07/25/2007 Env Bd Packet - Laserfiche WebLink

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• event (Paragraph 3(b)(ii)(b)). For sites with Hydrologic Soil Group (HSG) soil types A and B, the emphasis will be on infiltration. For sites with HSG soil types C and D, biofiltration and detention will be permitted to a greater extent. A property owner would be required to present an explicit analysis of site design practices that will limit impervious cover, preserve natural areas and retain the natural infiltration capacity of a property (Paragraph 3(b)(ii)(a)). If site volume retention and water quality criteria cannot be met because of unsuitable soils, high groundwater or circumstances that make infiltration undesirable (e.g., site contamination), a property owner then should seek to utilize biofiltration or filtration methods. Only after those methods have been explored may a property owner resort to detention in a natural or artificial basin as a means of meeting water quality criteria (Paragraph 3(b)(ii)(a), (e)). The existing requirement for use of a detention basin would be deleted. A prominent addition to the stormwater rule would be a separate set of volume control and water quality criteria for linear projects, such as roads and trails, built by road authorities and other public bodies (Paragraph 3(b)(ii)(f)). Because these projects very frequently involve constrained right -of -way in developed areas, infiltration may be challenging. Further, roadways and similar linear surfaces require periodic resurfacing or reconstruction and are subject to minor alterations. Applying the standard criteria to these activities in many cases would impose an expense on communities disproportionate to the work being undertaken and could create incentives for units of government to defer ordinary infrastructure maintenance. Accordingly, the proposed rule would subject a linear project to the standard criterion - infiltrating or retaining the 2.8 -inch storm -- if the project would create an acre or more of new impervious surface. However, if the project would not meet this threshold, if it involves simply conversion to urban section (curb and gutter), or if it involves expanding or reconstructing an existing roadway /trail, facilities need be sized only to capture the 0.8 -inch rain event. Again, mill and overlay without an increase in surface area would not be subject to the rule. Proposed site design practices and Best Management Practices will need to conform to standard reference documents. However, the proposed rule also would incorporate certain specific design criteria. First, pretreatment will be required before discharge to infiltration (Paragraph 3(c)(i)). Pretreatment lengthens the performance period of an infiltration practice and reduces its maintenance frequency. Numerous studies have observed that infiltration practices are susceptible to soil clogging and can fail due to the absence of pretreatment. 8