Manning's coefficients

Chris,

You have answered a couple other questions regarding RMS for our floodplain project. I initially provided Manning's coefficients (0.035) only at the downstream section (assuming subcritical flow) for the left, center and right banks, and analyzed the floodplain. Later, I added a different coefficient (0.028) for the right overbank for a portion of the floodplain that lies in a golf course. This involved 19 cross sections in the middle of the project (out of a total of 57 cross sections). Again, I ran the analysis and calculated the water surface. However, when I revised this coefficient in the golf course to 0.03, the water surface elevation process ended in a fatal error, and I had to change the cross sections back to 0.028.

My questions are:

1. Are Manning's coefficients entered for only some cross sections read by RMS (and HEC-RAS) instead of the default coefficients entered only at the downstream cross section?

2. Is it impossible to later change this coefficient?

3. Do have any idea why this coefficient does not display on the first 6 cross sections (from downstream), but does on the remainder? The first section to display is at the end of a row.

Thanks for your time,

Merle Miller
Cape Environmental Management, Inc.
10010 San Pedro Ave., Suite 345
San Antonio, TX 78216
(210) 377-2008

Dear Merle :

Note that when you define roughness coefficients at a cross-section, you need to define it for the left overbank, channel, and right overbank (unless you are using multiple horizontal roughness coefficients). RMS / RiverCAD will then propagate the roughness on upstream until you define it again at another cross-section. The roughness values will only be displayed at the cross-sections that you have defined them at, although the software will continue to propagate the defined values on upstream. However, don't make the mistake of defining only a left overbank
roughness and not the others. Otherwise, the HEC-2 and HEC-RAS software will assume that you have defined a roughness of 0.0 for the channel and right overbanks.

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

Thanks for the response- very helpful.

Merle Miller
Cape Environmental Management, Inc.
10010 San Pedro Ave., Suite 345
San Antonio, TX 78216
(210) 377-2008