Dimensionless erosion laws for cohesive sediment

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 ∼ ( τ − τ c ) m "> E ∼ ( τ − τ c ) m E∼(τ−τc)m , where E "> E E is erosion rate, τ "> τ τ is shear stress, τ c "> τ c τc is the threshold shear stress for erosion to occur, and m ≈ 7 / 4 "> m ≈ 7 / 4 m≈7/4 . This result contrasts with the commonly assumed linear erosion law E = k d ( τ − τ c ) "> E = k d ( τ − τ c ) E=kd(τ−τc) , where k d "> k d kd 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 "> k d kd and τ c "> τ c τc , but also of the erodibility parameter k d "> k d kd 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 "> m = 1 m=1 and, in the case of the JET, certain simplifying assumptions about the mechanics of jet scour.