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Chapter 2 Estimating Runoff Technical Release 55 Urban Hydrology for Small Watersheds Table 2-2a Runoff curve numbers for urban areas U 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,etc.) ': Poor condition(grass cover<50%).......................................... 68 79 86 89 Fair condition(grass cover 50%to 75%) .................................. 49 69 79 84 Good condition(grass cover>75%)......................................... 39 61 74 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) ................................................................................ 98 98 98 98 Paved;open ditches(including right-of-way).......................... 83 89 92 93 Gravel(including right-of-way)................................................. 76 85 89 91 Dirt(including right-of-way)...................................................... 72 82 87 89 Western desert urban areas: Natural desert landscaping(pervious areas only)-V..................... 63 77 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: 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).......................................................... 65 77 85 90 92 1/4 acre ................................................................................................ 38 61 75 83 87 1/3 acre ................................................................................................ 30 57 72 81 86 1/2 acre ................................................................................................ 25 54 70 80 85 1 acre ................................................................................................... 20 51 68 79 84 2 acres.................................................................................................. 12 46 65 77 82 Developing urban areas Newly graded areas (pervious areas only,no vegetation)5/................................................................ 77 86 91 94 Idle lands(CN's are determined using cover types similar to those in table 2-2c). 1 Average runoff condition,and I8=0.2S. 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. 4 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 design of temporary measures during grading and construction should be computed using figure 2-3 or 2-4 based on the degree of development(impervious area percentage)and the CN's for the newly graded pervious areas. (210-VI-TR-55,Second Ed.,June 1986) 2-5