A Method to Determine Lateral Effect of a Drainage Ditch on Wetland Hydrology: Field Testing

Abstract

An approximate method was previously developed to predict the lateral effect of a drainage ditch on wetland hydrology. The method predicts the lateral distance of influence of a single ditch constructed through, or adjacent to, a wetland in terms of T25 values, which are dependent on climatological conditions. The lateral effect, or distance of influence, is defined as the width of a strip adjacent to the ditch that is drained such that it no longer satisfies wetland hydrologic criteria. T25 represents the time required for the water table to be drawn down by drainage from the surface to a depth of 25 cm at the location on the landscape that will just barely satisfy the wetland hydrologic criterion. Data to test the method were collected at two wetland mitigation sites in eastern North Carolina: Mildred Woods in Edgecombe County and ABC near Pinetown in Beaufort County. The approximate method predicted lateral effects of 42.6, 7.2, and 14.1 m for Mildred Woods, ABC shallow ditch, and the ABC deep ditch, respectively. Compared to direct interpolation of 3-year average field results for Mildred Woods (41 m) and the deep ditch (12 m), the method performed well. The lateral effect predicted by the method for the shallow ditch at the ABC site was at least two times that measured in the field (<3.75 m). In this case, the ditch was located in a tight clay layer, which substantially reduced the effective transmissivity of the profile and the lateral effect of the ditch on the hydrology of adjacent wetlands.