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I am getting 1.7ft difference of peak elevation and about 1000cfs of outflow between HMS (155.5ft and 2185cfs) and RAS (156.7ft and 3239cfs) at the dam. I have ran a HMS model to route a outflow breach hydrograph to use it in RAS to simulate an overtopping dam breach scenario at downstream. If you are having stability problems due to an overly steep reach, HMS could be used for routing, but the peak flows should be finally run through steady flow RAS to get the flood inundation extents.
#Hec ras boundary conditions full
You could go with full unsteady RAS downstream of the dam and use the techniques I described in the dam breach class to get the final inundation mapping. What I usually suggest is that the breach can be modeled using HMS, but downstream of the dam, RAS should be used. Furthermore, there is no direct method for mapping flood inundation from an HMS model. In short, HMS is okay for routing water from A to B, but if you are interested at all in what is happening between A and B, HMS is not appropriate. Plus, backwater will not be fully accounted for using any of the HMS routing techniques-meaning you can not simulate flow attenuation properly. HMS uses hydrologic streamflow routing which is a simplification of full dynamic routing. However, routing the breach outflow downstream using HMS is very limited. Using HMS to perform a dam breach has its advantages, namely that it is easy to set up, the data requirements are minimal, and it is numerically stable. But you’ll have to model it in unsteady to know how much for sure.Written by Chris Goodell, P.E., D. If the proposed culvert is only slightly larger than the existing, then you are likely to see only a small increase in water surface elevation at the downstream bridge. With a bigger culvert, you’re likely to get less flow attenuation than with the existing culvert, so a higher flow and water surface at the downstream bridge 700 ft away. However, if the interested party is wondering about changes in flow attenuation due to the bigger culvert, you would have to run you simulation in unsteady flow and use an event inflow hydrograph for your upstream boundary condition. Thanks!Īs you indicated, a steady flow model will not show any differences downstream from the new culvert no matter how far downstream you extend your model. Can you think of a document I could send them? Any information that you may have would be beneficial. It seems like they’d want some kind of formal statement showing that the model wouldn’t catch impacts at the downstream bridge. However, I know that the 1D HECRAS is only modeling upstream impacts, so extending the model won’t give any different results. An interested party in the project wants to extend the cross sections downstream to see if there are any impacts to the downstream bridge which is over 700′ away. This is currently modeled with 1D steady flow. We have a medium size stream modeling a cross culvert which is to be turned into a slightly larger culvert in the proposed condition. I have a slightly related question in regards to boundary conditions. How far downstream you need to place the downstream boundary is variable, but you can test this by trying different downstream boundary assumptoins (critical depth and normal depth, or simply normal depth with a bunch of different normal slope assumptions) and seeing how far upstream you need to be to not see an effect from different downstream boundary assumptions. At some point upstream, the effect of that error will no longer be “felt”. In other words, if there is an error at a specific location (due to a bad boundary condition, improper n value, poor survey data, that error will diminish the further away from that location you move. One of the great things about the solution scheme in HEC-RAS is that it is “self-healing”. That is why it is important to maintain distance between your downstream boundary and the area of your model that you are interested in. The bottom line is both methods (in fact ALL methods) for assigning a downstream boundary have some inaccuracies.