hec-ras results

[firebird16]

Bridge replacement project. The results for a 2 yr and 100 yr discharge are 0.1 ft apart. 450 cfs compared to 1200 cfs. How reasonable can this be? The water does overtop the road in a sag. Any hec-ras users out there?

 

[gbam]

First off you need to examine the output from your model. are you modeling the dip as a conveyance region? Are you running the model in mixed regime or just sub-crit. Also look at your error messages. Make sure you fully examine you input data as well. It is not easy to comment on your situation without more information.

 

[cvg]

depends a lot on the shape of your channel cross section

 

[LHA]

Overtopping would let weir flow dominate the water surface determination. Yes, huge flow could be only 0.1 ft (that's more than an inch) above a small flow. One tenth of a foot or weir flow over a bridge deck...I'm guessing at least 20 feet of crest width?...is a lot of capacity, at macadam's resistance.

 

[rdj16]

could not post a thread under firebird16 so now i am rdj16. thanks for all your helpful comments gbam, cvg, Iha. The water surface is no longer overtopping the bridge or sag, I must have made a previous error or somthing......One recent run I made reduces the opening area from 370 ft2 to 195 ft2 using a 18 by 11 ft box. The 100 year event rises from 385.1 to 385.2. The state will accept it but my boss will not. If I put a 36" or 45" I-beam in it puts my 60' span on a carazy vertical curve which I am sure the client will not understand. I will try a 20 or 22 by 11 box on subsequent trials. What about setting the top of a box as the riding surface on a rural road?

 

[bltseattle]

I worked on a design with the top of the box culvert as the driving surface (- it was overlayed with pavement section that tied into the approach roadway). The big concern was that adjacent soils/backfill material would settle slightly, but the culvert wouldn't, leading eventually to an unintended bump in the road. The solution was to use CDF (controlled density fill) to backfill the excavation.

Hope this helps