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Corrugated canal linings and Manning's 'n'
2

Corrugated canal linings and Manning's 'n'

Corrugated canal linings and Manning's 'n'

(OP)
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

RE: Corrugated canal linings and Manning's 'n'

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

RE: Corrugated canal linings and Manning's 'n'

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.
WWW.amlinereast.com

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