Weirs Coefficients

[CMQ]

I am modeling a flooding situation where a local low point overtops and earthen berm. I have modeled the the low point as a pond and the existing berm (approx 40 feet long with a 6" high flow) as a custom weir with the default coefficient of 2.62. I was requested to provide a coefficient which considers the light brush and vegetation along the top of the berm. How would I best consider the "roughness" of weir material (i.e. vegetated).

 

[beej67]

At first glance, this Kindsvater paper would seem to address your concern directly:

http://pubs.usgs.gov/wsp/1617a/report.pdf

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[psmart]

Also note that different weir equations include different terms in their coefficients, resulting in different coefficient values being used for each equation. For example:

Rectangular Weir: C = 2/3 sqrt(2g) Cd (e.g. 3.26)

V-notch or Custom Weir: Cv = 8/15 sqrt(2g) Cd (e.g. 2.61)

Where g is the gravitational constant, and Cd is the unit-less discharge coefficient, which is typically 0.61, giving the typical C values shown above.

Note that the v-notch weir coefficient (which is also used for a custom weir in HydroCAD) is 80% of the rectangular weir coefficient. So you need to be careful when using a custom coefficient.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

When considering a custom coefficient, also be sure to do a sensitivity study to determine how accurate you really need to be. For example, how much difference is there in the peak flow and WSE as you increase the weir coefficient from 2.6, to 2.8, or 3.0? In many cases the effect is minimal, suggesting that less precision is required in the choice of C value.

Peter Smart
HydroCAD Software

http://www.hydrocad.net