Impact assessment of upstream flooding on extreme flood frequency analysis by incorporating a flood-inundation model for flood risk assessment

Abstract

Flood frequency analysis (FFA) is fundamental for providing hazard probability of flood risk assessment as well as for determining design flood. It is often the case that mega cities are located along downstream reaches of a large river basin in many areas of all over the world, and their extreme flood frequencies are assumed to be highly affected by dam operation and river overflow of its upstream areas. In particular, when upstream areas are also protected by river dike system, historical discharge samples cannot represent the impact of upstream river overflow on downstream extreme flood frequencies because it rarely occurs. FFA without this consideration, especially in large river basins which include several potential floodplains, would lead to inappropriate assessment of flood risk. To deal with this issue, FFA needs to incorporate flood-inundation modelling of upstream areas; however, previous studies on FFA have focused on smaller watersheds and combined rainfall-runoff models. Therefore, this study examined the impact of river overflow and dam operation of upstream areas on downstream extreme flood frequencies through a case study of the Yodo River basin (7280km2). To achieve FFA in a large river basin, this study combined a flood-inundation model of upstream Kyoto City area to a rainfall-based flood frequency model (RFFM) which accounts for the probability of spatial and temporal rainfall pattern over the river basin in a practical manner. The RFFM was validated with reproduced discharge samples of historical storm events and then applied to extreme flood frequency estimation. The application clarified that upstream river overflow causes much more drastic change of downstream flood frequencies beyond the design level than dam operation, which indicates that FFA for flood risk assessment needs to consider river overflow of its upstream areas otherwise flood risk of downstream areas will be overestimated. Furthermore, the scheme also produced the cumulative distribution function of flood area, which represents flood risk of upstream areas.