Thank you for inviting me to speak to you today on an exciting old technology – Porous Asphalt Pavements
Porous asphalt pavements with stone reservoirs have been around since the 70’s. Recently there has been an increase in the use of these pavements since they can address many of the goals of new stormwater regulations. With proper design and the right site conditions they can reduce stormwater runoff. Both peak flows and total volume of flow. They are one of the most effective solutions for reducing pollution from storm water flowing off pavements. Since they allow much of the stormwater to infiltrate through the soils below the pavement, the promote groundwater recharge. They may also reduce or eliminate the area needed for retention/detention ponds.
Here is a cross section of a porous pavements. The key components of this system starting at the top is an open-graded asphalt pavement. This pavement has interconnected voids that allow stormwater to flow through the surface into the heart of the system – the stone reservoir. This reservoir is made up of large singles size crushed stone which will typically have about 40% voids to temporarily store the stormwater. A permeable geotextile is typically used between the stone reservoir and subgrade to prevent mingling of the stoner reservoir and soil. We typically do not want to compact the subgrade to keep the infiltration has high as possible.
So why is this important to the bay? Here are some quotes taken directly from the Chesapeake Bay Foundations website. Click: Basically what these first two quotes are saying is that everything the drips and drops on the land can be carried by stormwater into the bay. Click: This includes oil and gas that may leak from our cars in addition to other pollutants. Click: With impervious surfaces large quantities of water may enter steams if left unabated which can erode stream beds and carry sediment to the bay where it can impact aquatic life.
Natural soils can be a great water treatment system due to the many microbes that will break down many of the pollutants. This graph of reduction in pollutants with different stormwater management techniques. Infiltration such as porous pavements is the best except for total nitrogen which is dissolved in water.
The water quality treatment generally excellent. Exceeds EPA’s recommended level of removal of total suspended solids Meets regional ambient water quality criteria for petroleum hydrocarbons and zinc. 0 to 25 percent of the salt routinely applied to impervious pavement
The porous asphalt system’s ability to manage runoff was exceptional. It has outperformed all systems tested at NHSC.
Porous Asphalt Pavements with Stone Reservoirs for Stormwater Management
Kent Hansen, P.E. National Asphalt Pavement Association
Why? Stormwater carries a host of contaminants from the land into the water Roadways, for example, release oil and grease, tailpipe emissions, and other toxics from motor vehicles. As impervious surfaces channel large quantities of rainwater into streams at high velocity, the runoff wreaks havoc. The flow scours stream banks, destabilizes stream contours and alters depths.