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engineering p c <br /> Table 1—Proposed Area Summary <br /> Surface Square Feet Acres <br /> Portable Roof 1,955 0.045 <br /> Asphalt Pavement 214 0.005 <br /> Total Impervious Surface Area 2,169 0.050 <br /> Perforated Metal Ramp and Stairs 274 0.006 <br /> (Pervious) <br /> Total"Surface Area 24443 °05O <br /> Refer to Figure 4—Proposed Conditions, for a depiction of the target surfaces. The new <br /> impervious area is less than 5,000 square feet;therefore, flow control will not be required(only <br /> MR#1-5 will apply). Water quality is also not required as the project does not propose more than <br /> 5,000 square feet of pollution-generating impervious surface(PGIS). The Target Area will require <br /> mitigation by on-site stormwater management BMPs to the maximum extent feasible. Refer to the <br /> On-site Stormwater Management Section below for additional information. Also refer to Appendix <br /> A, for the Grading and Drainage plan for the project. <br /> Pre-developed Site Hydrology <br /> Refer to the offsite analysis section of this report for a detailed description of the existing drainage <br /> conditions. In general, stormwater is collected from a concrete gutter and catch basins around the <br /> vicinity of the sand playfield and conveyed to a grass-lined stormwater swale. Stormwater <br /> eventually outfalls into Merrill and Ring Creek on the north side of Casino Road. <br /> Proposed Site Hydrology <br /> The overall drainage patterns throughout the site are not being changed by the proposed <br /> improvements. Rain falling upon the portable roofs will be collected via downspouts,then <br /> conveyed through storm drainage pipes to the existing catch basin located east of the proposed <br /> portable. Refer to the design drawings in Appendix A of this report for the drainage plans for the <br /> site. <br /> Conveyance System Analysis and Design <br /> Conveyance capacity was evaluated for the new 6-inch storm drain pipe from the portable to the <br /> existing catch basin to the east. The 100-year peak runoff rate from the tributary basin was <br /> determined using MGSFlood with a 5-minute timestep. The full flow capacity of the pipe was <br /> determined using Manning's equation. <br /> The new 6-inch storm drain pipe conveys runoff from the portable roof to the existing catch basin <br /> located to the east of the portable. The 100-year peak flow to the new 6-inch pipe was determined <br /> to be 0.07 cfs and the pipe full capacity at a slope of 1%was 0.66 cfs, demonstrating that the pipe <br /> LPD Engineering, PLLC Page 6 Horizon Elementary School — Classroom Portable Installation <br /> Storm Drainage Report,June 5, 2019 <br />