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