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Enhanced Treatment

is provided for reduction of dissolved metals (Volume I, Section 2.4.6)

because the project:

Discharges to a fish-bearing stream, lake, or to waters or conveyance systems

tributary to a fish-bearing stream or lake (excluding basic treatment waters

identified above); and

Is an industrial project site, or

A commercial project site, or

A multifamily project site, or

A high design year average daily traffic road (additional conditions apply, see

Volume I, page 4-9).

Project does not discharge untreated stormwater from pollution-generating impervious surfaces

to groundwater. Direct discharge of untreated stormwater from pollution-generating

impervious surfaces to groundwater must be prohibited.

Project provides necessary Phosphorus Treatment.

Project is a high-use site that provides Oil Treatment.

Project establishes a maintenance schedule in accordance with SWMMWW Volume V.

Minimum Requirement #7: Flow Control

Project Threshold: A flow control facility and/or land management BMPs that will achieve the

standard flow control requirement for Western Washington is required if a project’s:

Total effective, impervious surface is 10,000 sq ft or more in a Threshold Discharge Area

(TDA), or

The total of PGIS surfaces is three-quarters of an acre or more of native vegetation to lawn or

landscape, or convert 2.5 acres or more of native vegetation to pasture in a TDA from which

there is a surface discharge in a natural or man-made conveyance system, or

The project, through a combination of effective impervious surfaces and converted

pervious surfaces cause a 0.1 cubic foot per second (cfs) increase in the 100-year flow

frequency from a TDA as estimated by the WWHM or other approved model. (Section 2.5.7)

Verify if the project discharges directly or indirectly through a municipal stormwater system

with sufficient capacity (but not through a creek) into one of the following flow control-

exempt receiving waters. If so, no flow control facilities are required. (Additional

restrictions also apply, see Volume I, Section 2.4.7):

This list will be Municipality specific. See Volume I, Appendix I-E for a list of

Flow Control-Exempt Surface Waters and insert all that apply to your community.

Developments with a mix of land use types shall apply Enhanced Treatment requirements when the runoff

from the areas subject to Enhanced Treatment comprise at least 50% of the total runoff within a TDA.

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Sample Stormwater Site Plan Review Checklist - Washington State Department Of Ecology Page 4