Evidence and Implications of Nonlinear Flood Response in a Small Mountainous Watershed

Abstract

This study investigates the impact of event characteristics on runoff dynamics during extreme flood events observed in an 8.5-km2 experimental watershed located in South Korea. A high-quality data set containing the 31 most extreme flood events with event rainfall in excess of 50 mm were analyzed using an event-based rainfall-runoff model; the revitalized flood hydrograph (ReFH) routinely used for design flood estimation in the United Kingdom. The ReFH model was fitted to each event in turn, and links were investigated between each of the two model parameters controlling runoff volume and response time, respectively, and event characteristics such as rainfall depth, duration, and intensity, and also antecedent soil moisture. The results show no link between the parameter controlling runoff volume and any of the event characteristics, but identified a dependence between response time and rainfall depth. These results show that the linear unit hydrograph fails to adequately represent a reduction in watershed response time observed for the more extreme events. A new and dynamic link between the unit hydrograph shape and rainfall depth is introduced. The consequence of the observed nonlinearity in response time is to increase design peak flow by between 50% for a 10-year return period, and up to 80% when considering the probable maximum flood.