Stormwater filtering practice existing grade slope restriction?

[jtcrus31]

I have a proposed 5 lot subdivision in NY and it is a difficult site that needs stormwater approvals/permits from multiple agencies. The terrain is steep, which makes meeting water quality and runoff reduction requirements difficult. I have to follow the NYSDEC stormwater manual. I proposed cisterns for rooftop runoff which met the minimum runoff reduction volume. I proposed a stormwater filtering practice to meet the water quality volume. This filtering practice had to be placed over an area with existing slopes up to 22%; therefore, I regraded by proposing mostly fill to level the terrain and place my filter.

The NYCDEP reviewed my SWPPP and stated that the filtering practice can not be placed over existing slopes in excess of 6% regardless if we are regrading. I have been trying to argue this.

My opinion is:

[SIZE=11pt]A filtering practice is not a function of the existing slope. A filtering practice requires imported sand or organic media to be placed below an area designed to pond water. The ponded water provides head to push the water through the filter. Once the water is pushed through the filter it could then be designed to be conveyed through underdrains to discharge elsewhere. There is no practical need to have a slope restriction on the existing grade. [/SIZE]

The NYCDEP is quoting a planning matrix in Chapter 7 of the NYSDEC design manual; however, the matrix does not state the 6% slope requirement is for existing slopes.

Does any one have any thoughts on this? Am I missing something? Is there a practical reason for an existing grade slope restriction?

Any answers are much appreciated.

 

[NJmike PE]

so is this filter area that you are creating an inpoundment, likened to a detention/retention basin?

 

[jtcrus31]

I will have to impound about 4' of water for head. Their slope requirement doesn't differentiate between cut or fill. I also had to propose a detention basin adjacent to it for mitigating peak flows. They were not concerned about that minor embankment. I would like to load a picture, but can't find a way to.

 

[NJmike PE]

host the picture at

http://tinypic.com/?t=postupload

 

[jtcrus31]

http://i59.tinypic.com/2lsk13d.jpg

 

[NJmike PE]

is it possible that their concern is with creation of a dam? It would appear on the 22% slope, that you are filling and creating a dam condition. that being said, 6% seams a little excessive of a condition. I've never done anything NY so I'm not familiar with their regs, but here in NJ I don't recall a max slope condition. One method that we typically use is a straight forward sand bottom surface infiltration basin. It provides your WQ. We've not been required to design any pretreatment areas as you have here.

 

[jtcrus31]

I really appreciate your response and feedback. 6% seems very excessive to me. We are allowed to place an infiltration system on up to 15% (existing slopes) in NY. I believe they are incorrectly interpreting the NYSDEC regs.

In regard to the dam condition, we could propose a pond liner and a beefed up embankment detail since this is a relatively small practice.

Or if they are that concerned with creating a dam, I could regrade it so it is only a cut.

Thanks again.

 

[NJmike PE]

good luck. stick around the site. its a great resource pre and post exam.

 

[jtcrus31]

Will do. I was obsessively on this site waiting for the results, but kind of dropped off once I passed. Thanks again.

 

[John QPE]

Can you use snouts in your storm drain network to meet the required water quality? Lose that sand filter all together.

 

[NJmike PE]

how is that an effective WQ measure? Does it actually filter the water before it is discharged?

 

[John QPE]

Snouts are very effective in removing floatables, oils, and nutrients. I use them all the time, in conjunction with other practices.

What are the actual removal requirements?

I've found these to be very effective when putting them on the outfalls of basins

 

[jtcrus31]

We are in a very regulated area (NYC reservoir watershed) and this is new construction. There are specific practices allowed for meeting water quality requirements.

 

[bradlelf]

Have you thought about a regenerative Step pool conveyance system? These are meant for very steep terrain.

Here is a video of one that I designed with CWP: https://www.youtube.com/watch?v=PmmDJ3XG3SQ

Design manuals are here with removal rates:

http://www.dep.wv.gov/WWE/Programs/stormwater/MS4/Documents/Specification_4.2.7_Regenerative_Stormwater_Conveyance_WV-SW-Manual-11-2012.pdf

http://www.aacounty.org/DPW/Watershed/SPSCdesignguidelinesDec2012rev5.pdf

They are commonly used in EV/HQ watershed and Critical Areas of the Chesapeake Bay.

 

[NJmike PE]

I kind of like this idea. I may employ it in the future. is it an approved method in the state of NJ?

 

[bradlelf]

I have never used it in NJ, but I have in PA and MD.

They work really great actually; MD has been using them for years for coastal outfalls.

This is one I saw a few years ago near where I grew up: https://www.youtube.com/watch?v=xB2vb9Er5i4

A newer one during a rain storm: https://www.youtube.com/watch?v=vQP6BYErzeM

 

[NJmike PE]

I am very intrigued by this. If you have any additional information, feel free to pass it on.

 

[humner]

I really appreciate your response and feedback. 6% seems very excessive to me. We are allowed to place an infiltration system on up to 15% (existing slopes) in NY. I believe they are incorrectly interpreting the NYSDEC regs.

In regard to the dam condition, we could propose a pond liner and a beefed up embankment detail since this is a relatively small practice.

Or if they are that concerned with creating a dam, I could regrade it so it is only a cut.

Thanks again.

I have found the NYCDEP not interpreting things correctly. I still do not have an approved septic installation, after almost 4 years.

 

[bradlelf]

Yeah, I would call BS on NYCDEP. Most filtration practices are historically based upon sewage regulations. Again every state is different; but i would find it hard to believe that NYCDEP would not allow a SWM filtering practice on a slope greater than 6% but they would allow a sanitary sand mound on the same.

I have designed/permitted/constructed sand filters, rain gardens, retentive grading, etc on slopes much greater than 6%. You would think that the DEP would be more worried about the filter material/method than the underlying slope because you could always keep the BMP into the existing grade with a excavated flat bottom.

Just my 2 cents.

 

[humner]

Yeah, I would call BS on NYCDEP. Most filtration practices are historically based upon sewage regulations. Again every state is different; but i would find it hard to believe that NYCDEP would not allow a SWM filtering practice on a slope greater than 6% but they would allow a sanitary sand mound on the same.

I have designed/permitted/constructed sand filters, rain gardens, retentive grading, etc on slopes much greater than 6%. You would think that the DEP would be more worried about the filter material/method than the underlying slope because you could always keep the BMP into the existing grade with a excavated flat bottom.

Just my 2 cents.

I work for the NYCDEP, worked with those people he is dealing with. Proved them wrong while they were inspecting / reviewing my septic system. End result, I still have no DEP approval for my septic system and was transferred to another department. Not an easy group to deal with.