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<br /> <br /> <br /> <br />5 <br />• Simulating backwater effects due to hydraulic <br />structures and channel constrictions; <br />• Explicitly simulating channel/floodplain storage <br />(i.e., flood attenuation); and <br />• Simulating rapid transitions between subcritical <br />and supercritical flows. <br />Hydraulic Modeling using HEC-RAS <br />HEC-RAS is designed to perform one-dimensional (1D) <br />and two-dimensional (2D) hydraulic evaluations for <br />natural and constructed channels, overbank/floodplain <br />areas, levee-protected areas, reservoirs, etc. Some <br />typical components of a hydraulic model include <br />topographic data (i.e., cross-section or mesh), surface <br />roughness (i.e., Manning’s “n” roughness coefficients), <br />and inflow discharge (i.e., constant flow or <br />hydrograph). HEC-RAS cannot simulate precipitation, <br />watershed response, infiltration, or snowmelt; <br />however, some of its capabilities include: <br />• Simulating hydraulic characteristics within a <br />channel/floodplain (e.g., water surface profiles, <br />etc.); <br />• Simulating hydraulic characteristics at structures <br />such as bridges and culverts; and <br />• Developing flood extent and temporal based <br />hydraulic characteristics for inundation mapping. <br />HEC-RAS modeling is typically performed for either <br />steady (i.e., constant flow) or unsteady (i.e., flow <br />changes with time) simulations utilizing 1D (i.e., flow <br />travels only in the downstream direction) or 2D (i.e., <br />flow travels both longitudinally and laterally <br />downstream) geometric domains. The following <br />sections describe these modeling options and provide <br />insight on the advantages and disadvantages of each. <br />The RAS Solution [8] is also a great source for RAS- <br />related tips and tricks. Reference [9] is a good resource <br />for determining if your model results are reasonable. <br />Steady vs. Unsteady Flow <br />Steady flow analyses assume a constant discharge <br />through the entire reach and use the energy equation, <br />which does not account for changes in momentum. <br />Unsteady flow analyses using the St. Venant equations <br />(i.e., conservation of mass and momentum) are <br />capable of modeling changing discharge over time (i.e., <br />hydrographs). <br />Some advantages of steady flow analyses include: <br />• Greater stability; <br />• Shorter run times; <br />• Generally less time intensive overall; and <br />• Peak discharges are input rather than entire <br />hydrographs, making it easy to model many <br />scenarios in a short period of time. <br />Some disadvantages of steady flow analyses include: <br />• Reduced accuracy due to simplifying assumptions <br />of the energy equation; <br />• Inability to account for channel and floodplain <br />storage effects; and <br />• Inability to provide temporally based hydraulic <br />characteristics, like floodwave arrival times, <br />detention durations, overtopping durations, flood <br />volumes, etc. <br />Some advantages of unsteady flow analyses include: <br />• Greater accuracy given that the more sophisticated <br />St. Venant equations are used and account for <br />channel/floodplain storage effects on flood <br />attenuation; <br />• Temporally based results can be easily obtained; <br />and <br />• Reservoir routing and dam breach analyses can be <br />simulated. <br />Some disadvantages of unsteady flow analyses include: <br />• Increased computational intensity, longer run <br />times, and increased instability; <br />• Models can be especially unstable for some <br />geometric and hydraulic conditions like steep or <br />highly irregular reaches, low flood depths, and <br />flashy hydrographs, particularly with 1D models; <br />and <br />• Models can be significantly more time intensive <br />overall due to instability troubleshooting. <br />In general, unsteady analyses are more accurate and <br />appropriate if a higher degree of accuracy is required <br />and time/schedules allow. Steady flow analyses could <br />be more appropriate for very long or steep reaches as <br />well as stream networks with multiple watercourses <br />and junctions. Steady flow analyses may also be <br />appropriate for rating curve evaluations or simple