How to show no increase in runoff

[jimtheengineer10]

I am working on a project and we need to either show there is no increase in stormwater runoff to the NYSDOT ditch or install a detention pond to control the runoff so it matches the predevelopment rates. All of my other projects have required a detention pond but I think this one may not as the existing soils have a decent perc rate and the projects sits far enough from the road that the runoff will have time to perc into the existing soils prior to reaching the ditch.

My questions is how do I determine this? My pre development runoff is .078 af with a 1.07 cfs flow. My post development runoff is .081 af with a 0.91 cfs flow. So my volume is slightly more but my flow rate is less. So the volume of runoff increases by .003 but my flow rate decreases by .15 cfs. Therefore, the project will not have an impact on the DOT ditch as the post development flow entering the ditch will actually be less than the pre development flow. Am I interpreting things correctly?

Thanks.

 

[psmart]

I'm not exactly sure if you're asking how to model this, how to compare pre/post, the effect on the ditch, or something else...

If the DOT is only concerned with peak flow, they will presumably accept your design. Of course, peak flow isn't the only factor that may have downstream effects. Since the volume is increased, there must be an increase in flow duration, with a corresponding increase in erosion potential. It all depends on the specific site issues and the DOT requirements.

Peter Smart
HydroCAD Software

http://www.hydrocad.net