What are the controls for delta front slope failure? Insights from detailed monitoring at Squamish Delta, British Columbia.

Submarine slope failures pose a hazard to seafloor infrastructure and coastal communities. Given the high population densities, slope failures can have a particularly significant impact around river deltas, generating damaging tsunamis and breaking critical telecommunications connections. Despite the risks they pose, a lack of detailed monitoring means that the factors that lead to slope collapse remain poorly constrained. Numerical modelling is typically used to assess future slope stability. Still, sparse existing data ensure that we cannot yet determine how submerged delta slopes evolve and progress to failure at the field scale. Here, we aim to close this gap by analysing repeat seafloor surveys of the submerged Squamish prodelta, British Columbia, to determine the physical controls on slope instability. Multibeam bathymetric surveys were performed on 93 consecutive weekdays in 2011, during which time at least five large (>50,000 m³) delta slope collapses occurred, as well as numerous smaller slope failures. These surveys allow us to determine how the delta slope and geometry changes on an unusually detailed timeframe (i.e. daily) in the build-up to slope collapse and how it relates to variable sediment supply from the feeding river and tidal fluctuations. Analysis of the five large collapses reveals that a single mechanism is not responsible for every failure. So, we investigated how different parts of the delta encounter major failure at different times and locations by measuring and mapping out the delta head and associating it with sediment input and tide high. From this, we found that slope failure is likely due to a combination of enhanced slope geometry due to delta lip progradation and pore pressure fluctuations relating to sediment loading and tidal effects.