHOALS
Prior to evaluating the various volumes associated with an inlet, it is
instructive to make reference to the manner in which ebb and flood shoals
evolve at a newly cut entrance. Although each situation is obviously unique,
the following examples are illustrative of probable trends.
To trace the evolution of an inlet ebb shoal, a time history of the inlet
must be studied. St. Augustine Inlet, for example, has a unique history and
helps in understanding evolutionary trends as well as difficulties which are
typically encountered in precisely determining the shoal volume at any
particular point in time.
St. Augustine Inlet was cut 4 km north of an existing inlet in 1941.
Figure 4.9 depicts both the previous (1937) and recent (1985) shoreline and
shoal contours. Locations I and J represent the areas through which the old,
natural inlet meandered prior to the new inlet opening at location K, in
1940. The shoal contour lines delineate significant levels of sediment
annual littoral drift at an entrance to the maximum discharge through the
inlet during spring tide conditions. At entrances where the numerator
predominates, the offshore bar plays a major role in bypassing material. At
entrances where the denominator predominates, tidal flow bypassing occurs.
The ebb shoal or offshore bar in this latter case is usually limited in size
and volume.
Table 4.4 gives an evaluation of the bypassing effectiveness of each
entrance and its associated transfer system, as related to their combined
ability, i.e. natural and artificial, to aid in maintaining navigable depths
and in retarding downdrift erosion. These estimates are based upon shoreline
changes on both sides of the entrance, dredging data, and discussions with
individuals having local knowledge of the entrance behavior and shoreline
history. As a result, these estimates are essentially subjective, but are
believed to be fairly representative of actual performance. Unacceptable
bypassing effectiveness for prevention of beach erosion at several inlets in
Florida has led to strong recommendations for enhancing sand transfer
capabilities at these inlets (Dean and O'Brien, 1987a,b).
SEDIMENT VOLUMES NEAR AN INLET
Figure 4.8 shows a schematic of an inlet through a land barrier. This
description applies, for instance, to Florida's east coast inlet down to
Government Cut except Nassau Sound and Matanzas, which have no jetties or
dredged channel. Significant features are the sea or ebb shoal, A; bay or
flood shoal, B; updrift and downdrift beach fillets, C and D; and navigation
channel, E. For convenience in describing Florida's east coast inlets, the
updrift beach may be considered to be north and downdrift beach south of the
inlet. Among these features, the flood shoal is typically the most poorly
described area at most inlets, because it occurs in confined waters where
limited bathymetric information exists. Additionally, the history of dredging
or spoil deposition from the internal waterways is not documented well. The
beach fillets, which define alongshore distances corresponding to the updrift
and downdrift influences (up to points 1 and 2, respectively), of the inlet
are difficult to identify unambiguously. The dashed line between points 1 and
2 indicates shoreline position in the absence of the inlet. Point 2 is
particularly difficult to locate, with consequent limitation for the accuracy
of estimates of downdrift loss of sediment over the selected time interval.
At some inlets the ebb shoal distribution varies widely and shoal contours are
not defined clearly.
EVOLUTION OF EBB AND FLOOD SHOALS
Prior to evaluating the various volumes associated with an inlet, it is
instructive to make reference to the manner in which ebb and flood shoals
evolve at a newly cut entrance. Although each situation is obviously unique,
the following examples are illustrative of probable trends.
To trace the evolution of an inlet ebb shoal, a time history of the inlet
must be studied. St. Augustine Inlet, for example, has a unique history and
helps in understanding evolutionary trends as well as difficulties which are
typically encountered in precisely determining the shoal volume at any
particular point in time.
St. Augustine Inlet was cut 4 km north of an existing inlet in 1941.
Figure 4.9 depicts both the previous (1937) and recent (1985) shoreline and
shoal contours. Locations I and J represent the areas through which the old,
natural inlet meandered prior to the new inlet opening at location K, in
1940. The shoal contour lines delineate significant levels of sediment
New Shoreline
.........Old Shoreline
New
---- Old
Shoal Contours
Shoal Contours
/ \T
0 1200m
Figure 4.9. Ebb Shoal Changes at St. Augustine Inlet, Florida (After Marino and
Mehta, 1987).
deposition above an "ideal" offshore profile. The ideal profile is defined as
the natural offshore profile in that local area, as if the inlet were not
present. It can be seen that as a result of the opening of a new inlet, the
previous ebb shoal was caused to migrate. The old shoal formation moved both
westward, to form what is now known as Conch Island, and northward to the new
inlet. The old inlet was completely closed by sand deposition in 1957. The
elongated shape of the recent shoal is believed to be due to the presence of a
predominant longshore current to the south. The narrowest part of the shoal
directly east of the inlet is evidence of the dredging done by sidecast
dredges in the shoal area since 1940. The large bulge adjacent to the south
jetty is a direct result of jetty construction in 1957. The shoreline since
construction has moved eastward approximately 750 m adjacent to the jetty.
This suggests jetty sand-trapping during seasonal reversals of the littoral
drift.
This inlet is a mere example of the manner in which ebb shoals form and
how the coastline responds to inlet formation. By constructing jetties of
sufficient length to stabilize an inlet, as was done at St. Augustine, the
shoals are maintained a significant distance away from the inlet. It may also
be noted that dredging seems to have significantly affected the shape of the
shoal. Where a channel has been dredged, the shoal is divided into two
distinct lobes, rather than one large mass as is the case for example at Boca
Raton Inlet, where there is no dredged channel.
Figure 4.10 presents a history of St. Lucie Inlet interior shoaling
volumes (Dean and Walton, 1975). As observed this inlet shoaled rapidly in
its earlier years and gradually approached a much smaller "equilibrium"
shoaling rate as represented by the slope of the right-hand sides of the
curves. The two curves represent shoaling over different areas considered as
: Trend Line for Shoaling Pattern In
Si St. Lucle Inlet (From 3.2 km North of
' Inlet to 1.6 km South of Inlet In I.C.W.W.)
E
M Trend Line for Shoaling Pattern
-,"In St. Lucle Inlet (Vicinity of
S 1.2 km Radius of Inet)
i | Trend Line for Shoaling Pattern
I ?| /in St. Lucle Inlet (Vicinity of
!-- 1.2 km Radius of Inlet)
1883 1890 1900 1910 1920 1!
YEAR
Figure 4.10.
Time-History of Sediment Deposition in the Interior of St. Lucle Inlet
(After Dean and Walton, 1975).
\
* Leon 0
Canaveral
SC.
0
-t. Pierce
t---i T St. Lucie
-- __ Jupiter
.... Lake Worth
South Lake Worth
Boca Raton
S-'_Hillsboro
----\ -Port Everglades
S/'.-Bakers Haulover
| Govemment Cut
Figure 4.11. Nineteen Inlets along Florida's East Coast.
cn
>C
m
2 "
0
E n
U)
01 M
o
Qw
Z co
< F--
0)
x
n
E
a "bay". The widely differing results are indicative of problems inherent in
flood shoal calculations.
SAND TRAPPING
Selected Inlets and Physical Environment
Nineteen inlets along the 580 km shoreline between St. Marys Entrance at
the Florida/Georgia border to Government Cut, Miami, are listed in Table 4.5
(see also Fig. 4.11). St. Johns River Entrance and Ft. George Inlet are two
separate inlets. Ft. George, a small riverine entrance, occurs immediately
north of St. Johns. They are characterized together by a single large ebb
shoal and are therefore treated here as a single inlet system. Eleven inlets
were opened artificially, although three (St. Augustine, Boca Raton and Port
Everglades) have replaced inlets of natural origin in the vicinity. The
remainder are known to have existed naturally since the earliest recorded
history. All presently have two jetties except Nassau Sound and Matanzas. No
training works occur at Nassau Sound. During 1976-77 a portion of the bay at
Matanzas was closed by a dike at a location where a storm-induced breakthrough
had occurred in 1964. Inlet hydraulics and sediment distribution were
influenced measurably by this closure operation (Hayter and Mehta, 1979).
The tidal range and nearshore wave energy are reliable descriptors of the
coastal physical environment. The semi-diurnal spring tidal range varies from
2.1 to 0.8 m (Table 4.5). A nearshore wave energy characterizing parameter
can be defined as the square of the product of the wave height and the period.
Annual average significant height and modal period may be selected for the
present purpose (Marino, 1986). The range of wave energy parameter values are
from 29.1 to 3.8 m2 sec2. Thus both the tidal range and the wave climate
exhibit some variability along the coast, although this variability is
Table 4.5. Florida's East Coast Inletsa
Inlet Origin Training Spring Wave
works tidal energy
range parameter
(m) (m2sec2)
St. Marys natural jetties 2.1 10.9
Nassau Sound natural none 1.9 10.9
St. Johns/ natural jetties 1.7 18.3
Ft. George
St. Augustine opened, 1940b jetties 1.6 18.7
Matanzas natural closures 1.5 20.6
Ponce de Leon natural jetties 1.3 26.7
Port Canaveral opened, 1950 jetties 1.2 24.0
Sebastian opened, 1948 jetties 0.9 28.7
Ft. Pierce opened, 1921 jetties 0.9 26.5
St. Lucie opened, 1892 jetties 1.0 29.1
Jupiter natural jetties 0.9 27.5
Lake Worth opened, 1917 jetties 0.8 14.6
South Lake Worth opened, 1927 jetties 0.8 15.0
Boca Raton opened, 1925b jetties 0.8 16.0
Hillsboro natural jetties 0.8 5.2
Port Everglades opened, 1926b jetties 0.8 5.2
Bakers Haulover opened, 1925 jetties 0.8 5.2
Government Cut opened, 1902 jetties 0.8 3.8
aSource: Marino (1988)
bReplacing a natural inlet in the vicinity; two near
cStorm breakthrough closure inside the bay by a dike
Port Everglades
relatively minor in a global context. From the point of view of tide and
waves, Florida's east coast environment has been classified as moderate
(Walton and Adams, 1976; Marino, 1986).
The net littoral drift is generally from north to south, although a local
reversal is suggested at some inlets. At St. Marys, the net southward drift
is believed to be 420,000 m3/yr, while near Government Cut it is on the order
of 15,000 m3/yr (Marino, 1986). While these estimates are admittedly rough,
the littoral drift rate in the stretch between St. Marys and Jupiter is
considerably larger than that in the stretch between Lake Worth and Government
Cut. There is thus a general correlation with wave energy, which is
relatively low in southern Florida due to the intervening influence of the
Bahama Banks.
Volumetric Calculation
St. Marys, St. Augustine and Lake Worth may be selected as illustrative
examples. Sediment volumes have been calculated for each site by routine
procedures based primarily on bathymetric information, making allowances for
complicated bathymetry or lack of adequate data (Marino and Mehta, 1986).
Relevant quantities listed in Table 4.6 are self-explanatory.
Summary of Results
Three noteworthy quantities are given in Table 4.7 for all nineteen
inlets. These include the most recent, available (post-training) estimate of
the ebb shoal volume, the total material trapped due to training during the
approximate period indicated, the corresponding change of volume downdrift and
the quantity of sediment disposed at sea. The trapped volume in each case
represents the sum of ebb shoal volume change, flood shoal volume change
(where computed), updrift beach fillet volume change, and material disposed at
sea or placed upland, but not on the beach. A positive number indicates
accretion and a negative number implies erosion.
At shorelines where the littoral drift is predominantly unidirectional,
the total volume of sediment trapped by the updrift beach fillet, the ebb
shoal and the flood shoal must equal the volume of sediment denied downdrift.
Table 4.6. Sediment Volumes at Three Florida Inletsa
St. Marys St. Augustine Lake Worth
Quantity Period Quantity Period Quantity Period
(x10-6m3) (yr) (x10-n3) (yr) (x10-m3) (yr)
Ebb shoal 89.2 1870 59.4b 1924 0.0c 1917
95.1 1974 83.3 1979 2.9 1967
Updrift -1.3 1870-1975 1.1 1937-1970 4.8 1883-1957
Downdrift 8.8 1857-1957 5.5 1924-1976 -0.7 1883-1957
Flood shoal -d 0.0 1940 0.0 1917
_d 0.5 1970 _d
Deposit-sea 9.4 1903-1985 0.0 2.1 1929-1985
-beach 0.3 1982 1.2 1940-1976 0.5e 1929-1985
-inland 0.0 0.0 0.9 1970-1985
aSource: Marino and Mehta (1988)
bOld inlet
CInlet opened in 1917
dNot calculated; believed to be small compared to ebb shoal
eExcluding 1.1 x 106m3 bypassed from updrift to downdrift beach, 1968-1986
However, no strong correlation between trapped volume and downdrift volume
change is apparent from the data in Table 4.7, although a general (but not
uniform) trend of decreasing magnitudes of both quantities from north to south
can be discerned. At four inlets St. Marys, Nassau Sound, St. Augustine and
Ponce de Leon downdrift beach fillet volume showed an apparent increase.
Notwithstanding the likelihood of the effect of local reversals in the
direction of littoral drift at these sites, it must be noted that the
downdrift volumetric changes calculated are very approximate. Considerably
lower confidence can be placed in these values than in the estimates of
material trapped.
Over the indicated 99-year period, Nassau Sound trapped 6.3 x 106m3,
despite the fact that no modifications have been made at this large entrance.
a "bay". The widely differing results are indicative of problems inherent in
flood shoal calculations.
SAND TRAPPING
Selected Inlets and Physical Environment
Nineteen inlets along the 580 km shoreline between St. Marys Entrance at
the Florida/Georgia border to Government Cut, Miami, are listed in Table 4.5
(see also Fig. 4.11). St. Johns River Entrance and Ft. George Inlet are two
separate inlets. Ft. George, a small riverine entrance, occurs immediately
north of St. Johns. They are characterized together by a single large ebb
shoal and are therefore treated here as a single inlet system. Eleven inlets
were opened artificially, although three (St. Augustine, Boca Raton and Port
Everglades) have replaced inlets of natural origin in the vicinity. The
remainder are known to have existed naturally since the earliest recorded
history. All presently have two jetties except Nassau Sound and Matanzas. No
training works occur at Nassau Sound. During 1976-77 a portion of the bay at
Matanzas was closed by a dike at a location where a storm-induced breakthrough
had occurred in 1964. Inlet hydraulics and sediment distribution were
influenced measurably by this closure operation (Hayter and Mehta, 1979).
The tidal range and nearshore wave energy are reliable descriptors of the
coastal physical environment. The semi-diurnal spring tidal range varies from
2.1 to 0.8 m (Table 4.5). A nearshore wave energy characterizing parameter
can be defined as the square of the product of the wave height and the period.
Annual average significant height and modal period may be selected for the
present purpose (Marino, 1986). The range of wave energy parameter values are
from 29.1 to 3.8 m2 sec2. Thus both the tidal range and the wave climate
exhibit some variability along the coast, although this variability is
Table 4.7. Florida Inlet Sediment Volumesa
Ebb Material trapped Downdrift Disposed
Inlet Shoal Volume Period Vol.change Vol.at sea
(xlO-6m3) (xlO-6n3) (yr) (x10- 63) (x10-6m3)
St. Marys 95.1 14.0 1857-1979 8.8 9.4
Nassau Sound 40.5 6.3 1871-1970 3.2 0.0
St. Johns/ 131.3 120.9 1874-1978 -23.4 15.7
Ft. George
St. Augustine 83.3 25.6 1924-1979 5.5 0.0
Matanzas 4.8 5.4 1963-1978 -0.2 0.0
Ponce de Leon 17.0 0.7 1925-1974 1.7 0.0
Port Canaveral 4.3 13.8 1953-1985 -0.8 7.5b
Sebastian 0.1 3.2 1924-1976 -0.2 1.4
Ft. Pierce 22.2 66.3 1882-1983 -35.9 2.0
St. Lucie 16.4 20.3 1888-1984 -34.7 0.0
Jupiter 0.3 -3.0 1883-1978 -2.4 0.0
Lake Worth 2.9 4.3 1883-1985 -0.7 2.1
South Lake Worth 1.1 1.5 1927-1979 -0.4 0.0
Boca Raton 0.8 1.3 1920-1981 ~ 0.0 0.0
Hillsboro -0.2c -1.7 1883-1967 -0.5 0.5
Port Everglades ~ 0.0 6.0 1927-1981 -0.5 2.1
Bakers Haulover 0.5 0.3 1919-1969 -0.5 0.2
Government Cut ~ 0.0 3.5 1867-1978 ~ 0.0 0.0
aSource: Marino and Mehta (1988)
bExcluding 15.9 x 106m3 dredged during
at sea
harbor construction and disposed
CNegative sign is indicative of a scour hole at the site
An approximately 0.3 m relative mean sea level rise which has occurred during
this period is a possible cause. Furthermore, modifications carried out at
St. Marys are believed to have influenced sand distribution at Nassau Sound.
At Jupiter and Hillsboro, there was actually a post-training loss of sediment,
although in both cases the volume lost was small in comparison with the gains
at inlets between St. Marys and St. Lucie, with the exceptions of Ponce de
Leon and Sebastian.
At four inlets St. Marys, St. Johns/Ft. George and Port Canaveral -
sizeable quantities of sediment have been disposed at sea over decades. The
type and quality of the disposed sediment were not investigated in this study;
hence no conclusion can be drawn regarding the potential suitability of this
sediment for such uses as beach replenishment. It is significant, however,
that a total of 40.9 x 1063 have been disposed offshore. This number does
not include, for example, an additional 15.9 x 106m3 which also were deposited
offshore during the construction of Port Canaveral harbor. It is not clear
how much of this material was derived from upland dredging.
EBB SHOALS
Florida Inlets
Ebb shoals at eight out of the nineteen inlets contain a total of 405.8
x 106m3 of sediment (Table 4.7). These eight inlets St. Marys, Nassau
Sound, St. Johns/Ft. George, St. Augustine, Ponce de Leon, Ft. Pierce and St.
Lucie thus contain nearly 97% of the ebb shoal sediment. Out of these, the
five northernmost inlets St. Marys, Nassau Sound, St. Johns/Ft. George and
St. Augustine store 350.2 x 106m3, or 83% of the total sediment. Clearly,
most of the stored sediment is found in northern Florida, with relatively
small contributions from the south. Below St. Lucie there is practically
negligible storage of sediment in the ebb shoals.
The observed variability in the ebb shoal volume, ranging from as high as
131.3 x 106m3 at St. Johns/Ft. George to almost zero at Port Everglades and
Government Cut, is indicative of the influences of a wide variety of physical
factors that determine ebb shoal configuration and volume. Prominent among
these factors are tidal range, wave climate and littoral drift, offshore
bathymetry, type of sediment, inlet and bay geometries and runoff. For the
east coast of Florida, tidal range, wave climate and littoral drift, and inlet
and bay geometries are more important. At least in some cases however, an
overriding influential factor, as one would suspect, is likely to be the
holocene processes which have led to nearshore sand deposition ultimately from
riverine sources. Quite simply, sand seems to be available at shorelines where
it was deposited in the first place.
Georgia Inlets
The inlets of Georgia are of particular interest since they are
contiguous to those of Florida's east coast inlets, and because their ebb
shoals store significant quantities of sand. In Table 4.8 nine major inlets
are listed including representative spring tidal range at each inlet, the
corresponding wave energy parameter, and ebb shoal volume. The tidal range
places this shoreline in the mesotidal regime, as opposed to Florida's east
coast which, with the exception of St. Marys area, is microtidal (Table
4.5). On the other hand, the wave energy parameter values suggest wave action
similar to that along the northern part of Florida's east coast (Table 4.5),
which is moderate. Overall, therefore, these nine Georgia inlets are much
more tide dominated than for example Sebastian through Boca Raton.
The ebb shoal volumes, ranging from 15.1 x 106 m3 to 191.0 x 106 m3, are
quite large, and with the exception of St. Catherines Sound are comparable to
northern Florida inlets between St. Marys and St. Augustine. The sum of these
volumes, 828.3 x 106 m3, is double that stored in Florida east coast ebb
shoals. The two southernmost inlets, St. Simons Sound and St. Andrew Sound,
together account for 376.6 x 106 m3 or 45% of the total.
Table 4.8. Georgia Inlet Ebb Shoal Volumesa
Inlet Spring Waveb Ebb
tidal energy shoal
range parameter volume
(m) (m2 sec2) (x 10-6 m3)
Nassau Sound 2.5 13.7 86.6
Ossabaw Sound 2.6 17.1 51.3
St. Catherines Sound 2.5 13.3 15.1
Sapelo Sound 2.5 15.6 165.8
Duboy Sound 2.4 14.2 33.0
Altamaha Sound 2.3 14.2 66.7
Hampton River 2.4 14.2 33.2
St. Simons Sound 2.3 12.5 185.6
St. Andrew Sound 2.3 9.5 191.0
aData generated by Millard Dowd, graduate student, University of
Florida.
bEnergy parameter values derived from Jensen (1983).
Ebb Shoal and Nearshore Environment
Following the opening of a new inlet or the training of a natural inlet,
the rate of growth of the ebb shoal is mainly contingent upon the rate of
supply of sediment from the littoral drift. The larger the drift, the faster
the rate at which the ebb shoal will develop to its new equilibrium size (Dean
and Walton, 1975). It may therefore be argued that, for example, northern
Florida inlets have nearly attained equilibrium, while the southern inlets
have not, given the significantly lower drift in the south compared to the
north. In other words, as mentioned previously the availability of sediment
can be a factor influencing variations in the ebb shoal size as well as the
volume of material trapped. It is, however, noteworthy that, as noted in the
case of St. Lucie Inlet (Fig. 4.10), when a new inlet is dredged or a natural
inlet trained, sediment trapping usually occurs rapidly initially, followed by
a much slower rate of entrapment. It is believed that most of the nineteen
Florida inlets considered have passed the stage of rapid entrapment, that they
are approaching equilibrium sedimentary distributions at a slow rate, and
that, in most cases, the quantities (ebb shoal volume and material trapped) in
Table 4.7 are close to those at equilibrium.
This hypothesis, i.e. that inlet sediment distribution is in equilibrium
with the governing forces due to tides and waves, would imply that variability
in littoral drift may not correlate measurably with variability in ebb shoal
volumes. Without evaluating this hypothesis further, however, it is
worthwhile noting some observations by assuming that one is dealing with ebb
shoals of equilibrium size.
The assumption of equilibrium ebb shoal size was used by Walton and Adams
(1976) to empirically relate the ebb shoal volume to the spring tidal prism,
considering the prism to be the characteristic parameter representing inlet
hydraulics, encompassing the effects of tidal range and inlet-bay geometry.
By further assuming the variability in wave energy to be relatively small, all
Florida east coast inlets were treated as being influenced by a similar wave
climate. The result was a power law expression indicating the ebb shoal
volume to be proportional to prism raised to the power 1.3, approximately.
However, there was significant data scatter about this trend. Such scatter
suggests that the ebb shoal volume may not be related uniquely to prism, and
that the influence of additional parameters must be considered. One possible
candidate is the inlet width-to-depth aspect ratio. The influence of this
parameter is suggested by the data presented in Table 4.9. Three Florida
inlets, Matanzas, Ponce de Leon and Ft. Pierce, are characterized by similar
values of prism, wave energy parameter and channel throat or minimum flow
area. There is a slight increase in prism from Matanzas to Ponce de Leon, and
a significant decrease in the aspect ratio. The data suggest a stronger
Table 4.9. Influence of Inlet Aspect Ratio on Ebb Shoal Volumea
Inlet Spring Wave Throat Width/ Ebb shoal
tidal energy area depth volume
prism parameter
(m3) (m2 sec2) (m2) (m3)
Matanzas 1.42 x 107 20.6 910 123 4.8 x 106
Ponce de Leon 1.63 x 107 26.7 1,170 75 1.7 x 107
Ft. Pierce 1.73 x 107 26.5 980 64 2.2 x 107
aSource: Marino and Mehta (1987)
correlation between increasing ebb shoal volume and decreasing aspect ratio,
than with increasing prism.
Notwithstanding the fact that Matanzas channel is untrained while both
Ponce de Leon and Ft. Pierce have jetties and dredged channels, it may be
inferred from Table 4.9 that given the same tidal prism, wave energy and inlet
throat area, a wide and shallow inlet will have a smaller ebb shoal than that
a narrower and deeper inlet. Although depth at the channel throat is by no
means uniquely related to the natural, shoal-free depths in the ebb shoal
region, it is reasonable to associate a shallow throat with shallow offshore
depths and a deep throat with deeper waters offshore. In the ebb shoal region,
currents are relatively weak compared with those in the channel, and the
prevailing bed shear stress is predominantly due to waves. The minimum flow
depth over the ebb shoal is therefore determined mainly by waves (Mehta and
Joshi, 1988). Any excess material that may deposit over the shoal will be
carried shoreward by wave action (Walton and Adams, 1976). Consequently, all
other conditions being equal, the thickness of stored ebb shoal sediment will
be greater at an inlet with a small aspect ratio than at one with a larger
ratio. The inlets of Table 4.8, where the sediment size is similar (~ 0.2-0.4
mm), appear to illustrate this process, although this concept requires further
consideration including the role of geomorphologic factors.
28
at inlets between St. Marys and St. Lucie, with the exceptions of Ponce de
Leon and Sebastian.
At four inlets St. Marys, St. Johns/Ft. George and Port Canaveral -
sizeable quantities of sediment have been disposed at sea over decades. The
type and quality of the disposed sediment were not investigated in this study;
hence no conclusion can be drawn regarding the potential suitability of this
sediment for such uses as beach replenishment. It is significant, however,
that a total of 40.9 x 1063 have been disposed offshore. This number does
not include, for example, an additional 15.9 x 106m3 which also were deposited
offshore during the construction of Port Canaveral harbor. It is not clear
how much of this material was derived from upland dredging.
EBB SHOALS
Florida Inlets
Ebb shoals at eight out of the nineteen inlets contain a total of 405.8
x 106m3 of sediment (Table 4.7). These eight inlets St. Marys, Nassau
Sound, St. Johns/Ft. George, St. Augustine, Ponce de Leon, Ft. Pierce and St.
Lucie thus contain nearly 97% of the ebb shoal sediment. Out of these, the
five northernmost inlets St. Marys, Nassau Sound, St. Johns/Ft. George and
St. Augustine store 350.2 x 106m3, or 83% of the total sediment. Clearly,
most of the stored sediment is found in northern Florida, with relatively
small contributions from the south. Below St. Lucie there is practically
negligible storage of sediment in the ebb shoals.
The observed variability in the ebb shoal volume, ranging from as high as
131.3 x 106m3 at St. Johns/Ft. George to almost zero at Port Everglades and
Government Cut, is indicative of the influences of a wide variety of physical
factors that determine ebb shoal configuration and volume. Prominent among
The role of bed shear stress in reference to the relationship between the
inlet aspect ratio and ebb shoal volume may be formalized via an illustrative
example. The critical shear stress is that value of the bed shear stress that
is exerted at the point of incipient grain motion. When the actual bed shear
stress exceeds the critical shear stress, the bed material is put into motion.
Jonsson (1966) noted that the wave friction factor, fw, is in general
significantly larger than the current friction factor, fc. The constitutive
expressions representing the shear stress due current, Tc, and waves, Tw are
'T = 0.5 pfcuc2 (1)
and
W = 0.5 pfwuw2 (2)
respectively, where p is the density of seawater, uc is the depth-mean flow
velocity due to current and uw is the near-bed velocity amplitude due to
waves.
For the problem at hand, it is sufficient to consider two inlets of the
same cross-section, but having different width over depth aspect ratio, W/D.
Let inlet 1 be 3 m deep by 400 m wide, and inlet 2 be 6 m deep by 200 m
wide. Thus both inlets have a cross-sectional area of 1,200 m2, but the
corresponding aspect ratios are 133 and 33, respectively. It can be shown that
the maximum ebb velocity through both the inlets will be the same because the
flow areas, and therefore the tidal prisms, are equal (Marino and Mehta,
1987). Let us assume that the velocity, uc over the ebb shoal is as well the
same in both cases, in spite of the differences in the flow depth over the
bar. Let uc be 0.3 m/sec, a representative value. Select further, a
representative wave height of 1 m and a wave period of 7 sec applicable to ebb
shoals at both inlets. For current, a typical value of 4.1x10-3 may be
selected for fc. The magnitude of fw depends on the relative bottom rough-
ness, i.e. the maximum water particle displacement near the bed, Ab, divided
by the bed roughness, ds. fw can be estimated by using calculated Reynolds
Numbers of 2.85x106 and 1.33x106 and corresponding Ab/ds values of 2,264 and
1,586 for inlets 1 and 2, respectively (Marino, 1986). The f, values are
estimated to be 8.0x10-3 and 9.0x10-3 for inlets 1 and 2, respectively.
The current shear stress, Tc, and wave shear stress, Tw, are obtained for
the two inlets as follows:
Table 4.10. Bottom Shear Stress Calculation
Inlet D Tc Tw
(m) (N/m2) (N/m2)
1 3 0.18 3.23
2 6 0.18 1.62
It is observed that in the case of both inlets, the wave shear stress is
dominant. Hence the precise selection of the magnitude of uc for the inlets
is not a matter of critical importance, so long as reasonable values are
selected. Since the wave shear stress is twice as much in the shallower
inlet, it follows that the critical shear stress will be exceeded there more
often than in the deeper inlet. As the sand is put into motion, it is moved
by the longshore current and wave forces back towards the shore. This
movement of sand, therefore, occurs more significantly in shallower inlets
than in deeper inlets, allowing the shoals of deeper inlets to grow to greater
volumes than those of shallow inlets. This reasoning is in agreement with the
conclusion of Walton and Adams (1976), who state that more material is stored
in the shoals of low wave energy coasts than in high wave energy coasts. This
is because there is more energy available to drive the sand back to shore in
high energy environment after being deposited as a shoal.
30
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