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Tidal sand ridges are large-scale bedforms with horizontal dimensions of several kilometers and heights in the order of tens of meters, which occur on outer shelves of coastal seas. In order to study the long-term evolution of these ridges, an idealized nonlinear numerical model was developed. With this tool, the sensitivity of the characteristics of these finite-height ridges, in particular, their shape and growth time, to 1D/2D configuration (topography varies in one/two horizontal dimensions), tidal ellipticity and critical bed shear stress for sand erosion was investigated. In the case of a 1D configuration, the root mean square height hrms of the bedforms first grows exponentially and hereafter saturates. In the end, ridges in static equilibrium are obtained, i.e., hrms remains constant. In contrast, when the configuration is 2D, ridges are found with spatially meandering crests that oscillate in time. Initially the bedforms are composed of a finite number of bottom modes. The meanders occur if bottom modes with crests normal to those of the initially preferred bedform exist, and their topographic wavenumbers are in the order of that of the preferred bedform or smaller. In addition, the vertical distance between the crest and trough levels should be larger than around 80% of the maximum water depth. Generally, the global growth time, i.e., the time at which hrms stops increasing after the exponential growth stage of the bedforms, is slightly larger for a 2D than for a 1D configuration. The ridge shapes are sensitive to the tidal ellipticity, while they are hardly sensitive to the critical bed shear stress. The global growth time varies non-monotonically with the tidal ellipticity, and it increases if the critical bed shear stress is included. Comparison between the model results and field observations suggests that the model is able to simulate the gross characteristics of the Dutch Banks and the Flemish Banks in the southern North Sea and that these ridges may still be growing. [ABSTRACT FROM AUTHOR]

Tidal sand ridges and long bed waves are large-scale bedforms that are observed on continental shelves. They differ in their wavelength and in their orientation with respect to the principal direction of tidal currents. Previous studies indicate that tidal sand ridges appear in areas where tidal currents are above 0.5 m s −1 , while long bed waves occur in regions where the maximum tidal current velocity is slightly above the critical velocity for sand erosion and the current is elliptical. An idealized nonlinear numerical model was developed to improve the understanding of the initial formation of these bedforms. The model governs the feedbacks between tidally forced depth-averaged currents and the sandy bed on the outer shelf. The effects of different formulations of bed shear stress and sand transport, tidal ellipticity and different tidal constituents on the characteristics of these bedforms (growth rate, wavelength, orientation of the preferred bedforms) during their initial formation were examined systematically. The results show that the formulations for bed shear stress and slope-induced sand transport are not critical for the initial formation of these bedforms. For tidal sand ridges, under rectilinear tidal currents, increasing the critical bed shear stress for sand erosion decreases the growth rate and the wavelength of the preferred bedforms significantly, while the orientation angle slightly decreases. The dependence of the growth rate, wavelength and the orientation of the preferred bedforms on the tidal ellipticity is non-monotonic. A decrease in tidal frequency results in preferred bedforms with larger wavelength and smaller orientation angle, while their growth rate hardly changes. In the case of joint diurnal and semidiurnal tides, or spring-neap tides, the characteristics of the bedforms are determined by the dominant tidal constituent. For long bed waves, the number of anticyclonically/cyclonically oriented bedforms with respect to the principal current direction increases as the ellipticity of the cyclonic/anticyclonic tidal currents increases. Besides, under anticyclonic tidal currents, the growth rate of cyclonically oriented long bed waves increases as the tidal ellipticity increases. The model was also used to provide a possible explanation for the fact that the Dutch Banks have a larger wavelength than that of the Flemish Banks in the North Sea. [ABSTRACT FROM AUTHOR]

Field observations in estuaries reveal that phytoplankton blooms are strongly affected by advection processes related to river flow. To gain quantitative insight into this dependence, experiments were performed with a new idealised model that couples physical and biological processes. Advection of phytoplankton and nutrient by subtidal flow was explicitly accounted for, as well as longitudinal and vertical mixing processes. Results show that the idealised model is capable of reproducing the observed bloom. The specific spatial distribution of phytoplankton population emerges because the latter is suppressed in the upper reach by the advection processes, and the growth is limited in the lower reach by low nutrient concentrations. A sensitivity study of model results to different river discharges reveals the presence of three regimes. In the low discharge regime, blooms form because growth is faster than decay due to advection processes. In the high discharge regime, the situation is opposite and no blooms form. If time scales of growth and advection are comparable (in moderate discharge regime), phytoplankton population increases significantly slower compared to the low discharge regime. Results of additional model runs, in which water depth and the e -folding length scale of estuarine width convergence were varied, revealed that the three regimes occur in all these cases. [ABSTRACT FROM AUTHOR]

The effect of the joint action ofM2andM4tidal flow, residual flow and spatial settling lag on the lateral entrapment of sediment is examined in tidally dominated estuaries with an idealized model that assumes along-estuary uniform conditions. Approximate solutions are obtained for arbitrary cross-channel bed profiles by scaling and perturbation analysis. The hydrodynamics include externally drivenM2tidal flow, externally and internally drivenM4tidal flow and residual flow driven by horizontal density gradient, river discharge and nonlinear advection. The sediment concentration includes a mean component, anM2component driven by bed erosion and anM2component driven by both bed erosion and inertial terms. Sediment availability is calculated by imposing a morphodynamic equilibrium condition. The model is applied to a transect in the James River estuary where data of flow and suspended sediment concentration are available. Two types of sediment are separately considered, viz., fine silt and coarse silt. Residual advective transport of sediment by the lateral flow induces trapping of sediment over the left shoal (looking landward). Model results also show that the incorporation ofM4tidal flow and spatial settling lag leads to a second sediment trapping region over the right shoal. Model results are qualitatively in good agreement with the observations. [ABSTRACT FROM AUTHOR]

The residual circulation over the continental slope, and in particular, its vertical structure, is analysed by means of an idealised hydrodynamic model. The model is based on the depth-dependent shallow-water equations under uniform along-isobath conditions and is forced by a prescribed meridional density gradient and tidal velocities. By means of expansion in the small Rossby number ϵ solutions are analysed for conditions representative for the continental slopes off the Hebrides and in the Bay of Biscay. The steady solution at zeroth order consists of a linear density-driven flow. At order ϵ a tidally rectified flow is found and a stationary flow due to self-interaction of the zeroth-order density-driven flow. At order ϵ2 the leading-order effect of the interaction between the zeroth-order density-driven flow and the tides is found: the ‘interaction current’. The solutions up to and including order ϵ2 constitute an along-isobath steady slope current which is comparable to field data. The slope current and the accompanying cross-shelf circulation depend strongly on the shelf and flow characteristics. For the Hebridean case the density forcing predominates, but for the Biscay case the tidal effects are of the same order of magnitude as the density effects. Under those conditions the interaction current is significant which implies that linear superposition of density and tidal effects differs from the non-linear combination of both. It is also shown that the depth-average of the interaction current differs essentially from the solution obtained from a depth-averaged model. [Copyright &y& Elsevier]

Estuaries are often characterised by a complex network of branching channels, in which the water motion is primarily driven by tides and fresh water discharge. For both scientific reasons and management purposes, it is important to gain more fundamental knowledge about the hydrodynamics in such networks, as well as their implications for turbidity and ecological functioning. A generic 2DV estuarine network model is developed to study tides and river water transport and to understand the dependence of their along-channel and vertical structure on forcings, geometry characteristics and sea level changes. The model is subsequently applied to the Yangtze Estuary to investigate tides and the distribution of river water over channels during dry and wet season, spring tide, as well as prior to and after the formation of Hengsha Passage and the construction of the Deep Waterway Project and sea level rise. Increasing river discharge enhances the friction for tides by increasing both internal and bottom stresses. Changes in tidal forcing are correlated with the friction for both tide and river. A shortcut channel reduces the water level difference in adjacent channels, as well as tidal amplitudes difference. Sea level rise results in larger friction parameters and faster propagation of tides. The distribution of river water transport is hardly affected by above mentioned changes. Model results and current vertical structure are consistent with observations. • We developed a 2DV semi-analytical estuarine network model. • Various modifications are explained by the interaction between friction parameters. • Impacts of local geometric changes can be either local or global. [ABSTRACT FROM AUTHOR]

Abstract: To improve navigability of the major access channel into Shanghai Harbor, a large-scale Deep Waterway Project was carried out in the North Passage (NP) of the Yangtze Estuary. In this paper, we investigate how the navigational works affected morphological changes of this channel, as well as those of the adjacent North Channel (NC), Hengsha East Shoal (HES), Jiuduansha Shoal (JDS), and the South Passage (SP). Morphological changes were assessed by analyzing digitized bathymetric data of this area prior to and after execution of the engineering works. The qualitative relations between these changes, along with the hydrodynamic changes as a result of the construction of engineering works, were subsequently investigated. The results reveal that the construction of training walls, groins and jetties resulted in decreased ebb transport in the upper reach of the NP and increased ebb transport in the upper reach of the SP. In turn, this led to intensive upstream erosion with large amounts of sediment transported seaward in the SP and associated sedimentation in the downstream area. At the same time, intense siltation occurred in the upper reach of the NP, while the main channel of the NP mainly experienced erosion caused by the construction of training walls and groins that concentrated ebb flow in the main channel. The waterway deepened significantly in previously shallow areas. Small tidal channels in the HES, which used to connect the NP and the NC, were cut off by the northern training wall. Consequently, residual flow in the middle reach of the NC flowed directly toward the northern bank of the HES, leading to local erosion. Flow obstruction by the southern training wall reduced the upstream propagation of the flood tide in the SP. As a result, a collection of small flood channels evolved under the flushing action of flood currents in the JDS, preventing the natural trend of horizontal extension of the JDS. However, the flow obstruction enhanced the vertical accretion of the JDS. [Copyright &y& Elsevier]