Drain Commissioner

Accepted Coefficiency Factors

Listed below are the accepted coefficiency factors for roads, roofs and driveways.

Type	Coefficiency Factors
Roads
Asphalt Or Concrete With Curb-Gutters	95 -100%
Asphalt Or Concrete With Roadside Ditches	90 - 95%
Gravel Or Stone With Roadside Ditches	80 - 90%
Undeveloped Land
Flat Pervious Soils With Vegetation	5 - 10%
Rolling Pervious Soils With Vegetation	10 - 15%
Flat Moderately Pervious Soil With Vegetation	15 - 20%
Rolling Moderately Pervious Soil With Vegetation	20 - 30%
Flat Impervious Soil with Vegetation	30 - 40%
Rolling Impervious Soil With Vegetation	40 - 50%
Any Lands Without Vegetation Add An Additional	20%
Developed Land
Roofs, Driveways And Parking Lots That Are Asphalt Or Concrete	90 - 95%
Yards, Roadside Woods, Outlots, Playground, Swamps, Parks & Etc. Refer To Undeveloped Land	05 - 70%

Group Soil Types

A. Pervious Sand
B. Moderately Pervious Sandy Loam
C. Impervious Clay

Only with approval of the Drain Commissioner will a Group C (Impervious Soil) ever be used for retention basins.

Calhoun County
315 West Green Street
Marshall, MI 49068
Phone: (269) 781-0700
Fax: (269) 781-0140

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