×
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

Volume of trapezoidal channel

Volume of trapezoidal channel

Volume of trapezoidal channel

(OP)
I'm wondering if anyone has another method of determining the volume of water that is detained in a trapezoidal channel with check dams that measures 2.5'H x 8'BW x 18'TW (2H:1V sides). Check dams are the H (2.5') and are spaced 78' apart. Longitudinal slope of the swale is 0.032ft/ft.

I am just calculating the volume of a trapezoidal pyramid  to get the volume of water. (1/3A*h - Where A is the area of trapezoid and h is length of swale) So behind one check dam I can get 845cf of storage before overtopping. It seems right to me. Anyone see a flaw with this?

RE: Volume of trapezoidal channel

I would approach the storage volume by calculating the channel area at the check dam first. From some quick math, based on the dimensions you've provided I get an area of approx 32.5ft sq. Since the storage area will decrease to zero at the upstream limits due to the longitudinal slope of the channel, you could take an average area of 16.25 ft sq(32.5 +0)/2 and multiply by the length of 78ft to get a total volume of 1,267.5 ft3.
Hope this helps.

RE: Volume of trapezoidal channel

I agree that the 1/3 the area times the length versus 1/2 would be more accurate for a prism that tapers to a point, so I get the same value as you. Not sure i follow the dam being the same height as the channel, would this cause some flow around versus over the dam?

RE: Volume of trapezoidal channel

Rybo1 is correct on his approach using Average end area.  The volume is 1268 CFT, not 845.  You will be under estimating the volume.  Assuming a flat Trap 2.5' deep you will get about 2535 CFT. Half of that is the Volume.  Of course this does not account for slopes on the check dams.  This approach assumes vertical ends.

RE: Volume of trapezoidal channel

To be a bit more accurate:

V = L[BH/2 + zH^2/3]
Because it tapers to a line, not a point.

Solved by integrating the area B(xS)+z(xS)^2 from length x = 0 to L,
Where the water depth at any location = xS

L = length of channel
B = base width
H = maximum depth of water
z = channel side slope
S = channel longitudinal slope
 

RE: Volume of trapezoidal channel

(OP)
Thanks to all. I forgot I posted (being under the cosh), but I did end up realizing that the channel tapered to a line and did it the same way as described Ryb01. Thanks again guys.  

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