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