Tidal Asymmetry Research Articles

Projected Responses of Tidal Dynamics in the North Sea to Sea-Level Rise and Morphological Changes in the Wadden Sea

This study explores the projected responses of tidal dynamics in the North Sea induced by the interplay between plausible projections of sea-level rise (SLR) and morphological changes in the Wadden Sea. This is done in order to gain insight into the casual relationships between physical drivers and hydro-morphodynamic processes. To achieve this goal, a hydronumerical model of the northwest European shelf seas (NWES) was set-up and validated. By implementing a plausible set of projections for global SLR (SLRRCP8.5of 0.8 m and SLRhigh−endof 2.0 m) by the end of this century and beyond, the model was run to assess the responses of the regional tidal dynamics. In addition, for each considered SLR, various projections for cumulative rates of vertical accretion were applied to the intertidal flats in the Wadden Sea (ranging from 0 to 100% of projected SLR). Independent of the rate of vertical accretion, the spatial pattern of M2 amplitude changes remains relatively stable throughout most of the model domain for a SLR of 0.8 m. However, the model shows a substantial sensitivity toward the different rates of vertical accretion along the coasts of the Wadden Sea, but also in remote regions like the Skagerrak. If no vertical accretion is assumed in the intertidal flats of the Wadden Sea, the German Bight and the Danish west coast are subject to decreases in M2 amplitudes. In contrast, those regions experience increases in M2 amplitudes if the local intertidal flats are able to keep up with the projected SLR of 0.8 m. Between the different scenarios, the North Frisian Wadden Sea shows the largest differences in M2 amplitudes, locally varying by up to 14 cm. For a SLR of 2.0 m, the M2 amplitude changes are even more amplified. Again, the differences between the various rates of vertical accretion are largest in the North Frisian Wadden Sea (> 20 cm). The local distortion of the tidal wave is also significantly different between the scenarios. In the case of no vertical accretion, tidal asymmetry in the German estuaries increases, leading to a potentially enhanced sediment import. The presented results have strong implications for local coastal protection strategies and navigation in adjacent estuaries.

Role of ocean tidal asymmetry and estuarine geometry in the fate of plastic debris from ocean sources within tidal estuaries

Tidal asymmetry drives the transport of materials (e.g. plastic debris) within tidal estuaries. Most previous research has focused on the asymmetry that arises within estuaries and relates to sediment transport; however, estuaries exhibiting tidal asymmetry at the mouth and their relationship with materials other than sediments have received less attention. This study uses numerical hydrodynamic and Lagrangian transport models to assess the effect of estuary morphology on the propagation of asymmetric tides and how it influences the distribution of plastic debris that reaches tidal estuaries from ocean sources. A series of numerical experiments were conducted in idealised estuaries, specifically in tidal creeks. The results show that the asymmetry at the mouth results in flood/ebb dominance and influences the presence of plastic debris in areas where the main channel favours tidal flow. Flood-dominated estuaries show an import capacity 50% higher than those that show symmetric or negative asymmetric tides at their mouths. Numerical simulations show that the relevant role in defining flood or ebb dominance is played by the geometry of the estuary when increased friction that opposes flow and intertidal storage areas are present, although small influences of the external tidal asymmetry are also appreciated. The probability of plastic debris presence was 90% for positive asymmetric tides and varied between and 70%–80% for symmetric and negative-asymmetric tides. A remarkable finding of this study is the regulatory role of kurtosis which corrects the tendency induced by skewness in the fate of plastic debris. These findings were corroborated with the application of this methodology to a real estuary and a comparison of the obtained results with available field data on plastic debris.

The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

AbstractThe bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large‐scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large‐scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood‐ and ebb‐oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune‐induced directional bed roughness in numerical models of estuarine and tidal environments.

Understanding the Impact of Bathymetric Changes in the German Bight on Coastal Hydrodynamics: One Step Toward Realistic Morphodynamic Modeling

The hydrodynamic response to morphodynamic variability in the coastal areas of the German Bight was analyzed via numerical experiments using time-referenced bathymetric data for the period 1982–2012. Time-slice experiments were conducted for each year with the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM). This unstructured-grid model resolves small-scale bathymetric features in the coastal zone, which are well-resolved in the high-resolution time-referenced bathymetric data (50 m resolution). Their analysis reveals the continuous migration of tidal channels, as well as rather complex change of the depths of tidal flats in different periods. The almost linear relationship between the cross-sectional inlet areas and the tidal prisms of the intertidal basins in the East Frisian Wadden Sea demonstrates that these bathymetric data describe a consistent morphodynamic evolutionary trend. The numerical experiment results are streamlined to explain the hydrodynamic evolution from 1982 to 2012. Although the bathymetric changes were mostly located in a relatively small part of the model area, they resulted in substantial changes in the M2 tidal amplitudes, i.e., larger than 5 cm in some areas. The hydrodynamic response to bathymetric changes largely exceeded the response to sea level rise. The tidal asymmetry estimated from the model appeared very sensitive to bathymetric evolution, particularly between the southern tip of Sylt Island and the Eider Estuary along the eastern coast. The peak current asymmetry weakened from 1982 to 1995 and even reversed within some tidal basins to become flood-dominant. This would suggest that the flushing trend in the 1980s was reduced or reversed in the second half of the studied period. Salinity also appeared sensitive to bathymetric changes; the deviations in the individual years reached ~22 psu in the tidal channels and tidal flats. One practical conclusion from the present numerical simulations is that wherever possible, the numerical modeling of near-coastal zones must employ time-referenced bathymetry data. The second, perhaps even more important conclusion, is that the progress of morphodynamic modeling in realistic ocean settings with multiple scales and varying bottom forms is strongly dependent on the availability of bathymetric data with appropriate temporal and spatial resolution.

Propagation of tidal wave and resulted tidal asymmetry upward tidal rivers

Propagation of tidal wave and resulted tidal asymmetry upward tidal rivers

Mapping tidal current and salinity at a shallow tidal channel junction using the fluvial acoustic tomography system

Applying the fluvial acoustic tomography (FAT) system to shallow tidal junctions for studying the flow division and spatiotemporal difference of velocity and salinity is vital to understand the tidal hydrodynamics in multi-channel networks. To the best of our knowledge, this is the first study that monitors continuous 2D current and salinity distributions at a shallow tidal junction using six FATs for ~34.4 days. The horizontal distribution and spatiotemporal variation of the currents and salinity were efficiently estimated by the inverse method. These results demonstrate that FAT is a potential tool for the continuous mapping of variable 2D currents and salinity at shallow tidal junctions. The reciprocating patterns of the current and salinity during the spring tide at the junction responded well to the tide. High salinity occurred around high water, whereas salinity was negligible at low water. During flood tides, significant landward currents flowed with the maximum speed of ~0.4 m/s, and significant seaward currents with the maximum speed of ~0.55 m/s occurred during ebb tides. During neap tides, the salinity pattern began to develop landward from the low water and reached a mature phase around the high water; salinity at the ebb slack remained high. Inverted FAT results indicated a counter-clockwise circulation around the low water during neap tide; some of the currents continuously flowed landward at the downstream of the junction (S4–S5), whereas others continuously flowed to the Tenma River. The behaviors of the currents varied slightly with the tide. The behaviors of the current and salinity during the neap tide mostly resulted from the density-driven current phenomenon. Furthermore, tidal harmonic analyses of the reconstructed currents were performed to clarify the river-tide interactions at the tidal junction. The results reflected the increased tidal wave deformation that occurred with the gradually increasing tidal range and demonstrated the role of the limited river flow on the tidal asymmetry at this tidal junction. This study advances the understanding of river flow dynamics in shallow tidal junction systems.

Drag variations, tidal asymmetry and tidal range changes in a mangrove creek system

AbstractAboveground root structures enhance drag on tidal currents in intertidal mangrove forests, whereas the creeks dissecting such forests provide low‐resistance conduits for tidal flows. Here, observations from an established mangrove forest in the Whitianga Estuary, Aotearoa New Zealand, are used to investigate the variability of the drag experienced by tidal flows in a mangrove creek system and subsequent effects on tidal asymmetries and ranges.Tidal flow speed maxima in the creek occurred at overbank water levels during the sheet flow stage on rising tides, but at water levels below the creek bank (the creek flow stage) on falling tides. Inferred bulk drag coefficients for the creek were greater during the sheet flow than the creek flow stage, and were linearly correlated with the bulk drag coefficients at stations in the adjacent forest. Although falling tides, associated with larger bulk drag coefficients, had an increasingly longer duration than rising tides towards the back of the forest, we observed ebb‐dominant flow speed asymmetry that declined inland in the creek. Conversely, flow speeds within the forest were consistently flood‐dominant, in accordance with smaller bulk drag coefficients during rising tides. Along the full length of the mangrove system, high‐water levels were lowered by up to 12 cm/km within the creek and 36 cm/km within the mangrove forest.Creek bed roughness associated with bulk drag coefficients observed in deeper parts of the creek was much greater than the hydraulic roughness of the sediment. For accurate simulations of landscape‐scale feedbacks between the creek and mangrove forest, incorporating both direct and indirect contributions of the vegetated forest platform to creek bed roughness is essential. These findings show that the interaction between creek flow and sheet flow in a mangrove creek system is a key driver of tidal asymmetries as well as the attenuation of high‐water conditions.

Large‐Scale Scour in Response to Tidal Dominance in Estuaries

AbstractChannel beds in estuaries and deltas often exhibit a local depth maximum close to the river mouth. There are two known mechanisms of large‐scale (i.e., >10 river widths along‐channel) channel bed scours: width constriction and draw‐down during river discharge extremes, both creating flow acceleration. Here, we study a potential third mechanism: tidal scour. We use a 1D‐morphodynamic model to reproduce tidal dynamics and scours in estuaries that are in morphologic equilibrium. A morphologic equilibrium is reached when the net (seaward) sediment transport matches the upstream supply along the entire reach. The residual (river) current and river‐tide interactions create seaward transport. Herein, river‐tide interactions represent the seaward advection of tide‐induced suspended sediment by the river flow. Tidal asymmetry typically creates landward transport. Scours form when tidal flow is amplified through funneling of tidal energy. Scours simultaneously reduce the residual (river) current and the river‐tide interaction contribution to sediment transport, thereby maintaining morphologic equilibrium. When tidal influence is relatively large, and when channel convergence is strong, an equilibrium is only obtained with a scouring profile. We propose a predictor dependent on the width convergence, quantified as SB, and on the ratio between the specific peak tidal discharge at the mouth and the specific river discharge at the landward boundary (qtide/qriver). Scours develop if (qtide/qriver)/SB exceeds 0.3. Scour conditions were found to occur globally across a range of scales, which allows its prediction in estuaries under future changes.

Correction to: Tidal Asymmetry in Ocean-Boundary Flux and In-Estuary Trapping of Suspended Sediment Following Watershed Storms: San Francisco Estuary, California, USA

A Correction to this paper has been published: https://doi.org/10.1007/s12237-021-00937-y

Study on characteristics of tidal dynamics and vortex in Zhoushan Archipelago sea area

Numerical modelling is used to study the characteristics and mechanisms of tidal dynamics and vortex in Zhoushan Archipelago sea area. Results show that amplitudes and current speeds of the semidiurnal tides decrease significantly when passing through the islands. Conversion of tidal energy, archipelago topography and formation of shallow-water tides are important reasons for the evolution of tidal waves. The ebb tide has a longer duration in the archipelago sea area, particularly in narrow channels. The M2-M4 constituents play a major role in tidal asymmetry. The flow paths of tidal currents change much, causing the high speed flooding/ebbing near the north/south bank. The tidal vortex is mainly distributed on both sides of and behind the islands, which is related to the shape of islands and the flow velocity. Referring to the current directions, positive/negative vorticity generally appears on the right/left side of the islands.

Tidal Asymmetry in Ocean-Boundary Flux and In-Estuary Trapping of Suspended Sediment Following Watershed Storms: San Francisco Estuary, California, USA

Suspended-sediment flux at the ocean boundary of the San Francisco Estuary—the Golden Gate—was measured over a tidal cycle following peak watershed runoff from storms to the estuary in two successive years to investigate sediment transport through the estuary. Observations were repeated during low-runoff conditions, for a total of three field campaigns. Boat-based measurements of velocity and acoustic backscatter were used to calculate water and suspended-sediment flux at a location 1 km landward of the Golden Gate. Suspended-sediment concentration (SSC) and salinity data from up-estuary sensors were used to track watershed-sourced sediment plumes through the estuary. Estimates of suspended-sediment load from the watershed and net suspended-sediment flux for one up-estuary subembayment were used to infer in-estuary trapping of sediment. For both post-storm field campaigns, observations at the ocean boundary were conducted on the receding limb of the watershed hydrograph. At the ocean boundary, peak instantaneous suspended-sediment flux was tidally asymmetric and was greater on flood tides than on ebb tides for all three field campaigns, due to higher average SSC in the cross-section on flood tides. Shear-induced sediment resuspension was greater on flood tides and suggests the presence of an erodible pool outside the estuary. The storms in 2016 led to less export of discharge and sediment from the watershed and greater sediment trapping within one up-estuary subembayment compared to that observed in 2017. Results suggest that substantial trapping of watershed sediments occurred during both storm events, likely due to the formation of estuarine turbidity maxima (ETM) at different locations in the estuary. ETM locations were forced nearer the ocean boundary in 2017. Additional measurements and modeling are required to quantify the long-term sediment flux at the Golden Gate.

A Comprehensive Estuarine Hydrodynamics-Salinity Study: Impact of Morphologic Changes on Ria de Aveiro (Atlantic Coast of Portugal)

Shallow coastal lagoons driven by tidal processes are extremely dynamic environments prone to continuous natural and anthropogenic pressures. The hydrodynamics of these systems deeply depends on the effect of local morphology on the tidal propagation, so their permanent evolution constantly changes tidal dependent processes. For this reason, the present work aims to review the main characteristics of Ria de Aveiro hydrodynamics, a shallow lagoon located at the Atlantic Coast of Portugal, evaluating its evolution over the last 30 years (between 1987 and 2020) and investigating the main morphological changes in its origin. For this purpose, a comparative analysis is performed to determine the main process, including the observed hydrodynamic changes: Deepening of the inlet channel or of the main lagoon channels. To achieve these goals, the authors explored a remarkable database including bathymetric, tide gauge, and salinity data from 1987 until the present. This analysis is completed by the exploitation of a hydrodynamical model (Delft3D), validated against field data. Several simulations were performed to analyse changes in tidal propagation along the lagoon channels (considering the main semi-diurnal constituent M2), tidal asymmetry, tidal currents, tidal prism, and salinity patterns. The results show that the general deepening of the lagoon observed between 1987 and 2020 led to important changes in the lagoon hydrodynamics, namely the increase/decrease of the M2 constituent amplitude/phase, as well as the increase of tidal currents and salt intrusion within the entire lagoon, with the changes being amplified towards the head of the main channels. Although the inlet deepening partially contributed to the modifications found, the results revealed that the deepening of the main lagoon channels had the most significant contribution to the changes observed during the last 30 years.

Regime shifts in the Changjiang (Yangtze River) Estuary: The role of concentrated benthic suspensions

Channel deepening often triggers positive feedback between tidal deformation, sediment import and drag reduction, which leads to the regime shift in estuaries from low-turbid to hyper-turbid state. In this study, a transition in profiles of suspended sediment concentration (SSC) is hypothesised by including a positive feedback loop of vertical mixing and settling. Such a hypothesis is validated by the historical observations in the North Passage of Changjiang (Yangtze River) Estuary, with decreasing SSC in mid-lower layers and increasing SSC near the bed after the deepening. A mobile pool of concentrated benthic suspensions (CBS) develops in the North Passage, with a tidally averaged length of ~20 km and a mean thickness of ~4 m. The width of the CBS pool is limited (<1 km) as the CBS is concentrated in the Deepwater Navigational Channel. The movements of the CBS pool, combined with tidal asymmetry (e.g., slack-water asymmetry and lateral flow asymmetry), results in sediment trapping in the middle reaches and on the south flank of the channel. Observations by a bottom tripod system show the response of friction/drag coefficient to sediment concentration: (1) nearly linear decrease within low SSC (<10 kg/m3); (2) constant and minimum coefficient (with drag reduction up to 60–80%) in the presence of CBS (10–80 kg/m3). An empirical relationship was derived, which can be used to predict the friction coefficient and the magnitude of drag reduction for sediment transport studies, particularly for modelling regime shifts in estuaries.

Hydrodynamics of a hyper-tidal estuary influenced by the world's second largest tidal power station (Rance estuary, France)

The Rance estuary is a relatively small low-discharge steep-sided ria, located along the Brittany coast in northern France, with a maximum spring tidal range of 13.5 m. Taking advantage of this hyper-tidal regime, the first and currently the second largest operational tidal power station in the world was built at the estuary's mouth and has been in operation since the 1960s. Despite the well-known effect of damping of estuarine water levels, little attention has been given to quantifying the influence of the plant on the propagation and asymmetry of the tidal wave inside the estuary. In this study, hydrodynamics and tidal wave patterns were analyzed in this anthropogenically influenced estuarine system. A two-dimensional depth-averaged numerical model of the Rance estuary was developed. Two scenarios without the tidal power plant involving the dam's pre- and post-construction bathymetry (1957 and 2018 respectively) and present-day conditions scenarios were designed, to highlight the impact of bed evolution and the tidal power station on hydrodynamics and tidal asymmetry. Numerical results showed that, without the structure, bathymetric evolution did not substantially influence estuarine hydrodynamics. Nevertheless, on the estuary-side of the dam, the presence of the tidal power plant induced (i) a decrease in both tidal range and tidal prism, (ii) an increase of low water levels, and (iii) a decrease in both flood and ebb currents. Contrastingly, the region close to the structure reacted differently to plant operating modes, with an increase in flood currents (ebb currents) upstream of the sluice gates (downstream of the turbines). For both the natural condition and the artificially-induced hydrodynamic forcing due to the presence of the plant, numerical results showed that the Rance estuary mainly exhibits flood-dominant behavior, with a longer duration of falling than rising water and stronger peak flood currents than ebb currents. Spanning a period of approximately 60 years, this study presents a quantitative analysis of the influence of the tidal power station on the hydrodynamics in the Rance estuary, and its possible consequences for sediment dynamics. This approach is novel for this particular enclosed water body, characterized by the presence of a dam at its mouth and a lock at its uppermost limit.

Shifting Sediment Dynamics in the Coos Bay Estuary in Response to 150 Years of Modification

AbstractEstuaries worldwide have experienced modifications including channel deepening and intertidal reclamation over several centuries, resulting in altered fine sediment routing. Estuaries respond differently based on preexisting geometries, freshwater and sediment supplies, and extents and types of modification. The Coos Bay Estuary in Oregon is a relatively small estuary with complex geometry that has been extensively modified since 1865. A sediment transport model calibrated to modern conditions is used to assess the corresponding changes in sediment dynamics. Over ∼150 years, channel deepening (from ∼6.7 to 11 m), a 12% increase in area, and a 21% increase in volume have led to greater tidal amplitudes, salinity intrusion, and estuarine exchange flow. These changes have reduced current magnitudes, reduced bed stresses, and increased stratification, especially during rainy periods. Historically, fluvially derived sediment was dispersed across broad, deltaic‐style flats and through small tidal channels. Now, river water and sediments are diverted into a dredged navigation channel where an estuarine turbidity maximum (ETM) forms, with modeled concentrations &gt;50 mg/L and measured concentrations &gt;100 mg/L during discharge events. This “new” ETM supplies sediment to proximal embayments in the middle estuary and the shallow flats. Overall, sediment trapping during winter (and high river discharges) has increased more than two‐fold, owing to increased accommodation space, altered pathways of supply, and altered bed stresses and tidal asymmetries. In contrast to funnel‐shaped estuaries with simpler geometries and river‐channel transitions, these results highlight the importance of channel routing together with dredging in enhancing sediment retention and shifting pathways of sediment delivery.

Oceanographic and environmental characteristics of a coupled coastal bay system: Baía de Ilha Grande-Baía de Sepetiba, Rio de Janeiro, Brazil

Baía de Ilha Grande and Baía de Sepetiba are two connected coastal bays in southeastern Brazil just west of Rio de Janeiro. They are favored for tourism and recreation, provide most of the potable water for the city of Rio de Janeiro, are the site of the only two nuclear reactors in Brazil, a deep-water oil terminal, a Federal container port, and hundreds of industries. The bay system experiences mixed mainly semidiurnal tides and currents. Hundreds of sediment samples and cores plus side scan-sonar surveys were used to map the bottom sediment distribution. Long-term measurements from two moored instruments yielded two sets of time series of water level, currents, temperature, and salinity, which were used to analyze the salient ocean processes in the two bays. The temperature time series and extensive CTD profiling indicated the occurrence of cold-water intrusion events into Baía de Ilha Grande, with temperatures ranging from 16 °C to 20 °C, in spring to late summer (October–April). The intrusions are ascribed to coastal upwelling of South Atlantic Central Water as a result of synoptic northeasterly winds and the westward translation of cold core eddies and meanders from the region of Cabo Frio. The calculated mean sea level varies annually by 50 cm with a peak in June and a trough in November due to atmospheric pressure variations (5%), steric changes (28%), but mostly due to wind and Ekman effects. Spectral analysis was applied to analyze the water level and current time series. The results indicated both tidal asymmetry and the existence of a pronounced quarter-diurnal seiche oscillation in the coastal bay-system parallel to the coast.

Residual Sediment Transport in Tidally Energetic Estuarine Channels With Lateral Bathymetric Variation

AbstractThe residual sediment transport in tidally energetic estuarine channels is investigated by means of idealized cross‐sectional modeling. The lateral bathymetric variation follows a Gaussian profile, assuming longitudinal uniformity. The total along‐channel residual sediment flux is decomposed into contributions from an advective flux and a tidal pumping flux. Two important mechanisms are found to modify the tidal covariance between sediment concentration and current velocity, thereby contributing to the tidal pumping of sediment. First, longitudinal and lateral straining of salinity leads to tidal asymmetries in stratification and thus sediment resuspension. Second, lateral circulations directly redistribute suspended sediments within the cross section, which are then differentially transported by the along‐channel tidal currents. A general relationship between the phasing of the lateral circulations and the resulting lateral‐advection‐driven tidal pumping is proposed. Reduced‐physics experiments with lateral sediment advection turned off provide the first evidence that lateral‐advection‐driven tidal pumping plays a leading role in sediment transport for tidally energetic estuaries with nonnegligible lateral depth gradients. Additionally, a temporal decomposition breaks down the cross‐sectionally averaged tidal pumping flux into individual contributions from different tidal phases (early tide, peak tide, and late tide), providing a new perspective on tidal asymmetry in sediment resuspension and settling. The direction and strength of tidal pumping (both stratification‐driven and lateral‐advection‐driven) are shown to depend on lateral bathymetry, sediment grain size, and longitudinal buoyancy gradient forcing.

Coastal morphodynamic responses of a mixed-energy and fine-sediment coast to different sea level rise trends

The effects of sea level rise (SLR) are relevant to the future evolution of coasts and estuaries. This study analyses the importance of different SLR trends on the morphological evolution of a mixed-energy fine-sediment coast (the Caofeidian Sea in Bohai Bay, China) using a validated numerical model. Results show that SLR produces spatially non-uniform relative morphodynamic responses, depending on the local coastline (uninterrupted or inlet-interrupted) and wave exposure. In the Caofeidian Sea, the uninterrupted coast is exposed to waves and tidal currents, and SLR reduces the accretion of tidal flats via heightening waves. On the inlet-interrupted coast, the morphological response to SLR is dominated by changes in tidal asymmetry and current velocity. In low SLR scenarios, the enhanced flood tidal asymmetry makes the system initially resilient to SLR. For high SLR scenarios, tidal sediment transport capacity is dampened by the reduction in current velocities, which accelerates the degradation of tidal flats along the inlet-interrupted coast. Differences among results with eight theoretical SLR trends show that the magnitude of SLR-induced morphological change is influenced by the target value (0.5 and 1.0 m) and rising mode (abrupt growth, linear growth, parabolic growth, and exponential growth) of the SLR. A newly-introduced parameter (equivalent SLR, ESLR), which reflects the time-weighted average value of an SLR trend for a period of time, shows a strong linear relationship with SLR-induced morphological change. Results suggest that the morphological response to other SLR scenarios for the Caofeidian Sea is expected to be interpolated from the ESLR ratios without performing extra time-consuming simulations.

Estuarine tidal response to sea level rise: The significance of entrance restriction

Estuarine environments, as dynamic low-lying transition zones between rivers and the open sea, are vulnerable to sea level rise (SLR). To evaluate the potential impacts of SLR on estuarine responses, it is necessary to examine the altered tidal dynamics, including changes in tidal amplification, dampening, reflection (resonance), and deformation. Moving beyond commonly used static approaches, this study uses a large ensemble of idealised estuarine hydrodynamic models to analyse changes in tidal range, tidal prism, phase lag, tidal current velocity, and tidal asymmetry of restricted estuaries of varying size, entrance configuration and tidal forcing as well as three SLR scenarios. For the first time in estuarine SLR studies, data analysis and clustering approaches were employed to determine the key variables governing estuarine hydrodynamics under SLR. The results indicate that the hydrodynamics of restricted estuaries examined in this study are primarily governed by tidal forcing at the entrance and the estuarine length. In addition, SLR increases the average water depth and alters the nodal point location in a seaward direction, thereby significantly affecting tidal wave propagation patterns and reducing the degree of flood domination. In estuaries with restricted entrances, tidal range diminishes drastically in the restricted zone by 20–60%, and the maximum tidal current velocity is higher in the restricted part but lower in the upstream part compared to unrestricted estuaries. For estuary types examined, inundation extent under SLR will likely be greater than the extent expected through simply adjusting existing water levels upwards. The findings also underline that an engineered entrance restriction could regulate tidal wave propagation within an estuary to offset SLR induced tidal range amplification. However, this may pose additional management challenges.

The Fate of Mud Nourishment in Response to Short-Term Wind Forcing

In this study, results from a realistic 3D hydrodynamic and sediment transport model, applied to a channel in the Dutch Wadden Sea, are analyzed in order to assess the effect of short-term wind forcing, the impact of fresh water effects, and the variability induced by the spring-neap cycle on the transport of suspended sediment. In the investigated region, a pilot study for sediment nourishment, the so-called Mud Motor, is executed. This project aims for the beneficial re-use of dredged harbor sediments through the disposal of these sediments at a location where natural currents are expected to transport them toward a nearby salt marsh area. The model results presented in this study advance the understanding of the driving forces that determine sediment transport in shallow, near-coastal zones, and can help to improve the design of the Mud Motor. In the investigated channel, which is oriented parallel to the coastline, tidal asymmetries generally drive a transport of sediment in flood direction. It was found that already moderate winds along the channel axis reverse (wind in ebb direction), or greatly enhance this transport, up to an export of sediment over the adjacent water shed (wind in flood direction). The most beneficial wind conditions (moderate westerly winds) can cause an accumulation of more than 90% of the initial 200 tons sediment pool on the intertidal area; during less favorable conditions (northeasterly winds), less than a third of the dumped sediment is transported onto the mudflat. On-shore winds induce a transport toward the coast. Surprisingly, sediment pathways are only sensitive to the exact disposal location in the channel during wind conditions that counteract the tidally driven transport, and freshwater effects play no significant role for the dispersal of sediment.