Modeling the St. Francis dam break flood with 2-D shallow water, 3-D, and hybrid CFD methods

Computational Fluid Dynamics (CFD) is used to predict the extent of flooding and dam breach events more than ever before. Inputs to the governing equations include representations of topography and flow resistance. The inputs are often uncertain which can introduce difficulties in assessing a model’s accuracy. In other words, the numerical model and solution may not necessarily represent reality. This paper examines the effect of governing equation options and flow resistance inputs on the accuracy and efficiency of a model of the 1928 St. Francis Dam breach. A brief history of the dam breach is given, and a metric is proposed for comparing similar models of the same event. The breach is then modeled with two sets of governing equations in the commercial software package FLOW-3D. The Shallow Water equations are tested and calibrated independently and then combined and coupled with the 3-D Navier-Stokes equations. To our knowledge, this hybrid approach is unique and attempts to balance accuracy and efficiency by matching the governing equations to dominant flow characteristics. The results are analyzed and discussed, and some rules-of-thumb are proposed for modeling large-scale breach events.