Channel formation by flow stripping: large-scale scour features along the Monterey East Channel and their relation to sediment waves

The Monterey East system is formed by large-scale sediment waves deposited as a result of flows stripped from the deeply incised Monterey fan valley (Monterey Channel) at the apex of the Shepard Meander. The system is dissected by a linear series of steps that take the form of scour-shaped depressions ranging from 3.5 to 4.5 km in width, 3 to 6 km in length and from 80 to 200 m in depth. These giant scours are aligned downstream from a breech in the levee on the southern side of the Shepard Meander. The floor of the breech is only 150 m above the floor of the Monterey fan valley but more than 100 m. below the levee crests resulting in significant flow stripping. Numerical modeling suggests that the steps in the Monterey East system were created by Froude-supercritical turbidity currents stripped from the main flow in the Monterey channel itself. Froude-supercritical flow over an erodible bed can be subject to an instability that gives rise to the formation of cyclic steps, i.e. trains of upstream-migrating steps bounded upstream and downstream by hydraulic jumps in the flow above them. The flow that creates these steps may be net-erosional or net-depositional. In the former case it gives rise to trains of scours such as those in the Monterey East system, and in the latter case it gives rise to the familiar trains of upstream-migrating sediment waves commonly seen on submarine levees. The Monterey East system provides a unique opportunity to introduce the concept of cyclic steps in the submarine environment to study processes that might result in channel initiation on modern submarine fans.