Modeling Flow through Wetlands

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Modeling Flow through Wetlands

• Wayne Dodgens
• Chad E. Edwards
• Amy Gross

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Modeling Flow Through Wetlands

• Groundwater
• Surfacewater
• Groundwater-Surfacewater Interactions

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Hydrologic Cycle
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Groundwater

• The water stored in interconnected pores located
  below the water table in an unconfined aquifer or
  located in a confined aquifer. (Fetter 2001)
• That part of the subsurface water that is in the
  zone of saturation, including underground
  streams. (Glossary of Geology, 4th ed.)

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Groundwater
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Groundwater
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Reasons groundwater is of concern to wetlands
studies

• Ecological concerns
• Can store and also filter contaminated fluids
• Groundwater-surfacewater interactions

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• Animation from www.mhhe.com

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Saltwater Intrusion
www.mhhe.com
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How do you model groundwater flow in wetlands?

• Treated the same as any groundwater
  investigation.
• -Surface mapping
• -Subsurface characterization
• Soils
• Water
• -Modeling

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Darcys Law

• Q -k i a
• Q discharge
• k hydraulic conductivity
• i hydraulic gradient
• a area

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Soils cont.

• Data acquired from
• Soil borings
• Well cuttings
• Cores
• Geophysical techniques

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Soils
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Hydraulic Conductivity (Permeability)

• Gravel 10-2 1 cm/s
• Fine sand 10-5 - 10-3 cm/s
• Clays 10-9 - 10-6 cm/s
• Peat 10-3 108 m/day
• (Fetter 2001)
• (Wise et. al.)

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Hydraulic Conductivity cont.

• Flow rates from tests run during and after
  drilling of the monitoring wells
• Inferred hydrologic parameters based on
  inspection of samples.
• Assumed values for materials from published
  values in previous literature.
• Estimates based upon the grain size distribution
  curve for samples run through a sieve analysis

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Hydraulic Gradient

• Monitoring wells
• Piezometers
• Hydraulic head values
• Hydraulic gradient change in head over distance
  or (?h / ?l)

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Wetland flow possibilities
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Case Study
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Study Area Jensen Beach, Fla.
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Study Area

• Pine flatwoods of Savannas State Preserve
• Circular shape 60m diameter
• USFWS designation palustrine, persistent,
  emergent, nontidal and seasonally flooded wetland

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Vegetation - upland
Dahoon holly
Wax myrtle
Saw palmetto
http//www.gillespiemuseum.stetson.edu/grounds/lis
t.html
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Vegetation - interior
St. Johns Wort
Blue Maidencane
Duck potato
Maidencane
http//sofia.er.usgs.gov/virtual_tour/pgbigcypress
.html http//www.gillespiemuseum.stetson.edu/gr
ounds/list.html
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Geology

• Underlain by the surficial aquifer Upper
  Miocene to Pleistocene 45-52m thick
• Upper 12-18m fine to coarse grained sand
  intermixed with shell beds
• 3-6m layer of fine sand with a few shells
• Lower layer of limestone and calcarenite mixed
  with shells and sand

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Site Geometry

• Sediment surface contouring during flooded
  conditions
• 3m intervals along N-S E-W, NW-SE SW-NE
  transects
• Peat thickness was measured by pushing 1cm rebar
  through the peat until higher resistance
  indicated the sand layer

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Methods

• The basic idea behind this study is to pump
  enough surfacewater from the wetland so that its
  relationship to the underlying aquifer can be
  assessed based on the rate at which the wetland
  levels recover due to groundwater seepage from
  below.
• Monitoring of 6 wells in the marsh interior, and
  12 wells outside the area, for initial head
  values and the lowering and subsequent rise of
  head values throughout the experiment.

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WAIT well transects
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Results
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Results
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Conclusions

• Model agrees with data for smaller time
  increments while extrapolation to longer periods
  may involve inclusion of more variables
• WAIT quantifies the resistance to flow between
  wetland and aquifer
• AWIT to determine variability in the vertical
  hydraulic conductivity depending on direction

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Computer Modeling

• Computer models are used to help hydrologists
  understand how flow systems work and sometimes to
  project how flow systems might be affected by
  changes in the hydrologic cycle.
  http//ut.water.usgs.gov/modelsb.html
• More than 40 models have been developed or are
  being developed.

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Modeling

• Different programs solve for parameters dependant
  on the study design.
• Current programs are combining the capabilities
  of existing software into packages that can
  deliver results or predictions for numerous
  parameters

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GMS v.4.0
All images http//www.ems-i.com/GMS/gms.html
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Visual Modflow Pro v3.1
Animationhttp//www.visual-modflow.com/html/visua
l_modflow.html
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Modeling Surface Water Flow in Wetlands

• A non-mathematical explanation of a mathematical
  process

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Development and evaluation of a mathematical
model for surface-water flow within the Shark
River Slough of the Florida Everglades

• Carl H. Bolster, James E. Saiers

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Why develop a model for surface water flow
through wetlands?

• Wetlands are beginning to be appreciated for
  their value to society
• The future management and restoration of wetlands
  relies on a quantitative understanding of surface
  water flows over vegetation

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• Over the last 50 years, 1000 miles of canals, 720
  miles of levees, and nearly 200 water control
  measures have been implemented in the Florida
  Everglades

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• The restoration plan of 7.8 billion will include
  re-engineering the ecosystem to capture most of
  the water that is now being diverted to the ocean
  and use 80 of it for environmental restoration
  and the remaining 20 for societys water needs

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Planners need to be able to predict the effect on
wetlands of actions such as

• Removing canals and levees
• Removing dams
• Redirecting flow from canals to wetland sloughs

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The model developed in this study is a
two-dimensional model for surface water movement

• The model was tested against hydrologic data
  measured in Shark River Slough in the Fl.
  Everglades

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Assumptions of the model include

• Uniform rates of evaporation
• a constant ground surface slope
• spatially homogenous vegetation cover
• constant values for wetland porosity
• exchanges between surface water and subsurface
  water are negligible

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Overland flow models are determined by the
properties of the wetland Bed shape
irregularities (such as hummocks and depressions)
and vegetation density control resistance to flow
and the magnitude of the models friction
coefficient
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Variable data regarding the ground-surface slope
represent the effects of gravity on the movement
of water across the surface of the wetland Data
on evapotranspiration , rainfall, and groundwater
exchange also contribute to the designing of an
accurate surface water flow model for wetlands
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Field measurements of hydraulic head (water
level) were obtained from databases operated by
the USGS and Everglades National Park. Daily
measurements were compiled by averaging 15-minute
interval data
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Results of the Shark River study

• The model successfully predicted two observed
  decreases in hydrologic head occurring from Jan.
  17,1998-July 29, 1998 and from Aug. 14, 1998-Dec.
  30, 1998.
• Also, the model coincides with rainfall-induced
  head oscillations recorded at the monitoring
  sites

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• The model is not perfect, however
• Between May 1998 and July 1998 the model
  overestimated the observations at one recording
  station and underestimated the observations at
  another
• This was presumed to have been caused by
  violations of the uniform wetland properties
  assumption

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• The model did predict accurately the temporal and
  spatial changes in surface water levels over a 27
  km long area of Shark River Slough
• Results suggest that good predictions of wetland
  flow over relatively large scales can be obtained
  with simple mathematical models, without allowing
  for varying wetland properties

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• The authors of the study conclude surface water
  flow for extended time periods , over larger
  expanses, can be predicted with reasonable
  accuracy without the need to model changes in
  wetland parameters

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A 2-dimensional, diffusion-based wetland flow
model (WETFLOW) Ke Feng and F.J. Molz Two cases
are presented in this study a laboratory testing
of the model and the model applied to a wetland
pond in Talladega National Forest near
Moundville, AL This model was developed to be
applied to a general wetland type

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This small wetland was created when beavers
dammed a perennial stream
This is a Riparian wetland, one that is adjacent
to a body of water and is flooded on a regular
basis
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Flow domain boundaries (outlines of study area)
and outlines of the islands must be defined The
varying boundaries of a wetland provide a problem
to the mathematical modeling of surface water
flow The boundaries of a wetland may change with
time, due to flooding events and drought
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This model has many positive attributes

• The model allows for variations in wetland
  characteristics
• The model applies to both 1-D and 2-D flow fields
  (evidenced by the laboratory study and the
  wetland study)
• During drought and flood events, the model can
  identify changing wetland boundaries

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• This model can be used (as a hydrodynamic basis)
  for wetland research involving transport,
  chemistry, and biology
• The authors of the study concluded that
  micro-topography and the distribution of flow
  resistance are the two parameters that must be
  measured in detail, and not assumed, in order to
  build an accurate model

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Numerical Representation of dynamic flow and
transport at the Everglades/Florida bay
interfaceDr. Eric Swain USGS

• Southern Inland and Coastal Systems Numerical
  Model (SICS)

This model was developed by starting with the
USGS Swift 2-D model, and was then modified to
make it applicable to the Everglades
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Model input data

• The model area is characterized by topography,
  vegetation, wind friction coefficient, and
  bathymetry
• Hydrologic data is then incorporated rainfall,
  evapotranspiration, salinity time series data,
  and water discharge at the boundaries of the
  study area

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• There must be observed data on hand to compare to
  the results of the model the amount of water
  discharged at coastal creeks, at the boundaries,
  and within the study area
• Calibration data

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• The Southern Inland and Coastal Research Systems
  (SICS) will be discussed more by the next
  presenter (groundwater and surface water
  interactions)
• Several papers were researched in studying
  modeling surface water flow through wetlands,
  most of these papers deal with the mathematical
  equations of the models

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• The models usually contain a series of
  differential equations that work together
• There will be separate equations for different
  aspects of wetland hydrology

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conclusions

• There are a few mathematical models used for
  modeling surface water flow through wetlands
• These models may be modified to apply to a
  particular type of wetland Everglades, riparian,
  etc.
• All of these models attempt to provide a
  relatively simple means to model wetland flow
  without the need to account for minor changes in
  topography, porosity,etc.

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• Overall, the authors of the papers presented
  report relatively successful models, that have
  correctly predicted observed changes in the
  surface water flows in the wetlands studied
• It is important to have reliable models that
  allow us to understand and predict changes that
  may occur in surface water flows in a wetland due
  to human intervention whether those changes are
  for better (tearing down control structures) or
  for worse (building structures that resist
  wetland flow)

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References

• Development and evaluation of a mathematical
  model for surface water flow within the Shark
  River Slough of the Florida Everglades. Carl H.
  Bolster, James E. Saiers. Journal of Hydrology
  259 (2002)221-235
• A 2-D, diffusion-based, wetland flow model. Ke
  Feng, F.J. Molz. Journal of Hydrology 196 (1997)
  230-250
• Numerical representation of dynamic flow and
  transport at the Everglades/Florida Bay
  interface. Dr. Eric D. Swain, USGS

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Ground and Surface Water Interaction

• Examine the effects of fluxes in water between
  the ground and surface
• Study the effects of these movements on solutes
  Organic (carbon), inorganic (nitrogen),
  pollution (mercury)

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Ground and Surface Water Interaction

• Ecological effects salinity front movements
• Used to study the effects of management practices
  on hydrology

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Interactions Between Groundwater and Surface
Water Models

• Case Study
• The Tides and Inflows in the Mangroves of the
  Everglades (TIME)
• And
• Southern Inland and Coastal Systems (SICS)

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Introduction

• A critical goal of the Comprehensive Everglades
  Restoration Plan (CERP) is to restore and
  preserve the hydrology of the predrainage
  ecosystem to provide ecological conditions
  that are consistent with habitat requirements.

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Introduction

• SICS will investigate wetland response to
  freshwater inflows and to compute resultant
  salinity patterns and concentrations in the
  subtidal embayments of Florida Bay as functions
  of freshwater inflows

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SICS Study Area
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The dynamic surface-water model is connected to a
three-dimensional ground -water model
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SICS

• What effects hydrologic changes to Taylor Slough
  and C-111 will have on
• Hydroperiods and Hydropatterns
• Quantity, timing, and location of freshwater flow
• Development of hypersaline conditions and excess
  nutrients and contaminants

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SICS

• An existing, generic, two-dimensional
  surface-water flow and transport model was
  coupled to a fully developed, generic,
  three-dimensional variable-density ground-water
  flow and solute-transport model

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TIME

• TIME is an investigation into the interacting
  effects of freshwater inflows and coastal driving
  forces in and along the mangrove ecotone of
  southern Florida within Everglades National Park

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Satellite image of south Florida covering
Everglades National Park, 1500,000 scale
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Satellite image showing TIME model boundary Scale
1500,000
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Development of the TIME Model

• An extension of the SICS model westward
• Required many new, high resolution data sets to
  be created including, topography, vegetation, and
  other hydrographic data

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Primary Objectives of the TIME Project

• Develop, implement, and use a mathematical model
  to study the interaction of overland sheet flow
  and dynamic tidal forces
• Including flow exchanges and salinity fluxes
  between the surface- and ground-water systems
• In the mangrove-dominated transition zone between
  the Everglades wetlands and adjacent
  coastal-marine ecosystems

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Goals of the TIME project

• to provide
• new scientific insight,
• additional quantitative information,
• more comprehensive data
• a refined hydrodynamic model to help guide and
  assess restoration and management decisions for
  this critical ecosystem.

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Questions Addressed by TIME

• How do the Everglades freshwater-wetland and
  coastal-marine ecosystems respond concurrently,
  both hydrologically and ecologically, to
  regulation of inflow?
• Will upland restoration actions affect the
  transformation of freshwater wetlands to brackish
  and marine marshes and subsequently to mangrove
  marsh ecotones?

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Questions Addressed by TIME

• How will changes in inflows act in concert with
  predicted increases in sea level to affect
  migration of the freshwater/saltwater interface
  within the surface and subsurface flow systems?
• What key factors influence salt concentrations in
  the coastal mixing zone and how do these factors
  interact to affect wildlife habitat areas?

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Questions Addressed by TIME

• How will external dynamic forcing factors, such
  as sea level rise or meteorological effects,
  adversely affect upland regulatory plans?
• What concurrent changes in wetland hydroperiods
  and coastal salinities are likely to occur in
  response to various proposed restoration and
  management plans?

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Data sets used in model

• vegetation characteristics
• aquifer properties
• surface-water levels,
• ground-water heads,
• flow velocities,
• structure discharges,
• tidal fluctuations,
• salt concentrations,
• Rainfall events,
• and meteorological conditions

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Findings to Date

• Water management has increased recharge and
  discharge in the north-central Everglades above
  pre-drainage conditions
• Mercury is being recharged from surface water to
  groundwater and stored in the surficial aquifer

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Findings to Date

• Ungaged freshwater flows discharging from
  groundwater into Taylor Slough were quantified
  for the first time
• Significant recharge and discharge occurs by
  vertical flow through Everglades peat in areas
  that are far from boundaries with levees and
  canals

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Findings to Date

• Discharge of deep groundwater from relict
  seawater origin beneath WCAs cannot explain the
  contaminant-level concentrations of sulfate in
  Water Conservation Areas

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Conclusions

• Models are very useful and powerful tools
• Predict effects of management practices
• Allow officials to make management decisions
  based on more than speculation
• Predict effects of natural phenomena