HEC-RAS Encroachment Problems

[CivilMichael2318]

I gave up trying to get method 4 to work properly. I was trying to set a 1' rise and only the furthest downstream section would show a foot rise and by the time I got 150' upstream no rise at all. Now I'm tediously using method 1 but having another issue. In the cross section viewer the encroachments are showing up in the correct place but the water surface/filled in water surface is inside the encroachment. (Please see attached screenshot) Also the left and right bank offsets don't appear to be doing anything. I think I can work around having to use method 1 from scratch and the bank offsets not working but the water filling in the left encroachment is an issue. Has anyone else had this problem?

 

[Professional Engineer]

You have both encroachments inside of the bank stations. Note that encroachments cannot be in the channel--FEMA will not allow that. A work around would be to move the bank stations inward and that would effectively have the encroachments outside of the channel.

Note that you need to define encroachments at every cross sections, not just the downstream most cross section. You can watch this YouTube video and it describes modeling of HEC-RAS encroachments using GeoHECRAS, plus a lot of modeling tips when dealing with encroachment studies.

 

[CivilMichael2318]

Thanks for the help. I discovered why the water surface was filling in the encroachment. I had added a -100ft constant to each section so that my channel center line would be at zero to match my CAD drawing. For whatever reason (I assume a glitch) this caused the encroachment on one side to be filled in. If I remove the constant so that the bank stations run from 0 up the section shows the water filled in properly.

I had I started to suspect having encroachments in the channel may be the problem with the method 4 not working. The issue that's creating me so many headaches is the 100 year water surface elevation doesn't leave the channel. So the floodway and floodplain are both entirely within the channel. I've got an area that's zone A that I'm trying to do a LOMR for to get the limits of the floodplain pulled back away from a property. It's a small 100 acre drainage area. So if encroachments can't be in the channel would I just show the whole thing as floodway and not show floodplain at all?