Experiments on Upstream-Migrating Submarine Knickpoints

Abstract

Abstract Laboratory studies are commonly used to improve our understanding of physical processes that are very difficult or almost impossible to investigate in their natural settings. Erosion impacts on large-scale submarine morphology are good examples. We present the results of a preliminary laboratory study focused on submarine knickpoints, which are bed depressions bounded by steep walls upstream and a gentle slope downstream, originated by the passage of a turbidity current over an erodible bed. Eight experiments using different materials were performed in a facility capable of running continuous turbidity currents. The sediments used in these experiments were: kaolin clay, two different silica flours, and fine sand. Silica flours and fine sand were each used as single sediments in four experiments. The other four experiments were performed using a mixture of these sediment types. The bed deposit was formed by a net-depositional turbidity current. The total sediment concentration by volume in the experiments was 5% in one experiment and 10% in the other seven. The discharge ranged from 0.009 to 0.055 l/s. Each experiment started with an initial slope in the channel bed of 15% (8.53°). As the experiments progressed, the bed deposit inside the flume built up. Knickpoints were observed in four of the eight experiments. Knickpoints formed under a wide variety of conditions; they were found when one material or a combination of materials was used in the tests and when the turbidity current was partially and totally distributed over the channel. Measured width of the bed depressions ranged from 2 to 4 cm, length ranged from 3 to 6 cm, and the estimated maximum depth was close to 0.5 cm. Calculations of the speed of upstream migration indicate values in the vicinity of 0.5 mm/s. Upscaled geometric dimensions based on these laboratory experiments fairly reproduce bed depressions observed on a field scale in earlier studies. These laboratory observations suggest that the body of the turbidity current is responsible for the upstream migration of the knickpoint.