 However, for cohesive sediment, mostly clay but also silt, the "erosion" velocity increases with decreasing grain size, as the cohesive forces are relatively more important when the particles get smaller.The critical velocity for deposition, on the other hand, depends on the settling velocity, and that decreases with decreasing grainsize.The Hjulström curve shows that sand particles of a size around 0.1 mm require the lowest stream velocity to erode.