§ 24.30.310. Wetlands—Stormwater facilities—New and expanded stormwater facility.

New and expanded stormwater facilities (e.g., detention, retention, treatment, and conveyance facilities) may only be allowed in the outer twenty-five percent of Category III and IV wetland buffers, or in wetlands meeting the criteria of TCC 24.30.090(C) if all of the following are met:

A.

The facilities are consistent with the Drainage Design and Erosion Control Manual for Thurston County, as amended (Chapter 15.05 TCC).

B.

The facilities are designed and sized to only to accommodate stormwater from:

1.

Development allowed in the buffer pursuant to this chapter; and/or

2.

Existing or new impervious surfaces within or adjacent to the buffer when there is no available alternative outside of the buffer for accommodating stormwater due to topographic or other physical constraints.

C.

Design and Location.

1.

The facilities shall be designed and located to minimize impacts on the wetland or buffer; and

2.

The approval authority may require that the proposed development be redesigned or reduced in scale to avoid or minimize impacts to the wetland or buffer; and

3.

No other location is feasible; and

4.

The location of such facilities will not degrade the functions of the wetland and buffer; and

5.

Stormwater facilities shall be limited to the twenty-five percent of the standard buffer furthest from the wetland, unless another location is necessary to accommodate stormwater from a road or bridge.

6.

Stormwater facilities shall not be allowed in portions of the buffer that have been reduced in width pursuant to TCC 24.30.050.

7.

Portions of buffers expanded pursuant to TCC 24.30.055 shall not be used to accommodate stormwater facilities.

D.

Treatment. All stormwater from stormwater facilities, with the exception of conveyance facilities extending through the buffer, shall be treated prior to release to a wetland buffer, consistent with the Clean Water Act, the Drainage Design and Erosion Control Manual for Thurston County, as amended (Chapter 15.05 TCC), and other applicable state and federal standards pertaining to water quality and treatment of stormwater. Direct stormwater outfalls to wetlands are prohibited.

E.

Avoid Channelization. Stormwater flows released to wetland buffers, with the exception of conveyance facilities extending through the buffer, shall be dispersed as sheet flow at the outer edge of the buffer 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 be installed outside of the buffer to maintain sheet flow within the buffer.

F.

Open and Vegetated. Stormwater detention, retention, and treatment ponds in wetland buffers shall be open and, to the extent possible, vegetated with native plants. 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). Vegetation shall be maintained and, if necessary, planted adjacent to all open swales, channels, and ponds in order to retard erosion, filter sediments and 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.

G.

Protection of Wetland Hydrology. Wetland hydrology shall be protected through the development process, as determined by the director and pursuant to the Drainage Design and Erosion Control Manual for Thurston County, as amended (Chapter 15.05 TCC). Post-development wetland hydrology shall match pre-development wetland hydrology unless the approval authority determines that changes in hydrology will not harm wetland functions. The approval authority may require a hydrologic study if it is determined that the project has potential to significantly impact a wetland. The approval authority may call upon experts as needed, at the applicant's expense, to evaluate the study.

H.

Roadside Stormwater Conveyance Facilities. Roadside stormwater conveyance facilities (e.g., swales, ditches, and pipes) may be extended through wetland buffers within rights-of-way. When possible and practical, they shall be along the side of the road furthest from the wetland. If the conveyance facility must be located along the side of the road closest to the wetland, it shall be located as close to the road/sidewalk as possible, consistent with public safety. In no case shall facilities that infiltrate stormwater be less than one hundred feet from a Category I—III wetland or fifty feet from a Category IV wetland.

Stormwater conveyance facilities shall be designed and constructed consistent with the BMPs listed in the Regional Road Maintenance ESA Program Guidelines, 2002, and, if applicable, the Drainage Design and Erosion Control Manual for Thurston County, as amended (Chapter 15.05 TCC).

I.

Use of Wetlands for Stormwater Storage. Use of Category II-IV wetlands for storage of stormwater (not including stormwater facilities) is only allowed for public projects designed to halt or improve deteriorated wetland conditions, consistent with TCC 24.30.300, other applicable provisions of this chapter, and the following:

1.

The project shall be for the purpose of solving an existing problem, not to accommodate stormwater generated from new impervious surfaces; and

2.

The alteration in the timing, amount, duration and quality of stormwater reaching the wetland shall not be harmful to wetland functions, dependent aquatic life, wildlife, and native plants.

3.

Category I wetlands shall not be used for stormwater storage.

J.

Temporary Stormwater Management Facilities. If there is no alternative to avoid impacts to wetlands and buffers, surface water discharges may be allowed from new temporary sediment control ponds, retention/detention facilities, or other temporary surface water management structures located beyond the buffer and, if necessary, within the outer twenty-five percent of Category III and IV wetland buffers.

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