Dimensionless Erosion Laws for Cohesive Sediment: Fid-Bau Portal

Dimensionless Erosion Laws for Cohesive Sediment

Walder, Joseph S.

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 $E \sim {(τ - τ_{c})}^{m}$ , where $E$ is erosion rate, $τ$ is shear stress, $τ_{c}$ is the threshold shear stress for erosion to occur, and $m \approx 7 / 4$ . This result contrasts with the commonly assumed linear erosion law $E = k_{d} (τ - τ_{c})$ , 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 $τ_{c}$ , 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 $m = 1$ and, in the case of the JET, certain simplifying assumptions about the mechanics of jet scour.