Tidal Duration Research Articles

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.

Numerical Modeling of the Impact of Sea Level Rise on Tidal Asymmetry in Hangzhou Bay

The tidal asymmetry under the action of sea level rise in Hangzhou Bay. Coastlines dominated by the tide are not only directly affected by the rise of the mean sea level but also by the tidal dynamics. The computational domain of the hydrodynamic model covers the entire Hangzhou Bay and takes into account the feedback between the tidal motion and the erodible bottom. Its main application fields include: the simulation of different sea level rise (SLR) rates, the interaction between tidal duration and skewness and the interaction between tidal range and astronomical tide. The results on tidal asymmetry in Hangzhou Bay is a systematic process consisting of four aspects: Firstly, the tide increase, which is affected by the sea level rise, is between 25% and 50%. Secondly, the value of the sea level rise is about two times the added value of the tidal range, and the tidal range increased to the left side of the tidal wave propagation direction, which accelerated the propagation velocity. Thirdly, the sea level rise amplified the M2 tidal amplitude and delayed the M2 tidal phase in the inner bay, which reduced about 50%. Finally, the change of the tidal range caused by tide level had the same magnitude as the change caused by the mean sea level rise. The purpose of this study was to emphasize the importance of the predictions of the response area affected by tidal asymmetry based on the action of the sea level rise in Hangzhou Bay.

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.

Tidal effects on variations in organic and inorganic biogeochemical components in Changjiang (Yangtze River) Estuary and the adjacent East China Sea

Nutrients and dissolved organic carbon (DOC) including carbohydrates are essential components that drive the carbon cycle in estuarine and coastal ecosystems. The flood-ebb and spring-neap tidal variations in DOC, carbohydrate and nutrient concentrations were investigated in the Changjiang Estuary and the adjacent East China Sea. The DOC, monosaccharide (MCHO), SiO 3 2− , PO 4 3− , NO 3 − , and NO 2 − concentrations fluctuated notably with variations in tidal phases and tidal cycles. The SiO 3 2− , NO 3 − , PO 4 3− , and NO 2 − concentrations were significantly correlated with salinity at flood/ebb tides over a tidal cycle, which indicates that physical mixing might be a major factor affecting tidal variations in nutrient elements. However, there were no significant correlations between DOC, carbohydrate concentrations and salinity. Asymmetrical flood-ebb tidal variations in DOC, carbohydrate and nutrient concentrations were observed, which might result from variations in Changjiang freshwater input due to asymmetrical flood-ebb tidal duration time and velocities in the Changjiang Estuary. There were statistically significant flood-ebb tidal differences in SiO 3 2− , NO 3 − , and NO 2 − concentrations during spring tide in the Changjiang Estuary and neap tide in the East China Sea. DOC and carbohydrate concentrations showed more complex tidal behaviors due to their specific physical and biological characteristics. Principal component analysis indicated that physical mixing driven by tidal forces was an important factor influencing tidal variations in these organic and inorganic components. Other physical and biological processes, such as absorption and microbial degradation, might have also contributed to the tidal variations in these components. Tidal forcing plays a considerable role in controlling temporal variations in organic and inorganic nutrients and should be considered when sampling in estuarine systems and estimate the fluxes of these components. • Asymmetrical flood-ebb tidal variations in nutrients and DOC were observed. • PO 4 3− and NO 2 − showed significant variations between different types of tidal cycles. • DOC showed nonconservative tidal behaviors in the Changjiang Estuary. • Changjiang freshwater input was a main factor affecting tidal variation in nutrients.

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.

Modeling Tidal Characteristics in a Creek-Marsh Drainage System: Implications for Stormwater Management

The traditional goal of stormwater management is to reduce the threat of flooding to life and property, and so most landscapes are engineered to maximize the speed at which the unwanted water leaves the watershed. This has been effective in landscapes with some topographic gradient. This often involves the installation of drainage ditches that disperse runoff from urban areas to receiving water bodies; in coastal areas this means a tidal creek, estuary, bay, sounds, or the coastal ocean. This practice reduces flood hazards in some cases but results in unintended effects on the natural hydrology in the watershed and downstream tidal dynamics. For low-gradient watersheds in humid climates, ditch systems also lower the water table of an area, increasing infiltration to recharge and groundwater discharge to streams (baseflow), and larger volume of freshwater delivered downstream yearround. Ditches also create unintentional avenues for the incoming tide from a tidal creek or tidally-influenced waterway to reach further inland, thus reducing the hydraulic gradient between the inland areas and the receiving water body. The combination of these effects can exacerbate compound flooding events, increasing the flood probability if high tide and storm events coincide. Additionally, coastal communities face the challenge of mitigating more complicated flood hazards while land development increases to meet the needs of a growing population. This study analyzed the tidal influence within an inland drainage ditch in the central coast of South Carolina USA that is representative of thousands of artificially-drained coastal watersheds. The ditch-creek system investigated here is 12 km long in a 753-hectare (1860-acre) watershed of Church Flats Creek, a first-order tidal system. We monitored for 13 months a 0.75-km reach of the lower ditch portion of the system, just above the relatively undisturbed tidal creek and marsh. Prior to ditching in the 1960s this system had a wetland-rich floodplain but is now partially tidal. Field data collected were stream stage (depth), discharge, tidal range, tidal volume, incoming (flood) and outgoing (ebb) tidal durations, and water table hydrograph at a location about 50 m of mid-reach of the ditch. Multiple linear regressions were performed to best predict the flood and ebb tidal durations of the system based on tidal characteristics within the ditch. The mean values were 229 ± 2.5 and 182 ± 2.1 minutes for flood and ebb tide durations, respectively and the models explained 84% (residual standard error (RSE) of 25 minutes) and 80% (RSE of 23 minutes) for the flood and ebb conditions, respectively. The models were simulated for sea levels in 1993 and 2050, and results indicate that the flood tide within the drainage ditch is predicted to increase an average of 66 minutes and the total tidal duration (flood and ebb) an average of 139 minutes by 2050. These results suggest a loss in drainage functionality as sea level rises. Increases in the duration of tidal influence will induce a lower capacity for stormwater volume than the drainage infrastructure was constructed to manage, therefore resulting in an increased frequency of compound flooding events because of the lower storage volume and decreased hydraulic gradient in the system. This study fills a knowledge gap of tidal dynamics within coastal ditch-creek systems and we urge stormwater managers to consider the unintended consequences of using traditional stormwater methods in a region that does not benefit from gravity drainage practices like in other regions.

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.

High-resolution observations of a shear front in a tidal channel

The characteristics of a shear front system in a tidal channel have been investigated through a field study. High-resolution sampling, including three-dimensional measurements of current, temperature, salinity, and turbidity, was performed on repeated transects across the frontal zone both with fronts apparent and with fronts absent. The density measured using conductivity/temperature/depth sensors provides a field proof that the shear front is present whether a density gradient exists. It is distinguishable from conventional buoyant shear fronts, which require a strong along-channel density gradient. The velocities measured using an Acoustic Doppler Current Profiler (ADCP) and trapping Global Position System (GPS) drifters indicate that shear front evolution has a four-stage dynamic process in a tidal cycle: (1) a directional shear front of shoal-flood and channel-ebb during the slack before flood and early flood, (2) a speed shear front of shoal-quick and channel-slow after the early flood, (3) a directional shear front of shoal-ebb and channel-flood during the slack before ebb and early ebb, and (4) a speed shear front of shoal-slow and channel-quick ebb during the late ebb. Thus, the velocity shear, a variation in the current speed and direction on either side of the front, is the generation mechanism of the observed shear front system. It is caused by the unequal tidal durations between deep channels and shallow shoals in the channel discussed herein. The Coriolis effect is found to be negligible through the momentum balance analysis. The unequal tidal durations can be caused by the following alternative mechanisms, according to the analysis and model simulation: (1) differential friction, owing to the steep bathymetry between the shallow shoal and the deep channel, and (2) circulation, owing to the coastline bend of the shallow shoal.

A randomised control study comparing the efficacy of 2 techniques of preoxygenation - tidal volume breathing for one minute and 8 deep breaths in one minute

Introduction: The aim of preoxygenation is to replace nitrogen in the Functional Residual Capacity (FRC) with oxygen. This increases body oxygen store and increases tolerance to apnoea significantly. Materials and Methods: This study was conducted in PESIMSR, Kuppam between July 2012 and November 2013 in a selected population of 60 adults of either sex scheduled posted for elective surgery. The patients were distributed into 2 groups, Group Tidal Volume Breathing, T and Group Deep Breathing, D. Each group had 30 patients. Group T patients were preoxygenated with Tidal Volume Breathing and group D patients were preoxygenated with 8 deep breaths in 1 minute. The measured variables were SpO2, Heart rate, inspired oxygen concentration, Endtidal Oxygen concentration, End tidal Carbon Dioxide and Duration of Apnoea without desaturation (DAWD). Results: There were no statistically significant differences between both the groups in age, gender, height, weight and BMI. All the patients in both the groups attained ETO2 values of above 90 at the end of preoxygenation. The mean ETCO2 value in T group was 32.16 ± 1.51 and 27.36 ± 1.73 in Group D. This value was statistically significant with a p value of Conclusion: Preoxygenation with 8 deep breaths in 1 minute is a more effective method of preoxygenation because there is a significant increase in the time taken for desaturation. This method will be more effective in patients with full stomach posted for emergency surgery. Keywords: Tidal volume breathing, Deep breathing technique, Preoxygenation, Safe duration of apnoea.

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.

Dynamics of sediment transport and stratification in Changjiang River Estuary, China

Suspended sediment transport and stratification in the entire Changjiang River Estuary were examined using the field data of sea-surface level, currents, salinity, and suspended sediment concentration (SSC). The data were simultaneously observed at one-hour intervals and at six vertical layers at 23 stations covering two spring tidal cycles in May, 2004. The results showed that ebbing tides had a longer tidal duration and stronger tidal currents than flooding tides, due to a fresh water discharge. The tidal range reached about 4 m and the peak tidal current of about 2.9 m/s occurred in the north passage in the observed period. A stratification caused by salinity and suspended sediment appeared and varied with tides indicated by the overall Richardson number. The potential energy anomaly was larger when SSC was factored in to the water density. A stratification ratio (Sr), which is the ratio of potential energy required to induce complete mixing to the overall potential energy, was proposed to measure the degree of sediment stratification. Sr values indicated that mixing the stratified turbid water took as much as 0.25% of the overall potential energy at a particular field station. A turbidity maximum zone (TMZ) was observed in the north passage, where the salt-fresh-water interface and stratification appeared, due to the advective and tidal transport component. The main components controlling the vertical-averaged net sediment transport were the Eulerian velocity, Stokes drift and tidal pumping. Salinity intrusion creates the stratification in the north passage, which depresses the turbulence and re-suspension of sediment. The stratification is further enhanced by the vertical distribution of suspended sediment, which results in a TMZ and induces sediment deposition/trapping in the channel. Hence, the SSC was essential to be considered in the study of estuarine stratification, particularly in the highly turbid estuaries.

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.