Rip-Rap: Energy dissipation calculations

[waterguy]

Good Day,

I'd like some thoughts from anybody who has had some experience in determining/calculating the amount of energy dissipation from a large sized (300mm to 500mm) angular rip-rap stones.

I have a storm sewer outlet with a rip-rap "spillway" consisting of 300-500mm anugular rip-rap at a 10% slope (not ideal, but it's what I've been given to work with). I also know the lenght of the rip-rap spillway. I have already determined by depth of flow, velocity, and peak outflow at the upstream end of the spillway, but would like to know the best way to determine the velocity at the downstream end of the spillway due to erosion concerns. Would it be best to just apply Bernoulli's equation with a high loss coeffecient to account for the very "rough" rip-rap ? If so, what would be an appropriate loss coefficient ? Or, perhaps there are some equations or resources out there based on experimental work ?

Thanks for your time.

John

 

[sgwpe1]

Hi Wterguy:
Hope this isnt too late (probably is). At least it may be helpful on the next one. The USDA, Soil Conservation Service has all kinds of information on rip rap, conduit outlet protection, etc. They have nomographs and charts. I dont know if your application will fall within one of their charts, but generally mannings equation is the applicable formula (open channel flow). they do have charts for figuring the appropriate "n" factor. try their web site (use a search engine, eg., Google).
Scott