AAPG ACE 2018

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Surge Propagation in Debris Flows

Abstract

Submarine debris flows and associated turbidity currents are important geohazards that may damage seafloor infrastructure, whilst their deposits form important heterogeneities in many deep marine turbidite reservoirs. Turbidity current models are sensitive to the required prescribed initial conditions and although the mechanics of both laminar (non-Newtonian) debris flows and turbulent (Newtonian) turbidity currents have been extensively studied, the transition from laminar to turbulent states is poorly described by current theory. Thus understanding the transition from high to low concentration is essential for turbidity currents initiated by an underwater landslide. This work presents a coupled experimental and numerical study of designed to provide new insight into this transition and sediment transport.

The experiments entailed the release of dyed glycerol and water mixtures of varying concentrations into a 5 m long, 0.25 m wide tank via the phased removal of two pneumatically-controlled lock gates. The phased release generated a laminar flow with an internal surge. The surge propagated as a bore on the upper flow surface that, for certain concentrations, caused an abrupt transition from a relatively dense laminar flow to a dense turbulent flow upon reaching the flow front. For higher concentrations the pulse did not transfer suffcient energy to transition the flow; for lower concentrations the flow front transitioned through mixing and erosion before the pulse reached the front.

Numerical simulations were undertaken with single-layer and two-layer Lagrangian finite-difference schemes based on the depth-averaged, shallow water equations. Layers were coupled through additional hydrostatic pressure and inter-layer drag terms. Both formulations qualitatively captured some key flow features, but the two-layer formulation more accurately replicated experimental observations. The present work exposes the limitations of depth-averaged numerical models, but also demonstrates the promise in constraining the role of intra-flow surges in debris flow transition.

Felix and Peakall (2006) proposed that six transformation mechanisms could operate, singly or in combination, during the evolution of debris flows into turbidity currents. Three of these - erosion, mixing at the head and wave instability, are observed in the experiments. We assess the relative transformation efficiency of these mechanisms, and discuss the implications for the rock record.