HEC-RAS/FEMA Help?

[ashhafPE]

I am replacing a bridge built in the 40's. I determined the Q's using HEC-HMS (scs method) and compared it to the regression equ. (both were close). I used HEC-RAS and analyzed existing condition and proposed condition. The problem is the existing condition showed W.S.=79' while FEMA is showing 76'. I double cheched my Tc. Based on history, Water never overtopped the bridge or roadway. I know there is a difference of 1' in datum. Any suggestions? Also someone suggested I use HEC2. Doesn't HEC-RAS do what HEC2 did and more?

 

[LHA]

Since FEMA has a Detailed Study, there's will govern for regulatory purposes, unless you issue a Letter of Map Revision (LOMR).

Go to the FEMA website. Request the raw data used by the original consultant who did the HEC study. Sometimes, you get lucky, and the actual electronic file is available. But they always have printouts. Cost is nominal, usually $200 or so.

Recreate their sections or issue a LOMR, depending on whose sections best describe the current existing condition. from your name, you appear licensed; you will have to seal the data for the LOMR.

The original STUDY is almost certainly on HEC2, not RAS. HEC2 and RAS do the same things, RAS is just easier to use. You don't have to rerun it in HEC2; if it is electronic, RAS will convert HEC2 data nicely. If it is a printout, key it into RAS - but for that, you will need to learn to read HEC2 well enough to translate. That's probably what your friend meant.

HEC2 is also freeware, from the USACOE, same place you got the RAS software.

You can just explain the delta on the datum, because you probably can't adjust the roadway design datum, and your FEMA submittal will have to use their datum. Just explain in your H&H study for the bridge, correct for it and show the actual water depth is after the correction.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[ashhafPE]

Thanks for the information.

 

[gbam]

ashhafPE,
If the regulatory discharge is close to your discharge then I would use the regulatory Q. Also the 3 foot difference makes me wonder about the datum. Is the oregulatory model based on the 29 datum? What datum is your model based on? One last question; Did you use the regulatory model cross section locations?

 

[francesca]

Anecdotal evidence for 40 years should not sway your judgement. There's no guarantee there has been a 100-year storm in the past 40 years, or that someone was out there watching during the storm!

How old is the FEMA study? What has changed in the area since the FEMA study? (Other than modeling technology.)

You said you're replacing the bridge. Surely your challenge is to design a bridge that keeps the water surface elevation to 76' based on modern methods of estimation and your own professional judgement?