Recently, I discussed the most economical method to resurface parking lots, driveways, and roads out of the three techniques previously mentioned. As indicated, permeable pavement is roughly the same as the pavement presently being used. Only the percentages of the ingredients change. Hence, the expense of using permeable asphalt is essentially the same as the asphalt typically used today.

This week, I will discuss two other methods to minimize stormwater runoff on parking lots and other paved surfaces. Specifically, block, plastic or rock forms, which are filled with permeable soil and seeded with grass.

As with using porous asphalt, these two methods require a layer of Type B stone approximately 10.5 inches in depth and a top layer of Type A stone approximately 1.5 inches deep. Instead of the suggested 2.5 inches of porous asphalt, the layers of stone are covered with interlocking form blocks. These in turn are filled with gravel or topsoil. If topsoil is used, the blocks are seeded with grass.

Geoblock is one company that makes the blocks used in these two systems. The blocks produced by this company are made from recycled plastic which may be considered an added plus. As written in the brochure, there are several advantages to using one of these block systems:

° The interlocking block system is fully engineered to provide long-term success. In some areas, this system has been used since the early 1980s.

° It is a permeable design with an 87 percent open surface, which allows rain to percolate into the ground.

° The interlocking system maximizes load distribution capabilities.

° The blocks protect the crowns of the seeded grass (if using the second technique) and consequently reduces rutting and prevents soil compaction. Compact soil does not allow water to filter through it and consequently limits the amount of water entering the soil beneath.

° The blocks are easy to install.

° The blocks can be easily placed around obstructions such as trees

As one might gather, these two techniques are extremely similar and both are more expensive to install. Each site being considered for paving has different characteristics, and the best technique to use out of the three covered will depend on these site characteristics. In some cases, a combination of all three methods may be in order.

Regarding cost overrides, one can expect these two methods to run up to a maximum of 40 percent over the cost of traditional asphalt. However, there are advantages, especially when considering maintenance. Grass does need to be cut, but bare topped blocks can delineate parking spaces and hence cut down on repainting lines during the off seasons. Again, particular site characteristics will best determine what system or combination of systems to use.

Regardless as to what system one uses, stormwater runoff will be minimized if water can easily pass through the top layers of pavement and be temporarily stored in the gravel layer allowing the water to enter shallow groundwater.

Again, using environmentally sound practices in future and present paving projects make sense for a variety of reasons. Certainly, there are many communities that are beginning to question how long our present water supplies will last, especially with the continued growth of urban areas. Groundwater is an obvious source but groundwater has to be replenished. Rainwater replenishes that supply provided the ground surfaces are permeable.

If you would like more detailed information, I would suggest that you contact Bill Hunt in Raleigh, N.C., at 919.515.6751.

(Jamie Johnston, executive director of the Little Tennessee Watershed Association, wrote this article. He can be reached at LTWA, 5 West Main Street, Franklin, NC, 28734; or via e-mail at nbumppo@dnet.net.)

 

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