×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Calculating shear in Channels?

Calculating shear in Channels?

Calculating shear in Channels?

(OP)
The typical formula used to calculate shear in a channel bottom is:

62.4 * Depth * slope in ft/ft = Shear in PSF

This is listed in Hec 15, Design of Channels with Flexible Liners.

Is this just an empirical formula?  I don't see what slope has to do with shear in a channel, mathematically speaking.

I could see figuring the downward component of the gravity acting on the water to determine shear, but not raw slope.

RE: Calculating shear in Channels?

Chow's Open-Channel Hydraulics (1959) shows the derivation which I will not repeat here for brevity.  Your equation is correct except that it makes the assumption that in wide channels the hydraulic radius equals the flow depth. So:

t = wRS

  Where: t = tractive force, or shear force
         w = weight of water (62.4)
         R = hydraulic radius
         S = channel slope  

Chow also explains that, except in wide channels, the tractive force is not uniformly distributed along the wetted perimeter.

RE: Calculating shear in Channels?

note that velocity of the flowing water which creates the shearing force is proportional to the square root of the slope.  And as you indicated, gravity supplies the driving force and on a steeper slope, there is less of a vertical reaction force, therefor the velocity increases with steeper slopes.

RE: Calculating shear in Channels?

(OP)
Common sense would have you believe that velocity has something to do with shear.  But, I don't think it does.  

I wish I had a copy of Chow, but the $300 price of admission is a deterrant.

RE: Calculating shear in Channels?

proletariat,

Try www.gettextbooks.com I found a soft cover international edition for about $50 a couple of years ago.  Listed at about $65 now, but its one of the best references I own.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources