Maintenance dredging in a macrotidal estuary: Modelling and assessment of its variability with hydro-meteorological forcing

Type Article
Date 2021-09
Language English
Author(s) Lemoine J.P.1, 2, Le Hir Pierre2
Affiliation(s) 1 : GIP Seine-Aval, Hangar C - Espace des Marégraphes, Quai de Boisguilbert, 76000, Rouen, France
2 : IFREMER, DYNECO/DHYSED, Centre de Bretagne, 29280, Plouzané, France
Source Estuarine Coastal And Shelf Science (0272-7714) (Elsevier BV), 2021-09 , Vol. 258 , P. 107366 (17p.)
DOI 10.1016/j.ecss.2021.107366
WOS© Times Cited 1
Keyword(s) Seine estuary, Maintenance dredging, Mud and sand transport, Numerical model-hydrodynamic forcing
Abstract

This study used a hydrodynamic and sediment transport process-based model to simulate maintenance dredging in a macrotidal estuary: the Seine Estuary. This sandy-muddy estuary allows access to two major ports (Le Havre and Rouen). The model accounts for sand and mud dynamics and was previously validated for turbidity and morphodynamic coupling. Dredging is schematised as a physical process responding to simulated seabed evolutions. In coherence with port practices, (i) numerical dredging is conducted when sediment depositions exceed nautical depth limits determined by port authorities; (ii) dredged sediment are released in the water column above dumping sites. The model successfully reproduced the amount of sand and mud dredged in harbors and fairways (7MT/year) without any drift along 10 years. Moreover, the dredged quantities appeared to be considerably higher than the estuarine turbidity maximum (ETM) mass, which was successfully simulated by the model.

The model was used to study relationships between maintenance dredging requirements and hydro-meteorological forcings. Dredging requirements are related to forcings in different ways depending on the exposure of dredged areas to waves, currents and ETM. Le Havre harbor and fairway are sensitive to storms, as 75% of the dredging activity is due to waves higher than 1m. The entrance of Rouen fairway equally responds to tidal range, river discharge and waves. Variabilities in dredging requirements differ depending on the type of sediment. For instance, at the entrance of the Seine navigation channel, mud dredging is strongly correlated to tidal range and secondarily to river discharge, while sand dredging is dominantly related to waves. A specific simulation (not including local wind-induced circulation) showed that the low contribution of waves to mud dredging was explained by materials in suspension hauled off-site. This process is induced by westerly winds that occurred together with waves. On the other hand, mud dredging increases with tidal amplitude in areas of intense tidal currents, and presents a counter clock-wise hysteresis with tidal amplitude, which can be compared to the one followed by the ETM mass. In addition, the dependence of mud dredging on river discharge appears to be related to the proximity of the ETM, the main source of fine sediment, which location shifts downwards when river discharge increases.

At an inter-annual scale, variability in the temporal distribution of hydrometeorological forcings leads to 50% variation of annual dredged masses. In conclusion, this study has improved the understanding of the estuarine dynamics responsible of maintenance dredging.

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