Dam Inundation Map: HEC-GeoRAS, GeoHMS

TravisBailey

I am trying to make a dam inundation map using GeoRAS and GeoHMS. I have the programs successfully installed and have a DEM in my layer, but am having a hard time knowing where to go from here. Does anyone have any experience with this or know of any resources that can help me from here?
Thanks

MattMead

Are you looking at creating an unsteady or steady flow model?  I've included the basic steps below and will branch it out depending on the method.

1) delineate your watershed-either manually or using an automated system like GeoHMS.  Personally I dislike automated watershed delineation as either A) you put in a ton of preprocessing work burning in channels, culverts, bridges etc to make it not worthwhile time savings wise or you dont put in a ton of time and your results are useless.  Automated systems have their place(huge watersheds for example), but for many cases it is probably overkill and depending on the quality of your terrain data, potentially very flawed.  If the watershed is small enough to be done reasonably(most off-line dams should be ok depending on location and topography) go ahead and just draw the boundary manually. 

2)determine other watershed parameters required for whatever methodology you choose to use(ex, Curve Numbers, Time of Concentration)

3)determine your rainfall data and distribution.  Guidelines will vary depending on location and the purpose of your model.  If developing a "sunny day" or other non-rainfall related failure, this doesnt matter, just put in 0's for your rainfall depths

4) input all this information into HEC-HMS along with any information you have on the dam itself(crest length, failure method, breach time, etc)

5)run the HMS model to get either A) peak discharge or B)breach hydrograph or technically C)reservoir inflow hydrograph

HEC-GeoRAS
1)In the georas toolbar, RAS Geometry->Create RAS Layers-> then create Stream Centerline, banks, flowpaths and XScutlines

2)Trace an aerial or use the DEM to draw the centerline of the stream in the "River" file.  Banks are more easily modified in the model so just copy this centerline parallel on both sides so you have something in the file.  For the flowpaths, copy the centerline and then draw in two other lines at the approximate locations of the centroid of flow(just guess at this point.  the edges of the valley will likely be a good enough approximation) 

3)On the georas toolbar are buttons marked with "ID" and another with three vertical lines.  Select the ID button then click on your River centerline.  Assign the Stream name and reach ID.  Next select the three vertical lines and click on each of your flowpath lines.  Assign left, right and channel appropriately.

4A)Steady flow option: this will be the more conservative approach, but also has the potential to not be terribly accurate.  Does not give timing and needs to have the flows attenuated downstream outside of the HMS model.  Various equations exist for doing this, google around but I think the Bureau of Reclamation has a decent one that is fairly often used.  Draw in cross sections as if you are performing a normal detailed study(XS at contractions, try and keep them within a 200-500ft topography depending, all your bridge cross sections laid out appropriately, etc).  For the flow data put in your peak flow from the breach at the upstream most end and then run it.  May need to increase flows based on base flow or other flow conditions in downstream channel and to account for any tributary streams that may be entering the channel.

4)Unsteady flow(preferred, but costly and time consuming, gives timing and more accurate account of in channel/valley storage)-Setup cross sections in the XScutlines shapefile at both contractions and wide expansions(to account for the storage) as well as at other locations same as the steady flow option.

5A)In the georas toolbar-RAS Geometry->Stream Centerline Attributes->All (set the "River" centerline and terrain TIN accordingly)
5b)XS Cutlines Attributes->All(setting all options accordingly)

6)Georas Toolbar->Ras Geometry->Extract GIS data and then save it where ever you want.

7)import your geometry into RAS.  In HEC-RAS 4.1 in the geometry window->File->import geometrydata->GIS Data.  select your exported GIS file then hit ok.  Click ok or next until your data has been imported.  You shouldnt need to check or uncheck anything in any of those windows.

8)model away.

Mapping:
1)When your modeling is complete, in the HEC-RAS main window->file->export GIS data.  make sure it is set to export water surface profile if Steady flow and Max WS profile(should be profile 1) if unsteady.  Check the box to export "user defined cross sections" then click OK

2)import this data back into GeoRAS.  In the georas toolbar is a button with a circular shape on it that says "import RAS sdf file" click on this to turn your exported geometry into a .xml file.  Click on the RAS mapping->layer setup button.  Check new analysis and name it whatever, set the RAS gis Export file to your .xml file and output directory to a folder that you want it to store your data in.

3)RAS Mapping->Read RAS GIS export File.  This will create a geodatabase with your stream centerline, cross sections and I think a boundary file.  NOTE:these files do not have a projection.  Move the new XScutlines file to your original dataframe, then right click->data->export.  make sure to select the option that uses the .mxd's projection instead of the original file's.  This will create a copy of the data and at the same time create a new one in the correct coordinate system.

4)In 3dAnalyst, create a TIN from your new cross section file using the flood elevation data in the attribute table.

5)In 3dAnalyst->Tin Surface->Tin Difference.  This will create a polygon with above,below and same(or 1,-1,0) attributes.  If you have your water surface TIN(step 4) set as the first TIN you just need to delete the "below" or -1 portions and you have a polygon of your inundation area.

Let me know if you need anything else.