Based on model hindcasts of the bar cycle at two locations along the Dutch coast, the dominant processes and mechanisms that govern the bar amplitude growth and decay during net inter-annual offshore migration, the occurrence of bar switches and the inter-site differences in the bar cycle return period (Tr) are identified. Bar growth and decay are closely related to the wave-induced longshore current as it affects the distribution of the cross-shore sediment transport. The modelling results suggest that cross-shore processes may trigger a bar switch in the case of specific antecedent morphological configurations combined with storm conditions. The deceleration of the offshore migration rate as the bar moves to deeper water (the morphodynamic feedback loop) contrasts with the initial enhanced offshore migration behavior of the bar for steeper slopes. The bed slope in the barred zone is the most important parameter governing Tr.

Inter-annual bar dynamics may vary considerably across sites with very similar environmental settings. In particular, the variability of the bar cycle return period (Tr) may differ by a factor of 3 to 4. To date, data studies are only partially successful in explaining differences in Tr, establishing at best weak correlations to local environmental characteristics. Here, we use a process-based forward model to investigate the non-linear interactions between the hydrodynamic forcing and the morphodynamic profile response for two sites along the Dutch coast (Noordwijk and Egmond) that despite strong similarity in environmental conditions exhibit distinctly different Tr values. Our exploratory modeling enables a consistent investigation of the role of specific parameters at a level of detail that cannot be achieved from observations alone, and provides insights into the mechanisms that govern Tr. The results reveal that the bed slope in the barred zone is the most important parameter governing Tr. As a bar migrates further offshore, a steeper slope results in a stronger relative increase in the water depth above the bar crest which reduces wave breaking and in turn reduces the offshore migration rate. The deceleration of the offshore migration rate as the bar moves to deeper water—the morphodynamic feedback loop—contrasts with the initial enhanced offshore migration behavior of the bar. The initial behavior is determined by the intense wave breaking associated with the steeper profile slope. This explains the counter-intuitive observations at Egmond where Tr is significantly longer than at Noordwijk despite Egmond having the more energetic wave climate which typically reduces Tr.

A state-of-the-art process-based model is applied to hindcast the morphological development of a shoreface nourishment along the barrier island of Terschelling, The Netherlands. The applied morphodynamic model is Delft3D for fully three-dimensional flow and sediment transport in coastal environments. Owing to a complex geomorphological setting of the study area, the curvi-linear model includes adjacent tidal inlets and covers 40 x 70 km with an increasing grid size resolution towards the nourishment site in the center of the island. In 1993, a total of 2 Mm(3) of sand was supplied to the nearshore bar zone off Terschelling, filling up the trough between the middle and outer bar. By spring 1994, most of the nourished sediment had been redistributed onshore and welded onto the middle bar where it remained in the following years. The morphodynamic model is applied to the prediction of this rapid nearshore profile behaviour. The calibration of the model against an extensive set of full-scale hydrodynamic data at several locations throughout the nearshore bar zone shows a good representation of the measured hydrodynamics. Morphodynamic results show a dependency on spatial scale: on the scale of precise bed level evolution with respect to bar migration and growth, model predictions are poor as the nearshore bars are predicted to flatten out. Resorting to bulk volumes integrated over larger spatial scales, the model clearly has skill in predicting the overall effects of the nourishment. The lack of phase shift between sediment transport and bathymetry is identified as the key controlling factor for the poor sandbar predictions. (C) 2004 Elsevier B.V. All rights reserved.

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 gamma (a wave height-to-depth
ratio) are used. In this paper, a new functional form for gamma is
derived empirically by an inverse modelling of gamma 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, gamma
is not cross-shore constant, but depends systematically on the product
of the local wavenumber k and water depth It. 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 gamma are indeed in
better agreement with measurements than model predictions using the
cross-shore constant gamma. 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
gamma to be applicable to non-barred beaches as well. Our optimum gamma
cannot. be compared directly to field and laboratory measurements of
height-to-depth ratios and we do not know of a physical mechanism why
gamma should depend positively on kh.