Environmentally Critical Areas | Environment & Sustainability
of the adopted surface water design manual to minimize the risk of landslide or erosion within the no-disturbance area and minimize the risk of water quality impacts to any sensitive water body located downstream of the no-disturbance area; and d) Structural modification of, addition to or replacement of legally created single detached residences and improvements in existence before January 1, 2006, that do not increase the existing total footprint of the residence and associated impervious surface by more than 400 square feet over that existing before January 1, 2006, shall be exempt from the provisions of this subsection. d. Development Standards for Properties Draining to the No-Disturbance Area. The following development standards shall be applied, in addition to all applicable requirements of this chapter, to development proposals located within the erosion hazards near sensitive water body overlay that drain to a no-disturbance area: i. New proposed subdivisions, short subdivisions, public institutions, commercial site development permits, and binding site plans for sites that drain predeveloped runoff to the no-disturbance zone shall evaluate the suitability of on-site soils for infiltration. All runoff from newly constructed impervious surfaces shall be retained on site unless this requirement precludes a proposed subdivision or short subdivision from achieving 75 percent of the maximum net density as identified in SDC 21.04.030.
When 75 percent of the maximum net density cannot be met, the applicant shall retain runoff on site and a perforated tightline (per the adopted surface water design manual and SDC 21.03.050, Surface Water Management) shall be used to connect each lot to the central drainage system. The following drainage systems shall be evaluated, using the following sequential measures, which appear in order of preference: a) Infiltration of all site runoff shall be required in granular soils as defined in the adopted surface water design manual and SDC 21.03.050, Surface Water Management; b) Infiltration of downspouts shall be required in granular soils and in soil conditions defined as allowable in the KCSWDM when feasible to fit the required trench lengths on site. All flows not going to an individual infiltration system shall be detained on site using the most restrictive flow control standard; and c) When infiltration of downspouts is not feasible, the applicant shall design a drainage system that will detain flows on site using the applicable flow control standard and shall install an outlet from the drainage system designed using the best available science techniques to limit the risk of landslide or erosion to the no-disturbance area; provided, that in no case shall development proposals generating more than 2,000 square feet of impervious surface create point
Effective | January 1, 2022 Errata Revisions | November 17, 2022
Title 21: Sammamish Development Code | 83
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