Guest post by Ryan F. Swanson, University architect:
I hope everyone has enjoyed the permeable brick pavers installed this summer alongside Carroll Weathers Drive. By next summer, these paver walks will extend from the current location near Worrell, past Farrell Hall, the new Dining Hall and the two new Residence Halls. These pavers look great and add a sense of beauty that is not possible with plain concrete.
Yet, many may be surprised to know that the primary reason for using permeable pavers was not completely aesthetic – but rather to meet city storm water mitigation requirements for both water quantity and quality.
Rain falling in an undeveloped area causes natural runoff and ground absorption. In a developed area, the hard surface “footprint” of buildings, parking lots, sidewalks, driveways and roads prevent water from soaking naturally into the ground. This excess water creates a greater than natural surge downstream that increases stream bed erosion, flooding and property damage. Cities and counties now require owners to install storm water management mechanisms, such as pavers, that keep more rainwater on site and return downstream flows to more naturally occurring conditions.
There are other examples of storm water mitigation in our recent campus projects.
During construction of the Byrum Welcome Center, two large concrete cisterns were installed under the parking lot to receive, collect and then dissipate storm water at a slower rate . These cisterns, capable of handling 110,000 gallons, are used in conjunction with three bioswales – the hollowed out earth areas near the entrance drive – to slow water flow into Lake Katherine and Silas Creek. At the new South Residence Hall, two bio-swales were constructed that collect rain water at the surface in a 5’ deep crushed rock bed. We also have several small in-ground mini-cisterns with a rock filtering system connected to the storm water system under Jasper Memory Lane. Bioswales are excellent water mitigation devices but they take up a significant amount of land area – which is at a premium in the core of our campus.
As part of the North Campus construction project, there was an opportunity to move away from large cisterns and bioswales to more cost effective storm water strategies, such as the permeable pavers. If you look closely, these pavers all have notches on their sides which result in 3/8” gaps between pavers. These gaps allow rainwater to filter through the field of bricks and be absorbed by the 18-inch crushed rock bed beneath the pavers. This bed of crushed rock allows the pavers to remain relatively stable while at the same time serving as a rainwater collection device. Over time, the rainwater held under the pavers will naturally seep into the ground.
In a tight space where a large amount of sidewalk is programed, permeable pavers are much more cost efficient than underground cisterns, allowing water to naturally seep into the ground feeding nearby trees and grass – and as most would agree – they look great at the same time.