Spatially Index-based Assessment of Flash Flood Potential under Sparse Data Availability

Abstract

Flash flood hazard maps provide an essential support to mitigate flash flood risk. In the literature, various approaches have been proposed to estimate degrees of flash flood hazard. Most of them were developed in where large inventories of historic flash floods are available. However, none of them is proved to deliver a most suitable solution for flash flood hazard assessment in ungauged basins. Owing to unfavorable data conditions, it is difficult to prove which approach is more suitable than another one. This dissertation fills this gap by introducing a new spatial index-based approach to map potential flash flood hazard areas at high spatial resolutions preferably for ungauged basins but also for any basin, which needs a quick assessment of its flash flood potential. For comparison purposes, an integrated hydrologic-hydrodynamic approach was applied to model the upper Nan River basin in Thailand. With this model approach, the spatial distributions of flash flood hazard of the upper Nan was estimated and served as a reference for the development of the new approach. On the basis of the new index-based approach, physiographic variables of the study sub-basins were computed from available digital elevation models. These variables were linearly combined and automatically weighted by means of principal component analysis (PCA) to determine flash flood potential indices (FFPI). The FFPI values based on the sub-basins could be described well by a generalized extreme value (GEV) probability distribution. The 98th percentile of the GEV probability distribution (so-called lowest classifier) was found to be suitable for identifying flash flood extents in the sub-basins by comparison to results of the integrated modeling approach. In addition, the index-based approach was applied to the upper Ping River basin in order to test the transferability of the regional lowest classifier. The identified flood hazard areas were smaller than historic inundated areas extracted from the Landsat-7 satellite imagery and the classifier was adapted. It can be stated that the new spatial index-based approach is an effective procedure for flash flood hazard assessment in any basin including ungauged basins. Future work should focus on developing an analytical routing equation to transfer rainfall information in order to extend the approach towards a (real-time) forecast.