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