United States Department of Transportation
i
Determination of lateral effects of borrow pits on hydrology of adjacent wetlands.
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2007-09-28
[PDF-1.76 MB]
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Edition:Final report; Sept. 2004-Aug. 2007.
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Abstract:A three year research project was conducted to develop and test a method to predict the distance borrow pits should
be set back from adjacent wetlands to avoid detrimental impacts on wetland hydrology. The method is based on the
Boussinesq equation. Knowledge of soil hydraulic properties and the time, T25, required for characteristic water table
drawdown is necessary to calculate the setback. DRAINMOD simulations were conducted to determine the T25
values for all 100 North Carolina counties. A survey of 27 borrow pits indicated seepage from the wetland to the pit
in 42% of the pits, pit to the wetland in 31% and sometimes in one direction and vice versa in 27% of the pits. Eight
borrow pits were instrumented to determine the response of the water level in the pit, and the water table in adjacent
wetlands, to rainfall, evapotranspiration and seepage. The time required after closure for the water level in pits to rise
to equilibrium varied from 10 to 23 months with an average of 17 months. The equilibrium water level is a critical
factor affecting the impact of the pit on hydrology of adjacent wetlands. This depth depends primarily on the
elevation of surface water outlets, the depths of adjacent drainage ditches or other subsurface drainage sinks in close
proximity to the pit, and the natural depth of the water table prior to the construction of the pit. Equilibrium water
levels in 5 of the 8 instrumented pits were lower than the surface of adjacent wetlands, with seepage from the wetland
to the pit. The average water levels in the 5 pits ranged from 1.0 to 4.1 ft. below the surface of adjacent wetlands.
Water level elevations in the other 3 pits were equal to or above the elevation of adjacent wetlands. The method
developed herein was used to predict setback requirements for the 5 instrumented borrow pits having seepage from
wetlands to the pit. Results were compared to the potential lateral impacts determined from measured water table
data. The setback distances predicted using the measured pit water levels were about 60% greater than the measured
impacts on average. The method is based on conservative assumptions so the overestimation was not unexpected.
Use of a constant 2 ft. depth of water in the pit predicted setback requirements within 16% of measured potential
impacts for 3 of the 5 cases; the setback was overestimated by more than a factor of 2 for the other two pits.
However, the projected lateral impacts were less than 50 ft. in both cases. It was concluded that the proposed method
will provide conservative but reasonable estimates of the setback necessary to avoid impacts on the hydrology of
wetlands adjacent to borrow pits.
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