Flood Inundation Mapping In HEC-RAS?

Question

I'm getting this output as a result of discharge values while doing flood inundation mapping.

In some areas there are breaks as indicated by red arrows.

If this result is correct what would be the interpretation of it?

Answer 1

This is expected if the terrain model you load in RAS Mapper (let´s call it terrain) is above the results of water levels of your hec-ras model which has a geometry model.

You do not indicate enough of your data (input and output) to be more precise in our comments. My guess is that the geometry model of your hec-ras model does not match the raster terrain model you are using to produce the flood map, so inconsistencies like the discountinuities you show are possible and they are not necesary a wrong result of your modelling, but certainly they are a sign of poor input for your flood mapping.

In the picture below, the cross section of the geometry of hec-ras model (black line) and terrain (grey line) are different. Also, the water level of that simulation is below the terrain model, so it will produce a flood map with a discontinuity.

Answer 2

It is very unlikely that those results are correct.

Any flow put into your upstream tributaries will flow downstream with greater than zero depth.

To get accurate flood mapping with HecRas and Arc, you need to following:

1. Be a hydologist. Getting hydrology wrong can mean people die, so make sure you're qualified to work in this field. Here is a good test: could you write a backwater effect spreadsheet using the Manning equation for this location if you had to? If the answer is no then find someone who can to help you out. If the answer is yes, then you can geck these results for yourself using other tools.
2. Good elevation data, preferable LiDAR or photogrametry. If there is an existing water body/river, you'll need to tie in bathymetry below the water surface as LiDAR and photogrametry will give you the water surface elevation.
3. Use the HecGeoRas add-in to draw out stream center lines and cross sections, and export a HecRas model. The GeoRas manual is pretty good.
4. Run the model in HecRas
5. Import the results back to Arc using the HecGeoRas mapping tools. Again, the manual gives good examples.
6. Because you've used the same data to generate cross sections and present the data, you won't have zero depth locations
7. Pull imagery into the background. You should see a good match between your model results and the terrain, because terrain is heavily influenced by where water flows. If your results and the terrain (elevations/vegetation locations etc) don't match well, don't trust the results.

Note: for recent versions of Arc, you may need alternate GeoRas install files