Net Offshore Research Articles

Video monitoring of sandbar-shoreline response to an offshore submerged structure at a microtidal beach

In early 2013, an 800-m long and 12-m wide submerged breakwater with its crest in 2-m depth was implemented at the wave-dominated barred beach of Sète, SE France, to fight against erosion and submersion hazards. Daily video images from April 2011 to April 2016 covering an alongshore distance of 3.5km are used to analyse the response of both the sandbar(s) and the shoreline to the structure implementation. Results show that the breakwater had a profound impact on the nearshore system, both shoreward of the structure and well away from it alongshore. A progressive rotation and linearisation of the sandbar was observed shoreward of the submerged breakwater. This resulted in the splitting of the sandbar adjacent to the structure during a net offshore sandbar migration event driven by a severe storm in December 2013. The typical formation of a salient or tombolo was not observed. Instead, shoreline coupled to the sandbar geometry, which resulted in a slight seaward migration of the shoreline in front of the structure. Overall, this study highlights that the role of the sandbar is critical to shoreline response to the implementation of breakwaters on barred beaches.

Cross‐shore transport of nearshore sediment by river plume frontal pumping

AbstractWe present a new mechanism for cross‐shore transport of fine sediment from the nearshore to the inner shelf resulting from the onshore propagation of river plume fronts. Onshore frontal propagation is observed in moorings and radar images, which show that fronts penetrate onshore through the nearshore and surf zone, almost to the waterline. During frontal passage a two‐layer counterrotating velocity field characteristic of tidal straining is immediately set up, generating a net offshore flow beneath the plume. The seaward flow at depth carries with it high suspended sediment concentrations, which appear to have been generated by wave resuspension in the nearshore region. These observations describe a mechanism by which vertical density stratification can drive exchange of material between the nearshore region and the inner shelf. To our knowledge these are the first observations of this frontal pumping mechanism, which is expected to play an important role in sediment transport near river mouths.

Predicting net cross-shore total load transport: A phase-averaging, quasi-steady approach incorporating undertow contribution

Wave shapes that induce velocity skewness and acceleration asymmetry are usually responsible for onshore sediment transport, whereas undertow and bottom slope effect normally contribute to offshore sediment transport. By incorporating these counteracting driving forces in a phase-averaged manner, the theoretically-based quasi-steady formula of Wang (2007) is modified to predict the magnitude and direction of net cross-shore total load transport under the coaction of wave and current. The predictions show an excellent agreement with the measurement data on medium and fine sand collected by Dohmen-Janssen and Hanes (2002) and Schretlen (2012) in a full-scale wave flume at the Coastal Research Centre in Hannover, Germany. The modified formula can predict the net onshore transport of fine sand in sheet flows. In particular, it can predict the net offshore transport of medium sand in rippled beds through enlarged bed roughness, as well as the net offshore transport of fine-to-coarse sand in sheet flows with the aid of a new criterion to judge the occurrence of net offshore transport.

Sandbar Migration and Shoreline Change on the Chirihama Coast, Japan

Sandy beaches play a key role in regional tourism. It is important to understand the principal morphological processes behind preserving attractive beaches. In this study, morphological variation on the Chirihama Coast, Japan, an important local tourism resource, was investigated using two sets of field surveys. The objective was to analyze and document the multi-scale behaviors of the beach. First, long-term shoreline changes were examined based on shoreline surveys over the last two decades. Then, the middle-term behavior of multiple bar systems was analyzed based on the cross-shore profile surveys from 1998 to 2010. An empirical orthogonal function (EOF) analysis was conducted to capture the principal modes of the systematic bar migration. The shoreline analysis indicated a long-term eroding trend and showed that the seasonal variation has recently tended to increase. The profile analysis demonstrated that net offshore migrations of bars have been repeated with a return period of approximately four years. This general behavior of the bar system is similar to the net offshore migration phenomena observed at other sites in the world. EOF analysis revealed a relationship between bar configuration and middle-term variations in shoreline location; when a new bar is generated near the shoreline and a triple bar configuration is established, the shoreline tends to temporarily retreat, whereas the shoreline experiences an advance when the outer bar has most evolved.

The influence of seasonal to interannual nearshore profile variability on extreme water levels: Modeling wave runup on dissipative beaches

Wave runup, an important contributor to storm-induced extreme water levels, is commonly predicted via empirical formulations that parameterize coastal morphology using simple metrics such as the foreshore beach slope. However, spatially and temporally complex nearshore morphology, such as subtidal sandbars, have the potential to alter surf zone wave dissipation patterns and therefore influence setup, swash, and runup levels observed at the shoreline. In this study, a suite of numerical experiments using XBeach demonstrate reasonable skill in reproducing wave runup observations in dissipative settings, explore the relative influence of seasonal to interannual variability in nearshore morphology on runup and its constitutive components, and illustrate differences between empirical and numerically modeled estimates of runup. The numerical model results show that interannual variability in sandbar configuration, associated with net offshore sandbar migration, has a larger influence on wave runup than does seasonal sandbar variability. Although the particular configuration of sandbars was estimated to influence runup by as much as 0.18m during storm conditions, natural variability in subaerial beach topography has a stronger influence on runup than subtidal morphology. XBeach demonstrates that both wave setup and infragravity swash have morphologic controls. In experiments simulating storm conditions in which both nearshore and beach morphology was varied, natural interannual variability in beach topography explained about 80% of the variance in runup and its constituents. While XBeach predictions of setup, swash, and runup compare favorably with empirical predictors for low wave conditions, the numerical model predicts higher runup levels for storm-conditions on dissipative beaches raising potential concerns about coastal hazards assessments that use these empirical models to estimate extreme total water levels.

Impact of sea-level rise on cross-shore sediment transport on fetch-limited barrier reef island beaches under modal and cyclonic conditions

A one-dimensional wave model is combined with an analytical sediment transport model to investigate the likely influence of sea-level rise on net cross-shore sediment transport on fetch-limited barrier reef and lagoon island beaches. The modelling considers if changes in the nearshore wave height and wave period in the lagoon induced by different water levels over the reef flat are likely to lead to net offshore or onshore movement of sediment. The results indicate that the effects of SLR on net sediment movement are highly variable and controlled by the bathymetry of the reef and lagoon. A significant range of reef-lagoon bathymetry, and notably shallow and narrow reefs, appears to lead hydrodynamic conditions and beaches that are likely to be stable or even accrete under SLR. Loss of reef structural complexity, particularly on the reef flat, increases the chance of sediment transport away from beaches and offshore.

Regional scale sandbar variability: Observations from the U.S. Pacific Northwest

Understanding sandbar dynamics and variability is integral to developing a predictive capacity for nearshore flows, sediment transport, morphological change, and ultimately for determining coastline exposure to damaging storm waves. Here we report on a nearshore bathymetric data set from the U.S. Pacific Northwest (PNW) that stretches from Point Grenville, Washington to Cascade Head, Oregon, over approximately 260km in the alongshore and includes 8 distinct littoral cells. We describe and quantify the morphological variability of sandbars on a regional scale, using 560 individual cross-shore transects, as well as attempt to explain the inter-littoral cell variability via relationships to various environmental parameters. The cross-shore extent of the bar zone extends over 1km from the shoreline in the northern part of the study area, but only to about 600m from the shoreline in the southern part. Maximum bar crest depths are typically 7m below MLLW. Bar heights range from a step in the cross-shore profile to over 3m from crest to trough. The northernmost littoral cells typically have two or more subtidal sandbars per cross-shore profile whereas the littoral cells in the southern part of our study area have only one bar. The mean depths of the bars, however, are much more consistent across littoral cells even while the upper shoreface slope significantly increases from north to south, requiring that the maximum bar distance from the shoreline decreases from north to south. Results from a limited study of the temporal variability suggest that while data collected over large spatial scales captures significant amounts of overall sandbar variability, it does not completely characterize the variability over the entirety of the net offshore migration cycle.

Characteristics of Sediment Concentration and Suspended Sediment Transport Due to Horizontal and Vertical Asymmetric Waves

Wave phenomenon in ocean often has the shape in the horizontal and vertical asymmetric waves when propagating in the near-zone with high non-linearity. The orbital velocity due to the wave asymmetriesand nonlinearities could generate the flow patterns that drives cross-shore sediment transport in the near-shore zone. A new calculation method for bottom shear stresses and bed-load sediment transport rate for the horizontal and vertical asymmetries waves proposed by1,2was implemented to model the concentration sediment and suspended sediment transport. The characteristics of the concentration sediment and suspended load induced by the horizontal and vertical asymmetries waves have been examined. The modeling results showed that the smaller α and the higher Ni indicate more wave skew-ness and more non-linearity of waves causing a higher the bottom shear stress and the net bed load transport while the net suspended load transport decrease with the decreasing of the wave skew-ness. The higher skewed waves indicated the higher acceleration causing increase the value of the net onshore transport rate for bed load sediment transport and decrease the value of the net offshore transport rate for suspended sediment transport.

Process‐based modeling of kilometer‐scale alongshore sandbar variability

AbstractSubtidal 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.

Dynamical structure of the sea off the east coast of Peninsular Malaysia

The regional ocean modelling system (ROMS), at a resolution of 9 km, is used to investigate the dynamics of the sea off the east coast of Peninsular Malaysia (ECPM). The model is configured with two, one-way nested domains. Two simulations are performed in order to understand the dynamical structure of the sea off ECPM. The first simulation is a control run, using climatological monthly mean wind stress, surface freshwater flux, heat and observational oceanic inflow and outflow at open boundaries. The second simulation is an experiment aimed at presenting the seasonally averaged effect of isolated forcing in the absence of wind stress. This procedure allows understanding of the upwelling mechanism in the absence of wind stress forcing within the region. The model simulated the oceanographic features in the region reasonably well, in particular the circulation and temperature patterns conformed to those of Simple Ocean Data Assimilation and observations. Results show the possibility of upwelling in the summer monsoon along the sea off ECPM. This is due to the strong long-shore wind stress, which coincided with lower sea surface height and high baroclinic instability. The strong positive horizontal transport in accordance with the positive vertical transport within 104.5° E and 105.5° E, at latitude 3° N suggests net offshore transport and the occurrence of upwelling. Moreover, results demonstrate that summer upwelling rate at corresponding latitude induced by the long-shore wind stress is 3 × 10−5 m/s larger than the vertical velocity of 0.5 × 10−5 m/s induced by the wind stress curl. This reflects the importance of the long-shore wind stress for inducing the coastal upwelling. High concentrations of phytoplankton biomass through the proxy of chlorophyll-a concentration and the observed upward motion of denser water at the sea surface are due to the upwelled nutrient-rich water.

Evaluating the PEM passive beach drainage system in a 4-year field experiment at Egmond (The Netherlands)

The PEMs' (Pressure Equalizing Modules) passive vertical drainage system was tested at the coast just south of Egmond (The Netherlands) from November 2006 to December 2010. The present study aims at providing qualitative and quantitative insight into the impact of the PEMs on the nearshore morphological development at Egmond. The evaluation is based on a comparison of the annually observed temporal evolution of the nearshore morphology prior, during and after the PEM installation period. To this end a number of aggregated volumetric profile parameters derived from the surveys were compared for the test area and for an adjacent area which acted as a reference. Besides the PEMs, the study area is also influenced by beach and shoreface nourishments. Therefore, the nourishments carried out just north and south of the Test and Reference areas in 2004 and 2005 prior to the installation of the PEMs were specifically considered in the analysis. From 2008 onward the morphological behavior of the nearshore zone was dominated by the alongshore coherent net offshore migration of the multiple bar system and no obvious feeding of sand from the nourishments toward the Test and Reference areas could be identified. The influence of the nourishments, the PEMs, and the natural variability were difficult to distinguish at the Test and Reference areas. Therefore, the morphological development of the Test area relative to that of the Reference area was adopted as the primary evaluation criteria. Interestingly, the installation period (2007–2010) exhibited statistically significant different CSIs (coastal state indicators) compared with the 4years prior to installation (2003–2006) for both the Test and the Reference areas. Because both the Test and the Reference areas exhibited very similar behavior prior, during and after the PEM installation period, the analysis was unable to identify the impact of the PEMs. No influence of the PEMs on the local beach and dune morphology could be established. Considering the above and given the fact that the PEM system was also not able to induce a measurable accretion at a similar experiment in Denmark we consider it unlikely any efficacy of the PEM system will be identified in future experiments.

Investigating the role of complex sandbar morphology on nearshore hydrodynamics

ABSTRACT Cohn, N., Ruggiero, P., Ortiz, J., D.J. Walstra, 2014. Investigating the role of complex sandbar morphology on nearshore hydrodynamics. 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. 053–058, ISSN 0749-0208. Coastal environments are characterized by complex feedbacks between flow, sediment transport, and morphology, often resulting in the formation of nearshore sandbars. In many locations, such as Hasaki (Japan), the Netherlands, and the Columbia River Littoral Cell (CRLC, USA), these sandbars exhibit a net offshore migration (NOM) cycle whereby these features form in the inner surf zone, migrate seaward and decay offshore on interannual cycles. Depending on the stage of the cycle, the number and configuration of the bars may differ widely. It has long been recognized that sandbars act as natural barriers during storm events by dissipating wave energy through breaking far fr...

Microbial biogeochemistry of coastal upwelling regimes in a changing ocean

The coastal upwelling regimes located along the eastern boundaries of the Pacific and Atlantic Ocean basins are small in size, but make a disproportionate contribution to the productivity and microbial biogeochemistry of the ocean. These regimes include the California and Peru/Humboldt current systems in the Pacific, and the Canary and Benguela current systems in the Atlantic. Their characteristic equatorward wind patterns lead to the net offshore transport of surface water, allowing cold, nutrient-laden water to upwell into the illuminated zone where phytoplankton photosynthesis occurs. Rich upwelled supplies of nitrate, phosphate and silicate, and the tremendous blooms of resident phytoplankton they support, render these regimes the ‘new production factories’ of the world ocean 1

Spatiotemporal Variation in Cross-Shelf Exchange across the Inner Shelf of Monterey Bay, California

AbstractCross-shelf exchange resulting from wind- and wave-driven flows across the inner shelf has been the focus of a considerable body of work. This contribution extends recent analyses to the central California coastline using 5-yr of moored current observations. Acoustic Doppler Current Profiler (ADCP) data from stations across the Monterey Bay (two in the northern bay and one in the southern bay), in water depths of ~20 m, showed net offshore transport throughout the year. For the northern bay sites, cross-shelf exchange was dominated by Ekman transport driven by along-shelf diurnal sea breezes during the upwelling season. Intense stratification in the northern bay leads to very shallow observed Ekman layers (~5–8 m), and consequently no overlap between bottom and surface Ekman layers within a few hundred meters of the coast. The total transport is less than predicted by theory consistent with models of shallow-water Ekman transport. The observed transport (~42% of full Ekman transport) is shown to be caused by the influence of a positive vorticity that effectively increases the Coriolis parameter. Wave-driven return flow estimated from an offshore buoy was strongly correlated with observed transport during nonupwelling conditions for the northern, outer bay site, but not for the two inner bay sites (northern and southern). In the southern bay, winds and waves have a significantly reduced effect on the cross-shelf exchange. Internal tidal bores are believed to contribute most of the observed cross-shelf exchange in this region.

Net offshore bar migration variability at a regional scale: Inter-site comparison (Languedoc-Roussillon, France)

Aleman, N., Robin, N., Certain, R., Barusseau, J.-P. and Gervais, M., 2013. Net offshore bar migration variability at a regional scale: Inter-site comparison (Languedoc-Roussillon, France).The Languedoc-Roussillon coastline is a large unit stretching out over 200 km of sandy low coast in a wave dominated environment. The nearshore is characterized by a quasi continuous system of double sandbar that displays a wide range of typology. The interannual sandbar dynamic was investigated using 2D bathymetric profiles and 3D LiDAR imagery. This study has allowed determining the sandbar systems affected by the Net Offshore Migration (NOM). At a regional scale, conditions necessary to NOM development depend on the sandbar morphologies (crescentic or straight), the wave energy and the associated coastal orientation (low or high energy, sheltered area), the coastal structures (harbour and coastal defences) and the nearshore sedimentary budget. The areas where the NOM occurs show cycle dynamic differences. This regional inter-site comparison highlights that nearshore morphology and bar parameters seem to influence the nearshore bar behaviour. Interaction between the nearshore slope, width of the bar zone and the migration rate control the NOM duration. The sandbar volume and the regional wave climate influence also the migration rate of the system. On the Languedoc-Roussillon coast, the sediment grain size does not appear to influence the seaward bar migration.

石川海岸における沿岸砂州の形成・移動・消失に関する長期変動解析

This study analyzes a bathymetric dataset sampled annually for 51 years along the Ishikawa Coast, Japan, where the morphological variation is characterized by the cyclic cross-shore migration of bars with approximately 10-year return period. Five cycles have been identified from 1960 to 2010. The variability of spatial pattern during individual cycle is analyzed to quantify the characteristics of three-dimensional morphological evolutions. Considerable changes in plan shape were observed such as quasi-regular crescentic patterns, parallel alignment of oblique bars, disconnection and realignment of bars. This is in contrast to the quasi two-dimensional progress of net offshore migration (NOM) of sandbars reported in the Netherlands, New Zealand, Denmark, and Japan.

Net beach change in the swash zone: A numerical investigation

A range of bed-load sediment transport formulae are used to run fully coupled, morphodynamic simulations of one [1] swash cycle on an erodible plane beach. A system comprising shallow water equations and Exner equation is solved, in which sediment transport rate q is either dependent only on depth averaged velocity (u), or on u and water depth (h). The results are in agreement with equivalent uncoupled results [2] in that all sediment transport formulae considered applied to the event of [1] yield net erosion in the whole of the swash. Consistent with [3], however, full coupling yields significantly less erosion for all the q=q(u) formulae compared to the equivalent uncoupled results. It is shown that differences between uncoupled and coupled approaches (for most formulae) accumulate over the course of the swash event. The main reason for the reduced net erosion is the smaller maximum inundation. It is also shown that including a dependence on h in the bed load sediment transport formula for fully coupled simulations can result in net deposition in the upper swash.Bed shear stress described by a Chezy law is further included in fully coupled simulations to examine net beach change. Much reduced maximum inundation and net offshore sediment transport are predicted both for q=q(u) and q=q(h,u). It is shown that although the net sediment flux at the base of the swash under one [1] swash event is still offshore, deposition in the middle or upper swash may be predicted when bed shear stress is included, particularly when the drag coefficient in the backwash is reduced compared to that in the uprush, consistent with some in-situ measurements. The implication is that bed shear stress must be included not just to obtain correct quantitative beach change, but also to obtain correct qualitative beach behaviour.

Sediment transport processes and morphodynamics on a reflective beach under storm and non-storm conditions

New field experiments of sediment transport on the shoreface of a reflective beach (R=0.4–0.6) during pre-storm, storm and post-storm conditions are reported. Data were collected outside the breakpoint and include water surface elevations, cross-shore and long-shore flow velocity, suspended sediment concentrations, bedform dimensions and the morphological response. Instruments included two pulse-coherent ADPs, an ADV, multiple OBS and a pencil-beam sonar. The depth-averaged mean velocity was always offshore and increased by a factor five during the storm, and this increase is attributed to undertow being enhanced by strong backwash plumes from cusp bays. Estimates of cross-shore suspended sediment transport are landward during the pre- and post-storm phases, consistent with the morphological response. During the storm, the shoreface quickly appeared to reach equilibrium, despite energy levels being maintained, with sediment transport rates reducing rapidly after the initial phase of the storm. Streaming in the wave boundary layer was observed and strongest during the pre-storm phase, consistent with the observed shoreward transport. During the storm the near-bed streaming was too small to reverse the increased offshore current, consistent with the observed net offshore transport. The generally small orbital velocity skewness observed on this steep reflective beach suggests that wave reflection may reduce the overall skewness of the velocity, in turn contributing to the stability of reflective beach types.

On bar growth and decay during interannual net offshore migration

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, 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 transports peak landward of the bar crest forcing bar amplitude growth during offshore migration. For increased hXb-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 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.

Influence of Mixtures of Grain-Size Populations on the Parametric and Modal Characteristics of Coastal Sands (Herault, Mediterranean Sea, France)

Abstract Grain-size distributions in lower beach and nearshore sands of a segment of the coast of the Golfe du Lion (France) were analyzed by a parametric method and by modal analysis. The results have been considered both in a spatial framework delineating geographically distinct sedimentary compartments and cells, and in a morphodynamic framework separating the dynamic components of the shoreline (berm, collision zone, inner bar, outer bar, lower shoreface). Modal statistics indicate that a mixture of three dominant components (end members) contributes to the grain-size distribution (GSD) of the sediments: fine sand (Sedimentary Type I: Modal value: 0.195 mm; σI (Φ): 0.35), medium and coarse sand (ST II: Modal value: 0.680 mm; σI (Φ): 0.8) and very coarse sand and gravel (ST III I: Modal value: 2.3 mm; σI (Φ): 1.5). Quasi log-normal one-component GSDs form a minority group and mixtures produce various apparently unimodal sands and bimodal sediments. A large variation of grain-size indices results from mixing. A simulation of mixing of log-normal populations close to the end members shows the great sensitivity of parameters (sorting, skewness, and kurtosis) to small changes in the compositional formula of the mixture. The procedure followed supplies a good preliminary tool enabling rapid identification and localization of the main sand sources on a regional scale. These components derive from potential sources available in the region, well known from previous studies. ST I originates from the River Rhone and provides the textural base for most of the sediments. ST II and III form a local component originating either from fluvial input (Aude, Orb, and Herault rivers) or from the nearshore reworking of ancient Quaternary sediments and late Holocene beachrocks. Each ST takes on local aspects in relation to the morphological environment, but the regional scale of the study was too large to precisely reflect the local morphodynamic regime. A significant change in the ST I sorting of bar sands is, however, observed, probably due to the net offshore migration reported for the bar system.