Mixed sediment transport in a stratified estuary: first insights from a field study

Many estuaries are characterized by a mixture of clay, silt and sand. The erosion, (re-)suspension and transport of these sediments determine the bathymetry and stability of an estuary. Net estuarine sediment transport is the result of multiple processes. In stratified estuaries, gravitational circulation may lead to an inland near-bed sediment transport, which is directed opposite to the net sediment transport higher in the water column. Considering that coarse material is often transported near the bed, while suspended sediment usually consists of finer particles, gravitational circulation may cause a seaward flux of fine sediment and a landward flux of coarse sediment. The New Waterway in the Rotterdam Port area (The Netherlands) is such a stratified channel. Repeated channel deepening has intensified stratification, resulting in a strong salt-wedge type of flow. The channel is continuously dredged for navigation purposes, while the channel would naturally be gaining sediment (Cox et al., 2020). The amount of sediment entering the channel from sea and upstream, and the contribution of different sediment fractions however remain unclear. In this research, we combine  data analysis with numerical modelling to better understand and quantify sediment transport in stratified estuarine channels.

As a first step, we set up a field campaign which combines flow measurements with determination of suspended sediment characteristics. A measurement frame is equipped with a Sequoia LISST-200x and an YSI EXO Turbidity meter. Suspended sediment characteristics are determined every hour at three depths, next to water temperature, salinity and turbidity. Water samples are taken simultaneously to determine suspended sediment concentration, and flow is monitored continuously using a vessel-mounted ADCP. The full campaign includes two 13-hour measurements and covers two locations in the New Waterway.

The flow in the upper layer of the water column shows to be decoupled from the saline layer below. Before the flood acceleration phase, the upper and lower layer show an opposite flow direction, corresponding to the findings of De Nijs et al. (2010). The LISST-measurements confirm that suspended sediment in the upper water layer contains a high amount of clay and silt, while the material close to the bed is predominantly sand. This suggests a correlation between grain size and net transport direction. It should be noted that a major part of suspended sediment seems to be transported in the saline bottom layer, and that near-bed processes and local sediment availability could play an important role in the net sediment transport. Continued measurements and the modelling study will further reveal the sensitivity of the net sediment transport to sediment type, and provide insight in the effect of channel deepening.

 

Cox, J.R., Y. Huismans, J.F.R.W. Leuven, N.E. Vellinga, M. Van der Vegt, A.J.F. Hoitink, and M.G. Kleinhans (2020). “Anthropogenic effects on the Contemporary Sediment Budget of the Lower Rhine-Meuse Delta Channel Network.” Manuscript submitted to Earths Future.

Nijs, Michel A. J. de, Johan C. Winterwerp, and Julie D. Pietrzak (2010). “The Effects of the Internal Flow Structure on SPM Entrapment in the Rotterdam Waterway.” Journal of Physical Oceanography 40, no. 11: 2357–80.