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7.1 Permeable <br /> 7.1.1 Credits <br /> 7.1.1.1 Porous Asphalt or Concrete <br /> Description of public road or public parking lot Model Surface as <br /> (I)Base material laid above surrounding grade: <br /> (a)Without underlying perforated drain pipes to collect Landscaped area over underlying <br /> stormwater soil type(till or outwash) • <br /> (b)With underlying perforated drain pipes for stormwater collection: <br /> at or below bottom of base layer Impervious surface <br /> elevated within the base course Impervious surface <br /> (2)Base material laid partially or completely below surrounding grade: <br /> (a)Without underlying perforated drain pipes Option I:Landscaped area over <br /> underlying soil type <br /> Option 2:Impervious surface <br /> routed to an infiltration basin' <br /> (b)With underlying perforated drain pipes: <br /> at or below bottom of base layer Impervious surface <br /> elevated within the base course' Model as impervious surface <br /> routed to an infiltration basin' <br /> Description of private facilities(driveways,parking lots,walks,patios) <br /> I. Base material below grade without underlying perforated drain 50%landscaped area on <br /> pipes underlying soil:50%impervious <br /> 2. Base material below grade with underlying perforated drain pipes Impervious surface <br /> 7.1.1.2 Grid/lattice Systems (Non-concrete) and Paving Blocks <br /> Description of public road or public parking lot Model Surface as <br /> (I) Base material laid above surrounding grade <br /> (a)Without underlying perforated drain pipes Grid/lattice systems: <br /> landscaped area on underlying <br /> soil(till or outwash).Paving <br /> Blocks:50%landscaped area <br /> on underlying soil: <br /> 50%impervious. <br /> (b)With underlying perforated drain pipes Impervious surface <br /> 'See Section 7.8 for detailed instructions concerning how to represent the base material below grade as <br /> an infiltration basin in the Western Washington Hydrology Model. <br /> 2 If the perforated pipes function is to distribute runoff directly below the wearing surface,and the pipes <br /> are above the surrounding grade,follow the directions for 2a above. <br /> 142 o LID Technical Guidance Manual for Puget Sound <br />