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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.

RE: Mixing both SCS and Rational Method for design.

It is very common in my region to design pipes with rational and ponds with SCS. The rational method was basically invented to calculate peak flows from unrouted basins, which is perfect for evaluating pipe capacity.

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.

BTW, as an alternative to a Chicago-style storm, you can generate a synthetic rainfall distribution from your IDF data with HydroCAD or WinTR-20. I believe the NOAA PFDS also offers the ability to create a local rainfall distribution.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Mixing both SCS and Rational Method for design.

since this is a "retention" basin which retains all the runoff from the storm, than you really do not need a hydrograph to calculate the volume of water. A conservative estimate can be made with just the total rainfall depth times the area times a runoff coefficient. since it is an industrial site, you would normally assume about 90% of all the rainfall runs off to the basin. Of course this is highly dependent on the type of industry.

RE: Mixing both SCS and Rational Method for design.

(OP)
Thank you beej67.

@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.

To get the 24 hour rainfall depth, just take the 24-hour intensity and multiply by 24.


Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Mixing both SCS and Rational Method for design.

you could just size the spillway to handle the 100-year peak runoff. Still not sure why you would need to route a hydrograph to do that.

RE: Mixing both SCS and Rational Method for design.

(OP)
Thank you all for your help.

RE: Mixing both SCS and Rational Method for design.

Don't get confused with the multiplication advice above. 24 hour intensity (in/hr) times 24(hr) gives you depth of rainfall (in ), not a volume of runoff. The runoff (also in in) will be less, and based on the CN of the watershed. You can still calculate that volume directly, by simple equations listed in TR-55, without using any routing software, but it will not give you a sense of hydrograph timing or peak discharge rates.

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.

Implementing a routed hydrograph analysis to size an emergency overflow spillway can result in disaster.

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.

That varies by region, LincolnPE. I've seen some regions where your discharge pipe has to be sized for 125% of the 100 year undetained flow. I've seen some where it just has to meet 100% of the undetained flow. I've seen 125% the routed flow. I've seen 100% the routed flow. That's presuming the 100 year emergency overflow is being directed into the primary discharge culvert downstream of the outlet control.

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.

the basin is sized for a 10-year retention volume. unless the 100-year volume is very small, the basin will fill up very quickly. so in that case sizing the emergency spillway to handle the peak flow would be a conservative design. accounting for storage and routing of hte 100-year storm in a 10-year basin would likely not result in any economic reduction in the size of the spillway. In other words, yes, you would start the routing at the spillway crest and all flow would go over the emergency spillway (assume all pipes are gated or clogged) and it would have to handle the peak inflow. other than that, there is no information provided by the OP that would suggest flow through a pipe spillway would be considered in the routing of the 100-year storm or to indicate that an emergency spillway is located on an embankment. personally, I would avoid a spillway over an embankment if at all possible.

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