Turbidity currents on the open slope of the Fraser Delta

Turbidity currents have been observed and measured in submarine channels and canyons where the flows are confined, but with the exception of one serendipitous direct measurement, are only inferred to occur on unconfined delta slopes. In this paper, direct measurement of turbidity currents on the Fraser delta slope are reported and their dynamics compared to turbidity currents in channelized settings. Twelve turbidity current events occurring between May 2014 and June 2018 were identified from Acoustic Doppler Current Profiler instruments deployed at two sites (109 and 159 m water depth). All the events occurred in the months of May and June, in association with high spring river flow. Five of the events were recorded at both sites and the timing of first arrivals gives estimates of the current speed ranging from 0.4 to 3.6 m s−1. Direct current measurements confirm that most flow speeds exceed 1.5 m s−1, although one weak event did not exceed 0.4 m s−1. Current profiles indicate that the flows are of variable thickness, ranging from less than 3 m to 15 m. The turbidity current events varied in duration from 5 to 66 min and, in the cases where transit speeds were available, the durations were proportionally shorter when the transit speed was high. In stronger flows, the initial stage of the flow is highly variable, suggesting extreme turbulence, and the velocity structure of the flow is similar to previous experimental and field observations, with a distinctive relatively thin, high speed lower region being evident at times. Downslope translation of the instrument platform during two of the events suggest the presence of a dense basal layer up to 2.75 m thick, as suggested in the travelling wave model of Heerema et al. (2020). The fact that only approximately half the flows measured at the shallower site are observed at the deeper site suggest that the runout distances of open slope flows are limited. Weaker flows are barely distinguishable in speed and direction from ambient tidal velocities. Flow directions suggest that the open slope flows are unconfined and initiate in the upper slope area that is approximately the same as the area of upslope sediment waves. It is hypothesized that shallow gullies in the same area may be important for initiation and ignition of the flows that become unconfined on the lower part of the slope.