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

Targeted Drainage Report — Twentieth Ave Substation WO# 100026553 2.4 MINIMUM REQUIREMENT #4 — Preservation of Natural Drainage Systems and Outfalls The proposed development activity will not alter the existing drainage patterns. Stormwater will enter and exit the property as it does in the existing state. Rainfall within the station yard will infiltrate in -place. 2.5 MINIMUM REQUIREMENT #5 — On -site Stormwater Management The intent of the on -site stormwater management BMP's proposed is to infiltrate, disperse, and retain stormwater runoff on -site to the maximum extent feasible without causing flooding or erosion impacts. The proposed on -site stormwater management system consists of retention and on -site infiltration within the station yard and gravel perimeter. Utilizing the aggregates and their void ratios within the station yard foot print to infiltrate stormwater is an effective means for managing the stormwater. The expanded substation yard will be excavated to firm native soils then backfilled with approximately 8-inches of Crushed Surfacing Base Course (CSBC). The CSBC will have a minimum void ratio of approximately 0.2 in a state of 95% proctor compaction. The voids within the CSBC will be utilized to store the precipitation which falls within the yard and retain it while it slowly infiltrates into the native soil below. The existing catch basin (CB 42) which services the area to the west and north of the substation will be relocated to accommodate the station being expanded to the north towards Seaway Transit Center. A French drain will be installed at the toe of the slope west and north of the expanded substation yard. Any runoff collected will be conveyed to the relocated CB 42 and run through the existing drainage system along its existing route. Page 8