§ 24.15.170. Stormwater facilities on existing lots—New.

New stormwater facilities and swales proposed to store, treat and/or convey stormwater for single-family residential development on existing lots may be constructed within geologic hazard areas and associated buffers consistent with the Drainage Design and Erosion Control Manual, as amended (Chapter 15.05 TCC) under the following conditions:

A.

No Alternative. The applicant shall demonstrate that there is no alternative for accommodating stormwater with less impact to the hazard area and/or buffer due to topography or other physical constraint. The facilities shall be designed and located to minimize impacts on the geologic hazard area and associated buffer.

B.

Geologic Hazard Areas and Associated Buffers. If there is no alternative with less impact, stormwater facilities shall be constructed as follows:

1.

Surface drainage down the face of the slope must be avoided and must be consistent with the Drainage Design and Erosion Control Manual for Thurston County 15.05 TCC, as amended. If drainage must be discharged, it shall be collected upland of the top of the slope and conveyed by tight line drain through a high-density polyethylene pipe with fuse-welded joints, or similar product that is technically equal or superior. The pipe shall be located on the surface of the ground and properly anchored so that it will continue to function in the event of an underlying slide or active erosion conditions. The stormwater shall be treated prior to release by a method that meets clean water standards and poses the least risk of destabilizing the slope (e.g. a compost filter).

2.

If drainage must be discharged to a water body below the landslide or erosion hazard area, the conveyance system described in paragraph (1) above shall include an energy dissipating device at the edge of the water body and must comply with 15.05 TCC.

3.

Surface drainage not discharged per paragraphs (1) or (2) above, must be directed away from the slope and collected in a tight line or other approved method for discharge to an acceptable natural drainage. The tight line drain should terminate at the drainage course rather than at a point upslope within the natural drainage course to avoid erosion or destabilization.

4.

A maintenance and monitoring plan shall be developed for approved tight line drainages.

5.

If tight lining is not possible, stormwater retention and detention systems, such as dry wells and infiltration systems (including those utilizing buried pipe, French drains, or swales), within a landslide or erosion hazard area or associated buffers shall require approval of a Reasonable Use Exception, in compliance with Chapter 24.45 TCC and 15.05 TCC. Any such system receiving approval of a Reasonable Use Exception shall be designed by a licensed civil engineer and shall include a geological assessment indicating that such a system will not affect the stability of the slope. A monitoring plan shall be required through one wet season, at a minimum, for a proposed infiltration system and the results included in the geotechnical assessment.

6.

Proposals in geologic hazard areas and associated buffers located within jurisdiction of the Shoreline Management Act shall comply with the Shoreline Master Program for the Thurston Region, as amended.

(Ord. No. 14773, § 3(Att. B), 7-24-2012)