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    State of Washington (Western)

    Excerpt from Executive Summary of 2019 Revisions:

    "The Stormwater Management Manual for Western Washington (SWMMWW) provides guidance on the measures necessary to control the quantity and quality of stormwater. Local municipalities use this manual to set stormwater requirements for new development and redevelopment projects. Land developers and development engineers use this manual to design permanent stormwater control plans, create construction stormwater pollution prevention plans, and determine stormwater infrastructure. Businesses use this manual to help design their stormwater pollution prevention plans.

    The greatest use of the 2005 SWMMWW has been through National Pollutant Discharge Elimination System (NPDES) stormwater permits. The Municipal Stormwater General Permits for western Washington incorporate and reference the SWMMWW. The Industrial Stormwater General Permit, Construction Stormwater General Permit, Boatyard General Permit, and the Sand and Gravel General Permit reference the SWMMWW. Since 2005, Ecology has reissued or issued for the first time all of these NPDES stormwater permits. The 2012 revisions to the SWMMWW will help permittees comply with these permits.

    The method by which this manual controls the adverse impacts from quality and quantity of stormwater is primarily through the application of Best Management Practices. Ecology has revised many of the BMPs from the 2005 SWMMWW to improve their effectiveness for protecting water quality and to meet the intent of the anti-degradation provisions of the water quality standards.

    In addition, Ecology revised this manual to include low impact development (LID) related definitions, requirements, and an LID performance standard. Ecology made the LID revisions based on rulings by the Pollution Control Hearings Board, after consulting with LID advisory committees, and after providing opportunities for public input. The manual update also supports the new LID requirements in the Western Washington Municipal Stormwater Permits.

    Other major changes include revised guidelines on protecting wetlands and designing infiltration facilities, and numerous minor revisions for clarity."

    Volume V- Runoff Treatment Facilities

    V-1.4.2 Maintenance

    Maintenance is required for all types of runoff treatment facilities. See V-4.6 Maintenance Standards for Drainage Facilities for maintenance standards for the treatment facilities discussed in this volume

    V-4.1.3 Flows Requiring Treatment

    Runoff from pollution-generating hard or pervious surfaces must be treated. Pollution-generating hard surfaces (PGHS) are those hard surfaces considered to be a significant source of pollutants in stormwater runoff. PGHS includes pollution-generating impervious surfaces (PGIS) and pollution-generating permeable pavements. Permeable pavements subject to pollution-generating activities are also considered pollution-generating pervious surfaces (PGPS) because of their infiltration capability. The glossary in Appendix I-G: Glossary and Notations provides additional definitions and clarification of these terms.

    • PGHS, PGIS, and PGPS include those surfaces which are subject to: vehicular use; industrial activities; or storage of erodible or leachable materials, wastes, or chemicals, and which receive direct rainfall or the run-on or blow-in of rainfall. Erodible or leachable materials, wastes, or chemicals are those substances which, when exposed to rainfall, measurably alter the physical or chemical characteristics of the rainfall runoff. Examples include erodible soils that are stockpiled, uncovered process wastes, manure, fertilizers, oily substances, ashes, kiln dust, and garbage dumpster leakage. Metal roofs are considered to be PGIS unless they are coated with an inert, non-leachable material (e.g., baked enamel coating). Roofs subject to venting significant amounts of dusts, mists or fumes from manufacturing, commercial, or other indoor activities are also PGIS.
    • A surface, whether paved or not, shall be considered subject to vehicular use if it is regularly used by motor vehicles. The following are considered regularly-used surfaces: roads, unvegetated road shoulders, bike lanes within the traveled lane of a roadway, driveways, parking lots, unrestricted access fire lanes, vehicular equipment storage yards, and airport runways.
    • The following are not considered regularly-used surfaces: paved bicycle pathways separated from and not subject to drainage from roads for motor vehicles, restricted access fire lanes, and infrequently used maintenance access roads.
    • Pollution-generating pervious surfaces (PGPS) are any non-impervious surface subject to vehicular use, industrial activities (as further defined in the glossary); or storage of erodible or leachable materials, wastes, or chemicals, and that receive direct rainfall or run-on or blow-in of rainfall, the use of pesticides and fertilizers or loss of soil. Typical PGPS include permeable pavement subject to vehicular use, lawns and landscaped areas including: golf courses, parks, cemeteries, and sports fields (natural and artificial turf).

    Summary of Areas Needing Treatment

    • All runoff from pollution-generating hard surfaces is to be treated through the water quality facilities specified in Chapter V-2 - Treatment Facility Selection Process and Chapter V-3 - Treatment Facility Menus.
    • Lawns and landscaped areas specified are pervious but also generate run-off into street drainage systems. In those cases the runoff from the pervious areas must be estimated and added to the runoff from hard surface areas to size treatment facilities.
    • Runoff from backyards can drain into native vegetation in areas designated as open space or buffers. In these cases, the area in native vegetation may be used to provide the requisite water quality treatment, provided it meets the requirements in Chapter V-5 - On-Site Stormwater Management under the “Cleared Area Dispersion BMPs,” of BMP T5.30: Full Dispersion.
    • Drainage from hard surfaces that are not pollution- generating need not be treated and may bypass runoff treatment, if it is not mingled with runoff from pollution-generating surfaces.
    • Runoff from nonpollution-generating roofs is still subject to flow control per I-2.5.7 Minimum Requirement #7: Flow Control. The nonpollution-generating roof runoff that is directed to an infiltration trench or dry well must first pass through a catch basin as shown in BMP T5.10A: Downspout Full Infiltration. Note that metal roofs are considered pollution generating unless they are coated with an inert non-leachabale material. Roofs that are subject to venting of significant amounts of manufacturing, commercial, or other indoor pollutants is considered pollution-generating.
    • Drainage from areas in native vegetation should not be mixed with untreated runoff from streets and driveways, if possible. It is best to infiltrate or disperse this relatively clean runoff to maximize recharge to shallow ground water, wetlands, and streams.
    • If runoff from non-pollution generating surfaces reaches a runoff treatment BMP, flows from those areas must be included in the sizing calculations for the facility. Once runoff from non-pollution generating areas is mixed with runoff from pollution-generating areas, it cannot be separated before treatment.

    V-4.6 Maintenance Standards for Drainage Facilities

    The facility-specific maintenance standards contained in this section are intended to be conditions for determining if maintenance actions are required as identified through inspection. They are not intended to be measures of the facility's required condition at all times between inspections. In other words, exceedence of these conditions at any time between inspections and/or maintenance does not automatically constitute a violation of these standards. However, based upon inspection observations, the inspection and maintenance schedules shall be adjusted to minimize the length of time that a facility is in a condition that requires a maintenance action.


    Stormwater Management Manual for Western Washington


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