Floodplain mapping according to EGL

[dhvmed]

Hi All,

I have a 1D model of a canal in an urban area, that lies between highways.
The drainage of the city flows underneath the highways in more than 100 closed pipes, and discharges into the canal at the bottom (or very near the bottom).Most XSs have a pipe discharging into it and I made the assumption they all do.

The canal is typically dry but experiences flash floods of up to 600 CMS.
When water level rises to about 80% of the banks, before it overflows, it breaks through the urban drainage and floods the surroundings from the inlets.The water rises up to the Energy Grade Line level and therefore, I need to calculate the flood plain according to the EGL.

Alas, HEC-RAS cannot export EGL to GIS.

One solution I thought of is to create polygons in GIS of the flood plain according to the XSs, export the EGL to GIS via Excel, join the excel to the polygons and then subtract the polygons’s EGL from the DEM.
I have 2 problems with this approach :
1. It is quite cumbersome.
2. I think it might result in an over-estimation of the floodplain.

Is there a better solution to mapping floodplain by EGL?

[Anonymous]

Not sure if it would be any easier, but you could force the water surface to be the EGL.

Create a steady flow plan. From the steady flow data editor, go to options, go to set changes in WS and EG.

Click on Add Multiple RS Locations, click on Known WS Add all the cross sections. From the post processor results, select the Max WS profile. Copy the entire E.G. Elev column to the clipboard. Paste these values into the Known WS and run the model as a steady flow. The “WS” will now be the EGL. I presume you could then export this water surface map?

If you have any bridges or culverts, you may have to edit these out. Copy the column to Excel and delete the bridge/culvert lines and then copy over to known WS.

[Author]

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