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<br /> <br /> <br /> <br />4 <br />present and when rapidly varied flow conditions (e.g., <br />highly dynamic floodwaves, abrupt expansions and <br />contractions, mixed flow regimes, hydraulic jumps, <br />etc.) are anticipated. <br />Hydrologic Modeling using HEC-HMS <br /> <br />Figure 2. Schematic of typical HEC-HMS watershed <br />model[5] <br />HEC-HMS is designed to simulate hydrologic processes <br />of watersheds (refer to Figure 2), typically with the <br />intent of estimating runoff hydrographs at various <br />locations within a system. Some other functions that <br />can be performed in HEC-HMS include uncertainty <br />analyses (i.e., Monte Carlo simulations), erosion and <br />sediment transport, and water quality analysis. <br />Some typical components of a hydrologic model <br />include precipitation, infiltration, runoff <br />transformation (e.g., unit hydrograph), <br />evapotranspiration, snowmelt, and baseflow. The level <br />of effort to develop a HEC-HMS model will vary <br />depending on complexity of watershed and <br />precipitation inputs. Guidelines for selection of some <br />typical input parameters are presented in Western <br />Dam Engineering Technical Note (WDETN) Volume 2, <br />Issue 1 [6]. Some tips for calibrating and validating <br />hydrologic models are presented in WDETN Volume 5, <br />Issue 1 [7]. <br />Desired outputs from HEC-HMS typically include <br />precipitation, surface runoff volumes, water surface <br />elevations, and hydrographs. HEC-HMS is commonly <br />used for reservoir routing evaluations; however, if the <br />reservoir is relatively long and shallow, as is the case <br />with many run-of-the-river and low head dams, a <br />hydraulic model like HEC-RAS could be more <br />appropriate. <br />Hydrologic channel routing can also be performed to <br />provide coarse estimates of channel flood depths. <br />Hydrologic channel routing methodologies typically <br />utilize simplifying assumptions and empirical data to <br />implicitly simulate flood attenuation and routing. Some <br />of these methodologies include [5]: <br />• Kinematic Wave; <br />• Lag; <br />• Modified Pulse; and <br />• Muskingum/Muskingum-Cunge. <br />The most appropriate uses for HEC-HMS include: <br />• Estimating watershed runoff peak flow rates, <br />volumes, and hydrographs; <br />• Performing reservoir flood routing for deep, wide <br />reservoirs in which flow velocities are generally <br />negligible; <br />• Runoff timing and course estimates of channel <br />flow depth within stream networks; <br />• Screening-level hazard determinations for remote <br />dams without substantial downstream hazards; <br />and <br />• Hazard determinations for mountain dams with <br />steep downstream channels (as discussed later, <br />steep channels can be problematic in HEC-RAS). <br />Some applications for which HEC-HMS is not suitable <br />include: <br />• Explicitly simulating hydraulic structures (in some <br />cases, rating curves or other approximations can <br />be used as a supplement) like bridges and culverts; <br />• Simulating reservoir and diversion gate operations;