Analyzing inundation extent in small reservoirs: A combined use of topography, bathymetry and a 3D dam model

Numerous and widely distributed small reservoirs are vulnerable to flood risks and dam failures. This study focuses on the combined use of various common tools and GIS techniques to achieve rapid and effective estimation of inundation extent for both small reservoirs and associated hydraulic structures. We discovered that the reservoir terrain surface can be more realistic and coherent, if an airborne LiDAR-derived DEM, bathymetric measurements, and a 3D dam model are integrated. The source flooding algorithm based on a horizontal water surface can be enhanced by introducing the intersection detection from computational geometry, so that submerged areas could spread from terrain mesh cells to the upstream face of embankment dams, although the required neighboring relationship is absent in the 3D geometric model. Taking the Hengshan Reservoir in Southeastern China as the study area, the proposed methods and visualized results are tested by a historical rainfall event during the typhoon season. They are also proved to support extreme water stages, and better-founded to determine the reservoir stage-storage relationship, which is mostly based on bathymetric measurements and a polynomial extrapolation. As a first attempt to involve 3D geometric models of hydraulic structures into inundation extent analysis, this study is useful to flood risk mitigation in small reservoirs. It can also inform geospatial analysis and geocomputation that involve both natural terrain and man-made structures.