Permeable Paving

Toby Hemenway hemenway at
Sun Aug 29 11:26:24 EDT 1999

Here's a somewhat different approach to permeable paving in parking lots
that Ben Haggard uses in New Mexico; it may be less effective in non-arid
regions like Tennessee, but I'm sure it's better than straight asphalt. He
attempts to lay out parking lots on contour if the engineer in charge will
permit it (rather than laser-leveling it-though that works too, it's
unnecessary). Then they pave the lot with concentional asphalt, leaving
about 25% of the parking area unpaved. The unpaved areas are laid out as
short, shallow swales between parking spaces, and planted with appropriate
vegetation (i.e. useful shrubs and herbs: insectaries, wildlife, N-fixers,
craft supplies, you know . . . . Trees, if any, must have leaves that won't
stain or gum up auto finish and shed them slowly over a long season so that
cars don't get blanketed, which annoys many drivers). The swales serve as
infiltration areas for runoff as well as cleaning up oil-polluted
rainwater. This design eliminates the need for storm drains in the parking
lot, saving money (though Ben told me that the engineer on one wouldn't
believe it, and put a drain in anyway. Rainwater rarely makes it to the
drain). The soils there are pretty porous, but their rain can be extremely
heavy, an inch or two in an hour sometimes. You might want to fill your
swales with lighter soil if you've got clay; I'm sure a perk test would
tell you how much unpaved area would absorb a major downpour. It's an
inexpensive way to recharge groundwater, provides greenery, and helps cool
off the parking lot.


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