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Strickler: Developed based on data collected from gravel-beds in Switzerland and France, and fixed-bed laboratory data. Note: this form of expression for Manning's n is called the Strickler approach and equations using this approach are generally called Strickler-type equations. This equation only calculates boundary resistance because the only physical variable involved is bed material size. Strickler-type equations are applicable to wide-shallow channels (width-depthratio greater than 10) where the hydraulic radius can be replaced by the mean depth. Strickler-type equations are not suitable when the bedform effect is predominant (Yen, 1991). Also, since various numerical values of the constant exist, some are incorrectly converted to English units because of differing dimensions for the variables and constant (Yen, 1991).

Reference:
Chow, V.T., 1959, Open Channel Hydraulics: McGraw-Hill Book Co. New York, 680 p.

Yen, B.C., 1991, Hydraulic resistance in open channels; in Channel Flow: Centennial of Manning's Formula (B.C. Yen, editor), Water Resources Publications, Littleton, CO., p. 1-135.