A fall-velocity equation

Over the last several decades, there has been a conspicuous increase in the coastal engineering literature in the use of the fall velocity as a variable to characterize sediment. The increasing interest in fall velocity reflects acknowledgment of the fundamental physical importance of this variable in littoral sedimentation processes. One difficulty in using the fall velocity has been that there was no continuous equation to calculate fall velocity over a wide range of conditions. An equation is developed to calculate sediment fall velocities and is shown to fit a large data set well over a wide range of conditions. The equation was carefully calibrated to the quartz sand subset of the data and found to fit that subset very well; rms error was about 8%. The equation also approaches logical limiting values for laminar and turbulent flow regimes. Simple equations to calculate the kinematic viscosity of sea water and freshwater are also given.