SUSPENDED SEDIMENT FLUXES IN A SHALLOW, LOW-TURBIDITY, MACROTIDAL ESTUARY

Suspended sediment transport is important to the health of estuaries and adjacent coastal waters. Scientific understanding of sediment transport processes in macrotidal estuaries is dominated by studies of highly turbid estuaries like the Gironde (France). To broaden knowledge of sediment transport in macrotidal estuaries an observational study was conducted in the shallow, low-turbidity Aulne River estuary in western Brittany. Three-week current profiler deployments in summer and winter of 2013 enabled the calculation of residual suspended sediment flux. Total flux was compared to flux decomposition products generated by the Dyer equation and a simplified model based on 6 forcing mechanisms. On tidal timescales sediment transport is controlled by tidal resuspension of sediment and asymmetry in the duration of slack velocity. Spring/neap variations in tidal velocity drive a strong fortnightly variability in SSC and residual sediment flux. Total flux is seaward near the mouth, but distortion of the horizontal and vertical tides creates conditions favorable to landward flux in the middle estuary. SSC is higher and residual sediment flux is stronger during winter, under the influence of stronger river discharge and seaward flushing of the ETM. High river discharge is capable of reversing the direction of total flux in the middle estuary. The sum of advective and tidal pumping components from the Dyer equation equaled only 60-75% of the total flux, highlighting the importance of the two additional Dyer components controlled by vertical processes. The total flux was dominated by the Eulerian advective term near the mouth, but the development of a strong Stokes drift term allowed the total flux in the middle estuary to be controlled by the tidal pumping components. A simplified model of depth-average residual sediment flux indicated that the forcing mechanisms included in the model (mean water depth; M2 horizontal tide amplitude; residual, M2, and M4 velocity; depth-averaged SSC gradient) are insufficient to explain all of the variability or magnitude in sediment flux due to the importance of higher-order tidal constituents in the Aulne. Suspended sediment transport in the Aulne is consistent with shallow estuary theory, but none of the Dyer flux components can be ignored in evaluating sediment flux.