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Hydrologic Connectivity of Isolated Wetlands. Todd C. Rasmussen, Ph.D. Associate Professor of Hydrology Warnell School of Forest Resources University of Georgia, Athens GA 30602-2152 www.hydrology.uga.edu. Abstract.

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hydrologic connectivity of isolated wetlands

Hydrologic Connectivityof Isolated Wetlands

Todd C. Rasmussen, Ph.D.

Associate Professor of Hydrology

Warnell School of Forest Resources

University of Georgia, Athens GA 30602-2152

www.hydrology.uga.edu

abstract
Abstract
  • Recent changes in the judicial interpretation of isolated wetlands has caused the Savannah District of the U.S. Army Corps of Engineers to consider removing these wetlands from their jurisdiction.
  • If this policy is adopted, many coastal freshwater wetlands will be threatened because of their isolation from other waterbodies by surface water connections.
  • This paper examines the degree of subsurface hydrologic connection between shallow depressional coastal wetlands with other jurisdictional waterbodies.
abstract cont
Abstract (cont.)
  • It is shown that shallow groundwater flow between isolated wetlands and jurisdictional waterbodies occurs for a wide range of surficial aquifer conditions.
  • The magnitude of hydraulic linkage is a function of
    • the properties of the surficial aquifer,
    • the properties of the wetland,
    • the distance between the wetland and the jurisdictional limit.
  • Hydrologic analyses should be conducted prior to the removal of isolated wetlands to confirm their lack of subsurface connectivity with nearby waterbodies
background
Background
  • For the first thirty years of its history, the Clean Water Act was interpreted in a manner that afforded protection to nearly all waters and wetlands, including so-called isolated wetlands.
  • In 2001, however, the U.S. Supreme Court issued a decision in Solid Waste Agency of Northern Cook County v. U.S. Corps of Engineers (the SWANCC decision) that called into question the federal government's ability to regulate isolated waters.
  • Despite the growing trend to interpret the SWANCC decision narrowly, the Savannah District of the U.S. Corps of Engineers appears intent on using the decision as a basis for severely limiting federal protections for freshwater wetlands in Georgia.
background cont
Background (cont.)
  • The Corps appears unwilling to regulate wetlands unless a continuous surface water connection exists between the wetlands at issue and other jurisdictional waters.
  • Because the state of Georgia does not protect freshwater wetlands, the unwillingness of the Corps to protect wetlands that it deems to be isolated is a significant problem that exposes thousands of acres of Georgia's wetlands to many threats, including but not limited to mining, silviculture, and commercial and residential development.
approach
Approach
  • This presentation examines the degree of interconnection with U.S. waters even though they lack explicit surface-water connections.
  • Because these wetlands are located in coastal areas where subsurface hydrologic flow can be significant, they may still be hydrologically interconnected with U.S. waters due to subsurface flow.
  • Various subsurface factors are evaluated in determining the magnitude of the interconnections, including
    • the physical and hydraulic properties of the aquifer and the wetlands, and
    • the distance between the wetlands and U.S. waters
modeling approach
Modeling Approach
  • A two-dimensional (profile), steady flow domain.
  • Saturated ground-water flow within the flow domain.
  • The thickness of the aquifer varies
  • The distance from the wetland to U.S. waters varies
  • The depth of the wetland varies
  • The depth of the waterbody varies
  • The extent of the wetland varies
assumptions
Assumptions
  • Homogeneous: no variation in position
  • Isotropic: no variation in direction
  • Saturated: flow below the water table
  • Finite Extent: limited zone of influence
  • Two-dimensional: longitudinal features
  • Steady Flow: no aquifer storage
cvbem complex variable boundary element method
CVBEMComplex Variable Boundary Element Method
  • Mathematical Model that uses complex potential, w = h + i s
    • h is the hydraulic head, s is the streamline, i = sqrt (-1)
  • Uses Cauchy Integral Theorem to model flow within the domain
  • Requires specification of
    • domain size
    • material properties (hydraulic conductivity)
    • boundary conditions
  • Uses Ordinary Least Squares (OLS) to solve the resulting over-determined system of linear equations
  • Provides estimates of total head, streamlines, and fluxes within the flow domain
slide12
Cauchy Integral

Internal to domain:

Boundary:

base case other scenarios are compared to these conditions
Base Case:Other scenarios are compared to these conditions
  • Properties:
    • Aquifer thickness: b = 20 m
    • Separation distance: L = 60 m
    • Wetland depth: d = 1 m
    • Waterbody depth, D = 1 m
    • Wetland extent: w = 10 m
    • Waterbody extent: W = 20 m
    • Elevation difference: h = 20 cm
changes from base case
Changes from Base Case
  • Aquifer permeability:
    • Doubling permeability increased flow by 100%
    • Halving permeability decreased flow by 50%
  • Elevation difference:
    • Doubling elevation increased flow by 100%
    • Halving elevation decreased flow by 50%
changes from base case cont
Changes from Base Case (cont.)
  • Aquifer thickness:
    • Doubling thickness increased flow by 74%
    • Halving thickness decreased flow by 47%
  • Separation distance:
    • Halving distance increased flow by 78%
    • Doubling distance decreased flow by 47%
changes from base case cont18
Changes from Base Case (cont)
  • Waterbody depth:
    • Doubling waterbody depth increased flow by 0%
    • Halving waterbody depth decreased flow by 0%
  • Wetland depth:
    • Doubling wetland depth increased flow by 1.5%
    • Halving wetland depth decreased flow by 0.5%
  • Wetland extent
    • Doubling wetland extent increased flow by 17%
    • Halving wetland extent decreased flow by 7%
limitations
Limitations
  • Fails to account for dynamic conditions
    • Short term effects may be even greater
  • Fails to account for anisotropy
    • Effective aquifer thickness will be affected
  • Fails to account for limited longitudinal extent
    • Circular features may be affected to a greater degree
  • Overall, effects on wetlands are underestimated.
conclusions
Conclusions
  • Small, isolated wetlands can interact with coastal waters through the subsurface (i.e., ground water)
  • Even small wetlands at large distances from the coast are part of the coastal hydrologic continuum
  • Protecting small, isolated coastal wetlands should be considered.
recommendations
Recommendations
  • Prior to removal, isolated wetlands should be evaluated to assure that they are not actively contributing to the coastal hydrologic system
  • At a minimum, required studies should include the determination of:
    • The response to water level changes in nearby waterbodies
    • Aquifer properties such as hydraulic conductivity, anisotropy, and thickness
    • Wetland and nearby waterbody characteristics, such as depth, extent, etc.
    • Computer modeling to determine the quantity of subsurface flow that the isolated wetland contributes the regional hydrology
acknowledgement
Acknowledgement
  • I extend special thanks to Chris DeScherer, a staff attorney with the Southern Environmental Law Center in Atlanta, GA, for bringing this issue to my attention
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