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 Title Page
 Table of Contents
 Introduction
 Hydrographic data
 Physical parameters
 Hydraulic and fluvial consider...
 Design considerations on existing...
 Considerations on extending...
 Maintenance against siltation
 Conclusions and recommendation...






Group Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 79/020
Title: Study of siltation at Fernandina Beach Marina
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 Material Information
Title: Study of siltation at Fernandina Beach Marina
Series Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 79/020
Physical Description: Book
Creator: Mehta, Ashish J.
Christensen, B. A.
Affiliation: Coastal and Oceanographic Program -- Department of Civil and Coastal Engineering
Publisher: Dept. of Coastal and Oceanographic Engineering, University of Florida
Publication Date: 1979
 Subjects
Subject: Fernandina Beach Marina
Siltation
Sediment control
Spatial Coverage: North America -- United States of America -- Florida -- Fernandina Beach
 Notes
Funding: This publication is being made available as part of the report series written by the faculty, staff, and students of the Coastal and Oceanographic Program of the Department of Civil and Coastal Engineering.
 Record Information
Bibliographic ID: UF00076173
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.

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Table of Contents
    Title Page
        Title Page
    Table of Contents
        Table of Contents
    Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
    Hydrographic data
        Page 10
    Physical parameters
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
    Hydraulic and fluvial considerations
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
    Design considerations on existing marina
        Page 27
        Page 28
        Page 29
        Page 30
        Page 26
        Page 31
        Page 32
    Considerations on extending marina
        Page 33
        Page 34
        Page 35
    Maintenance against siltation
        Page 36
        Page 35
        Page 37
    Conclusions and recommendations
        Page 38
        Page 37
        Page 39
Full Text











A STUDY OF SILTATION AT
FERNANDINAi BEACH MARINA


by


A. J. Mehta
B. A. Christensen


Submitted to City of Fernandina Beach


August, 1979


I .











TABLE OF CONTENTS


INTRODUCTION . .. . ....

STUDY AREA . . . .

HYDROGRAPHIC DATA . . .

PHYSICAL PARAMETERS . . .

HYDRAULIC AND FLUVIAL CONSIDERATIONS .

Bed Shear Stress Regime . .

Percent of River Sediment Retained by

Rate of Siltation . . .

Piping (Quicksand) and Settlement .


DESIGN CONSIDERATIONS ON EX:

Basin . .

Bulkhead . .

Pier A . .

Pier B . .

Welcome Station .

Entrance . .

CONSIDERATIONS ON EXTENDING

Dredging . .

Bulkhead . .

T-Pier and Pier A .

Pierc C and D .


Basin

. .


LISTING MARINA .












MARINA .


Entrance .


MAINTENANCE AGAINST SILTATION .

CONCLUSIONS AND RECOMMENDATIONS


Page

1


7

. . 10

. . 10

. . 21

. . 21

. . 23

. . 25

. . 25

. . 26

S . 26

. . 28

. . 28

. . 32

. . 32

. . 32

. . 33

. . 33

. . 33

. . 33

. . 33

. . 35

. . 35

. . 37


. .







INTRODUCTION

Fernandina Beach Marina1 is owned and operated by the City of Fernandina

Beach. This marina (Fig. 1) was constructed in the mid-sixties and has been

heavily utilized by the recreational boat traffic in Florida. Because of its

unique location as the first Florida marina south of Georgia along the Intra-

coastal Waterway, the marina has served as an important stopover for the boat

traffic. Recognizing this need, the city has constructed a welcoming station

(Florida Marine Welcome Station) adjacent to the marina basin (Fig. 2). In

order to accommodate more boats, the city has also been considering expanding

the marina in the property immediately south of the existing basin. Fig. 3 is

a plan view of the marina indicating the existing basin, the area of proposed

expansion, the entrance, the welcoming station, the bulkhead (also Fig.4) and land

fill(also Fig.5). The basin is approximately 300 ft. long and 170 ft. wide. The

old T pier (Fig. 6) is unsafe for usage and has been condemned. Pier A (Fig. 7)

forms a partial barrier to flow between the existing basin and the extension as

a result of vertical timber planks and asbestos cement sheeting on the outside (not

visible in Fig. 7). Pier B (Fig. 8) likewise obstructs the flow partially due

to timber planks which extend down from the top to 1 1.5 ft. below mlw. The

concrete foundation of the welcoming station is supported by timber pilings so

that exchange of waters between the basin and the river is less hindered under

the welcoming station. At a number of places, portions of the timber planks

that are under water at high tide have deteriorated.

In the past 15 years the marina has experienced several engineering problems,

of which some of the more important are as follows:

1. Siltation of the basin

2. Settlement of the landfill behind the main bulkhead (see Fig. 5)

3. Settlement and cracking of the bulkhead in some spots (see Fig. 9)

4. Degradation of the timber planks lining the docks and the ramp

Located at Atlantic Avenue and Ash Street, Fernandina Beach, FL 32034


































Fig. 1 View of Marina


Fig. 2 Florida Marine Welcome Station











CITY FRONT REACH


APPROX.
LIMIT OF
EXTENSION


AMELIA RIVER


T PIER









PIER

PROPOSED
EXTENSION


EXISTING MARINA BASIN


Scale in Feet


Fig. 3 Plan view of Marina
































Fig. 4 Capped Bulkhead with King Piles (arrow) and Tee
Sheeting
























Fig. 5 Landfill behind the Bulkhead. Notice the recent
filling in foreground where settlement and piping
have occurred (arrow).

















Fig. 6 Old T Pier


Fig. 7 Pier A next to the Barge
5


I

































Fig. 8 Pier B in the Background


Fig. 9 Settlement and Cracking has occurred in the Portion
of the Bulkhead visible in the Center of the Photograph
(arrow)


:rl -- --







The first problem has been the most important one to the city inasmuch as

siltation of the basin has reduced depths in the basin to the point that at low

tide, portions of the basin bottom are exposed. Since the basin was originally

dredged to a depth of 4.4 ft. below mlw, this represents a substantial shoaling

in the past 15 years. In an attempt to alleviate this problem, the city has,

for the past 10 years, instituted a dredging effort. Each year a tug is made

to stir up the bottom with the help of its propeller. The suspended material

then disperses from the region around the tug and is transported elsewhere.

This effort has cost the city on the order or 4,000 6,000 dollars per year,

since the tug has to be operated for as long as 60 80 hours. With increasing

fuel prices, the dredging cost has been increasingieach year. The problem of piping

resulting in the settlement of the landfill has been handled by periodically

nourishing the landfill with additional sand, to maintain the level up to the

elevation of the bulkhead.

These problems, particularly the siltation problem, will be addressed in

this study with respect to

1. determination of the causes of the problem

2. suggestions concerning solutions which may help engineers involved in the

repair or design modification of the marina in their work

3. Suggestions concerning the proposed marina expansion

The proposed suggestions should be considered merely as guides to engineers

and should be followed up be architectural and structural design considerations,
if the suggested solutions are to be implemented.

STUDY AREA

As Fig. 10 shows the marina is located approximately 2.4nautical miles south

of Fort Clinch on Amelia River, at the waterfront of the city of Fernandina Beach.

Currents in the river are primarily tidal, and are driven by the rise and fall of

the water level in the Atlantic Ocean through St. Mary's Entrance. Currents on

the order of 1 3 fps have been reported in the river under a tidal range varying








CUMBERLAND
ISLAND


N \-
N <


ST MARY'S

ENTRANCE


0.13mm


AMELIA ISLAND


`3
K
-4
K


FERNANDINA
BEACH


FLOOD/EBB CURRENT
1.--- = 1 fps
TIDAL RANGE-:76-8.1 ft.


Scale: Nautical Mile


Fig. 10a Fernandina Marina is Influenced by the Tidal Movement
Entrance


through St. Mary's
























































Fig. 10b Aerial View of Marina (arrow)







9







from 7.6 to 8.1 ft. This is close to the highest tidal range that occurs in

Florida and if properly utilized it can be useful for the flushing of marina

waters. The Intracoastal Waterway runs adjacent to the marina with depths on

the order of 25 ft. (mlw) in the City Front Reach of the waterway.

Sediment movement in the river adjacent to the marina is influenced by

the relatively complex tidal circulation governed by the waterway and Lanceford

Creek. Near Fort Clinch the sediment is primarily composed of fine to medium

sand whereas in Amelia River the material is a mixture of sand, clayey silt,

organic matter and shells,with a greyish color. Near the marina, more than

50% of the sediment consists of clayey silt, the remaining 50% being fine to

medium.quartz sand and marine organic matter.

HYDROGRAPHIC DATA

Hydrographic data utilized for computations in this study are derived from

measurements including a bathymetric survey, bottom surficial sediment samples,

core samples of approximately 1 2 ft. depth, water surface elevations (both

inside and outside the marina) and current measurements.

These data have been supplimented by the following:

1. Aerial photographs (from the Corps of Engineers and the City of

Fernandina Beach)

2. Channel survey (from the Corps of Engineers)

3. Marina design plans (from the City of Fernandina Beach)

4. Previous studies at St. Mary's Entrance and Amelia River.

5. Some previously obtained laboratory test results on the sediment

movement

PHYSICAL PARAMETERS

Tidal water surface measurement in the river taken during April 25-26, 1979,

immediately outside the marina (Fig. 11) indicates a range of 8.1 ft. This

measurement was checked against the tide inside the marina as measured by an







from 7.6 to 8.1 ft. This is close to the highest tidal range that occurs in

Florida and if properly utilized it can be useful for the flushing of marina

waters. The Intracoastal Waterway runs adjacent to the marina with depths on

the order of 25 ft. (mlw) in the City Front Reach of the waterway.

Sediment movement in the river adjacent to the marina is influenced by

the relatively complex tidal circulation governed by the waterway and Lanceford

Creek. Near Fort Clinch the sediment is primarily composed of fine to medium

sand whereas in Amelia River the material is a mixture of sand, clayey silt,

organic matter and shells,with a greyish color. Near the marina, more than

50% of the sediment consists of clayey silt, the remaining 50% being fine to

medium.quartz sand and marine organic matter.

HYDROGRAPHIC DATA

Hydrographic data utilized for computations in this study are derived from

measurements including a bathymetric survey, bottom surficial sediment samples,

core samples of approximately 1 2 ft. depth, water surface elevations (both

inside and outside the marina) and current measurements.

These data have been supplimented by the following:

1. Aerial photographs (from the Corps of Engineers and the City of

Fernandina Beach)

2. Channel survey (from the Corps of Engineers)

3. Marina design plans (from the City of Fernandina Beach)

4. Previous studies at St. Mary's Entrance and Amelia River.

5. Some previously obtained laboratory test results on the sediment

movement

PHYSICAL PARAMETERS

Tidal water surface measurement in the river taken during April 25-26, 1979,

immediately outside the marina (Fig. 11) indicates a range of 8.1 ft. This

measurement was checked against the tide inside the marina as measured by an






























I-





0.- \ \--- f-
1.0 R=8



-2.0 -
.J
w w

-1.0- R=8.1 Ft.



-2.0 -



-3.0


9 10 11 12 I 2 3 4 5 6 7 8 9 10 II 12 I 2 3 4 5
4-25-79 TIME (EDT) 4- 26-79
TIME (EDT)



Fig. 11 Water Surface Record (Spring Tide)







11







existing staff. Since the marina is open to the river, and since the two locations

were in proximity to each other, the two recordings can not be expected to show any

measureable instantaneous difference in elevations. It is observed from Fig. 11

that actual low water was 0.6 ft. below mean low water, which in turn is 2.6 ft.

below the msl in this region. Tides in the region are affected by the prevailing

winds, particularly due to northeasters in fall and winter.

Fig. 12 shows the coarse grain size distribution of the sedimentary material

from the top of the landfill behind the bulkhead (two examples), and from the marina

basin (two samples). Whereas the landfill material consisted of sand alone, the

basin material was found to consist of 69% silt and clay. Grain analysis of the

samples is given in Table 1.

TABLE 1
COARSE GRAIN ANALYSIS

Median diameter Sorting coefficient
Location d50 (mnm) S


Landfill 0.18 0.20 1.16 1.17
Marina Basin 0.15 0.16 1.55 1.57


The data in Table 1 indicate that whereas the two sands are medium to fine,

the material from the top of the landfill is much better sorted indicating that

it is of different origin from the material in the marina.

The particle size distribution for the clayey silt portion of the marina

sediment is shown in Fig. 13. The two samples, one from the basin and one just

outside the basin in the river are observed to be closely similar indicating that

the source of the fine sediment in the marina is the river. The median particle

size is in the range of 0.0094 to 0.011 mm which is in the medium silt range.

Approximately 30% of the sediment consists of particles in the clay range.












SILT CLAY


>-
" 10



so



cr50
Z



z
w













-- 0 3 o\
r30I
a_

20



10


2 o f a 1 N
o 0 10 0 1


grain size in mm.


Fig. 12 Coarse Grain Size Distribution


SAND


GRAVEL









SILT CLAY


grain size in mm.


Fig. 13 Size Distribution of Clayey-Silt Portion of River/Basin Sediment


GRAVEL


SAND







Fig. 14 shows the weight distribution of the fine portion of the sediment

(less than 0.06 mm) relative to the sandy portion. As noted in the figure the

river contains between 70 and 85% fine sediment. This material has intruded

into the basin through the marina entrance. There is also a fine sediment

intrusion into the extension. Typically, as the current velocity decreases,

sand deposits first followed by fine sediment which, because it can remain in

suspension at lower levels of turbulence, intrudes further into the region of

low velocity. This is indeed observed to be the case in the extension region

where the fine material is observed to have been trapped between the shoreline

and the boat ramp. In the basin, however, there clearly appears to be a source

of coarse grain material intruding the basin from underneath the bulkhead, which

has caused the sediment in the immediate vicinity of the bulkhead to be coarser

than may be expected. This infact is due to the piping in the fill, or the

movement of the sand below the bulkhead, from the fill to the basin. The distri-

bution of the sediment near the northwest corner of the marina is, on the other

hand, less well-defined; no clear pattern has emerged there, inasmuch as the

operation of the tug propeller apparently has redistributed the sediment in this

region between the welcoming station and pier B.

Evidence gathered from the sediment distribution is corroborated by the

bathymetric map shown in Fig. 15. A 6 ft. deep hole created by the tug propeller

is clearly observed near the middle of the basin. Immediately adjacent to the

.bulkhead,landfill intrusion due to piping has caused the bottom to be elevated

there to as much as 1 ft. above mlw. This is also evident from Fig. 16 which shows

a cross-section of the bulkhead composed of king piles (to -55 ft.) and tee sheeting.

The original dredged depth was -7 ft., whereas the tee sheeting went down to -7.5

ft., i.e. 0.5 ft. more than the depth of the basin when it was constructed.

Referring again to Fig. 15, the bottom elevation in the basin adjacent to

pier B appears to be significant as well (almost 4 ft. higher than the hole),


I
















CITY FRONT REACH


Fig. 14 Fine Sediment Distribution in the Surficial Sediment




















CITY FRONT REACH
DEPTH -25'


DEPTH CONTOURS ARE
IN FEET MLW

0 40
Scale in Feet


Fig. 15 Bathymetric Map


-Z



























"
..I '
C'


r -
t.


-Original Dredged Depth
_ Elev-700'



.,11


|'
/- *. ^

*' : ^


Fig. 16 Cross-Section of Bulkhead


Elev. 6.50'



KING PILE





- TEE SHEETING















_MSL Elev.0.00




MARINA BASIN


Approximate Existing Depth


SMLW Elev-2.60'


2-
r'
I
?., .
c

::
4


.
I; I c~
r :'
,
'r 'I


,.


.-




4--

1'


'







and suggests that at least a portion of the sediment stirred up by the tug

propeller is deposited there. The timber planks of pier B effectively are

able to retain this sediment in the basin as evidenced by the drop from -3 ft.

on the basin side to -8 ft. on the river side across pier B.

Maximum depth in the entrance is on the order of 10-11 ft., whereas most of

the extension region is covered with sediment to mlw or more. Comparatively free

access to flow underneath the welcoming station has prevented any significant

accumulation of sediment in the basin area adjacent to the welcoming station.

Flow circulation in the marina is an important indicator of 1) the sediment

moving capacity of the basin and 2) the pollutant flushing ability of the basin.

Discharges measured at four different times are shown in Fig. 17. They indicate

a fair amount of flow circulation in the basin. Although the corresponding flow

velocities were rather small, on the order of 0.1 fps through the basin, the role

of the existing circulation is important inasmuch as it prevents stagnation of

waters and accumulation of pollutants such as petroleum hydrocarbons etc. Under

a 0.1 fps velocity, given a 300 ft. length of the marina, the flushing time is

on the order of(300y(0.1)(60) = 50 minutes, or approximately 1 hour, which is

nearly 1/6 the period of flood or ebb. Thus flushing is observed to occur in a

time period which is sufficiently small compared to the time between current

reversal such that reversal may not be expected to inhibit the flushing process

significantly.. The latter would be the case if the flushing time were of the

same order as the time between successive current reversals, i.e. approximately

6 hours, inasmuch as in the extreme case, if a pollutant volume at one end of

the marina moving toward the other end were to reach the end in 6 hours, the

subsequent current reversal would simply move the volume back toward the other

end. In this manner, the pollutant would remain in the marina and flushing action

would be minimal.






MARINA FLOW CIRCULATION


TIME(April 26,1979)
1630 EDT


1700 EDT








1730 EDT


// Mari// //na
Marina


49-
cfs


67 cfs 123 cfs



Marina
1264- --
=fs


120 cfs 129 cfs


////Marina////
Marine
"/^ ^ ----- ^


219 cfs


TIDE (MLW)
+ 0.23 Ft.


7cfs


+ 1.15 Ft.


117 cfs


+ 2.26 Ft.


146 cfs


127 cfs


1800 EDT


134 ->
cfs


I///////////////
Marina


245 cfs


+3.05 Ft.


243 cfs


132 cfs


Fig. 17 Flow Circulation in the Marina


-f-


I _


.. ......








HYDRAULIC AND FLUVIAL CONSIDERATIONS

Bed Shear Stress Regime

The hydraulic parameter that governs the movement of sediment is the bed

shear stress TO; hence, the sediment intrusion problem described by Fig. 14 will

be examined in terms of the distribution of TO, which is obtained from
T (n2 u- 2
0 1.49 Rh 1/3 (1)

where y = unit weight of water,

n = Manning's coefficient,
u = cross-sectional mean flow velocity and

Rh = hydraulic radius.

For water with near-sea salinity, y c 64 Ibs/ft.3 Manning's n was computed

from velocity measurements at the entrance. Some velocity profiles are shown

in Fig. 18. These approximately follow the well-known logarithmic law

u 5.75 log z (2)
ul zo
where u = current velocity measured at elevation z above the bed, u = /Vo/P

is the friction velocity and z0 = virtual origin of the profile. The bed
roughness ks = 29.7z0. Manning's n is then computed from
n = 0.263 Rhl/6 u*
n =0.263 Rh1/6 u' in English units (3)

where u is the mean-flow velocity. The average value of n was found to be

0.034. The shear stress TO computations are summarized in Table 2, for Amelia

River near the basin and for the entrance to the basin. These are based on

maximum measured velocities.
TABLE 2
MAXIMUM BED SHEAR STRESS IN RIVER AND MARINA


Location u Rh O.m
(fps) (ft) (psf)

River 2.00 10 0.0619
Entrance 0.17 9.4 0.00046































N 40
N

30 u_ z
30- =- 5.75 1og -




20-


o





0 5 10 15
u /u*



Fig. 18 Typical Velocity Profiles at the Entrance


__~~_









Note that in the river Rh was taken to be equal to the local depth near the

entrance. The critical bed shear stress for erosion, Tcr, may be computed for

sand from
Y
T r.s= 0.056Y( 1) d5 (4)

where ys = unit weight of the grain material (quartz) and d50 = median sand grain

diameter. Given ys/y = 2.65 for quartz sand in salt water and d50 = 0.155 mm from

Fig. 12, Tr.s= 0.00301 psf.

In some previously carried out tests, the natural sediment (sand plus silt)

was tested and a critical shear stress of 0.00293 psf was measured. The eroding

material in these tests was found to be the silt, but noting the closeness of

0.00301 psf and 0.00293 psf it may be concluded that the mixture and sand and silt

erodes when TO equals or exceeds the Tcr.s value given by Eq. 4, i.e. the critical

bed shear stress for the erosion of sand.

Comparing the critical shear stress Tcr.s, with TO.m it becomes apparent that

whereas the river has sufficiently strong currents to transport the sediment (TO.m

is much higher than the critical value), the material must begin to deposit as it

arrives at the entrance, since the maximum shear stress there is substantially less

than the critical value. This corroborates the description of the surficial sediment

distribution according to Fig. 14 which shows fine sediment intrusion in the basin

from the river. In the following the marina's present efficiency as a sediment

trap is evaluated, i.e. the fraction of incoming sediment retained in the marina
is determined.

Percent of River Sediment Retained by Basin

Given (qs)i = sediment in inflow, i.e. coming into the basin from the river,

and (qs)e = sediment outflow, i.e. leaving the basin, the ratio (qs)e/(q )i may

be obtained from Fig. 19, according to which

(qs)e wh1/6 w (5)
= f( ) (5)
qs i ung1/2' Th
where w = particle settling velocity

h = depth of basin












2







0.83.
8








0.20
0.12

I I I I I I I I I I

Wh
u nig


Fig. 19 Sediment Removal Function for Settling Basins
(Source: Hunter Rouse, "Engineering Hydraulics,"
McGraw-Hill, 1949)


Sediment Movement


Fig. 20 Experimental Setup Demonstrates Piping Effect


I- I








g = acceleration due to gravity
L = basin length

Selecting d50 = 0.01 mm, the settling velocity w = 0.00082 fps is found from

standard texts. Given u = 0.1 fps, L = 300 ft., h = 6 ft., n = 0.034 and

g = 32.2 ft./sec2, Fig. 19 yields (qs)e/(qs)i = 0.58 which means that (1 0.58)

x 100 = 42% of the incoming sediment from the river is retained in the basin.

Rate of Siltation

The existing volume of sediment above the original dredged depth of -4.4 ft.

(mlw) is 1,445 cubic yards. The volume of the propeller scour hole below -3 ft.

(assuming that in the absence of tug propeller dredging, the bed level would

be -3 ft. at the location of the hole) is 690 cubic yards. Thus the total

volume of sediment which would have deposited in the basin in the absence of

propeller dredging is 1,445 + 690 = 2,135 cubic yards.

Allowing 15 years of siltation, the rate of siltation (both from the

river and the bulkhead) is 2,135/15 = 142 cubic yards per year. Estimating 10

years of propeller dredging, the rate of removal of sediment is 690/10 = 69

cubic yards per year. Note that this is the maximum rate of removal; in actuality

some of the sediment suspended by the propeller probably has redeposited in the

marina itself. The minimum net rate of siltation is 142 69 = 73 cubic yards

per year. This signifies that despite propeller dredging, at least 73 cubic

yards of sediment have been moving into the basin per year, on the average.

Piping (Quick Sand) and Settlement

The phenomenon of piping under a bulkhead is demonstrated by the experimental

set up in Fig. 20, in which the landfill (on the left hand side) is separated from the
marina by a vertical bulkhead. Before the landfill was introduced in the

experiment, the ground elevation (dashed line) was equal on both the sides.

Notice the manner in which the piping effect has caused the bottom to rise in

the marina in the vicinity of the bulkhead (continuous line). Qualitatively,

this description is corroborated by the bathymetric map of Fig. 15.







Quantitatively, the flow net computations of Fig. 21 in which the upwards gradient

at the foot of the bulkhead were determined, indicate that the depth of the bulk-

head, is marginal in preventing piping. A piping factor may be defined as the ratio

i = mA x + e(6)
critical A 1

where i = flow potential gradient next to the bulkhead

critical = critical gradient next to the bulkhead

Ab = potential difference between adjacent equipotential lines

Al = length of flow path at the bulkhead between last adjacent equipotential

lines

e = void ratio

G = specific gravity of sand grains

m = fraction of a potential drop

A=- h Ah = h1 h2 ; where h h2 = total potential (head) difference

and n = number of drops. The criterion for piping is

i/icritical <<1, there is no piping

i/icritical 1, there is incipient piping

i/icritical > 1, there is piping
Using G = 2.65, e = 1.5, hI = 6.5 ft, h2 = -3.2 ft (low water), n'= 4.6 (see

Fig. 21), m = 0.6 and Al = 2 ft, i/icritical = 0.95 which is close to unity

implying that even under the existing depths in the marina, piping is only

marginally prevented and in fact is probably occurring where the actual depths

are more than 3.2 ft. This in turn also suggests that when depths in the marina

were greater everywhere, piping was a major problem, since i/icritical would be

typically greater than unity for h2 less than -3.2 ft.

DESIGN CONSIDERATIONS ON EXISTING MARINA

Basin

The inadequacy of existing depths in the basin suggests that there is a

need to dredge the marina. Selecting a depth of 5 ft below mlw, or 7.6 ft













DESIGN STORM WATER LEVEL


0 0.6AA




Ah= h1-h2

h, =6.5 FT, h2=-3.2 FT.
n =4.6, A- =2.0 FT.
i = 63


SOFT SAND


OF TEE SHEETING


iCRITICAL- I+e

G=2.65, e=1.5

CRITICAL = 0.66


Fig. 21 Piping Computations for the Existing Bulkhead


FILL ELEV.)







below msl, as the depth of the basin, Fig. 22 shows that 2,100 cubic yards would

have to be removed. Estimating a dredging cost of $3.00 per cubic yard, this

would cost $6,300 for the dredging operation. This number is merely a rough

estimate.

Bulkhead

Flow net computations similar to those shown in Fig. 21 have been made in

Fig. 23 for a hypothetical bulkhead to a depth of -15 ft msl, or -12.4 ft mlw.

For this bulkhead, the factor for piping is i/icritical = 0.55. This is

sufficiently lower than unity such that piping is avoided. The bulkhead should

reach at least this elevation.

The operation of extending the existing bulkhead to -15 ft would be very

costly. It is therefore, suggested that a new steel sheet pile wall properly

anchored be constructed with a top elevation of +6.5 ft (existing bulkhead level)

and down to -15 ft. There is an advantage to driving the sheet pile in front of

the existing bulkhead, in the marina, as only 15 7.6 = 7.4 ft of soil would

have to be penetrated. A new wall, at some distance from the bulkhead, would,

however, slightly reduce the size of the marina. An alternative is to drive

the sheet pile behind the bulkhead and immediately adjacent to it. A new concrete

cap covering the bulkhead and the sheet pile would than prevent any piping between

the two walls (see Fig. 24). The new bulkhead should be designed by a professional

engineer.

The length of the sheet pile wall required in the existing marina is approx-

imately 300 ft. Assuming a cost of $400 per linear foot of the 21.5 ft deep wall,

the estimated cost of construction would be $120,000.

Pier A

From the hydraulic point of view, two considerations are involved

relative to the role of pier A, namely 1) that it allows some flow exchange

























































REMOVE2,100 CUBIC YARDS IN BASIN
REMOVE,030 CUBIC YARDS IN EXTENSION

DREDGE TO-5 FT BELOW MLW(=-76FTBELOW MSL)
0 40
Scale in Feet


Fig. 22 Dredging Requirements for -5 ft. (mlw) Depth


-5 FT CONTOUR


5FT CONTOUR












STORM WATER


0.4A#

Ahn

Ah= h -h2

h, =6.5FT, h2=-3.2FT
n = 5.4, A= 2.0FT.
i =0.36


FILL ELEV.)




0.0 FT MSL

h2= -3.2FT. LW


BULKHEAD


SG-1
'CRITICAL I +e


G = 2.65, e= 1.5

CRITICAL = 0.66


-55 FT. .


Fig. 23 Piping Computations on a Bulkhead to -15 ft. (msl)







Quantitatively, the flow net computations of Fig. 21 in which the upwards gradient

at the foot of the bulkhead were determined, indicate that the depth of the bulk-

head, is marginal in preventing piping. A piping factor may be defined as the ratio

i = mA x + e(6)
critical A 1

where i = flow potential gradient next to the bulkhead

critical = critical gradient next to the bulkhead

Ab = potential difference between adjacent equipotential lines

Al = length of flow path at the bulkhead between last adjacent equipotential

lines

e = void ratio

G = specific gravity of sand grains

m = fraction of a potential drop

A=- h Ah = h1 h2 ; where h h2 = total potential (head) difference

and n = number of drops. The criterion for piping is

i/icritical <<1, there is no piping

i/icritical 1, there is incipient piping

i/icritical > 1, there is piping
Using G = 2.65, e = 1.5, hI = 6.5 ft, h2 = -3.2 ft (low water), n'= 4.6 (see

Fig. 21), m = 0.6 and Al = 2 ft, i/icritical = 0.95 which is close to unity

implying that even under the existing depths in the marina, piping is only

marginally prevented and in fact is probably occurring where the actual depths

are more than 3.2 ft. This in turn also suggests that when depths in the marina

were greater everywhere, piping was a major problem, since i/icritical would be

typically greater than unity for h2 less than -3.2 ft.

DESIGN CONSIDERATIONS ON EXISTING MARINA

Basin

The inadequacy of existing depths in the basin suggests that there is a

need to dredge the marina. Selecting a depth of 5 ft below mlw, or 7.6 ft

































PIER A


Fig. 24 Modifications in Existing Marina







between the marina and the river, which is useful for marina flushing and 2) that

it at least partially prevents a corresponding movement of sediment. Both of

these functions are important and necessary. It is suggested that the pier be

allowed to carry out these functions as at present. In the event that the

existing timber planks and the asbestos cement sheeting which is placed along a portion

of the outer side of the pier deteriorate and need replacing, the following

approach is suggested. The entire length of the pier A may be bulkheaded from

the bottom up to mlw wherever the back elevation is below mlw and, where the

bed is above the mlw, the bulkhead should be approximately 0.5 ft. above the

bed elevation. The asbestos cement sheeting should be removed at that time. In.this

way the tide can enter the marina above mlw through the spacing between the

planks but the sediment, whose concentration is high near the bottom, is pre-

vented from entering the basin. The timber planks should go down to the bulkhead.

Pier B

The role of pier B is similar to that of pier A, except that sediment

exchange here appears to be less of a potential problem. Should the existing

timber planks be replaced, the new ones, it is suggested, should go down to

-3.2 ft. below msl which has been selected as the design low water level here.2

The depth of water between -3.2 ft. and the bottom should be left open to allow

for flow exchange.

Welcome Station

The welcoming station end of the basin is currently open and should be

kept open as such, for flushing purposes.

Entrance

As noted, the entrance appears to allow fine sediment from the river to enter

and deposit in the basin. It is not certain as to what percent of the deposited

material in the past 15 years is derived from this source. Bathymetric evidence

seems to suggest that the major portion of the settled material is in fact derived
2 The existing planks are 8 in. wide with an 8 in. spacing between them. These
dimensions are adequate.


I









from piping near the bulkhead. Nevertheless, the mode of distribution of the

fine sediment shown in Fig. 14 suggests that the entrance does play a role in

transporting at least some fine sediment from the river. This role can be

altered by allowing the entrance to "jet-out" the sediment into the river.

Such a design is possible if two training walls are constructed in the manner

depicted in Fig. 24. The walls extend from the bottom up to msl. During the

period of time when the flow is into the basin, it will enter the basin as a

sink-type flow (dashed line) with comparatively weak bottom velocities, at a

time when the discharge is relatively high and the water elevation is above

msl. During ebb, the flow will issue from the basin in the manner of a two-

dimensional jet, with strong bottom velocities, transporting with the flow any

sediment that may have entered the basin during flood flow. Construction

material for the training walls may be concrete or steel.'

CONSIDERATIONS ON EXTENDING MARINA

Dredging

As shpwn in Fig. 22, dredging in the extension"area to -5 ft. (mlw) will

require a removal of 8,030 cubic yards of material in addition to the 2,100

cubic yards from the existing basin. If a $3 per cubic yard figure is assumed,

then the cost of dredging 8,030 cubic yards would be $24,090.

Bulkhead

The steel sheet pile wall would be extended by 160 ft. Backfill to +6.5

ft. (msl) would be required (see Fig. 25).

T Pier and Pier A

The condemned T pier would be removed completely and pier A would be cut

back by 90 ft. to accommodate boat traffic through the entrance.

Piers C and D

Pier C could be similar to pier B in design whereas pier D would be

analogous to pier A modified by the suggested improvements.


































C...


PIER D


Fig. 25 Considerations on Marina Extension







Entrance

With minor modifications, the entrance would be similar to the suggested

entrance to the existing marina with training walls up to msl.

MAINTENANCE AGAINST SILTATION

It is expected that the suggested modifications, after due consideration by

design engineers,will appreciably reduce the siltation problem in the marina.

Any residual siltation may be controlled by two means, namely 1) a silt pump for

periodically removing deposited sediment from the basin and/or 2) an air bubble

screen for preventing silt from entering the basin. The silt pump acts as a

"silt vacuum cleaner". It is an air lift pump in which an air compressor is util-

ized to mix air and water at the pump intake which is positioned on or near the

deposits on the marina bottom. The pipe leading from the pump transports the silt

from the bottom into a hopper on a barge. The bubble screen is an air bubble

curtain in water, produced by bubbles rising from a manifold or a hose with holes

at the marina bottom, and connected to an air compressor. Fig. 26 shows such a

manifold lying across the bottom. Fig. 27 shows the basic flow pattern near the

curtain. The flow circulation created by such a screen prevents suspended

sediment from crossing it. The same screen can also be used for preventing the

movement of hydrocarbons, and is a means to enhance the transport of oxygen to the

benthic layer near the bottom as well. At Fernandina Beach Marina, the problem

of primary importance is one of preventing the sediment from intruding the marina

through the entrance. This can be accomplished by utilizing a small, 4-5 hp

compressor required for generating the bubble screen. Piers A, B and the flow

connection under the welcoming station do not seem to allow any significant

sediment intrusion; hence no screen is required in these areas. On the other

hand, prevention of hydrocarbon transport through all the flow boundaries of

the basin and sediment influx through the entrance would require a screen along

the entire flow boundary of the marina. This would require a much larger air

compressor, on the order of 100 hp.













__ __ __


.. ...
* .. .a... *..*. *


From compressor.



.. ............ .. ...
. . .. .- .

:;::: h

m.


L--- I >m1
Air pressure po (in excess of atmospheric pressure)

Total air flow rate: AQ

Air flow rate per orifice: sQo


Fig. 26 Air Manifold Geometry


Basic Flow Pattern.


Oxygen Transfer.


MINOR OXYGEN UPTAKE


W.S.


..:...:.: .. .. :.. .....
.' ~ri;.~


..... .
.. .-


OXYGEN TRANSFER
TO BENTHIC LAYER


- --- -- '_-L


; '* *:* ; 0


Fig. 27 Air Bubble Screen







Entrance

With minor modifications, the entrance would be similar to the suggested

entrance to the existing marina with training walls up to msl.

MAINTENANCE AGAINST SILTATION

It is expected that the suggested modifications, after due consideration by

design engineers,will appreciably reduce the siltation problem in the marina.

Any residual siltation may be controlled by two means, namely 1) a silt pump for

periodically removing deposited sediment from the basin and/or 2) an air bubble

screen for preventing silt from entering the basin. The silt pump acts as a

"silt vacuum cleaner". It is an air lift pump in which an air compressor is util-

ized to mix air and water at the pump intake which is positioned on or near the

deposits on the marina bottom. The pipe leading from the pump transports the silt

from the bottom into a hopper on a barge. The bubble screen is an air bubble

curtain in water, produced by bubbles rising from a manifold or a hose with holes

at the marina bottom, and connected to an air compressor. Fig. 26 shows such a

manifold lying across the bottom. Fig. 27 shows the basic flow pattern near the

curtain. The flow circulation created by such a screen prevents suspended

sediment from crossing it. The same screen can also be used for preventing the

movement of hydrocarbons, and is a means to enhance the transport of oxygen to the

benthic layer near the bottom as well. At Fernandina Beach Marina, the problem

of primary importance is one of preventing the sediment from intruding the marina

through the entrance. This can be accomplished by utilizing a small, 4-5 hp

compressor required for generating the bubble screen. Piers A, B and the flow

connection under the welcoming station do not seem to allow any significant

sediment intrusion; hence no screen is required in these areas. On the other

hand, prevention of hydrocarbon transport through all the flow boundaries of

the basin and sediment influx through the entrance would require a screen along

the entire flow boundary of the marina. This would require a much larger air

compressor, on the order of 100 hp.








Maintenance measures such as the silt pump or the bubble screen are not

recommended unless siltation is not abated by the suggested modification in

marina design. Detailed design computations for the silt pump or the bubble

screen are beyond the scope of this study.

CONCLUSIONS & RECOMMENDATIONS

Conclusions derived from this study are summarized below.

1. There are two sources of sediment in the marina, 1) due to piping (quick

sand) near the bulkhead and 2) due to intrusion from Amelia River, through

the entrance. The first is primarily a source of sand, whereas the second

is a source of clayey-silt, which is finer than sand. The bottom surficial

sediment in the marina consists on the average of 69% fine sediment and

31% sand, by weight.

2. Flow circulation in the marina, under a comparatively high (8.1 ft.) range

of spring tide, is necessary for pollutant flushing of the marina.

3. While currents in Amelia River are strong enough to transport sediment,

they are too weak in.the entrance and the basin to move the sediment; hence,

approximately 48% of the sediment that enters the basin from the river deposits

there.

4. Piers A and B and the flow connection underneath the welcoming station allow

a comparatively less appreciable sediment influx, and do not require immediate

attention from this point of view.

5. In spite of tug boat propeller dredging in the past ten years, a net average of

72 cubic yards per year of sediment has been entering the basin, indicating

that propeller dredging may not have been the most effective way of removing the

sediment from the basin.

6. To prevent piping near the bulkhead, a new steel sheet pile wall with a top

elevation of +6.5 ft. msl and down to a minimum of -15 ft. is suggested.

This wall may be placed behind (landward of) the existing bulkhead. A new

concrete cap must cover the wall and the bulkhead in order to prevent piping









between the wall and the bulkhead. A rough estimate of the cost of wall

construction is $120,000.

7. A proposed dredging of the entire existing basin to -5 ft. (mlw) would

require a removal of 2,100 cubic yards of sediment. A rough estimate

of the cost is $6,300.

8. The entrance should have two training walls designed to prevent sediment

from entering the basin from the river. These walls, from the bottom up

to msl, will cause a flow pattern that will be conducive to transporting

any incoming sediment back to the river.

9. At a time when the timber planks at Pier A need replacement, the following

alterations are suggested: 1) the pier should be bulkheaded from the bottom

up to mlw wherever the bed elevation is below mlw. Near the shoreline,

where the bed elevation is above mlw, the bulkhead should be approximately

0.5 ft. above the bed elevation. THe asbestos cement sheeting should be removed,

and the timber planks should go down to the bulkhead. In this way, whereas

the spacing between the planks will allow flow circulation, the bulkhead

will prevent any appreciable anount of sediment from moving into the basin.

10. At a time when the timber planks on Pier B require replacement, it is sug-

gested that they go down to -3.2 ft. (msl), which is the selected design

low water elevation in this study. The space between the bottom and this

elevation must remain open, to allow flow circulation.

11. Design considerations on the extended marina are similar to those for the

existing marina. A total of 8,030 cubic yards would have to be dredged

from the extension region (to -5 ft. mlw) at an approximate cost of $24,000.

Considerations for the proposed Pier D are similar to those for Pier A,

whereas Pier A itself may have to be cut by 90 ft. to accommodate boat

traffic through the entrance.

12. The suggested changes should minimize the siltation problem in the marina.

However, should there be residual siltation in the basin, one of two possible








Maintenance measures such as the silt pump or the bubble screen are not

recommended unless siltation is not abated by the suggested modification in

marina design. Detailed design computations for the silt pump or the bubble

screen are beyond the scope of this study.

CONCLUSIONS & RECOMMENDATIONS

Conclusions derived from this study are summarized below.

1. There are two sources of sediment in the marina, 1) due to piping (quick

sand) near the bulkhead and 2) due to intrusion from Amelia River, through

the entrance. The first is primarily a source of sand, whereas the second

is a source of clayey-silt, which is finer than sand. The bottom surficial

sediment in the marina consists on the average of 69% fine sediment and

31% sand, by weight.

2. Flow circulation in the marina, under a comparatively high (8.1 ft.) range

of spring tide, is necessary for pollutant flushing of the marina.

3. While currents in Amelia River are strong enough to transport sediment,

they are too weak in.the entrance and the basin to move the sediment; hence,

approximately 48% of the sediment that enters the basin from the river deposits

there.

4. Piers A and B and the flow connection underneath the welcoming station allow

a comparatively less appreciable sediment influx, and do not require immediate

attention from this point of view.

5. In spite of tug boat propeller dredging in the past ten years, a net average of

72 cubic yards per year of sediment has been entering the basin, indicating

that propeller dredging may not have been the most effective way of removing the

sediment from the basin.

6. To prevent piping near the bulkhead, a new steel sheet pile wall with a top

elevation of +6.5 ft. msl and down to a minimum of -15 ft. is suggested.

This wall may be placed behind (landward of) the existing bulkhead. A new

concrete cap must cover the wall and the bulkhead in order to prevent piping








12. solutions may be considered; these being 1) the use of a silt pump for

periodically removing the deposited sediment from the marina and 2) the

use of an air bubble screen for preventing the sediment from entering the

marina through the entrance. Detailed design considerations on these two

methods are beyond the scope of this study

ACKNOWLEDGMENT

The authors wish to thank Mr. James Higginbotham, Public Works Director,

for his valuable assistance in providing information on the marina. The

corporation of Mr. Grady Courtney, City Manager and the interest of the members

of the City Commission is sincerely acknowledged.




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