Testing river surveying techniques in tidal environments: example from an actively meandering channel surveyed with TLS (Mont Saint-Michel bay, France)
Résumé
Tidal channel developed in mega-tidal salt marsh offer a unique set of characteristics to study the interaction between hydraulics, riparian vegetation and sedimentation using Terrestrial Laser Scanner (TLS). The recession of water allows a nearly complete survey of the channel that is otherwise impossible in rivers. Moreover, the predictability of tide amplitude allows to target surveys large events. Finally, the hydro-sedimentary processes and peak flow velocities in excess of 2 m/s in mega-tidal estuaries (e.g. Mont Saint Michel (MSM) bay) allow to explore conditions that are similar to river during flood conditions. This has motivated a 3 years study of a sinuous tidal channel located on the fringe of the marsh with the aim to understand its dynamics at daily to annual scales. We have acquired 36 high resolution topographic surveys with TLS, whose 13 daily surveys were acquired during annual largest tides. A local reference network of targets is used to yield a high registration accuracy with uncertainty varying between 1.5 mm and 3.4 mm. We use the CANUPO algorithm for classifying riparian vegetation and ground in 3D data, and use the point cloud comparison algorithm M3C2 to resolve 3D topographic changes down to 5 mm. ADCP, ADV and a turbidimeter were installed to constrain flow velocities and suspended sediment concentration (SSC). Our analysis is focused on three active compartments: (1) the inner bar on which riparian pioneer vegetation is developing and where sedimentation reaches up to 5 cm/tide; (2) the actively eroding outer bank which exhibits local retreat rates up to 2 m/tide; (3) the channel itself for which we document fluctuations of up to 0.2 m in elevation at daily to monthly timescales. We find that High Water Level (HWL) is a good predictor of the mean rate of evolution of these compartments with different empirical relationships. Spatially averaged sedimentation on the inner bend tends to increase linearly with HWL and is increased by a factor 2 during summer/autumn spring tides at the peak of pioneer vegetation development. Bank erosion and channel dynamics show a marked difference for tides reaching the salt marsh elevation. For tides below marsh elevation, bank erosion is negligible and the channel is systematically aggrading at a rate proportional to HWL. For tides flooding the marsh, mean bank erosion increases linearly with HWL and the channel shifts to erosion for over-marsh tides. Using flow velocity and SSC data we show that sedimentation on the inner bar results from the penetration of the turbid flood onto the inner bar. Spatial variability in sedimentation results from local interactions between flow and vegetation. On the contrary, bank erosion is dominated by the very large ebb peak velocity developing during spring tides. The very non-linear sensitivity to HWL of bank erosion and channel erosion means that the rate of evolution is largely controlled by the largest tides of the year. This in turn yields very large annual fluctuations in the rates of meander evolution. These results demonstrate that mega-tidal environment can offer an alternative setting to test new survey techniques aimed at river monitoring and can shed light in the elementary processes governing biogeomorphological interactions.