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Chapter 2 Stormwater Management Appendix - Fauquier County

Chapter 2 Stormwater Management Appendix - Fauquier County

Design Standards Manual

Design Standards Manual 1 st Edition NRCS Runoff Curve Numbers Refer to TR-55 for general assumptions and limitations. For specific footnotes, see TR-55 Table 2-2a - d. Figure 2.7 Runoff Curve Numbers for Urban Areas* Cover Description Curve Numbers for Hydrologic Soil Groups Cover Type and Hydrologic Condition Average Percent A B C D Fully developed urban areas (vegetation established): Open space (lawns, parks, golf courses, cemeteries, etc.) Good Condition (grass cover > 75%) Impervious areas: Paved Parking lots , roofs, driveways, etc. (excluding right-of-way) Streets and Roads: Paved; curbs and storm sewers (excluding right-ofway) Paved; open ditches (including right-of-way) Gravel (including right-of-way) Dirt (including right-of-way) Urban districts: Commercial and business Industrial Residential districts by average lot size: 1/8 acre or less (town houses) ¼ acre 1/3 acre ½ acre 1 acre 2 acres Developing Urban Areas: Newly graded areas (pervious areas only, no vegetation) Idle lands (CN’s are determined using cover types similar to those in TR-55 Table 2-2c) Impervious Area * Average runoff condition and Ia = 0.2S Source: TR-55 adapted Table 2-2a. Revisions Adopted: July 12, 2012 Design Standards Manual Chapter 2: Drainage D-44 85 72 65 38 30 25 20 12 39 98 98 83 76 72 89 81 77 61 57 54 51 46 77 61 98 98 89 85 82 92 88 85 75 72 70 68 65 86 74 98 98 92 89 87 94 91 90 83 81 80 79 77 91 80 98 98 93 91 89 95 93 92 87 86 85 84 82 94

Design Standards Manual 1 st Edition Figure 2.8 Runoff Curve Numbers for Cultivated Agricultural Lands* Cover Description Curve Numbers for Hydrologic Soil Groups Cover Type Treatment Hydrologic A B C D Condition Fallow Bare Soil - 77 86 91 94 Crop residue cover (CR) Poor 76 85 90 93 CR Good 74 83 88 90 Row Crops Straight row (SR) Poor 72 81 88 91 SR Good 67 78 85 89 SR + CR Poor 71 80 87 90 SR + CR Good 64 75 82 85 Contoured (C) Poor 70 79 84 88 C Good 65 75 82 86 C + CR Poor 69 78 83 87 C + CR Good 64 74 81 85 Contoured & Terraced (C&T) Poor 66 74 80 82 C&T Good 62 71 78 81 C&T + CR Poor 65 73 79 81 C&T + CR Good 61 70 77 80 Small grain SR Poor 65 76 84 88 SR Good 63 75 83 87 SR + CR Poor 64 75 83 86 SR + CR Good 60 72 80 84 C Poor 63 74 82 85 C Good 61 73 81 84 C + CR Poor 62 73 81 84 C + CR Good 60 72 80 83 C&T Poor 61 72 79 82 C&T Good 59 70 78 81 C&T + CR Poor 60 71 78 81 Closeseeded or broadcast legumes or rotation meadow Source: TR-55, Table 2-2b C&T + CR Good 58 69 77 80 SR Poor 66 77 85 89 SR Good 58 72 81 85 C Poor 64 75 83 85 C Good 55 69 78 83 C&T Poor 63 73 80 83 C&T Good 51 67 76 80 * average runoff condition and Ia = 0.2S Revisions Adopted: July 12, 2012 Design Standards Manual Chapter 2: Drainage D-45

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