HYDROLOGIC CONTROLS OF LARGE FLOODS IN A SMALL BASIN - CENTRAL APPALACHIAN CASE-STUDY

This paper reports the results of empirical and model-based analysis of extreme flood events observed in a small basin. The study catchment, WE-38 Mahantango Creek (7.2 km2), is situated in the North Appalachian Valley and Ridge province of eastern Pennsylvania. Data from four rain gauges and one water level station are available at 5 min intervals. From the observations, 12 flood events were selected for detailed analysis. The data include tropical storm Agnes (21-24 June 1972), which was responsible for one of the most devastating floods in the region. Model-based analysis of hydrologic response is used to study the dominating runoff processes in the catchment. In this study, we use a distributed version of Topmodel to model runoff generation, and a lumped hillslope and channel routing model to model overland flow and channel flow. The runoff generation model relies on a topographic index to predict saturation excess runoff and on Philip's infiltration equation to predict infiltration excess runoff. The runoff routing model is based on the channel network width function and on the drainage basin hillslope function. The relative roles of initial conditions, soil properties and rainfall rates in determining hydrologic response in the basin are investigated. The flood frequency distribution seems to be insensitive to scaled maximum rainfall intensity. However, the flood frequency distribution is strongly affected by the scaled initial storage capacity. Simulated hydrograph characteristics, such as peak discharge, are very sensitive to the overland flow velocity parameter in the routing model.