accomplishing No-Rise

[Anonymous]

I have a site where we are required cut/fill compensation in the floodplain fringe. We have about 40% extra cut to insure no rise. However, the increased flow area in left overbank reduces velocity in channel causing a slight 0.02 ft increase in the water surface profile. I tried changing mannings n all the way to 0.2 in area of cut
(when using actual “n” of 0.035 it makes it worse. (existing overbank is 0.1)

I achieved a no-rise by changing the channel n from 0.035 to 0.034 even though no work was being done in the channel.

unfortunately the reviewer questioned changing the channel n.

There is absolutely no reduction in flood storage volume so I know this is a no-rise situation.

Any ideas how to “massage” model without changing the channel n?

[huesmann]

Is the excavated area going to have the same Manning’s n as in existing conditions? Maybe you can justify your n change that way?

[Chris G.]

Joan-

The problem you bring up always frustrates me. Make a cross section’s flow area wider and you get a slight rise. It’s counter intuitive and many people don’t get it-unfortunately that includes reviewers. If you indeed increased the flow area, you should see a slight decrease in stage upstream of your cut. Can you plead your case to the reviewer given that?

You could insert some ineffective flow areas in the new cut area, to reduce the flow area back to pre-project conditions, thereby bringing the stage at that cross section back down. That may also increase the stage upstream slightly-just make sure it is still at or less than pre-project. You’ll need to justify the ineffective flow areas, but if you cut 40% over pre-project, I’m guessing most of that will be ineffective, unless you smooth out that expansion/contraction over a number of cross sections.

Good Luck-
@RASModel

[Author]

Posts