Impact of Flood Types on Superposition of Flood Waves and Flood Statistics Downstream

Abstract

Flood events may be caused by different runoff generating processes and can be differentiated in their genesis by the application of flood types. Additionally, the spatial interaction of catchments can play a crucial role in the flood generation. Flood wave superposition can increase the flood peak and volume downstream. The magnitude of increase and the probability of superposition depends on the shape of the upstream hydrographs. This study simulated different superposition scenarios, distinguished by flood types. These scenarios then were incorporated in a type-differentiated statistical model. Uncertainty ranges for design floods were obtained that can be used for an improved consideration of flood protection differentiated by the flood type. It was demonstrated that, although for central German catchments the superposition of floods caused by heavy rainfall is the least probable of all scenarios, this flood type leads to highest increase of the peak. Hence, heavy-rainfall floods could lead to underestimated design floods downstream. With the newly proposed range of design-flood return periods, the uncertainty caused by flood superposition can be incorporated in flood protection measures.