The construction of highways often requires soil to fill low areas and to build overpasses and ramps. When the required fill is unavailable from cuts made during construction, it is usually obtained from borrow pits located in the vicinity of the highway. Due to the relatively low elevations and flat topography, borrow pits in eastern North Carolina are often near or adjacent to wetlands. There is concern that the borrow pit may serve as a long-term drainage sink and that after closure of the pit, the wetland hydrology of a strip of land adjacent to the pit will be affected. An objective of this research is to determine the distance that a borrow pit should be set back from adjacent wetlands to avoid impacts on the hydrology of that wetland.
An approximate method was
developed in a previous NCDOT sponsored research project to estimate the
lateral effect of a roadside drainage ditch on adjacent wetland
hydrology. The lateral effect is defined as the width of a strip of land
drained such that it no longer meets the wetland hydrologic criterion.
It was determined that, for poorly drained soils in North Carolina,
sites that barely satisfied the wetland hydrologic criterion had
characteristic water table drawdown rates that depended on local weather
conditions and surface depressional storage, but were relatively
independent of soil type. The characteristic drawdown rates can be
quantified as the threshold time, T25, required for the water
table to be lowered by drainage from the surface to a depth of 10 inches
(25 cm). T25 was found to depend moderately on ditch depth
but was nearly constant among soils having a wide range of profile
transmissivities and drainable porosities. T25 was found to
depend strongly on surface depressional storage, decreasing as surface
storage increased.; T25 also depended strongly on location,
which affects both the growing season and weather variables. Now that T25 values have been determined for every county in North Carolina,
published solutions for water table drawdown due to a single ditch can
be used to estimate the lateral effect of a drainage ditch on wetland
hydrology. The
reader is referred to the following website for articles discussing the
development of the approximate method as well as a table listing T25 values for all North Carolina counties: http://www.bae.ncsu.edu/soil_water/projects_lateral_effect.htm
We believe that the approximate
method developed for estimating the lateral effect of a roadside ditch
can also be applied to determine the appropriate set back distance for a
borrow pit. 
For drainage ditches, the lateral effect was used to
calculate an area of required mitigation. For borrow pits, the goal is
to prevent impacts on adjacent wetlands. Because the approximate method
calculates the distance that will be drained such that it no longer
satisfies the wetland hydrologic criterion, applying the method to
borrow pits will allow prediction of the required setback for a pit to
prevent hydrologic degradation of an adjacent wetland system. The NCDOT
requires the use of our method in calculation of the required set back
distance for a proposed borrow pit if the boundary of the pit will be
within 400 feet of a stream or wetland. Information regarding NCDOT
procedures for applying the method can be found at the following: NCDOT Website.
NCDOT Contractor guidelines for application of the Skaggs Method
Phillips, B.D., R.W. Skaggs, G.M. Chescheir. 2007. Determination of Lateral Effects of Borrow Pits on Hydrology of Adjacent Wetlands. Written for presentation at the 2007 ASABE International Meeting in Minneapolis, Minnesota.
Phillips, B.D. 2006. Methods to Determine Lateral Effect of Drainage Ditch on Wetland Hydrology. Masters Thesis. Department of Biological and Agricultural Engineering, N.C. State University, Raleigh, N.C.
Phillips, B.D., R.W. Skaggs, and G.M. Chescheir. 2005. Determination of Lateral Effect of a drainage ditch on Wetland Hydrology for two field sites. Written for presentation at the 2006 ASABE International Conference on Hydrology and Management of Forested Wetlands in New Bern, North Carolina.
Skaggs, R.W., G.M. Chescheir, and B.D. Phillips. 2005. Methods to
determine lateral effects of a
drainage ditch on wetland
hydrology.
Transactions of the ASAE.
Vol. 48(2).
Skaggs, R.W., and G.M. Chescheir. 2001. Methods to
Determine Lateral Effects of a Drainage Ditch on Wetland Hydrology .
Written for presentation at the 2002 ASAE Annual International Meeting /
CIGR XVth World Congress.
