A method was developed in the first phase of this project to predict the lateral effect of a drainage ditch on adjacent wetland hydrology. The method predicts the distance of influence of a single ditch constructed through a wetland, i.e. the width of a strip adjacent to the ditch that is drained such that it would no longer satisfy wetland hydrologic criteria, in terms of T25 values which are dependent on climatological conditions. Main objectives of the second phase were to complete determination of the T25 values and to test the validity of the method.
T25 represents the time required for the water table to draw down from the surface to a depth of 25 cm at the location on the landscape that will just barely satisfy the wetland hydrologic criterion. Values were determined for all 100 North Carolina counties for ditch depths of 0.3 to 1.8 m (1 - 6 ft) and for surface depressional storage values of 2.5 and 5.0 cm (1 and 2 inches).
Data to test the method were collected at two wetland mitigation sites in eastern North Carolina: Mildred Woods in Edgecombe County and ABC near the town of Pinetown in Beaufort County. The 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 adjacent wetlands.
Long-term simulations, (1951 - 2004), were performed with DRAINMOD and WATRCOM for all transect wells to provide alternative assessment of the lateral effects. Results were in general agreement with the predictions of the new method, and, except for the ABC shallow ditch site, with lateral effects determined from field data.
