HydroCAD question 1

[cse53226]

Wisconsin DNR standards states that in order to meet their water quality requirement a detention pond must take over 24 hours to release the 1-yr 24-hr storm event. Does anyone know if HydroCAD models this and where to find the length of time for a storm event to drain from the pond? Any help would be great

 

[zabrab]

This may be somewhat simplistic, but doesn't your pond outflow hydrograph continue to at least 24-hours? If the pond isn't empty, then it is taking over 24 hours to release the 1-yr event.

I am not familiar with Wisconsin DNR standards, if a careful reading of the standard indicates that the above is acceptable, it should be easy to demonstrate compliance with an outflow hydrograph longer than 24 hours.

Be aware that in your question you say that to meet the water quality requirement “a detention pond must take over 24 hours to release the 1-yr 24-hr storm event.” This can be interpreted to mean 24 hours from the start of runoff from the 1-yr 24-hr storm and not the start of the storm itself. The start of runoff may take place many hours after the start of the event.

Unless a more sophisticated analysis is indicated in the standard, a complete outflow hydrograph would address the greater than 24-hr release from either the start of the event or the start of runoff.