how to model broadcrest trapezoidal weir in hydrocad

[yazoo]

need help modeling a broadcrested trapezoidal weir typical in dam emergency spillway designs. I use to run Dams2 for these and checked a hydrocad run against and old design. Upon comparison, i realized the emergency spillway calcs are not accounting for my sideslopes along centerline of the dam. Maybe Mr. Smart can help me out.
Thanks in advance.

 

[psmart]

The broad-crested weir option in HydroCAD allows the weir coefficient to vary with head. But this can only be used for specific rectangular weir profiles for which test data is available. Aside from the variable coefficient, it's just like a standard rectangular weir.

To handle your weir with side slopes, use the Vee/Trapezoidal weir option.

For further details see

http://www.hydrocad.net/weirs.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[DamEng1]

When modeling trapezoidal earthen emergency spillways for dams in HydroCad, I generally use the following spreadsheet (

http://www.mde.state.md.us/assets/d...delines/Spreadsheets/Hydraulics/SPILLWAY.XLS)

developed by the Maryland Department of Dam safety and import a rating curve into HydroCAD. The equations are from SCS TR-2 and Brater and King, the only limitation is that you are assuming that critical depth occurs at the downstream crest of the emergency spillway for all values of head. This may not be true in all cases and thus should be used with some caution.

 

[yazoo]

Thanks DamEng1- that closely matches my dams2 run.

 

[psmart]

Out of curiosity, how closely does it match the standard trapezoidal weir rating curve? And how much effect does it have on the actual hydrograph routing in HydroCAD?

In my experience, even modest chamges in the rating curve usually have a minimal effect on the final hydrograph routing. Of course, this depends on the exact situation, so I urge you to try both solutions and compare the results yourself.



Peter Smart
HydroCAD Software

http://www.hydrocad.net