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Subtidal nearshore sandbars may exhibit cyclic net offshore migration during their multi-annual lifetime along many sandy coasts. Although this type of behavior can extend continuously for several kilometers, alongshore variations in cross-shore bar position and bar amplitude are commonly observed. 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 attached to an inner bar, forming a phenomenon known as a bar switch. Here, we investigate such large-scale alongshore variability using a process-based numerical profile model and observations 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 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. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Input reduction is imperative to long-term (>years) morphodynamic simulations to avoid excessive computation times. Here, we introduce an input-reduction framework for wave-dominated coastal settings. Our 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, Netherlands and Hasaki, Japan) with multiple nearshore sandbars but contrasting long-term offshore-directed behavior: 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 long-term evolution. The performance of the model with reduced wave climates is referenced to a simulation with the actual (full) wave-forcing series. We demonstrate that input reduction can dramatically affect long-term predictions, even to such an extent that the main characteristics of the offshore bar cycle are no longer reproduced. This was particularly the case at Hasaki, where all synthetic series that no longer capture the initial chronology (steps 3 and 4) lead to rather unrealistic long-term simulations. At Noordwijk, synthetic series can result in realistic behavior, 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. We further demonstrate 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 behavior. Our input-reduction framework must be applied in an iterative fashion as to obtain a reduced wave climate that simulates long-term sandbar sufficiently accurately within an acceptable computation time. Given its potentially huge impact on the actual simulation, we believe it is imperative to consider input reduction as an intrinsic part of model set-up, calibration and validation. [Copyright &y& Elsevier]

Abstract: Multiple bar systems often show a cyclic net offshore directed migration with return periods on the order of years. Generally, a bar is generated near the shoreline, grows in height and width, while migrating offshore before finally decaying at the seaward limit of the surf zone. 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, this study identifies the dominant mechanisms that govern the bar amplitude growth and decay during net interannual offshore migration. The bar amplitude response is particularly sensitive to the water depth above the bar crest, h Xb , 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 transports peak landward of the bar crest forcing bar amplitude growth during offshore migration. For increased h Xb -values wave breaking becomes less frequent, consequently 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 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 h Xb and θ. Additional hindcast runs in which the wave direction was assumed time-invariant confirmed that h Xb and θ largely control the transient bar amplitude response. [Copyright &y& Elsevier]

Abstract: Although beach nourishment policy in the Netherlands is generally successful in maintaining the 1990 shoreline, the efficiency of nourishments at several locations along the North Sea coast is unacceptably low. These locations are identified as “erosion hotspots”. Egmond aan Zee is such a hotspot. The bathymetric longshore irregularities make this coastal stretch highly susceptible to rip currents and horizontal circulations, which transport large volumes of (nourished) sand to the sea. In 1999, an alternative nourishment design was implemented. This consisted of a shoreface nourishment at the outer bar (at 7.5-m depth) combined with a nourishment of the beach behind it. High-resolution bathymetric surveys, carried out with the WESP, revealed that the shoreface nourishment resulted in a significant shoreward migration of the outer bar during 1999–2001. After 2 years, the system started to return to its natural dynamic equilibrium and lost the positive influence of the shoreface nourishment area: shoreward movement of bars and increase of bar height, which forms the first defense line of the mainland against the sea, and positive contribution to the sand budget, which increased as a result of the nourishment. After 3 years (May 1999 to April 2002), the sediment volume still had an increase of 477,500 m3 (45% of the applied sand) relative to May 1999. The shoreface nourishment area did not have a direct effect on the beach. The shoreface nourishment area probably will have to be maintained to retain the sediment on the beach in the long run. In Egmond, the shoreface nourishment started to disappear before the sediment could reach the beach. The time scale of sediment feed to the beach zone probably is of the order of 5–10 years. The shoreface nourishment seems to function as a reef, creating a lee-side effect that influences both cross-shore and longshore sediment transport, resulting in two main hypotheses. Firstly, the longshore effect, in which large waves break at the shoreface nourishment, cause a calmer wave climate shoreward of the shoreface nourishment area and a reduction of the longshore current. The shoreface nourishment partially blocks the wave-driven longshore current. Secondly, the cross-shore effect, in which shoaling waves generate onshore transport. These hypotheses are supported by generated model results with both a process-based profile model, UNIBEST-TC, and a process-based coastal area model, DELFT3D-MOR. The UNIBEST-TC model was able to predict the detachment of the outer bar from the shoreface nourishment and, to some extent, the shoreward bar movements. In the lee of the shoreface nourishment area, the model found an increase in sediment, confirming the shielding effect of the shoreface nourishment area. The DELFT3D model found, as a result of the shoreface nourishment area, a decrease of the flow velocity and the wave height just shoreward of the shoreface nourishment area. The model was able to predict the similar trend in sedimentation/erosion pattern, although the intensities were smaller. In the lee of the shoreface nourishment area, a clear sedimentation was found. Erosion was found south and seaward of the shoreface nourishment area. The modeled profiles did not follow the measured profile changes. The separation of the shoreface nourishment from the outer bar and the shoreward migration of the bars could not be accurately modeled. Because the cross-shore transport caused by wave asymmetry is not taken into account in the standard version of DELFT3D, these results were in line with the expectations. [Copyright &y& Elsevier]