Stress analyses for numerical modelling of submarine flowslides

Abstract

This report is based on, and is a continuation of the NGI-report 522090-2 "An Approach to the Physics and the Modelling of Submarine Flows lides ", by Norem, Locat and Schieldrop (1989).Submarine flowslides are assumed to consist of two flow layers, a dense flow close to the bed and a turbidity current above that. Both reports concentrate on the dense part which is considered to behave as a granular material where visco-plastic behaviour is predominant. The present report establishes the assumptions and equations for the non-steady flow necessary for physical and numerical modelling of submarine flowslides. The equations developed are the momentum equation, the continuity equation with the corresponding boundary condition, as well as assumptions for the conditions at the upper and lower boundaries are presented. The boundary stresses at the upper surface are assumed to be comparable with the stresses in the turbulent boundary layer along a flat plate. The shear stresses have their maximum at the front of the slide and are reduced to an almost constant level towards the rear. The boundary conditions at the bed are assumed to be dependent on the roughness and the shear strength of the bed material. A non-slip velocity is assumed to exist for real slides, but may develop in model experiments with a smooth bed. On the other hand erosion may be significant if the bed consists of a soft material having a low shear strength. Equations to calculate the development of the slip velocity or the erosion depth are presented.