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Home My WebLink AboutSW6211206_Design Calculations_20220314BAYWOOD TERRACE SUBDIVISION SANDEROSA ROAD, EASTOVER STORMWATER NARRATIVE STORMWATER CALCULATIONS 11-28-21 Revised 1-03-21 Revised 1-19-22 PROJECT DESCRIPTION Baywood Terrace is a proposed 22-lot single-family subdivision on a 10.35 acre tract off Sanderosa Road in Cumberland County. The property is bounded by Sanderosa Road (SR 1833) to the south, vacant farmland to the east, one single-family property and vacant land to the west and the Stone Creek single-family subdivision to the north. The entire site is cleared with reasonably well - established ground cover. The site is extremely flat, slopes from north to south in the direction of Sanderosa Road, and ranges in elevation from approximate elevation 143 at the northernmost property line to approximate elevation 140 at Sanderosa Road. There are no existing impervious surfaces on the site. STORMWATER MANAGEMENT PLAN The property will be developed with one cul-de-sac street using NCDOT standards for a residential collector subdivision road. This standard calls for a 50-foot right-of-way with a 20-foot wide pavement section and side ditches. The topography of the site dictates that the side ditches will have an extremely flat longitudinal slope with the bottom of the ditch being approximately 18 inches below the edge of pavement. All of the lots will be graded for slab -on -grade home construction with approximately one-half of each lot draining to the rear (away from the roadway ditch) and one-half draining to the front (towards the roadway ditch). A typical house footprint and driveway section (see detail on sheet SP6) was used to calculate expected runoff from each lot once fully developed. For the purposes of the stormwater design, it was assumed that all of the impervious surfaces on each lot will drain towards the roadside ditch. The site is nearly 100% Candor Sand according to the Cumberland County Soil Survey. These are Group A soils with a high -to -moderately -high capacity to transmit water. Virtually the entire site is suitable for septic systems per a report by Southeastern Soil & Environmental Associates. PART A — STORMWATER CALCULATIONS FOR ROADWAY DITCHES Use the Rational method to determine runoff amounts to the roadway ditches and Manning's equation to determine stormwater velocity based on the ditch section. Area of the pavement is 22,212 sf with one-half flowing to the ditch on the western side of the roadway and one-half flowing to the ditch on the eastern side of the roadway. Lots 1-11 will drain the ditch on the eastern side and lots 12-22 will drain to the western side. Total area of lots 1-11 is 190,391 square feet or 4.37 acres. Total area of lots 12-22 is 207,725 square feet or 4.77 acres. Per the typical lot development detail on sheet SP6, each lot will have 3,540 square feet of impervious area draining to the roadway ditch. Total impervious area upon full development of the subdivision is (22)(3540) + 22212 = 100,092 sf = 2.30 acres. Percent impervious = 2.30/10.35 = 22.2% = low density project. Calculate total flow and velocity for the 10-year storm in the eastern roadway dam: Impervious = 11 lots x 3540 square feet per lot = 38,940 sf = 0.89 acres Yard area = (0.5)(190391) = 95,196 sf = 2.18 acres Total area draining to eastern ditch = 38940 + 95196 = 134136 sf = 3.08 acres Weighted C = 38940/134136(0.95) + 95196/134136(0.10) = 0.35 Determine precipitation intensity for 10-year storm using precipitation frequency data server at http://hdsc.nws.noaa.gov/hdsc/pfds/orb/nc pfds.html. Longitude and latitude of approximate center of property is 35-04-03N, 78-45-46W. 10-year (5 minute time of concentration, 24-hour duration) = 7.96 inches per hour Qio = CIA Qio = (0.35)(7.96)(3.08) = 8.6 cfs Ditch varies in elevation from 143 to 141 over an approximate length of 1007 linear feet. Longitudinal slope of roadway ditch = (143-141)/1007 = 0.2% Using Manning's Equation by trial and error, solve for area A Use 0.03 for Manning's n IfA=3.5 sf, then P = 7.29' R=A/P=0.48 Qio = 1.49/0.03(3.5)(0.482/3)(0.002)1/2 = 4.8 cfs If A = 5.47 sf, then P = 9.11' R=A/P=0.60 Qio = 1.49/0.03(5.47)(0.602/3)(0.002)1/2 = 8.6 cfs Q=VA V = Q/A = 8.6/5.47 = 1.6 fps therefore non -erosive Calculate total flow and velocity for the 10-year storm in the western roadway. dam: Impervious = 11 lots x 3540 square feet per lot = 38,940 sf = 0.89 acres Yard area = (0.5)(207725) = 103,863 sf = 2.38 acres Total area draining to western ditch = 38940 + 103863 = 142803 sf = 3.28 acres Weighted C = 38940/142803(0.95) + 103863/142803(0.10) = 0.33 Qio = CIA Qio = (0.33)(7.96)(3.28) = 8.6 cfs Ditch varies in elevation from 143 to 141 over an approximate length of 1007 linear feet. Longitudinal slope of roadway ditch = (143-141)/1007 = 0.2% From calculations for western ditch above: V = Q/A = 8.6/5.47 = 1.6 fps therefore non -erosive Calculate flow and velocitv for the 10-vear storm for the shoulder area Sanderosa Road: Distance from start of water line construction at Wild Pine Drive to inlet side of pipe at entrance to Baywood Terrace = 900'. Per Section A -A, shoulder of roadway is 6' wide at 8% slope, then 18" to silt fence location at 4:1 slope. Shoulder section area = (7.5)(900) = 6,750 square feet. Sanderosa Road is 21' of asphalt. Area of one-half of asphalt roadway = (21/2)(900) = 9,450 square feet. Total drainage area = 6750 + 9450 = 16,200 sf = 0.37 acres. By judgment use C = 0.60 for bare soil shoulders during water line construction. Weighted C = 9450/16200(0.95) + 6750/16200(0.6) = 0.80 Qio = CIA Qio = (0.80)(7.96)(0.37) = 2.4 cfs Shoulder ranges in elevation from approximate elevation 142 at Wild Pine Drive to approximate elevation 140 at the Baywood Terrace entrance. Longitudinal slope = 2/900 = 0.22% Using Manning's Equation by trial and error, solve for area A Use 0.03 for Manning's n If A = 5.5 sf, then P = 11.64' R=A/P=0.47 Qio = 1.49/0.03(5.5)(0.472/3)(0.0022)1/2 = 7.5 cfs IfA=2.Osf, then P=5.12' R=A/P=0.39 Qio = 1.49/0.03(2.0)(0.392i3)(0.0022)1/2 = 2.5 cfs This represents an area from the edge of the 8% shoulder slope to the silt fence. Horizontal distance = 4' of water flowing in the "ditch" created by the silt fence. V = Q/A = 2.5/2.0 = 1.24 fps therefore non -erosive velocity PART B — EROSION CONTROL NARRATIVE The site is extremely flat with sandy soils and with no on -site stormwater management facilities being utilized other than typical roadside ditches. Each lot will be constructed with a slightly elevated flat pad for slab -on -grade construction. Drainage is designed so that generally the rear one- half of each lot sheet drains away from the roadway and towards the perimeter lot lines. The front one-half of the lots will generally sheet drain towards the roadway and the roadside ditches. By judgment no sediment basins will be used for erosion control measures. Silt fence will be utilized around the property perimeter and the extremely flat slopes and sandy soil conditions will minimize any runoff during land -disturbing activities. Silt fence and construction entrances will be utilized on each individual lot during the home construction per the detail on sheet SP6. Individual erosion control plans will be submitted for individual lots prior to commencement of home construction. Water line construction along Sanderosa Road is in the roadway shoulder, and runoff from the disturbed area of the shoulder will be treated with silt fence. See Section A -A on sheet SP2.