An Analytical Model for the Structure of Turbidity Currents

Abstract

The characteristic velocity structure of turbidity currents shows a fast increase of velocity above the bed up to a velocity maximum, and a gradual decrease towards the stagnant ambient water some distance away. It is established in literature that this structure can be approximated with the superposition of two velocity profiles related to the two interfaces of the flow: the bed, with a wall bound logarithmic velocity profile; and the ambient water interface, with an error function velocity profile. We investigate the parameterisation of this classic analytical approximation of turbidity current structure, and establish that discharge, bed roughness, and slope are the three parameters that suffice to dimensionalise and shape the velocity profile of any turbidity current. The parameterisation is tested against turbidity current data from multiple series of experiments in two laboratories and measurements and estimations from assorted modern turbidity currents from the Monterey Canyon, the Congo Canyon, and the 1929 Grand Banks event. The parameterisation proves to be a reasonably successful predictor over flow thicknesses ranging from cm's to 100's of metres. The model is a static, local representation of the turbidity current structure and does not solve any equations of fluid motion, yet investigation of this simple process model can help us understand deepwater depositional systems. A surprising result is that the changing shape of the velocity structure with decreasing scale of the flow maintains shear stress at the base of the flow. This requires recalibration of the intuitive assumption that only large scale flows can transport sand in suspension: small turbidity currents have the competence to bypass sand in suspension on naturally occurring slope angles. The model is applied to a concave slope profile representative of large channel levee systems, to investigate a recent suggestion that changing velocity structure with decreasing slope in such systems allows maintenance of friction at the base of flow despite the decreasing gradient. Conspicuously absent from the simplest version of the model is the grainsize of sediment in suspension. In actual currents, the grainsize will determine the transition from a bypassing current into a depositing current, thus grainsize will have a significant impact on depositional style, even if not on flow structure.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014