Determination of lateral effects of borrow pits on hydrology of adjacent wetlands.

Details

• Creators:
  Skaggs, R. Wayne ; Chescheir, George M. ; Phillips, Brian D.
• Corporate Creators:
  North Carolina State University. Department of Biological and Agricultural Engineering
• Corporate Contributors:
  United States. Department of Transportation. Research and Innovative Technology Administration
• Subject/TRT Terms:
  • Pits
  • Wetlands
  • Simulation
  • Roads--Design and construction--Environmental aspects--North Carolina
  • Water quality management--North Carolina
  • Runoff--North Carolina.
• Publication/ Report Number:
  FHWA/NC/2006-50
• Resource Type:
  Tech Report
• Geographical Coverage:
  North Carolina
• Edition:
  Final report; Sept. 2004-Aug. 2007.
• Corporate Publisher:
  CTE/NCDOT Joint Environmental Research Program
• 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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• Format:
  PDF
• Download URL:
  https://rosap.ntl.bts.gov/view/dot/17593/dot_17593_DS1.pdf
• File Type:
  [PDF - 1.76 MB]
• Collection(s):
  University Transportation Centers ; US Transportation Collection
• Main Document Checksum:
  urn:sha256:3956f5f077a378fcee97ecb38a396f9ed7459b50bf25a68b09b075d3f27549f4