Closure to “Dimensionless Erosion Laws for Cohesive Sediment” by Joseph S. Walder

Abstract

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 ∼ ðτ − τ c Þ m , where E is erosion rate, τ is shear stress, τ c is the threshold shear stress for erosion to occur, and m ≈ 7=4. This result contrasts with the commonly assumed linear erosion law E ¼ k d ðτ − τ c Þ, 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 τ 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 ¼ 1 and, in the case of the JET, certain simplifying assumptions about the mechanics of jet scour.