High Tidal Range Research Articles

Mixing, hypersalinity and gradients in Hervey Bay, Australia

Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia, is a shallow tidal area (average depth = 15 m), close to the continental shelf. It shows features of an inverse estuary, due to the high evaporation rate (approx. 2 m/year), low precipitation (less than 1 m/year) and on average almost no freshwater input from rivers that drain into the bay. The hydro- and thermodynamical structures of Hervey Bay and their variability are presented here for the first time, using a combination of four-dimensional modelling and observations from field studies. The numerical studies are performed with the Coupled Hydrodynamical Ecological Model for Regional Shelf Seas (COHERENS). Due to the high tidal range (>3.5 m), the bay is considered as a vertically well-mixed system, and therefore, only horizontal fronts are likely. Recent field measurements, but also the numerical simulations, indicate characteristic features of an inverse/hypersaline estuary with low salinity (35.5 psu) in the open ocean and peak values (>39.0 psu) in the head water of the bay. The model further predicts a nearly persistent mean salinity gradient of 0.5 psu across the bay (with higher salinities close to the shore). The investigation further shows that air temperature, wind direction and tidal regime are mainly responsible for the stability of the inverse circulation and the strength of the salinity gradient across the bay. Due to an ongoing drying trend, the occurrence of severe droughts at the central east coast of Australia and, therefore, a reduction in freshwater supply, the salinity flux out of the bay has increased, and the inverse circulation has also strengthened.

Effects of intratidal and tidal range variability on circulation and salinity structure in the Cape Fear River Estuary, North Carolina

Tidal influences on circulation and the salinity structure are investigated in the largely unstudied Cape Fear River Estuary (CFRE), North Carolina, a partially mixed estuary along the southeast coast of the United States. During two different tidal conditions (high versus low tidal range) and when river flow was low, salinity and velocity data were collected over a semidiurnal tidal cycle in a 2.8 km long transect along the estuary axis. Water level data were recorded nearby. Mechanisms that influence salt transport characteristics are diagnosed from an analysis of the field data. Specifically, we investigated the relationship between tidal range and salinity through comparison of along‐channel circulation characteristics, computed salt fluxes, and coefficients of vertical eddy diffusivity (Kz) based on a parameterization and on salt budget analysis. Findings indicate up‐estuary tidally driven salt fluxes resulting from oscillatory salt transport are dominant near the pycnocline, while mean advective transport dominates near the bottom during both tidal range periods. Earlier research related to salt transport in estuaries with significant gravitational circulation suggests that up‐estuary salt transport increases during low tidal ranges as a result of increased gravitational circulation. In the CFRE, in contrast, net (tidally averaged) near‐bottom along‐channel velocities are greater during higher tidal range conditions than during lower tidal range conditions. Findings indicate stronger tidal forcing and associated mixing contribute to greater near‐bottom salinity gradients and, consequently, increased baroclinic circulation. Lower near‐bottom salinities during the higher tidal range period are a result of a combination of increased vertical turbulent salt fluxes near the pycnocline and increased bottom‐generated mixing.

Numerical modeling of tide-surge interaction along Orissa coast of India

The Orissa coast of India is one of the most vulnerable regions of extreme sea levels associated with severe tropical cyclones. There was extensive loss of life and property due to the October 1999 super cyclone, which devastated large part of the Orissa coast. The shallow nature of the head bay, presence of a large number of deltas formed by major rivers of Orissa such as Mahanadi and Dhamra, and high tidal range are responsible for storm surge flooding in the region. Specifically, rising and falling tidal phases influence the height, duration, and arrival time of peak surge along the coast. The objective of the present study is to evaluate the tide-surge interaction during the 1999 Orissa cyclone by using nonlinear vertically integrated numerical models. The pure tidal solution for the head bay region of the Bay of Bengal provides the initial condition for the fine resolution nested grid Orissa model. However, the feedback from the Orissa model does not affect the head bay model as the study provides a one-way interaction. Numerical experiments are performed to study the tide-surge interaction by considering various relative phases of the tidal waves with the surge-wave produced by 1999 Orissa cyclone. The comparison, although utilizing only the limited estimates of tidal data, appears adequate to assert that the principal features are reproduced correctly.

Holocene sea-level change in the Severn Estuary, southwest England: a diatom-based sea-level transfer function for macrotidal settings

The recent growth in the use of microfossil-based transfer functions in late-Quaternary sea-level reconstructions reflects their potential to accurately quantify palaeo sea-level changes. This study details the development of a diatom-based sea-level transfer function for the Severn Estuary, southwest England, a macrotidal setting that experiences the second highest tidal range in the world. This setting presents difficulties in representing the full tidal range from mean sea level (MSL) to highest astronomical tide (HAT). However, two separate transects were merged successfully and a statistically significant relationship between contemporary diatom assemblages and altitude (m O.D.) was established. A diatom-based transfer function for palaeoaltitude was developed using weighted averaging (WA), tolerance downweighted weighted averaging (WA-Tol) and weighted averaging partial least squares (WA-PLS). WA-Tol produced the lowest prediction errors for altitude and the transfer function was applied to a fossil diatom data set from Gordano Valley, a site adjacent to the Severn Estuary.

An unusual fluvial to tidal transition in the mesoarchean Moodies Group, South Africa: A response to high tidal range and active tectonics

The Moodies Group in the Dycedale Syncline, Barberton Greenstone Belt consists of a 100–130 m-thick upward-fining succession that exhibits a transition from fluvial to tide-modified sedimentation. A basal, 10–30 m-thick conglomerate–sandstone interval of alluvial origin is overlain by stacked upward-fining, decimeter- to meter-scale cycles within which three facies are recognized: 1) conglomerate lag; 2) cross-bedded sandstone; and 3) interlaminated sandstone–siltstone and mudstone. Within the cycles, the abundance of mudstone drapes increases upwards. Structureless conglomerates and cross-beds lacking mudstone drapes record braided-alluvial processes. In contrast, cross-beds with mudstone drapes and interlaminated sandstone–siltstone and mudstone are products of flows modified by various tidal beats. Sand and/or silt transport took place during the ebb and flood stages and mudstone accumulated during slack water phases. Alternating thick–thin laminations reflect dominant and subordinate, twice-daily tides. Thicker groupings of foresets and thicker intervals of vertically stacked sandstone–siltstone and mudstone laminations are interpreted as spring tide deposits whereas thinner groupings of foresets and thinner laminations record neap tides. Desiccated mudstone drapes on foresets indicate that bedforms rarely were locally exposed during some portion of the tidal cycle. Abundant exposure structures in the interlaminated sandstone–siltstone and mudstone facies indicate that the cycles are upward shoaling. The stacked upward-fining cycles are attributed to alternating subaerial exposure and fluvial influx followed by marine inundation, probably related to absolute sea level fluctuations. Lack of high-order vegetation on the Archean landscape promoted rapid lateral migration or avulsion of tidally influenced fluvial channels. The association of facies within the 100–130 m-thick upward-fining succession is comparable to Holocene and ancient paleovalley fills characterized by basal alluvial gradational upwards into estuarine facies. However, in the absence of vegetation, the land–ocean interface in the Archean probably consisted of laterally extensive fan or braid deltas rather than point sources of sediment characteristic of most modern rivers. The abrupt up-section change from syntectonic, high-energy, alluvial–fluvial flash flood deposits to tide-influenced sedimentation implies a proximal source that provided sediment to a shoreline influenced by strong tidal action. Possible Holocene analogues are orogenic settings such as the Canterbury Plains of New Zealand, the Indo-Gangetic Plains of India and strike-slip settings such as the Gulf of Aqaba but all three examples lack a direct transition to tidally influenced sedimentation.

Palaeoenvironmental evolution and sea‐level fluctuations along the southeastern Pampa grasslands coast of Argentina during the Holocene

AbstractHolocene pollen and diatom analyses and complementary data from δ18O and δ13C, malacology and sedimentology have provided a detailed record of vegetation history and palaeoenvironmental change at arroyo Las Brusquitas, on the southeastern coast of the pampas of Argentina especially in relation to past sea levels. Holocene palaeosalinity trends were estimated by Detrended Correspondence Analysis and by salinity indexes based on pollen and diatom data. As a consequence of sea‐level rise from the postglacial an extensive wave‐cut platform formed over which Holocene infilling sequences were deposited unconformably. In these sequences, variation in pollen and diatom assemblages occurred in agreement with changes in mollusc diversity and abundance, isotope values, and sediment deposits. Between ca. 6700 and 6190 14C yr BP (6279–6998 cal. yr BP) saline conditions predominated in an environment highly influenced by tides and salt water during the Holocene sea‐level highstand. Between ca. 6200 and 3900 14C yr BP (4235–4608 cal. yr BP) shallow brackish water bodies formed surrounded by saltmarsh vegetation that became more widespread from 5180 14C yr BP (5830–6173 cal. yr BP) to 3900 14C yr BP in relation to a sea‐level stabilisation period within the regression phase. Less saline conditions marked by frequent variations in salinity predominated between ca. 3900 and 2040 yr 14C BP (1830–2160 cal. yr BP). The intertidal saltmarsh environment changed into a brackish marsh dominated by freshwater conditions and sporadic tidal influence. Halophytic vegetation increased towards ca. 200014C yr BP indicating that saline conditions may be due to either desiccation or an unusually high tide range with rare frequency. After ca. 2000 14C yr BP the sedimentary sequences were buried by aeolian sand dunes. Changes in Holocene vegetation and environments in Las Brusquitas area are in agreement with data obtained from various southeastern coastal sites of the Pampa grasslands. Copyright © 2005 John Wiley & Sons, Ltd.

Submarine groundwater discharge (SGD) into the Yellow Sea revealed by 228Ra and 226Ra isotopes: Implications for global silicate fluxes

We estimated the magnitude of submarine groundwater discharge (SGD) into the Yellow Sea, which is one of the largest continental shelves in the world, using 226Ra and 228Ra isotopes. On the basis of 228Ra and 226Ra mass-balance models, we estimated the advective flux of 226Ra through SGD to be ∼270×1012 dpm yr−1 in the Yellow Sea. Using this 226Ra flux and the measured 226Ra activity in coastal groundwater, the submarine discharge of groundwater (mostly brackish groundwater) was calculated to be at least 40% of the river-water input (∼2.3×1011 m3 yr−1). Then, on the basis of the SGD and the concentration of Si in coastal groundwater, we estimated the flux of Si through SGD to be 20–100% of that associated with river discharge (∼23×109 mol yr−1). This large SGD is likely to be due to the high tidal range (up to 10 m) and the wide distribution of sandy sediments in the Yellow Sea, favorable for the recirculation of seawater through bottom sediments and rocks. This result from such a large area implies that the Si flux through SGD may be significant on a global scale.

Resource availability and meiofauna in sediment of tropical seagrass beds: Local versus global trends

Characterisation of productivity-diversity relationships forms an essential step towards a better understanding of biodiversity. In terrestrial systems this is a topical subject and most studies reported a hump-shaped relationship. For marine systems, however, the number of studies dedicated to this is low despite the high interest in this productivity-diversity relationship. The present study reports on meiofauna density/diversity patterns in relation to resource availability as an indicator for the productivity of the ecosystem. Standardised meiofauna samples were collected in tropical seagrass beds from three localities (Kenya, Mexico, the Philippines) in order to contrast local patterns with a more global scale. Although these sites were physically comparable, a range of resource availabilities was found. These differences between localities were mainly due to different tidal regimes and related input of organic matter. At all sites a significant positive effect of resource increase on meiofauna densities was found. This positive effect was less clear for meiofauna diversity. Highest density and diversity levels were reported for the Kenyan site and this is probably linked to a high tidal range. Pooling all localities together resulted in a significant positive linear relationship between resource availability and meiofauna density/diversity. Caution should be taken when choosing resource indicators. Chlorophyll a concentrations, for example, resulted in a positive density-productivity relationship while organic carbon content, an indicator for more refractory material, showed a negative relationship. In all cases, no hump-shaped relationship could be found suggesting that each ecosystem and each group of organisms may show a particular productivity-diversity/density relationship.

Modelling the effect of waves, weathering and beach development on shore platform development

AbstractA mathematical model was used to study shore platform development. Mechanical wave erosion was dependent on such variables as tidal range, wave height and period, breaker height and depth, breaker type, surf zone width and bottom roughness, submarine gradient, rock resistance and the elevational frequency of wave action within the intertidal zone. Also included were the effects of sand and pebble accumulation, cliff height and debris mobility, and downwearing associated with tidal wetting and drying. The occurrence, location and thickness of beaches often depended on initially quite minor variations in platform morphology, but owing to their abrasive or protective effect on underlying rock surfaces, they were able to produce marked differences in platform morphology. Generalizations are difficult, but the model suggests that platform gradient increases with tidal range. Platform width also increases with tidal range with slow downwearing but it decreases with fast downwearing. Platform gradient decreases and width increases with wave energy, and decreasing rock resistance and platform roughness. With low tidal range, platform gradient is generally lower and platform width greater with beaches of fine sand than with gravel, but the relationship is more variable with a high tidal range. Platform width increases and platform gradient decreases with the rate of downwearing on bare surfaces, particularly in low tidal range environments, but the pattern is less clear on beach‐covered platforms. Platforms with large amounts of beach sediment tend to be narrower and steeper than bare platform surfaces. Platform gradient increases and platform width decreases with increasing cliff height and with decreasing cliff debris mobility. Copyright © 2005 John Wiley & Sons, Ltd.

Impacts of storms and evolution of the coastline in western France

The morphological impact of storms on coastal accumulations varies considerably in space along the coast of western France. At some locations, the storm produces erosion, whereas at others, impacts may vary from erosion to accumulation, or no effect. This is explained by the variability of incident waves. Wave amplitude variability, cross-checked with wave direction variability, is presented on a phase diagram and shows that the response of the coast is not linked to the absolute values reached during the storm, but to changes in wave patterns that take place during the event. The morphology of the bottom (skerries, banks) is greatly forcing these changes because of the high tidal range and the complicated and fast-changing patterns of refraction. Locations with a flat seafloor have an almost predictable response to storms. Places with a complex submarine morphology have a response characterized by high variability. The variability of the wave pattern during a half tidal cycle storm (from low tide at 1 m to high tide at 13 m in 6 h) is a microscale equivalent of the variability of wave patterns changing during sea level rise in the Holocene (from −7 to 0 m in 6 ka). This fact explains why, during the late Holocene, the same locations (1) have displayed a very chaotic response to sea level rise and (2) are displaying, today, a highly variable response to storms. In that respect, seafloor morphology is a mesoscale to macroscale control on beach/barrier behaviour.

Numerical Modelling of Storm Surge in the Head Bay of Bengal Using Location Specific Model

The head Bay of Bengal region, which covers part of Orissa and west Bengal in India as well as Bangladesh, is one of the most vulnerable regions of extreme sea levels associated with severe tropical cyclones which cause extensive damage. There has been extensive loss of life and property due to extreme events in this region. Shallow nature of the Bay, presence of Ganga-Brahmaputra-Meghna deltaic system and high tidal range are responsible for storm surges in this region. In view of this a location specific fine resolution numerical modelis developed for the simulation of storm surges. To represent mostof the islands and rivers in this region a 3km grid resolution is adopted. Several numerical experiments are carried out to compute the storm surges using the wind stress forcings representative of 1974, 1985, 1988, 1989, 1991, 1994 and 1999 cyclones, which crossed this region. The model computed surges are in good agreement with the available observations/estimates.

Changes in saltmarsh surface elevation due to variability in evapotranspiration and tidal flooding

We examine the potential for diurnal variation in elevation of saltmarsh surfaces as a source of error in long-term experiments; errors particularly critical in high precision studies that employ the surface elevation table (SET) as a means to monitor elevations. The field study was carried out along the New Brunswick coast of the Bay of Fundy in high and low zones at three marshes with different tidal ranges. We used a total of 16 benchmark pipes and controlled for daily variability in evapotranspiration (ET), as well as timing of tidal flooding, two factors that affect soil water storage, and consequently soil volumes. In six of nine trials we detected significant elevation change over periods as short as 5 d. Marsh-wide averages ranged from 1.2 to 3.0 mm, greater than the yearly increase in relative sea level in many regions. Wood Point marsh had the highest tidal range, but lowest soil organic matter content, giving its soils the lowest compressibility and little sensitivity to ET during two of three trials; the average change in elevation in Wood Point high marsh stations was 4.0 mm during the last trial. Greater differences later in the growing season (while temperature changes were minor) at Wood Point and another site suggest that plant transpiration drove changes in water storage at those sites. Significantly greater differences in elevation with lower plant cover in the third marsh suggests that evaporation drove changes in water storage there. Surface elevation change due to ET should be of greatest concern to SET users in temperate regions where there are large changes in plant biomass and variable temperatures. Variation due to plant transpiration could be reduced if yearly monitoring is scheduled before the start of the growing season.

Offshore Tidal Power Generation—A New Approach to Power Conversion of the Oceans' Tides

Offshore tidal power generation (“OTP”) is a new approach to tidal power conversion that resolves the environmental and economic problems of the familiar “tidal barrage” technology. OTP uses a rubble mound impoundment structure and low-head hydroelectric generating equipment situated a mile or more offshore in a high tidal range area. Shallow tidal flats provide the most economical sites. Multi-cell impoundment structures provide higher load factors (about 62%) and have the flexibility to shape the output curve in order to dispatch power in response to demand price signals. The tides are highly predictable and permit tidal power to fit comfortably into existing electricity distribution grids. The author describes a computer simulation program that uses equipment performance characteristics and tidal data to create a detailed simulation of generation output, water flows and storage, and is used for design optimization. The article also contains a history of tidal power and discussion of tidal power's place in the renewables market.

The Validity of Allowing for Tidal Fluctuations in Offshore Wind Resource Predictions

Tidal action results in a rise and fall of the sea surface that effectively changes the position of an offshore wind turbine hub in relation to the wind shear profile. This effect is quantified using measured data from three offshore UK sites. Statistical evidence of the influence of tide on mean wind speed and turbulence is presented. The possibilities for improving correlation between onshore and offshore wind data are explored. It is found that improvements in correlation are negligible, even for high tidal range sites.

Peculiar Appearance of Nearly Symmetrical Tidal Flooding Time Along the Western Coast of Taiwan

ABSTRACTDuring the past several decades, nearly symmetrical tidal appearances, both the time of high water elevation and the tidal range, were noted along the western coast of Taiwan. Research corresponding to anomalous amplifications of tidal range have been well studied by Lin, et al. [1,2], but the particular symmetrical tidal appearance of high water time has not yet been clarified. In this paper, we firstly apply a harmonic analysis to tidal data compiling. Based on the results, the important symmetric appearances of high water time and tidal range can be located. Emphasis is given on the investigation of symmetrical distribution of high water time by superposing two progressive waves that interfere on a topographic shelf with specific width. A conceptual configuration of co-oscillating standing tides is proposed to demonstrate the symmetrical tidal range distributing in the space domain and oscillating in the time domain.

Hindcasting Coastal Sea Levels in Morecambe Bay

A method of variational data assimilation (by means of an adjoint model) is used to improve the boundary conditions for a high resolution depth-averaged two dimensional non-linear hydrodynamic model of Morecambe Bay. The modelled region contains a large inter-tidal area and has an unusually high tidal range. The assimilation procedure is robust and effective in reducing model error. This system performs considerably better than the previous simple method used for assimilating data into the same numerical model. Height errors as low as 10cm (root mean square) can be achieved.

FLOATING DOCK - BREAKWATER FOR REGIONS WITH HIGH TIDAL RANGE

It is essential to keep the clearance between the dock surface and the ships within the allowable limits to ensure easy access and serviceability. This is hard to achieve in locations with high tidal range. Using docks that can move in the vertical direction according to variation in water level is looking attractive, especially if the dock can serve as a breakwater at the same time. This paper introduces a simplified analytical approach to meet these requirements. The theory introduced predicts the wave height and water particle velocity in the leeward of the dock. The dimensions of the dock could be changed to obtain the suitable combination of both the wave height and-water particle velocity to fulfill the functionality of the harbor and minimizing the environmental impact resulting from the existence of the dock. The model eliminates the use of the variational principle and is bar—ad on mass and energy flux.

Metal fluxes in the Mersey Narrows

Abstract. Surveys of the Mersey estuary in north-west England were undertaken near the mouth of the estuary in the region known as the Mersey Narrows. Tidal fluxes of suspended and dissolved matter, particularly heavy metals, through the Mersey Narrows were investigated. This paper gives results of conducting four intensive cross-sectional surveys of the Narrows, during which currents, salinities, turbidity and water samples were obtained systematically at numerous positions, throughout selected tidal cycles. Over 300 water samples per survey were analysed to yield suspended and dissolved concentrations of the elements As, Cd, Cr, Cu, Hg, Ni, Pb and Zn, at all states of the tide. Suspended solids, concentrations and salinities were also measured. Suspended particulates account for the majority of each element present, except for cadmium, which was present in roughly equal dissolved and suspended fractions. From the tidal current and water quality data, calculations were made of hour-by-hour fluxes of each component, to show the detailed ebb and flow of heavy metals and the net tidal transport of each component. Although some differences between landward transport on the flood tide and seaward transport on the ebb were not significant, the more definite results consistently showed a seawards net transport. For spring tides of high tidal range, there was an indication of an opposite tendency, reducing the seawards transport or even reversing it, for certain suspended components. Keywords: Mersey estuary, surveys, tidal flux, dissolved metals, particulate metals, salinity, suspended particulate matter, suspended solids

Estuaries of the South Atlantic Coast of North America: Their Geographical Signatures

Estuaries of the southeastern Atlantic coastal plain are dominated by shallow meso-tidal bar-built systems interspersed with shallow sounds and both low flow coastal plain and high flow piedmont riverine systems. Three general geographical areas can be discriminated: the sounds of North Carolina; the alternating series of riverine and ocean dominated bar-built systems of South Carolina, Georgia, and northeast Florida, and the subtropical bar-built estuaries of the Florida southeast coast. The regional climate ranges from temperate to subtropical with sea level rise and hurri- canes having a major impact on the region's estuaries because of its low and relatively flat geomorphology. Primary production is highest in the central region. Seagrasses are common in the northern and southern most systems, while intertidal salt marshes composed of Spartina alterniflora reach their greatest extent and productivity in South Carolina and Georgia. Nuisance blooms (cyanobacteria, dinoflagellates, and cryptomonads) occur more frequently in the northern and extreme southern parts of the region. Fishery catches are highest in the North Carolina and Florida areas. Human population growth with its associated urbanization reaches a maximum in Florida and it is thought that the long-term sustainability of the Florida coast for human habitation will be lost within the next 25 years. Tidal flushing appears to play an important role in mitigating anthropogenic inputs in systems of moderate to high tidal range, i.e., the South Carolina and Georgia coasts. The most pressing environmental problems for the estuaries of the southeastern Atlantic coast seem to be nutrient loading and poor land use in North Carolina and high human population density and growth in Florida. The future utilization of these estuarine systems and their services will depend on the development of improved management strategies based on improved data quality.

Effects of extreme tidal range on characteristics of polychlorinated biphenyl distribution in sediment of industrial Incheon North Harbor, Korea

This study investigated the influence of the high tidal range on the polychlorinated biphenyl (PCB) levels and the compositional characteristics in the sediments of the North Incheon Harbor basin, Korea. The results indicated that the high tidal range could induce complicated effects on the distribution and the compositional characteristics of PCBs. Significant differences in the total PCB concentration were found between the tidal and subtidal zones. Higher total PCBs in the tidal zone established a total PCB gradient in a transversal direction along the harbor basin. The large tidal range created tidal currents strong enough to grade the sediment particles, establishing a gradient of total PCBs corresponding to an observed sediment particle size along the longitudinal direction of the basin. Heterogeneous congener patterns were observed in some parts of the tidal zone where diverse physicochemical and microbiological conditions are to be expected. However, tidally driven, active sediment transport promoted spatial homogeneity in the congener pattern. Sedimentation rate of suspended solids appeared to overwhelm any induced heterogeneity in the PCB congener pattern. Consequently, the congener pattern within the harbor basin was fairly uniform and similar to that of a dominant input source.