Submarine slope destabilization and gully formation by water sapping: Physical simulation of an underestimated trigger of subaqueous sediment gravity flows

Abstract

AbstractA laboratory tank experiment tested whether a subsurface flow from a confined aquifer causes slope instability and leads to the formation of pathways for sediment transfer from shallow to deep water when the subsurface flow discharges through the face of a subaqueous slope. A sandy slope with multilayer stratigraphy was built inside the tank, and a confined aquifer was simulated within the stratigraphy. To induce groundwater flow out of the face of the slope, water was injected in the proximal zone of the confined aquifer at progressive increased discharge. Sediment movement on the slope occurred by rolling of particles, fluidized flow, grain flow and slides. The fluctuation of phreatic pressure in the confined aquifer was measured by a set of piezometers, from which the hydraulic gradient generated by the water flow moving towards the slope was determined. This study determined that the mass movements started when the imposed injected flow rate was greater than the hydraulic conductivity capacity of the simulated aquifer, using the flow capacity calculated from the Darcy equation for porous media. The various physical parameters used in the experiment were found to scale well to natural prototypes. Moreover, the patterns of erosion and deposition in the physical simulation resembled natural features observed in seismic‐geomorphology maps and modern deep‐sea physiography. Therefore, water sapping by a confined aquifer flow is a potential mechanism for slope erosion and instability and for the formation of pathways connecting shallow‐water and deep‐water environments.