Tidal Duration Asymmetry Research Articles

How do morphological characteristics affect tidal asymmetry in the Radial Sand Ridges?

While it is widely recognized that the Radial Sand Ridges (RSR) in the South Yellow Sea are predominantly shaped by tidal forces, there remains a limited understanding of how this distinctive morphological configuration—characterized by an interlaced channel-ridge system—can subsequently influence local tidal dynamics. This study examines the effects of morphological features on tidal asymmetry, taking into account seabed slope, relative depths between ridges and channels, and channel convergence. Three principal indices—namely tidal-duration-asymmetry (TDA), peak-current-asymmetry (PCA), and slack-water-asymmetry (SWA)—are employed to quantify various dimensions of tidal asymmetry. The findings indicate that SWA serves as the most morphology-sensitive indicator, whereas TDA exhibits minimal sensitivity to morphological changes. Furthermore, seabed steepness emerges as a critical factor influencing tidal asymmetry within the RSR; steeper slopes enhance intrinsic energy conversion processes, thereby inducing tidal asymmetries. Additional analysis reveals that streamwise advection accounts for an average of 88 % of total advection scale while controlling for spatial heterogeneity. Specifically, the average integral sum of advection terms along submerged sand ridges is 2.53 times greater than that along the deepest section of the tidal channel line—a significant contributor to spatial variability in SWA. With a positive seabed slope, the apex of the RSR acts as a source for overtides which interact with incoming astronomical tides, consequently generating tidal asymmetries. Moreover, this study illustrates varying dependencies of tidal asymmetry on bottom stress across channels and ridges, contributing to spatial variability in arc direction among RSRs. Ultimately, this research elucidates complex interactions between tidal flow and morphological characteristics within RSRs and provides insights into tide evolution in analogous ebb-shoal systems.

Influence of tidal asymmetry on local scour near the offshore platform

An in-situ mooring measurement was conducted using a profiling sound of navigation and ranging (SONAR) system to reveal the influence of tidal asymmetry on the local scour near an artificial offshore platform, Southwest Offshore Windfarm Complex, Korea. During the mooring period, the flood tide was significantly dominated by positive tidal duration asymmetry and skewness. The lengths of scour hole were 9.8m and 5.1m along the flood and ebb directions, respectively. Flood-dominated currents intensified the horseshoe vortex, resulting in an approximately 10% deeper scour hole on the flood face than the ebb face. The periodic occurrence of bidirectional currents generated vortices that facilitated the mixing and redistribution of seabed sediments, forming a gentle slope within the scour hole. The average slope angles ranged from 14° to 16° on the flood face and 6° to 8° on the ebb face. Despite the strong influence of local scour, variations in scour depth (Ds) consistently remained around −2.15m, suggesting the exposure of the underlying consolidated sediment layer. During the intermediate period from spring to neap tide, the Ds on the lower slope of scour hole increased by 0.12m, while it decreased by 0.11m from neap to spring tide, suggesting that the net Ds gradually approached zero over the tidal cycle. These findings underscore the importance of understanding tidal impacts on the local scour morphology to enhance the stability and design of offshore wind turbine foundations.

The sensitivity of tidal asymmetry descriptors in the Ems estuary

Tidal asymmetry in estuaries and other tidally dominated coastal systems is commonly evaluated to assess system states or their development. Based on different methods, local states are classified as either flood or ebb dominant. An increasing number of descriptors for deriving tidal asymmetry in recent years calls for a comparison and discussion of their sensitivity on input data and its quality. We compared tidal asymmetry from water level and current velocity using various descriptors that deduce from harmonic, ratio, and skewness methods. Computed from one-year measurements at different stations along the Ems estuary, their comparability was enabled by a new approach of scaling. Our results on the variation of sampling intervals demonstrated a highly site-specific sensitivity of the descriptors that led up to changes in the asymmetry direction in tidal duration asymmetry and phase lag. The slack water asymmetry appeared most sensitive to the studied parameter settings. As expected, variability of tidal asymmetry reduced with an increasing number of analyzed tides. At the same time, uncertainty from the asymmetry during spring or neap phases compared to spring-neap periods remained in all analyzed descriptors. Hence, the characterization of the estuary in terms of flood- or ebb-dominance depends critically on the quality and extent of the input data. For all parameter settings, the impact of river discharge on tidal asymmetry was pronounced but varied depending on the location in the estuary. The actual characterization of the effect of asymmetry, e.g., on sediment transport, is not conducted in this study. We propose that this requires a more comprehensive dataset, such as depth and cross-sectional variability of currents and sediment concentrations.

Study on the spatiotemporal variation of the Yangtze estuarine tidal species

Owing to the nonlinear influence of river discharge, estuarine tides are nonstationary and more complicated than oceanic tides. This study investigated the spatiotemporal variation pattern of the Yangtze estuarine tides by considering tidal species separated using the Variational Mode Decomposition (VMD) model. The separated tidal species were compared with tidal species reconstructed using the NS_TIDE model. The results showed that the subtidal (D0) amplitude increased landward because of the enhanced influence of river discharge and river-tide interactions; diurnal (D1) and semi-diurnal (D2) tidal amplitudes consistently decayed landward because of frictional dissipation and nonlinear energy transfer to other tidal constituents; while quarter-diurnal (D4) and six-diurnal (D6) tidal amplitudes initially increased and then decreased in the landward direction because of the transformation of the dominant role of river discharge from nonlinear energy transfer to frictional dissipation. The spatiotemporal variations in different tidal species can illustrate other tidal processes, such as seasonal differences in tidal duration asymmetry. Although the VMD model neglects the separation of each tidal constituent, it can provide reasonable information, even when NS_TIDE may provide implausible results during the flood season because of the highly nonstationary conditions.

Contributors to tidal duration asymmetry with varied coastline configurations on western shelf of Yellow Sea

Coastal management in China is confronted with an urgent choice between natural restoration and maintenance of existing seawalls and reclaimed land for economic development. A key criterion for making this decision is the resilience to coastal flooding, which depends on the ability to predict tidal level. Tidal duration asymmetry (TDA) is a key parameter in determination of the arrival and duration of flood tides. This study selected the western inner shelf of the Yellow Sea (WYS) as the study area and investigated the responses of TDA to different shoreline configurations and relative sea level rise. The responses of TDA to shoreline reconstruction yielded spatial variability locally and remotely. In the nearshore area, the responses of TDA to the complex ocean environment mainly originated from the combined functions of reflection, bottom friction, and advection, which controlled the energy transfer from M2 or S2 constituents to their overtides or compound tides. The sensitivity of TDA to coastline typologies was not limited to coastal waters but could stretch over the entire inner shelf. The vulnerability of tidal responses was due to the displacement of the M2 amphidrome of the Kelvin wave on the WYS, which in turn changed tidal energy fluxes over the regime. The relative sea level rise could intensify the feedback of TDA to seawalls and land reclamation.

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.

Numerical study on tidal duration asymmetry and shallow-water tides within multiple islands: An example of the Zhoushan Archipelago

Tidal duration asymmetry (TDA) in Zhoushan Archipelago (ZA), East China, was investigated with a three-dimensional hydrodynamic model based on the finite volume community ocean model (FVCOM). The model was evaluated against observed sea surface elevations, currents, and harmonic constants, and the evaluation indicated that the model agreed well with the observations. As TDA is caused by high-frequency harmonics of the main astronomical tides, the generation mechanisms for the main shallow-water tides (M4, M6, MS4, and MS6) generated by nonlinearity were explored first. Shallow-water tides are primarily generated by the nonlinear advection and interactions in multi-island areas and shallow-waters with the M2 and S2 tides at the open boundary, and the S2 tide suppresses (or promotes) the generation of M4/M6 (or MS4/MS6) tides. The model results indicated that tides were flood-dominant in ZA, and the strength of asymmetry increased from the deep sea to the shallow-water area and the multi-island area. To explore the mechanisms leading to the observed TDA, seven sensitivity experiments were conducted by modifying the bottom friction term, removing the islands, removing the advection term in the model equations, and changing the water depth in the Luotou Channel. The sensitivity analysis results indicated that TDA was strongly related to the above four factors. Specifically, in the Luotou Channel area and Hangzhou Bay, decreasing the bottom friction term decreased TDA, and removing the islands enhanced TDA. In addition, the advection term also played an important role in determining TDA, especially at the entrance of the Luotou Channel. Furthermore, TDA was strengthened when the water depth decreased.

On the Role of Anthropogenic Activity and Sea‐Level‐Rise in Tidal Distortion on the Open Coast of the Yellow Sea Shelf

AbstractThis study investigates how varied coastline strategies and rising mean‐sea‐level influence tidal distortion along the open coast of the Yellow Sea shelf. The major contributors to tidal distortion were first identified. Then, the primary tidal constituents causing changes in tidal propagation and distortion in response to shoreline reconstruction management and near‐future sea‐level‐rise (SLR) were determined. The intrinsic mechanisms causing tidal duration asymmetry (TDA) were investigated and found to differ in various types of coastal systems. Specifically, the nonlinear tidal interaction between M2/M4, M2/S2/MS4 and K1/O1/M2 are the top three dominant contributors to TDA. The interactions between M2/M4 and M2/S2/MS4 are most sensitive to different coastline configurations and produce the largest evolution in tidal distortion. Two primary factors are (i) the location of the amphidromic point of the semidiurnal tides in the deep ocean, which controls the M2 and S2 energy fluxes into embayment, and (ii) the nondimensional amplitude (A/h), which importantly indicates tidal nonlinearity and determines shallow tide generation. For the coast with a progressive tidal system, tidal distortion and shallow‐water tide genesis depend mainly on the influx of semidiurnal tides from offshore, which is controlled by the position of the M2 amphidrome. For a coast with quasistanding tidal behavior, however, the scales and positions of tidal flats and shoals play vital roles in tidal distortion and the genesis of shallow‐water tides. Model simulations suggest that hardened shoreline or SLR could dampen the advection and bottom dissipation, thus reducing tidal distortion.

Impact of river discharge seasonality change on tidal duration asymmetry in the Yangtze River Estuary

The Yangtze River Estuary (YRE) is one of the world’s largest river-tidal systems with rapidly changing hydrology and morphology following the construction of multiple dams. The effects of dam construction may extend to the region close to the coast, where channel stability depends on the asymmetry of the tide. Here, we focus on the possible effects of changing discharge regimes on tidal asymmetry in the YRE. Specifically, we focus on the difference in duration between ebb and flood, quantified as tidal duration asymmetry, because it has strong implications for residual sediment transport and can be derived from available water level data. To cope with nonstationary tides under the influence of a time-varying river discharge, a nonstationary harmonic analysis tool (NS_TIDE) is applied to explore the spatiotemporal variations in tidal duration asymmetry, under the influence of different combinations of tidal constituents. Tidal duration asymmetry initially increases, then slightly decreases, in an upstream direction. It experiences significant seasonal variations in response to rapidly varying discharge: tides are more asymmetric upstream of Zhenjiang in the dry season and more asymmetric downstream in the wet season. The combined effects of discharge regulation and morphological changes cause seasonal alterations in tidal duration asymmetry. In the wet season, reduced river discharge caused by water storage and climate change enhance the asymmetry upstream (+11.74% at Wuhu, +7.19 at Nanjing) while the asymmetry is weakened downstream (−2.90% at Zhenjiang, −7.19 at Jiangyin) following the TGD’s operation. Downstream channel erosion caused by post-TGD lower sediment loads has become the dominant factor weakening tidal asymmetry in most parts of the YRE in the dry season. Understanding these evolutions of tidal duration asymmetry under the hydrological and morphological effects has important implications for the management of estuarine ecosystem and navigation.

Impacts of human activities on tidal dynamics in a sexta-diurnal tidal resonant bay

Using numerical modelling, we study changes in tidal dynamics in Daya Bay (DYB) between 1989 and 2014. During this period, a total water area of 30 km2 was reclaimed and the average water depth increased by 38 cm. As DYB is a sexta-diurnal tidal resonant bay, the sexta-diurnal tides respond differently to the coastline and bathymetry changes than other tides. Taking K1, M2, M4, and M6 as examples, model results show a decrease in tidal elevation amplitude, tidal current magnitude, and tidal energy flux for K1, M2, and M4 tides. For the M6 tide, however, the model predicted an increase in tidal elevation amplitude, tidal current magnitude in some parts of the bay, and the tidal energy flowing into the bay. Land reclamation leads to the enhancement of sexta-diurnal tidal resonance and thus the magnitude of the M6 tide. Furthermore, due to the magnification of M6, tidal duration asymmetry in DYB changed from ebb-dominance to flood-dominance, and water exchange became much more active. Therefore, owing to the sexta-diurnal tidal resonance, the impact of human activities on tidal dynamics in DYB is different from that in previously reported semi-enclosed bays where large-scale land reclamation has been carried out.

Influence of morphological changes on suspended sediment dynamics in a macrotidal estuary: diachronic analysis in the Seine Estuary (France) from 1960 to 2010

Estuaries are subject to extensive morphological changes through human activities, such as deepening and narrowing via dredging and channelization. The estuary sediment load, characterised by the estuarine turbidity maximum (ETM), can severely increase in response to channel deepening, shifting the estuary from a natural to hyperturbid state. The main processes driving the estuarine circulation, hydrology and sediment dynamics are relatively well known. However, their relative influence on suspended sediment concentration (SSC), as well as their role in the transition toward hyperturbid estuaries, is still a subject of debate. Therefore, this study aims at investigating the relative contribution of key estuarine drivers (gravitational circulation and tidal pumping mechanisms) on the ETM dynamics under significant estuary morphological changes. In a 3D numerical model of the Seine Estuary (France), three bathymetries from the last 50 years (1960, 1975 and 2010) were implemented, characterising the gradual estuary deepening and narrowing. The morphological changes induce an expected tidal amplification in the main channel, associated with a decrease of the tidal duration asymmetry. The salinity front slightly migrates up-estuary and the stratification strengthens, resulting in a higher and upper-estuary SSC distribution. Starting from a dominantly natural system driven by the tidal pumping mechanism in 1960, the contribution of the gravitational circulation strongly increased in the present-day anthropogenically-controlled system. Contrastingly, the maximum ETM mass barely changes with the morphological changes and dredging activities have hardly any influence. Hence, the maximum ETM mass in the Seine Estuary would principally result from the tidal forcing rather than the morphology.

A numerical study on the effect of tidal flat’s slope on tidal dynamics in the Xiangshan Bay, China

The Xiangshan Bay is a semi-enclosed and narrow bay, which is characterized by large scale tidal flats and has been historically utilized through coastal construction and aquaculture engineering. The hydrodynamic model using the Finite Volume Coastal Ocean Model (FVCOM) was constructed to examine the changes of tidal dynamics due to the variation of tidal flat slopes. According to the model results, a decreased slope of a tidal flat would amplify the M2 tidal amplitude and delay the M2 tidal phase in the inner harbor, due to an increased tidal prism, and vice versa. The amplitude of the main shallow-water tide M4 would be amplified/dampened in the entire bay due to the changed bottom friction, if the tidal flat’s slope were reduced/increased at the Tie inlet. The phase was advanced. The change of a tidal flat’s slope at the Tie inlet had greater impacts on tidal amplitude, phase and duration asymmetry, than that at the Xihu inlet. The impact of changes of the tidal flat slope at the Xihu inlet was small and was constrained locally. Changes in the tidal flats’ slopes at the Tie and Xihu inlets changed the tidal duration asymmetry, residual current and tidal energy via modulating tides. The ebb dominance decreased when the tidal flat’s slope at the Tie inlet was changed. Decreased/increased ebb dominance occurred when the tidal flat’s slope was reduced/increased at the Xihu inlet. The residual current and tidal energy density was amplified/dampened and more/less tidal energy was dissipated, with reduced/increased slope at both of the inlets. The findings in this study are instructive to coastal engineering and estuarine management.

Short-term tidal asymmetry inversion in a macrotidal estuary (Beira, Mozambique)

The distortion of the tide in estuaries, bays and coastal areas is the result of the generation of overtides due to the non-linear effects associated with friction, advection, and the finite effects of the tidal amplitude in shallow waters. The Beira estuary is classified as macrotidal, with a large ratio of S2/M2. Typical tides ranges from 6m and 0.8m, during springs and neaps tides, respectively. As a consequence of this large fortnightly tidal amplitude difference and the estuarine morphology, asymmetry inversions occur. Two types of tidal asymmetries were investigated in this paper, one considering tidal duration asymmetry (time difference between rising and falling tide) and the other, related to tidal velocity asymmetry (unequal magnitudes of flood and ebb peaks currents). In the Beira estuary when we examine the tidal duration asymmetry, flood dominance is observed during spring tide periods (negative time difference between rising and falling tide), while ebb dominance appears during neap tides (positive time difference between rising and falling tide). A 2DH hydrodynamic model was implemented to analyze this asymmetry inversion. The model was calibrated with water-level data measured at the Port of Beira and current data measured along the estuary. The model was run for different scenarios considering tidal constituents at the ocean boundary, river discharge and the morphology of the estuary. River discharge did not show significant effects on the tidal duration asymmetry. Through comparison of the scenarios, it was shown that the incoming ocean tide at the boundary has an ebb-dominant asymmetry, changing to flood-dominant only during spring tides due to the effect of shoaling and friction within the estuary. During neap tides, the propagation occurs mainly in the channels, and ebb dominance remains. The interplay between the estuary morphodynamics was thus identified and the relation between tidal duration asymmetry and tidal velocity asymmetry was observed. While fortnightly inversion in the tidal duration asymmetry is explained by the presence of channels and sandbanks, at the same time, the tidal velocity asymmetry acts as a positive feedback mechanism for bank formation and sediment retention.

Responses of water environment to tidal flat reduction in Xiangshan Bay: Part I hydrodynamics

Xiangshan Bay consists of a deep tidal channel and three shallow inlets. A large-scale tidal flat has been utilized through coastal construction. To ascertain the accumulate influences of these engineering projects upon the tidal dynamics of the channel-inlets system, this study uses FVCOM to investigate the tides and flow asymmetries of the bay, and numerically simulate the long-term variations of tidal dynamics caused by the loss of tidal flats. It was found that the reduction of tidal flat areas from 1963 to 2010 slightly dampened M2 tidal amplitudes (0.1 m, ∼6%) and advanced its phases by reducing shoaling effects, while amplified M4 tidal amplitudes (0.09 m, ∼27%) and advanced its phases by reducing bottom friction, in the inner bay. Consequently, the ebb dominance was dampened indicated by reduced absolute value of elevation skewness (∼20%) in the bay. The tides and tidal asymmetry were impacted by the locations, areas and slopes of the tidal flats through changing tidal prism, shoaling effect and bottom friction, and consequently impacted tidal duration asymmetry in the bay. Tides and tidal asymmetry were more sensitive to the tidal flat at the head of the bay than the side bank. Reduced/increased tidal flat slopes around the Tie inlet dampened the ebb dominance. Tidal flat had a role in dissipating the M4 tide rather than generating it, while the advection only play a secondary role in generating the M4 tide. The full-length tidal flats reclamation would trigger the reverse of ebb to flood dominance in the bay. This study would be applicable for similar narrow bays worldwide.

Contributions of different tidal interactions to fortnightly variation in tidal duration asymmetry

AbstractThe general framework for identifying tidal duration asymmetry proposed by Song et al. (2011) is extended to express fortnightly variability in duration asymmetry. The extended metrics are verified and studied using observed sea level data at 481 stations worldwide. The results reveal that fortnightly variability is universal and that duration asymmetry can be stronger during neap tide than during spring tide. The fortnightly variability in duration asymmetry is primarily induced by three types of tidal interactions: interactions within the principal tidal constituents, interactions between high‐frequency and principal tidal constituents, and interactions between long‐period and principal tidal constituents. Among these interactions, the first type is most important at most of the stations and is related to the form number F. The contributions of different interactions can be quantified using their frequencies, amplitudes and phases. Global patterns of the fortnightly variation are illustrated using TOPEX/Poseidon altimetry data. The findings show that remarkable fortnightly variation in the tidal duration asymmetry occurs in most open oceans and is significant around an amphidromic point. The metrics derived in this study can be used to examine any time‐varying characteristics in tidal asymmetry (not limited to duration asymmetry) by selecting a suitable frequency threshold.

Tidal asymmetry in a funnel-shaped estuary with mixed semidiurnal tides

Different types of tidal asymmetry (see review of de Swart and Zimmerman Annu Rev Fluid Mech 41: 203–229, 2009) are examined in this study. We distinguish three types of tidal asymmetry: duration and magnitude differences between flood and ebb tidal flow, duration difference between the rising and falling tides. For waterborne substance transport, the first two asymmetries are important while the last one is not. In this study, we take the Huangmaohai Estuary (HE), Pearl River Delta, China as an example to examine the spatio-temporal variations of the tidal asymmetry in a mixed semidiurnal tidal regime and to explain them by investigating the associated mechanisms. The methodology defining the tidal duration asymmetry and velocity skewness, proposed by Nidzieko (J Geophys Res 115: C08006. doi: 10.1029/2009JC005864, 2010) and synthesized by Song et al. (J Geophys Res 116: C12007. doi: 10.1029/2011JC007270, 2011), is utilized here and referred to as tidal duration asymmetry (TDA) and flow velocity asymmetry (FVA), respectively. The methodology is further used to quantify the flow duration asymmetry (FDA). A positive asymmetry means a shorter duration of low water slack for FDA, a shorter duration of the rising tide for TDA, and a flood dominance for FVA and vice versa. The Regional Ocean Modeling System (ROMS) model is used to provide relatively long-term water elevation and velocity data and to conduct diagnostic experiments. In the HE, the main tidal constituents are diurnal tides K 1, O 1 and semidiurnal tides M 2 and S 2. The interaction among the diurnal and semidiurnal tides generates a negative tidal asymmetry, while the interactions among semidiurnal tides and their overtides or compound tides result in a positive tidal asymmetry. The competition among the above interactions determines the FDA and TDA, whereas for the FVA, aside from the interaction among different tidal constituents, an extra component, the residual flow, plays an important role. The results show that the FDA exhibits a predominant tendency of shorter duration of low water slack, favoring the landward transport of fine sediment. The FVA demonstrates prevailing ebb dominance in the study period, favoring the seaward transport of coarse sediment. This ebb dominance is primarily induced by the interaction among the residual flow and the tidal constituents. The external TDA in the ocean experiences distinct cyclic variations with positive asymmetry when semidiurnal tides dominate and negative asymmetry during the periods when diurnal tides dominate. The funnel shape of the HE is advantageous for the development of positive tidal asymmetry as the semidiurnal tides are more amplified than the diurnal tides. The effect of river flow can enhance the ebb dominance, while the baroclinic effect is more complex. The existence of channel and shoals favors the development of residual pattern with seaward flow (ebb dominance) in the channel and landward flow (flood dominance) at the shoal when the tides are strong (semidiurnal tides dominate) and the residual pattern with landward flow (flood dominance) in the channel and seaward flow (ebb dominance) at the shoal when the baroclinic effect is dominant (diurnal tides dominate).

Evolution of sedimentary dynamic environment in the western Jiaozhou Bay, Qingdao, China in the last 30 years

Intensive reclamation and cross-sea bridge construction occurred in the Jiaozhou Bay, Qingdao, China in the last 30 years. These changes have not only reduced water volume but also altered sedimentation in the bay. Using the data collected in the 1980s, 2000s and early 2010s, bottom sediment evolution in the last 30 years in the western bay were studied. Furthermore, a numerical model was used to study the hydrodynamic environment change in the surrounding sea areas. The seabed sediment became finer in the northwestern area from the 1980s to 2003, which was mostly influenced by the reduced sediment discharge from the Dagu River. However, from 2003 to July 2014 during which the Jiaozhou Bay Bridge was built, the seabed sediment became coarser on the north side of the bridge, and became finer on the south side; in particular, two coarser sediment belts appeared across the channel bridge. Numerical simulation results indicated increased current velocity on the north side and decreased velocity on the south side of the bridge. The tidal-duration asymmetry was calculated from the tidal harmonic parameters of the model output. In the flood dominated JZB, the larger deceased tidal-duration asymmetry on the north side of the bridge could result in more coarser seabed sediment brought in from the northern bay. These could be the reasons for the coarser seabed sediment on the north side of the bridge and finer sediment on the south side after the bridge construction. Deeper water on the north side and shallower water on the south side of the bridge from 2005 to July 2014 further demonstrated the effect of hydrodynamic change caused by the bridge construction.

Land reclamation and its impact on tidal dynamics in Jiaozhou Bay, Qingdao, China

Over the past few decades, there has been large-scale land reclamation in Jiaozhou Bay, which inevitably has affected the hydrodynamics and sediment transport in the bay, and thus has consequences for its management. In this study we set up a three-dimensional barotropic hydrodynamic model based on the Finite Volume Coastal Ocean Model to investigate changes in tidal dynamic factors in Jiaozhou Bay from 1935 to 2008. According to the model, the M2 tide has experienced only minor changes over this period. Land reclamation on a large scale took place between 1935 and 1966, leading to significant hydrodynamic changes during this period. Again according to the model, the M4 tidal amplitude rose dramatically, by up to 80% at the northeastern part of Jiaozhou Bay, and this has caused a significant increase in the M2–M4 tidal-duration asymmetry. Concurrently, the M2 tidal-energy fluxes across the entrances to both the inner and outer bay were both reduced by more than 50%. From 1966 to 2008, land reclamation was conducted on a smaller scale. Over this period, the M2 tidal-energy flux continued to decrease but at a relatively smaller rate. Both the M4 tidal amplitude and M2–M4 tidal-duration asymmetry also experienced a gradual decrease during this period, probably due to the reduced M2 tidal energy, in contrast to the increase between 1935 and 1966. The changes in the M2–M4 tidal-duration asymmetry from 1935 to 2008 caused by the land reclamation were reproduced in the model by progressively removing tidal flats from the head of the bay to its mouth. However, the M2–M4 tidal-duration asymmetry showed a different variation when the tidal flats were removed progressively in the opposite direction, from the mouth to the head of the bay. Our study shows that the significant increase in the M2–M4 tidal-duration asymmetry between 1935 and 1966 was a consequence of the change in bottom friction when the tidal flats were land-filled; advection played only a secondary role.

Tidal variations in the Sundarbans Estuarine System, India

Situated in the eastern coastal state of West Bengal, the Sundarbans Estuarine System (SES) is India’s largest monsoonal, macro-tidal delta-front estuarine system. It comprises the southernmost part of the Indian portion of the Ganga–Brahmaputra delta bordering the Bay of Bengal. The Sundarbans Estuarine Programme (SEP), conducted during 18–21 March 2011 (the Equinoctial Spring Phase), was the first comprehensive observational programme undertaken for the systematic monitoring of the tides within the SES. The 30 observation stations, spread over more than 3600 km2, covered the seven inner estuaries of the SES (the Saptamukhi, Thakuran, Matla, Bidya, Gomdi, Harinbhanga, and Raimangal) and represented a wide range of estuarine and environmental conditions. At all stations, tidal water levels (every 15 minutes), salinity, water and air temperatures (hourly) were measured over the six tidal cycles. We report the observed spatio-temporal variations of the tidal water level. The predominantly semi-diurnal tides were observed to amplify northwards along each estuary, with the highest amplification observed at Canning, situated about 98 km north of the seaface on the Matla. The first definite sign of decay of the tide was observed only at Sahebkhali on the Raimangal, 108 km north of the seaface. The degree and rates of amplification of the tide over the various estuarine stretches were not uniform and followed a complex pattern. A least-squares harmonic analysis of the data performed with eight constituent bands showed that the amplitude of the semi-diurnal band was an order of magnitude higher than that of the other bands and it doubled from mouth to head. The diurnal band showed no such amplification, but the amplitude of the 6-hourly and 4-hourly bands increased headward by a factor of over 4. Tide curves for several stations displayed a tendency for the formation of double peaks at both high water (HW) and low water (LW). One reason for these double-peaks was the HW/LW stands of the tide observed at these stations. During a stand, the water level changes imperceptibly around high tide and low tide. The existence of a stand at most locations is a key new finding of the SEP. We present an objective criterion for identifying if a stand occurs at a station and show that the water level changed imperceptibly over durations ranging from 30 minutes to 2 hours during the tidal stands in the SES. The tidal duration asymmetry observed at all stations was modified by the stand. Flow-dominant asymmetry was observed at most locations, with ebb-dominant asymmetry being observed at a few locations over some tidal cycles. The tidal asymmetry and stand have implications for human activity in the Sundarbans. The longer persistence of the high water level around high tide implies that a storm surge is more likely to coincide with the high tide, leading to a greater chance of destruction. Since the stands are associated with an amplification of the 4-hourly and 6-hourly constituents, storm surges that have a similar period are also likely to amplify more during their passage through the SES.