4817 RUCKER AVE 2024-12-09 - Laserfiche WebLink

• Stormwater discharges shall match developed durations to existing durations for the range of existing discharge rates from the 10-year peak flow through the full 100- year peak flow • Peak runoff rates from the developed site shall not exceed existing runoff rates in the 25-year and 100-year return intervals • The existing condition to be matched shall be the site as it exists at the time of application. Calculations shall be performed using a continuous simulation modeling program approve by the Washington Department of Ecology for stormwater facility design in Western Washington. Modeling shall be performed using a 5-minute timestep. Site soils were investigated through a geotechnical study for the adjacent YMCA construction in 2018, by Associated Earth Sciences, Inc. This study included infiltration testing on the park site, and determined a design infiltration rate of 5 inches/hour (including applicable factor of safety) to be used for infiltration facilities. Stormwater management for the various surfaces is as follows.- Hard surfaces (NPGHS) Hard surfaces for the park include walkways and seating areas. Due to the sandy subsurface soils, runoff will be collected in a closed pipe system and routed to an underground infiltration facility as shown on the plan. This facility is essentially an expanded version of the downspout infiltration trench, modeled as a gravel trench bed in WWHM. As described above, a geotechnical study was performed to determine the appropriate infiltration rate, in accordance with Section 5-5 of the Manual. A 550 square foot trench bed, 6 feet deep was shown to infiltrate up to the 100 year storm runoff from Basins B and C, as shown in the attached WWHM modeling. Approximately 737 square foot trench bed, 6.5 feet deep, is provided, exceeding the volume required by the modeling. With all runoff from Basins B and C infiltrated, the flow control standards are achieved (see attached WWHM output). The bypassing of runoff from Basins A and D, located along the road frontages of the park, does not generate enough surface runoff to overcome the infiltration of all runoff from the much larger Basins B and C. MINIMUM REQUIREMENT #8 - DETENTION OR TREATMENT IN WETLANDS OR WETLAND BUFFERS There are no wetlands on or adjacent to the project site, nor anywhere in the downstream system. Therefore this Minimum Requirement is not applicable 7