APPENDIX C-3: RUNOFF CURVE NUMBERS AND PERCENT IMPERVIOUSNESS VALUES

Runoff Curve Numbers and Percent Imperviousness Values*
Cover Description		Curve Numbers for Hydrologic Soil Group**
Land Use/Cover Type	Average Percent Impervious Area	A	B	C	D
Agriculture:		Refer to Table 2-2b in source document (TR 55) by crop type and treatment
Impervious areas:
Paved parking lots, roofs, driveways and the like (excluding right-of-way)		98	98	98	98
Open space (lawns, parks, golf courses, cemeteries and the like):
Good condition (grass cover greater than 75%)		39	61	74	80
Residential districts by average lot size:
1/8 acre or less (townhouses)	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
Streets and roads:
Gravel (including right-of-way)		76	85	89	91
Paved; open ditches (including right- of-way)		83	89	92	93
Paved; curbs and storm sewers (excluding right-of-way)		98	98	98	98
Urban districts:
Commercial and business	85	89	92	94	95
Industrial	72	81	88	91	93
Woods		30	55	70	77
*   Source: Natural Resources Conservation Service Technical Release No. 55, Second Edition, June 1986
**   Hydrologic Soil Groups based on the county soil survey, latest edition

 

(Ord. 226-2007, passed 5-7-2007)