2514 RAINIER AVE 2020-04-02 - Laserfiche WebLink

NORTH MIDDLE SCHOOL VII. PERMANENT STORMWATER CONTROL PLAN As noted above in I, Project Description,this report will address the flow control and conveyance requirements for the North Middle School portables site. Area take-offs,calculations, and results discussed below under subsections Developed Hydrology and Stormwater Flow Control Plan reflect this approach. OFF-SITE ANALYSIS Existing stormwater runoff from the site is collected into a field underdrain system and is conveyed from the site to a series of catch basins and pipes which in turn are conveyed to an existing 10"and 12"storm main located in the middle of the parcel between both existing school buildings before being conveyed to the public combined sewer in 26th Street.The stormwater continues to flow in a series of public combined sewer pipes south and southeast of the site and eventually discharges to the Snohomish River approximately 2 miles downstream of the project site. PRE-DEVELOPED HYDROLOGY In compliance with the Interim Policy,the North Middle School portables project will use the existing site conditions to assess the pre-developed hydrology in lieu of using forested land cover. The existing site area where the portables and walking path will be located is a portion of the underdrained grass field. In Figure 2— Existing Conditions,the pervious and impervious areas are summarized for the existing field area DEVELOPED HYDROLOGY The impervious areas created by the portables and ATB walking path are shown in Figure 3—Proposed Conditions. STORMWATER TREATMENT PLAN In accordance with the Interim Policy, Minimum Requirement#6: Runoff Treatment is not applicable. However, by using a dispersion flow control method,some water quality control will naturally occur. STORMWATER FLOW CONTROL PLAN Minimum Requirement#7: Flow Control requires stormwater discharge to"match developed discharge durations to predeveloped durations for the range of predeveloped discharge rates from 50 percent of the 2-year recurrence interval peak flow up to the full 50-year peak flow." This project will be permitted under the understanding that the temporary portables are to be removed once the school modernization has been completed,which is anticipated to be around September 2019(approximately 1-year after Certificate of Occupancy has been granted for the portables). If the portables have not been removed by this time,the project will be required to install the sized flow control facility in addition to applying for any required permits associated with the portables for their extended • duration. The flow control calculations can be seen in Appendix C of this report. This portion of the project will be utilizing downspout dispersion measures with splash blocks and a 500 sq-ft. minimum vegetated flowpath for each. Based on section 12.3.5 of the 2010 COESWMM, a maximum of 700 sq-ft of area can drain to each splash block. The portables each have approximately 855 sq-ft of roof area, so two splash blocks will be provided for each portable. Because of this, 10,200 sq-ft of vegetated flow path is provided. Additional area is provided due to some overlap in flow paths. Due to the vast area of the underdrained field,the additional splash blocks will be properly dispersed. The proposed flow control measures can be seen in Figure 4 —Downspout Dispersion System. MAINTENANCE AND OPERATION PLAN A Maintenance and Operations Plan is included in a previous drainage report issued for permit for the main school project, dated October 20, 2017 which addresses Minimum Requirement#9:Operation and Maintenance. COUGHLINPORTERLUNDEEN 9