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UFL/COEL/-2003/010
SEDIMENT MANAGEMENT IN LOW ENERGY
ESTUARIES: THE LOXAHATCHEE, FLORIDA
by
Rashmi Ranjan Patra
Thesis
2003
Coastal & Oceanographic Engineering Program ,?i
Department of Civil & Coastal Engineering
575 Weil Hall* P.O.Box 116580 Gainesville, Florida 32611-6580
UNIVERSITY OF
FLORIDA
UFL/COEL/-2003/010
SEDIMENT MANAGEMENT IN LOW ENERGY
ESTUARIES: THE LOXAHATCHEE, FLORIDA
by
Rashmi Ranjan Patra
Thesis
2003
SEDIMENT MANAGEMENT IN LOW ENERGY ESTUARIES: THE
LOXAHATCHEE, FLORIDA
By
RASHMI RANJAN PATRA
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2003
ACKNOWLEDGMENTS
My sincere gratitude is reserved for Dr. Ashish J. Mehta for his guidance in my
education and research, which made my studies very precise and rewarding, as well as
the entire Coastal and Oceanographic Engineering Program faculty. Also deserving my
gratitude for their guidance and assistance are Dr. John Jaeger, Dr. William McDougal,
and Kim Hunt. Most of the analysis in the study was made possible by the valuable
assistance provided by Mr. Sidney Schofield, who taught me the basics of analysis.
Special thanks are due to Dr. Earl Hayter for setting up, supporting and guiding me
through the numerical model in its entirety.
Thanks are also due to Dr. Zal S. Tarapore, for his encouragement and guidance,
which marked my initial years as a coastal engineer and my studies here possible. My
wife Sumitra and my friend Anjana also deserve special kudos for their emotional and
editorial support. Finally, my mother and father merit unlimited praise for providing me
with mind, body and soul, as do my other friends and families for developing it.
TABLE OF CONTENTS
page
A CKN OW LED GM EN TS .............................................. .. .................................... ii
LIST OF TABLES ............................................................................ vi
LIST O F FIG U RES ................................................... ................................................ viii
L IST O F SY M B O L S ........................................... ... ...... ........................................... xii
A B STRA CT ............................................................ ................... ... ............................... xv
CHAPTER
1 IN T R O D U C T IO N ..................... .................. .... .....................................................
1.1 Problem Statement..................................................... .. ................
1.2 S tu dy T ask s................................................................... ................................3
1.3 O outline of C chapters ....................................................................................... 3
2 SEDIMENT MANAGEMENT ALTERNATIVES ......................................... ....4
2.1 Present Condition of the Loxahatchee Estuary................................................4
2.2 C -18 C anal .................................. .................. .. ............................ 12
2.2.1 Present C condition ............................................................ ............ .........12
2.2.2 M management Options....................................... ............17
2.3 Central Em baym ent ......................... ....... ... .. .. ......................... 21
2.3.2 Management Option: ....................................................27
2.4 N orthw est F ork: ............................... .............. ... ............................... ... ... 28
2.4.1 Present Condition:.............................................. ...... ..................28
2.4.2 Management Options.................................... ...............32
2 .5 N north F ork ....................................................... ............................................33
2.5.1 Present C ondition...................................................................................33
2.5.2 M anagem ent O ptions.................................................... ....................... 34
3 DATA COLLECTION................................ .......................35
3.1 Field Setup in the Southwest Fork........................... ..... ........................35
3.2 Instruments Deployed ................................. ................37
3.2 .1 C current .................................................................................................37
3.2 .2 T ide ........................................................................................................38
111
3.2.3 Salinity/Tem perature......................... ........ ............................... 38
3.2.4 Sediment Concentration..................... ............................39
3.3 Field Data Results in Southwest Fork....................... .................. ...........41
3 .3 .1 C current .................................................................................................. 4 1
3.3.2 Tidal Level........................................................................................ 44
3.3.3 Total Suspended Solids............................. .........................................47
3.3.5 Other Data Blocks.................................. .... ............................... 55
3.4 Field Data Results in Northwest Fork........................ ................... ...........56
3.4.1 Field Setup ........................................................................................ 56
3.4.2 Tidal Level........................................................................................ 56
3.4.3 Total Suspended Solids................................ ....................................58
3.4.5 Additional Data Blocks.................... .................................................59
3.4.5.1 Tidal Level ..................................................59
3.4.5.2 Total Suspended Solids ......................... .............................60
4 MODEL CALIBRATION AND VALIDATION ..................................... .....63
4.1 M odel D description ........................................ ................ ........................... 63
4.3 Grid Generation ........................ ..............................69
4.4 Boundary Conditions ....................... .............. .......................72
4.5 Model Calibration and Validation ...................... ................................78
4.5.1 Calibration........................... ...... ......................... 78
4.5.2 Model Validation ............................... ............................... 80
4.5.3 Simulation of trap scheme of Ganju, 2001 ...................................................86
5 EVALUATION OF SEDIMENTATION CONTROL ALTERNATIVES ..............87
5.1 D design B asis ...................... ....................... ...... ......................................... 87
5.1.1 General Principle ................................... ............................... 87
5.1.1.1 Sediment Entrapment .......................................................................87
5.1.1.2 Self-cleaning Channel ...................................................... ..............88
5.1.2 D esign A lternatives.......................................... ................................ 89
5.1.2.1 Alternative No. 2: C-18 Canal Trap............................................... 90
5.1.2.2 Alternative No. 3: Bay Channel .................................................91
5.1.2.3 Alternative No. 4: Bay Y-channel..................................... ...93
5.1.2.4 Alternative No. 5: Northwest Fork Channel ..................................94
5.1.3 Efficiency Analysis ....................... .............................................. 94
5.1.3.1 Velocity Vector Calculation... ...............................................94
5.1.3.2 Sediment Deposition Calculation...............................................95
5.1.3.3 Trap Efficiency....................... ... ..... ...................... 97
5.1.3.4 Channel Efficiency....................... .. ...............................98
5.2 D design Sim ulations..................................... ................................................98
5.2.1 D design Flow s ........................ .. .................................. ... .... 98
5.2 .2 A alternative 1 ................... .................................... .......................98
5.2.3 Alternatives 2, 3, 4 and 5 .............................. ..................... .............99
5.3 Deposition Equation Calibration........................ ........................................ 102
5.3.1 Calibration for Sand ...................................... ...........................102
5.3.2 Fine Sedim ent....................................................................................... 102
5.4 Sand Deposition due to Alternatives......................... .............................103
5.4.1 Bay Channel......................................... ........................................ 103
5.4.2 C -18 C anal ........................ ........................ ............................103
5.4.3 Bay Y-channel .......................................................................................104
5.5 Fine Sediment Deposition due to Alternatives ...........................................104
5.6 Sedim ent Rem oval ................................. .......... .......... ...................... 105
5.6.1 Calculation of D position ................................................................ 105
5.6.2 Calculation of Channel Efficiency....................................................... 106
5.6.3 Removal of Bay Sediment ............................... .................... ......... 107
5.7 Assessment of Alternatives......................... .............................107
6 C O N C LU SIO N S .............................................................................................. 109
6.1 Sum m ary ................................... ... ............................................................. 109
6.2 C onclusions................................. ..... ........................................................ 110
6.3 Recommendations for Future Work........................................................... 112
LIST OF REFERENCES ............................................................................................ 113
BIOGRAPHICAL SKETCH ..................................................................................... 116
LIST OF TABLES
Table page
2.1 Basin area distributions in the Loxahatchee River estuary watershed.......................7
2.2 Statistical tributary flow (based on Figures 2.6 a-c) ............................................... 14
2.3 Median and high flow concentration data and coefficients for equation 2.1 ...........15
2.4 Spring/neap tidal ranges and phase lags for three gauges......................................27
3.1 Instrumentation for data collection and data blocks...........................................36
3.2 Discharge data for the period 04/14/2002 to 04/21/200...................................41
3.3 Typical mean current magnitude values for data blocks..........................................44
3.4 Characteristic values of the tidal data .......................... ............................47
3.5 TSS concentrations for the representative data blocks..................... ......... 51
3.6 Characteristic salinity values......................................................... .....................51
3.7 Characteristic tem perature values ........................................................................54
3.8 Summary of parametric value (Days 37-59 in year 2003).....................................55
3.9 Summary of parametric value (Days 90-101 in year 2003) ...................................55
3.10 Summary of parametric value (Days 101-135 in year 2003)...................................56
3.11 Characteristic values of the tidal data .................................................58
3.12 TSS concentrations for the representative data blocks.................................. ...58
3.13 Characteristic values of the tidal data ................................. ..........................60
3.14 TSS concentrations for the representative data blocks......................................62
4.1 Definition of cell type used in the model input............................................69
4.2 Amplitude and phase correction factor for the tides ..............................................77
5.1 Alternative schemes for evaluation...................................................................89
5.2 Critical velocities for sand...................................................... 96
5.3 Design flows in tributaries ................................................................................. 98
5.4 Maximum currents at alternatives: calibration discharges...................................100
5.5 Maximum currents at alternatives: Different discharges .....................................100
5.6 Calibration for sedim ent fluxes........................................................................ 102
5.7 Rate of sand deposition in bay channel................................................................103
5.8 Rate of sand deposition in C-18 canal.............................................................. 103
5.9 Rate of sand deposition in Y-channel .......................................................104
5.10 Rate of fine sediment deposition in alternatives ..................................................105
5.11 Annual sand budget: Calibration discharge ...................................................105
5.12 Annual sand budget: Peak discharge....................................... ................ 105
5.13 Annual fine sediment budget: Calibration discharge ............................................ 106
5.14 Annual fine sediment budget: Peak discharge ...........................................106
5.15 Annual sedim ent loading........................................... ..................................106
5.16 Assessment of impacts of proposed alternatives..................................................108
LIST OF FIGURES
Figure page
2.1 Location m ap of the study area ............................. ................. ....................... 5
2.2 Loxahatchee River estuary and tributaries............................................................... 5
2.3 Hydrographic survey of the estuary (November 2001).......................................... 7
2.4 Loxahatchee River estuary watershed basins, estuarine limits (dashed arcs) and
central em baym ent ...................................... .... ...............................................8
2.5 Ariel photograph showing development of new shoal...........................................11
2.6 Cumulative discharge plot. a) Northwest Fork. b) North Fork.
c) Southw est Fork....................... ... .... ..... ....... ... ............................ 14
2.7 Dredging Plans for C-18 canal, 1956 ................................................16
2.8 Current variation under the effect of released discharge from the S-46
structure. ....................... ......... ... ..... ............. ................................. 19
2.9 Effect of S-46 discharge on the suspended sediment concentration......................19
2.10 Arial Photograph showing the Central Embayment, the Inlet and the
T tributaries ....................................... ........ .. ... ....... ...........................23
2.11 Location of tide gauges marked UFG1, UFG2 and UFG3.....................................26
2.12 Sample records of tidal measurements at three locations (09/14/00-09/15/00)-
D atum N A V D 88. ....................................................... ..................................27
2.13 Location of stream-gauging stations and sampling site for suspended
sedim ents, ......................... ........ ....... ....................................................... 30
2.14 Location indicating fresh mud depositions and the Shoals the estuary ..................33
3.1 Location of instrument tower in the Southwest and Northwest Forks...................35
3.2 Calibration plots used for calibration of OBS sensors...........................................40
3.3 Record of current magnitude: Days 94-114 (year 2002)........................................42
3.5 Record of current magnitude: Days 332-356 (year 2002)......................................43
3.6 Record of current direction: Days 332- 356 (year 2002). ......................................43
3.7 Water level time-series: All levels relative to NAVD 88. Days 94-114 (2002) .....44
3.8 Water level time series: Upper plot shows original time series with mean trend
and the lower plot is without the mean oscillations. All levels relative to
NAVD 88. Days 94-114 (2002). ...................... .. ..... .............. ................45
3.9 Water level time-series: All levels relative to NAVD88. Days 332- 365 (2002)
and D ays 01-35 (2003) ........................ ......... ... ...... ............................... 45
3.10 Water level time series: Upper plot shows original time series with mean trend
and the lower plot is without this trend. All level relative to NAVD 88. Days
332-365 (2002) and Days 01 35 (2003). ................................................46
3.11 TSS time-series at four elevations: Days 94-114 (year 2002). ..............................48
3.12 TSS time-series at three elevations: Days 352- 365 (year 2002) and 01-35
(year 2003). ..................... ...... ..................................................................... 48
3.13 Depth-mean TSS concentration time series: Days 94-114 (year 2002) ...................49
3.14 Depth mean TSS concentration time series: Days 352- 365 (year 2002) and
D ays 01 35 (year 2003). ..................... ... .............. ...................... ... .... 49
3.15 Depth mean TSS concentration time series and tidal trend indicating their
dependence: Days 352- 365 (year 2002) and Days 01 35 (year 2003). ................50
3.16 Salinity time series: Days 94-114 (year 2002)..........................................52
3.17 Salinity and Current magnitude time series: Days 94-114 (year 2002). .................52
3.18 Temperature time series: Days 94-114 (year 2002)............................................... 53
3.19 Salinity time series: Days 352- 365 (year 2002) and 01-35 (year 2003). ................54
3.20 Temperature time series: 352- 365 (year 2002) and 01-35 (year 2003)...................54
3.21 Record of water level variation. Days 245 -255 (year 2003). ..............................57
3.22 Water level time series: Upper plot shows original time series with mean trend
and the lower plot is without the mean oscillations. All levels relative to
NAVD 88. Days 245 -255 (year 2003). ........................ ................................57
3.23 Depth-mean TSS concentration time-series: Days 245-255 (year 2003).................58
3.24 Record of water level variation. Days 310.5 313.5 (year 2003). ........................59
3.25 Water level time series: Upper plot shows original time series with mean trend
and the lower plot is without the mean oscillations. All levels relative to
NAVD 88. Days 310.5 313.5 (Year 2003)........................................................... 60
3.26 TSS time-series at two elevations: Days 310.5 -313.5 (year 2003). ....................61
3.27 Depth mean TSS concentration time series: Days 310.5 313.5 (year 2003).........61
3.28 TSS time-series at three elevations: Days 315.5 318.5 (year 2003)....................62
3.29 Depth-mean TSS concentration time series: Days 315.5 318.5 (year 2003). ......62
4.1 Model domain showing input bathymetry and shoreline........................................71
4.2 Computational grid showing the flow boundaries ..................................................72
4.3 Tidal time series from UFG1, 09/14/00-10/13/00, a) Raw data, b) Tidal plot
after the mid-tide trend is removed. .......................... ...............................74
4.4 Tidal time series from UFG3, 09/14/00-10/13/00, a) Raw data, b) Tidal plot
after the mid-tide trend is removed. ............................................................... .............76
4.5 Flow time series applied at S-46 boundary ........................ ........................77
4.6 Flow time series applied at Northwest Fork boundary...........................................77
4.7 Model calibration measured vs. predicted current, a) Cold Start, b) Hot Start........81
4.8 Model calibration measured vs. predicted current direction..................................82
4.9 Model calibration measured vs. predicted water surface elevation (UFG2) Year
2000, a) Cold Start, b) Hot start. ........................ ..... ..........................................83
4.10 Model calibration measured vs. predicted water surface elevation (UFG3) Year
2000, a) Cold Start. b) Hot start. ....................... ... ............................ ................84
4.11 Model calibration measured vs. predicted water surface elevation
(Northwest Fork) Year 2003, a) Cold Start, b) Hot start........................................85
4.12 Validation results using trap used by Ganju, 2001.................................................86
5.1 Design concepts for sediment management...............................................88
5.2 Alternatives considered, with existing bathymetry..............................................90
5.3 Dredged bathymetry of the C-18 canal with plan form view of the proposed
trap. Trap considered by Ganju (2001) is also shown............................................91
5.4 Planform view of the proposed self-cleaning channel in the bay. .........................92
5.5 Location of the sea grasses indicated in model with increased roughness...............92
5.6 Planform view of the proposed self-cleaning Y-channel in bay............................93
5.7 Planform view of the proposed self-cleaning channel in the Northwest Fork.........94
5.8 Current comparisons for a model cell at the upstream end of the Northwest
Fork channel: calibration discharges..................................................99
5.9 Current velocity vectors over the modeled domain; maximum flood velocities
at spring tides. ............................................ .......................... ....................... 10 1
5.10 Current velocity vectors over the modeled domain; maximum ebb velocities at
spring tide.................................... ...................................................... 101
LIST OF SYMBOLS
A area
A, vertical turbulent viscosity
B width of the basin
C wave celerity
CO uniformly distributed initial sediment concentration (kg/m3)
C, sediment concentration (kg/m3)
C,, constant multiplier for u-velocity conversion to true east
c,, constant multiplier for u-velocity conversion to true north
CL, constant multiplier for v-velocity conversion to true east
Cc,, constant multiplier for v-velocity conversion to true north
D sediment deposition under reduced flow
De,, deposition at the entrance to the channel
Dx deposition at the exit of the channel
D,, dimensionless projected vegetation area
H total water column depth
L length of the channel/trap
K coefficient of conductance
QH volume source or sink
Rq Richardson number
T time
U steady mean flow velocity
U velocity vector
W width of the channel in equation 5.8
FW settling velocity
b buoyancy
cp vegetation resistance
f Darcy-Weishbach friction factor
f, coriolis acceleration
g acceleration due to gravity
h water depth
mx scale factor along x-axis
mV scale factor along y-axis
q turbulent intensity
(q,), amount of sediment in influent
(q ,), amount of sediment in effluent
r removal ratio
u velocity along the channel (x-axis)
u, friction velocity
ui. velocity amplitude under current
uc(n) current at the entrance
u(ex) current at the exit
ur, critical velocity for erosion
uco curvilinear-orthogonal horizontal velocity
u,, velocity in true east direction
v velocity across the length of the channel(y-axis)
v,.o curvilinear-orthogonal horizontal velocity
vI, velocity in true north direction
z variable water depth
p water density
Po reference water density
r, bed erosion shear stress
rbx x-component shear stress
r, y-component shear stress
rb bed bottom shear stress
r7 mid-tide elevation
r/71T high-tide elevation
r7LT low tide elevation
E vertical diffusivity
K Karman constant
Free surface potential
0 velocity angles
Abstract of Thesis to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
SEDIMENT MANAGEMENT IN LOW ENERGY ESTUARIES: THE
LOXAHATCHEE, FLORIDA
By
Rashmi Ranjan Patra
December 2003
Chairman: Ashish J. Mehta
Major Department: Civil and Coastal Engineering
Implementation of schemes for sediment entrapment and self-cleaning channels
was examined in the micro-tidal estuarine environment containing both sand and fine
sediment. The central embayment of the micro-tidal Loxahatchee River estuary on the
Atlantic Coast of Florida was chosen as the candidate location due to its unique
characteristics with respect to the influx of sand and fine sediment in its central
embayment, and concerns regarding the potential for long term impacts of this flux on the
embayment. An ideal sediment trap captures all of incoming sediment, i.e., the removal
efficiency is 100%. A self-cleaning channel allows no net deposition of incoming
sediment, which passes through, so that its removal efficiency is nil.
Hydrodynamic model simulations were carried out for selected trap/channel
alternatives, and their efficiency was calculated by relating sediment deposition to change
in the flow regime due to implementation of these alternatives. Calculations indicated
that the concepts of sediment entrapment and of self-cleaning can operate only
imperfectly in the study area due to the low prevailing forcing by tide and the episodic
nature of freshwater discharges in the tributaries.
Fine sediment accumulation in the central embayment can be reduced by dredging
the C-18 canal, as the trapped sediment would account for more than half of the total fine
sediment entering the bay. A channel close to the southern bank of the embayment could
improve bay flushing by ebb flow, reduce bay-wide sedimentation and serve as a
navigation route. Careful design with regard to channel alignment would be required to
avoid sea grass beds in the area. Long term simulations of flow and sediment transport
are required to assess sediment circulation patterns and the formation of shoals in the
central embayment and the Northwest Fork.
CHAPTER 1
INTRODUCTION
1.1 Problem Statement
Sedimentation due to the influx of fine and coarse particles is an issue affecting
numerous estuaries and coastal waterways. Often enough, these particles originate far
inland, and are transported into the coastal zone by runoff and stream flow. In the
estuarine regime, inorganic sediment almost never occurs in isolation, as it is
complemented by measurable organic fraction produced by either indigenous sources
(e.g., native phytoplankton, swamp vegetation, wind blown material), or allochthonous
sources (e.g., river-borne phytoplanktons, swamp vegetation, windblown material)
(Darnell, 1967). In turn, such organic-rich sediments can degrade water quality by
oxygen uptake and a reduction in light penetration. In this study, the question of
preemptive dredging of sediment prior to its deposition in an area of concern or, as an
alternative, preventing its deposition in the area of concern by channelizing flow, was
studied. The candidate water body was the estuarine segment of the Loxahatchee River
on the east coast of Florida.
Loxahatchee River, which discharges mainly through its Northwest Fork, supplies
mainly quartz sand and organic detritus. Clay mineral makes up less than 5% of the mud
in the estuary, but because this mud is rich in organic matter, its accumulation has
become a matter of concern in the central embayment of the estuary. This flow, in
addition to controlled discharges from the S-46 structure in the C-18 Canal at the head of
the Southwest Fork, brings in much of the sediment (mean concentration 0.014 kg/m3;
Sonnetag and Mcpherson, 1984) in the central embayment.
A commonly employed solution to reduce sedimentation is the implementation of a
trap scheme by trenching the submerged bottom. Such a trench-trap is a means to
increase the depth at the chosen location by dredging. Increased depth results in a
decreased flow velocity (and associated bed shear stress), thereby allowing incoming
sediment to settle in the trap, instead of being carried further downstream. The removal of
sediments becomes much easier as it can be then be removed from the trap, rather than
dredging the otherwise distributed deposits from a considerably broader area. As an
alternative to sediment entrapment, creating a self-cleaning channel in the area of concern
for sedimentation would mean that sediment would pass through the system, without
deposition. The degree to which both approaches can function depends on the flow
conditions, type of sediment and the morphology of the estuary.
Given the above background, the objectives of this study were: 1) to determine the
efficiency of traps installed at selected locations in the estuary, and 2) to evaluate the
efficiency of channels as a means to pursue the goal of a self-cleaning sedimentary
environment.
Shoaling has occurred the Loxahatchee in many areas, especially near the
confluences of the major tributaries (Northwest Fork and Southwest Fork) in the central
embayment where the velocities are typically low (Sonntag and McPherson, 1984).
Recent studies (Jaeger et al., 2002) suggest internal recirculation of sediments as an
important factor governing sediment transport within the estuarine portion of the river.
Accordingly, in order to manage sedimentation in the central embayment, it may be
desirable to test trap/channel deployments at multiple locations. The performance of these
schemes was evaluated with regard to efficiency of sediment removal.
1.2 Study Tasks
The tasks undertaken included:
1. Data collection from the site and scrutiny of data from the existing literature to
characterize the nature of flow, sediment transport and sedimentation. This
included measuring tidal elevations, current velocities, sediment concentrations and
bed sediment distribution (Jaeger et al., 2002) in the estuary, and obtaining stream
flow data for the tributaries from the literature.
2. Simulating the flow field using a hydrodynamic model, in order to determine the
velocities, water surface elevations and bed shear stress distributions.
3. Introduction of trap schemes in the calibrated flow model to determine flow
velocities with and without the trap, and development of relationships for
calculating trap efficiency.
4. Introduction of self-cleaning channels and an assessment of their viability.
5. A qualitative assessment of the usefulness of the approaches based on selected
criteria.
1.3 Outline of Chapters
Chapter 2 describes the sediment management alternatives including existing
conditions and the proposals for implementation. Chapter 3 deals with the field data
collection for this study including data analysis and interpretation. Flow model
calibration and validation is included in Chapter 4 and evaluation of management
alternatives is described in Chapter 5. Summary of the results and conclusions are made
in Chapter 6, followed by a bibliography of studies cited.
CHAPTER 2
SEDIMENT MANAGEMENT ALTERNATIVES
2.1 Present Condition of the Loxahatchee Estuary
Loxahatchee River empties to the Atlantic Ocean through the Jupiter Inlet located
in northern Palm Beach County on the south coast of Florida, about 28 km south of St.
Lucie Inlet and 20 km north of Lake Worth Inlet. The three main tributaries, which feed
the estuary, are the Northwest Fork, the North Fork, and the Southwest Fork. In addition,
the Jones Creek and Sims Creek, which are far lesser tributaries than the others, also feed
the estuary through the Southwest Fork. Figures 2.1 shows the general location map of
the study area.
The major surface flow in to the estuary historically was through the Northwest
Fork draining the Loxahatchee Marsh and Hungry land slough (refer Fig 2.1). The
upstream reach of the Southwest Fork, referred to as the C-18 canal, was created in
1957/58 in the natural drainage path in order to lengthen the area of influence of the
Southwest Fork and facilitate drainage of the westward swampland (Refer Figure 2.1 and
Figure 2.2). The flow in the canal is regulated by the S-46 automated sluice gate
structure. Whereas, the Southwest and the Northwest fork converge on the estuary
approximately 4 km west of the inlet, the North fork joins the central bay about 3 km
west of the inlet. Down stream of the Florida East coast Railroad (FECRR) Bridge the
Intracoastal Waterway (ICWW) intersects the estuary in a dogleg fashion. Five
navigation/access channels exist on the south shore of the central embayment
r
r--
Xy.
L
- J,
- r ~: ,^ r
1..
I I .
Figure 2.1 Location map of the study area (Source: U.S. Geological Survey report no.84-
4157, 1984)
4V
W.
*- /- '"'"" '- S (
S Dolph
6- 6 C otWi01 tf>uclue ff I I
Figure 2.2 Loxahatchee River estuary and tributaries
A detailed hydrographic survey of the central embayment (Figure 2.3) and the
Northwest and Southwest Forks carried out in November' 2001 (Lidberg Land
Surveying, Inc) indicates the depths in the estuary, which range between 0 m (reference
to North Atlantic Vertical Datum 1988, (NAVD 88)) near the sandy shoals to almost 6 m
13
I-
..
' .. ....... ....... . __-
-Ol*'r'i*tl ill ll(u~i3,
tq*...Z U.l.ltilU(*
;P '19 *:fl
in the entrance channel near the FECRR Bridge. The average depth over the embayment
is just over 1 m. The navigation channel (maintained by the Jupiter Inlet District) runs
westward from the Inlet, under the FECRR bridge, and through the central embayment
approximately 14 lkn upstream from the Inlet. The navigation Channel has a bottom
width of about 30.5m (100 feet) and is maintained at 1.75m(5.74feet) (reference to
National Geodetic Vertical Datum 1929, (NGVD 29) and 2.21m (7.24feet) with
reference to NAVD 88) with a side slope of 1:3. Flood shoals, which approximately
bisects the central embayment exists mainly due to the sand influx from the ocean, and
smaller shoals exist at the termini of the three main tributaries. Small shoal islands are
located west of the FECRR bridge, on both sides of the channel.
The Northwest Fork and North Fork are natural tributaries draining in to the central
embayment. However, as mentioned the Southwest Fork was lengthened westward by
construction of C-18 canal with a control structure (S-46), in order to divert flow from the
Northwest Fork to the Southwest Fork. A channel was then constructed allowing the
diversion of flow from the Northwest Fork to the Southwest Fork. For easy reference
from this point on, the C-18 canal will be indicated as the narrow channel section and the
broader section at the root will be called Southwest Fork (Figure 2.2).
S Bottom EI .
4" A. .
main tributaries, the ICWW, and several minor tributaries. The individual watershed
basins are shown in Figure 2.4 and listed in Table 2.1. The watershed constitutes
residential areas, agricultural lands, and uninhabited marsh and slough areas.
Table 2.1 Basin area distributions in the Loxahatchee River estuary watershed
Basin Area (kin2)
Intracoastal Water way 545
C- 18 Canal 278
Jonathan Dickinson 155
South Indian River 65
Loxahatchee Rive 6
i -' -i r i l H, .in .f.nil ...n.
'-'-ii-- -- -' '
i -:- .i ._ \. 11 1' 2 3^ m H"
Figure 2.4 Loxahatchee River estuary watershed basins, estuarine limits (dashed arcs)
and central embayment
Unlike more northerly estuaries, upland drainage in to the Loxahatchee provides
only quartz sand and organic detritus. Clay mineral makes up less than 5% of the mud in
the estuary (McPherson, 1984). Earlier studies indicate that the estuary was periodically
open and closed to the sea due to various reasons. Originally, flow from the Loxahatchee
River along with that from Lake Worth Creek and Jupiter Sound kept the inlet clean.
With the construction of the ICWW and the Lake Worth inlet and the modifications of
the St.Lucie Inlet in 1970, some flow was diverted. Subsequently, Jupiter Inlet generally
remained closed until 1947, except when it is dredged periodically. After 1947, it was
fonsihkin D~irkinwn ff-gr
maintained open by dredging by the Jupiter Inlet District and the U.S. Army Corps of
Engineers.
Dredge and fill operations have also been carried out in the estuary embayment and
forks. In the early 1900's, there was significant amount of filling at the present FECRR
Bridge, which narrowed the estuary from 370 m to 310 m. The areas east and west of the
bridge (and also under the bridge) were dredged in mid-1930's, and also in 1942. The
material was high is shell content and was used in construction of roads. In 1976-77,
additional estimated 23,000 m3 materials were removed from the estuary at the bridge
and from an area extending 180 m from the west. Some dredging was also carried out in
the Southwest Fork near the C-18 canal in the early 1970's (Wanless, Rossinsky and
McPherson, 1984). In 1980, three channels were dug in the embayment, and an estimated
23,000 m3 of sediment were removed.
After 1900, the estuary was greatly influenced by the dredging and alteration of the
drainage to the basin. With gradual lowering of the water table and resultant effect on the
water quantity, the direction and pattern of inflow (McPherson and Sabanskas, 1980)
were considerably affected. Historically, the major surface flow to the estuary was in to
the Northwest Fork from the Loxahatchee Marsh and the Hungry-land Slough (Figure
2.1), both of which drained north. A small agricultural canal was dug before 1928 to
divert a small amount of water from the Loxahatchee Marsh to the Southwest Fork. As
noted, in 1957-58, C-18 canal was constructed along the natural drainage way to divert
flow from the Northwest Fork to the Southwest Fork of the estuary.
Jaeger et al., (2001) carried out extensive studies in the estuary to reevaluate the
nature of enviromnental sedimentology in the lower Loxahatchee River Estuary and as a
companion study to Ganju et al. (2001). Specifically, new samples were collected in
order to 1) examine changes in surficial sediment types between 1990 and 2000, 2)
attempt to determine the sources of fine-grained muddy sediments accumulating within
the estuary; and 3) examine rates of sedimentation within the central embayment and
three forks (North, Northwest, and Southwest/C-18 canal) by collecting a suite of -1-m
long pushcores and -3 m long vibracores within the estuary. Grab samples were collected
in all regions of the estuary and were analyzed. One of the main findings of the study was
the internal movement of the sediments in the estuary system. With the growth of the
population on the shoreline and associated human activities the mangroves dotting the
shoreline started vanishing. The removal of these Mangrove cover from the shoreline
released a large quantity of sediments, which was otherwise trapped in their roots.
Essentially fine grained, these sediments moved with the flow and started getting
deposited in the estuarine bounds. According, to this study new shoals were
developed/grown by this process, especially the submerged one in the Northwest Fork,
down stream of the shoal identifiable from a satellite map and Figure 4 of the Report
(Jaeger et al., 2001). The aerial photograph reproduced in Figure 2.5 also indicates an
additional shoal developing from the root of the existing shoal, suggesting that the
general nature sediments being fed by the Northwest Fork is coarse grained with the fine
grained ones carried downstream with the current before deposition.
Tidal flow into and out of the estuary is much larger than freshwater inflow from
all the major tributaries. Fresh water flow is reported to be about 2 percent of the total
tidal inflow (Sonnetag and McPherson, 1984). Tides are mixed semidiurnal (twice daily
with varying amplitude) with a tidal range of about 0.6 to 0.9 m. Tidal waves advances
up the estuary at a rate of 2.23 m/s to 4.46 m/s (McPherson and Sonnetag, 1984) and
shows little change in the tidal amplitudes over to about 16 km river km. Winds have a
significant effect on the tidal ranges especially the strong northeast winds which prevails
during autumn and winter for example can push in additional water into the estuary
affecting the tidal ranges.
Figure 2.5 Ariel photograph showing development of new shoal
Estuarine conditions extends in the estuary from the inlet for about 8 river km into
Southwest Fork, 9.6 river km in to the North fork and 16 river kin into the Northwest
Fork.
Of late, the environmental condition of the Loxahatchee River and the estuary has
become a matter of great concern. The major factor affecting the environmental health is
the sediment transported in to the estuary. Large amount of the sediments settling in the
basin might affect the bottom life, alter circulation patterns, and accumulate shoals,
thereby impeding boat traffic (McPherson, Wanless and Rossinsky, 1984).
2.2 C-18 Canal
2.2.1 Present Condition
The C-18 canal drains the Loxahatchee Slough, a shallow swamp-like feature
containing diverse flora and fauna. However, estuarine conditions persist for 8 km up the
Southwest Fork/C-18 canal measured from the inlet.
Flow data obtained from USGS stream flow gage data, for all available years
(1971-2002 N.W. Fork, 1980-1982 N. Fork, and 1959-2002 S.W. Fork) indicate that C-
18 canal/Southwest Fork carries a maximum discharge of 61.54 m3/sec. Cumulative
frequency distribution curves were constructed to designate (Figure 2.6 a-c) median and
extreme flow events (Table 2.2) for all the tributaries. The C-18 canal is regulated at S-46
structure, which is basically a gated sluice. The criterion for controlling the flow at the S-
46 structure is based on water level behind the structure. When the level exceeds a
predetermined mark, the sluice gates are opened until the level recedes by 30 cm (Russell
and McPherson, 1984), at which point the gates are closed. This regulation has resulted in
a discontinuous flow record; with weeks of no flow passing the structure, and days when
storm flows have been released. During normal wet season, the level behind the S-46
structure is not always sufficiently high for releasing flow, while the other tributaries are
freely discharging to the estuary.
U.9
0
3 0.8
05
t-
C 0.-
0,
Eo-
a
0) -
0.2
0.1
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Flow rate (m3Is)
1.1
1-
a 0.9
o
I 0.8
- 0.7-
0
a 0.6-
0
0.5-
> 0.4
| 0.3
E
0 0.2
0.1
ti
b
- ---r
i i--
1 -i--
-----------4- ---1-_--------------
----------- -----l-----------------
1 1-
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Flow rate (m3/s)
2 2.2
rl~-rl~~~~I
14
1.1
C
0.9 --- ----------- ------- ---------------
-
0.9
5 0.8
I 0.7
O 0.6
S0.5 ---
2. 0.4
E0.3
E 0 .3 - - - --- --- -- - - - -- - - -
0.2
0.1 ------
0
0 5 10 15 20 25 30 35 40 45 50 55 60 65
Flow rate (m3/s)
Figure 2.6.Cumulative discharge plot. a) Northwest Fork. b) North Fork. c) Southwest
Fork.
Table 2.2 Statistical tributary flow (based on Figures 2.6 a-c)
Tributary Median Flow High Flow Maximum Flow
(50%) (90%) (98%)
(m3/s) (nm/s) (m3/s)
Northwest Fork 0.7 4.1 76
North Fork 0.1 0.21 1.9
Southwest Fork 1.3 7.8 61
Sonnetag and McPherson (1984) reported two values of suspended solid sediment
concentration (0.059 kg/m3, 0.017 kg/m3) with corresponding flow data for the C-18
canal (31 m3/s, 28 m3/s, respectively) and a mean concentration value for duration (1980-
82) of their study (0.014 kg/m3). The median flow for the C-18 canal (1.3 m3/s) from the
Figure 2.6c was correlated to this mean value of concentration in the present study. A fit
in the form of (Miiller and FOstner, 1968)
C = aQb (2.1)
was used (Ganju et al., 2001), where ac and b, are site specific coefficients, with a, is
indicative of the erodibility of the upstream banks/bed and exponent b, is indicative of the
intensity of the erosional forces in the river.
Table 2.3 Median and high flow concentration data and coefficients for equation 2.1
Median flow High flow
Tributary Concentration Concentration a
_(kg/m3) (kg/m3) Coefficient Coefficient
Northwest Fork 0.011 0.023 0.012 0.27
North Fork 0.01 0.018 0.018 0.02
Southwest Fork 0.014 0.059 0.012 0.49
Fieldwork, consisting of bottom profiling and sampling, was carried out during July
2001 (Jaeger et al.,) by collecting a suite of -1-m long push cores and -3 m long
vibracores within the estuary. A total of 110 samples were collected from sampling
locations covering the entire estuary and river (Figure 1, Final Report on Sedimentary
processes in the Loxahatchee River Estuary, 5000 Years ago to the Present, Jaeger et al.,
2001) including from outcrops of regional surficial geological unit (undifferentiated 1.8
million year-old Pleistocene sediments) in order to examine the potential sediment
sources (Loxahatchee River, C-18 canal, Inlet, and Pleistocene-Age (last 1.8 million
years) sediments exposed along the banks of the C-18 canal. 56 of the samples were
reoccupations of sites sampled in 1990 and were reported (Mehta et al.,1992). These new
samples were collected to examine the changes in sediment characteristics pattern over a
10-year period. Positions of all sampling sites were determined by differential GPS
providing a position accuracy of -1 m. Each grab sample recovered approximately
1,000-2,000 cm3 of sediment, removing approximately the uppermost 1-5 cm of the
sediment surface. Sediment distribution maps produced from these grab samples indicate
16
particle sizes reveal that the majority of the estuary is dominated (by weight) of fine,
well-sorted sand in the -150 micron (3 phi; 0.15 mm) size range (Jaeger et al., 2001)
In the same study conducted by Jaeger et al., (2001), poling depths (obtained by
pushing a graduated pole into bottom until a hard substrate is reached) in the C-18 canal
were determined to estimate sedimentation rates along the length of the canal. Since the
bottom was dredged at the time of construction of the canal in 1957/58, the bed thickness
can be considered to represent the subsequent accumulation. This is because, the
dredging of the canal in 1958 would have most likely left behind a hard, sand rich
horizon that could not be easily penetrated with the solid rod. Figure 2.7 shows these
thicknesses along the canal length. Sediment thickness increases with the distance from
the S-46 structure, possibly due to the large erosional forces near the structure (when
flow is released), and reduction of these forces as the flow moves along the canal,
allowing more deposition of sediment.
E Natural bottom (pre-1957)
0
SL_ 3rc,--nc-. canal bottom
UL -----------I----,---
0 "' I C 1i .. 2000
Distance from ;. .tr;: int (m)
Figure 2.7 Dredging Plans for C-18 canal, 1956 (Source: Ganju et al., 2001)
This coarse sand layer was sampled at the base of push cores (see Figures 21 and
22 from Jaeger et al., 2001). There appears to be a trend of increasing thickness away
from the S-46 control structure (Figure 19). Modeling of sediment transport in the canal
(Ganju et al., 2001) also supports such a trend. The overall sedimentation rates (10-50
mm/yr) in the canal are very high for most coastal areas, where sedimentation has kept
pace with the rise in sea level (3-5 mm/yr) (Davis, 1994). However, this sampling
technique of poling only provides mean sedimentation rates over this 42-year (1958-
2001) time period. Analyses of push cores collected in the canal document alternating
layers of clean sand and muddy sand/sandy mud (see Figures 21-22, Jaeger et al., 2001).
This inter-layering of sediment types is characteristic of time-varying deposition
rates/erosion rates. When the sluice gates are opened, fast currents can erode the
sediment surface followed by rapid deposition of sand and mud. The best way to evaluate
time-varying sedimentation rates is with either accurate annual bathymetric profiles or by
measuring naturally occurring radioisotopes in the sediment cores (Jaeger et al., 2001).
2.2.2 Management Options
Dredging plans for the C-18 canal from 1956 is shown in Figure 2.7 (U.S. Army
Corps of Engineers, 1956). The existing bottom was deepened to 3 m at some locations to
facilitate drainage. The depths refer to the National Geodetic Vertical datum of 1929
(NGVD). The present mean depth of the canal as measured along the length is 1.2 m.
Hence there has been substantial sedimentation in the canal, which in turn means that it
no longer serves as a sediment trap and allows sediment to be transported to the central
embayment. One way of maintaining the depth in the canal is to devise a suitable
dredging option coupled with a designed flow regime in order to maintain the canal in the
self-cleaning mode. However, one of the main difficulties in this is the lack of continuous
supply of water. As described earlier, the flow in the canal is erratic and controlled by the
S-46 control structure. Accordingly, although the median flow in this canal is higher than
the other tributaries, the flow is episodic and therefore not enough to overcome the bed
shear resistance of the deposited sediments. This situation can be illustrated by data
collected during between April 4th and April 24th, 2002.
Figure 2.8 indicates the dependence of the current velocity on the released
discharge. The sudden jump in the over all current magnitude recorded downstream of
the structure therefore exhibit strong erosional trend as can be seen from the Figure 2.9.
In addition, it indicates that, sediment concentrations in the bottom layers are much more
pronounced due to the obvious reason of erosion of the bed. It can therefore be concluded
that, a sustained and regular flow regime would help keeping the canal sediment free.
An option is to increase the depth in the canal by dredging part or all of it, thereby
recreating the sediment trap. As an alternative, a detailed study of the flow pattern can be
undertaken and a suitable flow regime worked out. This would involve redesigning of the
control structure and a better regulation of the flow. However, the following points
should be noted:
1. The capacity of flow from the structure appears to be insufficient to flush out
sediment beyond 1.2 km (Ganju et al., 2001) from the structure even under "high"
discharges when the gates are open.
2. A potential option is to change the gate configuration but not the flow regulation
schedule. If changing the gate configuration from sluice to weir is successful, it
would create a sediment trap upstream of the gate, which would "buy time" for
the downstream reach of the canal, but this upstream trap would eventually have
to be dredged to maintain it effectiveness. The volume of material trapped will be
restricted the weir height. Over-depth dredging upstream is a viable option.
3. However, because sediment transported across the gate is believed to be quite
heterogeneous (ranging from fine sand to clay and organic matter) and the organic
material is presently not found in the bed there, predictive modeling the transport
of sediment across the gate will be an uncertain exercise without extensive data
collection on both sides of the gate. An option would be to carry out gate
conversion and work with the new system based on a rough estimation of the new
flow/sediment regime. It is likely that some modification of gate opening schedule
may also have to be carried out to improve the efficiency of the upstream trap.
1.
\8 I
0.14-
U06
0.4-
o .. .. ]l' SI' __ __ _ ___ _ _ _
o9095 100 105 110 115
Days of the Year (2002)
Figure 2.8 Current variation under the effect of released discharge from the S-46
structure.
OBS 4
S* l'r,' OBS3 I3 15
150.
.. 100 0 O 2 s
O l{, ,,,f" r `',' I"'2 -
E .o- ,, .I, .' ,x,' ,,,,.,,
0 -.--.-- -- ------ .--...
90 95 100 OBS 2 10 110 115
200 -- --- ----
-J,
S1oo,'
E I
90 95 100 105 110 115
Days of the Year (2002)
Figure 2.9 Effect of S-46 discharge on the suspended sediment concentration
The present study envisages examining the option dredging the downstream canal.
Ganju (2001) carried out such an exercise by testing the effectiveness of a comparatively
short sediment trap. The trap design and results of the investigation are summarized
below.In order to quantify the sedimentation rate as a function of discharge in the C-18
canal investigations were carried out using calibrated sediment transport models. The
boundary conditions were designed to simulate the episodic unsynchronized (with
Northwest and North Fork discharges) discharges from the S-46 structure. The results
indicated that as discharge increases the change in the rate of sedimentation rate
decreases. However, they do not share a direct straight-line relationship. For instance,
doubling of flow from 2.5 m3/s to 5 m3/s results an increase of 71% in the sedimentation
rate and similar increase from 10 to 20 m3/s changes the rate only by 25% indicating that,
the sedimentation rate is more sensitive to lower discharges. This is evidently due to
increasing discharge is associated with increased concentration. The regulation of the C-
18 canal by the S-46 structure is manifested in the high frequency of zero-discharge
periods (54% of the days) and the spikes. The deposition rates were found to be 0.15 m
for a period of 10 years, which compared well with the poling results.
The study also compares the sedimentation in a regulated C-18 canal to that of
hypothetically unregulated canal by applying flow record for the Northwest fork for the
same period pro-rated so that the discharges over the 10 year flow period remains
identical. Resulting in a 10-year deposition thickness of 0.22m (0.022nm/yr), implying that
the episodic discharges in actuality reduced the rate of sedimentation. This is a direct
consequence of near constant high discharge attenuating the increasing trend of
sedimentation.
The study incorporates a trap near the area of greatest post dredging thickness, with
a poling depth of approximately 1.2 m. A dredging depth of 3 m (from the original bed
level) width of 60 m, and a length of 180 m were chosen for the trap, which was
considered sufficient to reduce the velocity in the canal, and allow a measurable amount
of sediments to settle. This trap configuration reduced the current magnitude by 67% over
the trap. As a consequence a number of factors were evaluated by the study namely,
* Simulations showed that the removal ratio, i.e., the ratio of sediment influx (into
the trap) minus out flux divided by influx), was maximum at an S-46 discharge of
approximately 1.7 m3/s. At higher discharges sediment was transported beyond the
trap, while at lower discharges sediment settled before the trap.
* The second simulation involved testing the trap efficiency as a function of sediment
concentration. It was observed that increase in sediment concentrations in the free
settling range in general increases the settlement. The increase in trapped load
followed a linear trend up to concentrations of 0.25 kg/m3 (free settling zone),
which is explained by the increase of deposition flux with concentration (with
constant settling velocity). Above this concentration, and below 7 kg/m3
flocculationn range), the increase in settling velocity yields a similarly increasing
trend for trapped load. In the hindered settling zone, however, (which lies above
this concentration) trapped load decreases as the settling velocity deceases. It was
therefore be inferred that trapped load is a function of concentration because
settling velocity (and hence the deposition flux) is also a function of concentration
at values greater than 0.25 kg/m3.
* The simulations on varying organic content indicated that, increase in organic
content led to decease in settling velocity, which resulted in lower removal ratio.
Sedimentation rate in the trap increased with increased organic content, due to
corresponding decease in dry density. In addition, the increase in influent load with
increasing organic content as less sediment was deposited upstream of the trap at
higher organic content.
2.3 Central Embayment
2.3.1 Present Condition
Jupiter Inlet, which is about 112 m wide and 3.9 m deep at the jetties, allows the
tidal flow in and out of the estuary. The channel starting at the jetties leading up to the
Florida East Coast Railroad Bridge is fairly uniform, with width varying from 206 m to
247 m and the mean depth varying between 3.92 m at the inlet and 2.6 m near the
FECRR Bridge. The ICWW meets the channel down stream of the FECRR bridge.
Upstream of the FECRR Bridge the embayment widens and the channel is divided in to
two parts by shoals often exposed under low water conditions. These shoals, presumably
created by the sands introduced in to the system through the inlet and the tributaries, and
carried by the flood tide, occur where the sediment carrying capacity of the flow reduces
with the reduction of current at wider sections. In addition, east of these sandy shoals
there occurs a small mangrove island. Similar Islands occur near the north bank close to
the FECCR Bridge. The deepest portion of the embayment lies to the north of the sandy
shoal, easily identifiable even from a areal photograph (Figure 2.9) is currently used for
navigation. The shoreline is basically sandy with little or no clay present. The percentage
of clay and silt is barely 5%. The average depth in the central embayment is 1.2 m. The
depth in the deeper portions along the flood channel however exceeds 3 m in patches. A
similar deep channel can be found along the south bank, which has been presumably
created by the ebb circulation. A clear ebb channel can also be seen from the satellite
photographs to the south of the sandy shoal. Boats returning to their docks use this
channel at high water. There are many private wooden docks along the entire coastline.
At the turn of the century, the Loxahatchee River estuaries along with its
immediate environ was a pristine ecosystem consisting of mangroves, salt marshes, and
scrubland. Prior to Word War II, agricultural interests transformed the area in to a rural
landscape with citrus groves and vegetable farms. As a result, a significant increase in
residential population occurred around this time. These developments ultimately
prompted the declaration of the estuary an aquatic preserve in 1984. Nonetheless, the
construction activities, especially of the residential homes, still continue along the
shoreline and the entire estuarial shoreline of the central embayment as well as a
significant portion of the tributary shorelines is residentially occupied. Recreational
boating is widely practiced in the estuary by the local residents. Access is necessary to
the upstream areas for recreational activities, and also to the open sea and the ICWW.
Many of the natural and artificial access routes have shoaled in recent years (Antonini et
al., 1998), leading to hazardous boating practices such as high-speed entry/ exist to
prevent grounding of vessels. The channels adjacent to the south shore of the central
embayment are more susceptible to shoaling (Sonntag and McPherson, 1984), directly
affecting the boaters who rely on these channels for access.
Figure 2.10 Arial Photograph showing the Central Embayment, the Inlet and the
Tributaries
Estimates with regard to grain size, composition and age of bottom sediments are
given by McPherson et al. (1984) for the entire estuary. The samples collected by vibro-
core boring were analyzed in the laboratory for micro-faunal and macro-faunal
assemblages, grain size distributions, constituent composition and radiocarbon age. With
regard to the grain size it was seen that, the characteristics of the bed material were
identical to those of the underlying sediments in the core. Fine-grained sediments
dominate the central bar at the lower reaches of the estuary; whereas medium to coarse-
grained sand dominates upper reaches of the bar. Patches of fine to medium sand draping
the muddy sediment surface can be seen in the main body of the estuary. The shell
content in the bed material varies from 0 to 5% at the eastern end to 20 to 30% at the
western end.
Grain-size analysis reveals that there are two distinct different populations. The
first, well-sorted sediment with a mode between 62.5 to 125 microns, and the second,
poorly sorted sediment commonly showing bimodality. The bimodal distributions
generally have one mode at about 300 microns and the other at 100 microns.
Jaeger et al, (2001) measured the particle sizes in the estuary, which, reveal that the
majority of the estuary is dominated (by weight) of fine, well-sorted sand in the -150
micron (3 phi; 0.15 mm) size range. This size sand is ubiquitous in the estuary and is
observed in Pleistocene-age coastal deposits exposed in outcrops within the study area.
The ultimate source of the sand accumulating within the upper estuary is from erosion of
these older deposits. The amount of mud-sized sediment (<63 microns) is minimal with
the exception of the upper reach of the Northwest Fork, the North Fork, and near the
junction of the C-18 canal and the Southwest Fork. Clay mineral analyses on the mud
fraction accumulating throughout the estuary reveals that the ultimate source of the clay-
sized sediment is from erosion of the Pleistocene-age deposits.
Comparison of the sediment characteristics (median particle-size, sorting) between
1990 and 2000 within the Central Embayment reveal that this region has not changed
significantly over the past decade. However, the navigation channels have become
coarser apparently due to the removal of fine sediment. Portions of the lower Northwest
Fork and the Southwest Fork have gotten finer.
Based on the analyses of 20 push cores, there does not appear to be a widespread
organic-rich flocculent "muck" layer within the three major forks of the estuary.
Although mud is a common component of the sediments in these locations, by weight it
usually represents less than 20% of the total core mass.
In addition, study by Jaeger et al., (2001) indicate that in the main navigation
channels, the sediments have become coarser and more poorly sorted over the last ten
years. The study attributes this to the likely inclusion of shelly material in the 2000
samples that was not sampled in 1990. It is possible that maintenance dredging during
this time period resulted in the exposure of older shelly material or that changes in the
shape of the navigation channel has led to stronger currents that have removed the finer
sands. Although the western portion of the Central Embayment has seen no change in the
median particle diameter, it has gotten marginally better sorted, and could reflect a
decrease in fine sediments accumulating.
Freshwater runoff enters the Loxahatchee River estuary by river and canal
discharges, by storm drains, and by overland subsurface inflow. Most of the freshwater
from the tributaries is discharged from the Northwest Fork of the estuary. These flows, as
expected, vary seasonally, occurring chiefly in the wet season. The median, high and
maximum flow discharges are given in Table 2.2.
Tidal flow into and out of the estuary is much larger than the freshwater inflow
from all the major tributaries. The combined freshwater flow into the estuary is found to
be about 2% or less of the average tidal inflow at the Jupiter inlet (McPherson, Sonnetag,
1984). However, during tropical storm Dennis, freshwater inflow per tidal cycle
increased to 18% of average tidal inflow (McPherson, Sonnetag, 1983). Tides are mixed
semi-diurnal with varying amplitudes, with a tidal range of approximately 0.6 to 1 m. The
tidal wave advances to the estuary at a rate of about 2.3 m/s to 4.5 m/s. Higher than usual
tides can be noted during the autumn and winter when strong northeast winds pushes
additional water in to the estuary causing higher than average tides.
Ultrasonic water level gauges (Model 220, Infinities USA, Daytona Beach, FL)
with stilling walls were installed to measure tidal elevations between September 14th and
October 18th, at three locations in the estuary one each in the Central embayment (tied to
the FECRR bridge pier), Northwest Fork, and Southwest Fork. The gauge locations are
shown in Figure 2.10. Tidal elevations were recorded with respect to North Atlantic
Vertical Datum 1988 (NAVD 88) and are reproduced in Table 2.3. Tidal ranges indicate
the total change in water surface elevation between low and high tides and phase lag
refers to the difference in time between high/low tide at UFG1 gauge and the other
gauges. In Figure 2.12 sample records from three tidal gauge locations are shown.
Figure
01
SUFG1
0 D- 0-N UFG2
3 0.6 0.4 2 6- UFG3
-0 1
-0.2 -
Ganju et al., (2001) compared the data obtained from these gauges to a station on
-05
Time (hour)
Figure 2.12 Sample records of tidal measurements at three locations (09/14/00-09/15/00)-
Datum NAVD 88.
Table 2.4 Spring/ ap tidal ranges ao d phase lags for three gauges
Gauge ID Spring range Neap range Phase lag from UFG1
(m)(filte (mi) (oin
UFG1 0.90 0.66 0 0
UFG2 0.85 0.65 21 60
UFG3 0.86 0.64 28 60
Ganju et al., (2001) compared the data obtained from these gauges to a station on
the Northeast Florida coast and inferred that trends in water surface elevation followed
similar increases and decreases in mid-tide elevations and the increased elevations in side
the estuary is a direct result of onshore winds. The wind records from two offshore
stations were averaged and correlated with the mid-tide elevation, resulting in a positive
correlation. Accordingly, mid-tide elevation was subtracted from the measured elevations
(filtering) in order to obtain tidal data without any variation.
2.3.2 Management Option:
Presently, the dredged spoil from the embayment is disposed on land. Land
disposal of marine sediment is often times not optimal for the environment, especially for
the ground water. According to earlier studies by Sonnetag and McPherson (1984) the
central embayment receives sediment from two main sources, the inlet and upland
discharge. Regular maintenance of the navigation channel is a clear indication of this
supply. Ideally, a large enough central shoal (if developed to correct contours) could
serve the process of self-cleansing of the bay. The shoal when developed would decrease
the water flow area and thereby, increasing the velocity of flow. The increased current in
the limiting case would develop erosional stresses equal to the critical bed shear of the
sediment and therefore would be able to prevent the further sedimentation of the bay.
However, numerical modeling for such an examination is outside the scope of this study.
The present study will however deal with the development of an additional
navigation/flow channels for improvement of ebb flow.
2.4 Northwest Fork:
2.4.1 Present Condition:
The Northwest Fork meanders through typical South Florida swampland within the
Jonathan Dickinson State Park (JDSP). The extensive swampland and scrubland east of
JDSP is drained by the North Fork. It is therefore evident that the watershed is
biologically productive, and the sediment carried by the runoff is rich in organic content
eventually finds its way in to the estuary (Sonnetag and McPherson, 1984).
Most of the freshwater from is discharged through this fork. From February 1st,
1980, to the September 30th, 1981, for example, 77.3 percent of the freshwater was
discharged into the Northwest Fork, 20.5 percent in to the Southwest Fork (C-18 Canal),
and 2.2 percent into the North Fork (Sonnetag and McPherson, 1984). The Loxahatchee
River (i.e., Northwest Fork) at SR-706, site 23 as shown in Figure 2.14 (Figure 2, U.S.
Geological report no 83-4244, 1984), contributed the greatest percentage of flow to the
estuary (37.4 percent) of all the tributaries.
Vertical variation of the sediments in the Northwest fork is Found at site 5 and 5E
(Figure 2, U.S. Geological report no 84-4157, 1984) during both incoming and outgoing
tides. Presumably, greater water velocities, particularly at 0.6 m above the bottom at the
mid depth, associated with higher tide stages contributed to the greater vertical variation
of suspended sediments (Sonnetag and McPherson, 1984). Concentration of the
suspended sediments and the percentage of sediments of organic origin were variable
with season and weather conditions as indicated by the data collected and listed in U.S
Geological Survey report 84-4157 (Sonnetag and McPherson, 1984). The greatest
increases were observed in Cypress Creek, lying upstream of the Northwest Fork.
Concentration of the suspended sediment in the tributaries also changed as a result of
man's upstream activities. During September 1981, suspended sediment concentration in
the Cypress Creek and Hobe Grove Ditch increased as much as 21 times over
concentrations in early September (Sonnetag and McPherson, 1984). Cleaning and
dredging operations on the irrigation canal connected to the Cypress Creek and Hobe
Grove Ditch were presumably responsible.
Suspended sediment load from the tributaries are highly seasonal and storm related.
The 5 major tributaries to the Loxahatchee estuary Loxahatchee River at SR-706,
Cypress Creek, Kitching Creek, Hobe Grove Ditch, and C-18 at S-46 discharged 1,904
tons of suspended sediments to the estuary during the 20-month period (February 1, 1980
to September 30, 1981) (Table 2.3). During the 61 days period of the above-average
rainfall (August 1 to September 30, 1981) that included tropical storm Dennis, the major
tributaries discharged 926 tons of suspended sediment to the estuary. This accounted for
49 percent of the suspended sediment discharged to the estuary during the 20-month
period and about 74 percent of the suspended sediment discharged during 1981 water
year (Sonnetag and McPherson, 1984). Sediment loads from C-18, Loxahatchee River at
SR-706, and Cypress Creek accounted for more than 94 percent of the total tributary
input of the sediment load.
-t-
TIR J0ARY $TREAM N4 ,;.: 1 Kr,
A000 211MILE.S NO P011. V 14M 44
"~" 59
r~r, i i.-~4*r s0i~r W
'I,(.1 (,14 14 l. 0*14
'41.33u I'34304 I
* ('4Y n~ll:I.I14 ,hD 14l1141
*4*4Ffi '4l~tlN Sf
r~ir~r, 5(r:0i Fm D'4"Ire~~
. w, _' ... -- . '.. .
.'. 4.. .. ... ... J _
Figure 2.13 Location of stream-gauging stations and sampling site for suspended
sediments, (Source U.S. Geological report no 83-4244 and 84-4157)
Unlike the central embayment concentration of mud was quite high (-50%) in the
Northwest Fork (Jaeger et al., 2001). The study by Jaeger et al., (2001) also analyses
vibracores takes which, reveal that there has been roughly 0.5-1 cm/yr of sedimentation
within a part of the Northwest Fork when compared to data from a USGS-sponsored
study completed, in 1984 (Sonnetag and McPherson). The study further concludes that,
these accumulation rates are close to those averaged over the past 50 years, assuming that
an observed change in the cores from layered sediment not mixed by organisms to those
that are well mixed by organisms occurred in 1947 when the inlet was stabilized. Inlet
stabilization would have led to increased tidal flushing that allowed for better
oxygenation of bottom waters and sediments permitting occupation of sediments by
organisms. However, this datum has not been substantiated as pre 1947 and the
accumulation rates are bulk averages. A comparison of the collected data and studies by
Ganju et al., (2001) showed that accumulation rates within the upper reaches of the three
Forks are about 2-3 times higher than the modeled fine-sediment budget prepared by
Ganju et al. (2001). Accordingly, the study concludes that, this discrepancy could be due
to poor age constraints of the core layers or to the substantial presence of sand in the core
sections, which was measured in this stratigraphic (i.e., core layering) approach but not in
the fine-sediment budget.
Upstream of the outfall point of the Northwest Fork is marked by a horseshoe-
shaped shoal (Figure 2.14). Presumably this shoal is formed due to the reduction in
current velocity of the sediment-laden flow by the ebb tide. In addition, the ebb flow
velocity gets reduced upon meeting a large body of water (central embayment). Upstream
of this shoal there occur a series of sand shoals also formed by the same processes.
Downstream of the shoal however, the depths are uniform gradually increasing as moves
in to the central embayment area. Formation of deposits presumably from the erosion of
old deposits in side the estuary was also reported by Jaeger et al., (2001). Figure no 4 of
the report are reproduced here for reference with regard to the deposition and material
composition.
In Figure 2.15 the mass percent of the mud (particles smaller than 63 microns) in
the upper -5 cm of the sediment surface is shown. Location of the sampling sites are
shown as dot symbols.
2.4.2 Management Options
Discounting the sedimentation from the internal sources of erosion, the Northwest
Fork contributes the maximum discharge as well as the maximum sediment into the
central embayment (Sonnetag and McPherson, 1984). However, Jaeger et al, (2001)
indicate that fresh deposits are found in the Fork (Figure 2.14), suggesting that the source
of such deposits may be mostly internal to the estuary, and most likely due to the erosion
of old deposits. Sediment from external sources entering the estuary with fresh water
discharge as reported by McPherson (1984) would have deposited in the proximity of the
horseshoe shoal.
In order to minimize the deposition of fine sediment in the area of high mud
percentage in the Northwest Fork (Figure 2.14), a self-cleaning channel will be
examined. According to Jaeger et al., (2001), the origin of deposits (Figure 2.14) is due to
the erosion of old deposits. Therefore the channel is proposed to be located downstream
of these deposits. Design aspects of the channel are considered in Chapter 4.
75
65
60
55
2.5.1 Present Condition
1-,45
40
trough mid-August. During the rest of the year the average flow was 0.12 /s, a very
--- 15
Figure 2.14 Location indicating fresh mud depositions and the Shoals the estuary. Source:
Sedimentary Processes in the Loxahatchee River Estuary: 5000 Years Ago to
the Present-FINAL REPORT, Jaeger et al., (2001)
2.5 North Fork
2.5.1 Present Condition
The North Fork is a natural tributary draining the eastern part of the Jonathan
Dickson State Park. Discharge as given in Table 2.2 is the least of the three main
tributaries (2.2% of total), and water depth is fairly uniformnn at around 2 min, with virtually
no shoals. McPherson and Sonnetag (1983) reported that in the 1981 water year the
tributaries of the North Fork were dry at the gauging stations (Figure 2.13) from March
trough mid-August. During the rest of the year the average flow was 0.12 m3/s, a very
small value. Discharge following Tropical Storm Dennis was also small for the amount of
rainfall associated with the storm. Daily discharges for the last 10 days of August 1981
averaged 0.31 m3/s but increased to 0.71m3/s in September. Jaeger et al. (2001) found
34
some mud deposits in the upper reaches. Depths in the fork appear to be adequate for the
recreational boating.
2.5.2 Management Options
The North Fork as indicated above has the least river inflow as well as the least
sediment contribution to the estuary. In addition, the depths are fairly uniform and good
for the types of boats presently using it. Hence no additional facility is believed to be
required for this area. Therefore no dredging is planned for this tributary nor appears to
be required.
CHAPTER 3
DATA COLLECTION
3.1 Field Setup in the Southwest Fork
Field data were collected at two sites, one in the Southwest Fork and the other in
the Northwest Fork. Section 3.3 collection effort and results in the Southwest Fork, and
Section 3.4 in the Northwest Fork.
The field data collection set up in the Southwest Fork of the estuary had
geographical coordinates of latitude 260 56' 36.78" N and longitude 800 07' 17.34" W. In
the Northwest Fork the corresponding coordinates were 260 59' 16.78" N and longitude
800 07' 56.34" W. These two locations are shown in Figure 3.1. The locations of the tidal
gages installed in the year 2000 were shown in Figure 2.11. The depth (below North
Atlantic Vertical Datum, 1988, (NAVD88)) at the sites ware 2.1m and 2.18, respectively.
Data in the Southwest Fork were collected in two phases. The first phase of the
data collection was carried out between 4th and 24th April 2002, and the second phase was
between 6th of February and 2nd of June 2003. The instrumentation deployed is given in
Table 3.1.
Table 3.1 Instrumentation for data collection and data blocks
Instrument Data Date
Data logger (*) Current (mag.) u Apr 04 to Apr 24 Nov 27 to Jun 2
Data logger (*) Current (dir.) u Apr 04 to Apr 24 Nov 27 to Jun 2
Data logger (*) Current (mag.) v No data Nov 27 to Jun 2
Data logger (*) Current (dir.) v No data Nov 27 to Jun 2
Data logger (*) Tide levels Apr 04 to Apr 24 Nov 27 to Jun 2
Data logger (*) OBS 1 Apr 04 to Apr 24 Nov 27 to Jun 2
(Poor quality)
Data logger (*)) OBS 2 Apr 04 to Apr 24 Nov 27 to Jun 2
Data logger (*) OBS 3 Apr 04 to Apr 24 Nov 27 to Jun 2
Data logger (*) OBS 4 Apr 04 to Apr 24 No data collected
Data logger (*) Temperature Apr 04 to Apr 24 Nov 27 to Jun 2
Data logger (*) Salinity Apr 04 to Apr 24 Nov 27 to Jun 2
*With ultrasonic current meter in April 2002 replaced with an electromagnetic current meter
Instruments were attached to a tower erected for this purpose and was powered by
rechargeable batteries. The instrument assembly consisted of a Marsh-McBirney
electromagnetic current meter, a Transmetrics pressure transducer for the measurement of
water surface elevation, a Vitel VEC-200 conductivity/temperature sensor for
measurement of salinity and temperature, and three Sea point Optical Backscatter Sensor
(OBS) turbidity meters for measuring the sediment concentration at 3 different levels. In
the first phase instrument setup, however, turbidity sensors were deployed at 4 different
levels. In addition Lidberg Land Surveys, Inc. carried out a hydrographic survey and
collected data with regard to the bottom bathymetry of the central embayment and the
tributaries.
In the Northwest Fork the data collection started on 14th of August 2003, with 3
level OBS sensors, one Conductivity Temperature sensor and one Pressure gauge
3.2 Instruments Deployed
3.2.1 Current
Current data were collected using Marsh-McBirney electromagnetic current meter
(Model 585 OEM). This meter consists of a 10 cm diameter spherical sensor, OEM
motherboard, and signal processing electronics (Figure 3.5). The instrument senses water
flow in a plane normal to the longitudinal axis of the electromagnetic sensor. Flow
information is output as analog voltage corresponding to the water velocity components
along the y-axis and x-axis of the electromagnetic sensor. The velocity sensor works on
the Faraday principle of electromagnetic induction. The conductor (water) moving in the
magnetic field (generated from within the flow probe) produces a voltage that is
proportional to the velocity of water. The Marsh-McBimey requires periodic cleaning of
the probe with mild soap and water to keep the electrodes free of non-conductive
material.
Since the instrument has essentially a cosine response in the horizontal plane, the
flow magnitude and the direction information are retained. In addition, the spherical
electromagnetic sensor has an excellent vertical cosine response. This unique
characteristic allows the sensor to successfully reject vertical current components that
may be caused by mooring line motions. As the flow changes direction, the polarities of
the output signal also change. So the u (velocity along axis of the channel flow) and the v
(velocity across the channel) velocities are stored and can be combined to give the
resultant magnitude and direction. It must however be noted that, the v velocity
component was largely insignificant due to the width of the channel at the tower location.
3.2.2 Tide
Water surface elevation was measured using a Transmetrics pressure transducer
installed at the instrument tower. The instrument incorporates three major design
elements that allow it to measure pressure accurately and reliably; bonded foil strain
gages configured in a Wheatstone bridge (for temperature stability), high precision
integral electronics for signal amplification, and stainless steel construction for durability
and corrosion resistance. The instrument was calibrated and temperature-compensated
against standards applicable for the region.
3.2.3 Salinity/Temperature
Conductivity is the measurement of the ability of a solution to carry an electric
current. It is defined as the inverse of the resistance (ohms) per unit square, and is
measured in the units of Siemens/meter or micro-Siemens/centimeter. The measurement
of conductivity is necessary for the determination of the salinity of a solution. Salinity is
proportional to the conductivity and is expressed in terms of concentration of salt per unit
volume (mg/l, or ppt). The field measurement of salinity was carried out following
similar procedures using a Greenspan Electrical Conductivity (EC) sensor substantially
eliminating a basic source of error arising out of the inaccuracies due to temperature and
electrode effects. In this instrument the electrical conductivity is a function of the number
of ions present and their mobility. The electrical conductivity of a liquid changes at a rate
of approximately 2% per degree Centigrade for neutral salt and is due to the ionic
mobility being temperature dependent. The temperature coefficient of the conductance
(or K factor) varies for salts and can be in the range 0.5 to 3.0. As electrical conductivity
is a function of both salt concentration and temperature, it is preferable to normalize the
conductivity measurement to a specific reference temperature (250C) so as to separate
conductivity changes due to salt concentration from those due to temperature changes.
* The instrument deployed consisted of the following primary elements:
* Toroidal sensing head (conductivity sensor)
* Temperature sensor
* Microprocessor controlled signal conditioning and output device
The conductivity sensor uses an electromagnetic field for measuring conductivity.
The plastic head contains two ferrite cores configured as transformers within an
encapsulated open-ended tube. One ferrite core is excited with a sinusoidal voltage and
the corresponding secondary core senses an energized voltage when a conductive path is
coupled with primary voltage. An increase in charged ion mobility or concentration
causes a decrease in the resistivity and a corresponding increase in the output of the
sensor.
A separate PT100 temperature sensor independently monitors the temperature of
the sample solution. This sensor provides both a temperature output and a signal to
normalize the conductivity output.
3.2.4 Sediment Concentration
The instrument deployed was a Sea Point turbidity meter. This instrument measures
turbidity by scattered light from suspended particles in water. The turbidity meter senses
scattered light from a small volume within 5 centimeters of the sensor window. The light
sources are side-by-side 880 nm Light Emitting Diodes (LED). Light from the LED
shines through the clear epoxy emitter window into the sensing volume, where it gets
scattered by particles. Scattered light between angles 15 and 150 degrees can pass
through the detector window and reach the detector. The amount of scattered light that
reaches the detector is proportional to the turbidity or particle concentration in the water
over a very large range.
The sensors were calibrated using a sample from the measurement site. Periodic
calibrations were conducted in order to evaluate the conditions of the windows and the
sensitivity to scattering. In addition, only black containers were used in calibration so as
to prevent any probable scattering events due to reflection off the container wall. The
calibration was carried out using known volume of sediments in known volume of water
and the voltage output of the instrument recorded. A linear fit curve was generated in
order to determine the accuracy of the calibration. The calibration plots are given below,
2.5
0
a 1
/ OBS 1
0.5 [
0
0.
0
2.3
S2 2/
1.5
1 OBS2
____________ ___ i
10 20 30 40 50 10 20 30 40
Concentration (mg/L) Concentration (mg/L)
2.5
-2
51.5
0
1 -
1,/ OBS 3
10 20 30 40 50
Concentration (mg/L)
Figure 3.2 Calibration plots used for calibration of OBS sensors
50
3.3 Field Data Results in Southwest Fork
3.3.1 Current
The electromagnetic current meter was located at a height of 96.5 cm from the
bed level. The velocity data in two directions, one parallel to the flow and the other
perpendicular to it, were combined vectorially to find the resultant magnitude and
direction. The ultrasonic current meter deployed in April 2002 collected the current
magnitude and direction directly. Based on these data the depth-mean magnitude time
series for Julian days 94-114 is shown in Figure 3.3 and the corresponding direction plot
is given as Figure 3.4. A sudden increase in the current magnitude in the plot is
attributable to the opening of control structure S-46. The directional plot indicates a uni-
directional flow driven by the discharge from the structure. The discharge record for the
period is given in Table 3.2 for ready reference.
Table 3.2 Discharge data for the period 04/14/2002 to 04/21/200
Date Julian Days of 2002 Discharge
(m3/s)
04.14.2002 104 0.03171
04.15.2002 105 0.00821
04.16.2002 106 0.01416
04.17.2002 107 0.01501
04.18.2002 108 0.03483
04.19.2002 109 0.05777
04.20.2002 110 0.03568
04.21.2002 111 0.00934
tJ
z
0.8
H-
w
UJ
0:
0
Maximum
"0,- Mean 004 m/s
Minimum 001 mis |
0.2 5 e P '.P Crer, ne
90 95 100 105
JULIAN DAYS IN YEAR 2002
Maximum
MPAeritu:.1
nVI
n115s
Figure 3.3 Record of current magnitude: Days 94-114 (year 2002).
200
150S | i
0 i FLOOD
z- 50
5-
z I
i I iI
D-10 0!
-150' I -
100 105
JULIAN DAYS IN YEAR 2002
Figure 3.4 Record of current direction: Days 94-114 (year 2002).
0351
S0.3
w
0 0.25;
0 02-
4-
I
n I
Z "
Maximum 0.17 m/s
Mean : 0.06 m/s
335 340 345 350
JULIAN DAYS IN YEAR 2002
Figure 3.5 Record of current magnitude: Days 332-356 (year 2002).
250
200-
150}
0 50"
z
ii
o
100r-
o
-150'
-100
-200^-
335 340 345 350 355
JULIAN DAYS IN YEAR 2002
Figure 3.6 Record of current direction: Days 332- 356 (year 2002).
Figure 3.5 is a representative plot of the current magnitude for the second data
block. This plot indicates a more uniform velocity pattern driven by the tidal flow in the
44
estuary. The current magnitudes reach a maximum value of 0.17 m/s with the mean
value at 0.06 m/s. In addition it is seen that the flow is predominantly along the estuary
with very low values observed for transverse current (v). In Table 3.3 typical mean
current values are summarized.
Table 3.3 Typical mean current magnitude values for data blocks
i *, Current magnitude (m/s)
Julian days in Current magnitude (mis) Velocity u Velocity v
Jhs2002 With S-46 Only tidal flow (rds) (lms)
2002 discharge (m/s) (m/s)
discharge
94-114 0.25 0.04
332 356 0.06 0.057 0.018
a No data
3.3.2 Tidal Level
2 r...-- -------
Maximum Level 1.7 m
Mean Level 12 m
1.8 Minimum Level 0.7 m
cj i I j I
ji
| i I :I
I- I *' !
i ,I 1 , ,,
061
90 95 100 105 110 115
JULIAN DAYS IN YEAR 2002
Figure 3.7 Water level time-series: All levels relative to NAVD 88. Days 94-114 (2002).
45 .i 4
15 '.4,4,i
Ii,.,.
4
Spring tide Range 0 8 m
Neap tide Range 05m
95 100 105
JULIAN DAYS IN YEAR 2002
Figure 3.8 Water level time series. Upper plot shows original time series with mean trend
and the lower plot is without the mean oscillations. All levels relative to
NAVD 88. Days 94-114 (2002).
3 ------
3
Z 2-.
I-
I-
1-
u j1
Maximum Level 2 92 nm
Min Level
Mean Level
1 49 m
2.21 m
340 350 360 370 380
DAYS OF THE YEAR (2002)
Figure 3.9 Water level time-series. All levels relative to NAVD88.
and Days 01-35 (2003).
Days 332- 365 (2002)
a os
0
-0 5
'I *
110 115
o
390 400
P
?
Iii '
i
S|'|i I'n tIIIII" '
-0.5
SSpring range 1.00 m
Figure 3.10 Water level time series. Upper plot shows oginal time series with mean
340 350 360 370 380 390 400
JULIAN DAYS IN YEAR 2002/03
Figure 3.10 Water level time series. Upper plot shows original time series with mean
trend and the lower plot is without this trend. All level relative to NAVD 88.
Days 332-365 (2002) and Days 01 35 (2003).
In Figure 3.7 the raw tidal time-series is shown for the period April 4th to April 24th,
2002. In Figure 3.8 the upper plot shows the original time series with the tidal trend and
the lower plot is with the tidal trend removed.
The tidal plots indicated in the Figure 3.7 to Figure 3.10 are representative plots
from the phase II and I. The characteristic values of the tidal data are given in Table 3.4.
In addition it can be noted that the tidal ranges compares well in both the phases with the
spring range equal to 1.0m and the neap range around 0.5m. As will be explained later,
the tidal fluctuations (as could be noted from Figure 3.9) between Julian days 360 to 365
in Year 2002, 01 to 5 and 17 to 25 in Year 2003, is likely to affect the sediment
concentration in the estuary.
Table 3.4 Characteristic values of the tidal data
J n ds Mean Water level/Tidal range
Julian days
in 2002/03 water depth Water level (m) Spring/nea range (m)
(m) Maximum Minimum Spring Neap
94-114 1.20 1.70 0.30 0.90 0.50
332-365
332-365 1.20 1.90 0.30 1.00 0.50
01 -35
3.3.3 Total Suspended Solids
Total Suspended Solid (TSS) was recorded at four elevations in the first phase and
three elevations in the second phase. The elevation of the OBSs relative to the bed level
was OBS-4 = 1.17 m, OBS-3 = 0.80 m, OBS-2 = 0.48 m and OBS-1 = 0.22 m. The
corresponding total suspended solid time series are reported in Figures 3.11 and 3.12 for
days 94-114 (Phase I) and 352- 365 in 2002 and 01 to 35 in year 2003 (Phase II),
respectively.
Table 3.5 provides the maximum, mean and minimum values of sediment
concentrations at different levels for each data block. Depth-mean concentration averaged
every 12 hours is presented in Figures 3.13 and 3.14. The mean concentration figures
(Figure 3.13 and 3.14) indicate the average variations in the concentration over time with
out the instantaneous variations (spikes).
OBS
200- --- -- ---- ----
S100
90 95 100 OBS2 105 110 115
200
90 95 100 OBS3 105 110 115
150
100-
10 ---1UA u
90 95 100 OBS4 105 110 115
300 r-1 --- - --- ------- ----i-
?2001
90 95 100 105 110 115
JULIAN DAYS IN YEAR 2002
Figure 3.11 TSS time-series at four elevations: Days 94-114 (year 2002).
OBS 3
80--------.---------
S1i Malnna n uW mg I
*" ar.Mrn 20 m.Lmi, 0i
350 *55 360 365 7 O 7'* 1 3 5 390 : 95 400
Md>rmum 80 i.,/L ) |
Mean II mr .j
350 355 360 3es 370 O 1 380 ses 390 39 400
S15001 Madimum 2500 mgLA
1,,, Mean 1600 mg/L
Minimum 270 mg/L
500i
350 355 360 365 370 375 380 385 390 395 400
JULIAN DAYS IN YEAR 2002/03
Figure 3.12 TSS time-series at three elevations: Days 352- 365 (year 2002) and 01-35
(year 2003).
150 --
100
501os
-J
OBS 1
95 100 OBS2 105 110 115
90 95 100 OBS3 105 110 115
150
50
90 95 100 OBS4 105 110 115
150--
z 100
1:11
E 50 -
S-90
90
95 100 105 110 115
JULIAN DAYS IN YEAR 2002
Figure 3.13 Depth-mean TSS concentration time series: Days 94-114 (year 2002)
OBS 3
100- Maximum 150 mg/L
Mean 17 mg/L
E Minimiu 7 mg/L
350 355 360 365 370 375 380 385 390 385 C
OBS 1
Maximum 2900 mglL
Mean 1900 mg/L
Minimum 400 mg/L
0 ---
350 355 360 365 370 375 380 385
JULIAN DAYS IN YEAR 2002/03
390 395 400
Figure 3.14 Depth mean TSS concentration time series: Days 352- 365 (year 2002) and
Days 01 35 (year 2003).
3000-
.2000
S1ooo00-
-40i
-J
S20 1 December 2 th
S50
IE30
-- DDriven by 46
355 360 365 370 375 380 385 390 395 400
JULIAN DAYS IN YEAR 2002/03
Figure 3.15 Depth mean TSS concentration time series and tidal trend indicating their
dependence: Days 352- 365 (year 2002) and Days 01 35 (year 2003).
It can be noted from Figures 3.11 and 3.13 that there is a sudden increase in
sediment concentration with the discharge from the S-46 structure on 14th of April 2002
(Refer Table 2.2 for discharge details). This clearly indicates that sediment concentration
is discharge driven. Results of Figures 3.12 and 3.14 indicate that the lowest OBS1
sensor was too close to the bed and recorded almost saturated sediment content. There
was no discharge from the structure between December 14th and February 20th, except for
0.01 m3/s discharge on the December 20th, 2002, which explains the increase in sediment
concentration recorded around Julian day 355 (December 20th). However, the increase in
TSS reported between days 17 and 27 (Year 2003) without any discharge from S-46,
could be attributed to spring tidal effects (Refer to Figure 3.15). In general, it appears that
TSS concentration is dependent on the local tidal current and flow discharges down the
S-46 structure. The TSS concentrations with regard to other data blocks are given in
Table 3.7 to 3.9.
Table 3.5 TSS concentrations for the representative data blocks
Julian Days Maximum TSS Mean TSS Minimum TSS
in 2002/03 concentration concentration concentration
(mg/L) (mg/L) (mg/L)
94-114 165 50 10
352-365 158 17 7
01-35_
3.3.4 Salinity and Temperature
The conductivity and temperature measurements carried out for the location is
presented in Figures 3.16 (days 94 -114 of 2002). The salinity curve indicates the effect
of the fresh water discharge. Due to this flow fresh water from the S-46 structure the
salinity values dropped to 11 mg/L from a mean value of about 28 mg/L. In order to
examine this hypothecation the current magnitude and the salinity was plotted together in
Figure 3.17, which, indicated a decrease in salinity with an increase in the current
magnitude. Accordingly, it can be concluded that the fresh water discharge reduces the
salinity in the estuary.
Table 3.6 Characteristic salinity values
Julian days Maximum Salinity Mean Salinity Minimum Salinity
in 2002/03 (mg/L) (mg/L) (mg/L)
94-114 34.2 24.9 11.1
352-365 39.5 36.5 26.7
01-35
OI I
Maximum 34 mg/L
Minimum 11 mgL
Mean 24 mg/L .'
100 105
JULIAN DAYS IN YEAR 2002
Figure 3.16 Salinity time series: Days 94-114 (year 2002).
30
25
I-
z
Lij
u-I
20
10
-i
2 10
-W
(0
Figure 3.17 Salinity and
i !; ; :-1 r i lI 1 -" I
Salinity ' r ,
II I
* ,
1 ] i f
...i.lL .;
Current Magnitude x 1 i
^^l^f.fA H 'l..A-** ^t*1 <*
95 100 105 110 115
JULIAN DAYS IN YEAR 2002
Current magnitude time series: Days 94-114 (year 2002).
30,
2 25.
E
I2
I20'
U,
n L -I
I
~I .*VUT~UY_~I.L~i
32
30 Maximum 310C
Minimum 210 C, i :
o 28- Mean 26 C
S' I iI I ,
26 *. l t ,li y
I ,I i' f
Li. 24 4 i
F-
22 -
20.
90 95 100 105 110 115
JULIAN DAYS IN YEAR 2002
Figure 3.18 Temperature time series: Days 94-114 (year 2002).
Similarly the temperature time-series shows a positive correlation with the
discharge, with temperature increasing with the discharge from S-46 structure. However
any definite conclusion could not be deduced from this the absence of adequate data on
temperature of the freshwater discharged.
For the second data block between days 352 and 365 (of year 2002) and days 01
and 35 (of year 2003) the Figure 3.19 indicates an apparent malfunctioning of the sensor
that seems to have contaminated the conductivity time series that calculates the salinity
by measuring its conductivity of the solution at a given temperature. Although the
temperature time series for the same period appears to give correct reading consistent
with the environment, the incorrect conductivity data have made the salinity
determination inaccurate. Therefore salinity values reported in this period appear to be
rather high. Tables 3.5 and 3.6 summarize the characteristic values of salinity and
temperature for both the data blocks. The results from the other data blocks are furnished
in Table 3.7 to 3.9.
70
o- Maximum 39 50 mg/L
60-
Mean : 35.60 mglL
50o Minimum 26.70 mg/L
-fJ
40
i)
20
10
.. ........ .. ... ........
355 360 365 370 375 380 385 390 395 400
DAYS OF THE YEAR 2002/03
Figure 3.19 Salinity time series: Days 352- 365 (year 2002) and 01-35 (year 2003).
30,
C)
o
v20-
uj
L1-
. 15
I-
Maximum 260 C
Mean 110C
Minimum 8 C
s50 3S5 360 365 370 375 380 3a5 390 395 400
JULIAN DAYS IN YEAR 2002/03
Figure 3.20 Temperature time series: 352- 365 (year 2002) and 01-35 (year 2003).
Table 3.7 Characteristic temperature values
Julian days Maximum Temperature
in 2002 (,0' C)
94-114 31.4
352-400 26.7
Mean Temp
C' C)
26.3
10.9
Minimum Temperature
C'' C)
21.6
7.8
3.3.5 Other Data Blocks
The foregoing discussions included the various aspects of data collection their
analysis and results for two representative data blocks (Julian Days 94 to 114, 330-365 in
year 2002 and 01 to 35 in year 2003). However since the second phase data collection
lasted from November 26th, 2002 to May 15th, 2003, it was considered necessary to
include the characteristic values obtained from the other data blocks, which would offer a
better insight in to the overall site conditions.
Table 3.8 Summary of parametric value (Days 37-59 in year 2003)
Parameter Maximum Mean Minimum
Depth (m) 1.9 1.2 0.5
OBS 1 (mg/L) *
OBS 2 (mg/L) 240 30 0.7
OBS 3 (mg/L) 110 20 1.0
Salinity (mg/L) 40 35 23
Temperature (C) 27 16 11
Current Magnitude (m/s) *
* Poor quality data
In Table 3.7 a summary of parametric values of the data collected between
February 6t" and February 281t is presented. The data obtained for the other two blocks
are presented in Tables 3.8 and 3.9.
Table 3.9 Summary of parametric value (Days 90-101 in year 2003
Parameter Maximum Mean Minimum
Depth (m) 1.6 1.1 0.6
OBS 1 (mg/L) 2670 1710 1470
OBS 2 (mg/L) 80 50 1.0
OBS 3 (mg/L) 96 51 20
Salinity (mg/L) *
Temperature (C) 20 11 3
Current Magnitude (m/s) *
*-Bad Data
Table 3.10 Summary of parametric value (Days 101-135 in year 2003)
Parameter Maximum Mean Minimum
Depth (m) 1.9 1.3 0.7
OBS 1 (mg/L) 2090 1670 1180
OBS 2 (mg/L) 170 46 2
OBS 3 (mg/L) 210 56 10
Salinity (mg/L) 26 19* 17*
Temperature (C) 22 12 4
Current Magnitude (m/s) 1.40 0.60 0.10
Bad Data
3.4 Field Data Results in Northwest Fork
3.4.1 Field Setup
In the third phase of data collection, in the Northwest Fork, the instrument tower
included three optical backscatter sensors (OBS), a pressure transducer (for water level)
and a conductivity/temperature sensor. Data collection began on 08/14/2003. Data on
water level and TSS are presented. The conductivity/temperature sensor malfunctions
during this phase and yielded values of questionable accuracy. Hence these data are not
reported.
3.4.2 Tidal Level
The pressure transducer was located 0.45 m from the bed. Figure 3.21 shows the
original time series of the water level.
Maximum Level 1 75 m
Mean Level 1.2 m
Minimum Level 070 m
II I; ~
2.
1 81"
1.41
w 1.4
o.0
08-
Z44
Figure 3.21 Record of water level variation. Days 245 255 (year 2003).
I-
0.5
0
05-
Sprng Range 0.8 m
Neap Range 0.5 m
246 248 250 252
JULIAN DAYS IN YEAR 2003
254 256
Figure 3.22 Water level time series. Upper plot shows original time series with mean
trend and the lower plot is without the mean oscillations. All levels relative to
NAVD 88. Days 245 -255 (year 2003).
In Figure 3.22 the upper plot shows 12-hourly mean trend with the original time
series, and in the lower plot this trend is removed. As can be seen from the latter plot, the
246 248 250 252
JULIAN DAYS IN YEAR 2003
254 256
ii
ii ;.
i
i
u i 4
ii:
rising mean trend indicates the effect of fresh water discharge. The tidal range was 0.80
m. Characteristic values are given in Table 3.10.
Table 3.11 Characteristic values of the tidal data
Mean Water Level/Tidal range
uian 2 s Water depth Water level (m) Spring/neap range (m)
(mi) Maximum Minimum Spring Neap
245-255 1.20 1.75 0.70 0.80 0.50
3.4.3 Total Suspended Solids
Total suspended solids (TSS) concentration was recorded at three elevations. The
elevations of the OBS sensors relative to the bed were OBS-1 = 1.04 m, OBS-2 = 0.66 m
and OBS-3 = 0.30 m. The corresponding depth-mean concentration time series is
reported in Figure 3.23. Characteristic values are given in Table 3.11.
240 : ..... ... .
220
-J
6 200
Maximum 230 mg/L
z 80 Mean 100 Mng/L
o Minimum 50 rg/L
S160.,
0 140V
S:JULAN IN YEAR 2003
(.ng/L) (mg/L) (mg/L)
245-255 230 100 50
454s 246 248 250 252 254 256
JULIAN DAYS IN YEAR 2003
Figure 3.23 Depth-mean TSS concentration time-series: Days 245-255 (year 2003).
Table 3.12 TSS concentrations for the representative data blocks
Julian Days Maximum TSS Mean TSS Minimum TSS
in 2003 concentration concentration concentration
245-255 230 100 50
3.4.5 Additional Data Blocks
3.4.5.1 Tidal Level
Two additional data blocks were collected between November 6th, 2003 and
November 24t", 2003. Tide data for Julian days 310 and 313 are presented here. The
remainder was found to be of poor quality.
1.9
18 Maximum 1.90 m
Mean 1.40 m
Min 1.00 m
1.7
1.6
1.5
S1.4-
1.3 -
1.2
1.1
1-
0.9
310.5 311 311.5 312 312.5 313 313.5
JULIAN DAYS IN YEAR 2003
Figure 3.24 Record of water level variation. Days 310.5 313.5 (year 2003).
1 V"
15
- 1
I-
o s-
I-
Or
.0.5
310.5
313 313.5
Figure 3.25 Water level time series. Upper plot shows original time series with mean
trend and the lower plot is without the mean oscillations. All levels relative to
NAVD 88. Days 310.5 -313.5 (Year 2003).
Table 3.13 Characteristic values of the tidal data
Mean
Julian days in Mean
2003 Water depth Water lev
(m) Maximum
310.5-313.5 1.40 1.90
Water Level/Tidal range
tel (m) Spring/neap range (m)
Minimum Spring Neap
1.00 0.90 0.50
3.4.5.2 Total Suspended Solids
Two data blocks for the TSS concentration was collected and are presented below.
Characteristic values are presented in Table 3.14.
311 311.5 312 312.5
JULIAN DAYS IN YEAR 2003
OBS 2
70
60 Maximum 834 mg/L
Mean 578 mg/L
50- Min 397 mg/L
40
30 .
20 -
3105 311 311.5 312 312.5 313 313
OBS I
800
700
--J
'9600
E
500
400-
1, 5 311 311.5 312 312.5 313 313.5
JULIAN DAYS IN YEAR 2003
Figure 3.26 TSS time-series at two elevations: Days 310.5 313.5 (year 2003).
OBS 2
45
40-
30
335-
E15
OBS I
750
700
650.
0600,
550 I
500-
JULIAN DAYS IN YEAR 2003
Figure 3.27 Depth mean TSS concentration time series: Days 310.5 313.5 (year 2003).
62
OBS 3
150
E: Ma0mu 144 mg.l
120! Mean 132 mg/L I
Min 119 mg/L
1 5.5 316 316.5 0S 2 317.5 318 318.
100r
-s
355
316 3165 1 317.5
318 318.5
220 . .. . . .-.. . .- --- ---. .. . -.... .
160- Mean 190 mg/L
Min 156 mg/L
3155 318 316.5 317 317.5
JULIAN DAYS IN YEAR 2003
TSS time-series at three elevations: Days 315.5
318 318.5
318.5 (year 2003).
OBS 3
140-
E ,1,
105
100:
95
90
316 316.5 0~ 1
3175
318 318.5
210-
10-J
3155 31G 316-5 317 317.5 318 318.5
JULIAN DAYS IN YEAR 2003
Figure 3.29 Depth-mean TSS concentration time series: Days 315.5 318.5 (year 2003).
Table 3.14 TSS concentrations for the representative data blocks
Julian Days Maximum TSS Mean TSS Minimum TSS
in 2003 concentration concentration concentration
(mg/L) (mg/L) (mg/L)
310.5 313.5 834 304 19
315.5 318.5 219 140 81
Figure 3.28
315.5
220
CHAPTER 4
MODEL CALIBRATION AND VALIDATION
The analyzed data presented in Chapter 3 give a qualitative insight into the
prevailing environmental conditions. However, in order to have a quantitative
understanding of the flow regime in the estuary, it is necessary to apply a numerical
simulation technique. This chapter includes a brief description of the numerical model,
generation of the computational grid, initial and boundary conditions and the model
operational scheme. Model calibration and validation are then carried out.
Certain aspects of the estuary have been idealized in the formulation of the model
in order to reduce the computational time and avoidance of potential errors. These
idealizations are as follows:
1. The central embayment domain is terminated at the FECRR bridge excluding the
ICWW (Intracoastal Waterway). This enables use of tide data from UFG1 gage
installed at the bridge.
2. The traps and the navigation channels have rectangular cross-sections.
4.1 Model Description
Flow simulations were carried out using Environmental Fluid Dynamics Code
(EFDC) maintained by the Environmental Protection Agency, and developed by
Hamrick, 1992. This code works through a Microsoft Windows-based EDFC-Explorer
pre- and post-processor. Developed on a Fortran platform, the physics of EFDC and
many aspects of the computational scheme are equivalent to the widely used Blumberg-
Mellor model (Blumberg and Mellor, 1987) and the U.S. Army Corps of Engineers'
Chesapeake Bay model (Johnson, et al, 1993). EFDC solves the three-dimensional
hydrostatic, free surface, turbulent averaged equations of motion of a variable density
fluid. The model uses a stretched or sigma vertical coordinate and Cartesian or
curvilinear, orthogonal horizontal coordinates. Dynamically coupled transport equations
for turbulent kinetic energy, turbulent length scale, salinity and temperature are also
solved. Externally specified bottom friction can be incorporated in the turbulence closure
model as a source term. For the simulation of flow in vegetated environments, EFDC
incorporates both two and three-dimensional vegetation resistance formulations
(Moustafa, and Hamrick 1995).
The numerical scheme employed in EDFC to solve the equations of motion uses
second-order-accurate spatial finite difference on a staggered- or a C-grid. The model's
time integration employs a second-order-accurate, three-time-level, finite-difference
scheme with an internal-external mode splitting procedure to separate the internal shear
or baroclinic mode from the external free surface gravity wave or barotropic mode. The
external mode solution is semi-implicit, and simultaneously computes the two-
dimensional surface elevation field by the preconditioned conjugate gradient procedure.
The external solution is completed by the calculation of the depth averaged barotropic
velocities using the new surface elevation field. The models' semi-implicit external
solution allows large time steps that are constrained by the stability criteria of the explicit
central difference or upwind advection scheme used for the nonlinear accelerations.
Horizontal boundary conditions for the external mode solution include the option for
simultaneously specifying the surface elevations, the characteristic of an incoming wave,
free radiation of an outgoing wave or the volumetric flux on arbitrary portions of the
boundary. The model's internal momentum equation solution, at the same time step as
the external, is implicit with respect to vertical diffusion. The internal solution of the
momentum equations in terms of the vertical profile of shear stress and velocity shear,
which results in the simplest and most accurate form ofbaroclinic pressure gradients, and
eliminates the over-determined character of alternate internal mode formulations.
The model implements a second order accurate in space and time, mass
conservation fractional step solution scheme for the Eulerian transport equation at the
same time step or twice the time step of the momentum equation solution. The advective
portion of the transport solution uses either the central difference scheme used in the
Blumberg-Mellor model or hierarchy of positive definite upwind difference schemes. The
highest accuracy up-wind scheme, second order accurate in space and time, is based on a
flux corrected transport version of Smolarkiewicz's multidimensional positive definite
advection transport algorithm, which is monotonic and minimizes numerical diffusion.
The EFDC model's hydrodynamic component is based on the three-dimensional
hydrostatic equations formulated in curvilinear-orthogonal horizontal coordinates and a
sigma or stretched vertical coordinate. The momentum equations are:
0(m, mHu)+ ed(myHuu)+ ,y(mxHvu)+ e3(mxm,wu)- femxmyHv
=-mHox (p + pa ()+m (+ m x + zxHzp + 0m, m, X u
( m
+.X HAH1ux) + y HAHyu m ycD(u2 + v /' (2
mx my (4.1)
d, (mX mHv)+ 9x (myHuv) +, (mxHvv) + z (mwmywv)+ fmxmHu
= -mxHi(p + p, + )+ mx dy* +z yH)zp +( m v m, zv (4.2)
+ mx+ m
mxmyf = mxmyf- umyn7x + vdxmy (4.3)
(V Z ) = AVH-'az (u, v) (4.4)
where u and v are the horizontal velocity components in the dimensionless
curvilinear-orthogonal horizontal coordinates x and y, respectively. The scale factors of
the horizontal coordinates are m, and m,. The vertical velocity in the stretched vertical
coordinate z is w. The physical vertical coordinates of the free surface and bottom bed
are z,* and z,, respectively. The total water column depth is H, and 0 is the free surface
potential which is equal to gz,*. The effective Coriolis acceleration f incorporates the
curvature acceleration terms, with the Coriolis parameter, f, according to (4.3). The Q
terms in (4.1) and (4.2) represent optional horizontal momentum diffusion terms. The
vertical turbulent viscosity A, relates the shear stresses to the vertical shear of the
horizontal velocity components by (4.4). The kinematic atmospheric pressure, referenced
to water density, is p,,,, while the excess hydrostatic pressure in the water column is given
by:
Ip = -gHb = -gH(p-p )p,,' (4.5)
where p and p., are the actual and reference water densities and b is the buoyancy.
The horizontal turbulent stress on the last lines of (4.1) and (4.2), with A, being the
horizontal turbulent viscosity, are typically retained when the advective acceleration are
represented by central differences. The last terms in (4.1) and (4.2) represent vegetation
resistance where c,, is a resistance coefficient and Dp is the dimensionless projected
vegetation area normal to the flow per unit horizontal area.
The three-dimensional continuity equation in the stretched vertical and
curvilinear-orthogonal horizontal coordinate system is:
(mm ,H) + 9 (mHu)+ dy(mxHv) + (mxmyw)= QH (4.6)
with Q, representing volume sources and sinks including rainfall, evaporation, infiltration
and lateral inflows and outflows having negligible momentum fluxes.
The solution of the momentum equations, (4.1) and (4.2) requires the specification
of the vertical turbulent viscosity, A,, and diffusivity, K,. To provide the vertical turbulent
viscosity and diffusivity, the second moment turbulence closure model developed by
Mellor and Yamada (1982) (MY model) and modified by Galperin et al (1988) and
Blumberg et al. (1988) is used. The MY model relates the vertical turbulent viscosity
and diffusivity to the turbulent intensity, q, a turbulent length scale, 1, and a turbulent
intensity and length scaled based Richardson number, R,, by:
A, = Aql
A A,(1+R 'RI
(1+ R'RqX + R3'Rq)
4,=A (1-3C- 6A',
R7;' =3A2 -1/3-
B, B,
(B2 3A2 I- -3C,(B2+ 6A,)
1-3C -6 B
Rj = 9AIA2
R; = 3A2(6A, + B2) (4.7)
K, = ql
K0
(1 + RA'Rq)
K 4 2(1 6A, (4.8)
gHdgb 12 (4.9)
q q2 H2
where the so-called stability functions, bA and 0, account for reduced and enhanced
vertical mixing or transport in stable and unstable vertically density stratified
environments, respectively. Mellor and Yamada (1982) specify the constants A,, B,, C,,
A2, and B, as 0.92, 16.6, 0.08, 0.74, and 10.1, respectively.
For stable stratification, Galperin et al. (1988) suggest limiting the length scale
such that the square root of R, is less than 0.52. When horizontal turbulent viscosity and
diffusivity are included in the momentum and transport equations, they are determined
independently using Smagorinsky's (1963) sub-grid scale closure formulation.
At the bed, the stress components are presumed to be related to the near bed or
bottom layer velocity components by the quadratic resistance formulation
(Z- 2 U (4.10)
(x Tyz-) = (Tbx,.b) = Cb 1l +v 1, ) (4.10)
where the 1 subscript denotes bottom layer values. Under the assumption that the near
bottom velocity profile is logarithmic at any instant of time, the bottom stress coefficient
is given by
C" )2
cb ln
ln(A, /2z) (4.11)
where K is the von Kannan constant, A, is the dimensionless thickness of the bottom
layer, and z,,=z,,/H is the dimensionless roughness height. Vertical boundary conditions
for the turbulent kinetic energy and length scale equations are:
q2 = B,2/3 :z =1 (4.12)
q2 = BZ/3 Tl: =1 (4.13)
1=0 : z=0,1 (4.14)
where the absolute values indicate the magnitude of the enclosed vector quantity which
are wind stress and bottom stress, respectively.
4.3 Grid Generation
The first step in the setup of the modeling system is to define the horizontal plane
domain of the region being modeled. The horizontal plane domain is approximated by a
set of discrete quadrilateral and triangular cells. Developed on a digitized shoreline, the
grid defines the precise locations of the faces of the quadrilateral cells in the horizontal as
well as in the vertical plane. However, all the computations are carried out at the center of
the cells. Since the model solves the hydrodynamic equations in a horizontal coordinate
system that is curvilinear and orthogonal, grid lines also correspond to lines having a
constant value of one of the horizontal coordinates. The shoreline as well as the cell
reference is provided by a local set of Coordinates in MKS unit, as the code uses MKS
system internally. Seven identification numbers were used to define the cell types. The
cell identification details are given in Table 4.1.
Table 4.1 Definition of cell type used in the model input
Cell ID Definition of cell type
0 Dry land cell not bordering a water cell on a side or corner of the model
1 Triangular cell with land to the northeast of the model
2 Triangular cell with land to the southeast of the model
3 Triangular cell with land to the southwest of the model
4 Triangular cell with land to the northwest of the model
5 Quadrilateral water cells of the model
9 Dry land cell bordering a water cell on a side or on a corer of the model
The type 9 dry land or fictitious dry land cell type is used in the specification of
no flow boundary conditions. The horizontal geometric and topographic (bottom
bathymetry) and other related characteristics of the region, files dxdy.inp and lxly.inp are
used. The program then directly reads these quantities expressed in meters. The lxly.inp
provides cell center coordinates and components of a rotation matrix. Cell center
coordinates are used only in graphics output and can be specified in the most convenient
units for graphical display such as decimal degrees, feet, miles, meters or kilometers. The
rotation matrix is used to convert pseudo east and north (curvilinear x and y) horizontal
velocities (u and v respectively) to true east and north for graphics vector plotting,
according to;
fcueCc, o1 4.15
vln Co.nCln Irov J
where the subscripts te and tn denote true east and true north, while the subscripts
co denotes the curvilinear-orthogonal horizontal velocity components. The coefficient C
is the multiplier term for conversion to true east and true north.
The width of the C-18 canal, which varies between 75 m at the Southwest Fork
junction to less than 40m at the S-46 structure, dictated the dimensions of the cells. It was
decided that a 25 x 25m cell would be accurate enough for representing the width of the
C-18 canal resulting in desired level of accuracy. The same cell size was then
conveniently extended to the rest of the model domain. The bottom bathymetry was
based on the Hydrographic survey carried out in November '2001 by Lidberg Land
Surveying, Inc. However additional data for areas not covered under this survey were
obtained from other available surveys. The roughness coefficient of the bottom
bathymetry in the model is composed of two components. A fixed component viscosity
(for the present model fixed at 0.020m) and a variable component, which is varied
uniformly on the entire model domain during calibration process, both the component
together constitutes the factor z0, defined in equation 4.11. The dimensionless thickness
of the bottom layer A,, defined in the same equation, equals to 0.25, since four vertical
layers are used. The fixed component of the roughness factor, how ever can be
increased/decreased in the areas of vegetation or other special features. The details of sea
grass locations in the central embayment can be referred from Drawing no LOX-001
(Cuthcher & Associates, Inc. Coastal Engineer, 2002) provided by the Jupiter Inlet
District. The sea grass was input in the model as an overlay file. In this way the cells
having the sea grasses are enclosed by a polyline so that, the roughness coefficient can be
easily edited. The sea grass was represented as cells having more roughness (fixed
component = 0.040m) than that of the surroundings. In Figure 4.1 the input bathymetry
and the shoreline as generated by the model are shown.
4 4*
Figure 4.1 Model domain showing input bathymetry and shoreline
..'-
Figure 4.2 Computational grid showing the flow boundaries
In the computational grid (Figure 4.2), each land cell was assigned number zero or
nine as the case may be and each water cell was assigned five. There were no triangular
cells used for this grid. Figure 4.2, in addition, indicates the locations of the tide gages
and the Instrument tower in the Southwest fork. The S-46 structure in the C-18 canal is a
flow boundary (black cells), as are the two main tributaries, and the FECRR bridge on the
East. The eastern boundary was restricted to the FECRR bridge. The flow boundaries
were kept straight; so as to allow flows perpendicular to the cell faces, as the model does
not allow non-orthogonal flows.
4.4 Boundary Conditions
In the beginning of the simulation, velocities throughout the model domain are
considered to be zero. It was observed that a full tidal cycle was required before the water
surface elevation reached a quasi-steady state. This was verified by recording water
surface elevations at the location of the two tide gauges (UFG2 and UFG3) over multiple
tidal periods.
Tidal forcing at the FECRR bridge (eastern boundary) is perhaps the most
important boundary condition in this system, because it is this mechanism by which the
majority of the water flows through the estuary. The data obtained from the UFG1 gage
(Figure 2.10) were used to simulate this forcing. The raw data were examined for the
mean trends in the water surface elevation (Figure 4.3). The raw data contains a sub-tidal
frequency trend, which was also noticed in the water surface elevation data of the Miami
Harbor. The trends were of a similar in nature and therefore it was hypothesized that
onshore winds may have created increased elevation in side the estuary. The wind records
from two offshore sites (37 and 221 kilometer east of Cape Carnival, Florida) were
correlated with the mid-tide elevation, which indicated a positive correlation (Ganju et
al., 2001). In order to overcome the effects of these variations imposed on the
astronomical tide, the mid-tide elevation was subtracted from each measured elevation in
the same tidal cycle. The mid tide elevation q, is given by Equation 4.1, where, Hr/ and
qLT are the water surface elevation at high and low tides respectively.
Hc rlH+LT (4.16)
2
I I
0.81
0.6
04
S-04
0
I-
1-
,0 -8
z
0
0
-J
ii ii; iI ;ii; 1i*
50 100 150 200 250
TIME (h)
300 350 400
Figure 4.3 Tidal time series from UFG1, 09/14/00-10/13/00, a) Raw data, b) Tidal plot
after the mid-tide trend is removed. Time origin 12:00 am.
50 100
150 200 250
TIME (h)
06-
o-g
CO04-
z02-
0
Z
028
I-
0 ...
0.p
(I)
300 350 400 450...................
300 350 4DO 450
I
; i :
: i I;i iiiii, i
,,,
i i
In Figure 4.3a the raw tidal time series is shown along with the tidal trend and in
Figure 4.3b the tidal time series is shown after subtracting the mean-tide trend. The
eastern boundary accordingly used this water surface elevation boundary condition.
For the boundary in the C-18 canal, two sets of boundary condition data were
available. The daily average flow time series of the S-46 structure and the water surface
elevation time series. The elevation time series was obtained from the tide gauge UFG 3
(same period as at UFG 1) installed in the Southwest Fork (Figure 2.10). In order to make
these data usable at the flow boundary (S-46 Structure) amplitude corrections were
carried out by trial and error till both predicted and measured time series matched. In
order to calculate the phase correction (lag) following calculations were carried out
assuming shallow water conditions. The tidal wave celerity C is given by,
C=-gh (4.23)
where, g is the acceleration due to gravity and h is the water depth. Then the phase
shift AT is given by,
C g = =AL. AL
AT AT = (4.24)
C
where, AL is the distance for which the water depth is considered uniform, accordingly
the phase lag for the distance between the UFG 3 gage station and the S-46 structure was
calculated and verified (0.13 hour). Figure 4.4 gives the plot of the raw data collected at
UFG 3, including the mean trend and the amplitude with trends removed. It was
hypothesized that these data, corrected for the phase and amplitude could be applied as
boundary condition to simulate actual flow conditions.
a
00
cO 04f
z
r -J
j -061
TIME (h)
'ii
ii iii'
ii
!1 1i
-08
0 50 100 150 200 250 300 350 400 450
TIME (h)
Figure 4.4 Tidal time series from UFG3, 09/14/00-10/13/00, a) Raw data, b) Tidal plot
after the mid-tide trend is removed. Time origin 12:00 am
Note that the flow discharge time series (Figure 4.5) from the S-46 structure was
selected, as the model is known to be giving better simulation results under discharge
boundary condition.
IIIi"I iji
ii tl j i ii ~i I
11;: ii1I i I1!
b
0.6~
> 04
Lo
Uj
z
02
LI-
O 0
w
*u 1, 0, ,150
-----.-- -~~~---~------~----~------i~-----~-,--~~
I iI j, i i 'i:
ir i. i jl I1 i
!r : i i I `i i i i
ii i I: I i'
i i i *
70.00
60.00
') 50.00
E
w 40.00
4 30.00
Q 20.00-
10.00
0.00 LJ
0.00 500.00 1000.0 1500.0 2000.0 2500.0 3000.0 3500.0 4000.0 4500.0
0 0 0 0 0 0 0 0
DAYS
Figure 4.5 Flow time series applied at S-46 boundary
70
60
M 50
W 40
< 30
_. 20
10
0
0 1000 2000 3000 4000 5000
DAYS
Figure 4.6 Flow time series applied at Northwest Fork boundary
In the Northwest Fork boundary as well, two sets of boundary conditions, namely,
the water surface elevation boundary condition (obtained from transferring the collected
data of the tidal station UFG 2) and flow discharge boundary condition were evaluated.
The flow time series used is shown in Figure 4.6.
Table 4.2 Amplitude and phase correction factor for the tides
Boundary Amplitude factor Phase correction
C-18 1.14 0.13 hour
Northwest Fork 1.18 0.042 hour
Per U.S. Geological Survey Report 84-4157 (Russell and McPherson, 1984) the
majority (77.3 %) of the fresh water flow in to the estuary enters through the Northwest
Fork. Therefore, the flow discharge boundary condition for this tributary was considered
as most appropriate as opposed to the water surface elevation. The corrected water
surface elevation data from UFG 2 tide gage was used for calibration.
The North Fork carries the least discharge (2.2%) of the total freshwater flow in
to the estuary in the mean, (Russell and McPherson, 1984)) hence at this boundary also
flow discharge boundary condition is applied. The flow discharge was worked out from
2.2
the Northwest boundary data applying a constant multiplier (-. = 0.0285).
77.3
4.5 Model Calibration and Validation
4.5.1 Calibration
In general calibration of the model aims at simulating conditions identical or close
to that in the prototype so that prototype conditions can be accurately replicated and
reproduced. Calibration involves matching multiple parameters, which is often times, is
practically impossible. However, depending on the nature experiments and the results
desired, the type of calibration differs. Since the present model simulation aims to relate
the velocity and the associated stress field to the erosion/accretion of the sediments in the
estuary, it would be highly desirable to calibrate the model with comparison of the flow
velocities. But current data for the model simulation period, between September 14th
2000 and 18th October 2000 was not available and therefore, it was decided to calibrate
the model using the data collected at the instrument station located in the Southwest Fork
(Figure 3.1) between November 26th and May 15th 2003 for which current as well as
water surface elevation data was available. The amplitude multiplier and phase lag
factors are given in Table 4.2.
Accordingly a simulation for this period was carried out using flow discharge
boundary conditions for the Southwest, the Northwest and North tributary boundaries and
water surface elevation boundary condition for the East boundary. For the eastern
boundary the tidal data from Miami Harbor were "transferred" to the FECRR bridge
boundary by applying suitable correction factors for the amplitude and the phase lag. This
procedure was carried out in two steps. In the first step, the Miami harbor data for the
period 14th September 2000 to 18th October 2000 were transferred to the boundary with
application of recommended coefficients (for method of calculation refer to NOS Tide
Tables for year 2000). The calculated tidal elevations were compared with the UFG 1
data and the final multiplication correction factor was obtained as 1.023. For the model
simulation period in year 2002 the same correction factor was used to transfer tidal
elevations of Miami harbor to the flow boundary.
Model calibration began with an initial run for 48 hours (referred to as 'cold start')
in order to make the tide and discharge mutually compatible throughout. In addition, the
flow attains stability in this period. The results of the cold start period were compared
with the current velocities as well as the water surface elevations obtained from the
instrument tower. The process was continued by changing the variable component of the
bottom friction coefficient (one component of z0) (applicable uniformly throughout the
model domain), until an approximate match of the current magnitude and phase was
obtained. In the second step RESTART.OUT and RSWT.OUT, the two output files of the
cold start were used as input, and model run was performed for a longer period (15days)
in order to obtain simulation for final calibration. The predicted and measured currents
were then compared and is given in Figure 4.7a (Cold start) and 4.7b (Hot start) for a
variable bottom friction factor of 0.027. It can be seen that the agreement is very good for
the current, with a maximum error of 1.8% of the total current amplitude. The water
surface elevation however differs by about 2.8 cm, which is about 3% of the tidal
amplitude. Since current is in better agreement with the measured data the calibration was
considered accurate enough for simulation. In addition, comparison of the predicted and
measured current direction exhibited good agreement as indicated in 4.8.
4.5.2 Model Validation
Model validation was carried out using the same calibrated parameters and
simulating the flow conditions of year 2000 (between September 14th and October 18th).
The measured as well as the model results at both the tidal gage stations after cold start as
well as hot start periods are compared and reproduced as Figures 4.9 and 4.10. As
indicated in the figure 4.10, the agreement is fairly accurate with a maximum variation of
2.7 cm, which is about 3.4% of the maximum tidal amplitude reported in the estuary.
Similar validation was also carried out using the Northwest Fork data collected between
September 3rd and September 12th which also showed equally good agreement as shown
in Figure 4.11.
0.06
0.04
0.02
0.00
331.00 331
-0.02 '
-0.04
-0.06
-0.08
-0.10
I*
JULIAN DAYS IN 2002
-- - -Model Run Measured
0.05
0.04
0.03
0.02
0.01
0.00
34600
-0.01 -
-0.02
-0.03
-0.04
-0.05
-0.06
JULIAN DAYS IN YEAR 2002
- - - Model run Measured
Figure 4.7 Model calibration measured vs. predicted current, a) Cold Start, b) Hot Start.
82
200.00
150.00
S100.00
H 50.00
0
5 0.00o
u 333.00 33.20 3 3.40 33 0 333.80 334.00 .20 334 0 334
Q -50.00
I--
z
S -100.00
-150.00
-200.00
-250.00
JULIAN DAYS IN YEAR 2002
...... Model --- Measured
Figure 4.8 Model calibration measured vs. predicted current direction.
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