a b s t r a c tMany estuaries worldwide are becoming more urbanised with heavier traffic in the waterways, requiring continuous channel deepening and larger ports, and increasing suspended sediment concentration (SSC). An example of a heavily impacted estuary where SSC levels are rising is the Ems Estuary, located between the Netherlands and Germany. In order to provide larger and larger ships access to three ports and a shipyard, the tidal channels in the Ems Estuary have been substantially deepened by dredging over the past decades. This has led to tidal amplification and hyper concentrated sediment conditions in the upstream tidal river. In the middle and outer reaches of the Ems Estuary, the tidal amplification is limited, and mechanisms responsible for increasing SSC are poorly understood. Most likely, channel and port deepening lead to larger SSC levels because of resulting enhanced siltation rates and therefore an increase in maintenance dredging. Additionally, channel deepening may increase up-estuary suspended sediment transport due to enhanced salinity-induced estuarine circulation.The effect of channel deepening and port construction on SSC levels is investigated using a numerical model of suspended sediment transport forced by tides, waves and salinity. The model satisfactorily reproduces observed water levels, velocity, sediment concentration and port deposition in the estuary, and therefore is subsequently applied to test the impact of channel deepening, historical dredging strategy and port construction on SSCs in the Estuary. These model scenarios suggest that: (1) channel deepening appears to be a main factor for enhancing the transport of sediments up-estuary, due to increased salinity-driven estuarine circulation; (2) sediment extraction strategies from the ports have a large impact on estuarine SSC; and (3) maintenance dredging and disposal influences the spatial distribution of SSC but has a limited effect on average SSC levels.
Flow expansions are a fundamental mechanism of sediment deposition. Here we define flow expansions as selfformed alluvial expanding deposits developing over a larger experimental delta that 1) are characterized by unchannelized flow, comprising a single sheet flow that covers the entire surface of the expansion, and (2) do not interact laterally with still fluid and thus are not related to jets. The flow-spreading mechanism is transverse topographic curvature rather than mixing with ambient fluid; hence we term these expansions ''topographic expansions'' to distinguish them from jet-related flow expansions. In topographic expansions, transverse bed convexity drives lateral flow, allowing topographic expansions to maintain much higher, and more variable, opening angles than those of jet-related expansions. The characteristic depositional body produced by the expansions is a relatively thin, tabular sand body with a flat base and slightly convex top, comparable to some ''sheet flood'' deposits in the stratigraphic record. Most of the deposition takes place as the topographic expansion develops, followed by a longer interval of stable sediment bypass, where all measured flow expansions show similar geometries. This stable-bypass phase is typically terminated by the flow abandoning the deposit or by channelization of the deposit itself, after which the cycle can begin again. One of the research questions raised by this study is the mechanism by which topographic expansions are able to maintain themselves in an unchannelized condition at flow aspect ratios that should be unstable according to current theories of bar and channel instability.
Mine tailings consisting of sands, silts and clays may segregate during deposition and flow, leading to problems with storage capacity and strength development of settling pond deposits. Non-segregating tailings (NST) may effectively limit the effect of segregation by increasing the amount of fines retained in deposited tailings. Segregation can be minimised once the settling velocity of the coarse fraction of the tailings is determined as a function of the rheological properties and shear rate. Shear cell testing provides a bench-scale test method to quantify settling velocities of the coarse fraction of slurries. Within the tested conditions, measured settling velocities are about twice as large as settling velocities calculated based on apparent viscosity and hindered settling.
In beaching of tailings, sand and clays may segregate. In laminar flow this is due to shear settling. First implementations of shear settling in numerical flow models are seen, offering unprecedented potential to conduct tailings management studies. In order to validate numerical codes, reference materials are necessary. For laminar flow, there is a small set of flume tests available from an earlier study. An analytical solution for transient sand concentration profile development with distance in laminar open channel flow appeared recently. This analytical method is more complete than an analytical model developed earlier at the author's institute. Data and analytical solutions are analysed and applied to serve for the validation of numerical flow simulation of beaching in tailings storage facilities. Fair agreement is observed between measurements and the analytical method. Moreover, fair agreement is obtained between an earlier produced computational outcome of the numerical model Delft3D-slurry and analytical solution. This contributes to building confidence in this model as an aid in supporting tailings deposition management.
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