Laserfiche WebLink
<br /> <br /> <br /> <br />8 <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