Dimensionless Erosion Laws for Cohesive Sediment

A method of achieving a dimensionless collapse of erosion-rate data for cohesive sediments is proposed and shown to work well for data collected in flume-erosion tests on mixtures of sand and mud (silt plus clay sized particles) for a wide range of mud fraction. The data collapse corresponds to a dimensional erosion law of the form E similar to (tau - tau(c) )(m), where E is erosion rate, tau is shear stress, tau(c) is the threshold shear stress for erosion to occur, and m approximate to 7/4. This result contrasts with the commonly assumed linear erosion law E = k(d) (tau - tau(c)), where k(d) is a measure of how easily sediment is eroded. The data collapse prompts a re-examination of the way that results of the hole-erosion test (HET) and jet-erosion test (JET) are customarily analyzed, and also calls into question the meaningfulness not only of proposed empirical relationships between k(d) and tau(c), but also of the erodibility parameter k(d) itself. Fuller comparison of flume-erosion data with hole-erosion and jet-erosion data will require revised analyses of the HET and JET that drop the assumption m = 1 and, in the case of the JET, certain simplifying assumptions about the mechanics of jet scour. (C) 2015 American Society of Civil Engineers.