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1.
Estuaries typically show converging planforms from the sea into the land. Nevertheless, their planform is rarely perfectly exponential and often shows curvature and the presence of embayments. Here we test the degree to which the shapes and dimensions of tidal sandbars depend on estuary planform. We assembled a dataset with 35 estuary planforms and properties of 190 tidal bars to induce broad‐brush but significant empirical relations between channel planform, hydraulic geometry and bar pattern, and tested a linear stability theory for bar pattern. We found that the location where bars form is largely controlled by the excess width of a channel, which is calculated as the observed channel width minus the width of an ideal exponentially widening estuary. In general, the summed width of bars approximates the excess width as measured in the along‐channel variation of three estuaries for which bathymetry was available as well as for the local measurements in the 35 investigated estuaries. Bar dimensions can be predicted by either the channel width or the tidal prism, because channel width also strongly depends on local tidal prism. Also braiding index was predicted within a factor of 2 from excess width divided by the predicted bar width. Our results imply that estuary planform shape, including mudflats and saltmarsh as well as bar pattern, depend on inherited Holocene topography and lithology and that eventually convergent channels will form if sufficient sediment is available. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

2.
We present an analytical model to decompose complex along-channel and transverse residual flows into components induced by individual mechanisms. The model describes the transverse distribution of residual flows in tidally dominated estuaries. Scaling and perturbation techniques are used to obtain analytical solutions for residual flows over arbitrary across-channel bed profiles. The flows are induced by horizontal density gradients, tidal rectification processes, river discharge, wind, channel curvature and the earth's rotation. These rectification processes induce residual flows that are up-estuary to the right and down-estuary to the left of an estuarine channel (looking up-estuary in the northern hemisphere). The tidal rectification processes fundamentally change the transverse structure of along-channel residual flows in many tidal estuaries, as these processes cause the flows to be internally asymmetric about the mid-axis of the channel for relatively large tidal velocities, steep channels or narrow estuaries. In addition, velocity scales are derived from the analytical solutions to estimate the relative importance of the various residual flow mechanisms from estuarine parameters. A case study of a transect across the Upper Chesapeake Bay showed that important features of the residual flow observed in that transect are reproduced and explained by the analytical model. The velocity scales were able to identify the relevant residual flow mechanisms as well. The tidal rectification processes considered here result from advection of along-channel tidal momentum by Coriolis-induced transverse tidal currents.  相似文献   

3.
The main regularities of hydrological and hydrological-environmental processes occurring within the complex estuary, the Chesapeake Bay and the mouths of its tributaries, are discussed. The peculiarities of the estuary morphological structure, including the structures of tidal and net currents, salinity and water turbidity fields and their variability, the environmental conditions, and their human-induced changes. Using the Chesapeake Bay as an example, it became possible to reveal the basic features of classical estuaries subject to a considerable impact of river runoff and featuring mixing of river and sea water and moderate stratification of the water mass. It is shown that the regularities of hydrological processes in the Chesapeake Bay are typical of many mouth water bodies of estuarine type (inlets, drowned river valleys, lagoons, and tidal estuaries proper).  相似文献   

4.
A review is given of suspended sediment dynamics in macrotidal regimes using examples of estuaries situated along the French coast of the English Channel. Characteristic features of estuarine turbidity maxima are described over a range of time-scales, which includes semidiurnal and neap-spring tidal cycles, and seasonal fluctuations of river discharge. The present behaviour of the fluvial sediment influx within these systems is described, taking into account the geological history of estuarine infilling.  相似文献   

5.
The dynamics of finite-amplitude bed forms in a tidal channel is studied with the use of an idealized morphodynamic model. The latter is based on depth-averaged equations for the tidal flow over a sandy bottom. The model considers phenomena on spatial scales of the order of the tidal excursion length. Transport of sediment mainly takes place as suspended load. The reference state of this model is characterized by a spatially uniform M2 tidal current over a fixed horizontal bed. The temporal evolution of deviations from this reference state is governed by amplitude equations: these are a set of non-linear equations that describe the temporal evolution of bed forms. These equations are used to obtain new morphodynamic equilibria which may be either static or time-periodic. Several of these bottom profiles show strong similarity with the tidal bars that are observed in natural estuaries. The dependence of the equilibrium solutions on the value of bottom friction and channel width is investigated systematically. For narrow channels (width small compared to the tidal excursion length) stable static equilibria exist if bottom friction is slightly larger than rcr. For channel widths more comparable to the tidal excursion length, multiple stable steady states may exist for bottom friction parameter values below rcr. Regardless of channel width, stable time-periodic equilibria seem to emerge as the bottom friction is increased.Responsible Editor: Jens Kappenberg  相似文献   

6.
The bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large-scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large-scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood- and ebb-oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune-induced directional bed roughness in numerical models of estuarine and tidal environments.  相似文献   

7.
Man-induced regime shifts in small estuaries—I: theory   总被引：3，自引：2，他引：1  
This is Part I of two papers on man-induced regime shifts in small, narrow, and converging estuaries, with focus on the interaction between effective hydraulic drag, fine sediment import, and tidal amplification, induced by river engineering works, e.g., narrowing and deepening. In this part, a simple linear analytical model is derived, solving the linearized shallow water equations in exponentially converging tidal rivers. Distinguishing reflecting and non-reflecting conditions, a non-dimensional dispersion equation is derived which yields the real and imaginary wave numbers as a function of the estuarine convergence number and effective hydraulic drag. The estuarine convergence number describes the major geometrical features of a tidal river, e.g., intertidal area, convergence length, and water depth. This model is used in Part II analyzing the historical development of the tide in four rivers. Part I also presents a conceptual model on the response of tidal rivers to narrowing and deepening. It is argued that, upon the loss of intertidal area, flood-dominant conditions prevail, upon which fine sediments are pumped into the river, reducing its effective hydraulic drag. Then a snowball effect may be initiated, bringing the river into a hyper-turbid state. This state is self-maintaining because of entrainment processes, and favorable from an energetic point of view, and therefore highly stable. We may refer to an alternative steady state.  相似文献   

8.
Winterwerp  Johan C.  Wang  Zheng Bing 《Ocean Dynamics》2013,63(11):1279-1292

This is Part I of two papers on man-induced regime shifts in small, narrow, and converging estuaries, with focus on the interaction between effective hydraulic drag, fine sediment import, and tidal amplification, induced by river engineering works, e.g., narrowing and deepening. In this part, a simple linear analytical model is derived, solving the linearized shallow water equations in exponentially converging tidal rivers. Distinguishing reflecting and non-reflecting conditions, a non-dimensional dispersion equation is derived which yields the real and imaginary wave numbers as a function of the estuarine convergence number and effective hydraulic drag. The estuarine convergence number describes the major geometrical features of a tidal river, e.g., intertidal area, convergence length, and water depth. This model is used in Part II analyzing the historical development of the tide in four rivers. Part I also presents a conceptual model on the response of tidal rivers to narrowing and deepening. It is argued that, upon the loss of intertidal area, flood-dominant conditions prevail, upon which fine sediments are pumped into the river, reducing its effective hydraulic drag. Then a snowball effect may be initiated, bringing the river into a hyper-turbid state. This state is self-maintaining because of entrainment processes, and favorable from an energetic point of view, and therefore highly stable. We may refer to an alternative steady state.

  相似文献   

9.
Analytical solutions of the momentum and energy equations for tidal flow are studied. Analytical solutions are well known for prismatic channels but are less well known for converging channels. As most estuaries have a planform with converging channels, the attention in this paper is fully focused on converging tidal channels. It will be shown that the tidal range along converging channels can be described by relatively simple expressions solving the energy and momentum equations (new approaches). The semi-analytical solution of the energy equation includes quadratic (nonlinear) bottom friction. The analytical solution of the continuity and momentum equations is only possible for linearized bottom friction. The linearized analytical solution is presented for sinusoidal tidal waves with and without reflection in strongly convergent (funnel type) channels. Using these approaches, simple and powerful tools (spreadsheet models) for tidal analysis of amplified and damped tidal wave propagation in converging estuaries have been developed. The analytical solutions are compared with the results of numerical solutions and with measured data of the Western Scheldt Estuary in the Netherlands, the Hooghly Estuary in India and the Delaware Estuary in the USA. The analytical solutions show surprisingly good agreement with measured tidal ranges in these large-scale tidal systems. Convergence is found to be dominant in long and deep-converging channels resulting in an amplified tidal range, whereas bottom friction is generally dominant in shallow converging channels resulting in a damped tidal range. Reflection in closed-end channels is important in the most landward 1/3 length of the total channel length. In strongly convergent channels with a single forward propagating tidal wave, there is a phase lead of the horizontal and vertical tide close to 90o, mimicking a standing wave system (apparent standing wave).  相似文献   

10.
Both natural changes (e.g., tidal forcing from the ocean and global sea level rise) and human-induced changes (e.g., dredging for navigation, sand excavation, and land reclamation) exert considerable influences on the long-term evolution of tidal regimes in estuaries. Evaluating the impacts of these factors on tidal-regime shifts is particularly important for the protection and management of estuarine environments. In this study, an analytical approach is developed to investigate the impacts of estuarine morphological alterations (mean water depth and width convergence length) on tidal hydrodynamics in Lingdingyang Bay, Southeast China. Based on the observed tidal levels from two tidal gauging stations along the channel, tidal wave celerity and tidal damping/amplification rate of different tidal constituents are computed using tidal amplitude and phase of tidal constituents extracted from a standard harmonic analysis. We show that the minimum mean water depth for the whole estuary occurred in 2006, whereas a shift in tidal wave celerity for the M2 tide component occurred in 2009. As such, the study period (1990–2016) could be separated into pre-human (1990–2009) and post-human (2010–2016) phases. Our results show that the damping/amplification rate and celerity of the M2 tide have increased by 31% (from 7 to 9.2 m−1) and 28% (from 7 to 9 m·s−1) respectively, as a consequence of the substantial impacts of human interventions. The proposed analytical method is subsequently applied to analyse the historical development of tidal hydrodynamics and regime shifts induced by human interventions, thus linking the evolution of estuarine morphology to the dominant tidal hydrodynamics along the channel. The observed tidal regime shift is primarily caused by channel deepening, which substantially enlarged the estuary and reduced effective bottom friction resulting in faster celerity and stronger wave amplification. Our proposed method for quantifying the impacts of human interventions on tidal regime shifts can inform evidence-based guidelines for evaluating hydraulic responses to future engineering activities.  相似文献   

11.
1INTRODUCTIONTheHaiheRiverBasinislocatedinNorthChinawithareaof262.6km2.Itisaquicklydevelopedareawithmanyimportantcitiesandindustrialhubs,includingBeding,Tianjin,Tangshan,Cangzhou,DezhouandHuanghua.Theareawatchedfastprogressesinurbanizationinthepastdecades,andhumanactivitieshaveresultedingreatinfluencesontheenvironment,riverhydrologyandsedimentbudget.Theareaisprojectedtobemoreprosperouswithmoreoilandgasfields,chemicalindustrybases,anddenserrailwaysandexpresshighwaysinthenextcent'Ury.T…  相似文献   

12.
Geomorphological characteristics of tidal basins control hydrodynamics and sediment transport potential within such basins, for example, by adjusting the balance in tidal asymmetry. In this study we examine the effects of entrance geometry on tidal velocity asymmetry, slack water asymmetry, bed shear stress patterns and hypsometric profile shapes by comparison of six shallow meso-tidal basins of Tauranga Harbour, New Zealand. Numerical model results show how tidal distortion increases with distance from a basin entrance. A simple ratio between basin width and entrance width defines levels of basin dilation. Sub-basins with a constricted geometry and deep entrance channels are associated with small bed shear stress values and high rates of flood-directed tidal velocity asymmetry in the sheltered basin centres, indicating a large potential for sediment deposition of larger particles. Moreover, slack water asymmetry within these basins is weakly ebb-directed, indicating a small potential for transport of fine sediments out of the basins. The constricted depositional basins are characterized by convex hypsometric profiles with elevated intertidal regions. Unconstricted geometries are associated with larger bed shear stress values and more ebb-directed tidal velocity asymmetry within basin centres, suggesting limited potential for overall sediment deposition. The slack tide duration asymmetry is weakly flood-dominant indicating that limited input of fine sediment into the basins is possible. The comparatively high-energy conditions within these exposed basins are associated with a less convex hypsometric intertidal profile. The ability to estimate tidal asymmetries is advantageous when developing management strategies related to ecosystem functioning, navigability or coastal protection in specific geomorphic settings. © 2019 John Wiley & Sons, Ltd.  相似文献   

13.
Estuaries, commonly, are densely populated areas serving the needs of the inhabitants in multiple ways. Often the interests are conflicting and decisions need to be made by the local managers. Intake of fresh water for consumption, agricultural purposes or use by industries may take place within a region not far landward of the limit of salt intrusion. Human interventions (e.g. deepening of the navigation channels) or climate changes (sea level rise, reduction of the river discharge) can bring these intake locations within the reach of saline or brackish water and consequently endanger their function. To support policy and managerial decisions, a profound knowledge of processes associated with the salinity structure in estuaries is required. Although nowadays advanced numerical three-dimensional models are available that are able to cope with the complexity of the physics there is still a need for relatively simple tools for quick-scan actions in a pre-phase of a project or for instructive purposes. The analytical model described in this paper may serve these needs. It computes the maximum salinity distribution using the dispersion coefficient in the mouth as the only model parameter. The model has been calibrated using observational data in a large number of estuaries and experimental data in a tidal flume. The dispersion coefficient was successfully related to geometric and hydrodynamic parameters resulting in an expression that can be used for convergent estuaries as well as prismatic channels, see Eqs. 25a and 25b. Application of the model in a predictive mode showed its promising capabilities. Comparison with three-dimensional numerical models indicates that the channel geometry in the estuary mouth largely influences dispersive processes. The analytical model for salt intrusion may be used in combination with the analytical model for tidal propagation in convergent estuaries and tidal channels by Van Rijn (part I). In this way, input is obtained on the tidal velocity amplitude and the Chézy roughness following calibration of this model on tidal amplitudes along the estuary.  相似文献   

14.
The most common definition of an equilibrium condition for estuaries and inlets makes use of the well documented relationship between tidal prism and the cross‐sectional area of the channel. However, the tidal prism is itself a function of the morphology of the tidal basin. It would therefore be useful to be able to define the key dimensions of estuaries and inlets based on properties that are external to the estuary itself. This would then provide a more rigorous basis for understanding how systems are being ‘perturbed’ by developments, or other influences such as climate change. An idealized representation of the three–dimensional (3D) form of an estuary has been proposed and is here applied to a wide range of UK estuaries to explore its ability to predict the gross properties of a range of different estuary types. When considering just tidal flow, the model was found to provide an adequate representation, however, the inclusion of wave action was found to significantly improve the predictive power of the model. The exogenous parameters therefore provide a basis for determining the estuary dimensions and how they are likely to change over time. This in turn provokes a broader definition of an estuary than those commonly cited. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

15.
Groundwater flow and chemical transport in subterranean estuaries are poorly understood despite their potentially important implications for chemical fluxes from aquifers to coastal waters. Here, a numerical study of the dynamics in a subterranean estuary subject to tidal forcing is presented. Simulations show that salt transport associated with tidally driven seawater recirculation leads to the formation of an upper saline plume in the intertidal region. Computed transit times and flow velocities indicate that this plume represents a more active zone for mixing and reaction than the dispersion zone of the lower, classical salt wedge. Proper conceptualisation of this surficial mixing zone extends our understanding of processes within the subterranean estuary. Numerical tracer simulations reveal that tidal forcing may reduce the threat of a land-derived contaminant discharging to the marine environment by modifying the subsurface transport pathway and local geochemical conditions. Mixing and stratification in the subterranean estuary are strongly affected by both inland and tidal forcing. Based on the estuarine analogy we present a systematic classification of subterranean estuaries.  相似文献   

16.
Human interference in estuaries has led to increasing problems of mud, such as hyper-turbidity with adverse ecological effects and siltation of navigation channels and harbours. To deal with this mud sustainably, it is important to understand its long-term effects on the morphology and dynamics of estuaries. The aim of this study is to understand how mud affects the morphological evolution of estuaries. We focus on the effects of fluvial mud supply on the spatial distribution of mudflats and on how this influences estuary width, depth, surface area and dynamics over time. Three physical experiments with self-forming channels and shoals were conducted in a new flume type suitable for tidal experiments: the Metronome. In two of the experiments, we added nutshell grains as mud simulant, which is transported in suspension. Time-lapse images of every tidal cycle and digital elevation models for every 500 cycles were analysed for the three experiments. Mud settles in distinct locations, forming mudflats on bars and sides of the estuary, where the bed elevation is higher. Two important effects of mud were observed: the first is the slight cohesiveness of mud that causes stability on bars limiting vertical erosion, although the bank erosion rate by migrating channels is unaffected. Secondly, mud fills inactive areas and deposits at higher elevations up to the high-water level and therefore decreases the tidal prism. These combined effects cause a decrease in dynamics in the estuary and lead to near-equilibrium planforms that are smaller in volume and especially narrower upstream, with increased bar heights and no channel deepening. This trend is in contrast to channel deepening in rivers by muddier floodplain formation. These results imply large consequences for long-term morphodynamics in estuaries that become muddier due to management practices, which deteriorate ecological quality of intertidal habitats but may create potential area for marshes. © 2018 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.  相似文献   

17.
The Mid-Atlantic Integrated Assessment (MAIA-Estuaries) evaluated ecological conditions in US Mid-Atlantic estuaries during the summers of 1997 and 1998. Over 800 probability-based stations were monitored in four main estuarine systems--Chesapeake Bay, the Delaware Estuary, Maryland and Virginian coastal bays, and the Albemarle-Pamlico Estuarine System. Twelve smaller estuaries within the four main systems were also assessed to establish variance at the local scale. A subset of the MAIA-Estuaries data is used here to estimate the extent of eutrophication, sediment contamination, and benthic degradation in mid-Atlantic estuaries. An Environmental Report Card and Index of Environmental Integrity summarize conditions in individual estuaries, the four estuarine systems, and the entire MAIA region. Roughly 20-50% of the region showed signs of eutrophication (high nutrients, excessive production of organic matter, poor water clarity, or depleted dissolved oxygen), 30% had contaminated sediments, and 37% had degraded benthic communities. Compared with the Environmental Monitoring and Assessment Program (EMAP)-Virginian Province study in 1990-1993, larger fractions of Chesapeake Bay (17%) and Delaware River (32%) had increased metals or organics in sediments.  相似文献   

18.
Salinity difference between terrestrial river discharge and oceanic tidal water plays a role in modifying the local flow field and, as a consequence, estuarine morphodynamics. Although widely recognized, recent numerical studies exploring the long-term morphological evolution of river-influenced estuaries with two-dimensional, depth-averaged models have mostly neglected salinity. Using a three-dimensional morphodynamic model, we aim to gain more insight into the effect of salinity on the morphodynamics of fluvio-deltaic systems. Model results indicate that the resultant estuarine morphology established after 600 years differs remarkably when a salinity gradient is included. A fan-shaped river-mouth delta exhibits less seaward expansion and is cut through by narrower channels when salinity is included. The inclusion of salinity tends to generate estuarine circulation, which favours landward sediment transport and hence limits the growth of the delta while enhancing the development of intertidal areas. The formation of deltaic channel–shoal patterns resulting from morphodynamic evolution tends to strengthen salinity stratification, which is characterized by an increased gradient Richardson number. The direction of the depth-averaged residual sediment transport over a tide may be opposite to the direction of residual velocity, indicating the significant influence of baroclinic effects on the net sediment transport direction (and hence morphological change). The effect of salinity on morphological evolution becomes less profound when the strength of tidal or fluvial forcing is dominant over the other. The effects of sediment type and flocculation, which are particularly important when salinity gradients are present, are also discussed. Overall, this study highlights that neglecting salinity to simulate long-term estuarine morphodynamics requires more careful justification, particularly when the environment is characterized by fine sediment types (favouring suspended transport), and relatively large river discharge and estuarine depth (favouring baroclinic effects). © 2020 John Wiley & Sons, Ltd.  相似文献   

19.
The factors that govern the distribution and transformation of tidal waves in the macrotidal estuary of the Mezen River have been considered, including tide range in the mouth section, water discharge in the river’s lower reaches, estuary shape, and bed resistance coefficient. Data on variations of water discharge over period 1920–2008 are given. The parameters of estuary channel narrowing in horizontal and vertical sections have been considered. The effect of narrowing and bed hydraulic friction on tide wave amplitude has been evaluated. Froude number values for the tidal estuary suggest that tidal bore can form at the Mezen mouth. The conditions of the propagation of tidal waves to the mouths of different rivers and tidal bore formation in them are considered.  相似文献   

20.
In tidal estuaries, quantifying freshwater discharge is still a difficult problem that has not yet been overcome due to the inherent difficulty in measuring and analysing the tidal discharge, especially during periods of low river flow. Because observations are often made in the stations further upstream, where the ratio of river to tidal discharge is large, it remains difficult to determine the discharge rate in the saline region. Freshwater discharge estimation is even more difficult in a branched estuary system having multiple diversion channels that connect with each other at a junction. To date, several methods have been developed for estimating freshwater discharge in estuaries. The most widely used are analytical and conceptual models that employ salinity as the principal trace and numerical simulations. However, these methods are very time consuming and costly as they require large sets of observations before the computations can take place. This paper presents a simple approach to investigating the discharge distribution over branched channels by considering the energy loss due to friction. We develop an analytical model that can obtain the discharge rate quantitatively at a junction where the main flow bifurcates into two branches. The model uses the bed roughness, tidal water level, and cross‐sectional profile under tidally averaged conditions as input data. Two selected estuarine systems in the Hiroshima delta in Japan and the Mekong delta in Vietnam have been investigated. Computations of the newly developed model show good agreement with earlier published results computed by sophisticated analytical and numerical models.  相似文献   

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