Floodway Analysis greater than 1'

[gopher669]

So, I'm going through RAS and doing the Method 4 & 5 Encroachment method to define my floodway. Everything is going fine for the first 3500 feet, until I get to the first cross section after it changes from concrete (upstream) to an earthen channel (also the first XSec after box culvert). Suddenly, after having reasonable encroachments and water surface changes, I get a +1.52' with the encroachment stations equal to the channel bank stations. I went back and changed the encroachments to Method 1 and moved each one out 200' (i.e., widened the floodway by 400'), only to have the change in water surface rise to +1.90'. Just for fun, went back and changed the encroachment stations to the very first and last point of the cross section, and got a +1.94'. The only way I'm getting to below 1' is to move the encroachment stations to 15' inside the channel bank stations, which isn't allowed.

Anyone else run into this, or have any suggestions? Seems counter-intuitive that the wider I make the encroachment stations, the higher my Prof Delta WS....

 

[francesca]

It seems to me that the only way you're getting your profile delta WSE less than 1 ft is to make the box culvert larger.

 

[gopher669]

It's an existing road crossing, so can't extend the box. I'm now in charge of doing the LOMR that the previous administration should have done 3 years ago.

 

[cvg]

didn't you have a CLOMR?

 

[gopher669]

There was a CLOMR, but it was only for lining part of the channel with concrete. Since then, the City took over the project, extended the concrete channel, some box culvert modifications... Enough so that the previous CLOMR was useless.

Next Question.... Wasted numerous hours until I finally found the magical combination that made that XSec work, and made it through the rest of the channel. Went back and imported the results into Method 1, only to find that the Prof Delta WS isn't the same between profiles. Although most are only a hundreth or two difference, there are a couple larger ones that push the Prof Delta WS over the allowable 1'. So, even though the Encroachment Stations for the Method 4/5 profile are the same as the Method 1 profile, I'm getting different WS Elevations.

 

[francesca]

At some point you need to ask yourself if you're trying to fudge the numbers to save the City from having to fix their screw-up.

 

[cvg]

said screwup being that they began work without an approved CLOMR. Who is the floodplain manager? Hope it isn't the City!