NYS SWPPP Question

[Apexpredetor]

Sorry for the duplicate post here... I was recommended to move it to this forum... Thanks in advance for any info and opinion.

I have been asked by a family member to help them with, what appears to be, a fairly simple SWPPP/SEQR permit.

He/She is expanding upon an existing Mobile Home Community. The expanded area is on slope, but will be terraced to properly build pads for the manufactured homes. there will only be about 30 pads over a 14 acre area. Gravel driveways, narrow roads, lots of green space left, with all trees left as best we can.

Here is my question:

Due to the terracing of the slope the post development runoff is less than that of the pre development. I am reading the NYS Stormwater Manual, but I can't figure out if I still need to account for the RRv, channel protection, or extreme flood conditions?

I actually signed up for a NYS Stormwater CEU class but it got cancelled.

Thanks all

 

[peter246]

Yes, you still need to provide RRv and Water Quality treatment.

You have to provide channel protection unless:
1) you reduce the entire Cpv volume through green infrastructure or infiltration systems; or
2) the site discharges directly tidal waters or fifth order (fifth downstream) or larger streams.

If you can demonstrate that the 10- and 100-year post-development flows are less than pre-development (or the site discharges directly tidal waters or fifth order (fifth downstream) or larger streams), you do not have to provide attenuation of those peak flows.

However, if you're clearing and grading an area, creating impervious roads, roofs and driveways and/or collecting any stormwater in catchbasins and culverts, you generally increase the post-development peak flows. You might have to back up the assertion with calculations.

 

[cvg]

I can't imagine you could ever get a grading permit without submitting a drainage report, which would generally include meeting the SWPPP water quality requirements as well. You should ask the development services department these questions, they will be approving the plans.

 

[Apexpredetor]

No development services department, just a planning board, that will use another local engineer to review the documents for approvals and comment.

I used TR-55 NRCS program to model the pre and post development conditions. It is a 14ac site, but only about 1.65 acres of impervious area (including gravel parking spots, single lane one-way drives, and trailer roofs). Most of the area will remain untouched, and worked on in multiple pregressive steps as not to disturbe more area than need be.

 

[Apexpredetor]

peter...

If I claim that the grading/terracing along with vegetative barriers to runoff, along with check dams in road culverts (if we even call them culverts, they will just be small depression along road between lots) could be used as green infrastructure, correct?

Thanks again for the guidance.

 

[beej67]

You could if you design them properly.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[peter246]

correct. The Design Manual states the design parameters.

 

[peter246]

but you still might not be able to accomplish the Runoff Reduction just by doing that.