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Primary Outlet Device--Culvert

Primary Outlet Device--Culvert

Primary Outlet Device--Culvert

(OP)
I am modeling a detention pond with a 12" culvert as my primary outlet (discharge pipe).  After routing, my results show a discharge above the capacity of my pipe.  I have a 12" ADS pipe with a .5% slope.  Modeled discharge is 4.78cfs and the full discharge of that pipe is approx 2.5 cfs.  I am not receiving an error message and I am wondering if any HW is being accounted for.  

RE: Primary Outlet Device--Culvert

HydroCAD takes into account the HW when calculating the flow through the pipe.  The 2.5 cfs is the capacity under gravity flow using mannings.  Based on your description the culvert likely has a significant ammount of head on the pipe causing it to flow under pressure.  If you want to double check the HydroCAD comps by hand recalculate the flow using the Darcy-Weisbach or Hazen Williams formula.  

RE: Primary Outlet Device--Culvert

A culvert outlet on a pond always considers headwater and inlet conditions.  Headwater can cause the flow to exceed the normal Manning's capacity of the pipe.  On the other hand, high values for the entrance energy loss coefficient Ke can result in flows that are less than the Manning's capacity.

Peter Smart
HydroCAD Software
www.hydrocad.net

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