§ 24.25.320. Stormwater facilities—New.

New and expanded stormwater facilities (e.g. detention, retention, treatment and conveyance facilities) shall not be constructed within priority prairie habitat. New stormwater facilities may be constructed within other important habitat areas and associated buffers consistent with the Drainage Design and Erosion Control Manual for Thurston County, as amended (Chapter 15.05 TCC) and the Clean Water Act, under the following conditions:

A.

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

B.

Accommodation of Stormwater Generated Within the Habitat Area. New roads and other development located adjacent to important habitat areas shall be designed and located so any associated stormwater facilities are located outside of the important habitat area. The approval authority may require that the proposed development be redesigned or reduced in scale to avoid or minimize impacts to the important habitat area.

Any new retention, detention, or treatment facilities shall be designed and sized to only accommodate stormwater generated from impervious surfaces (e.g., roads and bridges) within or immediately adjacent to the important habitat area or new impervious surfaces approved consistent with this chapter.

C.

Size Limits and Location. Use of riparian habitat areas, marine riparian habitat area or pond or lake buffers for stormwater retention, detention or treatment shall be limited to the twenty-five percent of the riparian habitat area, marine riparian habitat area, or buffer furthest from the water body, unless another location is necessary to accommodate stormwater from a road or bridge. In no case shall the size of the facility exceed twenty-five percent of the habitat area/buffer on the parcel(s) under development. The stormwater facility shall not cause an increase in water temperature or degradation of water quantity and quality of fish-bearing streams.

D.

Roadside Stormwater Conveyance Facilities. Roadside stormwater conveyance swales and ditches may be extended through important habitats within rights-of-way. When possible, they shall be located along the side of the road furthest from the habitat area. If the conveyance facility must be located along the side of the road closest to the important habitat area, it shall be located as close to the road/sidewalk as possible, consistent with public safety.

E.

Open and Vegetated. Stormwater detention, retention, and treatment ponds in important habitat areas shall be open and, to the extent possible, vegetated with native plants present in the area. Invasive vegetation shall not be planted. Stormwater conveyance facilities shall be open and vegetated with non invasive plants unless the approval authority determines, in consultation with the applicant's qualified engineer, that design constraints or protection of public safety warrant burying the conveyance facility (e.g., underground storage is needed or the facility would span a steep slope and must be "tight lined" to avoid slope failure - see Chapter 24.15 TCC, Geologic Hazards). Vegetation shall be maintained and, if necessary, planted adjacent to all open swales, channels, and ponds in order to retard erosion, filter sediments, pollutants, and (if warranted to maintain water temperatures necessary to sustain aquatic life) shade the water, consistent with the Drainage Design and Erosion Control Manual for Thurston County, as amended (Chapter 15.05 TCC), and the Clean Water Act.

F.

Avoid Channelization.

1.

With the exception of conveyance facilities extending through the important habitat area and/or associated buffer, stormwater shall be dispersed as sheet flow at the outer edge of the important habitat area to avoid channelization and allow filtration of sediment, nutrients, and pollutants and infiltration of water. The approval authority may require, if slopes exceed five percent, that obstructions or devices (e.g., perforated pipe) be installed to maintain sheet flow within the important habitat area and associated buffer.

2.

When an outfall to a stream, pond, or marine waters is necessary, it shall be designed to mitigate any adverse impacts to aquatic life. This may include, if warranted, a conveyance system and outfall structure that simulates natural conditions and provides habitat features necessary for fish feeding, cover and reproduction. If stabilization of an outfall along a Type S or F stream or marine shoreline is necessary, bioengineering techniques shall be used to the maximum extent practical, consistent with state and federal regulations (See TCC 24.25.300).

G.

Treatment. All stormwater from stormwater systems shall be treated prior to release to a water body consistent with the Clean Water Act and Chapter 15.05, TCC.

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