Suspended Sediment Transport and Fluid Mud Dynamics in Tidal Estuaries

Abstract

Cohesive sediments transport has been systematically studied for more than a century from field studies, laboratory experiments, and mathematical models. During the past decades, the accumulation of flocculated cohesive sediments and the formation of weakly consolidated mud deposits, including fluid mud, gained increased attention. Despite extensive research efforts, the governing processes of fluid mud formation are far from being fully understood. The primary objective of this study is to investigate tide-driven dynamics of fluid mud in estuaries. State of the art hydroacoustic subbottom (SES) and current velocity profilers (ADCP) are used, to measure fluid mud dynamics on appropriate temporal and spatial scales. Connected fields of research are to be taken into account, such as bedload transport and the influence of subaqueous dunes on the turbulent flow field. Technical aspects are considered, in particular the detection of suspended cohesive sediments. Measurements are conducted in the Ems and the Weser estuary, located along the North Sea coast of Germany. Other study sites in the North Sea are the Jade Bay in Germany and the Grådyb tidal inlet in Denmark. Not only the combined deployment of different hydroacoustic profiling devices but also the combined processing of collected data allows fluid mud dynamics to be studied in great detail. Combined processing is implemented in a software tool, programmed in MATLABTM. The software facilitates the acoustic backscatter calibration with respect to suspended sediment concentration (SSC) and thus joins information on hydrodynamic, near-bed density stratification and SSC in the water column. A large data set is collected in the Weser estuary and analysed regarding tide-driven dynamics of fluid mud. Fluid mud does not appear in the suggested form of a contiguous layer, but is deposited in depressions, in troughs of subaqueous dunes, as well as in the form of mud drapes during slack water. Entrainment is controlled by local production of turbulence, which is, in turn, influenced by local morphology. Fluid mud deposits in dune troughs are rapidly entrained, induced by strong turbulent stresses which are generated at the dune crest and advected in direction of the lutocline, i.e. the density gradient between fluid mud and the water column. Mobile mud layers are significantly resistant to entrainment and partly survive half a tidal cycle. Continuous feeding by slack water deposition induces a positive feedback of increasing concentrations and increased damping of turbulence, which inevitably leads to the formation of erosion-resistant estuarine mud deposits. A new method for the detection of density stratification is introduced, based on the backscatter gradient of acoustic current profiles. Furthermore, the gradient Richardson stability criterion is also expressed in terms of the backscatter gradient and proved to be applicable in order to assess lutocline stability under intricate hydrodynamic conditions in the Ems estuary. In the heavily engineered Ems estuary, weir closure during flood slack water and the subsequent release of captured water masses induce the flushing of the estuary and catastrophic downstream advection of fluid mud. By the time the estuary returns to flood-dominated conditions, fluid mud is rapidly advected upstream and re-established in the upper part of the estuary. In the Grådyb tidal inlet channel bedload transport in presence of large dunes is determined on the basis of highly accurate multibeam measurements. Bathymetrical changes are converted into bedload transport rates, which are not predicted by classical bedload transport formulae due to variations in grain-size composition of the mobilised sediment. Results from this study are applied to the Weser estuary to infer that lee-side deposition occurs simultaneously to the entrainment of fluid mud in dune troughs. An instrumental study conducted in the Jade Bay concerning suspended sediment dynamics reveals that suspended sediment is transported in form of turbidity clouds. Thereby, acoustic methods underestimate SSC when large aggregates are present. Several floc populations coexist in the water column, covering a wide range of sizes from a few microns to millimetre size.