As development increases pervious surfaces are decreased. This removal of vegetation and natural infiltration points creates more runoff and increases the likelihood of flooding. Historically, large amounts of rainwater were stored in detention systems. These large storage systems wouldn't be necessary if we allowed a portion of that rainwater to infiltrate rather than flow away from the surfaces we construct. Decreasing run off is especially important in the isthmus area as city storm sewers flow directly into our lakes carrying pollutants such as phosphorous and sediment into the water. Residential properties throughout Madison can impact our lakes water quality as if they are all waterfront properties. One of the best management practices we use to mitigate run off is permeable paving.
Permeable paving doesn't just reduce flooding, it also increases groundwater recharge by allowing what falls in a given area to recharge locally rather than directing the water to another area. The further rainwater is directed, the greater the risk of a back up or failure of the storm water system. We measure runoff as a C Value or runoff coefficient. Asphalt, for example, has a C value of .85 meaning that 85% of the water that falls on it runs off. A typical lawn has a C value of .15, a permeable patio has a C value of 0!
The design and construction of permeable patios requires specific tools and experience beyond what is required for a typical patio or brick driveway. Be sure your contractor is well versed in the materials and engineering required to take on a permeable paving project.
For more information read Unilock’s Directions in Sustainable Design. You can also take it a step further and collect the captured water as in the video below.