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SENT BY:DPRA St.Paul, MN 4-12-39 3:35AM 612 227,.5522 612 407 4191412 tVIOUS SURFACE COI Imperviousness also has a role in design related to mitigation of polluted runoff. "Best management practices" (BMPs) is the most commonly -used term to describe the wide range of an -sire options available cc manage stormwater runoff. BMPS are often divided inw two major types those involving structures such as stormwater detention ponds or infiltration wenches, and "nonstructural" practices that usually involve use of vegetated areas to buffer, direct, and otherwise break up the sea of asphalt, Maintenance measures like road sand sweeping and storm drain cleaning are also included. It is not within the scope of this article to give a thorough discussion of these practices; choosing the correct assemblage is a combination of tit and science, and involves many considerations. From the stand- point of imperviousness, however, BMPs can be viewed in terms of how well they replicate the natural hydrological functioning of the site. This perspective puts a premium on restoring infiltration, which has been suggested by Ferguson (1994) and others to be highly preferable to surface detention. Emphasizing infiltration and nonscrucrural solu- tions often comes into conflict with established devel- opment practices. Curbing is a goad example. Just as Southworch and Ben -Joseph (1995) found the over, engineering of road widths to be ingrained in local practice, our experience has been that to many town engineers, the necessity of curbing is a given. Safety and structural integrity of the road arc often given as reasons for curbing, above and beyond its drainage function. Highway engineers in our state, however, have told as that the sole purpose of curbing is to di- rect stormwater, and even their, is is only truly needed during the unstable construction phase (Connecticut Department of Transportation 1995). In many cases, :^ more pervious alternarives to directing runoff should be investigated- Grassy swales, for insamcc, might be constructed in the margin created when existing right- of-way widths are retained while road widths are re- r. dueed- .; Mitigating the impacts of polluted runoff in the "ultra -urban" inner city environment is a particularly thorny issue; Regional approaches like the Olympia ISRS may target these areas for increased impervious •cover (City of Olympia 1994a). Growth policies that encourage urban "infilling" may,resulc in higher inner- ; dry imperviousness in order co reduce sprawl and hl overall imperviousness, region -wide. In. effect, this is "clustering' on a regional scale. l :;,•::; Nonetheless, even, for these seemingly intractable 1 ,.areas, usingimperviousness as a planning framework " ,,i'ean be useful., Usually, this involves linking the reduc- rion of impervious surfaces to complementary urban initiatives. Parking is one example. Excess parking can be attacked from many angles ocher than water qual- ity, including air quality, traffic congestion, promotion of sprawl, and inefficient use of building lots. A park- ing reduction initiative could be combined with a plan to use the recouped paved area either for active starmwater treatment (infiltration basins, detention ponds) or for mare modest stormwater management (vegetated strips). Such a strategy could be combined with the creation of `vest packet" parks and other green spaces, shown by urban forestry research as hav- ing positive sociological and psychological effects on city dwellers (Gobster 1992; Schroeder and Lewis 1992). Research an the pollutant -processing capability of various types of vegetation suggests a slight twist on parking loc design that may reap large benefits in wa- ter quality for urban areas. Parking lots often incorpo- rate landscaped areas, usually in raised beds surrounded by asphalt curbing, However, these vege- rated areas can be planted below the level of the park- ing surface, serving as infiltration and treatment areas for runoff (Bitter and Bowers 1994) (figure 6). This idea can be extended to other areas where vegetated "islands" are traditionally used, such as in the middle of cul-de-sac circles, Another consideration for urbanized areas is per- vious alternatives m pavement. This includes various mixes of asphalt with larger pore spaces (e.g., "pop- corn" mix), and alternative systems such as open - framework concrete pavers filled with sand or gravel, or turf reinforced with plastic rings. These systems can become clogged with sediment, particularly during construction, but are often a suitable alternative in low traffic areas like emergency roads, driveways, and overflow parking areas. Cahill (1994) asserts that, con- trary to common belief, pervious pavement can be used successfully in many places if certain siting, con- struction, and maintenance practices are followed; for instance, he recommends vacuum cleaning at least twice per year. Granular surfacings are being pro- mored by some landscape archirecrs as attractive, inex- pensive, and more aesthetically -pleasing alternatives to paved pathways and trails (Sorvig 1995). One last important note about reducing impervi- ousness through planning and design —it can save money. Savings to both the private and public sectors in reduced construction and infrastructure costs can be considerable. For instance, a recent study done far the Delaware Estuary Program compared the impacts on twelve communities in the watershed, over a 25- year horizon, of a continuation ofcurrent "sprawl" de- velopmenc patterns versus the Program -recommended pattern of promoting mixed uses, open space, and APAJOURNAL•SPRINQ 1996 253