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Better Site Design Lots. The amount of quantity controL needed is therefore reduced. Thus, overaLL storm water management requirements are minimized and smaLLer quantity controls can be used. Table 10.2: Perceived Impedimei'i~ to Parrking lot SfcorrmwaJfcerr Mali'iagem~mfc ..Perceptiorn 1. Many of the storm water management techniques are relatively new and their long term performance uncertain. 2. The cost to provide onsite stormwat~r management may be more expensive than providing offsite management at one regional facility. 3. Maintenance requirements may be burdensome for lot owners. 4. The modifications to curbing around bioretention facilities, open channeLs, sand filters, and filter strips may cause the pavement to fail. CHALLENGE: Facts, Case Sfcucllies, aJ[I1}QJ (~aJ[[euiJ91e$ FACT: FACT: CHALLENGE: FACT: CHALLENGE: FACT: FACT: CHALLENGE: A recent study by Brown and Schueler (1997) found only 3 performance studies for perimeter sand filters and 4 for dry swales. However, preliminary monitoring and results from the limited number of monitoring studies suggest that these BMPs can significantly reduce stormwater pollutants (See Table 10.3). If not properly maintained, infiltration trenches can have failure rates as high as 50% (Galli, 1993). Bioretention facilities are relatively untested. Brown and Schueler (1997) identified only one performance study for bioretention facilities, but also found some studies on biofilters and surface sand filters, which have pollutant removal capabilities similar to on-site BMPs. The use of bioretention facilities and other on-site stormwater management facilities can significantly reduce the need for storm sewers, thus reducing stormwater infrastructure costs. Filter strips, bioretention facilities, and dry swales may be placed in dead space areas such as setbacks and traffic islands, minimizing impacts to usable (i.e., buildable) land. Sand filters are expensive, generally on the order of $10,000 - $50,000 per impervious acre. This cost may be offset by the costs for land acquisition, construction of the storm drain conveyance, and cor:struction for a large offsite facility. Bioretention areas can easily be maintained by commercial landscapers, but will require regular maintenance. Maintenance and physical plant workers may require special training to ensure that open channels, sand filters, and filter strips are properly maintained. Potential failure at the interface may be avoided through the use of a low-rising concrete lip. Curbing can be used as long as curb cuts or some similar device are provided to allow parking lot runoff to enter bioretention areas or sand filters. Care should be taken to ensure that runoff is conveyed away from the pavement. St3nding water and water beneath the pavement may caus~ the pavement to faiL Steps that can be taken to avoid pavement failure include providing a gravel subgrade and requiring geotechnical testing. - - 88 -