3721 OAKES AVE EVERETT MEMORIAL STADIUM 2018-12-21

Pi engineering p c Proposed Site Hydrology Refer to Figure 5—Proposed Conditions,which shows the proposed site conditions and a summary of the surface areas. The existing underdrain system for the natural grass outfield will remain.New perforated 4-inch lateral drain lines and 8-inch collector line will be installed for the infield. This collector line will connect to the existing outfield system,thus retaining existing drainage patterns,conveying flow to the existing catch basin in centerfield. Refer to Figure 1.3—Field Drainage Plan in Appendix A. Conveyance System Analysis and Design A conveyance analysis was performed 3 segments of the field collector pipe,as well as the overall outlet connection to the combined sewer system. Though the collector lines converge to a single catch basin,there is an 8-inch outlet connection to the existing public sewer system. Refer to the Conveyance Analysis Spreadsheet,attached in Appendix B of this report. Per section 6.2 of the 2010 Everett SWMM, conveyance requirements are stated as"the project's internal drainage system shall be designed for a 25-year recurrence interval peak flow form the contributing area."In this case,the conveyance analysis will be performed for the 100-year peak flow as a conservative measure.The 100-year peak flow will be determined using WWHM with 15-minute time steps,while the full-flow capacity of the pipe will be calculated using Manning's Equation. Each tributary area for conveyance is indicated on Figure 6. The first segment of the collector is the new portion underneath the infield.This will be an 8-inch PVC pipe(n=0.011)at slope of 0.33%,giving a full-flow capacity of 0.82 cubic feet per second (cfs).The 100-year peak flow from WWHM was determined to be 0.56 cfs. Therefore,this portion of the pipe will have adequate conveyance capacity for the 100-year storm. The second segment needing analysis is the southwest segment of the existing collector line,to which the proposed infield collector will connect.This 8-inch PVC pipe will be at 1.8%slope, which allows full-flow capacity at 1.92 cfs.The 100-year peak flow for the tributary area was 0.86 cfs. The third segment that was analysis included the NE portion of the collector,which will be an 8- inch PVC pipe at a minimum slope of 0.5%,giving full-flow capacity of 1.01 cfs;WWHM yielded 100-year peak flow of 0.30 cfs for the tributary area.Therefore,all segments of the existing collector drain line are shown to adequate conveyance capacity. The tributary area to the 8-inch outlet from the centerfield catch basin is the entire underdrained field. Based on field investigation done by D.A. Hogan&Associates(field designer),in which both field structures were located and observed,it was concluded that the outlet from the storm catch basin does not directly connect to the sewer manhole as survey indicates.From the field observed pipes,it is assumed that the 8-inch outlet connects to the combined sewer line.As a conservative measure,due to the depth of the sewer,it is assumed that the storm connects at near the top of the 48-inch combined sewer. Based upon the field measurements the site investigation and surveyor information for the existing sewer line,the slope of the 8-inch pipe was calculated at approximately 12.0%. Thus,the full-flow capacity is 4.96 cfs. The WWHM 100-year peak flow is 1.72 cfs, so this pipe will have adequate conveyance capacity. LPD Engineering, PLLC Page 8 Everett Memorial Stadium Baseball Field Replacement Storm Drainage Report, September 15, 2011