The Adour river mouth is located in Anglet, on the southwest coast of France, and it provides access to the commercial harbour of Bayonne. The navigation channel suffers from a recurring problem of silting and needs regular dredging. The construction of breakwaters and jetties has not solved the silting problem. Recently, a preventive trench was dug south of the channel to decrease the rate of siltation in the navigation channel.

Bathymetric data of the river mouth were monitored for 26 mo, and 40 bathymetric sets of data were analysed. During the investigation period, four dredging campaigns were carried out. The surveys provide a very unusual bathymetric record because the sampling in time is extremely dense for such data.

Eigenfunction analysis was performed along transects perpendicular and parallel to the direction of the river flow. These analyses are usually used to explain natural bathymetric or topographic evolutions. Here, there are also used to describe anthropogenic influences. The first spatial eigenfunction corresponds approximately to the mean bathy-metry over the study period. It does not show the preventive trench. This demonstrates that the trench is an ephemeral feature of the bathymetric record. The subsequent eigenfunctions represent the variations about the mean. The second eigenfunction explains the majority of the seabed evolution and reaches a maximum amplitude at the location of the preventive trench. The corresponding temporal eigenfunction presents important discontinuities during the dredging campaigns; its maximum amplitudes correspond to the maximum siltation, and its minimum amplitudes correspond to the dredging periods.

A two dimensional empirical eigenfunction analysis confirmed that there was little evidence for any directional bias in the seabed evolution. In conclusion, this study shows that empirical eigenfunction analysis methods can be a helpful tool for analysing the impact of dredging activities and could have a useful role in estuarine management planning.

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