Corrugated canal linings and Manning's 'n' 2

[nick269]

This is Nick again from Vietnam.

We have a canal that we are trying to upgrade, and the city authorities are sold on canal linings consisting of corrugated pre-cast concrete sheet pilings. The surface profile of the sections look like this:
__ __
_/ \__/ \_

The projections are 0.48m in height.

These linings will form vertical walls. The channel bottom is unlined. Canal is subject to tidal backwater, and velocities in both directions are 1 to 2 m/s maximum. Because of a low gradient (.001)and low lying ground, flow reversals occur in the combined sewer system outlets (periodically submerged), so I am concerned about raising the water surface /reducing velocities any more than necessary. (Tidal gates with pumping has been ruled out)

My question is - how to compute Mannings 'n' for this type of vertical surface. I looked at k, the height roughness factor, and also Chezy's roughness factor, and thought of computing a composite n. My aim is to compare this surface with a conventional smooth concrete surface. I'm not sure how to go about this. Is there some standard method developed for such surfaces?

I also thought of modeling with HEC Ras, with alternate cross sections representing the projected and indented surfaces, and using expansion/contraction coefficients. Is this a feasible approach?

Nick Lombardi

 

[cvg]

depending upon how wide your channel is, you will need to compute a composite n. If your channel is wide, the corrugations probably have minimal effect on the flow

 

[dicksewerrat]

The coorugations will fill with silt, sand within a fairly short time. then they will be very much like a dirt canal and I'd use n= .019

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com