Decadal-scale Evolution of Sediment Flux in the Aulne Estuary
Abstract
Estuarine sediment transport processes have the potential to evolve over time in response to alterations in various factors both internal and external to the estuary, such as sediment supply, river discharge, tidal forcing, or changes to bathymetry. Changes in sediment transport can affect many estuarine processes (e.g. budgets of sediment-adsorbed contaminants or nutrients) and ecosystem services, such as aquaculture, primary production and the need to dredge shipping channels. Most studies of decadal-scale changes in estuaries focus on geomorphology or bathymetry, or are performed using models calibrated by a limited set of observational studies. Because of the potential for sediment flux to both affect and be affected by geomorphology and bathymetry, observational studies oriented to sediment flux evolution are needed. This study focuses on two intensive observational studies separated by 30 years to quantify change in suspended sediment concentration (SSC) in the Aulne river, a shallow macrotidal estuary in western Brittany. Moored and vessel-mounted acoustic Doppler current profilers and YSIs were deployed over a three-week period in the winter of 2013 to examine hydrodynamic and sediment transport processes. The results of the modern study were compared to a 1977 investigation of currents, suspended sediment concentration, and erosion/deposition. The 1977 study found that SSC during spring tide and average river discharge was less than 30 mg/L near the mouth and above 300 mg/L landward, with near-bottom concentrations in the turbidity maximum zone occasionally greater than 1000 mg/L. SSC was highest during low tide and remained elevated throughout, in the upstream part of the estuary. Sediment deposition was stronger after flood tide due to a longer slack period, which implies landward sediment transport in the estuary. In the 2013 study, near-bottom SSC during spring tide and average river discharge was also highest during low tide, but SSC was above 1000 mg/L for a longer duration than in 1977 and SSC decreased during low tide slack before increasing again during maximum flooding velocity. Despite strong similarities in SSC, suspended sediment fluxes near the turbidity maximum zone were stronger in the seaward direction in 2013, due to stronger and longer-duration ebbing velocities. Furthermore, fluxes were higher at the upstream observational site and lower near the mouth, indicating a high likelihood of sediment deposition in the lower estuary. The results of this study indicate that decadal-scale changes in observed sediment transport in the Aulne estuary are significant, but this conclusion will be discussed by analyzing discrepancies between data sources and quality between 1977 and 2013.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP13A0862M
- Keywords:
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- 4235 OCEANOGRAPHY: GENERAL Estuarine processes;
- 4558 OCEANOGRAPHY: PHYSICAL Sediment transport