Differences in EGL elevation for multiple openings

Dear Chris,

I've run my three bridge opening in HEC-RAS and obtained an acceptable water surface elevation. Although the stagnation points don't seem to be listed correctly in the output, I just used the calculated flow rates for each bridge and ran them separately. The individual analyses have an energy grade elevation at the approach x-section that varies by almost 1' when I use those flow rates. Is this acceptable? What causes the difference?

Anne Daniels
lcdaniels@earthlink.net

Dear Anne :

The key items to remember is that at the approach cross section (at the cross section upstream of the bridge opening upstream face cross section), the water surface elevation is assumed to be level (the same value) across the cross section. However, the velocity through each of the openings is different. This gets translated upstream to the approach cross section. The sum of the water surface elevation and velocity head is the total energy head (EGL). Hence, this is why you are seeing different EGLs at the 3 bridge openings. You can easily explain this in your report.

If you have any additional questions or need any further information, please contact me again.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com

Dear Chris,

The water surface elevation is different too (in the individual models). I see that the problem may be that in the individual models, the channel (for two of the bridges) is in the ineffective flow area thus no velocity head is calculated. Do I need to redefine the channel for each model so that it falls within the bridge opening or effective flow area?

Anne Daniels

Dear Anne :

Umm....

Sounds like you have the answer. I agree.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com