The modeling of a dam break flood wave is one of the most difficult unsteady flow problems to solve. Previous discussions in this document have focused on modeling the reservoir pool, modeling the dam itself, and estimating breach parameters to be used in computing the breach outflow hydrograph. However, the most difficult part of performing a dam safety study is routing the dam break flood wave downstream.
Within HEC-RAS, the user can model the downstream area in the following manner: as a combination of 1D streams and storage areas; a combination of 1D streams, storage areas, and 2D flow areas; or as a single 2D flow area. There are many things that the hydraulic modeler must consider to get an accurate estimate of the downstream flood stages and flows. The following is a list of things that should be considered when developing an unsteady flow model for a dam break application. Most of these issues are concerns for 1D river reach modeling with cross sections.