On the anatomy of nearshore sandbars

Abstract

Nearshore sandbars have a lifetime of many years, during which they exhibit cyclic, offshore directed behaviour with strong alongshore coherence. A bar is generated near the shoreline and grows in height and width while migrating offshore, before finally decaying at the seaward limit of the surf zone. It may take 10 to 15 years for a bar to exhibit this cycle. Four to five bars may occur simultaneously within a cross-shore bed profile. Alongshore variations in cross-shore bar position and bar amplitude are commonly observed. A strong or abrupt alongshore variability is referred to as a bar switch. At large spatial scales, the inter-annual bar dynamics may vary considerably across sites with very similar environmental settings. In particular, the bar cycle return period (Tr, i.e. the duration between two successive bar decay events) may differ by a factor of three to four. This type of change in Tr appears to be always present in time and is characterized as a persistent bar switch. At smaller (kilometer) scales, bar switches typically occur in areas with similar Tr-values on both sides of a bar switch and occasionally disappear when the bars re-attach. These are characterized as non-persistent bar switches.

The assimilation of shoreface nourishments into the coastal system involves a strong interaction with the pre-existing sandbar system. Typically the placement of a shoreface nourishment just seaward of an outer bar reverses the bar cycle temporalily, inducing a landward migration of the bar system. The shoreface nourishment becomes absorbed in the coastal system as the new outer bar. At the distal ends of the shoreface nourishment bar switches often manifest, owing to a distinct difference in the bar migration cycle phase that is induced. Given the importance of the bar-nourishment interaction, an improved understanding of the nearshore bar dynamics is expected to improve the efficacy of shoreface nourishments. Furthermore, the long-term evolution of the nearshore barred profiles is generally considered indicative of the quality of the modelling for the response of the entire nearshore coastal system. Therefore, the ability to perform reliable and robust a-priori, long-term predictions has broad societal relevance in view of anticipated adverse impacts of climate change and sea level rise on the stability of coasts worldwide. Until now the anatomy of the nearshore sandbars has primarily been studied using field data. Although these studies have provided insight into how the geometric bar parameters respond to the external forcings, no comprehensive conceptual framework is available that explains the full life cycle of a sandbar and its associated characteristics. The overarching objective of this study is to elucidate the anatomy of the inter-annual bar morphology using a combined data and model approach. This overarching objective is in turn devolved into three objectives aiming to understand key features of bar morphology and a further objective to enable a comprehensive modelling approach based on the acquired insights. The latter objective involves the development of an input-reduction framework for advanced process-based forward modelling of the inter-annual bar morphology. 
 1) To elucidate the morphodynamic processes that result in cross-shore transient sandbar amplitude responses (i.e. the transition from bar growth in the intertidal and across surf zone to sandbar decay at the seaward edge of the surf zone). 2) To establish the role of cross-shore processes in non-persistent bar switches.
3) To identify the dominant environmental variables and the associated mechanisms that govern the bar cycle return period. 4) To develop an input-reduction framework to enable the application of state-of-the-art process based forward area models to simulate the multi-annual bar behaviour and nearshore morphology. 
 A comprehensive study approach is adopted in which observations of the nearshore morphology are combined with detailed forward modeling of the bar dynamics at Noordwijk (The Netherlands) utilizing wave and waterlevel observations as boundary conditions. The Noordwijk model acts as a reference for additional simulations at Egmond (The Netherland) and at Hasaki (Japan) to address the specific characteristics of the nearshore sandbar morphodynamics as outlined above. 
The transient cross-shore bar amplitude response Based on a three-year hindcast of a bar cycle at Noordwijk (Netherlands) and on additional synthetic runs using a wave-averaged cross-shore process model, the dominant mechanisms that govern the bar amplitude growth and decay during net inter-annual offshore migration are identified. The bar amplitude response is particularly sensitive to the water depth above the bar crest, hXb, and the angle of wave incidence, θ. These variables largely control the amount of waves breaking on the bar and the strength and cross-shore distribution of the associated longshore current. The longshore current has its maximum landward of the bar crest, inducing additional stirring of sediment on the landward bar slope and trough. The enhanced sediment concentration in the trough region shifts the cross-shore transport peak landward of the bar crest, forcing bar amplitude growth during offshore migration. For increased hXb-values wave breaking becomes less frequent, reducing the influence of the longshore current on sediment stirring. Therefore, the resulting dominance of the cross-shore current results in a sediment transport peak at, or just seaward of, the bar crest causing bar amplitude decay. All four types of bar response (viz. all combinations of onshore/offshore migration and bar amplitude growth/decay) can occur for a single wave height and wave period combination, depending on hXb and θ. Additional hindcast runs in which the wave direction was assumed time-invariant confirmed that hXb and θ largely control the transient bar amplitude response. 
The mechanics of non-persistent bar switches Intra-site alongshore variability is greatest when bars display km-scale disruptions, indicative of a distinct alongshore phase shift in the bar cycle. An outer bar is then, for example, attached to an inner bar, referred to as a non-persistent bar switch. This large-scale alongshore variability is investigated by applying the reference model at 24 transects along a 6 km section of the barred beach at Noordwijk (The Netherlands). When alongshore variability is limited, the model predicts that the bars migrate offshore at approximately the same rate (i.e. the bars remain in phase). Only under specific bar configurations with high wave-energy levels is an increase in the alongshore variability predicted. This suggests that cross-shore processes may trigger a switch in the case of specific antecedent morphological configurations combined with storm conditions. It is expected that three-dimensional (3D) flow patterns augment the alongshore variability in such instances. In contrast to the observed bar behaviour, predicted bar morphologies on either side of a switch remain in different phases, even though the bars are occasionally located at a similar cross-shore position. In short, the 1D profile model is not able to remove a bar switch. This data-model mismatch suggests that 3D flow patterns are key to the dissipation of bar switches. 
The mechanics of persistent bar switches and the bar cycle return period To date, data-analytic studies have had only partial success in explaining differences in Tr, establishing at best weak correlations to local environmental characteristics. In the present approach the process-based profile reference model is utilized to investigate the non-linear interactions between the hydrodynamic forcing and the morphodynamic profile response for two sites. Despite strong similarity in environmental conditions, the sites at Noordwijk and Egmond on the Holland coast exhibit distinctly different Tr values. The detailed comparison of modelling results enables a consistent investigation of the role of specific parameters at a level of detail that could not have been achieved from observations alone, and provides insights into the mechanisms that govern Tr. The results reveal that the bed slope at 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 hXb 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 behaviour of the bar. The initial behaviour is determined by the intense wave breaking associated with the steeper profile slope. These mechanisms explain 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. 
Input reduction for inter-annual advanced forward model applications In order to avoid excessively long computation times, input reduction is imperative for the application of advanced forward morphodynamic area models to consider long-term (>years) predictions. Here, an input reduction framework for wave-dominated coastal settings is introduced. The framework comprises 4 steps, viz. (1) the selection of the duration of the original (full) time series of wave forcing, (2) the selection of the representative wave conditions, (3) the sequencing of these conditions, and (4) the time span after which the sequence is repeated. In step (2), the chronology of the original series is retained, while that is no longer the case in steps (3) and (4). We apply the framework to two different sites (Noordwijk, The Netherlands and Hasaki, Japan) with multiple nearshore sandbars but contrasting long-term offshore-directed behaviour: at Noordwijk the offshore migration is gradual and not coupled to individual storms, while at Hasaki the offshore migration is more episodic, and wave chronology appears to control the long-term evolution. The performance of the model with reduced wave climates is compared with a simulation with the actual (full) wave-forcing series. It is demonstrated that input reduction can dramatically affect long-term predictions, to such an extent that the main characteristics of the offshore bar cycle are no longer reproduced. This was the case at Hasaki, in particular, where all synthetic series that no longer retain the initial chronology (steps 3 and 4) lead to rather unrealistic long-term simulations. At Noordwijk, synthetic series can result in realistic behaviour, provided that the time span after which the sequence is repeated is not too large; the reduction of this time span has the same positive effect on the simulation as increasing the number of selected conditions in step 2. It is further demonstrated that, although storms result in the largest morphological change, conditions with low to intermediate wave energy must be retained to obtain realistic long-term sandbar behaviour. The input-reduction framework must be applied in an iterative fashion to obtain a reduced wave climate that is able to simulate long-term sandbar behaviour sufficiently accurately within an acceptable computation time. These results imply that it is essential to consider input reduction as an intrinsic part of any model set-up, calibration and validation effort.

The study outcomes indicate clearly that a relatively simple model can be utilized to study the highly non-linear interaction between the nearshore hydrodynamics and morphology in great detail. This was achieved through carefully designed numerical experiments in which the influence of a specific process or environmental variable was isolated and identified. Although the model only considers cross-shore processes, the numerical experiments generated new insights into the importance of 3D processes under particular morphological conditions of the nearshore barred profiles. Even though the model was successfully calibrated at Noordwijk, the application at Egmond showed a significantly reduced predictive capacity. The model was able to reproduce the main characteristics of the inter-annual bar morphodynamics, but the bar cycle return period was under-estimated by about 30%. This suggests that the model can capture trends fairly well, but is unable to produce accurate absolute predictions - a finding that has broader implications. As stated earlier, accurate predictions of the long-term evolution of the nearshore barred profiles are generally considered indicative of the quality of the modelling of the entire nearshore coastal system. Consequently, further improvement of morphodynamic process-based models, particularly for the nearshore zone, constitutes a major research priority.