1802 75TH ST SW 2021-12-30 - Laserfiche WebLink

Targeted Drainage Report — Twentieth Ave Substation WO# 100026553 EXECUTIVE SUMMARY The proposed development activity is to expand the footprint of -existing Twentieth Avenue Substation in order to install a new switchgear and maintain maintenance accessibility for the substation. The substation is not a manned facility and is infrequently visited for maintenance. The existing station security fence encloses approximately 22,400 square feet. The expansion of the station will push the existing north security fence an additional 10 feet to the north and the existing west fence 30 feet to the west. The expanded area will result in 5,900 square feet of new enclosed area within the substation fence which will increase the total enclosed area approximately 28,300 square feet. The expanded station fence lines will require a shoulder around the exterior of the station, both for access and electrical safety. The shoulder area is approximately 2,000 square feet and is surfaced the same as the substation. The station expansion will be construction within the District's Operation Center property, parcel 392200000300. The Operations Center property is approximately 55.5 acres, the site is approximately 59.3% impervious surface, per 2014 SMMWW Figure 1-2.4.1 a site as a whole exceeds 35% impervious coverage, therefore Figure 1-2.4.2 is required for determination of minimum requirements. The project results in approximately 11,000 square feet of disturbed area considering the grading required in order to expand the substation. The area within the substation fence and the shoulder does not meet the definition of a hard surface nor impervious surface as defined by EMC 14.28.020. These areas are surfaced with highly permeable open graded aggregate which does not retard the entry of water into the soil mantle. The proposal to expand Twentieth Avenue Substation would create approximately 1,500 square feet of new impervious surface and has an insignificant effect upon the site impervious area. Utilizing Figure 1-2.4.2 from the 2014 Ecology Stormwater Management Manual; project is subject to Minimum Requirements #1 through #5. An on -site soils investigation was performed in 1991 and revealed the soils at the site to generally consist of un- weathered glacial till. Temporary erosion and sediment control measures will be implemented during construction in accordance with the approved SWPP Plan prepared per the 2014 SMMWW. All on -site soils disturbed by construction activities will be stabilized with grass and or landscaping prior to the removal of any temporary erosion and sediment control measures. The proposal does not include the installation of a point source to address. Rainfall within landscaped and naturally vegetated areas will remain dispersed and infiltrate naturally. Precipitation which falls within the substation will be stored in the voids of the crushed surfacing base course layer and slowly percolate into the below soils. No effect upon the natural drainage system or outfall is expected. On site stormwater management will serve to store and slowly infiltrate any runoff generated. Page 1