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8 acceptable to ignore, or assume the obstruction is washed out during an extreme flood or dam breach event. If the study area is steep and flow is expected to be mostly supercritical, backwater effects are unlikely. If there is uncertainty about backwater impacts, one simple technique employed in the past is to perform hydrologic flood routing (using HEC-HMS) to the obstruction and use the peak flow at that location as an input to a steady flow HEC-RAS model of the obstruction. What are your Objectives? The objectives of a study will dictate which software (i.e., HEC-HMS vs HEC-RAS) and type of analysis (e.g., steady vs. unsteady, 1D vs 2D, kinematic wave vs. diffusion wave vs. dynamic wave) is most appropriate. Some study objectives to consider when pondering the choice between software and analysis type include required study accuracy (e.g., client/regulatory requirements), schedule, and budget. Do you want to evaluate detailed flood impacts resulting from a 1 in 100 annual exceedance probability (AEP) flood event within a relatively flat urban area? If so, you’ll likely want to develop a 2D unsteady HEC-RAS model using the full dynamic wave equation, which will provide the most accurate estimate of the flood extent and hydraulic characteristics within the study area. However, if you want an approximate estimate of the flood velocities and depths in an urban area where hydraulic structures and backwater effects are present and flood flows are generally contained within a channel, an unsteady, 1D HEC-RAS model could be appropriate. Are you responding to a time-sensitive event where somewhat conservative estimates of channel flood depths are required to evaluate downstream levee overtopping potential? If so, HEC-HMS could be appropriate as a first pass at assessing overtopping potential. A steady flow, 1D HEC-RAS model could also be appropriate depending on channel slopes and downstream hydraulic characteristics. Do you want to estimate the runoff volume into a reservoir resulting from a 1 in 100 AEP precipitation event? If so, you’ll likely want to develop a watershed model in HEC-HMS. However, if you want to understand the potential downstream flood impacts resulting from spillway overflows, you might initially want to consider a simple 1D HEC-RAS model using a steady flow analysis. Q & A Can I make conservative assumptions and use a simpler model, like HEC-HMS or 1D, steady HEC-RAS? Channel routing functionality in HEC-HMS does not explicitly account for flood attenuation, channel storage, or backwater effects and should not be used on flat slopes or areas with significant floodplain storage unless coarse estimates of flow depth are all that is required. However, this coarse level of analysis is often acceptable, particularly for screening-level hazard determinations in remote areas or similar applications. HEC-HMS can be a good initial screening tool. Since the model is easy to set up and not as data intensive as HEC-RAS, running a coarse conservative HEC-HMS dam breach model can provide direction for further analysis. Often, it is not necessary to go the extra mile with a hydraulic analysis - hydrologic modeling could be sufficient. HEC-RAS should not be used to estimate the hydraulic characteristics associated with steep slopes (i.e., greater than 10 percent) like those associated with drop structures, spillways, steep mountain streams, etc. HEC-RAS results associated with rapidly varied flow conditions (i.e., hydraulic jumps, etc.) may also be questionable. More sophisticated modeling, like computational fluid dynamics (CFD) or a physical model could be required for these types of conditions. In the interest of time and economy, simplified and conservative assumptions can be applied to many water engineering applications; however, such assumptions must be justifiable to ensure that results are conservative, yet reasonable. Furthermore, the reader is cautioned to forecast the time required to develop such assumptions, as this time plus the time required to develop a simpler model could be greater than the amount of time required to develop a more