×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

HEC-RAS/FEMA Help?

HEC-RAS/FEMA Help?

HEC-RAS/FEMA Help?

(OP)
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?

RE: HEC-RAS/FEMA Help?

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

RE: HEC-RAS/FEMA Help?

(OP)
Thanks for the information.

RE: HEC-RAS/FEMA Help?

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?

RE: HEC-RAS/FEMA Help?

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?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources