THE DYNAMIC DEVIATION OF FLUID PRESSURE FROM HYDROSTATIC PRESSURE IN TURBIDITY CURRENTS

The simplifying assumption that the pressure in a flowing fluid is equal to the hydrostatic pressure is justified in many classic free surface hydraulic analyses such as flow around a bend in a river. While there has been no rigorous test of its suitability in turbidity current modeling, the assumption has traditionally been applied to research concerning turbidity current flow through sinuous submarine channels.

Here we present the first experimental measurements of fluid pressure beneath turbidity currents, generated under controlled laboratory conditions. Measurement of flow density and velocity as well as fluid pressure allows us to relate gravity flow fluid pressure to gravitational and kinematic flow dimensions. Basal fluid pressures are much lower (30-45 %) than those predicted from bulk flow density, in proportion to the kinematic energy density of the flow, a parameter usually referred to as the “dynamic pressure”. This finding invalidates the commonplace assumption of a hydrostatic pressure regime in turbidity currents, and necessitates the incorporation of dynamic fluid pressure as physical parameter in the analysis of turbidity currents.

Stepping away from the hydrostatic pressure assumption allows a fresh view of some current enigmas of turbidity current flow around bends in sinuous submarine channels. Our results suggest that velocity asymmetry between the outer and inner bend can play an important role in setting the lateral pressure gradient that drives flow curvature. Specifically, slower flow in the inner bend and faster flow in the outer bend can counteract the pressure effect of super-elevation, an explanation that is consistent with the exaggerated super-elevation observed in published experimental datasets. Further advances in resolving the controls on secondary flow cell orientation (helical flow) in turbidity currents can only be expected after the primary flow pressure field is understood.