Observation and analysis of shear instability in the Fraser River estuary

Abstract

We investigate the occurrence of shear instability in the Fraser River estuary. Instabilities observed with an echo sounder are compared with a linear stability analysis based on observed velocity and density profiles. We find that each set of observed instabilities coincides with an unstable mode predicted by the Taylor‐Goldstein equation. Each of these instabilities occurs in a region where the gradient Richardson number is less than the critical value of 1/4. Both the Taylor‐Goldstein predictions and the echo soundings indicate the instabilities are concentrated either above or below the density interface. This “one sidedness” is in contrast to the archetypal Kelvin‐Helmholtz instability. Although the dominant source of mixing in the estuary appears to be caused by shear instability, when the tide produces strong near‐bed velocities, small‐scale overturning due to boundary layer turbulence is apparent throughout the depth.