Merging water-surface profiles from adjacent streams

[Anonymous]

I am using HEC-RAS to simulate water-surface profiles on two streams approximately 4,000 ft apart that share the same floodplain as they flow though a densely-populated urban area. I am simulating a 100-year flow on each stream with the objective of creating a single 100-year flood map for the entire area. Of course the simulations produce water-surface profiles that don’t match (one profile is higher than the other)and I have a discontinuity in flood depths along the arbitrary boundary between the two streams. There are many bridges along both channels so I don’t believe a 2D model is an option. One bit of advice that I have received thus far is to simply project the highest of the two water-surface elevations generated by either stream across the floodplain between them. Another suggestion was to reassess the way that the flood magnitudes were computed and determine whether one flow could be tweaked to better approximate the water levels produced by the stream with the more reliable flow estimate. Any ideas?

[Anonymous]

Why is this odd though?

Two streams almost a mile apart would certainly produce WSEL’s that are different due to the terrain. Though both streams share a watershed, does each stream have an independent peak flow? For inundation mapping it would be incorrect to project the higher of the profiles a mile away. Each streams inundation boundaries should have their own limits and be generated accordingly

[Anonymous]

I agree. It is no surprise that the water-surface elevations are different and if all I wanted to do was create a map of the combined flood extents, there would be no problem. Previously created FEMA Flood-Insurance maps have been drawn this way and presumably they were created from hydraulic models that generated water-surface profiles that did not match in the floodplain between the two streams. However, if possible, I also want to create a depth grid for the flooded area. This is the issue I’m seeking advice on. Is there a precedent for merging depth grids in this case?

[Anonymous]

In RAS Mapper, right click the plan under the “results” tab and select “add new results map Layer”

Select Depth and the time desired or just “Maximum”. Select “Raster (with associated Terrain) then click “Add Map”

Right click the plan again and “Manage all results maps” and click “compute updated stored maps”

The depth grid should be created. Now right click it and export raster. With both rasters available you can do the rest within GIS

[ldguethle]

How much is the difference in depth/elevation at the boundary b/w the two floodplains?

You could do a combined 1D/2D model where the channel is 1D up to the banks and the shared floodplain is 2D. Whether this is acceptable depends on whether your project and client (and FEMA in this case?) will accept a combined 1D/2D model.

[Author]

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