Flood Envelope Curves for the Setouchi Region Derived by Spatial Distribution Analysis of Radar Rainfall

This paper presents the estimation procedure of a flood envelope curve originally developed by Kadoya and Nagai (1979) on the basis of depth-area-duration (DAD) analysis of radar precipitation data in order to examine the availability of spatial distribution data of precipitation for statistical flood peak analysis. The estimated flood envelope curves by applying the presented procedure to the Yoshii River Basin located in Okayama Prefecture of the western Japan well enveloped the flood peaks observed in the Hiroshima and Okayama Prefectures; the curves tended, however, to overestimate flood peaks for small catchments. It was explained by the fact that very limited areas are with regionally maximum areal rainfall arising maximum flood peak discharge and the other areas are with smaller areal rainfall than maximum in the objective region. Flood envelope curve equation considering spatial probability of regionally maximum areal rainfall was presented and applied to the Yoshii River Basin to show the effect of adjusting the spatial probability for mitigating overestimation of maximum flood peaks for small basin. This result shows probabilistic aspects of flood envelope curves deterministically estimated from hydrological records, which should be considered when determining design flood discharge for dam planning.