Influence of re-circulation dredging on fluid mud dynamics in seaport emden

Maintenance of the nautical depth in the seaport of Emden, Germany, is achieved by re-circulation of fluid mud using a trailing suction hopper dredger. Continued re-circulation has proven to maintain low settling rates and to keep yield stresses of re-circulated fluid mud below 50-100 Pa, in combination with densities of 1.15- 1.2 t/m³ enabling safe navigation through these layers. It is assumed that low-density extracellular polymeric substances (EPS), produced by a highly adapted microbial community, are responsible for the desired effect of keeping fine-grained sediment in suspension and that the regular contact with the oxygenated water phase during recirculation dredging supports thriving of this community. Climate change models have indicated increased future discharge of fresh water from the hinterland into the saline port of Emden. It is hypothesized that increased freshwater discharge alters the composition and activity of the microbial community and hence changes the prerequisites for maintaining the currently favourable properties of fluid mud. In this paper we evaluate this hypothesis by elucidating the main factors governing the success of the current practice of re-circulation dredging in the Port of Emden by means of field and laboratory investigations. Initial results obtained in this ongoing research suggest that the low yield stresses of fluid mud in the Port of Emden are not the result of a specialized microbial community producing high concentrations of EPS, but can be solely attributed to a reduction in density as achieved by re-circulation dredging. Secondly, it appears that future possible increases in the share of freshwater do not affect the sediment’s rheological response, settling behaviour and concentration of extracellular polymeric substances and will hence not adversely impact the maintenance of the nautical depth.