Stormwater filtering practice existing grade slope restriction?

[OnionSkin]

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 tried to argue this.

My opinion is:

A filtering practice is not a function of the existing slope if we are not proposing to infiltrate into in-situ soils after being filtered. 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.

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. I did further research and came across the EPA's website and they state the following "Sand filters can be used on sites with up to about 6% slopes." I can not imagine that existing slopes need to be limited to 6%!!

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.

 

[ACtrafficengr]

Some of the water could infiltrate into the hillside, then exfiltrate out the slope and cause erosion or slope stability problems. We had an issue like that due to stone diaphragm filters above a roadway embankment.

 

[racookpe1978]

Over time, the filter will NOT be maintained, NOT be cleaned nor replaced, and should be designed to "operate" as a solid mass of "decaying gunk".

Yeah. I'm a cynic. And that's being optimistic. It might be stolen, then it will simply be an empty pipe or box.

 

[brandonbw]

So what do you do if the entire site is 7% or greater? You must have other options.

B+W Engineering and Design | Los Angeles Civil Engineer and Structural Engineer

http://bwengr.com

 

[OnionSkin]

I may have not have worded my question well. There is always other options, but I do not understand why they would believe that 6% EXISTING GRADE is a limit. Why couldn't one cut a steep site level and then place a filtering practice. It makes no practical sense to me. Also, as racookpe1978 stated, it may not be maintained, which I believe is true for most practices. However, the possibility of not maintaining a practice should not be grounds to deny it.

The review agency will not allow any filtering practices (bioretention, surface sand filter, underground, etc) over an existing grade of 6%. Regardless if we are cutting the slope back or filling to be less than 6%. Why? Any practical reason I am missing?