INCREASED "n" FACTOR TO REDUCE VELOCITY??

[tincan]

I have an open channel between 2 fixed elevation RCB's. To connect these two I can use a drop structure or a slope of 1.33% which produces high velocities. Has anyone roughened the interior of an RCB, sandblast or water pressure on setting concrete to obtain a higner "n" value?

Would have to go from the normal value of 0.013-0.014 to 0.025 to reduce the velocities to a workable limit.

i would prefer not construction dragon teeth or blocks within the RCB because of possible trash hangup, maintenance is poor.

Anybody tried this?

 

[cah2oeng]

What's the problem with using the drop structure?

 

[cvg]

I agree - would prefer to keep the velocity in the box higher to prevent sedimentation and maintenance. A drop off the end of the box is a common method of reducing the channel slope. How will roughening the box slow the velocity in the channel?

 

[gbam]

I agree, use a drop and the equilibrium slope of the channel. I usually try to minimize drops to 2 feet for safety.

 

[tincan]

I agree with the drop but client doesn't. Will retry.
I was curious if anyone had tried anything but baffle blocks as per HY8.

Thanks for the replys.

 

[cah2oeng]

Have you calculated the expected velocities if you just go with standard RCB? If so,what's the problem with having the high velocities?

 

[gbam]

Tincan,
What is the distance between the boxes? Can you riprap, grouted riprap or ? between the cbcs?

 

[tincan]

Sorry i never could tipe. The slope is 13.32% missed a decimal. The drop depth will be 13-14' +-

gbam, have used the rip rap and grouted rip rap on other installations but owner wants to pave paradice and put in a parking lot, so RCB is required.

cah2oeng, have a problem at the exit, trying to keep the velocities as low as possible. May just have to buy adjacent area and put in an energy dissipator.

thanks

 

[cvg]

how about drops within the box? I haven't done this but it seems that it could be done.

 

[gbam]

Is continuous box out of the question, Ok to park cars on top of most cbcs

 

[dicksewerrat]

what about some corrugated pipe lined with CIPP? Liner for corrosion and abrasion, corrugated for 'n' value.

 

[tincan]

Thanks for the replies;
Cah2oeng---have done drops before and they would be applicable here, but the client doesnt want them.

gban--no problem with parking, want to put in a shallow RCB as much as possible for the economy, then a steep section(or drop) and then flatten out to the normal channel slope.

dicksewerrat--(love that moniker)no CGMP or internal rings allowed.

It looks like I'm going to end up with a broken back RCB with a CSU enegergy dissipator at the end.

Thanks for all your input, Tincan.