Tidal Propagation Research Articles

Subseasonal Tidal Variability in the Gulf of Tonkin Observed by Multi-Satellite Altimeters and Tide Gauges

Exploring multi-timescale tidal variability is fundamental and necessary for numerous practical purposes, such as flood protection, marine cultivation, and ocean transport. It is well known that tides show significant seasonal, inter-annual, and 18.61-year nodal variability. Less known and less discussed is the subseasonal tidal variability (i.e., ter-annual, quarter-annual, and penta-annual cycles) in the coastal ocean. In this study, we explore subseasonal tidal modulations in the Gulf of Tonkin via the combination of four tide gauges and 27-year multi-satellite altimeter observations. Both tide gauges and satellite altimeters indicate that tidal subseasonality is significant in the Gulf of Tokin, although the amplitudes of subseasonal variations are much smaller than those of seasonal variations. Compared to spatially limited tide gauges, satellite altimeters successfully derive the basin-scale tidal subseasonality in the Gulf of Tonkin. The largest amplitude of subseasonal tidal constituents originated from the subseasonality of main tidal constituents, and can reach as high as 31.8 mm. It is suggested that subseasonal variations in ocean environments (e.g., sea levels and ocean stratification) induce tidal subseasonality through changing tidal propagation and dissipation. Although powerful, satellite altimeters also have some defects. Due to tidal aliasing related to long-period sampling intervals, some subseasonal tidal constituents are indistinguishable in satellite altimeter records.

Numerical simulations of generation and propagation of internal tides in the Andaman Sea

The generation and propagation of internal tides in the Andaman Sea are investigated using a three-dimensional high-resolution numerical model. Three categories of experiments, including driving the model with four main semidiurnal tides (M2, S2, N2, and K2), four main diurnal tides (K1, O1, P1, and Q1), and eight main tides (M2, S2, N2, K2, K1, O1, P1, and Q1), are designed to examine the effects of barotropic tides. The results show that the semidiurnal internal tides are dominant in the Andaman Sea, and the inclusion of diurnal barotropic tides negligibly modulates this result. That is partly due to the strength of the diurnal barotropic tides is generally one order smaller than that of the semidiurnal barotropic tides in this region. The sensitivity experiments put this on a firmer footing. In terms of the internal tidal energy, the experiments driven by the diurnal barotropic tides are three orders and one order smaller than those driven by the semidiurnal barotropic tides, respectively, during the spring and neap tides. In addition, the experiments result in total barotropic-to-baroclinic energy conversion rates over the Andaman Sea 29.15 GW (driven by the eight tides), 29.24 GW (driven by the four semidiurnal tides), and 0.05 GW (driven by the fourdiurnal tides) in the spring tidal period and 3.08 GW, 2.56 GW, and 0.31 GW in the neap tidal period, respectively. Four potential generation regions of internal tides are found, three of which are in the Andaman and Nicobar Islands and one in the northeastern Andaman Sea.

Internal tides off the Amazon shelf during two contrasted seasons: interactions with background circulation and SSH imprints

Abstract. The Amazon shelf break is a key region for internal tide (IT) generation. It also shows a large seasonal variation in circulation and associated stratification. This study, based on a high-resolution model (1/36∘) explicitly forced by tide, aims to better characterize how the ITs vary between two contrasted seasons. During the season from March to July (MAMJJ) the currents and mesoscale eddies are weak while the pycnocline is shallower and stronger. From August to December (ASOND) mean currents and mesoscale eddies are strong, and the pycnocline is deeper and weaker than in MAMJJ. For both seasons, semi-diurnal M2 ITs are generated on the shelf break mainly between the 100 and 1000 m isobath in the model. South of 2∘ N, the conversion from barotropic to baroclinic tide is more efficient in MAMJJ than in ASOND. Local dissipation of the coherent M2 at the generation sites is higher in MAMJJ (30 %) than in ASOND (22 %), because higher modes are favourably generated (modes 2 and 3), making the internal wave packet more dispersive. The remaining fraction (70 %–80 %) propagates away from the generation sites and mainly dissipates locally every ∼ 100 km, which corresponds to the mode 1 reflection beams. About 13 %, 30 %, and 40 % of the M2 coherent IT dissipates at the first, second, and third beams. M2 coherent baroclinic flux propagates more northward during MAMJJ while it seems to be blocked at 6∘ N during ASOND. There is no intensified dissipation of the coherent M2 that could explain the disappearance of the coherent flux. In fact, the flux at this location becomes more incoherent because of strong interaction with the currents. This has been shown in the paper using 25 h mean snapshots of the baroclinic flux that shows branching and stronger eastward deviation of the IT when interacting with mesoscale eddies and stratification during ASOND. Finally, we evaluated sea surface height (SSH) frequency and wavenumber spectra for subtidal (f<1/28 h−1), tidal (1/28 h−1 < f<1/11 h−1), and supertidal (f>1/11 h−1) frequencies. Tidal frequencies explain most of the SSH variability for wavelengths between 250 and 70 km. Below 70 km, the SSH is mainly incoherent and supertidal. The length scale at which the SSH becomes dominated by unbalanced (non-geostrophic) IT was estimated to be around 250 km. Our results highlight the complexity of correctly predicting IT SSH in order to better observe mesoscale and sub-mesoscale eddies from existing and upcoming altimetric missions, notably the Surface Water Ocean Topography (SWOT) mission.

High-resolution, non-hydrostatic simulation of internal tides and solitary waves in the southern East China Sea

The slope area northeast of Taiwan Island in the southern East China Sea (ECS) is well known as a hotspot for both internal tides and internal solitary waves (ISWs) despite uncertainties regarding their specific source and generation mechanism. This study investigates the generation and evolution processes of internal tides and ISWs northeast of Taiwan Island in the southern ECS, based on a super-high-resolution and non-hydrostatic numerical simulation. ISWs create a complex spatial pattern in both satellite images and our simulation, with wave propagation in multiple directions, including onshore and offshore. These ISWs, with the largest amplitudes reaching ∼50 m, are mainly generated by two mechanisms: local tide-topography interactions and disintegration of internal tides remotely generated over I-Lan Ridge. Locally generated ISWs propagate in various directions owing to the complex topography, whereas ISWs generated by the disintegration of remote internal tides are more consistent in their pathways. Strong barotropic to baroclinic energy conversions occur over both Mien-Hua Canyon and North Mien-Hua Canyon. For the baroclinic tidal energy fluxes, the advection flux is comparable to the pressure-work term in shallow water, while the pressure-work flux dominates in deep water. The nonlinear energy flux terms in shallow water are more important for the supertidal than the tidal signals. The semidiurnal energy flux features a clockwise pattern over the double-canyon region, indicating the formation of partial standing waves.

The influence of dredging on estuarine hydrodynamics: Historical evolution of the Santos estuarine system, Brazil

We assess changes in estuarine hydrodynamics due to the deepening of navigation channels by dredging. A validated numerical hydrodynamic model (Delft3D) was used to analyze scenarios (historical bathymetry data from 1876 to 2014) and applied to assess the hydrodynamic evolution of the Santos estuarine system (SES), Brazil. During the studied period, the SES underwent significant morphological changes, mainly related to the dredging of the Santos harbor navigation channel. The unprecedented results of the hydrodynamic evolution of the SES show that the gradual process of deepening the channels by dredging resulted in modified estuarine circulation parameters such as tidal propagation and stratification of the water column, as well as the residual water and materials transport through the SES channels. Channel deepening resulted in greater saline intrusion, a decrease in the volume of water transported through the Santos channel, an increase in the influence of the tide in the system and an increase in its asymmetry over the years in the upper estuary.

Projected climate variability of internal waves in the Andaman Sea

The Andaman Sea, in the northeast Indian Ocean, is renowned for large-amplitude internal waves. Here, we use a global climate model (CanESM5) to investigate the long-term variability of internal waves in the Andaman Sea under a range of shared socioeconomic pathway (SSP) scenarios. SSPs are future societal development pathways related to emissions and land use scenarios. We project that mean values of depth-averaged stratification will increase by approximately 6% (SSP1-2.6), 7% (SSP2-4.5), and 12% (SSP5-8.5) between 1871-1900 and 2081-2100. Simulating changes in internal tides between the present (2015-2024) and the end-century (2091-2100), we find that the increase in stratification will enhance internal tide generation by approximately 4 to 8%. We project that the propagation of internal tides into the Andaman Sea and the Bay of Bengal will increase by 8 to 18% and 4 to 19%, respectively, under different SSP scenarios. Such changes in internal tides under global warming will have implications for primary production and ecosystem health not only in the Andaman Sea but also in the Bay of Bengal.

Energy balance and momentum budgets due to the internal tides generated by a group of submarine canyons

A field multibeam survey discovered a group of regularly-spaced submarine canyons on the continental shelf in the northern South China Sea. However, they are not included in the bathymetry datasets commonly used by the oceanographic community. In this study, we employ a very-high-resolution (as fine as 54 m) numerical model that resolves these submarine canyons to investigate their impacts on the local internal tide generation and propagation. Results show that the barotropic to internal tidal conversion rate exhibits an alternating distribution on each flank of submarine canyons. Rotational direction of the tidal flow determines a positive conversion rate on the west flank and a negative rate on the east flank. The rough seafloor due to the submarine canyons does not appreciably change the dissipation of barotropic tides, but significantly affects the dissipation of internal tides, leading to an averaged dissipation rate of 1.2 × 10−9 W kg −1. Internal tides also induce a bottom intensified mean flow, which causes westward mean transport on the continental slope and a clockwise circulation inside each submarine canyon. The mean flow satisfies a momentum budget between the Coriolis force, pressure gradient and divergence of momentum flux of barotropic and internal tides. We highlight that the divergence of the pseudo-momentum flux of internal tides acts as a stress and is especially reinforced near the sea bottom and surface. Its effect on the sea surface is even comparable with moderate winds (∼13 m s−1).

On the Spatial Variability of the Mesoscale Sea Surface Height Wavenumber Spectra in the Atlantic Ocean

AbstractThe wavenumber spectral slope of sea surface height (SSH) computed within the mesoscale range (70–250 km) from satellite altimetry exhibits a large spatial variability which, until now, has not been reproduced in numerical ocean models. This study documents the impacts of including internal tides, high‐resolution bathymetry, and high‐frequency atmospheric variability on the SSH wavenumber spectra in the Atlantic Ocean, using a series of 1/50° North and equatorial Atlantic simulations with a realistic representation of barotropic/baroclinic tides and mesoscale‐to‐submesoscale variability. The results show that the inclusion of internal tides does increase high frequency SSH variability (with clear peaks near 120 and 70 km) and flattens the spectra slope in the mesoscale range in a good agreement with observations. The surface signature of internal tides, mostly in the equatorial Atlantic but also in subtropical regions in the eastern North Atlantic, is the primary reason behind the observed large spatial variability of the spectral slope in the Atlantic. Internal tides are stronger in the tropical regions when compared to higher latitudes because of the stronger barotropic tides and stronger stratification in the upper layer of the water column. High‐resolution bathymetry does play an important role in the internal tide generation on a local scale, but its impact on the modeled large‐scale SSH variability and SSH wavenumber spectra is quite small. High‐frequency wind variability plays only a minor role on the generation of the simulated high‐frequency SSH variability. Impact of altimetry sampling on the computation of the SSH wavenumber spectra is also discussed.

Responses of tidal duration asymmetry to morphological changes in Lingding Bay of the Pearl River Estuary

Tidal asymmetry is one of the main factors for generating net transport for waterborne materials in tidal estuaries, and thus, this phenomenon has significant influences on controlling morphological development and the ecological environment. Tidal propagation is sensitive to changes in the coastline and geometry of estuarine regions. Moreover, tidal waveforms vary with various factors, such as coastline changes and bathymetry evolution due to local anthropogenic activities. The topography of Lingding Bay (LDB) of the Pearl River Estuary (PRE) has greatly changed since the 1960s because of human interventions, but the response of tidal duration asymmetry (TDA) to morphological changes is still poorly understood. Utilizing the two-dimensional Delft-3D flexible mesh numerical model, the spatial pattern of TDA and its primary contributors in LDB of the PRE were reproduced for 1964, 1989, and 2016, accounting for the changes in both shoreline and bathymetry owing to human interventions. The results reveal that as the tidal wave propagates upstream, the tidal skewness increases from negative values to positive values longitudinally, indicating the transition from a shorter ebb-duration state to a shorter flood-duration state. Additionally, a prominent shift in TDA and its primary contributors takes place approximately in the period of 1989. In 1964-1989, the tidal skewness increased by at least 0.1 throughout the LDB, indicating that the flood duration of the entire bay was shortened significantly. However, in 1989-2016, the tidal skewness decreased by at most 0.15 throughout the LDB, representing a longer flood duration in the entire LDB. The scenario simulations reveal that reclamation-induced shoreline changes control the increase in TDA and its primary contributors by enhancing width convergence of estuary in the period of 1964-1989. Conversely, the increase in water depth plays a vital role in the decrease of TDA in the period of 1989-2016. The results obtained from this study are particularly useful for understanding the controlled factors contributing to net sediment transport and the associated long-term morphological evolution in estuaries heavily impacted by human interventions.

Vertical Structure of Barotropic‐to‐Baroclinic Tidal Energy Conversion on a Continental Slope

AbstractHorizontal distribution of the vertically integrated barotropic‐to‐baroclinic energy conversion has been widely studied to examine the generation of internal tides at steep topography. The vertical structure of the energy conversion that provides insights into the associated dynamics, however, is masked by the often used depth‐integrated approach. Here, we reveal the vertical profile of barotropic‐to‐baroclinic energy conversion by employing an idealized ocean model in a slope‐shelf context forced by M2 barotropic tidal flow. The model shows two vertically separated hotspots of energy conversion, one near the sloping bottom and the other at the thermocline, resulting from the stronger vertical velocity and enhancement of the density perturbation, respectively. Isolation of the hotspots demonstrates that baroclinic energy generated in the bottom layer radiates toward onshore and offshore primarily in the form of internal wave beams, whereas that generated at the thermocline propagates away in the form of internal wave modes. Although energy converted at the thermocline contributes to only a small portion of the total energy conversion, it plays an important role in onshore baroclinic energy radiation and can be significantly affected by the internal wave activity at the bottom layer. With a fixed bottom topography, the percentage of baroclinic energy generated at the thermocline is linearly related to a body force exerted by the barotropic tidal flow over topography that can be estimated analytically. This provides a convenient way to estimate the overall barotropic‐to‐baroclinic energy conversion over a continental slope in the real ocean by measuring the energy conversion in the thermocline only.

Incoherent signatures of internal tides in the Tokara Strait modulated by the Kuroshio

Internal tides are ubiquitously generated from disturbances between barotropic tides and bottom topography in stratified oceans. Incoherence is routinely considered as non-stationary phase discrepancy between internal and astronomical tides. Internal tides, especially incoherent constituents, contribute substantial energy to dissipation and turbulent mixing. Using ∼8-month near-full-depth moored current observations, we investigated internal tides in the Tokara Strait, where the Kuroshio exits the East China Sea. Semidiurnal internal tides, especially the M2 internal tides, dominate the diurnal internal tides by approximately one-order of magnitude. We decomposed the bandpass-filtered tidal currents into coherent and incoherent constituents, both of which presented a spring–neap cycle; however, the former was largely coupled with local barotropic tides, while the latter had an incoherent phase. The coherent semidiurnal internal tides were intensified and had larger variability near the bottom; nevertheless, the intensity and variability of incoherent internal tides decreased with depth. These results suggest that the incoherent internal tides are exogenous and are possibly affected by the background flow field during propagation. The mean incoherence (above 54 % for both semidiurnal and diurnal internal tides) was expectedly higher than the global mean (∼44 %), implying a robust effect of nonlinear Kuroshio-tide interactions. Moreover, the relationship between the variability of the incoherence and the north–south shift of the Kuroshio further revealed the impact of the background flow intensity variation on internal tides. A ray tracing model demonstrated different refraction indices during the propagation of internal tides, leading to converged (diverged) ray paths in the mooring when the Kuroshio was at the northern (southern) axis position, thus explaining the differential incoherence caused by the variability of the Kuroshio.

Development History of the Numerical Simulation of Tides in the East Asian Marginal Seas: An Overview

As a ubiquitous movement in the ocean, tides are vital for marine life and numerous marine activities such as fishing and ocean engineering. Tidal dynamics are complicated in the East Asian marginal seas (EAMS) due to changing complex topography and coastlines related to human activities (e.g., land reclamation and channel deepening) and natural variability (e.g., seasonal variations of ocean stratification and river flow). As an important tool, numerical models are widely used because they can provide basin-scale patterns of tidal dynamics compared to point-based tide gauges. This paper aims to overview the development history of the numerical simulation of tides in the EAMS, including the Bohai Sea, the Yellow Sea, the East China Sea, the East/Japan Sea, and the South China Sea, provide comprehensive understanding of tidal dynamics, and address contemporary research challenges. The basic features of major tidal constituents obtained by tidal models are reviewed, and the progress in the inversion of spatially and temporally changing model parameters via the adjoint method are presented. We review numerical research on how a changing ocean environment induces tidal evolution and how tides and tidal mixing influence ocean environment in turn. The generation, propagation, and dissipation of internal tides in the EAMS are also reviewed. Although remarkable progresses in tidal dynamics have been made, nonstationary tidal variations are not fully explained yet, and further efforts are needed. In addition, tidal influences on ocean environment still receive limited attention, which deserves special attention.

Incoherence of the M2 and K1 internal tides radiated from the Luzon Strait under the influence of looping and leaping Kuroshio

The Kuroshio in different paths is considered to have different impacts on the generation, propagation and energetics of internal tides (ITs) originated from the Luzon Strait (LS). However, its influence on the incoherence of ITs remains unknown. Based on a regional model simultaneously simulating ITs and background circulations, this study examines the incoherence of the M2 and K1 ITs radiated from the LS under the influence of looping and leaping Kuroshio. The looping Kuroshio causes stronger incoherence of the M2 and K1 ITs than the leaping Kuroshio. The reason is that the looping Kuroshio causes a larger variation of eigenspeed, which results in stronger incoherence of ITs than the leaping Kuroshio does. For both the looping and leaping Kuroshio, the low-frequency background currents play a more important role than the stratification in changing the eigenspeed. We believe that the results of this study can deepen our understanding of the incoherent nature of ITs near the LS.

Interannual variability of internal tides in the Andaman Sea: an effect of Indian Ocean Dipole

A marginal sea in the north eastern Indian Ocean, the Andaman Sea, has been known for the presence of high-amplitude internal waves since the nineteenth century. In this study, we explored the interannual variations of the internal wave activity in this complex region. We found that the Dipole Mode Index, which represents the Indian Ocean Dipole (IOD), influences the circulation in the Andaman Sea, which in turn impacts its density stratification on interannual scales. Ocean Reanalysis System 5 data (1993–2018) is used to see an increasing trend in the sub-surface stratification, whereas it showed a decreasing trend in the near-surface waters. Numerical model simulations carried out from 2009 to 2018 have shown that the interannual variability in the generation of semidiurnal internal tides is governed by distinct parameters (tidal forcing and stratification) at different sites in different months. Enhanced upwelling (downwelling) is observed during positive (negative) IOD events. Sensitivity experiments conducted between extreme IOD events (2006 and 2016) revealed an increase in internal tide generation from positive IOD to negative IOD. Furthermore, a sharp decrease in local baroclinic dissipation is seen during negative IOD, increasing baroclinic flux into the Andaman Sea. An increase in the strength of positive IOD could lead to enhanced diapycnal mixing due to strong local dissipation, whereas an increase in the intensity of negative IOD could result in amplified propagation of internal waves.

Updated parameterization of internal tidal mixing in the deep ocean based on laboratory rotating tank experiments

The Coriolis effect plays an important role in the generation and dissipation of internal tides. In this study, the effects of the Coriolis parameter on internal tides generated by tidal current flowing over rough subsurface topography were investigated in the laboratory. The results show that the variation in the internal tidal local dissipation rate with variation in the Coriolis parameter in the deep layer follows a Gaussian distribution. The strongest dissipation occurs near the critical latitude where the local Coriolis parameter is half of the tidal frequency. A modified internal tidal mixing parameterization was developed by introducing a correction function related to the Coriolis effect on internal tidal local dissipation in the deep ocean, based on the original version developed by St. Laurent et al. (2002). The new parameterization was then implemented in a parallel ocean model. The results indicate that the deep global meridional overturning circulation (GMOC) is strengthened in the low and middle latitudes and the Southern Ocean by the change, compared with the original parameterization.

An Eco-Morphodynamic Modelling Approach to Estuarine Hydrodynamics & Wetlands in Response to Sea-Level Rise

Tidal inundation is the primary driver of intertidal wetland functioning and will be affected by sea- level rise (SLR). The morphology of estuaries and friction across intertidal surfaces influences tidal propagation; accordingly, sea-level rise not only increases inundation frequency, but will also alter other tidal parameters, such as tidal range. To investigate responses of estuarine intertidal vegetation, primarily mangrove and saltmarsh, to SLR an eco-morphodynamic modelling approach was developed that accounted for some of the feedbacks between tidal inundation and changes to wetland substrate elevations. This model partially accounts for adjustment in estuarine hydrodynamics, and was used to examine the potential effect of SLR on mangrove and saltmarsh distribution in a micro-tidal channelised infilled barrier estuary in southeast Australia. The modelling approach combines a depth-averaged hydrodynamic model (Telemac2D) and an empirical wetland elevation model (WEM) that were coupled dynamically to allow for eco-geomorphological feedbacks. The integrated model was parameterised to consider two SLR scenarios, and two accretion scenarios within the WEM. Time series of observed water levels, tidal inundation and flow velocity were used to validate the hydrodynamic model for present-day sea level, whereas wetland mapping was used to verify predictions of mangrove and saltmarsh distribution. Tidal range varied along the estuary, increasing in response to low and high SLR scenarios (by up to 8%), and responded non-linearly under high SLR. Simulations of low and high SLR scenarios indicated that wetlands mostly withstand modest SLR rates (+ 5mm yr-1) through sedimentation, but submerge and convert to subtidal areas under fast SLR rates (> 10mm yr-1). Projected changes in tidal range are linked to eco-geomorphological feedbacks caused by changing wetland extents and adjustments of intertidal wetland geomorphology through sedimentation. Potential changes arising from morphological change at the entrance and in the tidal channels is not obtained from the model. The results of this study demonstrate interconnections between hydrodynamics and intertidal wetlands, which need to be accounted for when estimating wetland response to SLR in channelised estuaries. Integrated models of estuarine-wetland systems are more precise as they account for the dynamic feedbacks between hydrodynamics and wetlands. For example, they also consider alterations to tidal range resulting from SLR and the effects of these on wetland inundation and sedimentation.

The transition of tidal Kelvin waves to hybrid Kelvin edge and internal waves in the global ocean

In this study, we investigate the transition of semidiurnal Kelvin waves into Hybrid Kelvin-Edge (HKE) waves and associated generation of internal tides at widening shelves using theory, a realistic global baroclinic ocean model simulation, and quasi-realistic regional barotropic model simulations. Using the global model simulation, we identify several areas where a tidal HKE wave transition co-exists with internal wave generation. Of all areas considered, the Celtic Sea/Bay of Biscay shelf has the widest shelf and the strongest internal tide generation. We find that the global simulation agrees better with the theoretical Kelvin modes on the narrow than with the hybrid edge modes on the wide shelves. To help us understand the effect of complex, realistic bathymetry on the HKE wave transition, we perform quasi-realistic 1/25° barotropic simulations of the Celtic Sea/Bay of Biscay shelf areas. In these simulations, we gradually change the realistic bathymetry to a more idealized bathymetry. The idealized simulations show that the complex bathymetry steers the barotropic energy flux and causes standing wave patterns, which mask the HKE wave transition. Based on this analysis, we conclude that the HKE wave transition in the Celtic Sea/Bay of Biscay and other shelf areas in the global ocean is most likely masked by the effects of complex bathymetry and that offshelf baroclinic fluxes cannot be exclusively attributed to the HKE wave transition.

Simulation of freshwater transport network and salt flux in the Bangladesh delta

Circulation of saline water is important for maintaining water quality in the Ganges-Brahmaputra-Meghna (GBM) delta because of its vulnerability to the threat of climate change. We applied a numerical model to examine the volume and salt transports within the GBM delta, Bangladesh. To understand the components of salt water intrusion driven by tidal and subtidal (residual) transports, we selected 19 cross-sections to represent the complex delta circulation in a simplified network model. Our results show that over 82.51% of GBM river water drains through the eastern estuarine system (EES) in the wet season, increasing to 98.37% in the dry season. Residual transport can be comparable in size with the tidal transport in the wet season, and one order of magnitude smaller in the dry season. The western estuarine system (WES) experiences serious salinity intrusion in the dry season, and strong seasonal variability in both tidal and subtidal transport, with suppression of tide-driven transport observed during the wet season. Our results show the sub-channels area of the Lower Meghna River also faces the risk from salinity intrusion issues, as stronger tidal salt flux is estimated in the dry season. Tidal volume transport varies seasonally, corresponding to the variability of river discharge. A simplified solution by means of polynomial expansion was applied to describe the tidal propagation within river channels. Inland penetration of tidal energy is reduced with large river discharge, and additionally the propagation speed of the tidal wave increases in the wet season. Our analysis helps understand the response of the three estuarine systems to seasonal and tidal controls, and can be used to inform river management about the upstream-downstream linkages.

A generalized analytical solution of groundwater head response to dual tide in a multi‐layered island leaky aquifer system

AbstractThe hydraulic properties of coastal aquifer systems are relevant to various hydrogeological, hydro‐ecological and engineering problems. This study presents an analytical solution for predicting groundwater head fluctuations induced by dual‐tide in multi‐layered island aquifer systems, consisting of an unconfined aquifer on the top and any number of leaky aquifers below. The solution was derived via the methods of matrix differential calculus and separation of variables. It is more general than any existing analytical solutions for the tidal pressure propagation since the new solution can consider multi‐layered aquifer systems along with the effects of leakage and aquifer length. Using this solution, we illustrated potential errors that may occur due to neglecting one or more vital factors affecting groundwater fluctuations. Besides, we articulated the groundwater response to the dual‐tide in complex coastal aquifers. Considering that some thin semipermeable layers may be ignored in practical field investigation, we also demonstrated the effects due to simplification of aquifer layers. The results showed that with the increase in the number of overlapped leaky layers, the tidal propagation in the bottom part of multi‐layered aquifer system approaches that in a single confined aquifer with the same transmissivity and storage.

Response of tidal dynamics to successive land reclamation in the Lingding Bay over the last century

Globally, many estuaries are suffering from land reclamation that drastically influences estuarine hydrodynamics. It has been reported that reclamation may induce tidal regime shifts in small converging estuaries, but it is not clear whether such regime shifts can also occur in wide estuarine bays. Lingding Bay (LDB) in Pearl River Estuary, southern China has a sufficient width (ca. 60 km) and a long history of reclamation (approximately 3.99 × 10 8 km 2 over the past century), forming an ideal area to study the mechanism of tidal regime shift in wide bays. Based on a state-of-the-art modeling tool (Delft3D Flexible Mesh), tidal propagation pattern in 1917, 1964, 1989 and 2016 was reproduced accounting for reclamation-induced shoreline changes. Our results reveal a prominent shift in tidal amplitude, residual current, and tidal energy flux in the period of 1989–2016, even though the reclamation area in this period was the smallest among the three studied periods. The reason for such shift is that reclamation of key areas amplified the imbalanced tidal energy distribution between the western and eastern LDB, and such imbalance exceeded a tipping point around 1989. This regime shift may increase the relative tidal forcing in the eastern LDB, which means stronger saltwater intrusion and higher flooding risk. Our findings indicate that tidal dynamics of LDB has been greatly altered over the last century in the LDB, where the scale and location of the reclamation are of key importance. The insights obtained from this study are valuable to the future management of LDB and other estuaries that are under reclamation stress. • The continuous land reclamation triggered a different response of the tidal dynamics in the Lingding Bay (LDB) around 1989. • The increased lateral difference resulted in regime shifts in the tidal motion in the LDB. • The tidal motion in the LDB showed threshold behavior in response to reclamation. • The altered tidal motion may lead the morphology to a new pattern as a snow ball effect requiring further studies.