by Don Green, LEED AP Biologist
Undoubtedly one of the most major impacts of urban built environment is the increase of impervious surfaces such as roof tops, sidewalks and roads. While the imperious surfaces provide ease of access and dry places to live and work, there is an impact to what happens with rainfall that can lead to trouble.
A natural undeveloped site usually allows approximately 10% of a rain event to runoff, 50% to infiltrate into the ground, and as much as 40% to evaporate from the trees and other vegetation. But as development increases, the landscape has less vegetation. As impervious surfaces increase the amount of runoff from a storm event could be higher than 50%.
The built urban environment also has the potential to have more pollutants on the surface. These pollutants come from oil and gasoline spilled from vehicles and service stations. It comes from fertilizer and pesticides used on lawns. All of these chemicals get picked up by rain water runoff and it is all carried directly to our water ways. The fact that water is trapped and runs off these paved surfaces into smaller areas creates an increase in volume of water trying to be absorbed into smaller parcels of land. This also is a problem.
The increase of amount and speed of stormwater running into our streams and rivers result in the erosion of our stream and river banks and the corresponding widening of the bank. This cuts down the stream bed increasing the load of sediment which destroys wildlife habitat.
Increased runoff also has a detrimental effect on ground water levels which starves streams of water during low flow conditions. In other words, when the banks widen and sediment increases streams that never used to go dry can become dry creeks during times when there is not a lot of rain, but are subject to flooding when there is rain.
According to research from the Center for Watershed Protections, when the impervious cover of a watershed approximates 10%, aquatic resources are impacted and when the imperviousness reaches 25%, the streams are so impacted that they cannot support aquatic life.
What can we do?
Decreasing the hard surfaces in the city can significantly address urban pressure. Installing pervious pavement such as pervious concrete, permeable pavers, pervious asphalt and xeripave (pictured above) can go along ways in addressing these problems.
- Pervious Concrete is a highly porous concrete mix that has little or no fines creating substantial voids on the surface allowing a majority of the water hitting it, and running onto it, to infiltrate into gravel base and the ground. It is used in parking areas, pedestrian walkways and light traffic areas. It is often used in driveways.
- Permeable pavers are interlocking concrete or stone units with open spaces between the units that allow rain water to infiltrate into the base and groundwater. The blocks have small gravel separating them filtering and capturing most of the pollutants that are carried with the stormwater. Pavers are easy to maintain. They also can have a curb appeal of the architectural appearance and coordinating colors.
- Xeripavers are a porous pavement system made of aggregate natural stones with an inert polymer binder. The pavers can be used in place of or with traditional paving materials such as concrete, bricks or asphalt. Like other porous surfaces they can be installed on sidewalks, patios, pool decks, courtyards, and surrounding tree.
Most of these applications have a variable base of stone beneath them for water storage and pollutant removal. When considering using a pervious application you should consider what you will do underneath them to allow the water to flow down and then disperse into the ground. You also should consider the surrounding area to understand what and how much sediment is going to be carried across the pavers or concrete. Sediment can clog the porous areas of these pavements. Installing them away from disturbed areas or trees is advisable.
But when installed on your property there is no question that you will decrease the percentage of runoff in the watershed, allowing groundwater to replenish, and treating the pollutants that the stormwater carries. Many of these pervious applications can be put in light use areas, walking and cycling paths, areas of parking and light traffic, and pedestrian walk ways and many have non-slip ADA compliant surfaces.
As any stormwater control measures, continuous inspection and regular maintenance and sweeping of these applications can be important to continue their high level of function.
Photo credit: Site Supply Inc.