Protecting Watersheds with Permeable Paving

As urban and suburban areas grow to accommodate larger populations, pavement follows. As the amount of impervious paved surfaces increase, watersheds become more vulnerable to pollutants.

When rains fall on a conventionally paved impervious surface, the precipitation is generally directed down storm drains, which can result in flooding and overwhelming the storm water management system.

Urban flood events damage both the interior and exterior of buildings. Water travels up sewage pipes and seep through walls to cause significant damage to both the building ant its infrastructure. As storm water makes its its way to storm water management systems or bodies of water, it picks up pollutants such as petroleum based chemicals, pesticides, fertilizers, and all kinds of inorganic particulate that significantly pollute watersheds, aquifers, and aquatic habitats. Urban flooding events carry a heavy financial toll in the form of costs for repair of public and private infrastructure and buildings, increased insurance rates, lower property values, and a decrease of local businesses unable to recover after significant flooding damage. And with the climate change, heavy precipitation events are becoming more and more frequent and intense.

In some areas of our country, (our headquarters town of Cleveland, Ohio included) storm sewers combine with waste water sewer systems and during storm events, the combined systems overflow. When combined sewer overflows occur, sewage can be pushed into fresh water bodies instead of arriving at the treatment plant. As with many environmental concerns resulting from buildings, roadways and developed land, the US Environmental Protection Agency has regulations controlling storm sewers. It is important for design teams to know what regulations are in place regarding stormwater so that they can advise owners and developers of the life-cycle cost of storm water management.

So what can be done do to reduce the impact of impervious paved surfaces? Perhaps one of the best and most cost effective solutions is the installation of permeable surfaces. Permeable surfaces help protect watersheds and stressors on municipal storm water systems by reducing storm water runoff at the source. Permeable pavement options dramatically reduce runoff by allowing water to infiltrate the surface while allowing the earth and stone to act as a filter to remove harmful pollutants from the water. A simple strategy is to lay a stone bed 18 to 36 inches deep (deep enough for water to not to collect on top of the pervious surface) and then percolate through the soil. This provides a groundwater recharge, similar to how water behaves in natural systems. Depending on the type of permeable surface, a large portion of the stormwater is effectually diverted from sewer systems and bodies of water.

Choosing a pervious surface comes with considerable financial incentives. For example, pervious pavement options can reduce costs in large construction projects that require a large amount of paved surface area. Because permeable pavements effectively manage stormwater, they reduce the need for retention ponds and larger storm sewers. Also, pervious pavement may decrease costs in locations where the local government imposes stormwater fees. And if you are in a cold weather climate, you will use less salt and plow less because snow and ice melt quicker on pervious pavement.

You may find yourself overwhelmed by the variety of permeable pavement options. Here’s a quick primer to help you become familiar with a few of the most widely utilized pavement options.

Pervious concrete: Pervious concrete is simply a special type of concrete with high porosity because of the ratio of large to small aggregates. Porous concrete can retain 99 to 100 percent of stormwater volume initially, though clogging can significantly reduce this percentage. Pervious concrete is more prone to damage from freezing/thawing than porous asphalt. See this pervious concrete demonstration during which 1,500 gallons of water was poured at the UNH Stormwater Center without any runoff.

Porous asphalt: Porous asphalt has been in use for over forty years. Like pervious concrete, porous asphalt has a high porosity because of the ration of large to small aggregates. Of the three permeable surface options, this material has the lowest recorded stormwater retention ability, retaining from about 25 percent to 100 percent of stormwater volume. Porous asphalt is a little coarser than conventional asphalt, but it so closely resembles regular asphalt that the average person standing on it wouldn’t notice any difference. With proper installation and maintenance, porous asphalt can last for up to twenty years. On a commercial scale, porous asphalt requires cleaning with regenerative air or vacuum sweepers. One drawback of porous asphalt is that it is weaker than regular asphalt, so it is not recommended for use in high volume areas. Of the three listed options, porous asphalt is the least expensive to install.

Permeable Interlocking concrete pavers: Permeable interlocking concrete pavement (PICP) is create with concrete jointed units that connect while leaving spaces between units. Porous pavers have been recorded to retain 34 percent to 100 percent of stormwater volume. PICP is best suited for walkways, driveways, parking lots and roads where speeds do not regularly exceed 35 mph.

The type of permeable surface you choose is highly dependent on your site selection, project size, and stormwater management needs. Because permeable pavements reduce urban flooding and increase the health of aquatic habitats, permeable pavements can help earn points towards various green certifications. Many third party certifications such as LEED, NGBS, Enterprise Green Communities, INVEST and SITES include credits for implementing permeable pavement (each rating system has its own specific requirements and performance metrics) and they do so in recognition of the environmental benefits of storm water mitigation.

In some areas of the country, financial incentives exist for implementing stormwater management systems. For example, to combat the increasing number of combined sewer overflow events, some sewer districts NEORSD in Northeast Ohio has a program to offset the storm water fees assessed to all commercial and residential customers (fees are determined based on the square feet of impervious pavement within a property boundary).

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