Evaluation of Drainage Area

Definition: Drainage area, or watershed, is the total number of acres which drain to a common point, such as a culvert, creek crossing or bridge. Determining the acreage in the watershed is important in sizing culverts, locating stream crossings or locating bridges.

Using topographic maps: Topographic maps show changes in elevation by a series of contour lines. These lines can be used to determine which slopes drain through an area. To determine the watershed, it is helpful to remember two things:

• On hilltops, contour lines will form a small, roughly circular shape.
• On contour lines with fingerlike projections, the fingers point uphill.

The watershed can be defined by drawing arrows in the direction of drainage to the common point.

Determining areas: After the watershed is drawn, the number of acres in the area can be estimated. For a topographic map with a scale of 1:24,000 (a 7.5 minute map) the table below can be used as a quick guide.

Guide for Area Estimation on 7.5 Minute (1:24,000 scale) Topographic Maps

Facsimile/Shape	Acres
Pencil Eraser Head	5
Dime	40
Nickel	50
Quarter	70
1” by 1” Square	90

• Using appropriate maps, estimate the drainage area for the stream crossing site.
• Using either a soils map or testing the texture by feel, determine if the drainage is predominantly sandy, loamy or clayey.
• Determine the average slope class (flat, moderate or steep) of the drainage area. Although most drainage areas will be either flat or moderate, do not consider only the crossing site but the whole drainage area.
• Using the table below, determine the runoff for a 5-inch, 24-hour storm for the appropriate drainage area, soil type and slope class.

Peak Runoff from A 5-Inch Storm, in Cubic Feet per Second

Acres		Flat Slopes				Moderate Slopes				Steep Slopes
		Sand	Loam	Clay		Sand	Loam	Clay		Sand	Loam	Clay
5		4	9	15		6	12	19		8	14	20
10		7	14	24		11	21	35		15	27	40
15		9	19	32		14	29	47		21	39	57
20		11	23	40		17	35	60		26	50	75
25		12	26	47		20	42	70		30	60	92
30		14	30	52		23	47	80		35	70	110
35		15	32	60		25	52	90		40	77	125
40		17	35	65		27	57	100		43	85	140
45		18	37	70		30	62	120		47	95	150
50		19	40	75		32	67	140		50	100	165
75		25	52	100		42	87	160		67	130	230
100		30	65	120		50	110	190		80	170	290