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A study of siltation at Fernandina Beach Marina

Material Information

Title:
A study of siltation at Fernandina Beach Marina
Series Title:
UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 79/020
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:

Subjects

Subjects / Keywords:
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.

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University of Florida
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University of Florida
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All applicable rights reserved by the source institution and holding location.

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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

ISTING MARINA MARINA ...

Entrance .

MAINTENANCE AGAINST SILTATION CONCLUSIONS AND RECOMMENDATIONS

Page
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0 O I




INTRODUCTION
Fernandina Beach Marina 1 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 Intracoastal 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 (Mso 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 I 1.5 ft. below m1w. 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. I 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




.44

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)




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. -Thep prob lem 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 bpimplemented. STUDY AREA
As Fig. 10 shows the marina is located approximately 2.4.nautical 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

ST MARY'S ENTRANCE

0.13mm

AMELIA ISLAND

0

FERNANDINA
BEACH

FLOOD/EBB CURRENT
1.*- 01 = 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., Previousstudies' 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
w w 1.0 i,
z
0
0.0
w
-J
w
-1.0- R=8.1 Ft.
-2.0
-3.0
9 t10 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
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 measurable 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 I
COARSE GRAIN ANALYSIS
Median diameter Sorting coefficient
Location d50 (mm) S 0
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 fr om 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

70
LI 60
o o
6r 50 o
w
1.1
F- 4O
Z
w 30
w
20
I
0 5 0l
rain size in mm.

Fig. 12 Coarse Grain Size Distribution

GRAVEL

SAND




SILT CLAY

50
Z
Z
I0
LI
40
o
Io co w
d S6 Od c; C
gramin 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 distribution 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 n Feef

Fig. 15 Bathymetric Map

-Z




-~ II

II

II

Elev. 6.50'
KING PILE
-TEE SHEETING
_MSL Elev..00 MARINA BASIN
Approximate Existing Depth
7 MLW Elev.-2.60'

-Original Dredged Depth.
.1 P low _7tY)

Fig. 16 Cross-Section of Bulkhead




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 o-n 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(300)/(0.l)(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 I730 EDT

///Marina 1/
Marino

49cfs
67cfs 123 cfs
Marino
1264
cfs
120 cfs 129 cfs
//// 7Marina////////
Marino

219 cfs

TIDE (MLW)
+ 0.23 Ft.

7cfs

+ 1.15 Ft.

117 cfs

+ 2.26 Ft.

146 cfs

127cfs

1800 EDT

134-.cfs

I/I/1//1////////
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 ( n )2 2
0 1.49T 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 64 lbs/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)
u lo
where u = current velocity measured at elevation z above the bed, u= /o/P is the friction velocity and z0 = virtual origin of the profile. The bed roughness ks = 29.7zO. Manning's n is then computed from
n = .26 Rh/6 u*
n=0.263 R 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 T0 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 TO.m
(fps) (ft) (psf)
River 2.00 10 0.0619
Entrance 0.17 9.4 0.00046




0
N40
N
30 -u_ z
30- . 5.75 log -20
10,
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
Tcr.s = 0.056Y(y 1) d50 (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, Tcr.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 T0 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 T cr.s with T0.m it becomes apparent that whereas the river has sufficiently strong currents to transport the sediment (T0.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 substantitally 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 s)i may be obtained from Fig. 19, according to which
(qs)e Ah1/6 L (5)
qse- =f(w -)l
where w = particle settling velocity
h = depth of basin




o2o0
...-" -0.8-%
05-7
0.4--

.2
to10 0 h10 1.0 100
WnhG

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

NOENW




g = acceleration due to gravity
L = basin length
Selecting d 50 = 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 bulkhead, is marginal in preventing piping. A piping factor may be defined as the ratio
i mAp (I + e (6)
critical Ai (G- 1)
where i = flow potential gradient next to the bulkhead
critical = critical gradient next to the bulkhead
A = 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- Ah Ah = h- h ; where h, h= total potential (head) difference
n 1
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-

OF TEE SHEETING

G-I
ICRITICAL- I+e

S = n G=2.65, e=1.5
Ah= h -hI =0 066 I 2 CRITICAL II
=6.5FT, h2_-3.2FT. i II I
= 4.6, A&=2.0 FT. II

SOFT SAND

TO 55 FT.

Fig. 21 Piping Computations for the Existing Bulkhead

0.6 A A=

i =0.63

FILL ELEV.)

5




below ms], 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 m1w. For this bulkhead, the factor for piping is i/i critical 0.55. This is sufficiently lower than unity such that piping is avoided. The bulkhead sould 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 approximately 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




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




n =5.4, L {= 2.OFT i =0.36
II II
II II II II
SOFT SAND I
S FIRM GREY CLAY. I-,KING
Fig. 23 Piping Computations on a Bulkhead to -15 ft. (ms])




PIER A
OLD E
TO -7.
L BE LO
BOAT BL
S RAMP
FoCONCRETE CAP
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 -whereever 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 prevented 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.




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 twodimensional 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 shopwn 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. Bul khead
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 accomodate 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.




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 siltt vacuum cleaner". It is an air lift pump in which an air compressor is utilized 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.
A
h.. ........
. . . . .. .

L Air pressure p0 (in excess of atmospheric pressure)
Total air flow rate: PQo
Air flow rate per orifice: sQo

Fig. 26 Air Manifold Geometry

Basic Flow Pattern.

Oxygen Transfer.

MINOR OXYGEN UPTAKE

W.S.

. :. .: .

.. .
.. .

OXYGEN TRANSFER
TO BENTHIC LAYER

; ., ",j

Fig. 27 Air Bubble Screen




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 whereever 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 suggested 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 accomodate 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




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.