Confinement width controls the morphology and braiding intensity of submarine braided channels: Insights from physical experiments

Abstract. Submarine channels conveying sediment gravity flows are often topographically confined, but the effects of confinement width on channel morphodynamics is incompletely understood. We use physical experiments and a reduced-complexity model to investigate the effects of confinement width (B) and inflow-to-sediment discharge ratio (Q_in/Q_s) on the evolution of submarine braided channels. The results show that a larger confinement width results in increased active braiding intensity (BI_A), and that BI_A takes longer to stabilize (i.e., a longer critical time, t_c). At a fixed confinement width, a higher Q_in/Q_s slightly decreases the BI_A. Digital Elevation Models of Difference (DoD) of the experiments allow measurement of the morphological active width (W_a) of the submarine channels and the bulk morphological change (V_bulk) within an experiment, defined as the sum of total erosion and deposition. We find that W_a and V_bulk are proportional to B. We further confirm that BI_A is proportional to both dimensionless sediment-stream power (ω^**) and dimensionless stream power (ω^*). These trends are consistent for submarine braided channels both with and without confinement width effects. Furthermore, we built a reduced-complexity model (RCM) that can simulate flow bifurcation and confluence of submarine braided channels. The simulated flow distribution provides reliable predictions of flow depth and sediment transport rate in the experiments. Using kernel density estimation (KDE) to forecast the probability and trends of cross-sectional flow distribution and corresponding BI_A under extreme events, we find that skewness of the flow distribution decreases as discharge increases. The development of braided submarine channels, shown here to extend to conditions of topographic confinement, suggests that factors not modelled here (e.g., fine sediment) may be necessary to explain the abundance of single-thread submarine channels in nature.