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Watershed Size Method
Procedure:
1) From Table D below, for the given watershed area, soil and slope conditions , read corresponding drainage area i.e., cross-section area of pipe required for 2-1/2 inches of rainfall per hour.
2) Select culvert size from Table E (next page) that has at least the required area determined above.
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Table D: Drainage for 2 1/2 Inches/Hour Rainfall (Sq.Ft.)*
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Watershed
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Site Conditions (Soil Type & Slopes)
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Watershed
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Steep
(25%+)
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Moderate
(15-25%)
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Gentle
(0-15%)
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Flatland
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Soils
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Impervious
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Heavy
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Moderate
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Light
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Sandy
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10
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3.4
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2.6
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1.9
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1.2
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0.7
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20
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5.8
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4.3
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3.2
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2.0
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1.2
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30
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8.0
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5.9
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4.4
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2.8
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1.6
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40
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9.9
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7.3
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5.4
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3.5
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2.0
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50
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11.6
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9.7
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6.4
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4.1
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2.3
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60
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13.4
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10.1
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7.4
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4.7
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2.7
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70
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15.0
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11.2
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8.3
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5.3
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3.0
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80
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16.6
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12.4
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9.2
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6.3
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3.3
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90
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18.2
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13.6
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10.1
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6.3
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3.6
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100
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19.7
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14.7
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10.8
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6.8
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3.9
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200
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33.2
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24.9
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18.4
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11.7
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6.6
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300
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45.7
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33.6
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27.1
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15.8
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9.0
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400
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56.0
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42.0
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30.5
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19.5
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11.2
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500
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66.8
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49.4
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36.6
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23.2
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13.2
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1000
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113.0
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88.9
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62.1
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39.4
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22.4
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* Modified from Talbot's Formula for a 2 1/2-inch/hr. rainfall event.
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