Suspended sediment fluxes in a shallow macrotidal estuary

Residual suspended sediment flux in estuaries is dependent on water level, velocity, and suspended sediment concentration (SSC), but complex interactions between these variables and other forcing mechanisms can lead to drastic differences in the magnitude and direction of sediment flux. The goal of this study was to quantify residual suspended sediment flux in a shallow, macrotidal estuary, and to determine its most important forcing mechanisms, using the Dyer flux decomposition equation and a simplified analytical model. Water level, velocity, and acoustic backscatter were measured in the Aulne River estuary in Brittany, France, and acoustic backscatter converted to SSC. The vertical tide was slightly flood dominant near the mouth, but strongly flood dominant upstream. Velocity was ebb dominant throughout the estuary. The magnitude and direction of total residual suspended sediment flux changed with position in the estuary and seasonally. The Eulerian flux was dominant at the mouth, but the tidal pumping and Stokes drift components increased in importance landward. Residual suspended sediment flux in the Aulne is dependent on several processes in addition to those included in the simplified model. The strong spring-neap control and tidal resuspension of sediments in the Aulne and the presence of higher-order tidal velocity terms contribute in a non-negligible way to residual suspended sediment flux. Finally, all of the first five components of the Dyer flux decomposition equation are needed to accurately represent residual suspended sediment flux in the Aulne. •Magnitude and direction of total residual flux change with position and discharge.•Eulerian flux is the main component of total flux near the mouth.•Tidal pumping and Stokes drift flux components gain importance landward.•The 6 processes in the model are insufficient to replicate residual flux in Aulne.•First 5 Dyer flux components were all needed to estimate total residual flux.