Saturday, October 17, 2009

Where Does the Rain Go?

Water is an essential component of human life and an important resource for the natural environment. It is also a primary concern when designing and constructing new facilities or renovating old structures. The whole point of a building facade is to keep rain and other precipitation from compromising the interior structure and finishes. Water also has to be dealt with on a site-wide basis, as large puddles are not only undesireable but can be harmful to the foundation and hamper access to the site itself. For these reasons, we have a history of designing buildings and sites to take the water "away". Roof gutter systems lead to asphalt parking lots which slope appropriately to gather all water falling on the site and funnel it neatly to a storm drain, which connects to the municipal sewage system, which connects to a local stream or river and ultimately the ocean. This system of stormwater management, when applied on such a large scale, can have dramatic effects on local ecosystems and requires extremely expensive utility systems. The two main issues with storm water control are quality and quantity. The quality of stormwater runoff, after it has gathered dirt, oil and other inorganic compounds from a parking lot or other paved surface, can be toxic to local plant and animal life and can compromise whole ecosystems if released straight into local water sources. The quantity of stormwater runoff is equally important because large and sudden influxes of stormwater from large nonpervious surfaces causes erosion and other damage to local watersheds.

Paved surfaces are the largest source of stormwater runoff, and several systems have been developed that deal with both stormwater quality and quantity simultaneously. Pervious concrete can be used as a replacement for asphalt in most commercial and residential paving applications. Pervious concrete minimizes the amount of fine aggregate (i.e sand) in the mix, which allows void content of 15-20% to develop. Water can easily pass through these voids and permeate the soil underneath. This minimizes runoff to municipal sewage systems and allows the water to be filtered naturally. The photo below illustrates a demonstration pervious concrete pour at the Breakell, Inc. office on Patterson Avenue in Roanoke, VA. To prepare for the pour several inches of gravel are laid on top of leveled and compacted soil. The concrete is then poured similar to normal applications with the exception of steel reinforcing.

Pervious paving systems offer an alternative to pervious concrete for residential and commercial applications. They offer the same benefits for stormwater quality and quantity while offering a different look for unique applications. These systems allow for plant material to be grown in the spaces between pavers to create a "green parking lot". Below is an example of a pervious paver system being installed at Breakell, Inc. The space between the pavers will be packed with engineered topsoil and will eventually sprout drought resistant grasses.

In conclusion, pervious paving systems are an exciting innovation in contemporary pavement technology. By employing the types of systems discussed above, we can improve the quality of runoff from our parking surfaces while decreasing the quantity of runoff. If adopted on a wide scale, these systems can have a dramatic effect on municipal water supplies and drastically improve local ecosystems. Pervious paving and pourous concrete systems can contribute to LEED credit for heat island reduction, site selection, stormwater management, and recycled content.


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