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When narrow continental shelves are stressed by extreme weather events, nearshore currents dominate the coastal circulation leading to complex flow patterns that can result in previously unforeseen cross-shelf exchange of water and sediment. Here we present a series of detailed studies carried out to investigate the nature of turbidity currents that impacted upon a submarine pipeline offshore Philippines, nearby tropical river deltas, after the landfall of intense typhoons. These rivers debouch into a shelf only a few hundreds of meters wide that is interrupted by steeper continental slopes carved by multiple submarine canyons. Turbidity currents were detected through regular pipeline monitoring, which showed lateral displacements and sea-floor erosion where the pipeline crosses some of these canyons. Seabed assessments indicated signatures of the occurrence of turbidity currents as opposed to landslides or ground motion due to earthquakes. Particularly, the submarine canyons were covered with regular sediment patterns that indicated the passage of deep-water turbulent flows, suggesting the local occurrence of turbidity currents. Meteorological data pointed at river floods and meteocean conditions, and associated fluvial sediment delivery and coastal sediment transport, as the most likely leading mechanisms for the triggering of turbidity currents. Hydrological modelling and related sediment transport calculations show these rivers were not capable to debouch into the sea with sediment concentrations high enough to generate hyperpycnal flows. Nevertheless, river plumes played an active role as source of sediment available on the shelf. Conversely, the role of the coastal circulation was found to be crucial for the triggering of turbidity currents. Our simulations show the development of exceptional rip currents (megarips) that flush out water and sediment from the inner shelf in the cross-shore direction towards the canyons’ heads, ultimately triggering turbidity currents into deep ocean waters. Such extreme nearshore circulations require the passage of intense typhoons in proximity to the trigger area inducing shore-normal incoming waves at peak conditions that in association with shoreline concavity at the river deltas favour the formation of erosional megarips, whose dynamics strongly depends on typhoon's approach latitude. The turbidity current modelling confirmed such an interpretation, matching field observations in the form of pipeline displacements. These evidences support our hypothesis that typhoon-induced megarip circulations could be responsible for the triggering of turbidity currents in submarine canyon systems offshore tropical river deltas. This newly identified mechanism has wide implications on the threatening of seafloor infrastructures and the assessment of frequency and duration of turbidity currents. |
