Approximate Calculation of Flash Flood Maximum Inundation Extent in Small Catchment with Large Elevation Difference

Abstract Inundation analysis is one of the core aspects of flash flood prevention and control in mountainous area. Remote sensing technology can conveniently extract submerged region of a large flood, but infeasible to dynamically obtain flood inundating process in a high temporal resolution. Meanwhile, it is difficult to establish a suitable hydrodynamics-based inundation model under conditions of complex terrain. Moreover, such model usually requires huge input data and long simulation time. The Bulk Method (BM) can quickly calculate flood inundation extent, given the total water yield in the catchment and local Digital Elevation Model (DEM). However, the traditional BM is only applicable for relatively flat terrain. In order to apply BM to calculation of the flash flood inundation extent in the catchment of large elevation difference, the differential BM based on differential element theory is proposed in this paper. In the method, the main flood channel is divided into a certain number of differential channel segments, in which the local elevation difference is relatively small, and the BM can be soundly applied. The flood inundation extent of the whole channel can be obtained by combining the maximum inundation extents of all differential channel segments. Furthermore, it is the key to determine the optimal number of channel segments by repeating the prior steps of the proposed method until the area change ratio of the total inundation extents calculated in two consecutive loops is less than the specified threshold value. Taking Zharugou Catchment as the study area, we obtained the approximate maximum inundation extent of a once in 10-year flood occurred there on July 10th, 2018. As the calculation results were generally in good agreement with the field measuring data, it demonstrated the feasibility and effectiveness of the differential BM in approximately calculating the maximum flood inundation extent in a small ungauged catchment with sharp elevation variation.