Abstract

Subaqueous channels are formed on subaqueous fans in oceans or lakes by turbidity currents. The subaqueous channels are laid on gentle slopes and are bounded by levees. The sediment in subaqueous channels differs in grain-size distribution in the channel and levee. The process by which a turbidity current flowing over an initially featureless surface creates channels by itself is still incompletely understood. I have addressed the following research question: which combination of turbidity-current factors leads to channelization? In this paper the discharge of turbidity currents, the bottom gradient of the turbidity currents, the settling velocity of sediment, and the range of grain-sizes of the sediment in the turbidity currents were considered for the turbidity-current factors. Some factors such as the sediment concentration of turbidity current, the width of the fan, the kinematic viscosity of water, the density of the particles, and the density of the ambient fluid were not considered. This meant that I had to determine the proper interaction of the following factors: the discharge of turbidity currents, the bottom gradient of the turbidity currents, the settling velocity of sediment, and the range of grain-sizes of the sediment in the turbidity currents. The following (model) testing arrangements were used: a test basin 6 m long with a slope 5 m long that can be inclined between 20% and 0.3%, sediments with four different densities and seven different grain-size distributions were used, with various discharges of turbidity currents. The model tests showed that subaqueous depositional channels can be formed when the dimensionless discharge is an empirical linear function of the gradient of subaqueous fans. The dimensionless discharge of the turbidity current is the product of the square of the grain size and the settling velocity of the sediment, divided by the discharge of the turbidity current. Formulas were developed to narrow down the range of sediment and flow conditions required for channel formation. It is shown that these formulas are applicable not only on the laboratory scale but also on the field scale.