Modeling Riprap Abutment Protection

[Anonymous]

Assume you have a square channel–bridge with vertical abutments. You need to model sloped riprap abutment protection in the channel. Is it reasonable to just use Sloping Abutments from the Bridge Culvert Data screen to model that?

[Marulke]

If you want to include the riprap in the geometry, I believe using a sloping abutment to represent the riprap is OK. RAS combines the US and DS cross section and the bridge geometry to make two CS inside the bridge. You may check them in the bridge editor to see if they represent the actual geometry.

However, I dont know how RAS handles the difference in friction between bridge (concrete) and river bed / riprap, when calculating the friction loss through the bridge.

[Anonymous]

But does HEC-RAS actually model the abutments as concrete?

[Marulke]

I don’t know how HEC-RAS handles the difference in Manning number between river bed and abutments for bridge energy loss compuations. I think, but I am not sure, that RAS ignores this.

Anyway, the flow distance under a bridge is normally quite short, and the friction loss small.

[Chris G.]

By default, RAS will use the Manning’s n values from the bounding cross sections for the bridge internal cross sections. However, you have the ability to go into the bridge internal cross sections and change Manning’s n values, bank stations, and station-elevation points. Changing the n values for the internal cross sections will allow you to simulate concrete abutments. To access the bridge internal cross sections, go to the bridge/culvert editor, and under the Options menu item, select “Internal Bridge Cross Sections…”.

[Author]

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