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Home My WebLink About05-003 Swift Stormwater Supplemental Information 3-2-17SUPPLEMENTAL INFORMATION TO ACCOMPANY STORMWATER REPORT #63 TEACHER SHORE ROAD, YARMOUTHPORT, MA OF MA8 8 9cti �c) DANIELA. �N o OJALA U CIVIL No.46502 G/STrcO�� SS/ONAL EAG DATE: 3-2-2017 Prepared by: down cape engineering, inc. 939 Route 6a Yannouthport, MA 02675 Ph. 1-508-362-4541 Fax 1-508-362-9880 05-003 #63 Thacher Shore Rd, ® -7_ ® 1-7 Prepared for: David Swift Subdivision#1648H # 63 Thacher Shore Road, Yarmouthport Date: 3-2-17 DRAINAGE SUMMARY- Supplemental Information This summary is designed to supplement the drainage calculations and information already submitted in support of the Yarmouth Conservation Commission's review of the Yarmouth Planning Board approved Subdivision #1648H for the gravel roadway serving a single family home at #63 Thacher Shore Road in Yarmouthport. The drainage system specified for the 12' wide gravel road (probably better described as a driveway) consists of a low impact stormwater design which utilizes many of the Best Management Practices suggested by the State Stormwater Management Guidelines, such as a pervious gravel surface and grass lined swales. Because the roadway follows a natural ridge, and will be built up slightly in places, very little runoff from outside the roadway will affect the drive. Guidance from standard TR55/TR20 tables inside the HydroCAD program were utilized in selecting the Curve Numbers for modeling the stormwater performance of the proposed system and existing conditions. The existing conditions consist of a compacted dirt driveway and shoulders of approximately 8 feet in width from Thacher Shore Road to the crown of the hill. There are existing wooded areas off the shoulders of the existing drive. The Curve Number for the woods was selected as 36, based on generally pervious soils and the moderate slopes. The existing dirt drive and shoulders was judged to represent a Curve Number of 45. A HydroCAD model of the watershed DA1 which is the area from the crown of the hill to Thacher Shore Road was run utilizing a time of concentration of six minutes, estimating approximately 1.5 ft/sec. of sheet and overland flow on the 560' area. A 25 year storm generated a peak flow of only 0.04 cfs due to the largely pervious nature of the watershed. This run is attached and labeled DA1 Existing. The proposed gravel base and washed stone wearing surface is estimated to be quite pervious, with a drainage Curve Number (CN) of 30. The grass lined swales and check dams are anticipated to be able to infiltrate the small amount of water which flows from the crown to either side of the narrow pervious gravel surface during rain events. These swales were modeled with a CN of 25 due to the presence of the check dams, please see design plans for details on construction and cross sections. A robust species of grass such as New England Erosion Control Mix will be utilized. A HydroCAD model of the watershed DAIwas run utilizing a time of concentration of eight minutes to account for the slightly flatter runs, check dam storage, and washed stone surface. A 25 year storm (Type III Barnstable County 24 hour rainfall of 5.7") yielded a maximum flow at the study point adjacent Thacher Shore Road of only 0.01 cfs. This HydroCAD output is attached and labeled DA1 Proposed. This modeling indicates that by eliminating impervious pavement, and instead selecting pervious materials and utilizing grass lined retention swales with check dams, the runoff from the site is reduced significantly over existing conditions. Thus based on the attached analysis and previously submitted materials, the design is shown to meet the spirit and intent of the Yarmouth Conservation Stormwater Management Regulations as well as the State Stormwater Management Guidelines, even though the project does not technically trigger review by these standards. Prepared by: \A OF All C o� DANIELA. yes o OJALA U CIVIL Cn No.46502 o �0 FG/STE' �SS/ONAL EAG L Daniel A. Ojala PLS, PE Down Cape Engineering, Inc. date DRAINAGE o exa jSF Fl- Appendix A2: Runoff Curve Numbers (continued) Table 2-2c Runoff curve numbers for other agricultural lands 1/ Farmsteads—buildings, lanes, driveways, — and surrounding lots - 1 Average runoff condition, and Ia = 0.2S. 2 Poor: <50%) ground cover or heavily grazed with no mulch. Fair.' 60 to 75% ground cover and not heavily grazed. Good.' > 75% ground cover and lightly or only occasionally grazed. 3 Poor. <50% ground cover. Fair. 50 to 751/6 ground cover. Good: >75% ground cover. 4 Actual curve number is less than 30; use CN = 30 for runoff computatio asttlre cover. Other combinations of conditions maybe computed r, CN's shown were computed for areas with 50%woods and 50% grass (p re) cover. the CN's for woods and pasture. or regular burning. s Poor. Forest litter, small trees, and brush are destroyed by heavy grazin g gul Fair. Woods are grazed but not burned, and some forest litter covers the soil. Good: Woods are protected from grazing, and litter and brush adequately cover the soil. This appendix reprinted from S.C.S. TR -55, revised 1986. Curve numbers for _ ---- hydrologic soil group Cover description ------ ___ — ------- Hydrologic C D condition A B Cover type Pasture, grassland, or range—continuous Poor Fair 68 49 79 69 86 79 89 84 forage for grazing. V Good 39 61 74 80 30 68 71 78 Meadow—continuous grass, protected from — grazing and generally mowed for hay. Brush—brush-weed-grass mixture with brush Poor 48 35 67 66 77 70 83 77 the major element. a Fair Good 30V 48 65 73 Woods—grass combination (orchard Poor Fair 57 43 73 65 82 76 86 82 or tree farm). -I/ Good 32 68 72 79 Woods. V Poor Fair 45 '3 66 60 77 73 83 79 Good 30 Al 55 70 77 59 74 82' 86 Farmsteads—buildings, lanes, driveways, — and surrounding lots - 1 Average runoff condition, and Ia = 0.2S. 2 Poor: <50%) ground cover or heavily grazed with no mulch. Fair.' 60 to 75% ground cover and not heavily grazed. Good.' > 75% ground cover and lightly or only occasionally grazed. 3 Poor. <50% ground cover. Fair. 50 to 751/6 ground cover. Good: >75% ground cover. 4 Actual curve number is less than 30; use CN = 30 for runoff computatio asttlre cover. Other combinations of conditions maybe computed r, CN's shown were computed for areas with 50%woods and 50% grass (p re) cover. the CN's for woods and pasture. or regular burning. s Poor. Forest litter, small trees, and brush are destroyed by heavy grazin g gul Fair. Woods are grazed but not burned, and some forest litter covers the soil. Good: Woods are protected from grazing, and litter and brush adequately cover the soil. This appendix reprinted from S.C.S. TR -55, revised 1986.