Sediment threshold

Abstract

The flow condition that is just adequate to initiate the motion of sediment particles at the bed surface is called sediment threshold. Albert Frank Shields carried out his doctoral research study on sediment transport at the Technischen Hochschule Berlin, Germany. He is well known for proposing a useful diagram, known as 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 in the literature. It has provided an enormous inspiration to initiate a sizable number of researches over last seven decades. Since his pioneering work, numerous attempts have so far been made to quantify the required flow condition for the beginning of sediment motion. In this chapter, the important experimental and theoretical studies on sediment threshold under steady stream flow are furnished, highlighting the empirical formulations and semitheoretical analyses. Both deterministic and probabilistic models of sediment threshold are discussed. The special feature of this chapter is a discussion on the influence of turbulent bursting on threshold of sediment entrainment. Latest experimental findings evidenced that the mechanism of sediment entrainment is governed by the sweep events. The concept of sediment threshold is applied to determine the stable-ideal section of a channel, known as 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.