Methods to Predict the Lateral Effect of a Drainage Ditch on Adjacent Wetland Hydrology

Abstract

Four methods were studied to predict the lateral effect of a drainage ditch on wetland hydrology — a field method based on threshold drainage conditions, long-term simulations in both DRAINMOD and WATRCOM, and the approximate method (also known as the Skaggs Method). As defined in this study, the lateral effect is the width of a strip of land drained such that it no longer meets the wetland hydrologic criterion as set forth in the U.S. Army Corps of Engineers Wetland Delineation Manual.

Three years (2002 – 2004) of data were collected at field sites located at the Mildred Woods mitigation site in Edgecombe County, North Carolina and the ABC mitigation site located in Beaufort County, North Carolina. Hourly water table depths were recorded at several locations on transects perpendicular to one drainage ditch (1.2 m depth) at Mildred Woods and a shallow ditch (0.9 m depth) and deep ditch (1.3 m depth) at the ABC site. Rainfall was recorded at each site and temperature data were collected from nearby weather stations.

DRAINMOD simulations were performed for a 54-year period for each ditch to determine the threshold drain spacing, i.e. a spacing associated with water table fluctuations that would just barley satisfy the wetland hydrologic criterion in one half of the years. Next DRAINMOD was used with the threshold ditch spacing and depth along with recorded rainfall data for 2002-2004 to predict the maximum consecutive duration that the water table would be above the 30 cm depth for those specific years. Based on the measured durations for each year the estimated lateral effect was 41 m for Mildred Woods, <3.75 m for the ABC shallow ditch, and 12 m for the ABC deep ditch.

DRAINMOD and WATRCOM were calibrated for each study transect by comparing model predicted water table depths and observed water table depths. The calibrated models were then used to simulate water table depths at each observation well for a 54-year period, and the results were analyzed to determine the distance from the ditch where the criterion was satisfied in exactly one-half of the years. Based on these simulation results the lateral effect was estimated to be 38.6 m for Mildred Woods, <3.75 m for the ABC shallow ditch, and 18.0 m for the ABC deep ditch. Simulation results with WATRCOM estimated a lateral effect of 41.5 m for Mildred Woods, and 8.9 m and 20.3 m ABC shallow and deep ditches, respectively.

Results of the approximate method estimated the lateral effect to be 42.6 m for the ditch at the Mildred Woods site. This was close to the values obtained from the observed data and the two simulation models. A lateral effect of 14.1 m was estimated for the ABC deep ditch using the approximate method. This value is slightly larger than obtained from the field data, 12 m, 3.9 m less than the value predicted by DRAINMOD, and 6.2 m less than the value predicted by WATRCOM. The approximate method estimated a lateral effect of the ABC site shallow ditch of 7.2 m, 1.7 m less than that predicted by WATRCOM, which is two times greater than results from the field method and predicted by DRAINMOD. The ABC site shallow ditch resides in a tight clay layer that apparently cut off most of the drainage from the lower, higher conductivity layer. Additional research is needed to determine how the method should be modified for shallow ditches confined in a low conductivity layer.