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FS-139-96
U.S. Department of the Interior
U.S. Geological Survey
FS-139-96
Coupling Models for Canal and Wetland Interactions in the South Florida Ecosystem
Introduction
The U.S. Geological Survey is one of several agencies participating in the
scientific effort to provide knowledge that can help protect and preserve the
ecosystem of south Florida. One project of the intergovernmental
South Florida Ecosystem Program (SFEP)
is focused on developing a computer model to simulate the flow of water and
analyze the transport of waterborne chemical constituents between canals and
wetlands. Quantification of dynamic flows within the south Florida ecosystem is
vital to understanding the implications of the residence time of water,
potentially nutrient-enriched (with nitrates or phosphates) or contaminant-laden
(with metals or pesticides), that can alter plant life and affect biological
communities. Nutrients carried in the water conveyed by canals draining
agricultural areas and dispersed into wetlands by canal discharges, by levee
overflows, or by seepage are considered to be a major contributor to changes in
the types of vegetation found in the Everglades. Freshwater inflows, typically
of varying magnitudes and durations, not only influence the salinity of Florida
Bay but also potentially carry toxic substances that can affect and alter the
Bay's aquatic biota. The simulation capability being developed within the SFEP
can be useful for identifying approaches to alleviate adverse impacts of
excessive or deficient flows and transported constituents. Through strategic use
of a simulation model, cause-and-effect relations between discharge sources,
flow magnitudes, transport processes, and changes in vegetation and biota can be
investigated. The effects of driving forces on nutrient cycling and contaminant
transport can then be quantified, evaluated, and considered in the development
of remedial management plans.
Ecosystem Flow/Transport Characteristics
Flow behavior and transport properties in low-relief environments such as
south Florida are complex. Flows from canals to neighboring wetlands and
reciprocal runoff flows are the combined result of hydraulic, inertial, and
meteorologic forces. Because velocities are extremely low, water movement is
highly susceptible to winds that potentially can reverse the flow direction and
even cause a wetland area to drain or flood. Wetlands adjacent to Florida Bay
also are subject to tidal effects that further complicate flow analyses.
Moreover, flow and transport are integrally linked meaning that precise
quantification of the fluid dynamics is required to accurately evaluate the
transport of waterborne constituents. In other words, in order to investigate
the movement of constituents, it is essential to understand the forces governing
the flow of water that is the transporting media.
Additional complications are introduced in the south Florida ecosystem by a
diversity of natural and man-made flow controls. A complex canal and levee
system, designed to control flooding and provide a continuous supply of fresh
water for household and agricultural use, has altered naturally occurring flow
patterns through the Everglades and into Florida Bay. Flow in the straight,
uniform canals is predominately confined to one direction and can be readily
characterized in terms of mean cross-sectional properties. Flows in the
wetlands, however, can be highly irregular in direction in response to varied
topographical patterns and vegetative features. Improved numerical techniques
are needed not only to accurately evaluate discrete forces governing flow in the
canals and wetlands but also to analyze their complex interaction in order to
facilitate coupled representation of transport processes.
Canal C-111 Drainage Basin
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Figure 1 - Aerial photograph, approximately 1:88,000
scale, of portion of canal C-111 drainage basin taken on January 15, 1994,
showing gaps cut in spoil mound along southwest bank. (click on image
to view larger image.)
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One area of particular interest in terms of flow-distribution analyses is
the drainage basin of the canal identified as C-111 in southern Dade County
(fig. 1). A dam and water-control structure, S-197, which
regulates freshwater flows and prevents saltwater intrusion from Florida Bay, is
located near the bend in the canal at the middle right in the aerial photograph.
Gaps have been cut in the spoil mound along the southwest bank of the canal in
an attempt to reestablish shallow surface-water flow, referred to as sheet flow,
to the south into Florida Bay. Tree islands (dark red, teardrop-shaped, forested
areas) and elongated patterns of vegetation along channel banks identify the
historical direction of sheet flow to the Bay. These indicate that impediments
to the natural surface runoff and drainage patterns have been created by the
construction of roads, canals, levees, and hydraulic control structures. Major
water-management issues having to do with flow-control measures for the C-111
drainage basin are:
- What are the cause-and-effect relations between tides,
winds, and altered freshwater flows on neighboring wetlands, mangrove
ecosystems, and coastal water bodies?
- What are the effects of outflows from C-111 on salinity dynamics of
Florida Bay?
- What processes control the fate of nutrients or contaminants and govern
their dispersal into neighboring wetlands and adjacent ecosystems?
- What are the consequences of various C-111 redesign alternatives on
inflows to Manatee Bay, Barnes Sound, and other coastal water bodies?
- How do the dynamics of outflows from C-111 affect sheet flow through
wetlands in the adjacent Everglades National Park?
- How does canal C-111 interact with other hydrologic and hydrodynamic
systems?
Model-Implementation Requirements
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Figure 2 - Ground contours in the Glades topographic
quadrangle of south Florida. (Click on map for full-sized version.)
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From precisely measured land-surface elevations, computational grids that
depict flow systems can be developed for implementing a mathematical model.
Special projects of the SFEP, such as the Elevation
Data project, are employing advanced mapping techniques to collect data
defining topographic patterns and land-surface features at the high resolutions
required for model implementation. Roads, canals, levees, and wetlands need to
be accurately described in the model grid and features such as culverts, pumps,
levees, and other hydraulic-control structures must be properly represented.
Ground contours of one preliminary model grid encompassing the Glades
topographic quadrangle of the C-111 drainage basin are illustrated in
figure 2. Ground contours in
figure 2 are derived from one-hectare (100 meter square)
grid cells interpolated from land-surface elevations collected using the
satellite-based NAVSTAR Global Positioning System (GPS). The preliminary model
grid of 125 by 130 one-hectare cells is illustrated in
figure 3.
Roadways, canals, and levees have yet to be integrated in this preliminary
computational grid. Land-surface elevations in Barnes Sound and bordering
mangroves (cross-hatched area in figure 2) are also
presently undefined due to the lack of data. A slight northwest to southeast
gradient is evident in the land-surface elevations. Precise resolution of this
gradient and definition of other topographic features are required to accurately
simulate sheet flow through wetlands of the south Florida ecosystem. Mangroves
and other types of dense vegetation present unique mapping difficulties and new
techniques are being explored within the SFEP to survey these areas of limited
accessibility.
In addition to required topographic data, flow levels and rates, needed for
model calibration and for the conduct of numerical simulations, are being
determined by other SFEP projects, such as the Freshwater Discharge to East
Coast and Florida Bay projects, using conventional and advanced acoustic
measurement techniques.
Model Development
The capability of a mathematical model to simulate flow and transport in a
complex environment such as the south Florida ecosystem is highly dependent on
its representation of important forces and processes. Of particular concern in
the development of models for the south Florida ecosystem is representation of
precipitation gains from rainfall, frictional effects due to vegetation, driving
forces of winds, and losses by evapotranspiration and ground-water infiltration.
New techniques for expressing these processes in numerical simulation models are
being investigated and data-collection efforts by complementary projects of the
SFEP, such as the Vegetative
Resistance to Flow and
Evapotranspiration
Measurements projects, are seeking improved equation representations founded
on field-defined parameters. As new expressions for these processes are defined,
numerical approximations are developed and tests are conducted using the model.
Tests are typically made using known theoretical solutions and, eventually,
using fine-scale grids of limited canal and wetland areas for which detailed
data can be collected.
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Figure 3 - Grid of 125 by 130 one-hectare cells of
land-surface elevations. (Click on image above for full-sized version.)
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Very often the nature of flow and transport conditions also dictate the need
for much finer grid resolution than the one-hectare cell size illustrated in
figure 3. Geometric conditions that typically require
fine-scale resolution are transition areas between deep canals and shallow
wetlands. Such a transition area from the C-111 drainage basin is depicted in
figure 4. Velocity vectors, simulated by the model over a
290 meter square area, illustrate a flow pattern through one of the 54 gaps in
the C-111 southwest spoil mound. In this grid, each arrow indicates the
direction of flow and average speed in a 100 meter square area. The approximate
in-bank flow condition of the C-111 canal is identified by the parallel dashed
lines in
figure 4. For visualization purposes, lengths of vectors
representing flow velocities in the wetland have been increased by a factor of
ten over those in the canal. This was necessary because of the large disparity
in flow velocities, which are a maximum of 0.32 m/s in the canal and 0.07 m/s in
the wetland for this particular simulation. Note the uniformity of flow
velocities in the canal and the non-symmetrical, residual effect of the
left-to-right flow direction through the gap and into the wetland. Such subtle
but important flow features can easily be lost in coarse-scale model grids or in
models that do not appropriately treat all relevant forces affecting the flow
through such transitions.
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Figure 4 -- Grid of 29 by 29 model-simulated velocity
vectors illustrating flow pattern through one gap in spoil mound along southwest
bank of canal C-111. (click on image above for full-sized version.)
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Summary
The interconnected canals, wetlands, and underlying shallow aquifer of south
Florida represent a geometrically complex, physically diverse, and dynamically
changing hydrologic environment. Mathematical models of this interconnected
hydrologic system must include terms in their governing equations that represent
all relevant hydraulic forces and transport processes. Moreover, high-quality
data that accurately depict the topographic properties are required for proper
implementation and use of resultant models. In this regard, several hydrologic
investigations and efforts, directly related to flow and transport model
development for the south Florida ecosystem, are underway. Model-related
activities being conducted within this project and(or) by other complementary
projects of the SFEP are intended to:
- Improve fundamental concepts and numerical expressions by
which to represent the variable effects of precipitation, constituent sources,
winds, resistance caused by vegetation, and losses due to evapotranspiration and
ground-water infiltration.
- Evaluate existing techniques and formulate new approaches for coupled
treatment of flow and transport conditions between canals, wetlands, and the
underlying aquifer system.
- Develop and appraise new techniques and approaches, using
state-of-the-art instrumentation and systems such as acoustic Doppler
velocimeters and Differential Global Positioning System (DGPS) technology, to
collect basic data required to implement and calibrate numerical models.
- Design and develop computer tools, using the latest Geographic
Information System (GIS) and scientific visualization products, to facilitate
integration of model input data and scientific interpretation of simulation
results.
- Provide techniques to identify and quantify flow losses and
canal/wetland exchanges due to leakage through and(or) beneath spoil mounds and
levees.
- Collect data and develop ratings to quantify freshwater flows through
primary channels into Florida Bay.
- Test and evaluate enhanced and new advanced mapping techniques with
which to collect high-quality elevation data needed to identify topographic
features at spatial-resolution scales required for model implementation and
numerical simulation.
It is anticipated that this model-development effort will provide improved
tools for water managers to use in their analysis and evaluation of alternative
management plans and strategies for the south Florida ecosystem.
FS-139-96 June 1996
By Raymond W. Schaffranek
Click here for a printable version of this fact sheet (note: document will
open in a new browser window)
For more information contact:
Raymond W. Schaffranek
U.S. Geological Survey
National Center, MS 430
Reston, VA 20192
(703) 648-5891
rws@usgs.gov
Related information:
SOFIA Project: Canal and Wetland Flow/Transport Interaction
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