D.J.R. Walstra

The predictability of cross-shore bed evolution of sandy beaches at the time scale of storms and seasons using process-based Profile models

Deterministic and probabilistic Profile models have been compared with hydrodynamic and morphodynamic data of laboratory and field experiments on the time scale of storms and seasons. The large-scale laboratory experiment is a pure 2D case and offers and ideal test case for cross-shore Profile models, as disturbing alongshore non-uniformities are absent. The field experiments are performed at the Egmond site (The Netherlands) during the EU-COAST3D project and represent storm time scale (Oct.–Nov. 1998) as well as seasonal time scale conditions (May 1998–Sep. 1999). The objective of the paper is to present information of coastal processes on these time scales and to assess the predictive capabilities Coastal Process-based Profile models with respect to hydrodynamics and morphodynamics at sandy beaches on the time scales of storms and seasons. Profile models can quite accurately (errors smaller than 10%) represent the cross-shore significant wave height distribution in the surf zone, if the wave breaking model is properly calibrated. The wave breaking coefficient should be a function of local wave steepness and bottom slope for most accurate results. Profile models can reasonably represent the cross-shore and longshore currents (undertow) in a pure 2D case and in 3D field conditions. Profile models including cross-shore mixing effects and breaker delay effects do not produce better predictions of the longshore and cross-shore current velocities. Profile models using default settings can quite reasonably simulate the behaviour of the outer and inner bars on the storm time scale; the behaviour of the beach cannot be modelled with sufficient accuracy on the storm time scale. Profile models can reasonably simulate the post-storm onshore bar migration, provided that the near-bed orbital velocities and wave asymmetry-related sand transport are represented in a sufficiently accurate way (using non-linear wave theories). Profile models cannot simulate the beach recovery processes on the post-storm time scale, because these essentially 3D processes are not sufficiently known to be included in the models. Profile models using default settings cannot simulate the behaviour of the outer and inner bars and the beach on the seasonal time scale; the behaviour of the outer bar on the seasonal time scale can only be represented properly after tuning using measured bed profiles. The simulation of the inner bar and beach morphology on the seasonal time scale could not be improved by tuning.

Calibration and verification of a parametric wave model on barred beaches

Abstract Since its introduction in 1978, the Battjes and Janssen model has proven to be a popular framework for estimating the cross-shore root-mean-square wave height H rms transformation of random breaking waves in shallow water. Previous model tests have shown that wave heights in the bar trough of single bar systems and in the inner troughs of multiple bar systems are overpredicted by up to 60% when standard settings for the free model parameter γ (a wave height-to-depth ratio) are used. In this paper, a new functional form for γ is derived empirically by an inverse modelling of γ from a high-resolution (in the cross-shore) 300-h H rms data set collected at Duck, NC, USA. We find that, in contrast to the standard setting, γ is not cross-shore constant, but depends systematically on the product of the local wavenumber k and water depth h . Model verification with other data at Duck, and data collected at Egmond and Terschelling (Netherlands), spanning a total of about 1600 h, shows that cross-shore H rms profiles modelled with the locally varying γ are indeed in better agreement with measurements than model predictions using the cross-shore constant γ . In particular, model accuracy in inner bar troughs increases by up to 80%. Additional verifications with data collected on planar laboratory beaches show the new functional form of γ to be applicable to non-barred beaches as well. Our optimum γ cannot be compared directly to field and laboratory measurements of height-to-depth ratios and we do not know of a physical mechanism why γ should depend positively on kh .

Modeling cross-shore sandbar behavior on the timescale of weeks

We compare predictions of a coupled, wave-averaged, cross-shore waves-currents-bathymetric evolution model to observations of onshore and offshore nearshore sandbar migration. The observations span a 10- and 44-day period with onshore/offshore bar migration at Duck, North Carolina, and at Hasaki, Kashima Coast, Japan, respectively, a 3.5-month period of onshore bar migration at Duck, and a 22-day period of offshore bar migration at Egmond, Netherlands. With best fit parameter values the modeled temporal evolution of the cross-shore bed profiles agrees well with the observations. Model skill, defined as 1 minus the ratio of prediction to no-change error variances, ranges from 0.50 at Egmond to 0.88 for the prolonged onshore bar migration at Duck. Localized (in time and space) reductions in model skill coincide with alongshore variations in the observed morphology. Consistent with earlier observations, simulated offshore bar migration takes place during storms when large waves break on the bar and is due to the feedback between waves, undertow, suspended sediment transport, and the sandbar. Simulated onshore bar migration is predicted for energetic, weakly to nonbreaking conditions and is due to the feedback between near-bed wave skewness, bedload transport, and the sandbar, with negligible to small effects of bound infragravity waves and near-bed streaming. Under small waves and conditions, when breaking and nonbreaking conditions alternate with the tide, the sandbar is predicted to remain stationary. The intersite differences in the optimum parameter values are, at least partly, induced by insensitivity to parameter variations, parameter interdependence, and errors in the offshore wave forcing.

Vertical variation of the flow across the surf zone

Abstract This paper reviews recent advances that have been made in the numerical modelling and measurement techniques of the surf zone. The review is restricted by the assumption of a long and uniform coastline case. Therefore, the frame of reference is the 2DV case, but including tree-dimensional processes important for this topic. During the last two decades, new measurement techniques have become available (e.g. Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV)), which have successfully been applied in numerous laboratory experiments. These methods have enabled detailed measurements of, for instance, the production, transport and dissipation of turbulence and have made a valuable contribution to our understanding of the processes in the surf zone. The first models that were developed were primarily based on assumptions directly derived from such observations. Since the development of the first numerical models in the mid-eighties, much research effort has been put into trying to improve these wave-averaged models because they can be applied at relatively low computational cost. The improved understanding of the surf-zone processes has also led to the development of more advanced intrawave models such as the Boussinesq-based models as well as the use of Navier–Stokes solvers. These new modelling techniques give a detailed description of the processes in the surf zone.

Modeling of a mixed‐load fluvio‐deltaic system

Present?day observations and classical classification schemes of alluvial deltas address feeder channel dynamics and multiple sediment fractions. However, high?resolution physics?based mathematical models have not been applied to address formation of both fluvio?deltaic links (channels) and nodes (diffluences and confluences), and their stratigraphy. Here, we present a simulated delta system under riverine forcing that shows striking similarity to its counterparts recognized in field and laboratory studies. These findings include distinct shifts in river planimetrical mode and altimetry, deltaic mouth bar and distributary formation, lateral fining in migrating?meander bend axes and fining?upward patterns in passive delta?plain distributaries.

MORPHOLOGICAL MODELLING OF KETA LAGOON CASE

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

ROLLER CONTRIBUTIONS AS INFERRED FROM INVERSE MODELLING TECHNIQUES

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

INITIAL MORPHOLOGIC AND STRATIGRAPHIC DELTA EVOLUTION RELATED TO BUOYANT RIVER PLUMES

Abstract : This paper presents the results of a numerical model study on initial delta formation in basins of different water depths. A process-based model (Delft3D-Online) is used to calculate fluid flow and sediment transport patterns at a newly formed river effluent. In abse nce of wave and tidal forcing, the river effluent is fully river dominated. The model results are interpreted in terms of morphology and stratigraphy. For the shallow basin case, the delta develops as a complex pattern of channels and shoals analogous to Wax delta. The simulated delta development in the deep basin is markedly different and resembles a classic prograding bird foot delta morphology. Typical clay-rich prodelta deposits underlie sandy bar and channel deposits. The top of the sedimentary sequence consists of a clay -rich delta plain deposit. The potential of the process-based model to simulate typical delta morphologies and a realistic stratigraphy provides many new opportunities to bridge typical engineering and geological research approaches.

Time and spatial variability of sediment grading in the surfzone of a large scale nourishment

ABSTRACT Huisman, B.J.A., Sirks, E.E., van der Valk, L., Walstra, D.J.R., 2014. Time and spatial variability of sediment gradingin the surfzone of a large scale nourishment. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 127–132–134, ISSN 0749-0208. Temporal and spatial variations in sediment gradation in the surfzone were investigated for a large scale nourishment (Sand Motor) at the Dutch coast (~ 21.5 million m3 sand). This nourishment provides an excellent opportunity to investigate the effect of hydrodynamic conditions on the spatial and temporal development of sediment grading. A strong point of the Sand Motor is that it has a well-known initial sediment composition. Measurements of the sediment grading at the Sand Motor were carried out during and after its construction to investigate the sorting process. Samples were taken for six cross-shore rays at ten depth contours (from MSL -1m to MSL -10m). A weighted average of the grain size distribution was determined for each cross-shore transect to investigate the alongshore sorting process. Similarly, cross-shore averaged sediment properties were determined for the samples above and below MSL -4m to assess the cross-shore sorting process. The sieved samples showed that: (1) significant sediment sorting takes place across the surfzone of the Sand Motor. Typically, the sediment in the surfzone of the exposed part of the Sand Motor is 20 to 30% coarser than the average for the survey; (2) a large depositional area with relatively fine material (150 to 200 μm) from the Sand Motor has developed on the Northern side of the Sand Motor; and (3) an alongshore band with finer sediment was found at water depths of 4 to 6 meter below MSL as a result of selection processes at the waterline.

THE EFFECTS OF BED SLOPE AND WAVE SKEWNESS ON SEDIMENT TRANSPORT AND MORPHOLOGY

The potential relevance of wave skewness and sloping beds on morphodynamic predictions is investigated. Based on a comparison with measurements it was shown that the phase lag is potentially important for modeling nearshore bar dynamics as it influences the suspended wave related transports (magnitude and direction). Velocity skewness results in a phase shift between the orbital motion and the instantaneous bed shear stress increasing onshore bed load transport. It was seen that its influence is relatively small but does affect bar migration and helps to predict the observed asymmetric bar shape. The bed slope affecting the initiation of motion and sediment bed-load transport magnitude and direction are investigated by comparing scour hole predictions. The initiation of motion, which is also included on the suspended transports, only has an effect for somewhat steeper slopes. The modifications of bed-load direction and magnitude significantly affect the horizontal and vertical scour hole equilibrium dimensions.