In-situ characterization and dynamics of Suspended Particulate Matter (SPM) in the Rhine river front during neap tide

A 13 hour field campaign was carried out off the Dutch coast near the Sand Engine on September 17, 2014, on a neap tide, 10 km north of the river mouth [1,2]. Samples were taken with a van Dorn water sampler every half hour at 0.6 m above bed (mab). These samples were analysed with a LabsFLOC-2 camera [3]. In addition an underwater camera took videos of flocs. LISST, OBS and water velocity data were also obtained. The flocs size and shape found from the two cameras is compared.

From LISST and OBS data, it was found that the mean D50 of the particles is varying in correlation with the cross shore velocity: the mean particle size is smallest at highest offshore directed velocity and highest at highest onshore directed velocity, where a peak in SSC is also observed.

It was found that the velocity change from North-East to South-West, South-West to North-East or from onshore to offshore triggered the presence of elongated particles with low density in the water column. These particles could be identified as algae or algae-sediment particles from the underwater camera images.

In order to study the binding of algae to sediment particles, laboratory experiments were performed with Skeletonema costatium, which is one of the abundant algae living in coastal areas. The experiments confirmed that the algae could easily bind to sediment particles and form large flocs that are prone to reconform under different hydrodynamic conditions.

[1] R.P. Flores, S. Rijnsburger, A.R. Horner-Devine, and A.J. Souza. The impact of storm and stratication on sediment trasport in the rhine region of fresh water influence. Journal of Geophysical Reseach: Oceans, 122, 2017.

[2] A.R. Horner-Divine, J.D. Pietrzak, A.J. Souza, M.A. McKoen, S. Meirelles, M. Henriquez, R.P. Flores, and S. Rijnsburger Cross-shore transport of nearshore sediment by river plume frontal pumping. Geophysical Reseach Letters, 44, 2017.

[3] Manning, A.J. (2006). LabSFLOC - A laboratory system to determine the spectral characteristics of flocculating cohesive sediments. HR Wallingford Technical Report, TR 156.