§ 20.32.070. Stormwater facilities.

Stormwater flow control and treatment facilities may be allowed as open space subject to the provisions of this chapter and the criteria contained in Section 20.32.070 TCC. In the event of a stormwater system failure subsequent to final plat approval, the department may authorize, with the consent of the affected property owner(s) and consistent with Section 18.04.060, necessary stormwater facilities in designed open space. Also, the department may authorize new or expanded stormwater facilities in open space tracts to accommodate stormwater generated by a federal, state or county project if there is no other reasonable alternative.

The design of such facilities shall minimize adverse impacts on the function (e.g., tree preservation, play area) and quality of the open space. If the pond must have steep side slopes (greater than 3H:1V) to provide needed capacity, thorny vegetation or fencing shall be used to restrict access (per Thurston County Drainage Design and Erosion Control Manual, Volume V, Appendix V-E, Site Design Elements). Thorny or other impenetrable vegetation shall be at least a ten-foot wide strip that will provide a barrier to entry of small children or others who may be incapable of climbing out of the facility. Vegetation type must be approved by the jurisdiction and must be in place within one growing season of facility excavation.

Thorny or other impenetrable vegetation is preferred, rather than fencing unless the approval authority determines that it would not adequately protect public safety.

Prior to authorizing use of established open space to accommodate stormwater facilities in a manner that would significantly diminish its function or aesthetic quality, all viable alternatives must be explored and be found to be impractical or cost prohibitive (taking into account initial construction cost and long term maintenance cost). Prior to authorizing use of such open space, the approval authority must determine that, overall, the use of the open space to accommodate stormwater facilities is beneficial to the affected neighborhood or the general public. Not more than twenty-five percent of designated open space outside of critical area buffers may be used for stormwater facilities, except that areas used for dispersion will not count toward the twenty-five percent limit.

The approval authority may allow the following types of stormwater facilities, if designed to allow for either passive or active recreation, to be considered for open space use:

• Bioretention facilities and rain gardens

• Constructed treatment wetlands

• Wetponds

• Sheet flow and concentrated flow dispersion areas

• Retention ponds

• Detention ponds

• Bioswales and filter strips

Active recreation design of stormwater facilities includes facilities that are designed to provide play areas (including, but not limited to, a soccer field, volley ball court) that are usable during portions of the year.

Passive recreation design of stormwater facilities includes facilities, where wildlife habitat has been planned (e.g., nesting areas provided) or water features provided and reasonable passive viewing facilities are provided (e.g., trails, picnic facilities).

(Ord. 11398 § 3 (part), 1997: Ord. 9859 § 12, 1991: Ord. 8034 § 17 (part), 1985)

(Ord. No. 15355, 1(Att. A, § PP), 10-18-2016)