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7 Other benefits of a 2D analysis include [4]: • 2D models directly incorporate topographic and land cover data and do not required third party GIS software for mapping; • The ability to produce detailed animations of flood wave progression in 2D space, including depth, velocity, shear stress etc.; • Flood characteristics can be obtained at locations other than cross-sections more easily than in a 1D model; • 2D models eliminate the need for subjective components such as ineffective flow areas, levee markers, and cross-section orientation; • Inundation mapping is much easier than 1D; and • Unsteady flow analyses are often more stable with a 2D model than a 1D model. There is often a misconception that a 2D analysis is much more time-consuming and expensive than a 1D analysis. This is not always the case, as a highly complex system can be much easier to analyze with a 2D model, while simpler systems may be better suited for a 1D model. Often it can be prudent to combine these models, using 2D where detailed results are required and 1D elsewhere. Ultimately, there is not necessarily a right or wrong answer when deciding between a 1D and 2D analysis. Often, the 1D or 2D decision is based on the personal preference of the modeler as well as the study objectives and requirements. Dam Breach Analyses Both HEC-HMS and HEC-RAS are capable of modeling dam breaches. As discussed above, HEC-HMS is intended to model hydrologic systems, while HEC-RAS is better suited for hydraulic analyses. However, the choice of which software to use when modeling a dam breach will vary depending on the application. Some advantages of using HEC-HMS are that it is a simple setup, data requirements are minimal, and it is numerically stable. However, hydrologic streamflow routing does not account for backwater, and the results cannot be easily used to develop inundation maps. Figure 5. Image of Teton Dam failure [10] HEC-RAS uses full dynamic routing to perform breach analyses, which accounts for backwater effects. Also, outputs can be easily and quickly used to develop inundation maps directly in the software. However, the data input for dam breaches in HEC-RAS is more complex than HEC-HMS and simulations can become numerically unstable, especially in steep reaches. HEC-HMS is commonly used to develop a breach hydrograph unless tailwater is expected to significantly influence breach outflow. The hydrograph from HEC- HMS can then be used as an input to an unsteady HEC- RAS model. However, if backwater is anticipated to be significant, such as with low head dams, mild slopes, or abruptly converging downstream reaches, HEC-RAS is likely to be more appropriate. The piping dam breach event at Teton Dam occurred at location on the embankment well above the valley floor (refer to Figure 5). Breach outflows were not constrained by the downstream valley geometry. As such, it would likely be appropriate to use either HEC- HMS or HEC-RAS to model this breach event as backwater impacts were likely negligible. Backwater Bridges, dams, and other stream obstructions can create backwater, which influences flow conditions upstream of the obstruction. Before immediately embarking on a HEC-RAS model to evaluate backwater, review the area proximate to the obstruction to determine if the results of a HEC-RAS model could be potentially beneficial. For example, if the backwater does not cause any flooding hazards, it could