225 N CABOT RD 2019-10-04 - Laserfiche WebLink

i project will use List #2 of SMMWW Volume I, Section 2.5.5 to mitigate storm water run-off. • Full Dispersion is infeasible due to the location of the project site. • Downspout Full Infiltration System is feasible in accordance with BMP T5.10A. Minimum Requirement#6: Runoff Treatment The proposed project will add less than 5,000 sf of PGIS; therefore, a storm water treatment facility is not required. Storm water run-off will meet water quality requirements through existing on-site soils. Minimum Requirement#7: Flow Control Flow control shall be provided for the proposed project if any of the following thresholds are meet: • 10,000 sf or more of effective impervious surface o The proposed project does not exceed 10,000 sf of effective impervious surface. • 3/4 acres of more of forest converted to lawn or landscape and surface water is discharged from the site into a conveyance system of receiving water o The proposed project does not convert forest to lawn. • 2.5 acres of forest converted to pasture and surface water is discharged from the site into a conveyance system or receiving water o The proposed project does not convert forest to pasture. • A combination of hard surfaces and converted pervious surfaces cause a 0.1 CFS, one-hour time step, or 0.15 CFS, 15-minute time steps, or greater increase in the 100 year flow frequency from a threshold discharge area as estimated using WWHM o WWHM2012 (15-minute time step) was used to determine the pre- developed and mitigated discharge volumes from the site under developed conditions. The proposed project will not increase storm water run-off, for the 100 year return period, more than 0.15 CFS. Calculations are found in Section III of this report. Minimum Requirement#8: Wetlands Protection Storm water run-off from the proposed project will not directly discharge storm water run-off into on-site wetlands. 5