In a recent column, I discussed what stormwater runoff is and how it negatively effects the water quality of our streams, rivers, ponds, lakes and even oceans. As I indicated in the previous article, stormwater runoff is most visible and is the greatest environmental hazard in urban and developed areas due to the amount of paved surfaces, including roads, driveways and parking lots.

There are alternatives to the present, conventional methods of paving ground surfaces that would lead to environmental and economic benefits to urban communities.

Presently, there are three approaches which a community can take to minimize stormwater runoff and its negative effects on water quality.

Communities can apply one - or a combination of all three - new techniques described here and in future articles when building roads, parking lots and even driveways.

The simplest system is installing what is referred to as semipermeable asphalt (porous asphalt), block, plastic or rock forms filled with a gravel type of material, or block, plastic or rock forms filled with permeable soil and seeded to grass. The differences between the systems, other than the obvious construction material used, are the cost of installing the product.

Porous asphalt, the first technique mentioned, is perhaps the most economical from the perspective that the material used to create it is the same as in conventional asphalt. Only the composition of the ingredients differs. Hence, dollar for dollar, the cost of installing either the conventional type of pavement and the porous asphalt is practically identical. For all intents and purposes, porous asphalt looks and acts like conventional asphalt pavement except that it happens to be permeable. The percentage of tar is reduced and the smaller particles found in conventional asphalt are left out. The tar provides bonding of the large particles, which is one ingredient found in conventional asphalt. Using only the larger particles in the mixture allows for water to seep through the pavement into the ground.
One might ask how effective is this method in comparison with conventional pavements?

In laboratory tests, porous asphalt was able to pass “rainfall” over 60 inches per hour. This is certainly more rain than anyone would expect at any time. The rates of water passing through the material may vary, as the underlying soil must be able to absorb the water as well. The more water the soil can absorb, the more water can filter through the porous asphalt.

Obviously, questions about this technique are going to be asked.

How strong is this porous material as the composition of the material used to create it is different? The porous pavement has been found to be extremely strong. A test site in England happens to be a major airport runway. The porous asphalt has withstood the abuse of hundreds of planes landing on it.

As the porous pavement allows for the movement of water into the underlying soil, what about freezing and thawing?

The key to this is that water moves through the material and is not retained within the layer of asphalt. Hence, there has been no evidence that problems associated with freezing and thawing have occurred. One of the earliest demonstration sites in the United States using porous asphalt is at Walden Pond.

Walden Pond is located near Boston, Massachusetts, which has more freeze-thaw cycles during a typical winter than most regions of the world. The porous asphalt was put down in 1977 on a busy visitor’s parking lot. Twenty years later, which is a long time for even conventional pavement, the parking lot still looks good and works well. Certainly, if freezing and thawing were to be problems with this technique, some evidence would be observable.

Can local paving companies make this porous asphalt?

This is perhaps the toughest question. Theoretically, a local asphalt company can produce this porous asphalt, as the material is the same as that found in conventional pavements. In practice, local companies may actually have to “play” with the recipe to obtain the proper composition. This may cost a bit more initially. However, with practice, producing this porous pavement would be no more expensive than the conventional material. A possible benefit to the asphalt plant owners who take on the job of producing this material may not be realized for 10 to 15 years. With the growing awareness of what stormwater runoff is and how it negatively affects water quality, government agencies may require, by law, that any new or replaced pavement on streets, parking lots, and even driveways must be surfaced with permeable pavement. Asphalt plants, which have produced the material for years, would have a distinct advantage over their competition as they will already have the expertise in producing material and can produce it more cheaply.

Next week, I will cover the remaining techniques briefly mentioned in this installment. Again, all three methods work in reducing stormwater runoff. The remaining techniques may be, on the surface, more expensive than porous asphalt. However, all three methods have their distinct advantages.

(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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