Triggering Mechanism of Subaqueous Sediment Density Flows on the Fraser Delta Slope: What Can be Gained through Continuous Observation from Venus?

Abstract:

Recent observations on the Fraser Delta slope (British Columbia, Canada) confirm that unconfined turbidity currents occur on the delta slope close to the mouth of the main river channel. However the conditions that trigger these events are not understood. This study attempts to characterize the triggering conditions using data from the Delta Dynamics Laboratory (DDL), part of the VENUS Coastal Network in the Ocean Networks Canada Observatory, collected over four years. Data collection focused on the freshet season (May-July), a period believed to have the highest occurrence of submarine sediment density flows due to a high river discharge from snow melt combined with high tidal excursions resuspending bottom sediment. ADCP backscatter profiles reveal sediment settling through the water column from the river plume to the delta slope during ebb tide when the salt wedge is likely pushed out of the river channel and into the basin. Combined with measurements of tidal excursions and Fraser River discharge, quantitative measurements of the sediment settling process were analyzed on a daily and seasonal time scale. It is found that while sediment settling events occur throughout the entire year, the most intense and longest duration events occur on a higher frequency during the freshet season. In addition, the intensity of the ADCP backscatter has a strong positive correlation with the height of the major tide each day. This suggests that the tidal excursion has a larger direct control on the near bottom sediment concentration the delta slope than the river discharge. Continuous data collection is able to capture a clearer picture of the conditions in which submarine sediment density flows are created. From this, the aim is to produce more accurate predictions of when and where the flows will occur. With this information, improved geohazard monitoring on delta slopes can be achieved.