Modeling Flow through Wetlands

Wayne DodgensChad E. Edwards

Amy Gross

Modeling Flow Through Wetlands

Groundwater

Surfacewater

Groundwater-Surfacewater Interactions

Hydrologic Cycle

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.)

Groundwater

Groundwater

Reasons groundwater is of concern to wetlands studies:

Ecological concerns

Can store and also filter contaminated fluids

Groundwater-surfacewater interactions

Animation from www.mhhe.com

Saltwater Intrusion

www.mhhe.com

How do you model groundwater flow in wetlands?

Treated the same as any groundwater investigation.

-Surface mapping-Subsurface characterization:1. Soils 2. Water

-Modeling

Darcy’s LawQ = -k * i * a

Q = dischargek = hydraulic conductivity

i = hydraulic gradienta = area

Soils cont. Data acquired

from: Soil borings Well cuttings Cores Geophysical

techniques

Soils

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.)

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

Hydraulic Gradient Monitoring wells

Piezometers

Hydraulic head values

Hydraulic gradient = change in head over distance or (Δh / Δl)

Wetland flow possibilities

Case Study

Study Area – Jensen Beach, Fla.

Study Area Pine flatwoods of Savannas State

Preserve

Circular shape – 60m diameter

USFWS designation: palustrine, persistent, emergent, nontidal and seasonally flooded wetland

Vegetation - upland

Saw palmetto

Dahoon holly

Wax myrtle

http://www.gillespiemuseum.stetson.edu/grounds/list.html

Vegetation - interior

St. John’s Wort

Blue Maidencane

Maidencane

Duck potato

http://sofia.er.usgs.gov/virtual_tour/pgbigcypress.html

http://www.gillespiemuseum.stetson.edu

/grounds/list.html

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

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

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.

WAIT – well transects

Results

Results

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

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.

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

GMS v.4.0

All images: http://www.ems-i.com/GMS/gms.html

Visual Modflow Pro v3.1

Animation:http://www.visual-modflow.com/html/visual_modflow.html

Modeling Surface Water Flow in Wetlands

A non-mathematical explanation of a mathematical process

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

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

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

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 society’s water needs

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

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

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

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 model’s friction coefficient

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The models usually contain a series of

differential equations that work together

There will be separate equations for different aspects of wetland hydrology

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.

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)

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

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)

Ground and Surface Water Interaction Ecological effects: salinity front movements Used to study the effects of management

practices on hydrology

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

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)

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.

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

SICS Study Area

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

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

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

QuickTime™ and a Cinepak decompressor are needed to see this picture.

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

Satellite image of south Florida covering Everglades National Park, 1:500,000 scale

Satellite image showing TIME model boundary Scale 1:500,000

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

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

Goals of the TIME projectto 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.

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?

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?

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?

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

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

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

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

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