V4 11 Stormwater Report Rev 3 1 26 2023

Chapter 2 Estimating Runoff, ‘Technical Release 5i Urban Hydrology for Small Watersheds Table 2-2a — Runoff curve numbers for urban areas / Curve numbers for — Cover description hydrologic. soil group Average percent Cover type and hydrologic condition impervious area ¥ A B c D Fully developed urban areas (vegetation established) Open space (lawns, parks, golf courses, cemeteries, ete.) Poor condition (grass cover < 50%) .. 79 86 89 Fair condition (grass cover 50% to 75%) 69 79 84 Good condition (grass cover > 75%) 61 74 380 Impervious areas: Paved parking lots, roofs, driveways, etc. (excluding right-of-way) 98 98 98 98 Streets and roads Paved; curbs and storm sewers (excluding right-of-way) 98 98 98 98 Paved; open ditches (including right-of-way) 83 89 92 93 Gravel (including right-of-way) 76 85 389 OL Dirt (including right-of-way) .. 72 82 87 39 Western desert urban areas: Natural desert landscaping (pervious areas only) -! 63 17 85 88 Artificial desert landscaping (impervious weed barrier, desert shrub with 1- to 2-inch sand or gravel mulch and basin borders) .. 96 96 96, 96 Urban distric Commercial and business 85 89 92 94 95 Industrial .. 72 81 88 91 93 Residential districts by average lot size: 1/8 acre or less (town houses)... 77 85 90 92 WA acre .. 61 75 83 87 1 acre .. 57 72 81 86 1/2 acre .. 54 70 80 85 lacre .. 51 68 79 84 2 acres 46 65 7 82 Developing urban areas Newly graded areas (pervious areas only, no vegetation) . 7 86 91 94 Idle lands (CN's are determined using cover types similar to those in table 2-2c). 1 Average runoff condition, and I, + The average percent impervious area shown was used to develop the composite (N's. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. CN's for other combinations of conditions may be computed using figure 2-3 or 2-4. 's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type. ! Composite CN’s for natural desert landscaping should be computed using figures 2-3 or 2-4 based on the impervious area percentage (CN = 98) and the pervious area CN. The pervious area CN’s are assumed equivalent to desert shrub in poor hydrologic condition. 5 Composite CN's to use for the

based on the impervious area percentage (CN = 98) and the pervious area CN. The pervious area CN’s are assumed equivalent to desert shrub in poor hydrologic condition. 5 Composite CN's to use for the design of temporary measures during grading and construction should be computed using figuré based on the degree of development (impervious area percentage) and the CN's for the newly graded pervious areas. a or 24 (210-VETR- », Second Ed., June 1986)