Mixing both SCS and Rational Method for design.
Mixing both SCS and Rational Method for design.
(OP)
Hello,
I'm designing a drainage system for a small industrial area consisting of a closed drainage system (pipes and catch basins) that will discharge into a retention (not detention) pond.
Right now I only have the IDF curves which can not be used to calculate the volume to be retained in the pond, therefore I will derive a storm using the Chicago distribution and use the SCS method to calculate the volumes.
Since the pond will be sized using the SCS method, must I also design the closed drainage system using the SCS method or can I use the Rational Method for simplicity?
Thank You.
I'm designing a drainage system for a small industrial area consisting of a closed drainage system (pipes and catch basins) that will discharge into a retention (not detention) pond.
Right now I only have the IDF curves which can not be used to calculate the volume to be retained in the pond, therefore I will derive a storm using the Chicago distribution and use the SCS method to calculate the volumes.
Since the pond will be sized using the SCS method, must I also design the closed drainage system using the SCS method or can I use the Rational Method for simplicity?
Thank You.





RE: Mixing both SCS and Rational Method for design.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Mixing both SCS and Rational Method for design.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Mixing both SCS and Rational Method for design.
RE: Mixing both SCS and Rational Method for design.
@psmart: Thank you. I will look into WinTR-20.
@cvg: Even if I'm designing a retention basin which should retain all the runoff, I still need the hydrograph to design an emergency spillway, or am I missing something?
Anyways if I want to size the basin using the (Depth)x(Area)x(Runoff Coefficient)relation, the depth here refers to the IDF (Intensity) multiplied by (Time Difference) right? So if I want to design the retention pond to hold the 10-Year 24-Hour runoff, I would tabulate the 10-Year IDF curve, multiply each storm intensity by the time difference up until 24 hours to get the depths, then add up all of these calculated depths to get the total rainfall depth for this time period, correct?
RE: Mixing both SCS and Rational Method for design.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Mixing both SCS and Rational Method for design.
RE: Mixing both SCS and Rational Method for design.
RE: Mixing both SCS and Rational Method for design.
Presuming your retention requirement isn't to retain the entire 100 year storm volume, but rather to retain some regulatory limit and then pass the remainder through a spillway, you will indeed have to do a hydrograph analysis.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Mixing both SCS and Rational Method for design.
http://zonums.com/online/hydrology/curve_number.ph...
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Mixing both SCS and Rational Method for design.
Spillways should be sized using non-routed peak flow (i.e. 100 yr peak) from the developed tributary area, plus any offsite flows that need to be conveyed through the pond.
RE: Mixing both SCS and Rational Method for design.
I'd have a hard time scientifically justifying why an emergency overflow spillway benched into a berm shouldn't take routing effects into account. What case is routing not going to apply? 500 years from now when the whole thing has been silted in and there's a new channel cut into the silt running to the spillway?
I would think a safe assumption for spillway routing on a retention pond would be to clog all outlets other than the spillway and set the initial stage of the pond at the spillway invert. Then route as usual. If you're directing all pond discharge to a culvert, then it would be smart to consider the unrouted flows in case the outlet control box fails. Floats off its base or something.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Mixing both SCS and Rational Method for design.