Tjerk J. Zitman

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

Abstract 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 SLRlimit), and the system will not be able to reach a state of dynamic morphological equilibrium any longer and drown. Because the SLRlimit 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 SLRlimit predictions quasi-probabilistically to gain quantitative insight into the inherent uncertainties. This technique leads to an estimate of the probability distribution for the SLRlimit 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.

Ebb and flood channel systems in the Netherlands tidal waters

VAN VEEN, J.; VAN DER SPEK, A.J.F.; STIVE, M.J.F., and ZITMAN T., 2005. Ebb and flood channel systems in the Netherlands tidal waters. Journal of Coastal Research, 21(6), 1107–1120. West Palm Beach (Florida), ISSN 07490208. The present paper, ‘‘Eben vloedschaarsystemen in de Nederlandse getijwateren’’ (Ebband 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 morphodynamics 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. ADDITIONAL INDEX WORDS: estuaries, sediment transport, morphology, morphodynamics.

Predicting long-term and short-term tidal flat morphodynamics using a dynamic equilibrium theory

Dynamic equilibrium theory is a fruitful concept, which we use to systematically explain the tidal flat morphodynamic response to tidal currents, wind waves, sediment supply, and other sedimentological drivers. This theory stems from a simple analytical model that derives the tide- or wave-dominated tidal flat morphology by assuming that morphological equilibrium is associated with uniform bed shear stress distribution. Many studies based on observation and process-based modeling tend to agree with this analytical model. However, a uniform bed shear stress rarely exists on actual or modeled tidal flats, and the analytical model cannot handle the spatially and temporally varying bed shear stress. In the present study, we develop a model based on the dynamic equilibrium theory and its core assumption. Different from the static analytical model, our model explicitly accounts for the spatiotemporal bed shear stress variations for tidal flat dynamic prediction. To test our model and the embedded theory, we apply the model for both long-term and short-term morphological predictions. The long-term modeling is evaluated qualitatively against previous process-based modeling. The short-term modeling is evaluated quantitatively against high-resolution bed-level monitoring data obtained from a tidal flat in Netherlands. The model results show good performances in both qualitative and quantitative tests, indicating the validity of the dynamic equilibrium theory. Thus, this model provides a valuable tool to enhance our understanding of the tidal flat morphodynamics and to apply the dynamic equilibrium theory for realistic morphological predictions.

Response of large-scale coastal basins to wind forcing: influence of topography

Because wind is one of the main forcings in storm surge, we present an idealised process-based model to study the influence of topographic variations on the frequency response of large-scale coastal basins subject to time-periodic wind forcing. Coastal basins are represented by a semi-enclosed rectangular inner region forced by wind. It is connected to an outer region (represented as an infinitely long channel) without wind forcing, which allows waves to freely propagate outward. The model solves the three-dimensional linearised shallow water equations on the f plane, forced by a spatially uniform wind field that has an arbitrary angle with respect to the along-basin direction. Turbulence is represented using a spatially uniform vertical eddy viscosity, combined with a partial slip condition at the bed. The surface elevation amplitudes, and hence the vertical profiles of the velocity, are obtained using the finite element method (FEM), extended to account for the connection to the outer region. The results are then evaluated in terms of the elevation amplitude averaged over the basin’s landward end, as a function of the wind forcing frequency. In general, the results point out that adding topographic elements in the inner region (such as a topographic step, a linearly sloping bed or a parabolic cross-basin profile), causes the resonance peaks to shift in the frequency domain, through their effect on local wave speed. The Coriolis effect causes the resonance peaks associated with cross-basin modes (which without rotation only appear in the response to cross-basin wind) to emerge also in the response to along-basin wind and vice versa.

RECONSTRUCTION OF THE HOLOCENE EVOLUTION OF THE DUTCH COAST

Tanah Lot Temple is situated in Tabanan Regency - Bali, on the coast of the Indonesian Ocean. Due to continuous wave attack, wind force, and weathering of the rock bank where the Temple stands, abrasion has occured which is more and more threatening the existence of the Temple. Considering that Tanah Lot Temple is a sacred place for the Hindu Balinese people and a place of high cultural value, and also an important tourism, steps to save the Temple are imperative. The Central as well as the Regional Authorities, and also the Bali nese community are very much interested in the effort to keep the Temple intact. Measures have been undertaken to protect both the seaside and land-side banks of the Temple rock bank. This paper only discusses counter measures of the sea —side bank of the Temple.

EFFECT OF ASSUMING A LOGARITHMIC FLOW PROFILE NEAR THE BOTTOM ON COMPUTING TIDE-RESIDUAL SEDIMENT TRANSPORT

A common assumption in modelling tidal flow in coastal areas is that in the vicinity of the bottom the vertical flow profile attains a logarithmic shape. Some consequences of this type of assumption are studied in the present paper. In a diagnostic setting, the tide-residual transport of sediment is depicted as a function of the thickness of the assumed log-layer. In this respect, the tide is schematised to a combination of the M2 and M4 constituents and the water depth is chosen in a range typical for the shoreface (10-25m). The computational results indicate that assuming a log-layer may lead to an error in the cross- shore transport in the order of 1% of the alongshore transport.