Have you been to the Chisago County Courthouse lately? If so, you may have noticed the construction work that was going on late this summer as the employee parking lot was reconfigured and resurfaced.  Within this parking lot is a very exciting water quality practice that you might not even notice.  A portion of the parking lot, about the width of 2 parking stalls wide and the length of one row, was installed as pervious asphalt rather than typical asphalt.

Pervious asphalt doesn’t look much different on the surface except that there are pore spaces between the bits of asphalt. Vehicles can still drive and park on it just as they would on a normal asphalt surface.  The important workings and the biggest difference between pervious and regular asphalt is below the surface.  Under the pervious asphalt layer, a choker course of small rock transitions to a deep layer of porous rock material.  This allows water to be temporarily stored in the space between rocks during a large rain event.  There is an underdrain system that slowly allows water to leave the storage area and enter the nearby North Center Lake.

Pervious asphalt has two main water quality benefits: slowing water down by temporarily storing it, and filtering water through small rock to remove contaminants. The faster water runs, the more power it has to cause erosion and carry sediment into the lake bed.  By slowing water down, erosion is prevented.  This often requires a place to store a large amount of water for a short time, such as a holding pond or under a parking lot.  Between rains, the holding area dries out.  The water is also filtered as it travels through the asphalt and choker course layers.  Larger debris, such as leaves and sediment get trapped here rather than making it into the lake.

Pervious asphalt also has some other benefits. During a rain, the pervious asphalt area is dry because the water is able to quickly drain into the storage area.  No puddles to worry about when you walk to or from your vehicle.  In the winter, the pervious asphalt areas tend to clear of snow and ice faster than the conventional asphalt, making winter walking safer and requiring less use of salt, which can also travel into the lake via runoff water.

Check out the SWCD’s website (www.chisagoswcd.org) or Facebook page to see a video of the new Government Center pervious asphalt at work during a hard rain storm this past summer. For more information about pervious pavement, contact the SWCD at 651-674-2333.

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