HEC-RAS ENCROACHMENT

[mhiw]

Using HEC-RAS method 4 to determine encroachments for floodway definition, seting target ws elev as shown in Application Guide, Example 6. But response seems backwards: increasing target elev results in smaller topwidth (as expected) but lower ws. Decreasing target elev results in larger topwidth but higher ws.

I can see that as encroachment increases, main channel n value (lower than overbank n) starts to dominate, overcoming reduced area for greater flow and lower elevation. But I don't see where to go from here. WS elev response seems to be from d/s x-sects, rather than x-sect of interest.

 

[Crystalct1]

That is true. In order to get the WSE at the section of interest to match the tolerance desired (1 foot higher than 100-year WSE for most states, but only 0.2 feet higher for NJ), you do have to move the encroachments outward on the downstream sections. I pulled my hair out on this too, but it takes time to smooth the encroachments out and get the best difference for each section. Just keep at it.