Impact evaluation of geomorphic changes caused by extreme floods on inundation area considering geomorphic variations and land use types

Extreme floods caused by dam breaches, dike breaches, and rainstorms cause significant erosion and deposition in the flooded area. Furthermore, geomorphic changes have various impacts on different land use types, which is an important aspect extreme flood outcomes. The impact type and degree depend on geomorphic variations and land characteristics. However, neither the amount of geomorphic variations nor its impact on the inundation area have been fully understood. Firstly, we propose the use of a numerical simulation method to calculate erosion and deposition depths of the whole inundation area caused by extreme floods. Secondly, combined with the characteristics of erosion, deposition, and land use types, the impact type of geomorphic changes on different land use types were divided into positive, negative, and negligible impacts, and the impact degree was expressed by two indices of impact grade and impact score. In addition, the calculation methods of the two indices were put forward. Then, we propose a method for evaluating the impacts of geomorphic changes on the whole inundation area from five aspects of mesh, land use type, overall erosion region, overall deposition region, and overall inundation area. Combined with the simulation of the flood process caused by dam breach of Luhun Reservoir in China, this method was verified, and the results showed that: (a) geomorphic changes had a negative impact on 94.7% of the inundation area, and only part of the water bodies were positively affected and the towns were not affected, accounting for 2.1% and 3.2% respectively; (b) the negative impact degree of each land use type in descending order was grassland, town, cropland, forest, shrubland and water body; and (c) the area of deposition was larger than that of erosion, whereas the severity of negative impact was opposite.