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

The 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%). Plain Language Summary: When exposed to a certain flow velocity, sediment particles are dislodged from the seabed, namely, the erosion threshold. Attributed to sizes, shapes, minerals, etc., sediments of different grain sizes behave differently, resulting in various erosion behaviors. Sand particles (62.5–2,000 μm) are not sticky and erode particle by particle, whereas clay particles (<4 μm) are sticky and collectively erode as chunks. Silt (4–62.5 μm), of which the grain size is between sand and clay, exhibits either sand‐like or clay‐like behavior, suggesting transitional erosion behavior. Coastal sediments are usually mixtures of clay, silt, and fine sand. Different bed compositions result in different erosion behaviors, leading to a variety of bed forms, morphological patterns, etc. Therefore, a thorough understanding of the erosion behavior of sediment mixtures is of great significance to the topic of coastal sediment transport, which further benefits coastal geomorphology, ecology, etc. This study indicates that, for sand‐silt mixtures, a critical silt contents exists, beyond which the bed mixtures shift from a sand‐like erosion behavior to a clay‐like erosion behavior. The widely adopted parameterized expression of the erosion threshold (namely, the Shields curve) is modified to mimic the transitional erosion behavior of sand‐silt mixtures by considering the effects of silt content. Key Points: Silt content is an important factor controlling the erosion of sand‐silt mixtures in regards to noncohesive or cohesive propertiesThere exists a critical silt content of 35%, beyond which a stable silt skeleton is formed that increases the erosion thresholdA modified critical bed shear stress is proposed to mimic the transitional erosion behavior of sand‐silt mixtures [ABSTRACT FROM AUTHOR]