Stormwater Report 1

Chapter 2 Estimating Runoff Technical Retease 55 Urban Hydrology for Small Watersheds Table 2-2a Runoff curve numbers for urban areas / Curve numbers for a Cover description hydrologic soil group Average percent Cover type and hydrologic condition impervious area Y A B c D Fully developed urban areas (vegetation established) Open space (lawns, parks, golf courses, cemeteries, etc.) Poor condition (grass cover < 50%)..... 68 79 86 89 Fair condition (grass cover 50% to 75%) 49 69 8 84 Good condition (grass cover > 75%). 39 61 14 80 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) . sesseeesees 98 98 98 98 Paved; open ditches (including right-of-way) 83 89 92 93 Gravel (including right-of-way) 76 85 89 OL Dirt (including right-of-way). 72 82 87 89 Western desert urban areas; Natural desert landscaping (pervious areas only) + . 63 WW 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 districts: Coramercial and business .. 85 89 92 S4 95 Tadusttial .... 72 81 88 ot 93 Residential districts by average lot size: V8 acre or less (town houses) . 65 17 85 90 92 14 acre 38 61 75 83 87 1 acre 30 57 72 81 86 V2 acre 25 54 70 80 85 Jacre .. 20 51 68 79 84 2 acres. 12 46 65 7 82 Developing urban areas Newly graded areas (pervious areas only, no vegetation) ¥... 17 86 a1 o4 Idle lands (ON's are determined using cover types -_ similar to those in table 3-2c). } Average runoff condition, and I, = 0.28. 2 The average percent impervious area shown was used to develop the composite CN'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, 3 CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type. | Composite GN’s for natural desert landscaping should be computed using figures 23 or 2-4 based on the impervious area percentage (CN = 98) and the pervious area CN. The pervious area CN's are assured equivalent ta desert shrub in poor

landscaping should be computed using figures 23 or 2-4 based on the impervious area percentage (CN = 98) and the pervious area CN. The pervious area CN's are assured equivalent ta desert shrub in poor hydrologic condition, 5 Composite CN's ta use for the design of temporary measures during grading and construction should be computed using figure 23 or 2-4 based on the degree of development (impervious area percentage) and the CN's for the newly graded pervious areas. (210-V-TR-5, Second Ed., June 1986)