Hydrology of Alluvial Stream Channels in Southern Coastal Plain Watersheds

THIRTEEN years of recorded rainfall, streamflow, and groundwater elevation data were used to analyze the hydrologic characteristics of the alluvial stream channels for three subwatersheds in the Coastal Plain of the southeastern U.S. Drainage areas of these subwatersheds range from 16.7 km2 to 50 km2. Digitized soil maps and cross-sectional transects were used to determine alluvial areal extent and alluvial depth, respectively. These data were used to compute the total alluvial volume in each sub watershed. Total alluvial depths and total alluvial volumes with groundwater elevation data were then used to determine the alluvial antecedent conditions and available alluvial storage for three subwatersheds. Results indicate that the available alluvial storage varies with both time of year and antecedent rainfall conditions. Lower available storage was observed for late winter and early spring due to higher rainfall and lowered evapotranspiration. Gradually increasing values were observed during summer and autumn due to lower rainfall and higher evapotranspiration. Results also indicate the effect of alluvial antecendent conditions on instantaneous peak discharge responses. For comparable storms, instantaneous peak discharge rates were higher by an order of magnitude for wet conditions than for dry conditions.