Effect of Minor Drainage on Hydrology of Forested Wetlands

Abstract

Results of a simulation study to determine the impacts of minor drainage for silviculture on wetland hydrology are presented in this article. Long-term DRAINMOD simulations were conducted to determine the threshold drainage intensity (ditch depth and spacing) that removes wetland hydrology from forested wetlands. Analyses were conducted for 13 soil series and profile combinations at ten locations from Norfolk, Virginia, to Baton Rouge, Louisiana, in the Atlantic and Gulf coastal states. Threshold ditch spacings (LT) were obtained for five ditch depths for all combinations of soil profiles and locations. Analysis of the results showed that LT can be approximated as LT = , where T is the horizontal hydraulic transmissivity of the soil profile, and C is a coefficient dependent on ditch depth and geographic location. The C values for all combinations of ditch depth and location are given in this article. The threshold spacings can be used as benchmarks to directly evaluate the impact of drainage alternatives on wetland hydrology. They were also used herein to determine T25 inputs for previously developed methods to predict the lateral impact of a single ditch on wetland hydrology. Lateral impacts were determined and presented for a 0.9 m (3 ft) deep drainage ditch for all soils and locations considered. The T25 values presented can be used to determine lateral impacts for other ditch depths and soils. The analyses in this study were conducted for a surface depressional storage of 5 cm. More work is needed to define T25 values for smaller surface storages, including those smaller values needed for application to agricultural cropland.