Zoning and Pervious Pavement

19 03 2010

This month, the topic of APA’s Planning Advisory Service’s column, “You Asked, We Answered”, is how zoning codes across the country are handling pervious pavement for parking and sidewalks. The links to the various zoning codes were very useful. The following are excerpts from some of the more interesting ones.


“Porous paving blocks and pervious paving materials are permitted and encouraged as material for parking lots. The use of grass as a parking lot surface is permitted for overflow and intermittent parking. Pervious paving systems are required for parking spaces which exceed the maximum number of spaces required by subsection 7-11-2(c). The use of grass or other vegetation as a parking surface is permitted only for parking spaces which are provided in excess of the maximum number of parking spaces required by subsection 7-11-2(c) or used for intermittent or overflow parking. Parking lots associated with arenas, sporting facilities, amphitheaters, fairgrounds, and religious institutions may, however, use grass or other vegetation for the entire parking lot.”


Impervious Surface Replacement. Existing properties exceeding the standards for impervious surface coverage present a distinct management challenge from that of newly developed properties and there is a need to establish clear and consistent guidelines for how re-development of these lots may occur.

1. The applicant removes existing impervious surfaces at a ratio of one and one-half (1.5) square feet removed for every one (1) square foot added and restores these areas to a permeable surface…

…a. Permeable pavement systems are encouraged in the management of sites currently over the impervious surface limit and shall be credited as twenty-five (25) percent pervious for these sites when installed according to the requirements of

Section V.L.4.a.(2.)(d.)iii. Applicants are encouraged to replace existing impervious surfaces with natural vegetation at the 1.5 to 1 ratio listed above, however, permeable pavement systems may also be used. In these cases they are to replace existing impervious surfaces at a ratio of at least four (4) square feet converted for every one (1) square foot of new impervious surface being added;

2. The applicant removes existing impervious surfaces at a 1:1 ratio and restores those areas to a permeable surface and in addition, submits a comprehensive stormwater management plan that emphasized infiltration and onsite retention of stormwater for at least the two year 24-hour storm event through a combination of methods including buffer strips, swales, rainwater gardens, permeable pavement systems and other low impact development methods. The stormwater management plan must be designed by a registered engineer or landscape architect and installed as designed by a qualified professional.

a. Permeable pavement systems may be considered as 100% pervious when submitted as part of a stormwater management plan consistent with this section…


If construction techniques such as pervious pavement, block and concrete modular pavers, and grid pavers are used for off-street parking surfaces, each space provided as a result may serve in lieu of two (2) required off-street parking spaces, up to a maximum of 10% of the number of required spaces…

…Paving and drainage. All land which is placed in use for off-street parking and all driveways serving parking, delivery, and loading areas, shall be paved with asphalt, concrete, or other approved all-weather hard surface, including construction techniques such as pervious pavement; block, concrete, and similar modular pavers, and grid pavers; and shall be drained with materials and in a manner which meets the current minimum standards and specifications for parking areas adopted by the Board.

-Brian Phelps

Pervious Concrete and Solar Reflectance

10 02 2010

Pervious Concrete A=20% Fly Ash Mix B=50% Slag Mix
Center of the photo is the stalite base of the concrete

Our office was invited to tour the Tennessee Concrete Association’s (TCA) new headquarters in Nashville. The focus of the tour was to learn more about their experience with pervious concrete. They had a few examples of pervious concrete pours on site, and I thought the one in the photograph above was particularly interesting. TCA is experimenting with mixes to increase the Solar Reflectance Index (SRI) value of pervious concrete. Mix A in the left side of the photograph is a pervious concrete mix that replaces 20% of the required cementitious content with fly ash. Mix B to the right has replaced 50% of the required cementitious content with slag. Both substitutes are by-products of industrial process. Fly ash is from the process of burning coal for energy and slag is from the production of both iron and steel.

As you can see there is a noticeable difference in the color between the two samples. The slag sample is much lighter in color. We were told it was difficult to determine the SRI of pervious concrete due to the voids. As a result, TCA could not definitively tell us the SRI value. They are interested in finding someone to perform more testing.

In regard to cost, TCA stated the slag is readily available in Tennessee but not all producers carry it. Fly Ash is the less expensive option compared with cement and slag, but slag is typically less expensive than cement (especially white cement)

It does appear the slag mix  provides a great option to improve solar reflectance. I hope more data will be available in the near future that supports this.

-Brian Phelps

EPA to test porous pavement and raingarden benefits

11 11 2009

1700 Charlotte
1700 Charlotte in Nashville
Combines Porous Concrete & Bioswales

Traditional asphalt parking lots may seem to be the most cost efficient, but underlying costs such as increased pollution and water load on our sewer systems need to be considered as well. In an attempt to measure those underlying costs the EPA has replaced nearly 43,000 SF of their traditional asphalt parking with 3 different types of permeable pavement systems and several raingardens with different planted vegetation. At their Edison, NJ facility they will conduct a decade long study to evaluate and document the performances of these permeable systems on the basis of removing pollutants and filtering capabilities. Having these systems all in the same location will likely result in more balanced testing of each material.

This study comes at an ideal time as many cities are beginning to re-evaluate old paving methods in order to reduce the load on existing sewer systems or just to reduce the amount of toxin runoff from paved surfaces to our nearby rivers and lakes. Traditional asphalt parking lots collect oil, grease and other debris over time, after a heavy rain or snowstorm these toxins are washed from the parking surface to the nearest storm drain or permeable surface. Replacing this impervious surface with a permeable pavement or raingarden will allow plants and soils to naturally filter the pollutants, while re-charging the ground water table.

Porous for Blog
Porous Concrete

-Will Marth

Sustainable Site Strategies: Rosemary Beach, Florida

2 11 2009

Rosemary Beach-050
Rosemary Beach-099

I recently returned from a relaxing week long family vacation in Rosemary Beach, Florida, located on the panhandle just west of Panama City Beach. For those of you not familiar, this is a traditional neighborhood development established in 1995 complete with a Town Center that includes a post office, retail and restaurant space, a hotel, neighborhood parks and more. Many people choose to spend their vacation here for obvious reasons: sun, sand, surf and swimming pools but let’s review what visitors may not notice during their stay.

The following is from the RB website…

“Natural foliage creeps into the pervious pavement streets to slow what little motorized traffic finds its way into town, and keeps drivers alert – a conscious effort to create a safe environment for foot and bicycle traffic. Go green and get pampered at Rosemary Beach!”…

As stated, the streets are constructed of pervious concrete, which allows stormwater from frequent rain showers to penetrate through the pavement and directly into the sandy soil base rather than sheet drain to the typical/conventional system of numerous curb and gutters, inlets and concrete pipes. This isn’t to say there aren’t any storm sewer pipes as part of the infrastructure, just less because of an alternative stormwater solution. The average person visiting Rosemary may only notice that the driving surface is not asphalt and is more ‘bumpy’ that typical concrete.

In addition to pervious concrete, the use of native vegetation is another sustainable site strategy. Using native vegetation requires less water and maintenance while supporting bio-diversity of local wildlife species. Sidewalks, neighborhood boardwalk paths and parks are lined with plants that seem to have been there prior to development and give the impression that the buildings were somehow built around them. Live Oaks create a dramatic tunnel effect over the sidewalks while providing habitat for wildlife such as birds and squirrels. Lizards appear to love hanging out under the low growing palm shrubs and other native plant material. The careful planning of plant material size, location and scale of streetscape planting also aids in the slowing of motorist traffic by psychological affect.

These are only two simple sustainable site strategies/design elements that may go unnoticed by the typical visitor but could serve as an example of how to develop in the future. By the way, I did have fun and relax with my family and didn’t spend they whole week analyzing better ways to develop.

Brian Hudson