02/23/2011 Env Bd Packet - Laserfiche WebLink

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FINAL DRAFT 02 -23 -2011 (d) An outfall structure discharging to a wetland, public water or public water wetland must incorporate a stilling- basin, surge- basin, energy dissipater, placement of ungrouted natural rock riprap or other feature to minimize disturbance and erosion of natural shoreline and bed resulting from stormwater discharges. (e) All new residential, commercial, industrial and other habitable or non - habitable structures, and all stormwater basins, must be constructed so that the lowest floor and lowest entry elevations of structures comply with the following table: Within a landlocked basin, lowest floor elevations must be at least one foot above the surveyed basin overflow elevation. Where a structure is proposed below the runout elevation of a land- locked basin, the low -floor elevation will be a minimum of two feet above the high water level as determined from an estimate of high water levels determined from the highest of either the 100 - year, ten -day runoff event or back -to -back 100 -year, 24 -hour rainfalls. Aerial photos, vegetation, soils, and topography will be used to derive a "normal" water elevation for the basin for the purpose of computing the 100 -year elevation. (f) All stormwater management structures and facilities must be designed for maintenance access and properly maintained in perpetuity to assure that they continue to function as designed. (g) Before work under the permit is deemed complete, as- builts must be submitted demonstrating that at the time of final stabilization, stormwater facilities conform to design specifications. (9) Drainage And Utility Easements (a) If a stormwater management plan involves direction of some or all runoff off of the site, it shall be the responsibility of the applicant to obtain from adjacent property owners any necessary easements or other property interests concerning flowage of water. (b) Easements are required for all stormwater management facilities, stormwater conveyances and on -site floodplain and shall extend a minimum of 10 feet beyond the basin's 100 -year storm high water level elevation. 18 • • • Regional Elevations Local Detention Basins & Wetlands Infiltration Basins Rain- gardens Ground Water Elevation 100yr EOF 100yr EOF Bottom 100yr EOF EOF Bottom Low Floor Freeboard 2 -ft 1 -ft 0 -ft NA 0 -ft NA NA NA 4 -ft Low Entry ' Freeboard NA NA 2 -ft 1 -ft NA 2 -ft 1 -ft 0.5 -ft NA Within a landlocked basin, lowest floor elevations must be at least one foot above the surveyed basin overflow elevation. Where a structure is proposed below the runout elevation of a land- locked basin, the low -floor elevation will be a minimum of two feet above the high water level as determined from an estimate of high water levels determined from the highest of either the 100 - year, ten -day runoff event or back -to -back 100 -year, 24 -hour rainfalls. Aerial photos, vegetation, soils, and topography will be used to derive a "normal" water elevation for the basin for the purpose of computing the 100 -year elevation. (f) All stormwater management structures and facilities must be designed for maintenance access and properly maintained in perpetuity to assure that they continue to function as designed. (g) Before work under the permit is deemed complete, as- builts must be submitted demonstrating that at the time of final stabilization, stormwater facilities conform to design specifications. (9) Drainage And Utility Easements (a) If a stormwater management plan involves direction of some or all runoff off of the site, it shall be the responsibility of the applicant to obtain from adjacent property owners any necessary easements or other property interests concerning flowage of water. (b) Easements are required for all stormwater management facilities, stormwater conveyances and on -site floodplain and shall extend a minimum of 10 feet beyond the basin's 100 -year storm high water level elevation. 18 • • •