Sequential Checkdam modeling

[stioffan]

I have been trying to model 4 checkdams in sequence 100ft apart on 6% slope. The swale is trapezoidal with a 12'BW x 30'TW x 3'D & 3:1 sides. The check dam H is 2' and spans the width of the swale. My problem is that I can't get the peak flow rate down without overtopping the swale at each Check Dam. The dimenions used above are the last ones I tried but I have used other dimensions with the same results. I started with 6bw x 22tw x 4d & 2:1 sides with a 3'H check dam - which I did get to work! I want to flatten out the sides and not be as deep for safety reasons. I have tried putting pipes through the weir, and exfiltrating but that's not doing the job. Any ideas would be appreciated.
Thanks
Stioffan

 

[TerryScan]

How do you have this defined in Hydrocad? Is it a series of ponds? or do you have reaches? How is the storage defined? What kind of flow rate are you dealing with?

 

[stioffan]

A series of ponds defined with incremental storage at the contour elevations. The flow rate for 100yr storm is 20 cfs. After looking at it some more I think I am going to need a pond.

 

[psmart]

I'm not really clear about your question. Unless you can retain the entire flow behind the check-dams, I would expect them to overtop. Compare the inflow volume to the available storage.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TerryScan]

stioffan -

Ok, I ran this with your geometry and flow rate.

At a 6% slope, the storage length at 3 ft deep is 50 ft, so there is too small of a storage volume to make any significant attenuation before the water reaches the top of the weir.

Your conclusion about a pond is correct. At this slope, there is too little storage. An outlet large enough to prevent overbank will never produce significant storage. An outlet that restricts flow enough for attenuation will quickly cause a water surface elevation rise at this slope (a culvert that would halve the flow rate would bring the water surface to about 8 ft deep)