The viscosity of a fluid has a significant affect on the fall velocity of a particle within that fluid. In clear water, the kinematic viscosity is on the order of 1 X 10-5 ft2/s, however, when a high concentration of fine sediment, particularly clay particles, is present, the viscosity will increase, in much the same way as when the water temperature is reduced. Colby (1964) proposed an adjustment factor to account for high concentration of fines, as well as temperature, which is shown in the figure below.
HEC-RAS provides and field for the user to enter the concentration of fine sediments. This is an optional field, and, if left blank, bypasses the Colby adjustment factor calculations. Concentration magnitudes are entered in parts per million (ppm).
