M. van der Vegt

Morphodynamics of ebb-tidal deltas: a model approach

Abstract The results of 2DH numerical models of the Frisian Inlet (located in the Dutch Wadden Sea) are discussed to gain further knowledge about the physical mechanisms causing the presence of both ebb-tidal deltas and of channels and shoals in tide-dominated inlet systems. A hydrodynamic model, extended with sediment transport formulations, was used to verify earlier conceptual models that deal with ebb-tidal delta characteristics. The model does not confirm their hypothesis concerning the observed spatial asymmetry of ebb-tidal deltas and suggests that long-term morphological simulations are needed to understand this aspect. Furthermore, the model indicates that the initial formation of the ebb-tidal delta is mainly due to convergence of the tidally averaged sediment flux related to residual currents, whilst the net sediment transport in the basin is mainly caused by tidal asymmetry. A second model (accounting for feedbacks between tidal motion and the erodible bottom) was used to simulate the long-term bathymetric evolution of the Frisian Inlet under fair weather conditions. This model reproduces the gross characteristics of the observed morphology: the presence of a double-inlet system with two distinct ebb-tidal deltas having different sizes and the presence of channels and shoals. The role of the ‘Engelsmanplaat’, a consolidated shoal in the middle of the Frisian Inlet, was not found to be crucial for the morphodynamic stability of this inlet system.

Numerical simulation and analysis of saltwater intrusion lengths in the Pearl River Delta, China

ABSTRACT Zhang, W.; Feng, H.C.; Zheng, J.H., Hoitink, A.J.F.; Van Der Vegt, M.; Zhu, Y., and Cai, H.J., 2013. Numerical simulation and analysis of saltwater intrusion lengths in the Pearl River delta, China. In recent years, large-scale saltwater intrusion has been threatening the freshwater supply in the metropolitan cities surrounding the Pearl River delta (PRD). Therefore, a better understanding of the saltwater intrusion process in this region is necessary for local water resource management. In this paper, a one-dimensional flow and salinity model of the Pearl River networks was established to improve our understanding of saltwater intrusion problems in deltas. The model has high spatial resolution, discretized into 328 reaches and 5108 cross-sections, and the time step is 300 seconds for the hydrodynamic model and 30 seconds for the salinity model on the basis of the Courant–Friedrichs–Lewy condition. The model is calibrated and validated against the field measurements of the water surface elevation, discharge, and salinity at around 40 gauges in 2005 and 2001, respectively. The estimated results are in reasonable agreement with the observational data, suggesting that the model is sufficiently robust to simulate the movement of flow and salinity in the Pearl River networks. The simulated 0.5 parts per thousand salinity isohaline in the Pearl River networks displays a shape similar to “S” and slanting to the right, indicating that the maximum saltwater intrusion length occurs at the Humen outlet. In 2005, the saltwater intrusion lengths intruded far upstream at an average length of 32.4 km from the eight outlets, which is nearly two times that in 2001. Four representative upstream flows were also simulated to acquire quantitative knowledge of the response of the saltwater intrusion to discharges. Finally, historical data were collected to compare the situations of saltwater intrusion in the river networks in the 1960s and 2005. The result implies that the abrupt change in topography due to intensive dredging campaigns in the river networks is probably the most crucial factor leading to the saltwater intrusion outbreaks in large areas of the PRD in recent years.

Turning the tide: experimental creation of tidal channel networks and ebb deltas

Tidal channel networks, estuaries and ebb deltas are usually formed over a period longer than observations cover. Much is known about their characteristics and formation from linear stability analyses, numerical modelling and field observations. However, experiments are rare whilst these can provide data-rich descriptions of morphological evolution in fully controlled boundary and initial conditions. Our objective is to ascertain whether tidal basins can be formed in experiments, what the possible scale effects are, and whether morphological equilibrium of such systems exists. We experimentally created tidal basins with simple channel networks and ebb deltas in a 1.2 by 1.2 m square basin with either a fixed or self-formed tidal inlet and initially flat sediment bed in the tidal basin raised above the bed of the sea. Rather than create tides by varying water level, we tilted the entire basin over the diagonal. The advantage of this novel method is that the bed surface slopes in downstream direction both during flood and ebb phases, resulting in significant transport and morphological change in the flood phase as well as the ebb phase. This overcomes the major problem of earlier experiments which were entirely ebb-dominated, and reduces the experiment time by an order of magnitude. Ebb deltas formed in sand were entirely bedload dominated whereas the lightweight plastic sediment was intermittently suspended. Channels bifurcated during channel deepening and backward erosion to form a network of up to four orders. For initially dry tidal plains, the tidal prism increased as more sediment eroded from basin to ebb delta, so that evolution accelerated initially. The rate of change, the size of the channels and the final length of channels and delta were very sensitive to the tidal amplitude, tidal period and initial water depth in the basin. Most experiments with sand terminated with all sediment below the threshold for motion, whilst lightweight sediment remained mobile in the inlet region and firstorder channels, suggesting that sustained morphodynamics are feasible in experiments. We discuss how this novel experimental setup can be extended to produce tidal deltas, estuaries and other tidal systems and study their dynamics as a function of their forcing.

Wind forcing controls on river plume spreading on a tropical continental shelf

The Berau Continental Shelf is located close to the Equator in the Indonesian Archipelago, hosting a complex of coral reefs along its oceanic edge. The Berau coral reefs have a very high biodiversity, but the area is under serious risk due to river-derived nutrients and sediments. The region is characterized by weak winds, moderate tides, and almost absent Coriolis forcing. Existing knowledge about river plume behavior in tropical environments is limited. The aim of this paper is to investigate the influence of the subtle physical forcing on the dynamics of the Berau river plume. A three-dimensional model (ECOMSED) was calibrated with observational data. The model was forced by freshwater input from the Berau river distributaries, tides at the open boundaries, and measured hourly wind. The model reproduces the freshwater dynamics on the shelf adequately and highlights that the river plume spreads symmetrically for river forcing only. Tides cause vertical mixing and suppress the cross-shelf spreading of the river plume. However, the spreading of the river plume over the shelf is mainly controlled by the weak monsoonal winds, resulting in a seasonal development. During the Southeast Monsoon, the southerly winds push the plume northeastward and cause a stratified water column in the northern part of the continental shelf. Northerly winds during the Northwest Monsoon disperse the plume to the south, promoting a vertically well-mixed water column. The results can be used to predict the possible impact of land-use changes in the steadily developing Berau region on coral reef health.

Geometry of tidal inlet systems: A key factor for the net sediment transport in tidal inlets

The net transport of sediment between the back-barrier basin and the sea is an important process for determining the stability of tidal inlet systems. Earlier studies showed that in a short basin, tidal flats favor peak ebb-currents stronger than peak flood currents, implying export of coarse sediment, while shallow basins favor stronger flood currents. The new elements considered in this study are (1) arbitrary basin lengths, (2) a narrow inlet that connects the basin to the sea, (3) an asymmetric tidal forcing, and (4) radiation damping. The objective is to gain fundamental insight in how the geometry of a tidal inlet system affects the net sand transport in a tidal inlet. For this purpose, a width-averaged and depth-averaged analytical model was constructed. It is found that the length of a back-barrier basin controls the effect that nonlinear hydrodynamic processes have on the tidal asymmetry, and consequently controls whether the currents in the inlet are flood-dominant or ebb-dominant. Furthermore, the cross-sectional area of the inlet controls the ratio between the net sediment transports that results from tidal asymmetry and that caused by the interaction of the principal tide with the residual current. Finally, it is shown that the effect of an asymmetric tidal forcing on the net sand transport depends on the length of the back-barrier basin with respect to the tidal wavelength in that basin.

Morphodynamics of a storm-dominated, shallow tidal inlet: the Slufter, the Netherlands

In this article we study the morphodynamics of the Slufter on the short-term (months) and long-term (years to decades). The Slufter is a small, shallow tidal inlet located on the island of Texel, the Netherlands. A narrow (tens of meters) channel connects the North Sea with a dune valley of 400 ha. This narrow channel is located in between a 400-700 m wide opening in the dunes. Approximately 80% of the basin of the Slufter is located above mean high water level and is flooded only during storms, when a threshold water level is exceeded. Analysis of historical aerial photographs revealed that the inlet channel migrates about 100 m per year. In the 1970s it migrated to the south, while since 1980 it is migrating to the north. When the channel reached the dunes at the north side of the dune breach the channel was relocated to the south by man. The channel inside the backbarrier basin was less dynamic. It shows a gradual growth and southward migration of a meander on a decadal time scale. The short-term dynamics of the Slufter were studied during a field campaign in 2008. The campaign aimed at identifying the dominant hydrodynamic processes and morphological change during fair weather conditions and during storm events. During fair weather flow velocities in the main inlet channel were 0.5-0.8 m/s at water depths of 0-1.5 m, slightly ebb-dominant and associated morphological change was small. When water levels were above critical levels during a storm period the hydrodynamics in the main channel drastically changed. The flow in the main channel was highly ebb dominant. Long ebb periods with typical flow velocities of 2 m/s were alternated by much shorter flood periods with typical velocities of 0.5-1 m/s. This resulted in a net outflow of water via the main channel, while we measured a net inflow of water at the beach plain. During the storm period in 2008 we measured a 10 m migration of the channel to the north. We conclude that the Slufter is a storm-dominated tidal inlet system.

Observations of waves and currents during barrier island inundation

Overwash and inundation on barrier islands can transport sediment onshore, leading to vertical accretion. These processes could ensure barrier island growth in times of sea-level rise, but wave and current fields during overwash and inundation are not well understood. Field data of water levels, waves, and currents were collected on a barrier island in the Netherlands to investigate the hydrodynamics during island inundation. Observations show that even in shallow water depths (<0.5 m) wave energy was not completely dissipated as waves propagated from the North Sea onshore. Additionally, locally generated wind waves entered the field area from the Wadden Sea and propagated offshore. Infragravity waves were an important part of the wave field, particularly onshore of the beach crest. They were observed to be onshore progressive and displayed a bore-like shape when water depths were shallow. Wave breaking was the dominant dissipation mechanism for high-frequency waves as well as for infragravity waves, which is in agreement with prior research on infragravity wave energy dissipation on mild sloping (closed-boundary) beaches. A large-scale offshore directed water-level gradient between the Wadden Sea and the North Sea side, caused by elevated water levels in the Wadden Sea during the storms, frequently drove an offshore flow if it was large enough to exceed the cross-shore gradient due to wave setup. In addition, elevated water levels in the Wadden Sea decreased current velocities due to a decrease in water-level gradients. This study highlights the influence of back-barrier processes on the hydrodynamics during inundation.

Hydrodynamics of the Rhine ROFI near IJmuiden

The paper will focus on hydrodynamics of the Rhine ROFI in its most northerly extent near IJmuiden. A bottom-mounted ADCP was deployed near IJmuiden and measured for almost 6 months. Observations show that the largest difference in ellipticity between surface and bottom mostly occurred during neap tide, suggesting that the water column was stratified, leading to a cross-shore circulation. However, for 3 out of the 11 neap tides no ellipticity difference was observed and during 1 out of the 10 spring tides a strong ellipticity difference occurred. To understand the causes of these irregularities a model was set up with GOTM. Based on the depth-averaged currents the barotropic pressure gradient is determined. Using a simple advection equation the salinity profile was estimated at the measurement location and used as input for the model. We were able to simulate the overall characteristics of the observed flow patterns. Model results show a strong link between wind stress magnitude and direction and reduced stratification during low energetic neap tides. An increased fresh water discharge was the cause for the strong ellipticity difference during spring tide. The strong effect of wind speed and direction on the onset of stratification of the Rhine ROFI has never been shown before. Furthermore, the results show that during the 150 days of observations the plume always reached IJmuiden.

Modeling the growth and migration of sandy shoals on ebb-tidal deltas

Coherent sandy shoals that migrate toward the downdrift coast are observed on many ebb-tidal deltas. In this study, processes that cause the growth and migration of shoals on ebb-tidal deltas are identified. Moreover, the effect of the incident wave energy and the tidal prism of an inlet on the migration speed of these shoals is investigated. For this, a numerical morphodynamic model with an idealized geometric setup is employed. The model computes the bed level evolution due to local erosion and deposition of sand driven by tides and waves. Analysis of model results shows that shoals grow when there is a local imbalance between the bathymetry and the wave conditions, which in this study was imposed by manually breaching the ebb-tidal delta or by adding storms to the wave forcing. There are thresholds for shoal formation that depend on the distribution of the sand and the incident wave energy. Wave refraction over the shoals leads to focusing of wave energy and increased wave energy dissipation around the location of the local minimum water depth. This generates residual currents over the shoal and increased skin friction toward the local minimum water depth, which together create a sand transport pattern that induces the growth and migration of the shoal. Sand transport due to asymmetric waves contributes to keeping the shoal a coherent structure. The shoal migration speed increases with increasing incident wave energy and decreasing tidal prism; this is because tidal residual currents oppose the wave-driven residual currents that cause shoal migration.

Discharge distribution and salt water intrusion in the Rhine-Meuse river delta network

Tidal river networks form complex environments in which both river and tidal processes play an important role in determining the hydrodynamics. Some research has been conducted on singlejunction tidal rivers, but for tidal networks the available research is very limited. This paper aims to take a first step in the understanding of the development of hydrodynamics and morphology of tidal river networks by analysing flow velocity data in the Rhine-Meuse tidal network in the western Netherlands. The Rhine-Meuse tidal river network is a heavily engineered system in which both hydrodynamics and morphology are in large part determined by anthropogenic influences. In May 2011, 13-hour measurements of flow and salinity have been conducted at twelve different tidal junctions. Analysis of the data shows that seaward junctions are subject to a distinct vertical layering of the flow, while upstream junctions display an increasingly horizontal flow differentiation. Although tidal flow generally decreases when moving upstream, local morphology is an important factor in determining tidal flow velocities in individual channels. The contribution of salt water to the total river flux depends highly on the stage of the tide, but decreases quickly upstream.