spillway capacity calculation

[olemiss]

I have a piece of property with an earth dam impounding a 4.7 acre surface reservoir, the runoff area is 516 acres but no hydrograph is available. The dam has a concrete spillway 42 feet bottom and sides sloping 10 feet horizontal and two feet vertical. The slope is 2 ft. 9 in. and length is 79 feet. The reservoir stays full to spillway invert most of the time, no control system in place. I need to determine what the capacity of the spillway is at say one foot and two feet depths at overflow point. There is a formula given by Bureau of Reclamation in a book entitled Design of Small Dams Q = CL(H3/2 power) that gives a much smaller capacity than simple open channel formulas, for one foot 180 cfs vs 934 cfs, for two feet 650 cfs vs 3117 cfs. Anybody help to straighten me out on this point will be appreciated.

 

[prblmfxr]

If I understand you correctly, your spillway is set at a very gradual slope, 2'-9" drop over 79'. What is the terrain like downslope of the spillway? Depending on this, backwater effects may limit the capacity of your spillway as compared to general open-channel flow equations or the equation you mentioned from the dam design book. A complete hydraulic analysis of the system with proper assumptions of conditions downslope of the spillway would be an adequate way to assess the spillway capacity.

 

[PECPESC]

Get the goverment publication "Design of Small Dams"