3146. **Lacey`s silt factor (f ) : **A factor introduced by Lacey in 1930 based on the theory that in all regime channels of the same silt grade the ratio V2/R in which V denotes the mean velocity and R the hydraulic mean depth, characterizes turbulence and is constant. When the silt factor is unity, as given by the equation 57.0 = fV2/R, the grade of sand closely approximates to that of regime canals in the northern Punjab, the corresponding value of either Kutter`s or Manning`s N when the hydraulic mean depth is equal to one metre (3.28 feet) being 0.0225 approximately. The Lacey (1958) silt factor is in terms of the mean depth Dm and is written as- f = 0.75 V2/Dm It is thus proportional to the square of the Froude number. It is now fully recognized that the transported load as well as the silt grade must enter in any determination of the silt factor. For a constant charge, the silt factor varies approximately as the square root of the sand particle diameter. |