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.