|
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
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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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