A LABORATORY PERSPECTIVE ON THE GROWTH OF SUBMARINE CHANNEL BENDS BY TURBIDITY CURRENTS

We summarize the connections between depositional and bypassing turbidity currents and the evolutions of channel bends they traverse. The experimental subaqueous channels have sinuosities of 1.04 and 1.32. Each channel is trapezoidal in cross-section, 0.10 m deep and between 3.60 m and 4.00 m in length. Inlet thickness for every turbidity current is 0.10 m. By varying the sediment composition of both the turbidity currents and the channel substrate, we address the following questions tied to the development of sinuous submarine channels: (1) what controls the development of bar forms constructed by bed load versus suspended-load deposition along the inner banks of channel bends; and (2) what controls of the occurrence of sediment erosion versus deposition on the outer banks of channel bends?

We observed two distinct styles of bar construction at the inner banks of channel bends. One bar type is constructed by deposition of bed load only. The sediments composing these bars are worked into trains of bed forms that are actively migrating down flow. Each bed-load bar begins up flow of a bend apex and persists past the point of minimum channel curvature. The second bar type is constructed of sediment deposited from suspension. These bar forms are connected to zones of flow separation that occur along the inner banks of bends. Only fully suspended sediment is mixed into these low-velocity separation eddies where it then settles down to the bed. The bars accumulating in low-velocity eddies are constrained by the points of flow separation and reattachment. In the laboratory these separation points are at or immediately downstream of a bend apex.

Laboratory currents document how the outer banks of channel bends can switch from sites of focused sidewall erosion to sites of deposition depending on whether the sediment-transporting currents are producing bypassing or channel-filling conditions. The lateral movement of channel bends and growth of channel sinuosity only occurs when the turbidity currents are at least weakly net erosional in their transport properties. Depositional currents accumulate sediment at the site of the outer bank, preserving this sidewall and potentially reducing the overall sinuosity of a submarine channel as it aggrades.