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Home My WebLink AboutSD-23-07 - Supplemental - 0600 Spear Street (36)Stormwater Management Narrative 600 Spear St South Burlington, VT 1.Introduction Trudell Consulting Engineers (TCE) is writing on behalf of the applicant, 600 Spear FJT, LLC, to apply for a State Stormwater Discharge Permit pursuant to General Permit 3- 9050 for the above referenced project. 2.Project Description The proposed project involves a multi-family residential development including a new city street, eight multi-family homes, ad associated parking and drives. In total, the project proposes 1.60 ac of new impervious and 0.443 ac of redevelopment. Five gravel wetlands are proposed to treat all new and redeveloped impervious as well as 0.092 ac of existing rooftop. An additional 0.018 ac of existing impervious will remain on-site, however, runoff from this impervious will not be directed to stormwater treatment practices. As the project proposed more than ½ ac of new impervious, the site is subject to state stormwater management rules – Environmental Protection Rules, Chapter 22, as well as the City of south Burlington’s stormwater management regulations. 3.Existing Conditions The parcel is currently developed with 0.45 acres of impervious surface including buildings, a small paved driveway and a gravel driveway with parking area. Topography throughout the site is relatively flat with slopes ranging from 0% to 5% in the area proposed to be developed. An existing drainage ditch follows the northern property boundary between Class II wetlands. Soils within the developed project area are very well drained loamy sands, transitioning o fine sandy loam within the wetland buffer. Three soils are mapped within the project area: o AdA – Adams and Windsor sandy loams, Hydrologic Soil Group (HSG) A o AdD – Adams and Windsor sandy loams, HSG A o HnB – Hinesburg fine sandy loams, HSG A Soils on site have been field verified during soil excavations for environmental assessments and are included herein for reference. No ledge was encountered during test pit excavations. Due to existing disturbances and sub-surface contamination this property is considered a stormwater hotspot and therefore all drainage practices must be lined and no infiltration may be proposed. 4.Proposed Stormwater System: a)Description of Impervious Area: The proposed site will include a new City street, eight (8) new multi-family dwellings, one existing dwelling, an existing steel building and associated paved parking and drives. Under proposed conditions, total site impervious will be approximately 2.764 ac. Of the total site impervious, 2.578 ac of impervious is proposed for treatment via one of five Gravel Wetlands with discharge to an unnamed wetland tributary of Potash Brook (S/N 001). Drainage Area to S/N 001:3.868 ac Total Impervious to S/N 001:2.135 ac New:1.600 ac Redevelopment:0.443 ac Existing:0.092 ac b)Receiving Body: Unnamed Wetland Tributary of Potash Brook c)Fish Habitat Designation for Receiving Water: Warm (Wetland) d)Description of compliance with each of the treatment standards: i)Post-Construction Soil Depth and Quality Standard: Sheet C4-02 highlights all areas disturbed during site development that are applicable to the Post Construction Soil Depth and Quality Standard as well as a proposed sampling scheme. Each option for methods to meet this standard is thoroughly described on C8-403 Stormwater notes to warrant contractor compliance during construction. ii)Groundwater Recharge Standard: This standard is waived due to a hot spot site designation. iii)Water Quality Treatment Standard (WQV): (1)S/N 001: The WQV is met for discharge point one, via a series of five (5) individual gravel wetlands, a Tier 2 treatment practice. Due to prior land use and the hot spot designation, Tier 1 infiltrative practices are not feasible. All gravel wetlands discharge to a subsurface drainage system with outlet to an unnamed wetland tributary of Potash Brook. iv)Channel Protection Standard (CPV): (1)S/N 001: The CPV is met for discharge point one, via 24-hour Extended Detention or maximum detention though a minimum 1” orifice throughout the five (5) individual gravel wetlands. All gravel wetlands discharge to a subsurface drainage system with outlet to an unnamed wetland tributary of Potash Brook. v)Overbank Flood Protection Standard (QP10): (1)S/N 001: The QP10 is met for discharge point one, via peak runoff detention within the five gravel wetlands and subsurface storage pipe. Pre- development peak discharge rate for the 10-yr, 24-hr rain event is 1.87 cfs, post-development peak discharge rate for the 10-yr, 24-hour rain event is proposed to be 0.57 cfs. vi)25-Yr, 24-Hr Rain Event: (1)S/N 001: Peak discharge detention is provided for the 25-yr, 24-hr rain event within the five gravel wetlands and subsurface storage pipe. Pre- development peak discharge rate for the 25-yr, 24-hr rain event is 2.41 cfs, post-development peak discharge rate for the 25-yr, 24-hour rain event is proposed to be 0.91 cfs. vii)Extreme Flood Protection Standard (QP100): (1)S/N 001: As this site involves less than 10ac of impervious, the QP100 standard is waived. All stormwater treatment areas and conveyances have been designed to safely pass the 100-yr, 24-hr rain event.