Properties of Fluid Mud and Precention of Sedimentation
Type:
Presented during:
WODCON XVIII - 2007 Orlando, Florida, USA
Authors:
Nasner
Abstract: The open tidal harbours of Emden, Bremerhaven and Brunsbuettel on the German North-Sea Coast, are situated in the brackish zone of the tidal rivers Ems, Weser and Elbe. In these zones the intermixture of fresh water coming from inland with salt water coming from the North-Sea results in high concentrations of fine cohesive sediments. In these harbours fluid mud layers can reach a thickness of up to several metres in areas with low current intensity. The annual rates of sedimentation are an order of magnitude higher than in the harbours of Bremen and Hamburg, both situated upstream of the seawater influence. The causes for the accumulation of solid material are the superposition of tidal and density currents.
A research project was initiated (German Ministry of Education and Research, BMBF, registration: 03 KIS 051) to explore the sedimentological and rheological fluid mud properties of different brackish water harbours at the German North-Sea Coast.
The analysis of dual frequency echo soundings (210 kHz and 15 kHz) have shown that the fluid mud volumes in the outer harbours of Emden, Bremerhaven and Brunsbuettel are nearly constant over a period of years. The key for the creation of fluid mud is the presence of a bar at the harbour entrances.
The paper will present results of the in situ measurements and methods for prevention of sedimentation in harbours. The behaviour of the cohesive sediments is determined not only by physical but also by chemical and micro-biological processes. With changing rheological properties (viscosity and shear strength) fluid mud adopts increasing plastic attributes inside a transition zone.
With the knowledge of the current and morphological conditions strategies have been developed with water injections or remedial dredging to Keep the Sediment in the System (KSIS). Management of fluid mud is possible either by Keep the Sediment Moving (KSM) or Keep the Sediment Navigable (KSN).
With these measures conventional dredging in the outer harbours has been reduced to zero, saving costly deposition or dumping of dredged material.
Keywords: fluid mud pattern, navigability, solid matter, ignition loss, density, viscosity.