Pervious concrete and lot coverage

[dandy_warhol]

A developer is currently before the ZBA requesting a varianace for lot coverage. Maximum coverage is 70%. Developer is proposing 85%. Existing has ~92% (combined for 3 lots).

The developer has received a lot of feedback that the proposed 85% will not be approved. The developer is now suggesting using pervious concrete on a portion of the lot to help with stormwater management.

The developer is now proposing (requesting?) a 30 - 50% credit towards the lot coverage ratio due to the proposed pervious concrete.

The city has limited experience with the application of pervious concrete. We've been doing research into it and recognize that there are some issues with its application in the Northeast.

Our code defines lot coverage as any paved surface. There is no mention of pervious v. impervious and credits for pervious.

Thoughts on how to coach the ZBA to respond to his proposed credit?

[Cardinal]

The first thought I had is to ask whether the lot meets requirements for greenspace and landscaping with im/pervious surfaces covering 85% of it. The second thought is to ask the question you already did. Is the pervious concrete going to function in your climate. I have not seen any definitive answer to the question. Minnesota is doing quite a bit of research but has not produced results yet. Wisconsin has researched the topic and does not recommend use of pervious concrete. Then again, this is the Wisconsin DOT. I would not expect them to embrace anything new.

[NHPlanner]

Given the NH Climate, whenever pervious pavement has been proposed here, we require "redundant" conventional drainage systems to be installed as well. We simply do not think pervious pavement will function as intended in winter conditions.

IMHO, NY isn't too different, and I'd hesitate to support a request like you have described.

[ColoGI]

Given the NH Climate, whenever pervious pavement has been proposed here, we require "redundant" conventional drainage systems to be installed as well. We simply do not think pervious pavement will function as intended in winter conditions.

IMHO, NY isn't too different, and I'd hesitate to support a request like you have described.

Our climate has periods of warm weather in winter (shhhh) allowing pervious, but we have a number of municipalities whose PW folks still require the settling ponds and the pervious is seen as a buffer. That credit is way too high at 30% and may be doubled what he'd settle for. If he can produce something that will back his numbers, I might give him 10% as it is too early for your place to have experience with this offer. Better to err on the side of caution.

[Tide]

Have you involved your city/county engineers to get their take on it. Though pervious the actual perc rate is not anywhere near 100%. I would be interested to hear an engineers take on this.

[cdub]

I'm not an engineer, but we've been looking a lot into pervious systems for a green infrastructure master plan we're developing for our Metro Water dept.

In terms of pervious in colder climates, I've heard that Chicago did a study related to their green alleys program of pervious pavements (either concrete or pavers). In winter conditions, pervious pavement actually raised the temperature of the pavement by allowing air to circulate in the sub-base, which allowed any precipitation to work it's way down and not freeze at the surface. I'm trying to find a link to back this up and may need to contact a product rep to find the info and get back to ya on that. The problem with pervious systems in colder climates comes from either poor design/ installation and/or poor maintenance that allows water to stay in the pervious concrete layer.

Additionally, the percolation rate of most pervious systems can take about anything you throw at it. According to PerviousConcrete.org, the perc rate appears to be 3.5 gal/sq.ft./ min. and would need a storm intensity of 340 in/ hr to be a factor. A video of paver installation shows that system can handle over 500 in/ hr. If we hit storm events of that magnitude, we have bigger problems than worrying about the pavement system Of course, this is also dependent on subgrade conditions (ie. sub-soil infiltration rate, drainage system, etc.)

I don't have any experience with the primary issue of this being considered open space since it's pervious, but would think it's not what you're after based on your ordinance coverage.

[southsideamy]

We require redundant drainage systems in my fair city (suburban Chicago) because #1: our soil conditions are clay, clay, clay and #2: the stormwater regulatory agency (Metropolitan Water Reclamation District) does not give any credits for porous pavement. Our engineers are still on the fence about it -- there's some evidence that the pores in the pavement can actually plug over time. I don't have any studies to prove that up, though (and they haven't given me any so that I can cross-check their current stance).

I'm anxious for more studies to be done and more time to pass so that we can see how these porous pavements are holding up. We encourage it on all projects from a water quality standpoint -- and we always tell people that if it is ever recognized from the storm water perspective, their site will be that much more valuable.

[cdub]

Check with some of your engineers on the Morton Arboretum's recent study of their pervious paver parking area. They 'dissected' the parking lot to see how the lower gravel bases have faired over the years that it's been in. I think the study was done in May. I've been trying to find a resource for that info online and have yet to stumble across it.

The product rep who mentioned some of these Chicago figures (I'll check this week to see if data is available online) attended and they found that in the case of the paver system only the top gravel area in the joints accrued the sediment and it didn't travel farther into the system. Routine maintenance (3-4 times per year with a sweeper truck and once with a vacuum truck) helps reduce or eliminate the clogging at the paver level.

[CRTAPT]

I was a part of the group providing the workshop at Morton Arboretum and also was part of the company that constructed this and many other projects across the United States and would offer that this demonstration has provided evidence that particulate travel is confined to the upper 2" of the joint and void aggregates.
There are many perceptions about pervious and porous pavements that do not apply to permeable pavements.I have listened to the same concerns from various parts of the US, Canada and other parts of the world regarding these perceptions and belive that PICP offers a simple solution to these concerns especially when coupled with LID to provide a treatment train that will satisfy most redundant requirements. Most projects we build are on clay soils and are working as designed. We now have conclusive evidence as a result of the forensic work performed at Morton Arboretum that these fine particles will not reduce the effectiveness of void ratio storage in the sub-base aggregates as percieved. In addition, when areas are clogged, the sediment and aggregates may be removed and replaced reinstating infiltration rates without removing the pavement surface.
As Dr. Hunt has stated, whther PICP's work or not, that train has left the station. The only question remaining is when will you get on board.

[luckless pedestrian]

also remember, the impervious cover % requirement isn't just for storm water, it is also for aesthetic reasons - requiring some non-built upon areas allows for landscaping and streetscape design, a visual break maybe -

in office parks it allows for picnic areas or walking trails for workers, in residential areas it prevents someone from paving their whole lot (which in most suburbs, that's not something that is done) -

so if someone is seeking a variance, I would definitely throw that up to say that yeah, it's great he doesn't need to put in a retention/detention basin but the place is going to look like hell...

[transguy]

They are looking for a variance right? Go to your City procedures (which of course will be in accord with State statutes) for what factors must be present in order for a variance to be granted. The discussion thus far seems more appropriate for the City to review their requirements, not to grant a variance.