Conditions of submarine levéed channel inception: Examination by flume experiments

Submarine levéed channels are often observed in submarine fans, although submarine fans without continuous levéed channels are also common depending on the composition of supplied sediments, particularly the proportion of muddy/sandy materials. However, parameters governing the inception of levéed channels have not yet been studied. Furthermore, depositional levéed channel topography has not been simulated in experimental flumes in any previous study conducted with dilute flows (flow concentration <10%). Herein, four experimental series were conducted (Series A, B, C and D) to simulate depositional submarine channels and to study their formative conditions. A mixture of sediment and saline water was used in the experiments, and both the width of an outlet diffuser and flow discharge rate were varied in the experimental series. A topography composed of a channel with two ridges resembling natural depositional submarine channels with levées was formed in all experiments. Comparisons between the experimental topographies and natural submarine fans revealed that two of the four experimental series produced channel levées with length to width ratios similar to those produced by natural systems. An experiment with half the outlet size produced a channel two times deeper (4.5 cm against 2.0 cm and 2.2 cm) as compared to other experiments. The flow discharge rate and outlet width were half of those observed in Series A and B. Furthermore, salt was removed from the initial mixture for one of the experiments, resulting in high natural levées with a short channel. This study demonstrated that dilute flows could form purely depositional channel levées without precursor or resultant erosive features. In addition, results revealed that the formation of submarine channels is related to two factors: channel width and muddy suspended sediment.