GIS and Stormwater Management

Stormwater Runoff
The process by which rainfall becomes runoff is a complicated process with many variables. Theoretical models pour rainfall amount and distributions over the losses associated with land characteristics into a computation and produce runoff amounts. Two methods for determining runoff in the United States are generally used - the Rational Method and the Soil Conservation Service (SCS) Hydrologic Method. Both methods have their advantages, disadvantages, and limitations and will produce good results if applied correctly. The Rational Method uses a simple equation to produce peak discharges for drainage basins up to 200 acres, while the SCS Method requires several equations but can produce hydrographs for drainage basins as large as 2000 acres. Both methods use a variable to relate the land characteristics such as soil types and coverage to the losses such as interception and infiltration that occur prior to runoff. The Rational Method refers to this variable as a runoff coefficient “C” and the SCS Method calls it a Curve Number, CN. To determine a “C” value for a drainage basin, a weighted average is calculated for the entire basin using the runoff coefficients in Table 1.

Table 1. Runoff Coefficients for Various Land Uses

Description of Area		Runoff Coefficients
Downtown Areas	Commercial	0.70 – 0.95
	Neighborhood Areas	0.50 – 0.70
Residential	Single-Family	0.30 – 0.50
	Multi-Family Attached	0.60 – 0.75
	Multi-Family Detached	0.40 – 0.60
	Suburban	0.25 – 0.40
	˝ acre lots or greater	0.30 – 0.45
	Apartments	0.50 – 0.70
Industrial:	Light Areas	0.50 – 0.80
	Heavy Areas	0.60 – 0.90
Parks, Cemeteries		0.10 – 0.25
Playgrounds		0.20 – 0.40
Unimproved Areas		0.10 – 0.30
Railroad Yard Areas		0.20 – 0.40
Streets:	Asphalt	0.70 – 0.95
	Concrete	0.80 – 0.95
Drives and Sidewalks		0.75 – 0.85
Roofs		0.75 – 0.95

Source: Hydrology, Federal Highway Administration, HEC No. 19, 1984

Similarly, a curve number for a drainage basin requires the calculation of a weighted average; however, unlike the range of Runoff Coefficients, the curve number has specific values for land uses on different soil types. The Soil Conservation Service defines the four Hydrological Soil types as follows:

Group A	Soils having a low runoff potential due to high infiltration rates. These soils consist primarily of deep, well-drained sands and gravels.
Group B	Soils having a moderately low runoff potential due to moderate infiltration rates. These soils consist primarily of moderately deep to deep, moderately well to well drained soils with moderately fine to moderately course textures.
Group C	Soils having a moderately high runoff potential due to slow infiltration rates. These soils consist primarily of soils in which a layer exists near the surface that impedes infiltration or soils with moderately fine to fine textures.
Group D	Soils having a high runoff potential due to very slow infiltration rates. These soils consist primarily of clays with high swelling potential, soils with high water tables, soils with claypan or a clay layer near the surface, and soils with bedrock at or near the surface.

Using the different soil types and the appropriate land use, a Curve Number can be determined for a drainage basin based on the values in Table 2.

Table 2. Runoff Curve Numbers for Urban Areas

Curve Number for Hydrologic Soil Group
Description of Area		% Imp	A	B	C	D
Lawns, open spaces, parks, golf courses cemeteries, etc.
	Good condition 75% or more grass coverage		39	61	74	80
	Fair condition 50% to 75% grass coverage		49	69	79	84
	Poor condition less than 50% grass coverage		68	79	86	89
Parking Lots, Roofs and Driveways			98	98	98	98
Streets and Roads:
	Paved w/curbs & storm sewers excl. ROW		98	98	98	98
	Gravel including ROW		76	85	89	91
	Dirt including ROW		72	82	87	89
	Paved w/open ditches including ROW		83	89	92	93
Commercial and Business Areas		85%	89	92	94	95
Industrial and Manufacturing Districts		72%	81	88	91	93
Residential
	1/8 acre lot or less, Apartments and Townhouses	65%	77	85	90	92
	Ľ acre lot	38%	61	75	83	87
	1/3 acre lot	30%	57	72	81	86
	˝ acre lot	25%	54	70	80	85
	1 acre lot	20 %	51	68	79	84
	Agricultural Land Uses
	Pasture or Range Land
	Good condition, protected from grazing		39	61	74	80
	Fair condition, some grazing, mowed for hay		49	69	79	84
	Poor condition, used for grazing		68	79	86	89
Cultivated land with conservation treatments			62	71	78	81
Cultivated land without conservation treatments			72	81	88	91
Wood or forested land
	Mature stand with good cover		25	55	70	77
	Thin stand, with poor cover		45	66	77	83

Source: National Engineering Handbook, Section 4, “Hydrology” Chapter 9, August 1972.