Tidal Propagation in a Branching System: Effects on Sediment Transport

Abstract:

Wax Lake Delta is a prograding delta formed where Atchafalaya River water is diverted down the Wax Lake Outlet into Atchafalaya Bay. The delta, which is preferentially composed of sand, has multiple distributary channels leading to the bay. Sand transport and deposition within this channel network is the primary driver of the delta’s progradation. We present results of a three-month field campaign to analyze channel velocities and turbidities in a pair of distributary channels in the Wax Lake Delta to understand controls on sand transport in the system. Background river velocities and tidal heights in the channels align with each other in both phase and magnitude, but tidal velocities differ greatly. This difference leads to flow in one channel that reverses on tidal timescales while the other channel is always outflowing. This unexpected behavior is explained using a simple, analytical model of tidal propagation into and through a branching system. Differences in tidal velocities drive differences in sediment transport, as shear velocities in the reversing channel are generally lower than that in the channel with unidirectional flow, and are commonly lower than settling velocities for sand grains typically found in the system. This effect is observed with turbidity measurements during the field campaign and with bed observations of the two channels.