Pervious Concrete Pavement Design for Sustainable Porous & Permeable Stormwater Drainage

Appendix A:

Curve Numbers

The Curve Number of a site with a variety of different surface covers can be estimated as a composite of Curve Numbers of smaller sections of the site, weighted by area. For example, the composite, or weighted CN of an urbanized site overlying an HSG C soil horizon with an R-4 residential area occupying 60% of the total area, a maintained greenway occupying 20%, and a commercially developed area (with 85% impervious surface) occupying the remaining 20% is 83 [(0.60) 83 + (0.20) 74 + (0.20) 94 =83 (not 83.4; the estimate is not that accurate)]. The CN of an agricultural area in an HSG C soil horizon area composed of 40% wooded lot and 60% meadow would be 71 [(0.40) 72 + (0.60) 71]. In urbanized areas, a further adjustment of the composite CN is possible if desired and if the percent of connected impervious area is known, using the methods provided in TR-55 (SCS 1986).

Cover description		Curve numbers for hydrologic soil group
Cover type & Hydrologic condition	% Impervious area	A	B	C	D
Fully-developed urban areas (vegetation established) Open space (lawns, 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, streets, roofs, etc. (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)		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
⅛ acre (500 m²) or less (townhouse)	65%	77	85	90	92
¼ acre (1000 m²)	48%	61	75	83	87
½ acre (2000 m²)	25%	54	70	80	85
1 acre (4000 m²)	20%	51	68	79	84
2 acres (8000 m²)	12%	46	65	77	82
Agricultural lands:
Meadow – continuous grass; protected from grazing & generally mowed for hay		30	58	71	78
Woods – grass combination (orchard or tree farm)
Poor	< 50% cover	57	73	82	86
Fair	50% to 75% cover	43	65	76	82
Good	> 75% cover	32	58	72	79
Developing urban areas:
Newly graded areas (pervious areas only, no vegetation)		77	86	91	94

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