Hydrologic and Hydraulic Models for Performing Dam Break Studies

The U.S. Army Corp of Engineers, in conjunction with the State of Hawaii, is evaluating the risks associated with potential failures of 11 Hawaii dams. An important question to be answered in any Dam safety study is "What will happen if the dam fails?" This question can be answered through the use of hydrologic and hydraulic computer models. The Hawaii Dam Break studies were performed by a team of federal, state, and consulting engineers. Very early in this study, it was decided that a consistent methodology would need to be used for all of the Dam Break analyses performed by the team. Part of the methodology was to use a specific set of computer models to perform the analyses. A Dam Break study consists of precipitation analyses; hydrologic modeling; dam breaching analysis; dam break floodwave routing; and inundation mapping of the resultant flood. Precipitation analyses were performed by utilizing Technical Paper Number 43, Rainfall-Frequency Atlas of the Hawaiian Islands (NOAA, 1963); Rainfall Frequency study for Oahu (Giambelluca et al., 1984); and Hydro-Meteorological Report 39, Probable Maximum Precipitation in the Hawaiian Islands (NOAA, 1963), in conjunction with several GIS tools and techniques. Hydrologic modeling was performed with the HEC-HMS (Hydrologic Modeling System) software. Dam breaching analysis and routing of the resulting flood wave was performed with the HEC-RAS (River Analysis System) software. In areas where one-dimensional river hydraulics models were thought to be potentially inadequate, the two-dimensional model FLOW-2D was used for comparison of results. Inundation mapping of the resulting floods from various dam break scenarios was performed with the HEC-GeoRAS software, an ArcGIS extension that processes HEC-RAS model results. This paper discusses the use of HEC-HMS, HEC-GeoHMS, HEC-RAS, HEC-GeoRAS, and FLOW-2D for performing the hydrology, hydraulics, and inundation mapping required for performing a dam break analysis.