3721 OAKES AVE EVERETT MEMORIAL STADIUM 2018-12-21

PI engineering p c Stormwater for the field will percolate vertically through the permeable surface materials as described above and gravel aggregate base course materials and come in contact with the native subgrade soils beneath the field. There is no infiltration of stormwater anticipated into the native soils beneath the field,based upon a letter prepared by Associated Earth Sciences,dated September 13,2017(which a copy of the letter is included in Appendix B of this report). Therefore, infiltration has not been included in the MGS Flood stormwater modeling calculations,presented in Minimum Requirement#7 below. After a buildup of stormwater gradient along the native subgrade between the trenches,the stormwater will then percolate horizontally through permeable gravel base to the nearest perforated underdrain pipe. The attenuation of the stormwater through the field sub-base materials both horizontally and vertically ranges between 12 and 160 minutes,assuming that there is a previous stormwater gradient present along the subgrade. If the storm event occurs without an existing gradient,the time of concentration would be significantly increased to allow for adequate precipitation to provide a gradient for the horizontal movement of stormwater through the base material. Once the flows reach the lateral perforated underdrain pipes,they are then conveyed to a main solid wall collector pipe(located roughly through the center of the field),that will tie into an existing catch basin followed by combined sewer manhole underneath centerfield. Due to the various layers that the stormwater has to vertically pass through,there limited to no sediment within the system thus limited to no potential of plugging the subsurface layer. Minimum Requirement#6—Runoff Treatment(MR6) The proposed synthetic turf field is considered as a pollution generating surface per the City of Everett,however the project site is located within a combined sewer area.Per Public Works Policy #2-11,MR6,water quality treatment will not be required in the combined sewer area. Minimum Requirement#7—Flow Control(MR#7) Per the City of Everett Municipal Code(EMC) 14.28.060.G.3,the proposed project has been evaluated to determine if there is a 0.15 cfs increase in the 100-year flow frequency from the existing conditions,estimated using the Western Washington Hydrology Model(WWHM), 15- minute time steps. Based upon the Everett Public Works Director Interim Policy#2-11,for Minimum Requirement#7,the existing site conditions shall be used instead of forested land cover, to represent the predeveloped conditions. Therefore,the true existing conditions are compared to the proposed conditions when comparing the 100-year peak flow rates. The underdrained sports field has been considered as a hard surface for only determining the project minimum requirements. For stormwater modeling purposes there is no specific guidance is provided within the Everett SWMM or the 2014 DOE SMMWW for the modeling of underdrained pervious sports fields. Per coordination with the City of Everett,it has been requested that for the proposed conditions,the underdrained field be modeled with the"porous pavement"element in WWHM is used to represent the permeable synthetic turf field and the gravel base layers. The "infield mix"surfacing consists of the pitching mounds for the field itself and the bullpens has been included in as an impervious surface. For the existing conditions,the cinder surfacing(warning track and perimeter),along with the compacted dirt infield were considered as impervious surfaces.Per the City's request,the existing LPD Engineering, PLLC Page 5 Everett Memorial Stadium Baseball Field Replacement Storm Drainage Report, September 15, 2017