Tidal Bore Research Articles

Physical Modeling of Unsteady Turbulence in Breaking Tidal Bores

A tidal bore is an unsteady flow motion generated by the rapid water level rise at the river mouth during the early flood tide under macrotidal and appropriate bathymetric conditions. This paper presents a study that physically investigates the turbulent properties of tidal bores. Results from some experimental measurements of free-surface fluctuations and turbulent velocities conducted on smooth and rough beds are reported. The free-surface measurements were conducted with Froude numbers of 1–1.7. Both undular and breaking bores were observed. Using an ensemble-averaging technique, the free-surface fluctuations of breaking tidal bores are characterized. Immediately before the roller, the free-surface curves gradually upwards. The passage of the bore roller is associated with some large water elevation fluctuations; the largest free-surface fluctuations are observed during the first half of the bore roller. The turbulent velocity measurements were performed at several vertical elevations during and shortly after the passage of breaking bores. Both the instantaneous and ensemble-averaged velocity data highlight a strong flow deceleration at all elevations during the bore passage. Close to the bed, the longitudinal velocity component becomes negative immediately after the roller passage, implying the existence of a transient recirculation. The height and duration of the transient are a function of the bed roughness, with a higher and longer recirculation region above the rough bed. The vertical velocity data presented some positive, upward motion beneath the front with increasing maximum vertical velocity with increasing distance from the bed. The transverse velocity data show some large fluctuations with nonzero ensemble average after the roller passage that highlight some intense secondary motion advected behind the bore front.

State-Space Solution of Estuary Bed Responses Subjected to Extreme Hydrodynamic Conditions. I: Tidal Bore

The evaluation of the extreme hydrodynamic forces-induced dynamic response in bed is particularly important for coastal geotechnical engineers involved in the design of the foundations of marine structures at estuarine area. As the two main sorts of extreme hydrodynamic forces in Qiantang estuary, tidal bore and storm surge are considered in this work. This paper presents an analytical solution of the tidal bore induced estuary bed response. State-space method together with Laplace-Fourier transform techniques are used to solve the governing equations. The tidal bore-induced response of estuary bed can be calculated by using the matrix theory, boundary conditions and inverting integral transform. Compared with the special solution in preceding literatures, it’s shown that the proposed method has the advantage of high calculation efficiency and accuracy.

Signal Analysis of Impact Test Due to Tidal Bore Using Empirical Mode Decomposition (EMD)

Tidal bore is a series of waves propagating upstream as the tidal flow turns to rising. The Qiantang Bore has aroused the curiosity of scientists and tourists all over the world in the past centuries as a strong turbulent bore. We consider here tidal bore impact measurements, recorded in the Qiantang River using pressure transducers. Based on the empirical mode decomposition (EMD) algorithm, a signal processing method for impact test due to the Qiantang Bore is proposed. As a self-adaptive approach, EMD method can be used to decompose the local characteristics of the no-stationary and nonlinear signals into several fluctuations and trend items with different time scales step by step. The separation of the quasi-static and dynamic pressures is conducted by using the empirical mode decomposition method.

Mechanisms and challenges for managing disaster in Bangladesh

Bangladesh is a developing country in South Asia which has approximately fourteen core population. It is also known as a disaster prone country and is surrounded by thousands of rivers such as the Ganges, the Brahamaputra, and the Meghna and so on. The Himalayan range to the North and the Bay of Bengal in the South creates a complex weather system. Bangladesh is a common victim of natural calamities due to this unfavorable weather condition. Major natural calamities are tropical cyclones, tidal bores, floods, tornados, river bank erosions, earthquakes etc. A large number of poor people live in the vulnerable areas in the southern part of Bangladesh. The vulnerability is so miserable that they must resettle in the newly accreted land in the Bay of Bengal and its surrounding areas which are occasionally hit by tidal bores or devastating cyclones. The adverse impacts of these natural hazards affecting the socio-economic condition need to be reduced for sustainable development. Realization of this reality, the Government of Bangladesh (GoB) has undertaken a lot of plans and programs for disaster reduction through disaster management. Key words: Disaster, mitigation measures, community, disaster management mechanisms.

Momentum Considerations in Hydraulic Jumps and Bores

A hydraulic jump is the turbulent transition from a high velocity into a slower flow. A related process is the hydraulic jump in translation. The application of the equations of conservation of mass and momentum in their integral form yields a series of relationships between the flow properties in front of and behind the jump. The effects of the cross-sectional shape and bed friction are investigated. The effect of the flow resistance yields a smaller ratio of conjugate cross-section areas for a given Froude number. The solutions are tested with some field measurements of tidal bores in natural channels, illustrating the range of cross-sectional properties in natural systems and irregular channels.

Linking long-term, large-scale climatic and environmental variability to patterns of marine invertebrate recruitment: Toward explaining “unexplained” variation

The rate of input of new individuals into communities is a key component of community dynamics. Detection of ecosystem responsiveness to climatic perturbations such as El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) can provide insight into how ecosystems will respond to climate change. Although these climate patterns are ocean-based, understanding of their influences on key ecosystem processes in temperate coastal marine ecosystems is limited. Recent analyses documented orders-of-magnitude increases in mussel recruitment in the 2000s relative to the previous decade. To evaluate the generality of this pattern across other intertidal species with planktotrophic dispersive larvae, and determine the extent to which variation in recruitment was explained by climatic and environmental (upwelling) variation, we analyzed patterns of barnacle and mussel recruitment across space (at up to 10 sites across 250 km) and through time (up to 20-years). Compared to mussel recruitment, barnacle recruitment varied far less interdecadally, and showed minimal change in seasonality. Using three analytical approaches, multiple regression, quantile regression and wavelet analysis, we found that recruitment of the barnacles Balanus glandula and Chthamalus dalli and mussels Mytilus spp. varied in generally similar ways with climate variation as expressed in ENSO, NPGO, and PDO indices, and with upwelling, as expressed in the Bakun upwelling index. In regression approaches, NPGO and upwelling had the strongest associations with recruitment of all species, with MEI and PDO also having an influence on C. dalli. The strength of the association with the environment varied with recruitment magnitude, however, being generally stronger at intermediate and in some cases high recruitment densities and weak at low recruitment densities. Time series (wavelet) analysis showed that with all environmental indices, the most consistent strong relationships occurred at annual periodicities, but multi-year periodicities were also important with PDO and upwelling indices. Patterns were complex, however, with periods of strong and weak associations at different time periodicities varying differently among the different environmental measures, and in some cases varying from strongly positive to strongly negative associations at the same periodicities through time. These relatively consistent relationships between large-scale climate and upwelling, and recruitment explained a substantial fraction of the variance (37–40%). The ~ 60% of the variance in recruitment not explained in our analyses is likely due to myriad other factors such as shorter-scale variation in upwelling, internal tidal bores, sea breezes, wave forces, larval behavior, and post-settlement processes. Thus, our analysis adds insights into potential influences on recruitment that occur at ocean basin- to regional scales, and complements the body of work that focuses on the more local-scale and shorter-term mechanistic influences that also help determine the pace at which larvae recruit to the adult habitat.

A Numerical Simulation of Tidal Bores of MPS with the Pressure Gradient Term Calculated by a Precise Gradient Operator

This paper shows the accuracy comparison of wave pressure of tidal bores with the improved MPS and original MPS. The authors simulate two basic tests for verification of accuracy. In the test of progressive wave, the authors show that the attenuation of progressive wave is reduced by the improved MPS. In that of wave shoaling, the atiltness of wave of improved MPS is different from the original MPS. In investigation of wave pressure of tidal bores, the authors show the relationship between the pressure of tidal bore and the bore height. The present study suggests that the improved MPS is useful to simulate wave pressure of tidal bores.

Numerical Modeling of Suspended Sediment Transport Affected by Tidal Bore in Qiantang Estuary

PAN, C. and HUANG, W., 2010. Numerical modeling of suspended sediment transport in Qiantang River: an estuary affected by tidal bore. Journal of Coastal Research, 26(6), 1123–1132. West Palm Beach (Florida), ISSN 0749-0208. Study of suspended sediment transport in an estuary affected by tidal bore is important for coastal engineering and management because the tidal bore can cause strong sediment resuspension and scour in shallow waters. Numerical modeling of suspended sediment transport in a natural estuary affected by tidal bore is such a challenging research topic that there are few articles on the subject available in the literature. In this study, a two-dimensional, numerical model was developed to investigate suspended sediment transport induced by a tidal bore. The hydrodynamic component of the model employs the Godunov-type scheme with second-order accuracy in space, which effectively describes the rapid supercritical flow and sharp horizontal pressure gradients of the tidal bore. To preserve balance between the source terms and the internal forces, both the water level–bottom topography formulation (WLTF) method and a special technique for triangular mesh have been applied to solve the source term in the model equations to account for the irregular bottom topography. The wet/dry boundary issue was solved by using the improved exact-Riemann solver on the dry bed. The coupled sediment transport model incorporates more reliable equations from recent publications to characterize the rapid increase of sediment resuspension in the water column. The model test against an analytical solution of convection transport shows that the sharp gradient of scalar transport is satisfactorily estimated in the model simulations. The model in the application has been validated to simulate hydrodynamics and suspended sediment transport affected by a tidal bore in the Qiantang River of China. The results compare well with a time series of observations to characterize the rapid increases of surface elevation, currents, and suspended sediment concentration resulting from the tidal bore. Results of spatial distributions of water levels and currents indicate that the model adequately describes the sharp horizontal gradients of the surface elevation and the tidal currents during the passage of the tidal bore and characterizes the suspended concentrations in the estuary.

Turbulent mixing of particles under tidal bores: an experimental analysis

A tidal bore develops in an estuary when the tidal range exceeds 4.5–6 m and the estuarine bathymetry amplifies the tidal wave. The bore is an abrupt rise in water depth associated with a discontinuity in velocity and pressure fields at the front. Herein the free-surface properties and the turbulent mixing of light-weight particles were investigated during the passage of tidal bores. The free-surface properties were recorded using a non-intrusive technique, while particle tracking was performed under undular and breaking bores. A basic result was the identification of a broad spectrum of particle trajectories, linked with the existence of large-scale vortical structures. These turbulent structures were responsible for the vertical water mixing as a tidal bore propagates upstream in an estuary. The large-scale eddies were also responsible for the rapid longitudinal dispersion of particulates, such as fish eggs, with some form of preferential motion, depending upon the particle's vertical elevation.

Lagrangian modeling of tidal bores passing through bridge piers

The process of tidal bore passing through bridge piers is numerically simulated in an open channel using a pure Lagrangian method, i.e. Smooth Particle Hydrodynamics (SPH). Due to its “gridless” feature, SPH is intrinsically well-suited for the complex hydrodynamic simulation when the fluid encounters a structure. The behavior of bores propagation along the channel is illustrated herein and includes the bore's free surface properties and flow field, and in particular, the intricate behavior of unsteady flow during the passage of tidal bore. The generation of shock waves between the structures can be clearly observed in the numerical experiment. Intense vortical structures behind the bore front are generated as the bore propagate through the constriction.

INTERNAL WAVES DYNAMICS IN THE LOMBOK STRAIT STUDIED BY A NUMERICAL MODEL

A baroclinic 3D hydrodynamic model with the non-hydrostatic approximation called Massachusetts Institute of Technology Global Circulation Model (MIT gcm) has been applied to simulate the generation of internal tidal bores and their disintegration into internal solitary waves in the Strait of Lombok. Numerical simulation have been carried out by incorporating seasonal variations of the stratification of the water body, which exist during the first transitional monsoon, the east monsoon, the second transitional monsoon, and the west monsoon. Our simulation yields the results that the existence of the sill at the southern part of the Lombok Strait, strong tidal flow, and a stratified fluid, play an important role in forming some short of divergence and convergence area as an indication of the birth of internal waves, which are simulated on the both sides of the sill. The simulated results reproduce reasonably well the basic features of internal waves in the Strait of Lombok as captured by the Synthetic Aperture Radar (SAR) from the European Remote Sensing (ERS) satellites ERS 1 and ERS 2, such as a north-south asymmetry, propagation speeds, average amplitudes and wavelengths, and solution packets. Similiar to observations made by National Oceanic and Atmospheric Administration (NOAA) satellites, the simulation results also showed the intrusion of warmwater from thePacific Ocean into the Indian Ocean and the exitence of well-developmed thermal plume at south of the sill. Seasonal variations of interface depth of thermocline and the density difference between the stratified layers influence magnitudes of the amplitudes and wavelengths of the internal waves and solitons, and the distance of thermal plume in the Lombok Strait. It is found that during the monsoon transition periods and the west monson, the amplitudes of internal waves and solitons at the southern part of the strait is apparently larger than those at the northern one, whereas during the east monsoon, the wave amplitudes is large north of the sill than south of it. Meanwhile, the propagation speeds of northward propagating internal solitary waves (0.71-2.67m per s) are stronger than southward propagating ones (0.21-1.53 m per s) throughout the monsoon periods. Key words: internal waves, non-hydrostatic approximation, solitary waves, thermal plumes

Tide‐influenced fluvial bedforms and tidal bore deposits (Late Jurassic Lourinhã Formation, Lusitanian Basin, Western Portugal)

AbstractIn a sedimentological sense a fluvial to tidal transition zone can be defined in rivers as a zone that separates the upstream fluvial from the downstream estuarine zone. Characteristic sedimentary structures within this zone are notoriously difficult to recognize. This study demonstrates the influence of tidal modulation within the most proximal part of the fluvial‐tidal transition zone (i.e. the ‘backwater zone’) of an ancient fluvial system. Criteria have been established to differentiate between purely fluvial facies and those modulated by tidal energy. The stratigraphic interval from which the data were derived is the Lourinhã Formation (Late Jurassic) of the Lusitanian Basin, Western Portugal. An analysis of sedimentary features at four key localities has identified a temporal spectrum of tidal influence ranging from the daily modulation of fluvial flows to the effects of tidal bore passage. A combination of quantitative and qualitative data reveals systematic changes in the internal architecture of dune‐scale bedforms deposited in a channel‐floor setting. The key co‐occurring features are: (i) increasing‐decreasing organic particle concentration; (ii) increasing‐decreasing bottomset thickness; (iii) increasing‐decreasing foreset dip and shape (from convex to concave); and (iv) increasing‐decreasing brinkpoint height. Collectively, these features are interpreted as having been produced by successive fluctuations in flow regime conditions from lower (during flood tidal retardation) to higher (during ebb tidal drawdown) current velocities. Bedforms showing these features occur in both meandering fluvial channels and straighter distributary systems. In addition, several examples of a specific type of stepped erosion surface and draping sediment have been recognized, the interpretation of which strongly suggests generation by the passage of tidal bores. If this interpretation is correct, then it represents one of the first published examples of tidal bore propagation in ancient fluvial systems. Palaeoclimatic evidence (cellular analysis of woody tissue, palaeosol character and plate reconstruction) indicates a warm, seasonal, winter wet to summer dry climate during deposition of the Lourinhã Formation. From this evidence it is suggested that tidal modulation and tidal bore effects are more likely to develop in the ‘dry season’, when fluvial flow in the main river channels was reduced.

Undular tidal bores: effect of channel constriction and bridge piers

A tidal bore may occur in a macro-tidal estuary when the tidal range exceeds 4.5–6 m and the estuary bathymetry amplifies the tidal wave. Its upstream propagation induces a strong mixing of the estuarine waters. The propagation of undular tidal bores was investigated herein to study the effect of bridge piers on the bore propagation and characteristics. Both the tidal bore profile and the turbulence generated by the bore were recorded. The free-surface undulation profiles exhibited a quasi-periodic shape, and the potential energy of the undulations represented up to 30% of the potential energy of the tidal bore. The presence of the channel constriction had a major impact on the free-surface properties. The undular tidal bore lost nearly one third of its potential energy per surface area as it propagated through the channel constriction. The detailed instantaneous velocity measurements showed a marked effect of the tidal bore passage suggesting the upstream advection of energetic events and vorticity “clouds” behind the bore front in both channel configurations: prismatic and with constriction. The turbulence patches were linked to some secondary motions and the proposed mechanisms were consistent with some field observations in the Daly River tidal bore. The findings emphasise the strong mixing induced by the tidal bore processes, and the impact of bridge structures on the phenomenon.

Undular Tidal Bores: Basic Theory and Free-Surface Characteristics

The present study examines the free-surface properties of undular tidal bores observed for 1<F<1.5–1.8. The analysis is based on some new experimental results together with a reanalysis of both field and laboratory data. The free-surface profiles followed a pattern somehow similar to the sinusoidal and cnoidal wave shapes, although neither captures the fine details of the undulation profiles. The wave properties highlighted some maxima in amplitude and steepness for a Froude number of about 1.3–1.4 corresponding to the apparition of some wave breaking at the first wave crest.

Unsteady Turbulence in Tidal Bores: Effects of Bed Roughness

A tidal bore is a wave propagating upstream as the tidal flow turns to rising. It forms during spring tide conditions when the flood tide is confined to a narrow funneled channel. To date, theoretical and numerical studies rely upon physical experiments to validate the developments, but the experimental data are limited mostly to visual observations and sometimes free-surface measurements. Herein turbulent velocity measurements were obtained in a large-size laboratory facility with a fine spatial and temporal resolution. The instantaneous velocity measurements showed rapid flow deceleration at all vertical elevations, and large fluctuations of all velocity components were recorded beneath the bore and secondary waves. A comparison between undular (nonbreaking) and breaking bores suggested some basic differences. In an undular bore, large velocity fluctuations were recorded beneath the first wave crest and the secondary waves showing a long-lasting effect after the bore passage. In a breaking bore, some large turbulent stresses were observed next to the shear zone in a region of high velocity gradients, while some transient flow recirculation was recorded next to the bed. The effects of bed roughness were tested further. The boundary friction contributed to some wave attenuation and dispersion, and the free-surface data showed some agreement with the wave dispersion theory for intermediate gravity waves. The instantaneous velocity data showed however a significant effect of the boundary roughness on the velocity field next to the boundary (z/do<0.2) for both undular and breaking bores. Overall the findings were consistent with field observations of tidal bores and highlighted the significant impact of undular (nonbreaking) bores on natural systems.

Orientated plant fragments revealing tidal palaeocurrents in the Fumanya mudflat (Maastrichtian, southern Pyrenees): Insights in palaeogeographic reconstructions

Leafy axis fragments of Frenelopsis sp., leaves of Sabalites longirhachis (Unger) Kvaček et Herman and logs are common in the Maastrichtian of the basal Tremp Formation in the Vallcebre Syncline (southern Pyrenees). The largest plant remains, i.e. leaves of S. longirhachis and logs, appear scattered and randomly orientated, whereas the smallest plant remains, i.e. Frenelopsis sp., are aligned in a clear east–west trend. This orientation (based on 117 measurements) is consistent with the basin palaeogeography, which was an E–W elongated foreland trough that confined tidal palaeoflow and deposits such as tidal bores. In the absence of usual sedimentary indicators, e.g. tractive structures and base marks, the accurate study of fossil plant orientations can largely assist palaeoflow determinations in palaeogeographic reconstruction.

Large Eddy Simulation of turbulence generated by a weak breaking tidal bore

A tidal bore is a natural and fragile phenomenon, which is of great importance for the ecology of an estuary. The bore development is closely linked with the tidal range and the river mouth shape, and its existence is sensitive to any small change in boundary conditions. Despite their ecological and cultural value, little is known on the flow field, turbulent mixing and sediment motion beneath tidal bores. Indeed, some striking features can be highlighted in two-dimensional simulations, such as large velocity fluctuations and flow recirculation structures. Using Large Eddy Simulation method, the numerical results emphasised the complicated turbulent structures and their unsteadiness under a tidal bore.

Current knowledge in tidal bores and their environmental, ecological and cultural impacts

A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising. It forms during the spring tide conditions when the tidal range exceeds 5–6 m and the flood tide is confined to a narrow funnelled estuary with low freshwater levels. A tidal bore is associated with a massive mixing of the estuarine waters that stirs the organic matter and creates some rich fishing grounds. Its occurrence is essential to many ecological processes and the survival of unique eco-systems. The tidal bore is also an integral part of the cultural heritage in many regions: the Qiantang River bore in China, the Severn River bore in UK, the Dordogne River in France. In this contribution, the environmental, ecological and cultural impacts of tidal bores are reviewed, explained and discussed.

Numerical simulation of a weak breaking tidal bore

A tidal bore is a natural and fragile phenomenon, which is of great importance for the ecology of an estuary. The bore development is closely linked with the tidal range and the river mouth shape, and its existence is sensitive to any small change in boundary conditions. Despite their ecological and cultural value, little is known on the flow field, turbulent mixing and sediment motion beneath tidal bores. Indeed, some striking features can be highlighted in two-dimensional simulations, such as large velocity fluctuations and flow recirculation structures. Using Large Eddy Simulation method, we present numerical results that show the complicated turbulent structures and their unsteadiness under a tidal bore.

Internal solitary waves induced by flow over a ridge: With applications to the northern South China Sea

The generation of internal solitary waves by barotropic tides over a ridge is studied in a nonhydrostatic numerical model under idealized oceanographic settings. The experiments examine the effects of ridge width, barotropic tidal strength, and stratification on wave generation. The barotropic tidal flow produces internal wave beams emitting from the ridge top if the slope of the ridge exceeds a critical value equal to the slope of the wave beam. Reflection and refraction of a wave beam in an upper ocean waveguide associated with a strong shallow thermocline produce horizontally propagating internal tides. When the local Froude number over a ridge is not small, lee waves generated on the ridge convert enough energy from the barotropic tides to the internal tides to form tidal bores and solitary waves. Increasing stratification at ridge depths enhances the generation of internal waves, particularly at the diurnal periods. In the Luzon Strait, the slope of the wave beam decreases in spring and summer as stratification at the ridge depths increases, favoring the generation of internal tides. Without the presence of a strong shallow thermocline, internal solitary waves are not observed east of the Luzon Strait. In the northern South China Sea, internal solitary waves are likely observed from April to July when a strong shallow thermocline is present. A deep mixed layer in winter suppresses the production of internal solitary waves.