The influence of flocculation on the development of a sediment plume

Abstract

A lot of uncertainties exist about the impact of deep-sea mining on the Benthic environment. One of these uncertainties concerns the sediment plumes created by the mining operation. The return slurry released above the sea floor creates a large plume of sediments. With numerical models the behavior of such plumes is modelled in order to minimize these uncertainties. Particle size distributions in these plume models are currently based on individual particle sizes. In situ, at the bottom of the ocean in an salt water environment, flocculation can occur. Flocculation can have effects on the particle size distribution, settling rates and floc structure. The objective of this research is to investigate the significance of flocculation on the settling behavior and if it should be taken into account in these plume models. Shear rate and particle concentration, as function of time, are the most governing processes that have effect on the flocculation behavior. The influence of these parameters on the flocculation process are investigated. Different methods are used in the search for a suitable technique to measure the in situ particle size distribution. Next to the particle size distribution the floc shapes, structure and settling rates are measured under the influence of the factors mentioned above. In the shear range of 6 s-1 and particle concentration of 0.7 to 40 g/l flocculation does occur and has a large impact on the settling behavior of the sediment. Flocculation at other shear rates and particle concentrations remain unknown. Figure \ref{fig:vol_dem_salt_floccu1} shows the time until 100 $\%$ of the mass is settled when the suspension is released at a height of 10 m. This graph shows the difference in settling time between a suspension in demi and salt water. A settling time difference of 85 days (25 days for salt water and 110 days for demi water) shows the importance of including flocculation behavior in plume models.