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 |
Estimating Storm Runoff for Culvert Sizing
- 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 |