A Comparative Study of Confined and Unconfined Turbidity Flows and their Associated Deposits

Abstract

Turbidity currents are a type of sediment gravity flow where sediment is held in suspension purely by fluid turbulence, and the driving force is the density difference between the sediment body and ambient medium. Turbidity current research is multidisciplinary by nature, affecting fields as diverse as pollutant dispersal, marine geology, and oil exploration. The main objective of this thesis was to detail the similarities and differences between confined and unconfined turbidity flows and resulting depositional evolutions in an experimental setting. In order to achieve this objective, a flume and basin were constructed at The University of Auckland to allow physical modelling of the dual flow regimes. Test turbidity flows were run through the basin and flume, with the only independent parameter being the level of confinement. Acoustic Doppler velocimetry (ADV), photo-series, and video recording techniques were used to capture the dynamics of the flows, with the results analysed to describe and quantify flow behaviour. The deposits for the dual flow regimes were described and measured, using the flume photos and videos for the confined deposits, and ultrasonic bed profiling for the unconfined deposits. The confined flows exhibited flow deceleration and thinning across the flume distance, with associated fluctuations typical of turbidity flows. The flows deposited sediment as they traversed the flume, resulting in uniform deposits which decreased in thickness along the flume distance. MATLAB was used to model two-dimensional flow velocities in the unconfined basin flows using the captured photo-series. It was observed that the unconfined flows travelled through the basin in a semicircular fashion, with flow deceleration occurring along all directions relative to the flow origin. The maximum velocity (and minimum deceleration) occurred along the main axis of the basin. The velocity trends decreased away from the main axis in both directions, with the minimum velocities (and maximum decelerations) occurring in the lateral directions. The unconfined depositional evolutions reflected this flow behaviour. Smooth, symmetrical lobes formed close to the lockbox, decreasing in thickness across all directions away from the lobe tip. Scouring in the lobe tip of the first experiment was observed, potentially analogous to submarine channel formation in natural deep ocean deposits.