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UFL/COEL-2004/002
SEDIMENTATION ISSUES IN LOW-ENERGY
ESTUARIES: THE LOXAHATCHEE, FLORIDA
by
Rashmi R. Patra
and
Ashish J. Mehta
Submitted to:
Jupiter Inlet District
400 N. Delaware Blvd.
Jupiter, FL 33458
February 2004
UFL/COEL-2004/002
Sedimentation Issues in Low-Energy Estuaries: The Loxahatchee,
Florida
Rashmi R. Patra1
and
Ashish J. Mehta
Submitted to:
Jupiter Inlet District
400 N. Delaware Blvd.
Jupiter, FL 33458
Coastal and Oceanographic Engineering Program
Department of Civil and Coastal Engineering
University of Florida, Gainesville, FL 32611-6580
February 2004
1 Present address: Coastal Planning and Design, Inc., 849 Cormier Road, Green Bay, WI 54304.
Summary
Sedimentation issues in low-energy estuaries are examined with reference to the
Loxahatchee, a micro-tidal, coastal plain estuary in southeastern Florida. The
construction of a railroad bridge has effectively divided the estuary into two zones, with
the eastern zone consisting of sandy littoral sediment, whose accumulation has been
contained by an efficient plan involving periodic dredging and transfer. The central
embayment is the principal feature of the western zone, within which sediment consists
mainly of fine sand.
There is concern for slow accumulation of organic-rich muck derived from local
sources and supplied mainly by two tributaries, the Northwest Fork (the main stem river)
and the Southwest Fork. The effectiveness of sediment control by dredged traps and
channels is examined. While by definition traps accumulate sediment, channels can be
self-cleaning, i.e., they prevent the deposition of sediment, or they can serve as traps that
can be "cleaned" by dredging. It is found that tributary freshwater discharges are too low,
even at their peak values, to prevent designed channels from acting as accumulators. The
overall effectiveness of the channel alternatives has been examined on a qualitative basis
by weighing in improvement in navigation for small vessels afforded by these channels.
It is concluded that only two alternatives require future consideration towards
implementation; these being the maintenance of a second navigation channel close to the
southern rim of the central embayment and dredging the C-18 Canal. In case impacts on
local sea grass preclude the southern channel as sediment collector and an access route to
the interior waters, the existing navigation channel may be extended upstream to provide
access. Periodic removal of sediment from either channel should help maintain depths
and possibly also the clarity of water in the central embayment.
Future decisions concerning sediment management in the estuary, especially
within the western zone, would be significantly facilitated by a program of data
collection. At the least, we recommend a detailed survey of the entire zone within the
boundaries of the Jupiter Inlet District every four years, and concurrent pushcore-based
data from pre-specified sites.
Acknowledgment
Long-term assistance provided by Taylor Engineering, Inc. of Jacksonville for
field data collection is sincerely appreciated.
Table of Contents
page
Sum m ary ........................................................................................................... ............ ii
Acknowledgm ent ......................................................................................................... iii
List of Tables....................................................................................................................... v
List of Figures .............................................................................................................. vi
1. Introduction.................................................................................................................... 1
2. The Loxahatchee Estuary ............................................................................................... 3
3. Sedim entry Regim e ................................................................................................ 8
3.1 Sedim ent Properties.......................................................................................... 8
3.2 Sedim ent Loads ................................................................................................ 8
3.3 Traps and Channels ........................................................................................ 20
3.3.1 C-18 Trap............................................................................................. 23
3.3.2 FERRC Trap ........................................................................................ 25
3.3.3 C-18 Channel North ................................................. ......................... 26
3.3.4 C-18 Channel South ................................................. ......................... 26
3.3.5 Bay Channel South ................................................... ......................... 27
3.3.6 Bay Channel N orth................................................... ......................... 28
3.3.7 Northwest Fork Channel........................................... ......................... 28
3.4 Self-Cleaning Behavior of Channels....................................... ..................... 29
3.5 Dredging Frequency....................................................................................... 30
4. Qualitative Assessm ent of Alternatives ................................................ ....... ..... 32
4.1 Sand Shoal and N orth Fork Dredging............................................. ...... .... 32
4.2 M onitoring...................................................................................................... 34
5. Concluding Com m ents........................................................................................... 36
References ......................................................................................................................... 39
List of Tables
Table page
2.1 Watershed zones of the Loxahatchee River ...................................... ............. 3
2.2 Annual mean tributary freshwater discharges.................................... ............. 5
2.3 Characteristic tide ranges ....................................... ............... .......................... 5
2.4 Characteristic parameters: Southwest Fork (April-May, 2003).......................... 6
2.5 Characteristic parameters: Northwest Fork (November, 2003).......................... 7
3.1 Annual mean sand volumetric transport rates in the eastern zone .................... 11
3.2 Annual mean fine sediment mass transport rates in the eastern zone ................... 12
3.3 Annual mean sand volumetric transport rates in the western zone................... 12
3.4 Annual mean fine sediment mass transport rates in the western zone................ 17
3.5 Selected trap/channel dimensions................................................. ........ ..... 23
3.6 Surficial sediment composition upstream of S-46 (from Jaeger and Hart, 2001). 27
3.7 Effect of river discharge on trapping efficiency............................... ........... .. 31
4.1 Matrix for overall assessment of the effectiveness of the alternatives.................. 33
4.2 Pushcore coordinates ........................................ ....... ....................................... 35
5.1 Dredging in the central embayment navigation channel................................... 37
List of Figures
Figure page
1.1 Seaward reach of the Loxahatchee on the Atlantic Coast of Florida................... 2
2.1 Sand trap and the Intracoastal Waterway (ICWW)......................................... 4
2.2 Measurement stations, areas of comparatively high mud (muck) in
surficial sediment (adapted from Jaeger and Hart, 2001) and sandy
shoal in the central embayment............................................ ........................... 6
3.1 Textural classification of surficial sediment in the Loxahatchee
estuary (after Jaeger and Hart, 2001).............................................. ................ 9
3.2(a) Schematic rendition of a natural estuary such as the Loxahatchee
in sedimentary equilibrium .............................................. .............................. 10
3.2(b) Schematic rendition of urbanized Loxahatchee Estuary. About 260 km2
of the watershed are undeveloped wetland and the remaining 260 km2
are divided approximately evenly between urbanized area and the
agricultural and forested zones mainly in the north watershed. The bridge
was constructed in the early 1900s.............................................................. 10
3.3 Schematic drawing shown formation and sustenance of shoal due to
tide-induced secondary flows in the central embayment .................................. 13
3.4(a) An example of modeled instantaneous flood flow vectors in the shoal area........ 14
3.4(b) An example of modeled instantaneous ebb flow vectors in the shoal area......... 15
3.5(a) Measured and modeled water level at station UF-2 in the Northwest Fork.......... 15
3.5(b) Measured and modeled depth-mean current speed at station UF-3 in
the Southwest Fork.................................................................................. ... 16
3.5(c) Measured and modeled depth-mean current direction at station UF-3 in
the Southw est Fork.......................................................................................... 16
3.6(a) Effect of S-46 operation (beginning April 14th, 2002) on the transport
regime in the Southwest Fork: 12-hour-mean current speed ............................ 18
3.6(b) Effect of S-46 operation on the transport regime in the Southwest Fork:
12-hour-m ean salinity ..................................................................................... 18
3.6(c) Effect of S-46 operation on the transport regime in the Southwest Fork:
w after tem perature............................................................................................ 19
3.6(d) Effect of S-46 operation on the transport regime in the Southwest Fork:
12-hour-mean TSS concentration data at three elevations from bottom............ 19
3.7 Poled sediment thickness as a measure of accumulation in C-18 Canal
since 1958 (adapted from Jaeger and Hart, 2001)............................................ 20
3.8 Stratigraphy of pushcores C00-5 (1.0 km from S-46), C00-6 (1.2 m
from S-46) and C00-7 (2.1 km from S-46). Core lengths approximately
correlate with local deposit thickness since 1958 (adapted from Jaeger
and H art, 2001)................................................................................................ 21
3.9 Channel and trap alternatives in the central embayment................................... 22
3.10 Design channels and traps in the western zone of the estuary .......................... 24
3.11 Dependence of sediment removal ratio in (fine sediment) trap on S-46
discharge (after Ganju, 2001).............................................. .......................... 25
3.12 Surficial sediment sampling sites in C-18 Canal upstream of S-46 structure....... 27
3.13 Representative flood current vectors in the area of Bay Channel North.............. 29
3.14 Estimation of time-scale for trap filling......................................... ............... 31
4.1 Pushcore measurement sites for monitoring ................................... ............. 34
1. Introduction
In Florida, as elsewhere, low energy estuaries in urban areas have been
experiencing human-induced increase in the incoming sediment load (Kirby, 2003). Such
estuaries are characterized by weak tides (<1 m range) and low freshwater outflows.
Under natural conditions, the long-term net rate of shoaling due to sediment supply from
the sea and the tributaries will in general equal the rate of relative sea level rise, so an
increased load will mean a decrease in water depths, which is typically manifested as
filling-up of flood and ebb channels. Since these channels are preferred conduits for
exchange between the sea and the inner waters, their role in flushing of embayed waters
is thereby altered.
When channels begin to shoal at rates that are thought to be high, e.g., for water
quality or convenience of navigation, several remedial options can be considered:
1) elimination or reduction of sediment influx, 2) channel relocation, 3) decrease in flow
velocity to capture sediment before it reaches the zone of interest, and 4) increase in flow
velocity to prevent or minimize sedimentation by enabling sediment to pass through. In
what follows, strategies of this nature and their consequences are discussed in reference
to the estuarine reach of Florida's Loxahatchee River on the Atlantic Coast (Fig. 1.1).
Figure 1.1 Seaward reach of the Loxahatchee on the Atlantic Coast of
Florida.
2. The Loxahatchee Estuary
The present Loxahatchee River estuary, about 16 km in length, is a drowned river
valley that functioned as a shallow tidal wetland about 5,000 years BP, as determined
from core dating (Wanless et al., 1984). By global standards it is a very small body of
water fed by five watershed basins (Table 2.1).
Table 2.1 Watershed zones of the Loxahatchee River
Area
Zonesa
(km2)
Intracoastal Waterway 545
C-18 Canal 278
Jonathan Dickinson 155
South Indian River 65
Loxahatchee River 6
a Not sub-basins. Formally there are seven hydrologic sub-basins totaling about 520 km2.
During the past five millennia the relative sea level rose at a mean rate of about
0.08 m per 100 y, i.e., by about 4 m, forming the present mouth (Jupiter Inlet), where the
natural depth in the channel was 0.9-1.5 m and a sand bar blocked flow much of the time
(Fineren, 1938). As a result the littoral sand drift was almost wholly bar-bypassed
without much interference by the inlet (Bruun, 1978). Navigation between the ocean and
the river was made permanent in 1947 by stabilizing Jupiter Inlet with two jetties.
Dredging on a regular basis began at that time in the area of the sand trap (Fig. 2.1) of the
Jupiter Inlet District (JID), the custodial agency, to a depth of about 6 m below mean
water level. Presently the trap is dredged annually and material transported to the beach
downdrift (south) of the inlet to contain local beach erosion. This procedure, as well as
periodic dredging of the Intracoastal Waterway (ICWW) by the U.S. Army Corps of
Engineers, has provided an effective means to transfer littoral sand from the updrift
(north) to the downdrift (south) side of the inlet.
//Atlantic
Ocean
-a d trp Jupiter
Figure 2.1 Sand trap and the Intracoastal Waterway (ICWW).
During the 1990s the inlet jetties were modified to reduce littoral sand influx, and
the trap enlarged to intercept additional sand that would otherwise deposit in the
Loxahatchee central embayment (Grella, 1993). This embayment (Fig. 1.1) is fed mainly
by two natural streams, the Northwest Fork (main stem Loxahatchee River) and the
Southwest Fork. A 2.1 km reach of the main stem is federally classified as wild, 9.3 km
as scenic and 0.81 km as recreational. The -60 m wide C-18 Canal (Fig. 1.1) was
constructed in 1958 to divert floodwater from the main-stem to the Southwest Fork. A
sluice-gated control structure (S-46) discharges water during spates when the water level
behind the gate exceeds a designated level. The outcome is that flow in the Southwest
Fork is entirely tidal except when the structure is open. Nonetheless, as Table 2.2
indicates, on an annual average basis discharge from the structure is comparable to that in
the main stem. In contrast the third tributary, the North Fork, has negligible freshwater
flow. The instantaneous water level within the embayment itself remains practically
(spatially) uniform because the bay dimensions are small relative to the (semi-diurnal)
tidal wave length (Table 2.3 and Fig. 2.2).
Table 2.2 Annual mean tributary freshwater discharges
Median (50%)b High (90%) Peak (98%)
Tributary
(m3/s) (m3/s) (m3/s)
Southwest Fork 1.3 7.8 61
Northwest Fork 0.7 4.1 76
North Fork 0.1 0.2 1.9
a Based on data reported by the South Florida Water Management District (Patra, 2003).
b Cumulative percentile. Example: 98% of time the discharge is less than 61 m3/s.
Table 2.3 Characteristic tide ranges
Location Spring range Neap range
Location
(m) (m)
UF-1 0.90 0.66
UF-2 0.85 0.65
UF-3 0.86 0.64
Current speed and total suspended solids (TSS) concentration (representing <63
ljm, i.e., fine-grained, sediment fraction) in the Southwest Fork (Fig. 2.2 and Table 2.4)
are influenced by the episodic release of water from the S-46. The result is that strong
outflows occur under this condition. As seen from Table 2.4, a peak 1.4 m/s ebb flow
occurred during April-May in 2003. During the same period, TSS concentration near the
bottom rose to 2,090 mg/L due to the formation of a dense suspension at the bottom. In
the Northwest Fork (Fig. 2.2 and Table 2.5) during November, 2003 the conditions were
milder with regard to TSS concentration. To a degree these differences in TSS reflect the
flow regimes in the two tributaries regulated by gate operation in the Southwest Fork
with intense transport versus unregulated and usually milder conditions in the Northwest
Fork.
Tide station
A Current, salinity, temperature, TSS station
Ii
...... --- Jupiter
Figure 2.2 Measurement stations, areas of comparatively high mud (muck) in
surficial sediment (adapted from Jaeger and Hart, 2001) and sandy shoal in the
central embayment.
Table 2.4 Characteristic parameters: Southwest Fork (April-May, 2003)
Parameter Maximum Mean Minimum
Water depth (m) 1.9 1.3 0.7
Current (m/s) 1.40 0.60 0.10
TSS concentration near-surface (mg/L) 210 56 10
TSS concentration mid-depth (mg/L) 170 46 2
TSS concentration near-bottom (mg/L) 2,090 1,670 1,180
Table 2.5 Characteristic parameters: Northwest Fork (November, 2003)
Parameter Maximum Mean Minimum
Water depth (m) 1.90 1.40 1.00
Current (m/s) 0.70 0.35 0.18
TSS concentration near-surface (mg/L) 144 132 119
TSS concentration mid-depth (mg/L) 109 96 81
TSS concentration near-bottom (mg/L) 219 190 156
3. Sedimentary Regime
3.1 Sediment Properties
The main focus of this study is the central embayment, within which the sediment
is mainly fine, well-sorted sand (0.15 mm) mixed with silt- and clay-sized, i.e., fine-
grained (<63 glm), material (Fig. 3.1). The sand appears to have been deposited in this
region during the late Pleistocene (last 1.8 million years). The fine material, or mud, is
rich in carbon derived mostly from natural organic matter and is often referred to as muck
(Kirby, 2003). Loss-on-ignition analysis of samples from the C-18 indicated a mean
value of 15% of organic matter (Ganju, 2001). In some patches close to but not quite
within the embayment (Fig. 2.2), the fine fraction is comparatively high, ranging between
15 and 25% by weight of sample. This material appears to have been derived from local
sources within the watershed, whose input has been exacerbated in recent decades by
effects of urbanization (Jaeger and Hart, 2001).
3.2 Sediment Loads
An assessment of sediment loads is complicated by the presence of both coarse
and fine material, and the multiplicity of sources and sinks despite the small areal extent
of the region. A noteworthy feature is that given the characteristically low rates of
sediment transport in Florida, the response of estuaries to natural or anthropogenic
forcing of sediment loads tends to range from decadal to centennial time scales. As a
consequence, the estuarine morphology can be thought of.as being in a state of quasi-
equilibrium with the prevailing flow, sediment transport and relative sea level rise.
Clay
Inlet and ICWW
0 100 U North Fork
A Central Embayment
1 \* Northwest Fork
v Southwest Fork
Upper C-18 Canal
20 80 Pleistocene Outcrop
30/ 70
40 60
5o 50
70 7 V S V ,lay"\ iyy 30
80 20
1010
Sand 0 10 20 30 40 50 60 70 80 90 100 Silt
Figure 3.1 Textural classification of surficial sediment in the Loxahatchee estuary
(after Jaeger and Hart, 2001).
Conceptually it is convenient to view the Loxahatchee as a highly engineered
estuary in the pre-Columbian environment, for all practical purposes until about a century
ago [Figs. 3.2(a)-(b)]. In the present context, the flow constriction imposed by the
FECRR bridge can be conveniently taken as the zonal boundary between the seaward
reach of the estuary up to the inlet due east and the central embayment in the west.
Accordingly, in relation to the present sediment budget for the embayment, we will
consider sand and fine sediment transport loads separately for the zones eastward and
westward of the bridge.
F.7 -
Figure 3.2(a). Schematic rendition of a natural estuary such as the Loxahatchee in
sedimentary equilibrium.
Sand
Fine sediment
NM1firh die-hck ----
Bridge 1
cons action,~
- I> s>ElI
4I
>
Figure 3.2(b) Schematic rendition of urbanized Loxahatchee Estuary. About 260
km2 of the watershed are undeveloped wetland and the remaining 260 km2 are
divided approximately evenly between urbanized area and the agricultural and
forested zones mainly in the north watershed. The bridge was constructed in the
early 1900s.
-- -- -- -- -- -- --------
Annual sand volumetric transport rates for the eastern zone are given in Table 3.1.
Note that to obtain sand mass for a given volume, the latter must be multiplied by (1-n)ps,
where n (= 0.4) is the porosity and p, (= 2,650 kg/m3) is the sediment granular density.
These rates rely on data reported elsewhere (Dombrowski and Mehta, 1993; Thieke and
Harris, 1993), and it should be noted that values of 4,000 m3/y and less are essentially
best estimates. Note also that the according to this tabulation there is no net accumulation
of sand within the eastern zone, because sand inflow rates balance outflow rates due to
natural- and dredging-determined pathways.
The transport rates for fine sediment in the eastern zone are given in Table 3.2
based on mass rather than volume following usual convention. Mass must be divided by
sediment dry density, Pd (= 400 kg/m3) to obtain volume. The constant rate of 50 tons per
year is a best guess. There is no significant accumulation of fine material in this zone.
Table 3.1 Annual mean sand volumetric transport rates in the eastern zone
Volumetric rate
Transport from/to (m3/y)
(m /y)
Net southward littoral drift 176,000
Entering the channel from littoral drift 46,000
Bar-bypassed around the inlet 128,000
Bypassed by dredging from JIDa trap and ICWW 33,000
Tidally bypassed by entering and then leaving the channel 4,000
Ejected from the channel to offshore by ebb flowb 4,000
Transported offshore from drift by ebb flowb 2,000
Transported to ICWW channels north and south of inlet 4,000
Transported to the central embayment 1,000
a Jupiter Inlet District.
b Deposited seaward of the littoral system.
Table 3.2 Annual mean fine sediment mass transport rates in the eastern zone
Mass rate
Transport from/to Mas rat
(tons/y)
Transported from the central embayment 50
Ejected out of the inlet 50
On the western side, the sandy shoal (Fig. 2.2) is the main accumulation feature.
In general, a flood shoal begins to develop as soon as a new inlet connected to a
reasonably well-defined bay is opened and littoral sediment enters.
During the early decades of the last century, attempts were made to open the inlet
for navigation, but without long-term success. The effective date of opening is 1947
when, as mentioned earlier, the inlet was widened permanently by dredging and
stabilized with jetties. Carr de Betts (1999) estimated the shoal volume in the central
embayment to be 7.55x105 m3 in 1983. It is believed that much of this material arrived
from the ocean-side during the initial years of inlet opening, and from the forks following
the construction of the C-18 Canal. The present rate of accumulation is estimated to be on
the order of 3,000 m3/y at most; Table 3.3 (Antonini et al., 1998 estimate between 1,050
and 3,150 m3/y between 1967 and 1996), which would amount to -2 mm/y of uniform
rate of shoaling within the central embayment.
Table 3.3 Annual mean sand volumetric transport rates in the western zone
Volumetric rate
Transport from/to (m3/y)
(m /y)
Transported to the central embayment from east of FECRR bridge 1,000
Transported from Northwest and Southwest Forks 2,000a
a Upper limit estimate.
The redistribution of otherwise uniformly distributed sand by the prevailing
currents is believed to be collectively responsible for the "visibility" of the shoal since the
1970s. The embayment created by the FECRR bridge is idealized in Fig. 3.3. An outcome
of such a configuration is the generation of secondary flow cells due to shoreline
curvature. In the upper part of the water column this flow is always directed inward, i.e.,
both during flood flow and ebb tides. This is a "self-organized" mechanism by which
sand grains at bottom are redistributed by lateral flows to form and sustain a shoal
(Hibma et al., 2003).
Central > Secondary flow
embayment in upper water column is
directed inward during
both
flood and ebb
flows
Figure 3.3 Schematic drawing shown formation and sustenance of shoal due
to tide-induced secondary flows in the central embayment.
Flow patterns in the shoal area simulated by the three-dimensional hydrodynamic
model EFDC (Environmental Fluid Dynamics Code) of Hamrick (1992) indicate the
presence of reasonably well-defined (primary and secondary) flood and ebb channels
[Figs. 3.4(a)-(b)]. As a starting point of these simulations, the model was calibrated by
using water level and current data from sites UF2 and UF3 (Fig. 2.2). Examples of
comparison between measured and modeled results are shown in Figs, 3.5(a)-(c). Details
of the application are provided by Patra (2003). Some of the calculations presented
subsequently also rely on another (two-dimensional, depth-averaged) code for flow
dynamics and sediment transport developed by Marvdn (2001) and applied to the present
study by Ganju (2001) and Ganju et al. (2001).
A seen from Table 3.4, fine sediment mass transport rates in the embayment are
characteristically very low. This is consistent with the observation (Mulder and Syvitski,
1996) that for small, coastal plain rivers such as the Loxahatchee, the predicted total
(sand+mud) annual sediment input for a low-relief (5m) river with a -500 km2 area
watershed is only -300200 tons/yr. Furthermore, while muck deposition is found in the
forks, much of the material that enters the embayment is believed to be transported
offshore by strong ebb flows. The transport of 600 tons/y of sediment by the C-18 Canal
makes it the main contributor of fine material, which since its construction has served as
a shunt for both water and sediment [Fig. 3.2(b)]. As a result, fine material from the flood
plains of the main stem bypasses the Northwest Fork and is introduced directly into the
embayment.
Figure 3.4(a) An example of modeled instantaneous flood flow vectors in the shoal
area.
al'ea.
-
. 4 A-a-\-
Figure 3.4(b) An example of modeled instantaneous ebb flow vectors in the shoal
area.
0.8
0.7
0.6
0.5
S0.4
- 0.3
S0.2
0.1
0.0
-0.1 1
253.0
253.5
254.0
254.5
255.0
Julian days (2003)
Figure 3.5(a) Measured and modeled water level at station UF-2 in the Northwest
Fork.
Measured - Modeled
7 I
~
2
B
1
u
0.04
0.03
0.02
0.00
-0.01
-0.02
-0.03
-0.04
Measured
-0.05 ........
- Mod'led
0- 06
345.5
346.0
346.5 347.0
Julian days (2002)
347.5
348.0
Figure 3.5(b) Measured and modeled depth-mean current speed at station UF-3 in
the Southwest Fork.
150 ---
SMeasured
.-....... ...............,. ................... iI ..................
100 ......................V-11.............................. ............. ..... .....................
_n ^-- o-- -- --- ---i--i i-- -----*--
5.
-100
-150
-200
333 334 335
Julian days (2002)
Figure 3.5(c) Measured and modeled depth-mean current direction at station UF-3
in the Southwest Fork.
Table 3.4 Annual mean fine sediment mass transport rates in the western zone
Transport from/to Mass rate
(tons/y)
Transported from the C-18 Canal into Southwest Fork 600
Transported from the Northwest Fork 50
Transported to the North Fork 140a
Transported east of FECRR bridge 50
a Upper limit estimate. To examine consequences a better estimate based on measurements is required.
Data presented in Figs. 3.6(a)-(d) highlight the episodic nature of sediment
advection down the Southwest Fork governed by the operation of the S-46. Except for the
water temperature record, these data on current velocity, salinity and TSS concentration
from site UF-3 (Fig. 2.2) are 12-hour averaged, so that tidal oscillations have been largely
filtered out from them. On Julian day 104 (April 14th), 2002 the sluice gate was opened.
Until then the current meter recorded a small (almost negligible) flow velocity
downstream. Yet, over the ensuing ten days the velocity rose to as high as 1 m/s [Fig.
3.6(a)] and the salinity dropped by as much as 12 psu due to freshwater flow [Fig.
3.6(b)]. Water temperature rose somewhat [Fig. 9(c)] presumably because the canal water
was slightly warmer than the ocean, and TSS concentration increased dramatically from
the usual 10-20 mg/L (Ganju, 2001) to 130-160 mg/L [Fig. 3.6(d)]. The TSS response
lagged the current by several days because of the -2.5 km distance between the sluice
and the measuring station UF-3. This time lag reflects the retarding effect of tidal
oscillations and associated sediment settling and resuspension lags on sediment advection
from the sluice to UF-3. Nevertheless it is noteworthy that despite this lag, TSS
concentration rose by an order of magnitude over the ambient, implying that, ultimately,
sediment transport down the canal is largely determined by the frequency and intensity of
rainfall within the contiguous watershed.
1.2
1 .0 .............................
S 0 .... ... ...................... ..............................
S0.
S 4 ...... ........ .................. ...........................
0 .2 ........................ .......... ..............................
0.0
95
105
Julian days (2002)
Figure 3.6(a) Effect of S-46 operation (beginning April 14th, 2002) on the transport
regime in the Southwest Fork: 12-hour-mean current speed.
35
30
25
.=
c 20
15
10
105
Julian days (2002)
Figure 3.6(b) Effect of S-46 operation on the transport regime in the Southwest
Fork: 12-hour-mean salinity.
30
U 28
0
28
2 26
24
22
20
[-- Sluice discharge
M. ean trend
'5 100 105 110 1
Julian days (2002)
Figure 3.6(c) Effect of S-46 operation on the transport regime in the Southwest
Fork: water temperature.
120
.2 100
" 80
0 60
A n
0 L.
95
100 105 110
Julian days (2002)
Figure 3.6(d) Effect of S-46 operation on the transport regime in the Southwest
Fork: 12-hour-mean TSS concentration data at three elevations from bottom.
An additional impact of the released water is that sediment deposition decreases
as one approaches S-46, as seen from the poled depth data in Fig. 3.7, which may be
considered as a measure of the thickness of the deposit in the canal since its construction
in 1958 (Jaeger and Hart, 2001). It needs to be pointed out that much of the fine material
from the release eventually enters the Southwest Fork where lower velocities than those
in the canal allow it to deposit, while in the canal itself mostly sandy material has
accumulated. Stratigraphy of three representative pushcores from the canal shown in Fig.
3.8 suggest a measurable winnowing effect of the S-46 discharge, which leaves sand in
place but advects fine material downstream.
2.2
2.0
1 .8......... .. ...... .....
C. 1.6high ,
S1.2
1.0
0.8
S0.6 7
0.4 *
0.2 -
0 0.5 1.0 1.5 2.0 2.5
Distance from S-46 (km)
Figure 3.7 Poled sediment thickness as a measure of accumulation in C-18
Canal since 1958 (adapted from Jaeger and Hart, 2001).
3.3 Traps and Channels
While a dredged trap is evidently meant to capture moving sediment, a dredged
channel can either act as a trap when the through-flow is low, or as a "self-cleaning"
conduit at high flows for passage of sediment without deposition. Both traps and channels
are appropriate means for sediment management in the Loxahatchee, where the JID
(Jupiter Inlet District) trap and the turning basin of the ICWW (both in the eastern zone)
have served as efficient means to capture and hydraulically transfer sand. A segment of
the existing navigation channel adjacent to the shoal in the embayment (Fig. 3.9) also
traps sediment, which is periodically removed and deposited on land. Likewise, the C-18
Canal, since its inception in 1958, has trapped considerable sediment (-50,000 m3) (Figs.
3.7 and 3.8).
C00-5 C00-6 C00-7
0.0 m -
L0.0 m Predomiantly sandy
0.1 m -[--
0L m Siltysand
0.2 m Sandy silt
0.3m Organic silt
0.4 m
0.5 m -
0.6 m -
0.7 m
0.8 m
Figure 3.8 Stratigraphy of pushcores COO-5 (1.0 km from S-46), COO-6 (1.2 m from
S-46) and COO-7 (2.1 km from S-46). Core lengths approximately correlate with
local deposit thickness since 1958 (adapted from Jaeger and Hart, 2001).
Figure 3.9 Channel and trap alternatives in the central embayment.
Table 3.5 lists the trap and channel alternatives examined. Results are
summarized here; details are found in Patra (2003), Ganju et al. (2001) and Ganju (2001).
Table 3.5 Selected trap/channel dimensions
Feature Length
(m)
C-18 Trap 180
FECRR Trap 60
C-18 Channel North 1,890
C-18 Channel South 1,000C
Bay Channel South 1,430
850 (E-arm + N-arm)
Bay Channel North
710 (S-arm)
NW Fork Channel 890
a Relative to ambient depth.
b Minimum for a small craft.
C Nominal value.
d Present design depth.
e Below NAVD88.
3.3.1 C-18 Trap
This trap (Fig. 3.10 and Table 3.5) would be placed at the site of the maximum
deposition in the canal (Fig. 3.7). The trap depth (3 m) would be approximately equal to
the original dredged depth plus over-depth allowance. The width (30 m at the base) is
governed by the canal width and the need to account for side slopes. The length (180 m)
is based on the need to optimize trap efficiency without making it excessively long.
Even though the C-18 Canal was initially filled with locally derived sand possibly
soon after its construction, one can expect that the trap would capture fine sediment that
appears to be presently in motion in that reach of the canal. The objective of testing the
function of a small trap at this particular site (as opposed to a full-length dredged
channel) was to assess its efficiency, defined in terms of the sediment ratio R = (Sediment
deposition flux in trap)/(Sediment influx) (Ganju, 2001). As seen from Fig. 3.11, with
Depth
(m)
3.00a
2.50a
3.50e
3.50e
2.13b
2.20d
2.13b
Bottom width
(m)
30
150
25"
25b
25"
25"
25b
--
increasing discharge R (in percent) increases at first, reaches a peak value (= 0.52) at 1.7
m3/s and then decreases. This characteristic discharge is close to the median discharge 1.3
m3/s at the structure, and implies that the selected trap dimensions in terms of length and
depth could be expected to be reasonably functional.
OP
IA
7-. S~5
Figure 3.10 Design channels and traps in the western zone of the estuary.
0.5
a4
o 0.4
0.
> 0.3
o
0.2
0.1
0.5 1 1.7 10 3 20 50
S-46 discharge (m3/s)
Figure 3.11 Dependence of sediment removal ratio in (fine sediment) trap on S-46
discharge (after Ganju, 2001).
The left arm of the R-curve indicates that as the discharge increases more
sediment arrives and settles. However, when discharge increases beyond the peak value,
high velocities reduce the efficiency of capture. In general, -50% efficiency would be
acceptable, but because of the variability in discharge and uncertainty in the effect of
heterogeneity in the composition of the arriving sediment (not considered in determining
the response shown in Fig. 3.11), the actual value of R may range between 35 and 52%.
3.3.2 FERRC Trap
The purpose of this trap (Fig. 3.9 and Table 3.5) was to examine its efficiency in
capturing sand arriving from the eastern zone of the estuary through the channel
underneath the FECRR bridge. The justification for trap dimensions was essentially along
the same basis as that for the C-18 trap, but the much shorter length (60 m) was dictated
:I
i. .... ... ... ... .. .. .. .. .. ..
I I
Increasing Decreasing sedimentation
increasing sediment discharge
influx
influx !I
by the need to capture sand as opposed to fine sediment. Calculations indicated a very
high efficiency (85%). However, we note that the incoming sediment load in recent
times is estimated to be on the order of 1,000 m3/y only (Tables 3.3). Consequently, a
strong justification for capturing sand is not apparent at this location, as opposed to the
landward reach of the present channel in the embayment (Fig. 3.9), which also receives
sand from upstream sources and is dredged out as needed.
3.3.3 C-18 Channel North
The historic role of the C-18 Canal as a sediment trap and the fact that the C-18
trap was found to have an efficiency that may conceivably be less than desirable (as low
as 35%) led to the issue of dredging the entire canal close to its original depth (Fig. 3.10
and Table 3.5). Calculations indicated that the resulting channel would have a trapping
efficiency of 77%, which would mean that it would capture 460 tons/y out of the
incoming 600 tons/y (Table 3.4).
3.3.4 C-18 Channel South
There is an incentive to preempt the arrival of sediment downstream of the S-46
structure by dredging a channel upstream. Sediment sampling carried out there (Fig. 3.12
and Table 3.6) indicated that very little muck is present close to (and upstream of) the
structure, but as a norm muck fraction can be expected to increase with distance further
upstream as the effect of the structure wanes. Also, the sand grains seem to be coarser (as
high as 0.36 mm versus 0.15 mm downstream). It appears that fine sand and finer
material is winnowed out and moved downstream of the structure during spates. Under
such conditions, a channel of dimensions given in Table 3.5 could be a feasible
alternative to consider, especially because such an action would minimize dredging needs
downstream.
Figure 3.12 Surficial sediment sampling sites in C-18 Canal upstream of S-46
structure.
Table 3.6 Surficial sediment composition upstream of S-46 (from Jaeger and Hart, 2001)
Sample Sand Mud Median size
(%) (%) (mm)
C18-01 99 1 0.29
C18-02 99 1 0.15
C18-03 98 2 0.36
3.3.5 Bay Channel South
This channel (Fig. 3.9 and Table 3.5) would take advantage of the prevailing
flood and ebb flow pathways [Figs. 3.4(a)-(b)] close to the southern rim of the
embayment, and thereby provide an efficient means for flushing of embayed waters. The
trapping efficiency was found to be 79%. It needs to be note, however, that taking in to
account the fact that this area transports only around 600 tons/y of sand and fine
sediment, the rate of shoaling (which will be unevenly distributed lengthwise) is likely to
be very low overall.
An alternate route for this channel would be via a natural channel to the east of
mangrove island (Fig. 3.9), which is longer but is likely to experience lower
sedimentation than the arm west of the mangrove island. A potential hindrance to its
construction may be sea grass beds, which are believed to be more abundant west of the
island than east (Antonini et al., 1998).
3.3.6 Bay Channel North
The evident purpose of this channel (Fig. 3.5 and Table 3.5) is to extend the
present channel westward and allow better excess to the Northwest and the Southwest
Forks. The trapping efficiency is found to be 80%, which would mean that the channel
would draw in significant sediment as opposed to its transport elsewhere. With regular
dredging the channel would function as an efficient means to exchange waters between
the forks and the ocean. A noteworthy drawback is that the south arm of this channel is
oriented almost orthogonally to the prevailing flow direction (Fig. 3.13), which would
mean that shoaling in this reach of the channel most likely will be rapid enough to require
annual dredging.
3.3.7 Northwest Fork Channel
The basis for dredging in this channel (Fig. 3.10 and Table 3.5) would be to draw
fine sediment that appears to deposit in this area (Fig. 2.2), and thereby prevent its
eventual transport into the embayment. The trapping efficiency is found to be high
(96%). However, because the incoming sediment load is low (50 tons/y), the shoaling
rate would be low as well. In other words, there seems to be insufficient cause to
construct this channel unless and until a time the sediment load increases considerably.
** h Beam current and
4*. sedimentation potential
Figure 3.13 Representative flood current vectors in the area of Bay Channel North.rm
Fiue31 epeettv fodcretvetr nte rao a CanlNrh
3.4 Self-Cleaning Behavior of Channels
The function of a self-cleaning channel within the embayment would be to
transport sediment, particularly fine-grained, from the forks to the eastern zone of the
estuary, without allowing the material to deposit in the embayment. Similarly, such a
channel in the C-18 Canal or the Northwest Fork would prevent deposition there. The
sediment would enter the embayment from where the northern or the southern channel
(or both) would transport the material to the estuary east of the FECRR bridge. From
there the material could be expected to be exported to the ocean, as little organic matter
appears to deposit in the eastern zone of the estuary.
For a self-cleaning channel the removal ratio R would be zero. Calculations
summarized in Table 3.7 indicate that even at the design peak discharge condition in the
three forks, self-cleaning is unlikely not occur. The filling of the dredged C-18 Channel
and also a segment of the present navigation channel in the embayment when it is
dredged (Fig. 3.9), are manifestations of the absence of self-cleaning.
As mentioned earlier the rates of sediment transport in the western zone of the
estuary are low as signified by the low rates of sedimentation in the embayment (< 3
mm/y overall), and changes in the bathymetry are not detected easily from surveys more
frequent than 3-5 years (Ganju et al., 2001). Thus, the existence of self-cleaning channels
is not an imperative here in contrast to estuaries with high rates of sedimentation.
3.5 Dredging Frequency
Uncertainty in the composition of sediment, i.e., fractions of sand and fine
sediment, makes it difficult to estimate the duration over which a dredged channel would
be filled and maintenance dredging required.
In general, the rate of filling of traps and channels is proportional to the difference
between the original depth and the depth to which filling has occurred. Using the analysis
of Vincente (1992) for estimation purposes, an example is given in Fig. 3.14 of the filling
of the C-18 trap (Ganju et al., 2001). Considering the time required for filling to be 90%
of the dredged depth as giving the redredging frequency, it is seen that the duration would
depend significantly on the type of sediment. For sand redredging would be required
every 8 years, whereas for fine material it would be 53 years. These widely differing
time scales highlight the need to rely on measurements of deposition (rate and
composition of deposited material) at selected sites in the estuary in the coming years as
an essential basis to consider alternatives for sedimentation control.
Table 3.7 Effect of river discharge on trapping efficiency
Removal ratio, R
Incoming
(%)
Feature loadc
(tons/y)
@ Calibration discharged @ Peak discharge
C-18 Channel N 600 52 43
Bay Channel N 1600d 80 13
Bay Channel S 500d 79 13
NW Fork Channel 50 96 95
a Refers to discharge time series in the forks during measurements at sites UF-2 and UF-3 (Patra, 2003).
b Refers to discharge in the three forks corresponding to the 98% cumulative values (Table 2.2).
C Sand plus fine sediment at the particular site.
d Any load reduction due C-18 Channel or NW Fork Channel are not considered.
3.0
2.5
S2.0
4-.
1.5
[..
0.5 -
0.3
0
0
010 20 30 40
Time (y)
Figure 3.14 Estimation of time-scale for trap filling.
4. Qualitative Assessment of Alternatives
In Table 4.1, an attempt has been made to assess the overall effectiveness of the
alternatives considered, including "no-action", by weighing in, on a qualitative basis,
improvement in navigation (provided by the design channels) equally with sedimentation
control. Bay Channel South and scores well, followed by Bay Channel North. The
Northwest Fork option is unlikely to be useful. It should also be pointed out that the -1
mark for present condition, i.e., "no-action", does not necessarily convey urgency of
action.
4.1 Sand Shoal and North Fork Dredging
A corollary issue of concern in the central embayment is the growth of the sand
shoal and whether it requires dredging. As we have noted, the sand shoal is a natural
consequence of the arrival sand and its redistribution by the prevailing flows (Fig. 3.3).
Its location is determined by the morphodynamics of the embayment, which appears to be
dividing the wide body into two channels, one north of and the other south of the shoal.
As the shoal continues to grow, albeit at a slow rate (2,000-3,000 m3/y maximum),
currents in the two channels can be expected to increase and, in the absence of arrival of
sediment from fresh sources, stabilize the flow cross-sections.
Ganju et al. (2001) examined the effect of lowering the elevation of a large
portion of the shoal on the flow pattern in the embayment. Although the two-dimensional
(depth-averaged) flow model they used showed no significant impacts, one must temper
this observation by recognizing that there appears to be a correspondence between the
shoal and sea grass distribution (Antonini et al., 1998). If indeed a remedial action is
sought to contain shoal growth, it can be accomplished quite conveniently by maintaining
the proposed Bay Channel North or Bay Channel South. Either can be expected to trap
sediment in addition to the amount being trapped by the present navigation channel.
Table 4.1 Matrix for overall assessment of the effectiveness of the alternatives
Sediment
Alternative control Navigation Overall Remarks
control
Overall, -1 does not imply
No-action -1 0 -1 urgency of action since
sedimentation rates are low.
Dredging of C-18 Channel
C-18 Channel N +2 0 +2 South should be considered on a
concurrent basis.
A strong candidate, as the
channel will be along a natural
flow pathway with low rates of
Bay Channel S +1 +2 +3 sedimentation over most of the
channel and significant
improvement in navigation
access to Southwest Fork.
The south arm, required for
navigation access to Southwest
Fork, will also require
Bay Channel N 0 +1 +1
significant redredging due to
cross-flow induced
sedimentation.
No noteworthy benefit to
NW Fork Channel 0 0 0
sediment control or navigation.
a +3 very good; +2 good, +1 fair, 0 no change, -1 weak, -2 poor; -3 very poor.
Due to lack of data, we have not investigated sedimentation in the North Fork,
although flood and ebb flows have been simulated (Patra, 2003). The value 140 tons/y of
fine material transported in to fork (Table 3.4) from the central embayment is based on
turbidity current-induced sediment movement thought to be the mechanism by which the
lower reach of the fork receives muck (Ganju et al., 2001). However, this value may be
an upper limit. The actual modes and rates of sediment transportation remain unknown.
4.2 Monitoring
As a continuing basis for an assessment of sediment quantity and quality, it will
be essential to carry out bathymetric surveys and sediment sampling by coring at selected
sites on a regular basis. The following recommendations are made:
1. Bathymetric survey of the western zone of the estuary within the boundaries of
the Jupiter Inlet District. The approximate region is shown in Fig. 4.1. Spacing
between survey lines must be no more than about 30 m. Frequency once every
four years.
Figure 4.1 Pushcore measurement sites for monitoring.
2. Concurrent survey of the eastern zone, including the open coast.
3. Pushcore collection every four years at 20 sites noted in Fig. 4.1 and in Table 4.2.
Analysis to include vertical profiles of physical and textural properties (bulk
density and grain size), organic content, and radioisotopic chronologies where
mud content is > 10% (Jaeger and Hart, 2001).
Table 4.2 Pushcore coordinates
Station
C00-1
C00-2
C00-3
C00-4
C00-5
C00-6
C00-7
C00-8
C00-9
C00-10
C00-11
C00-12
C00-13
C00-14
COO-15
C00-16
COO-17
C00-18
C00-19
C00-20
Longitudea
-80.13537780
-80.12649902
-80.11015473
-80.10413349
-80.13222221
-80.13107952
-80.12240888
-80.12626562
-80.11519199
-80.11961375
-80.11312490
-80.12221510
-80.12001757
-80.11878391
-80.11516746
-80.11192239
-80.10779360
-80.10198353
-80.10825158
-80.10462085
a NAD83 Datum.
Latitude
26.97835886
26.97201160
26.97088019
26.96399885
26.93913071
26.93971051
26.94332223
26.96569957
26.94388203
26.94264576
26.94543090
26.96702861
26.96068628
26.95738335
26.95605165
26.95362077
26.95213217
26.95433425
26.94770232
26.94568413
5. Concluding Comments
Due to the low rates of sediment transport, and because the main-stem river has
been largely protected from human encroachment, the western zone of the estuary
including the central embayment and the three forks appear to function reasonably close
to "quasi-equilibrium" as far as sedimentation is concerned. To the extent that deviations
in this condition exist, in particular with respect to net sedimentation in the forks and the
embayment, the main causes appear to be:l) the effect of relative sea level rise creating
room for net sedimentation, 2) exposure of erodible substrate at river banks and river
flood plain by removal of mangroves, top vegetation and topsoil, and 3) release of
organic-rich sediment from die-back of vegetation by salinity intrusion due to diversion
of freshwater, e.g., by the construction of the C-18 Canal bypass.
Regular dredging of sediment in the eastern zone of the estuary for navigation
purposes "invites" littoral sediment due to the ensuing sedimentary disequilibrium.
However, as mentioned the material is transferred back to the littoral system by hydraulic
dredging. This protocol has functioned well and has maintained depths in the majority of
this zone.
The present protocol of maintaining the existing navigation channel in the central
embayment by dredging is tantamount to removal of sandy sediment that arrives from the
eastern zone and the forks. This procedure is essential for preventing excessive shoaling
in the embayment, although it appears that the amount presently removed (which is not
annual but works out tol,200 m3/y over the period 1988-2003; see Table 5.1) is less than
the -3,000 m3/y deposited in the embayment.
Table 5.1 Dredging in the central embayment navigation channel
Dredged volume
Year
(m3)
1988 1,910
1994 3,820
1997 5,730
2003 7,650
Of the management alternatives considered, only two appear viable from the point
of view of the combined effect of sediment control and navigation. The first of these is
dredging Bay Channel South, and the second dredging of the C-18 Canal. Bay Channel
South is a strong candidate because it would take advantage of the existing flood and ebb
flow patterns and thereby act as a means for efficient flushing of waters. Secondly it
would draw sediment presently not captured by the existing channel, and thereby act as a
additional means for maintaining depths throughout the embayment.
If at the time of its design it is found that Bay Channel South would disturb sea
grass beds to an unacceptable degree, Bay Channel North should be considered as the
second best option. The south arm of this channel will require regular dredging (Fig. 3.9);
however, sea grass beds do not seems to occur in its path. This channel will also serve as
an efficient conduit for flushing of Southwest Fork.
Some concern may exist regarding the effect of dredging additional channels in
the embayment on the flow distribution in adjacent areas, especially those where sea
grass occurs. However, model-based calculations (Ganju et al., 2001; Patra, 2003) do not
indicate strong effects because in all cases, with the exception of the south arm of Bay
Channel North, the channels would be generally laid along existing flood and ebb flow
pathways. In that regard it needs to be noted that the shoal in the central embayment
serves to define the natural flow channels. In particular, its presence allows
comparatively strong flows to occur through the present navigation channel, and also
along the southern rim of the embayment where Bay Channel South is proposed.
With regard to the trapping potential of the C-18 Channel, both the North and the
South Channels must be considered concurrently in order to optimize the trapping
efficiency of the North Channel. Decision concerning the implementation of the project
must await the next suite of monitoring data on bathymetry and coring. Since such
monitoring was carried out in 2001, the next collection should be in 2005.
A noteworthy feature of the low-energy Loxahatchee estuary is that it has low
currents and low rates of sedimentation, and consequently long time scales of response to
changes. Thus, while it cannot maintain self-cleaning design channels, low rates of
shoaling make it convenient to manage sedimentation in the existing channels.
With respect to the fine-grained sediment, the rates of shoaling are
characteristically supply controlled, and the load carrying capacity of existing flows is
believed to be substantially higher than at present because the system is sediment-starved.
This in turn means that if supply is increased, e.g., due to further effects of urbanization,
the material will deposit and zones of comparatively high concentrations of muck will
increase. Analysis of recent sedimentation based on pushcores appears to support this
inference (Jaeger and Hart, 2001). Continued increase in fine sediment supply may
eventually degrade water quality in the embayment by increasing turbidity.
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