Environment & Sustainability | Surface Water Management (Title 13)
requirements set by the director until such time as the community hazard is alleviated. b. Single-family development located in landslide hazard drainage areas shall be limited to a maximum of 35 percent impervious surface on each lot until such time as the director has determined that infrastructure is on- line and operational to mitigate the risk to downslope hazards. c. Single-family development located in areas that drain to Pine or Beaver Lake shall be subject to minimum yard areas and maximum impervious surfaces in accordance with SDC 21.04.030.C. d. Low impact development techniques shall be used to the maximum extent feasible for all critical drainage areas. e. Development proposed within a critical drainage area shall not qualify for exemption or exception from core requirements in the Surface Water Design Manual except for director approved exemptions and exceptions for Core Requirement Nos. 1, 3, and 8 as provided below: i. The director may approve an exemption or exception request, prepared by a Washington State licensed professional engineer using sound engineering principles and judgment, from tightline requirements in Core Requirement No. 1 (Discharge at the Natural Location) which can demonstrate the following condition:
a) Discharge is to an acceptable location where surface and groundwater will not damage downslope properties, will not increase the risk of landslides or severe erosion on such properties, will not discharge to streams of gradient 15 percent or more, and will not be located upgradient of a septic drain field or its reserve field as measured within a 100-foot radius around the discharge location. Examples of acceptable discharge locations include, but are not limited to: ditch and culvert systems that meet capacities in accordance with Core Requirement No. 4 (Conveyance System) for full build-out conditions and/or locations where the cumulative effects of discharges will not increase the risk of landslides or severe erosion and is supported by a geotechnical engineering report prepared by a Washington State professional licensed geotechnical engineer and peer reviewed by the City engineer or his/her designee. b) For projects proposing a net increase in impervious surface and located in critical drainage areas in the Tamarack or Inglewood Historic Plats, no exemption to tightline requirements shall be approved. c) Where a tightline system is proposed, discharge shall be to a City owned, operated, and maintained system or where the City has or is granted an easement for the inspection, maintenance, repair, and replacement of the
Revisions | June 29, 2022 Effective | January 1, 2022
146 | Title 21: Sammamish Development Code
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