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Home My WebLink AboutSD-23-10 - Supplemental - 0255 Kennedy Drive (41) Attachment 1: Narrative, Location Map, and Soils Map Hillside @ O'Brien Farm Lots 11-15 1. Introduction Krebs and Lansing Consulting Engineers Inc. (K&L) are writing on behalf of O’Brien Brothers to apply for a State Stormwater Discharge Permit pursuant to General Permit 3- 9050 for the next phase of the Hillside @ O’Brien Farm project located on Kennedy Drive, O’Brien Farm Road, and Two Brother. 2. Project Description The next phase of the Hillside @ O’Brien Farm project involves the construction of six multi-family apartment buildings on Lots 11-15. The first phase of development will be buildings on Lots 10 and 11. We expect buildings on Lots 13 and 15 to be permitted and constructed shortly after. Future buildings on Lots 12 and 14 are included as part of this application. As part of this project, O’Brien Farm Road will also be extended towards Old Farm Road. A portion of the proposed parking for Lots 13 and 15 will extend onto Lot 17. Each building features underground parking. The proposed stormwater treatment practices include existing Wet Pond A and Infiltration Basin A, two new Gravel Wetlands, and four infiltration trenches that were previously permitted but not constructed. Forebays are provided for pre-treatment of non-rooftop runoff for each Gravel Wetland. 3. Existing Condition Lots 11-15 of the project were previously permitted as lots for future development. Portions of O’Brien Farm Road, Two Brothers Drive, and associated utilities have been either already constructed, or are currently under construction. Existing soil types on this portion of the site are Adams-Windsor loamy sands. These soils are classified as type A by the U.S. Soil Conservation Service, which normally indicates a low degree of runoff and high infiltrative capacity. However, on-site soil testing, as well as observation during construction, indicates a high seasonal groundwater table at less than three feet below existing ground surface throughout the site. The site slopes from east to west, at times steeply. The pre development condition was divided into five watersheds corresponding with the natural stormwater discharge points of the property. Study point 1 is located at the intersection of Kennedy Drive and Kimball Avenue and drains to S/N005; Study point 2 is a culvert crossing under Kennedy Drive north of Windridge Condominiums, draining Wetland E and eventually to S/N 004; Study point 3 is a culvert crossing under Kennedy Drive south of Windridge draining to S/N001; Study point 4 is the where Wetland A exits the property, Hillside @ O’Brien Farm – Lots 11-15 Stormwater Narrative 2 near Wellsley Grove Condos draining to S/N 002; and Study point 5 is the southwest corner of the property in a swale along Hinesburg Road and drains to S/N003. The proposed development areas and associated stormwater treatment practices all drain to S/N 004. We have redone pre-development stormwater modeling for this watershed. To account for the high seasonal water table, we have calculated half the area as “A” soil and the other half as “B” soil. We believe this better simulates existing conditions on the site due to high water table. Our site visits during construction have indicated there is far more existing runoff from the site than is typically associated with a site that is all “A” soils. 4. Existing Stormwater System The original Eldredge Street development received State of Vermont Stormwater discharge Stormwater Permit #7815-INDS was obtained in September 2017 for Phase 1 of the Hillside development. This permit also incorporated previously permitted infrastructure from Stormwater discharge Permit #1-1520. Existing permitted stormwater infrastructure includes Extended Detention Ponds 1, 2, and 3, Wet Ponds A and B, Infiltration Basin A, six grass channels, and four infiltration trenches. Wet Pond B and the infiltration trenches have not been constructed at this time. 5. Proposed Stormwater System: The proposed stormwater collection, treatment, and detention system will utilize a combination of existing and new stormwater treatment practices (STPs). The watershed draining to SN 004 features the majority of Two Brothers Drive and the majority of O’Brien Farm Road. The proposed buildings on Lots 10 and 11 will drain to the existing/permitted collection system on O’Brien Farm Road and discharge to Wet Pond A/Infiltration Basin A. A portion of the paved parking on Lot 12 will also discharge to Wet Pond A/Infiltration Basin A via a previously installed storm pipe. Because Infiltration Basin A is a “Tier 1 practice”, no modifications are necessary to bring it into compliance with the 2017 State of Vermont Stormwater Manual. The rest of Lot 12, including the building, and Lot 14 will drain to Gravel Wetland #1. Gravel Wetland #1 replaces previously permitted (but not constructed) Wet Pond B. This brings the project into compliance with the 2017 Stormwater Manual. Lots 13 and 15, and the extended portion of O’Brien Farm Road drain to Gravel Wetland #2. We previously showed and modeled Wet Pond C in this location, but it was neither permitted nor constructed. a) Description of Impervious Area: The existing permitted area of impervious surface is 16.24 acres. An additional 6.6 acres of impervious surface is proposed as part of this application. This includes allowances for future buildings and impervious areas on adjacent lots that will likely drain to the treatment practices. All of this is new impervious surface. The new impervious surface is from building roofs, paved roads, driveways, and parking, and concrete sidewalks and pads. The areas are further summarized below: (1) 1.81 additional acres of impervious surface will drain to Wet Pond A/Infiltration Basin. A. This area was previously accounted for in the Hillside @ O’Brien Farm – Lots 11-15 Stormwater Narrative 3 modeling, so no changes were made to the model. (2) An additional 0.45 acres of impervious surface will drain to Gravel Wetland #1. This is in addition to the previous 0.86 acres of impervious surface that was previously permitted to drain to Wet Pond B. (3) 4.34 acres of impervious surface will drain to Gravel Wetland #2. b) Receiving Body: S/N004: Wetlands in Potash Brook watershed c) Fish Habitat Designation for Receiving Water: Warm d) Description of compliance with each of the treatment standards in the 2017 VSMM including the treatment practices or waivers used to meet each of the following standards: i) Post-Construction Soil Depth and Quality Standard: This standard will be met via two options outlined in the VSMM. Option 1: Areas outside of construction will be left undisturbed and protected from compaction during construction. This option will apply only to the far westerly portion of the site. Option 3: Remove and stockpile existing topsoil during construction. On site soil testing indicates and existing sandy loam topsoil layer on the site ranging from 7-13 inches in depth. Given that much of the finish site will be covered in building or paving, there will be an excess of existing topsoil to re store other disturbed areas such as setbacks, vegetated islands, swales, and side-slopes. Compost will be incorporated into the existing topsoil stockpile if needed to achieve 4% organic content. ii) Groundwater Recharge Standard: The Groundwater Recharge standard is met site wide by the use Infiltration Basin A. The standard is met for the Potash Brook watershed. iii) Water Quality Treatment Standard (WQV): S/N004: WQv will be met for S/N 004 via the use of Wet Pond A, Infiltration Basin A, and Gravel Wetlands #1 & #2. The stone voids in the gravel wetland will store 50% of the WQv draining to each wetland. The remaining 50% WQv is provided by extended detention using small diameter orifices to release the remaining WQv over a 24 hour period. Pre-treatment is provided forebays. iv) Channel Protection Standard (CPV): S/N004: The Cpv standard is met by the use of Wet Pond A, Infiltration Basin A, and Gravel Wetlands #1 and #2. Infiltration Basin A will completely infiltrate the 1 year, 24 hour storm. The gravel wetlands will feature outlet structures using low flow orifices to provide the required detention times. Hillside @ O’Brien Farm – Lots 11-15 Stormwater Narrative 4 v) Overbank Flood Protection Standard (QP10): S/N004: The Overbank Flood Protection Standard is met by the use of Wet Pond A, Infiltration Basin A, and Gravel Wetlands #1 and 2. Infiltration Basin A will completely infiltrate the 10 year, 24 hour storm. The gravel wetlands use controlled outlet devices to limit post development peak flows to below pre development levels for the 10 year, 24 hour storm event. vi) Extreme Flood Protection Standard (QP100): S/N004: The Extreme Flood Protection Standard is met by the use of Wet Pond A, Infiltration Basin A, and Gravel Wetlands #1 and 2. The gravel wetlands and infiltration basin use controlled outlet devices to limit post development peak flows to below pre development levels for the 100 year, 24 hour storm event. The following items are attached for review: • Complete NOI form • Attachment 1: Narrative: Narrative, Location Map, and Soils Map. • Attachment 2: Workbooks: STP Selection Tool and Standards Compliance Workbook • Attachment 3: Worksheets: STP and waiver worksheets, grouped by discharge point • Attachment 4: Modeling: Runoff modeling and calculations demonstrating compliance with the applicable treatment standards. • Attachment 5: Plans: Pertinent plan sheets with all required information outlined in Part 7 of the Application Requirements for Operational Permit Document. • A check in the amount of $5916.00 Payable to “State of Vermont”. Hillside @ O’Brien Farm – Lots 11-15 Stormwater Narrative 5 Location Map [Insert project specific location map here. You may download topographic map from the Natural Resource Atlas. Please show the site outline, the location of the discharge point(s) and receiving waters. The scale of the location map should be between 1:20,000 and 1:40,000.] See Attached Location Map. Potash Brook KENNEDY D R I V E OLD FARM ROADK IM B A L L A V E N U EPotash BrookHINE SBURG ROAD INTERST A T E 8 9 X X XStudy Point 2 X SN004 SITE DATE: 9/10/21 LOCATION MAP SCALE: 1" = 1000' KENNEDY DRIVE & TWO BROTHERS DRIVE South Burlington, Vermont Consulting Engineers, INC. 4 Main StrHHt SuitH 20 &oOFhHstHr VT 044 P 802 880 _ F 802 8898 HPaiO#krHEsandOansinJFoP _ krHEsandOansinJFoP HILLSIDE @ O'BRIEN FARM LOTS 11-15 Hillside @ O’Brien Farm – Lots 11-15 Stormwater Narrative 6 Soils Map [Insert project specific soils map here. Soils information can be found at the Web Soil Survey website. Hydrologic Soil Groups— “HSGs” shall be overlaid with site outline. Soils information can also be provided as data layer on an existing or proposed condition plan sheet (if included as a data layer on one of the plan sheets please indicate that here] See Attached Soils Map. MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:15,800. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Chittenden County, Vermont Survey Area Data: Version 19, Sep 15, 2016 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Aug 28, 2010—Oct 8, 2011 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Soil Map—Chittenden County, Vermont (O'Brien Home Farm) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/11/2017 Page 2 of 3 Map Unit Legend Chittenden County, Vermont (VT007) Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI AdA Adams and Windsor loamy sands, 0 to 5 percent slopes 52.6 15.3% AdB Adams and Windsor loamy sands, 5 to 12 percent slopes 52.3 15.2% AdD Adams and Windsor loamy sands, 12 to 30 percent slopes 24.5 7.1% AdE Adams and Windsor loamy sands, 30 to 60 percent slopes 9.0 2.6% Be Beaches 0.3 0.1% BlB Belgrade and Eldridge soils, 3 to 8 percent slopes 2.5 0.7% Cv Covington silty clay 7.6 2.2% DdA Duane and Deerfield soils, 0 to 5 percent slopes 38.7 11.2% DdB Duane and Deerfield soils, 5 to 12 percent slopes 1.8 0.5% EwA Enosburg and Whately soils, 0 to 3 percent slopes 5.0 1.4% EwB Enosburg and Whately soils, 3 to 8 percent slopes 50.1 14.6% FaC Farmington extremely rocky loam, 5 to 20 percent slopes 13.7 4.0% FsB Farmington-Stockbridge rocky loams, 5 to 12 percent slopes 30.4 8.8% HnB Hinesburg fine sandy loam, 3 to 8 percent slopes 10.0 2.9% Lf Limerick silt loam, very wet 17.4 5.1% Lh Livingston clay 2.6 0.7% VeB Vergennes clay, 2 to 6 percent slopes 20.1 5.8% VeC Vergennes clay, 6 to 12 percent slopes 5.5 1.6% VeD Vergennes clay, 12 to 25 percent slopes 0.1 0.0% Totals for Area of Interest 344.2 100.0% Soil Map—Chittenden County, Vermont O'Brien Home Farm Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 4/11/2017 Page 3 of 3