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