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HEC-RAS ENCROACHMENT

HEC-RAS ENCROACHMENT

HEC-RAS ENCROACHMENT

(OP)
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.

RE: HEC-RAS ENCROACHMENT

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.

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