Sediment Threshold

The flow condition that is just adequate to initiate the motion of sediment particles at the bed surface is called the sediment threshold. Albert Frank Shields carried out his doctoral research study on sediment transport at the Technische Hochschule Berlin, Germany. He is well known for proposing a useful diagram, known as the Shields diagram. This diagram provides the criterion for the sediment threshold, which is an essential requirement for the determination of sediment motion in a loose boundary stream. His diagram becomes famous and is most frequently referred to in the literature. It has provided an enormous inspiration to initiate a sizable number of researches over last eight decades on the beginning of sediment motion. Since his pioneering work, numerous attempts have so far been made to quantify the required flow condition for the initiation of sediment motion. In this chapter, the important experimental and theoretical studies on sediment threshold under steady stream flow are discussed, highlighting the empirical formulations and semitheoretical analyses. Both deterministic and probabilistic models of sediment threshold are described. The turbulence phenomenological approach provides the origin of the scaling law of the threshold of sediment entrainment. The special feature of this chapter is a discussion on the influence of turbulent burst[aut]Burstturbulent burst on sediment threshold. Latest experimental findings evidenced that the mechanism of sediment entrainment is governed by the sweep event. The concept of sediment threshold is applied to determine the stable-ideal section of a channel, known as the threshold channel. It has a bank profile for which the sediment particles along the wetted perimeter are in a state of incipient motion. The design of threshold channel is demonstrated through numerical examples.