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Field survey at La Lambarde


The port of Nantes Saint-Nazaire in France needs to carry out maintenance dredging works in order to maintain navigability of its channel and basins. The sediments are dredged with a trailing suction hopper dredger (TSHD) and dumped at the disposal site of La Lambarde, situated in the external Loire estuary. An earlier study suggests however that 80 % (i.e. 4 million m³) of the sediments dumped disappear from the disposal site on the long-term.

In order to increase the understanding of the behaviour of released sediments, IMDC was asked to carry out a series of field measurements. The objective is to determine the fate of the disposed sediments and to quantify as much as possible the physical parameters and phenomena involved. In particular the field survey should investigate dredge plume behaviour and the existence of fluid mud at the disposal site.

In order to determine the short-, medium- and long-term behaviour of disposed sediments (resp. ST, MT and LT), the following extensive measurements have been carried out :

  • Plume tracking (ST);
  • Sediment fluxes through transects (MT);
  • Tracer experiment (LT);
  • Sediment distribution and fall velocity (sediment properties).

Additionally the background conditions have been measured. The large range of instruments used allowed for cross-verification of most measurements.

For the plume tracking the survey vessel was equipped with an ADCP and the SiltProfiler, a rapid-descent suspended sediment concen-tration profiler developed by IMDC. Additionally, density profiles of the unconsolidated bed were measured with a gamma density meter. This combination of instruments and the use of four measuring protocols allowed to clearly visualize the plume and identify near-bed density currents. Spatial dimensions, lifetimes and physical parameters could be quantified, as well as the short-term sediment loss.

Sediment samples were taken and the size distribution determined by laser diffraction and classical sieving. Fall velocities were measured at regular time intervals in the plume with the LabSFloc methodology. These measurements gave better insight into flocculation characteristics and plume dynamics per type of dredged sediments.

On the medium-term, three stations on the sea-bed, equipped with a series of instruments measuring turbidity, current velocity and direction, wave characteristics and erosion-sedimentation, provided the necessary data for results analysis and to determine the re-suspension capacity of sediments.

On the long-term, the tracer experiment and transect measurements during a complete tidal cycle were used to investigate observed sediment sorting processes and to confirm the preferred trajectories of suspended sediments from existing model results.

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