Weir Coefficient

[limno]

Hello,
How is the weir coefficient (Cd) for a lateral weir calculated in the HEC-RAS model. HEC-RAS had a default value of 1.11 (SI units), which I decreased to 0.611. Is 0.611 a low value for Cd?

What is a reasonable value or a range for Cd (in SI units) that one can apply.

Also, how Cd is different from the "Weir Coef" used for Bridge Deck in the HEC-RAS.

Thanks!

-Pranesh.

[jarvus lunt]

I believe the weir computations over a lateral structure are basically the same as for a bridge, with the added computation that a lateral structure has a sloping water surface parallel to the length of the weir.

There is a discussion of the lateral weir computations in chapter 8 of the Hydraulic Reference manual. This should be available under help, or from HEC's web site.

I'm not familiar enough with SI units to recommend any values.

[Paul M]

Maybe your well past this problem, but if you go back to hydraulics 101, the weir coifficient is some root of 1 over g or something, i will leave that to you to look up. so for a broadcrested wier C-standard = 3.09 and C-metric = 1.704; so C-standard x 0.55 = C-metric. So your coefficient of C-metric = 1.11 is C-standard=2.0. so in your problem, C-metric=0.61whatever is about C = 1.0 in standard, which is extremely low. So C-metric = 1.11 is as conservatively low as you need to go.

Have fun.