Uncertainty in complex three-dimensional sediment transport models: equifinality in a model application of the Ems Estuary, the Netherlands

Estuarine suspended sediment transport models are typically calibrated against suspended sediment concentration data. These data typically cover a limited range of the actual suspended sediment concentration dynamics, constrained in either time or space. As a result of these data limitations, the available data can be reproduced with complex 3D transport models through multiple sets of model calibration parameters. These various model parameter sets influence the relative importance of transport processes such as settling, deposition, erosion, or mixing. As a result, multiple model parameter sets may reproduce sediment dynamics in tidal channels (where most data is typically collected) with the same degree of accuracy but simulate notably different sediment concentration patterns elsewhere (e.g. on the tidal flats). Different combinations of model input parameters leading to the same result are known as equifinality. The effect of equifinality on predictive model capabilities is investigated with a complex three-dimensional sediment transport model of a turbid estuary which is subject to several human interventions. The effect of two human interventions (offshore disposal of dredged sediment and restoration of the tidal channel profile) was numerically examined with several equifinal model settings. The computed effect of these two human interventions was relatively weakly influenced by the model settings, strengthening confidence in the numerical model predictions.Environmental Fluid Mechanic

Sediment trapping in the Zeebrugge Coastal Turbidity Maximum

The mechanisms leading to the formation and maintenance of a Coastal Turbidity Maximum (CTM) along the Belgian coastline was investigated using a complex 3D numerical model. Interpretation of model results suggests that the sediment concentration in this CTM may have strongly increased as a result of human interventions in the 1980’s. The large amount of sediment that became availablethen triggered self-organizing mechanisms that maintain or strengthen the CTM.Environmental Fluid Mechanic

Residual flow and tidal asymmetry in the Singapore Strait, with implications for resuspension and residual transport of sediment

The Singapore Strait connects the South China Sea, where tides are dominantly diurnal, to the dominantly semidiurnal Indian Ocean. At this transition, the tidal water level oscillations are observed to be semidiurnal while the tidal current oscillations are mixed, diurnal to fully diurnal. Due to the interaction of the diurnal constituents with the semidiurnal M2 tide, the tides are strongly asymmetric. Both residual flows and subtidal flows, with periodicities of 2 weeks to 1 year, are strong. In order to analyze and explain the hydrodynamics around Singapore, a well-documented and calibrated regional tidal model application was further improved and validated. Analysis of the results of this model shows that the diurnal tidal wave is primarily standing, with an amphidromic point close to Singapore, explaining the dominantly diurnal current and semidiurnal water level oscillations. Analysis of the model results further indicates that the fortnightly constituents in the subtidal flow are probably compound tides, with a combined amplitude over 10 cm/s. Pronounced yearly and half-yearly cycles in spring tidal current amplitude and asymmetry exist, resulting from interaction of the diurnal and the semidiurnal spring-neap cycles, compound tides, and the monsoon currents. A simple analytical transport formula was applied to determine the relative importance of tidal asymmetry and residual flows, verified with a full sediment transport model. With fine sediment being more sensitive for residual flow and coarser sediment for tidal flow, a pronounced divergence in sediment transport pathways may exist, depending on the grain size.Hydraulic EngineeringCivil Engineering and Geoscience

The shape of fringing tidal flats in engineered estuaries

For the management of estuaries and the preservation of tidal flats it is crucial to understand the tidal flat shape and development. Previous work focused predominantly on the quasi-equilibrium shape of tidal flats along open coasts with a dominant cross-shore flow and wave exposure. This paper evaluates the shape of fringing tidal flats in engineered estuaries, where longshore velocities generally dominate. Using a long-term (20 years) topographic data set of an anthropogenically modified estuary in the Netherlands (the Western Scheldt estuary), we relate key profile shape parameters and changes over time to natural and anthropogenic processes. In an engineered estuary, the tidal flat shape depends on the estuary geometry, hydrodynamic forcings and human interventions. In contrast to open coast tidal flats, the presence of the channel and dominant longshore flow determines the available cross-shore length (accommodation space) of the tidal flat and the shape of the tidal flat. This accommodation space defines the maximum tidal flat height and opportunity for marsh development. We propose the use of the Index of Development, indicating to what extend tidal flats have space to develop. This index is not only influenced by longshore and cross-shore flow, but also (or even more) by hydraulic structures, dike realignments and channel migration. Especially the latter two strongly influence the accommodation space and thereby the maximum tidal flat height and the opportunity for marsh development. For large stretches of the Western Scheldt, the accommodation space is too small, and the majority of the tidal flats do not vertically extent to mean high water. The success of tidal flat and marsh restoration projects depends on the accommodation space.Coastal EngineeringEnvironmental Fluid Mechanic

Tidal response to polder construction in the Pussur-Sibsa estuary

Environmental Fluid Mechanic

Observations of cross-shore chenier dynamics in Demak, Indonesia

Cheniers are important for stabilising mud-dominated coastlines. A chenier is a body of wave-reworked, coarse-grained sediment consisting of sand and shells overlying a muddy substrate. In this paper we present and analyse a week of field observations of the dynamics of a single chenier along the coast of Demak, Indonesia. Despite relatively calm hydrodynamics during the one-week observational period, the chenier migrated surprisingly fast in the landward direction. The role of the tide and waves on the cross-shore chenier dynamics is explored using velocity moments as a proxy for the sediment transport. This approach shows that both tide and waves are capable of transporting the sediment of the chenier system. During calm conditions (representative for the south-east monsoon season), the tides generate a landward-directed sediment transport when the chenier crest is high relative to mean sea level. Waves only generate substantial sediment transport (direct, via skewness, and indirect, via stirring) when the chenier is submerged during periods with higher waves. The cross-shore chenier dynamics are very sensitive to the timing of tide and waves: most transport takes place when high water levels coincide with (relatively) high waves.Environmental Fluid Mechanic

Tidal Response to Land Reclamation in Single- and Multichannel Estuarine Systems

Intertidal areas play a crucial role in controlling tidal hydrodynamics and morphodynamics in estuaries and tidal inlets. As a consequence, widespread land reclamation of the intertidal zone has led to alterations in tidal dynamics and the associated morphodynamics of estuaries worldwide. Comparatively little research has focussed on the impact of width changes on tidal hydrodynamics, and results that do exist are highly ambiguous (Talke and Jay, 2020). Tie channels in between parallel inlets complicate the tidal motion, as multichannel systems exhibit a hydrodynamic response that differs significantly from the response observed in single channel systems. In this study, we combine idealized process-based modelling and historical data analysis to investigate the effects of intertidal land reclamation on the tidal dynamics of single- and multichannel systems. Specifically, we focus on the Scheldt and Pasur-Shibsa estuaries, which represent a single- and multichannel system, respectively.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Environmental Fluid Mechanic

Chenier Formation Through Wave Winnowing and Tides

Cheniers are ridges consisting of coarse-grained sediments, resting on top of the fine sediment that forms the otherwise muddy coast. In this paper, we use Delft3D to explore how cheniers are formed through wave winnowing. We identify three phases of chenier development: (a) a winnowing phase, during which mud is washed out of the seabed initially consisting of a mixture of sand and mud, (b) a sand transport phase, when the sand in the upper layer is transported onshore, and (c) a crest formation phase, during which a chenier crest rapidly develops at the landward limit of onshore sediment transport. The main mechanism driving onshore sand transport is wave asymmetry. During calm conditions, sand transport takes place within a narrow band limiting the volume of sand delivered nearshore, and therefore no chenier develops. In contrast, average storm conditions mobilize sufficient sand for a crest to develop. Our results thus reveal that chenier formation through wave winnowing does not require extreme storm conditions. Furthermore, our study showed that chenier formation through wave winnowing is a relatively slow process, with the largest time scales associated with the winnowing and sand transport. Once sufficient sand is available in the intertidal zone, the crest develops rapidly.Environmental Fluid Mechanic

Fine sediment transport into the hyperturbid lower Ems River: The role of channel deepening and sediment-induced drag reduction

Deepening of estuarine tidal channels often leads to tidal amplification and increasing fine sediment import. Increasing fine sediment import, in turn, may lower the hydraulic drag (due to a smoother muddy bed and/or sediment-induced damping of turbulence), and therefore, further strengthen tidal amplification, setting in motion a process in which the sediment concentration progressively increases until the river becomes hyper-turbid (Winterwerp and Wang, Ocean Dyn 63(11–12):1279–1292, 2013). To advance our understanding of the relative role of bed roughness and bed topography on sediment import mechanisms and sediment concentration, a Delft3D numerical model has been setup for an estuary which has been deepened and as a consequence experienced a strong increase in suspended sediment concentration: the lower Ems River. This model is calibrated against present-day hydrodynamic and sedimentary observations, and reproduces the basic sediment transport dynamics despite simplified sedimentological formulations. Historic model scenarios are semi-quantitatively calibrated against historic high and low water observations, revealing that changes in hydraulic roughness and deepening are probably equally important for the observed tidal amplification. This model is subsequently used to better understand historic changes in the hydrodynamic and sediment transport processes in the lower Ems River. Import of fine sediment has increased because of larger tidal transport, even though the degree of tidal asymmetry may not have significantly changed. The resulting rise in suspended sediment concentration reduced hydraulic drag, amplifying the tidal range. Export of fine sediment became less because the river-induced residual flow velocity decreased with deepening of the channel.Hydraulic EngineeringCivil Engineering and Geoscience

On the hindered settling of silt-water mixtures

The sedimentation behavior of silt particles in the hindered settling regime has been considered. Therefore, laboratory experiments are carried in especially designed settling columns. Silt-water mixtures are prepared and allowed to settle at various initial concentrations. During the settling process, a continuous vertical concentration profile is measured. Settling velocities are obtained from these profiles and are compared with the widely used Richardson and Zaki (1954) expression. This comparison showed that the Richardson and Zaki expression underestimates the settling velocities for the finest silts at high concentrations. For the coarsest silt the Richardson and Zaki expression performed will with the measured data. We anticipate that this is the effect of the particle size on the apparent viscosity of the settling silt-water mixture.Hydraulic EngineeringCivil Engineering and Geoscience