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HomeMy WebLinkAboutSD-23-11 - Supplemental - 1200 Dorset Street (18) 1 Attachment 1: Narrative, Location Map, and Soils Map Heald Subdivision – Lot 6 1. Introduction Civil Engineering Associates, Inc. is writing on behalf of 1200 Dorset, LLC to apply for a State Stormwater Discharge Permit pursuant to General Permit 3-9050 for the above referenced project. 2. Project Description The project includes the proposed construction of site improvements, in the form of two new single-family residences and related impervious areas, on Lot 6 of the Heald Subdivision, located at Foulsham Hollow Road in South Burlington. Lot 6 is proposed to be subdivided into two lots. The lots, 6A and 6B, will be accessed via a proposed shared driveway off of Foulsham Hollow Road. Due to topographic constraints, site balancing is proposed for a portion of the project as follows: 265 square feet of the driveway apron, 30 square feet of driveway, and 300 square feet of patio and walkway will be site balanced with portions of the existing sidewalk and recreation path. These areas all discharge to the same received water, S/N 001, Potash Brook. Please refer to enclosed Attachment 5 - Plans for additional detail regarding locations. The project requires permit coverage under the jurisdiction of the Vermont Stormwater Management Rule – Environmental Protection Rules, Chapter 22. 3. Existing Condition Lot 6 currently hosts a condemned single-family home and a related outbuilding, all of which will be demolished as part of this project. The majority of Lot 6 drains to the east, eventually discharging to Potash Brook. The existing impervious area, which exceeds 1-acre, within the common plan of development, the Heald Subdivision, includes Foulsham Hollow Road and five (5) single-family homes. Underlying Soils Underlying soils for the project area were determined using USDA soil mapping for Chittenden County. The soils in the area of the proposed site improvements on Lot 6 are Vergennes clay, 2 to 6 percent slopes (VeB), and Stockbridge and Nellis soils, 8 to 15 percent slopes, stony (SuC). The on-site soils have been assigned to Hydrologic Soil Groups D and C, respectively. The soil borings conducted and shown on sheet SW1 appear to indicate a silty clay beginning 6 - 12 inches below grade in the project area. This silty clay is heavily mottled and indicates high water table for the soil profile. Based on a very shallow depth to water table and limiting 2 soil texture the project site appears to contain HSG D soils based on the TR-55 definition of Hydrologic Soil Groups. 4. Proposed Stormwater System: a) Description of Impervious Area: 0.29 acres of new impervious area is treated via a proposed subsurface gravel wetland. The gravel wetland is designed to treat the water quality storm event via water quality orifice and extended detention for QP10 mitigation. The forebay is sized for greater than 10% of the water quality volume. b) Receiving Body: Potash Book 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: The accompanying plan set includes notes and specifications outlining the requirements and options for the contractor to meet the Post-Construction Soil Depth and Quality Standard. ii) Groundwater Recharge Standard: This site wide standard is waived for the project as the entire site is located on HSG D soils based on the on-site soil investigations. iii) Water Quality Treatment Standard (WQV): (1) S/N 001: The proposed subsurface gravel treatment wetland provides the required treatment for the Water Quality Treatment Standard. iv) Channel Protection Standard (CPV): (1) S/N 001: The pre-routed, post-development discharge from the 1-year, 24-hour storm event is 1.56 cubic feet per second (cfs), which is less than less than 2 cfs; therefore, this criterion is waived. v) Overbank Flood Protection Standard (QP10): (1) S/N 001: QP10 is met via detention within the within the gravel treatment wetland facility and outlet control structure. As a result, the post-development peak discharge rate of 2.24 cfs is less than the pre-development peak discharge rate of 2.63 cfs. vi) Extreme Flood Protection Standard (QP100): (1) S/N 001: The proposed impervious expansion is 0.29 acres, which when combined with the existing 0.83 acres of impervious within the common plan of development, the Heald Subdivision, is less than 10 acres; therefore, this criterion is waived. 3 Location Map 31,683 1,609.5 Location Map Vermont Agency of Natural Resources 24,000 © Vermont Agency of Natural Resources 1,219.0 1: WGS_1984_Web_Mercator_Auxiliary_Sphere Meters1,219.00 NOTES Map created using ANR's Natural Resources Atlas LEGEND 610.00 vermont.gov DISCLAIMER: This map is for general reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. ANR and the State of Vermont make no representations of any kind, including but not limited to, the warranties of merchantability, or fitness for a particular use, nor are any such warranties to be implied with respect to the data on this map. August 22, 2023 THIS MAP IS NOT TO BE USED FOR NAVIGATION 1" = 2000 1cm = 240Ft.Meters Parcels (standardized) Roads Interstate US Highway; 1 State Highway Town Highway (Class 1) Town Highway (Class 2,3) Town Highway (Class 4) State Forest Trail National Forest Trail Legal Trail Private Road/Driveway Proposed Roads Town Boundary Project Location 4 Soils Map Soil Map—Chittenden County, Vermont (Polygon) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/16/2023 Page 1 of 3492200049220104922020492203049220404922050492206049220704922080492209049221004922110492212049221304922010492202049220304922040492205049220604922070492208049220904922100492211049221204922130645140645150645160645170645180645190645200645210645220645230 645150 645160 645170 645180 645190 645200 645210 645220 645230 645240 44° 26' 16'' N 73° 10' 34'' W44° 26' 16'' N73° 10' 29'' W44° 26' 12'' N 73° 10' 34'' W44° 26' 12'' N 73° 10' 29'' WN Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 18N WGS84 0 30 60 120 180 Feet 0 5 10 20 30 Meters Map Scale: 1:649 if printed on A portrait (8.5" x 11") sheet. Soil Map may not be valid at this scale. 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. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. 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 26, Feb 17, 2023 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Jun 18, 2020—Jun 20, 2020 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 (Polygon) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/16/2023 Page 2 of 3 Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI SuC Stockbridge and Nellis soils, 8 to 15 percent slopes, stony 1.0 74.8% VeB Vergennes clay, 2 to 6 percent slopes 0.3 25.2% Totals for Area of Interest 1.4 100.0% Soil Map—Chittenden County, Vermont Polygon Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 8/16/2023 Page 3 of 3 A–1(210-VI-TR-55, Second Ed., June 1986) Exhibit A: Hydrologic Soil Groups for the United StatesAppendix B Synthetic Rainfall Distributions and Rainfall Data Sources The highest peak discharges from small watersheds in the United States are usually caused by intense, brief rain- falls that may occur as distinct events or as part of a longer storm. These intense rainstorms do not usually ex- tended over a large area and intensities vary greatly. One common practice in rainfall-runoff analysis is to develop a synthetic rainfall distribution to use in lieu of actual storm events. This distribution includes maximum rainfall intensities for the selected design frequency arranged in a sequence that is critical for producing peak runoff. Synthetic rainfall distributions The length of the most intense rainfall period contributing to the peak runoff rate is related to the time of concen- tration (Tc) for the watershed. In a hydrograph created with NRCS procedures, th Appendix A Hydrologic Soil Groups Soils are classified into hydrologic soil groups (HSG’s) to indicate the minimum rate of infiltration obtained for bare soil after prolonged wetting. The HSG’s, which are A, B, C, and D, are one element used in determining runoff curve numbers (see chapter 2). For the conve- nience of TR-55 users, exhibit A-1 lists the HSG classifi- cation of United States soils. The infiltration rate is the rate at which water enters the soil at the soil surface. It is controlled by surface condi- tions. HSG also indicates the transmission rate—the rate at which the water moves within the soil. This rate is controlled by the soil profile. Approximate numerical ranges for transmission rates shown in the HSG defini- tions were first published by Musgrave (USDA 1955). The four groups are defined by SCS soil scientists as follows: Group A soils have low runoff potential and high infil- tration rates even when thoroughly wetted. They consist chiefly of deep, well to excessively drained sand or gravel and have a high rate of water transmission (greater than 0.30 in/hr). Group B soils have moderate infiltration rates when thoroughly wetted and consist chiefly of moderately deep to deep, moderately well to well drained soils with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission (0.15- 0.30 in/hr). Group C soils have low infiltration rates when thor- oughly wetted and consist chiefly of soils with a layer that impedes downward movement of water and soils with moderately fine to fine texture. These soils have a low rate of water transmission (0.05-0.15 in/hr). Group D soils have high runoff potential. They have very low infiltration rates when thoroughly wetted and consist chiefly of clay soils with a high swelling poten- tial, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious material. These soils have a very low rate of water transmission (0-0.05 in/hr). In exhibit A-1, some of the listed soils have an added modifier; for example, “Abrazo, gravelly.” This refers to a gravelly phase of the Abrazo series that is found in SCS soil map legends. Disturbed soil profiles As a result of urbanization, the soil profile may be con- siderably altered and the listed group classification may no longer apply. In these circumstances, use the follow- ing to determine HSG according to the texture of the new surface soil, provided that significant compaction has not occurred (Brakensiek and Rawls 1983). HSG Soil textures A Sand, loamy sand, or sandy loam B Silt loam or loam C Sandy clay loam D Clay loam, silty clay loam, sandy clay, silty clay, or clay Drainage and group D soils Some soils in the list are in group D because of a high water table that creates a drainage problem. Once these soils are effectively drained, they are placed in a differ- ent group. For example, Ackerman soil is classified as A/D. This indicates that the drained Ackerman soil is in group A and the undrained soil is in group D.