A Comparison of Thresholds: Shields and Erodibility Index

Abstract:

The threshold between stasis and mobility, and the forms and processes associated with the interfacial instabilities it defines, is a fundamental component of most morphodynamic systems. Scientists and engineers who focus on the interface of the earth’s dynamic surface and the turbulent fluids of the atmosphere and hydrosphere, have approached this interface from a diversity of perspectives. In the field of sediment transport, Shield’s 1936 paper stands as a benchmark for the theoretical understanding of the threshold of incipient motion of non-cohesive granular transport. However, in processes such as the erosion of cohesive material or bedrock erosion, consensus has yet to be reached among researchers, as to which method most aptly encompasses these processes. The lack of consensus is reasonable, considering the diversity of the applications, and implications, of interface dynamics. When thresholds may depend on the sequence of antecedent conditions, as in cohesive sediment entrainment, or the time scale of perception ranges from many Hertz to geologic eras, as in the processes of rock scour, a diversity of conceptual models is to be anticipated. In Scour Technology (Annandale 2006), the Erodibility Index Method (EIM) is presented as a threshold paradigm that spans the breadth of all earth materials that may experiences erosion by fluid forces. The authors present a comparison of the modern expression of the benchmark theoretical understanding of the threshold of incipient motion of non-cohesive granular material with the EIM. Results illustrate that each threshold can provide insight into the other and that the Shields parameter can be expressed within the vocabulary of the EIM.