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Modeling a Rip Rap Spillway in HydroCAD

Modeling a Rip Rap Spillway in HydroCAD

Modeling a Rip Rap Spillway in HydroCAD

(OP)
I have done a quick search for similar topics and haven't found anything specific, so I apologize if this is a reposting of a previous question.

I am assisting one of my longterm clients by analyzing several of his existing properties in an attempt to bring these projects into compliance with the current drainage ordinance. While reviewing some as-built information for an existing detention basin, I have noticed that there are significant variances in the flow leaving my basin depending on how I model an existing emergency spillway and would really appreciate any suggestions that someone may have:

Originally, this basin was designed with a concrete emergency overflow to pass the 50-year storm event, per the local requirements. However, when it was constructed, the contractor used riprap in lieu of concrete and it wasn't caught by the original designer. (This basin was constructed approximately 10 years ago). To make the problem worse, the basin that was constructed is significantly smaller than the original design, so now this spillway is passing portions of much smaller storm events (i.e 10-year storm). While initially modeling this existing condition in HydroCAD, I used a secondary broadcrested weir for this overflow, but this type of outlet does not seem to differentiate at all between a concrete or riprap lined weir. Just based on the Manning's coefficient, it would seem that the riprap would provide significantly lower release rates for smaller storm events than would be expected from a concrete overflow. Is there a better way to model this type of outlet or am I flawed in my assumption and the release rates won't be significantly different?

 

RE: Modeling a Rip Rap Spillway in HydroCAD

Is the spillway crest lined/constructed of rip-rap, our just the discharge channel?  If you have rip-rap along the crest, the cross-sectional flow area will be much less at low heads.  You won't get full weir flow until the rip-rap is submerged.

The most accurate solution would be to generate a stage-discharge curve based on field observations, and enter this into HydroCAD as a special outlet device.

A picture or sketch would help a lot.
 

Peter Smart
HydroCAD Software
www.hydrocad.net
 

RE: Modeling a Rip Rap Spillway in HydroCAD

You could use Mannings Equation to develop a depth-discharge relationship for a trapezoidal channel of variable depth, and use that as your stage-discharge relationship for your pond.

It's not a perfect method, but it aught to be close.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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