Erosion Behavior of Sand-silt Mixtures: the Role of Silt Content

It is well known that critical clay content is key factor controlling overall cohesion of the mixtures and increasing erosion threshold. Besides cohesive clay fraction, existing studies show that non-cohesive silt can enhance bed stability of a sandy bed as well. Many studies were conducted with an artificial bimodal mixture with distinct peaks of sand and silt fraction to understand the role of silt particles. However, it is unclear to what extent a bimodal mixture represents natural sand-silt mixtures. In this study, we conducted series of annular flume experiments on sediment samples collected from typical silt dominated tidal flat to study role of the silt on erosion behavior of sand silt mixtures. Two types of sand-silt mixtures were used: a silt-sized mixture with median grain size of 52 nm, and a very fine sand-sized mixture with median grain size of 82 nm. The results show mixtures with 30% silt content behaves as sand, and a Shields curve of fine sands (van Rijn, 2007) is applicable for prediction of the critical bed shear stress. However, bed stability is enhanced when silt content increases to 60%, since the critical bed shear stress is twice of bed materials with 30% silt. Since the fractional grain size ratio (Dm,sand/Dm,silt) of each mixture is the same, silt content is considered as the important factor on increasing the threshold of motion. More studies should be conducted to understand the critical silt content and the underlying mechanisms.