Erosion threshold of sand–mud mixtures

Abstract

Results of a large number of erosion tests on artificially generated and relatively dense sand–mud mixtures are presented. Soil sample compositions are varied concerning clay–silt and sand–silt ratio, and clay mineralogy. The experimental set-up consists of a re-circulating small-scale rectangular erosion flume with unidirectional flow conditions. The erosion threshold and erosion rate are studied through step by step increasing the flow rate during a test. Results clearly indicate time-decreasing erosion during which individual flocs are randomly eroded, and time-independent (steady) erosion during which both sand and mud particles are continuously and uniformly eroded. These two erosion types appear to be floc and surface erosion, respectively ( Winterwerp and Van Kesteren, 2004). Floc erosion relates to the stochastic character of both the flow conditions and (surficial) sediment strength, whereas surface erosion relates to the plasticity index, which is a bulk soil mechanical parameter characterizing cohesiveness. The surface erosion threshold is discussed following a geotechnical approach, which argues that surface erosion is a drained process. This implies that cohesiveness rather than packing density is important for the erosion threshold, which is confirmed by the experimental data. Simultaneously with the erosion tests, also the undrained shear strength of the applied soil samples was determined. A model is proposed and validated to predict the undrained shear strength as function of the granular porosity in combination with the plasticity index. The comparison of the undrained shear strength with the surface erosion threshold further confirms the applicability of a geotechnical approach to understand the erosion of mixed sediments. Finally, the study provides a valuable data set that can be used as a reference for future research on erosion behavior of (natural) sediment mixtures.