Rainfall Space-Time Organization and Orographic Control on Flash Flood Response: The Weisseritz Event of August 13, 2002

This work provides an analysis of the Weisseritz 2002 flood event in Eastern Germany, which was characterized by intense upslope storm movement and associated orographic enhancement of precipitation. Examination of the event is based on the availability of weather radar observations and post-event surveys to document the peak runoff rates. The analysis is based on the use of a set of statistics that clarify the dependence existing between spatial rainfall distribution, basin morphology, and runoff response. The statistics provide a description of overall spatial rainfall organization at the catchment scale in terms of concentration and dispersion statistics and storm motion, as a function of the flow distance coordinates, i.e., distance to the outlet measured along the flow path. The statistics are based on the observation that runoff routing through branched channel networks imposes an effective averaging of spatial rainfall at equal flow distance, despite the inherent spatial variability. A spatially distributed rainfall-runoff model is used to evaluate the effects of the rainfall spatial variability and storm velocity on flood modeling at various river sections. It is shown that storm velocity exhibits rather moderate values, despite the strong kinematic characteristics of individual storm elements. Consistently with this observation, hydrologic simulations show that oro-graphically controlled rainfall spatial variability and storm motion have an almost negligible effect on flood response modeling. The drainage network structure filters out the effects of spatial rainfall variability for storm event hydrologic response, resulting in Weisseritz exhibiting striking uniformity of response to a storm with strongly varying precipitation. Overall, this shows the value of using the rainfall statistics introduced in this paper to isolate and describe the features of rainfall space-time variability that have significant impacts on runoff simulation. DOI: 10.1061/(ASCE)HE.1943-5584.0000569. (C) 2013 American Society of Civil Engineers.