Erosion Behavior of Sand‐Silt Mixtures: Revisiting the Erosion Threshold

Abstract

AbstractThe erosion threshold, beyond which bed sediments start to move, is a key parameter describing sediment transport processes. For silt‐dominated mixtures, in which the grain size is between sand and clay, existing experimental studies exhibit contradictory observations. That is, the erosion was either sand‐like or clay‐like, suggesting transitional erosion behavior. To explore the underlying mechanism of the transitional erosion behavior of silt‐sized sediment, we revisited the topic of the erosion threshold of sand‐silt mixtures by carrying out a series of erosion experiments for different bed compositions. The results suggest that there exists a critical silt content of approximately 35%, separating two domains. Below this critical value, the critical bed shear stress follows the Shields criterion, whereas above this value, the erosion threshold of a mixed bed increases abruptly and remains relatively constant with a further increase in silt content. By combining with existing data, we found that the proposed critical silt content acts as a tipping point, beyond which the mixed bed shifts from a sand‐dominated to a silt‐dominated domain. For the silt‐dominated domain, a stable silt skeleton can be formed by attraction forces that resist erosion. However, the attraction forces are too weak to form a stable silt skeleton when the silt content is too small. Based on this finding, a modified critical bed shear stress formula is proposed for silt‐dominated mixtures, which results in a better agreement with experimental data (an averaged bias of 10%), performing better than existing formulas (larger than 30%).