Tidal Basin Research Articles

Ebb and Flood Channel Systems in the Netherlands Tidal Waters1

The present paper, “Eb- en vloedschaarsystemen in de Nederlandse getijwateren” (Ebb- and Flood-Channel Systems in the Dutch Tidal Waters), which was published in 1950, should be considered as Van Veen's most important publication since his thesis. It summarizes the results of 20 years of intensive study of estuarine and tidal-basin mor-phodynamics in The Netherlands. Unfortunately, Van Veen's paper was published in Dutch, with only a brief summary in English. Luckily, the figure captions were given in both Dutch and English, allowing international researchers coming across the paper to read it as a kind of “cartoon.” Understandably though, the paper has received very limited recognition in the international literature. We have seized this occasion to publish an English version of Van Veen's paper. The paper is testimony to Van Veen's keen observational and artistic skills. His approach is nearly “Da Vincian,” in the sense that he is not only a fascinated observer of nature, but a sharp one as well, and he tries to capture the essentials of the dynamic behavior of complex coastal systems in apparently simple sketches. Many of the natural systems that Van Veen studied have been regulated since; thus, this paper contains a set of irreplaceable, high-quality observations on the natural dynamics of tidal systems. It forms an excellent introduction to the study of channel dynamics in estuaries, tidal inlets, and tidal basins.

A combined wind wave–tidal model for the Venice lagoon, Italy

A numerical model that combines wind waves with tidal fluxes in a tidal basin is presented and validated. The model couples a hydrodynamic finite element module based on the shallow water equations with a finite volume module that accounts for the generation and propagation of wind waves. The wave module solves the wave action conservation on the same triangular mesh used in the hydrodynamic module, thus efficiently reproducing the physical relationships between waves and tide propagation. The combined wind wave–tidal model is applied to the Venice lagoon, Italy. The highly irregular bathymetry of this tidal environment, characterized by deep channels, emergent salt marshes, and extensive tidal flats, suggests the introduction of specific hypotheses that simplify the governing equations with a noteworthy increase in efficiency and robustness of the algorithm. Particular attention is devoted to the dissipation of wave energy at the steep boundaries between channels, tidal flats, and salt marshes. Simulations of wave fields generated under specific wind conditions are presented and discussed. The model results are compared, with good agreement, to field data collected in different stations inside the lagoon of Venice. Finally, evidence of the complementary effect of tidal currents and wind waves on bottom shear stresses is presented using the results of different simulations.

The contribution of seagrass beds (Zostera noltii) to the function of tidal flats as a juvenile habitat for dominant, mobile epibenthos in the Wadden Sea

Since the substantial loss of subtidal eelgrass (Zostera marina L.) in the 1930s, seagrass beds in the Wadden Sea are limited to the intertidal zone and dominated by Z. noltii Hornem. This study deals with the effect of vegetated tidal flats on quantities of mobile epifauna and proves empirically the function of seagrass canopies as a refuge for marine animals remaining in the intertidal zone at ebb tide. Drop-trap samples were taken in the Sylt-Romo Bight, a shallow tidal basin in the northern Wadden Sea, on vegetated and unvegetated tidal flats during July and August 2002, and during the entire growth period of Z. noltii from May to September in 2003. The species composition in Z. noltii and bare sand flats showed minor differences since only two isopod species (Idotea baltica and I. chelipes) occurred on Z. noltii flats exclusively. Juvenile shore crabs (Carcinus maenas L.), brown shrimps (Crangon crangon L.) and common gobies (Pomatoschistus microps Kroyer) were also found abundantly on bare sand flats. However, the results showed significantly higher abundances and production of these dominant species on vegetated tidal flats. Additionally, the analyses of faunal size classes indicated higher percentages of small individuals in the seagrass bed during the entire sampling period. Despite drastic diurnal fluctuations of dissolved oxygen at low tide, faunal density in the residual water layer remaining in seagrass canopies at ebb tide was found to be consistently higher than that found in artificially created tide-pool units. Although species composition of mobile epifauna did not basically differ between vegetated and unvegetated tidal flats, Z. noltii beds are considered to contribute quantitatively to the function of tidal flats, as an extended juvenile habitat for some of the most important species of the Wadden Sea food web.

Development and Preservation of a Mid-Holocene Tidal-Channel Network Offshore the Western Netherlands

Recently obtained high-resolution seismic records, collected offshore the western Netherlands in a kilometer-size grid across the lower shoreface and inner shelf, suggest the presence of a dense, previously unknown network of tidal-channel fills in the shallow subsurface of the North Sea. Seven channel fills constituting this network were analyzed in detail to infer relationships among the fills in time and space, and to identify factors governing channel development and tidal-basin evolution. The majority of the channels are oriented perpendicular to the coast, but parts of some have a strong coast-parallel component. Shell associations and sedimentological characteristics in most channel fills are typical of back-barrier depositional environments. All but one of the channel fills are part of an Atlantic-age tidal-channel system, draining a tidal basin protected by barrier islands. One channel fill, which is correlated to an incision recognized onshore, represents a younger phase in coastal development. The presumed barrier-island chain was positioned at least 12 km seaward of the present coastline in the northern part of the study area, and at least 6 km seaward of the present coastline in the southern part. The tidal channels grew and migrated actively before partially being filled in between 7300 14C yr BP and 5500 14C yr BP, reflecting an initial tidal-prism increase followed by a substantial decrease. The tidal-prism increase was governed by an upsurge of the tidal amplitude between 8000 14C yr BP and 7000 14C yr BP, and the subsequent decrease by a gradual decrease in the rate of sea-level rise. Rapid retrogradation of the coastline put a sudden end to the existence of the tidal basin. The ensuing shoreface erosion removed all but the deepest parts of the channel fills, which do not show any evidence of systematic landward migration. This field observation corroborates recent modeling results suggesting that rapidly rising sea level may create the conditions necessary for overstepping of sandy barriers. Copyright © 2005, SEPM.

Sediment Transport in a Tidal Lagoon Subject to Varying Winds Evaluated with a Coupled Current-Wave Model

Abstract The sensitivity of estimated suspended sediment transport rates was studied by applying existing current-wave fields within a coupled numerical model using the Bagnold and the Soulsby-van Rijn formulae. The study area was the Sylt-Romo bight, an enclosed back barrier tidal basin located in the North Sea. The bight has an area of some 400 km2 in which sand flats prevail. This work provides numerical estimates of sediment transport rates and shear velocities throughout the Sylt-Romo tidal basin. Water levels, vertically-averaged two-dimensional current velocities and wave spectra were used as input from validated numerical model calculations. Four days covering a full storm period in April 1997 were selected for the calculations of total bed shear velocities and sediment transport rates. The simulation shows that during moderate wave conditions sediment transport mainly takes place in the tidal channels. However, with high wave energy present, sediment transport becomes more important in shallow ar...

Modeling of channel patterns in short tidal basins

We model branching channel patterns in short tidal basins with two methods. A theoretical stability analysis leads to a relationship between the number of channels and physical parameters of the tidal system. The analysis reveals that width and spacing of the channels should decrease as the slope of the bottom profile and the Shields parameter increase and as the mean water depth decreases. In general, the channel depth should halve at every bifurcation. These theoretical results agree well with the field data from the Dutch Wadden Sea. A numerical model based on Delft3D, a software system of WL/Delft Hydraulics, is used to simulate the time evolution of a channel network in a geometrically simplified basin of similar dimensions as the Wadden Sea basins. The resulting channel network displays a three‐times branching behavior, similar to the three‐ to four‐times branching patterns observed in the Wadden Sea. The simulated channel pattern satisfies the relation derived from the theoretical analysis. The results of this pattern analysis provide for additional validation of two‐dimensional/three‐dimensional process‐based morphodynamic models of tidal basins.

Archaeology: a major tool in the reconstruction of the coastal evolution of Westergo (northern Netherlands)

Westergo is the name of the coastal area in the northern Netherlands between the former tidal systems of Marne and Middelzee. The area silted up and changed into a salt-marsh area during the Subatlantic (1000 BC–500 AD). The history of seaward migration of the salt marshes in the area has been reconstructed with the help of the archaeologically dated settlements on dwelling mounds ( terps). Terps were constructed on the salt marshes when the process of silting had reached the stage of the middle marsh. This zone is flooded less frequently than ca. 50 days per year. The construction of terps during the middle stage of marsh development is indicated by the thickness of the salt-marsh deposits (on which terps were built) and by the diatoms found in these marsh deposits. The oldest phase of terp settlement on the marshes of Westergo is dated between the Late Bronze Age and Early Iron Age (ca. 600 BC) and is found in the southern part of Westergo (the “old land”). During the next period (500 BC–500 AD) the “funnel-shaped tidal basin” in the northern part silted up to the supratidal level. The middle marsh zone in the funnel-shaped basin expanded during this period (1000 years) by ca. 5 km at the western and eastern sides of the basin and by ca. 10 km in the central part. This implies a mean marsh extension in this former tidal basin of ca. 5–10 m per year. In this paper, a reappraisal of the nature and genesis of the salt-marsh ridges of the northern Netherlands is presented.

Observations on silt and sand transport in the throat section of the Frisian Inlet

The Frisian Inlet is one of the tidal basins of the Dutch Wadden Sea. In 1969, its basin area was reduced by 30%. As documented by bathymetric surveys, this has led to an import of sediment of 30×10 6 m 3 over the first 18 years. The study presented in this paper seeks to establish the mechanisms responsible for the passage of the sediment through the throat cross-section of the inlet channel. Emphasis is on a 14-day period of relative calm when sediment transport can be attributed solely to tidal currents. Use is made of continuous measurements of velocity, sand and silt concentration. The measurement station was located on one side of the throat cross-section in a water depth of approximately 6 m. For both the sand and silt fraction of the sediment, suspended load transport is the dominant transport mode. It is shown that for sand, concentration variations and net transport are determined by the local (in the throat section) velocity. Especially the residual velocity and tidal velocity asymmetry play an important role in the net sand flux. For silt, except for transport associated with locally generated vertical mixing, the net transport is largely determined by sedimentation–erosion processes in the basin and the silt concentration in the North Sea. Comparison with measurements in a station located in the middle of the throat section shows considerable difference in residual velocity and tidal velocity asymmetry. As a result, the sediment fluxes also differ. Accurately determining the net sediment flux in the throat section would require a dense net of measurement stations.

Biologically induced differences in erodibility and aggregation of subtidal and intertidal sediments: a possible cause for seasonal changes in sediment deposition

This study was carried out to describe the difference in erodibility and aggregation in a tidal basin including both subtidal and intertidal study sites and to use these results to explain the shifting erosion/deposition cycles at the sites. Erosion thresholds, erosion rates and settling velocities of the eroded material were measured at a mudflat transect and at sediment cores taken from a nearby tidal channel during surveys made in May 2000 and March 2002. Surface samples were analysed for grain-size, chl. a content, faecal pellet content, dry bulk density and organic content. Additionally, surface samples were taken at eight occasions in the period January 2002 to May 2003 from shallow tidal channels in the area. These samples were analysed for mud content and showed that major shifts in sediment distribution occurred in the period. The erodibility of the mudflat was generally high due to pelletization by the mudsnail Hydrobia ulvae but close to the salt marsh much lower erodibility was found, probably due to stabilisation by microphytobenthos. In contrast, the erodibility of the channel bed seemed to be very little influenced by biological activity and the relatively low erodibility found here was caused by physical characteristics of the sediment. The sediment eroded from the mudflat was generally strongly pelletized and showed high settling velocities whereas less aggregation and lower settling velocities were found for the channel bed sediments. Temporal variations of the mudflat stability and hydrodynamics resulted in temporal variations of deposition and erosion and the changing stability at the mudflat is likely to be one of the main reasons for a general transport of fine-grained sediment from the mudflat to the channel in the cold seasons and vice versa during the rest of the year.

Tidal dynamics of dissolved and particulate matter and bacteria in a tidal flat ecosystem in spring and fall

Tidal flat ecosystems are highly dynamic environments with tremendous short‐term and long‐term changes in physical, chemical, and biological properties. We measured salinity, temperature, oxygen, dry weight (dry wt), inorganic (PIC) and organic carbon (POC) fractions of suspended particulate matter (SPM), and chlorophyll a (Chl a); counted numbers of free‐living and particle‐associated bacteria; and measured dissolved amino acids and carbohydrates, and ectoenzymatic activities throughout tidal cycles in the Spiekeroog tidal flat system of the German Wadden Sea in November 1999 and May 2000. Bacterial production was measured only in May 2000. In November, high sediment resuspension, as indicated by increased amounts of SPM, PIC, and POC, mainly controlled bacterial dynamics and led to enhanced numbers of aggregate‐associated bacteria. In contrast, in May, phytoplankton‐related processes mainly influenced bacterial dynamics. Increased concentrations of oxygen, Chl a, and dissolved combined neutral monosaccharides resulted in high numbers of free‐living bacteria, elevated β‐glucosidase activities, and high bacterial production rates. The introduction of foreign water masses, either from the open North Sea or adjacent tidal basins, as reflected by different signatures of SPM and the mole percent composition of particulate combined amino acids, appeared to be a regular event in this highly dynamic system and rapidly changed physical, chemical, and biological processes.

Determining effective stress and permeability equations for soft mud from simple laboratory experiments

Applying models to describe the self-weight consolidation of soft mud soils requires specification of constitutive equations for the vertical effective stress and permeability. A wide range of constitutive equations exist, each involving soil-dependent coefficients. This paper presents a method for determining the constitutive equations for soft mud from low-cost and easy-to-do laboratory settling column experiments. The method assumes the validity of Gibson's consolidation equation and of the power law constitutive equations proposed by Merckelbach & Kranenburg. Analytical expressions, which take into account partial segregation that may have occurred during the suspension phase of the experiment, were derived for the initial settlement of the mud–water interface as a function of time and for the volume fraction of solids as a function of the depth below the mud–water interface when consolidation is (almost) completed. Determining the coefficients in the constitutive equations with these expressions requires only recording the interface height and a single density measurement at the end of the consolidation process. In addition, an expression for the final bed height is given. This expression can be used as an alternative for the expression for the volume fraction of solids as a function of the depth below the mud–water interface if no density measurement is available. The methods were applied to a data set of settling column experiments with a natural mud from the Dollard tidal basin (The Netherlands). The results were compared with conventionally determined constitutive equations for effective stress and permeability. It is concluded that the coefficients that are determined by new methods compare well with the coefficients that are conventionally determined, even if significant segregation occurred during the suspension phase of the experiment.

A Simplified Model for Predicting the Pollution Exchange Coefficient of Small Tidal Embayments

In recent years there has been a significant increase in the development and application of ever more sophisticated multi-dimensional models for solving the hydrodynamic and constituent transport equations which govern the flushing of pollution. However, advanced numerical modelling techniques can sometimes be augmented by alternative mathematical approaches which use simplified analytical solutions to predict the dispersion of contaminants. In the present article, a novel analytical tidal prism model is described for predicting the pollution flushing characteristics of small tidal embayments. The model relates the water quality response of the basin to the external forcing effects of the tide, the initial pollutant loading and the freshwater inflow rate. In general, the pollution flushing not only depends upon the geometry of the embayment and the tidal range but also on the proportion of effluent water which leaves the basin on an ebb tide, mixes with the surrounding coastal water and returns on subsequent flood tides. This effect has been taken into account in the mathematical model by the inclusion of a ‘pollution-return’ parameter. The analytical approach offers a viable and computationally inexpensive alternative to conventional multi-dimensional pollutant transport simulations and, more importantly, provides an increased understanding of the flushing characteristics of semi-enclosed tidal basins. The efficiency of the tidal flushing can be expressed in terms of the pollution exchange coefficient which measures the proportion of water exchanged with the sea each tidal cycle. To demonstrate the accuracy of the proposed mathematical formulation, analytical water quality predictions are compared against experimental pollution data from a 1:400 scale laboratory model of a generic square harbour. The results demonstrate that the analytical approach provides a simple and robust method of determining the water quality response of well-mixed tidal embayments.

Estuarine morphodynamics

Estuaries and tidal lagoons display complex morphological characteristics on both the meso- and macroscale. Single and multiple channel–shoal patterns and branching or braided patterns can be encountered on the mesoscale. On the macroscale, hypsometry of tidal basins may differ widely. Our insight in and understanding of morphodynamic processes leading to these patterns is limited. While observations and empirical relations have been established, it is only during the last two decades that macroscale models have been introduced to explain macroscale features. The emergence of mesoscale features in the long-term macroscale evolution, however, was limited to empirical and qualitative findings. Recently, mesoscale, process-based models have been introduced that show ability to reproduce mesoscale patterns in a macroscale evolution. This contribution attempts to highlight this recent capacity by bridging meso- and macroscale observations and process knowledge to increase our understanding of estuarine and tidal basin morphological evolution. It is shown that positive feedback processes leading to self-organization may be derived from first physical principles on smaller scales. In fact, this may be seen as a first successful step in the issue of aggregation of smaller estuarine process scales to larger ones.

Sand?mud morphodynamics in a short tidal basin

The influence of sand and mud transport on the morphological behaviour of a short tidal basin is investigated in this paper. For this purpose, a morphological model is applied in which sand and mud transport are included and the temporal and spatial bed composition variations are taken into account. Initially, the morphological development shows a sand wave near the entrance of the basin and a mud deposition wave more landward. A quasi equilibrium bed level profile is found after a long period (order century) with a sandy bed surface over almost the entire basin and only a small muddy area near the landward end. The dimensionless ratio between the deposition and erosion flux turns out to be a crucial parameter for the understanding of the observed behaviour. Comparison with previous studies on short tidal basins for sand indicates only that the presence of mud in a combined sand mud model does not change the equilibrium bed level profile considerably for the applied parameter settings herein, but drastically decreases the morphological time scale. Comparison between model results and field data of the Wadden Sea suggests that the obtained bed level and bed composition profile are realistic, indicating that the process-based sand mud model is a first step towards a better understanding of sand mud distributions in tidal basins.

Effect of advective and diffusive sediment transport on the formation of local and global bottom patterns in tidal embayments

Bathymetric field data of tidal basins reveal two main classes of bottom patterns: (1) tidal bars, located near the entrance of the basin (length scale determined by the embayment width) and (2) global channel-shoal patterns which scale with the basin length. Previous models were able to describe only either one of these patterns. In this paper it is shown that both of them can be investigated within the framework of an idealised model of a rectangular tidal embayment, with fixed side walls and an erodible bed. The water motion is described by the depth-averaged shallow-water equations and is forced by a prescribed vertical tide at the seaward entrance. Sediment is transported as suspended load and only realistic values of the bottom friction parameter are considered. By assuming the ratio of embayment length over tidal wave length to be small, the model allows for a morphodynamic equilibrium, characterised by a spatially uniform tide moving over a bottom which slopes upwards toward the landward boundary. This equilibrium is unstable for a range of values of the model parameters, such that growth of bedforms occurs. Both global and local bottom patterns are found. In this study particular emphasis is laid on the mechanism governing the growth of a new type of localised bottom pattern. These patterns consist of small bars located near the entrance of the basin, resembling multiple row bars, and are found when advective sediment fluxes prevail over diffusive sediment fluxes. The formation process of these new bedforms is discussed in detail. The results agree well with field data. Comparison of the results with those obtained with a process-based, numerical model shows that, although the idealised model is strongly simplified, it is capable of producing the essential morphodynamics. Therefore, the idealised model is a useful tool to investigate mechanisms of bottom pattern growth.

Morphological response of tidal basins to human interventions

This study focuses on the prediction of the long-term morphological evolution of tidal basins due to human interventions. New analytical results have been derived for an existing model [ASMITA, Aggregated Scale Morphological Interaction between a Tidal inlet and the Adjacent coast; Stive, M.J.F., Capobianco, M., Wang, Z.B., Ruol, P., Buijsman, M.C., 1998. Morphodynamics of a Tidal Lagoon and adjacent Coast. 8th International Biennial Conference on Physics of Estuaries and Coastal Seas, The Hague, September 1996, 397–407.]. Through linearisation of the model equations a set of time scales is obtained that describe the main features of the morphological evolution of tidal inlets. The magnitude of these system time scales is determined by inlet geometry and sediment exchange processes. The nature and degree of interventions determine which time scales are dominant. We focus on five different tidal inlets in the Wadden Sea. For these inlets, the system time scales have been estimated. The model has been applied to simulate the morphological response of the Marsdiep and Vlie inlets to the closure of the Zuiderzee in 1932. In this way, the model and associated system time scales for each of these inlets have been validated. Results show that in both inlets, the channels display the largest adaptation time. It will take at least a century before the channels and hence the tidal inlet systems reach a new morphological equilibrium.

Analytical Study of Longitudinal Mass Flux Due to the Shear Effect in a Tidal Basin

The fundamental nature of the mass flux due to the shear effect is examined analytically in a basin with steady and oscillatory currents to promote a better understanding of the mass transport process in coastal waters. The currents are given from solutions of the simplified motion equation so as to be consistent with the diffusion equation. The matter concentration used is given by an analytical solution of the diffusion equation with the settling flux term contained. Mass flux, yielding the depth-averaged dispersion coefficient, is rather varied vertically in both steady and oscillatory currents. In the oscillatory current with a Stokes layer in particular, the vertical profile of flux is more complicated and even negative flux is induced near the basin floor. This negative flux does not necessarily yield a negative value of the vertically averaged dispersion coefficient. The exact dispersion coefficient given by the flux analysis is realized only in the steady state of the matter concentration distribution, though we can scarcely observe the steady state in the actual sea. The vertically uniform longitudinal dispersion coefficient at the stationary stage is shown to be caused from the vertical complexity of mass flux by the action of the vertical diffusing and the settling flux.

Sediment and carbon accumulation in a small tidal basin: Yuehu, Shandong Peninsula, China

A small sedimentary basin (the Yuehu lagoon), located at the eastern tip of Shandong Peninsula, China, was selected to study its filling by sediment and the vertical flux of particulate organic carbon in response to natural/anthropogenic processes. Surficial and short core sediment samples were collected and analyzed to obtain data sets of grain size, organic carbon content, deposition rates and vertical fluxes of sediment and organic carbon. The analytical results show that the lagoon is covered mainly with fine-grained sediments with high deposition rate and particulate organic carbon content being found from the central part of the mud deposit. The sediment balance of the lagoon indicates high denudation rates of the catchment basin, which may be related to soil erosion in response to farming and land use pattern changes. Furthermore, preliminary analysis of the organic carbon fluxes of the Yuehu lagoon and other embayments of the region shows that these coastal systems make an important contribution to the regional shallow sea carbon burial.

Impact of sea-level rise on the morphological equilibrium state of tidal inlets

This study addresses the question whether the geomorphology of a tidal inlet (i.e. the coastal inlet and associated tidal basin) can maintain equilibrium under a rising relative sea level. When a tidal inlet system is exposed to a constant rate of sea-level rise ( SLR), the system will be permanently in a state that deviates from the Equilibrium State corresponding to zero SLR. This is a physical requirement to create a permanent incentive to import sediment into the system. In case the rate of sediment import matches the rate of sea-level rise, a new state of dynamic morphological equilibrium is reached. If the actual import rate is less than this, the system’s morphological state will deviate increasingly from its equilibrium and finally degenerate. In this case SLR has exceeded an upper boundary (named ‘state limit’, denoted as SLR limit), and the system will not be able to reach a state of dynamic morphological equilibrium any longer and drown. Because the SLR limit predictions are sensitive to a variation of internal sediment exchange rate and availability of sediment at the seaward boundary of the system, we assess the SLR limit predictions quasi-probabilistically to gain quantitative insight into the inherent uncertainties. This technique leads to an estimate of the probability distribution for the SLR limit of two selected tidal inlet systems in the Dutch Wadden Sea. The calculated probabilities comply with the available fragmentary geological data, which suggest that the former tidal inlets in the western region of the Netherlands were drowned under the influence of a sea-level rise of 80 cm to a few metres per century.

First- and higher-order dynamical controls on water exchanges between tidal basins and the open ocean. A case study for the East Frisian Wadden Sea

Observational data, high-resolution numerical modelling results and a simple analytical theory are combined in this paper to demonstrate the dependence of the volume transports through tidal inlets on topographical or morphological parameters of a Wadden Sea system. The area of interest covers the East Frisian Wadden Sea and consists of seven weakly connected tidal basins. The observations include time series of tidal gauge data and surface currents measured at a pile station in the backbarrier basin of the island Langeoog, as well as several ADCP transects in the Accumer Ee tidal inlet. The numerical simulations are based on the 3-D primitive equation General Estuarine Transport Model (GETM) with a horizontal resolution of 200 m and terrain following vertical coordinates. The model is forced at its open boundaries with sea-level data from an operational model for the German Bight (German Hydrographic Office). The simple theoretical concepts presented illustrate the effect of topography (hypsometry) in the tidal basins on the temporal variability of the exchange of water. This topographic control is effectuated through the bottom slope in the areas prone to drying and flooding. For our study area it takes about twice as long from slack water to maximum flood current than from slack water to maximum ebb current. The underlying physics of this signal modulation from a more or less harmonic forcing at the open-sea boundary and the quantification of the contributing physical processes are the major results of this paper. Estimates based simply on volume conservation are consistent with observations and results from numerical modelling, but they do not completely capture the actual non-linear tidal response. Our analysis shows that at least during part of the tidal cycle characteristic topographic parameters of the inlet/bay system have a major impact on the rate of exchange of waters between the Wadden Sea and the open ocean. This impact is especially strong during the transition between flood and ebb conditions. The possible morphodynamic responses are also addressed focusing on some common (“universal”) topographic features in seven tidal basins.