Acoustic Imaging of the Transition from Fluvial Suspension to Density Underflow on a Marine Delta Lip

English Abstract:

The abrupt delta lip transition from continuous fluvial bedload and suspended load transport to episodic gravity-driven underflows has long been a focus of speculation. Hyperpycnal flows have been observed in lacustrine systems where there is minimal density contrast between the inflow and receiving watermasses. The same mechanism, however, is much harder to achieve given a typical 20-30 kg/m³ contrast between fresh and salt water.

To directly address this process, repetitive surveys were conducted off the lip of the Homathko delta at 15 minute intervals around low-water when the off-delta sediment flux was most intense. Simultaneous bathymetry from 5 to 80m as well as acoustic volume scattering of the water mass was imaged using 90 and 500 kHz multibeams. A rapidly dipping optical backscatter probe was deployed to relate the volume scattering to suspended load.

The acoustic imaging clearly identified the highest suspended load in the overlying outflowing freshwater. The load never at any time approached the level at which it could result in direct hyperpycnal flow. Rather, the suspended load was seen to drop through the base of the freshwater layer into the underlying saltwater forming a near continuous descending flux. This avoids the need to overcome the bulk of the density contrast. Although the descending plume appeared continuous, the development of seabed surges in this flow was intermittent with only 2 surge flows in a single tidal cycle.

The simultaneous bathymetry indicated that just one of the delta lip chutes was intermittently active with both bulk accretion and deflation as well as upslope migrating bedforms. Beyond the mouth of the chute, however, only one flow was sufficiently powerful to cause bedform migration in the downstream channel. That movement was clearly correlated with the development of a discrete turbidity current flow visible in the water column imaging.

The presence of a near continuous descending plume, but only intermittent surges, suggests that there is an intervening step where the sediment flux builds up on the proximal delta lip, before initiating a subsequent flow. The details of that step remain imperfectly understood.