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i x <br /> • <br /> Runoff Model Representation <br /> In WWHM 3, enter sidewalk area as lawn/landscaped area over the underlying soil type. <br /> For WWHM 2012, see Appendix III-C:Washington State Department of Ecology Low <br /> Impact Development Flow Modeling Guidance (p.587). <br /> BMP T5.19: Minimal Excavation Foundations <br /> Purpose and Definition <br /> Low impact foundations are defined as those techniques that do not disturb, or minimally <br /> disturb the natural soil profile within the footprint of the structure.This preserves most of <br /> the hydrologic properties of the native soil. Pin foundations are an example of a minimal <br /> excavation foundation. <br /> Applications and Limitations <br /> To minimize soil compaction, heavy equipment cannot be used within or immediately <br /> surrounding the building. Terracing of the foundation area may be accomplished by <br /> tracked, blading equipment not exceeding 650 psf. <br /> Runoff Model Representation <br /> . Where residential roof runoff is dispersed on the up gradient side of a structure in <br /> accordance with the design criteria and guidelines in BMP T5.10B: Downspout Dis- <br /> persion Systems(p.905),the tributary roof area may be modeled as pasture on the <br /> native soil. <br /> . Where "step forming" is used on a slope,the square footage of roof that can be <br /> modeled as pasture must be reduced to account for lost soils. In "step forming,"the <br /> building area is terraced in cuts of limited depth.This results in a series of level <br /> plateaus on which to erect the form boards.The following equation (suggested by <br /> Rick Gagliano of Pin Foundations, Inc.)can be used to reduce the roof area that <br /> can be modeled as pasture. <br /> Al ddp5) x AZ = A2 <br /> Al =roof area draining to up gradient side of structure <br /> dC = depth of cuts into the soil profile <br /> dP =permeable depth of soil (The A horizon plus an additional few inches of the B <br /> horizon where roots permeate into ample pore space of soil). <br /> A2 =roof area that can be modeled as pasture on the native soil <br /> • If roof runoff is dispersed down gradient of the structure in accordance with the <br /> 2014 Stormwater Management Manual for Western Washington <br /> Volume V-Chapter 5-Page 938 <br />