Predicting beach rotation using multiple atmospheric indices

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Wave and Tidal Controls on Embayment Circulation and Headland Bypassing for an Exposed, Macrotidal Site

Headland bypassing is the transport of sediment around rocky headlands by wave and tidal action, associated with high-energy conditions and embayment circulation (e.g., mega-rips). Bypassing may be a key component in the sediment budget of many coastal cells, the quantification of which is required to predict the coastal response to extreme events and future coastal change. Waves, currents, and water levels were measured off the headland of a sandy, exposed, and macrotidal beach in 18-m and 26-m depths for 2 months. The observations were used to validate a Delft3D morphodynamic model, which was subsequently run for a wide range of scenarios. Three modes of bypassing were determined: (i) tidally-dominated control during low–moderate wave conditions [flux O (0–102 m3 day−1)]; (ii) combined tidal- and embayment circulation controls during moderate–high waves [O (103 m3 day−1)]; and (iii) multi-embayment circulation control during extreme waves [O (104 m3 day−1)]. A site-specific bypass parameter is introduced, which accurately (R2 = 0.95) matches the modelled bypass rates. A 5-year hindcast predicts bypassing is an order of magnitude less than observed cross-shore fluxes during extreme events, suggesting that bypassing at this site is insignificant at annual timescales. This work serves a starting point to generalise the prediction of headland bypassing

Rip current types, circulation and hazard

AbstractRip currents are narrow and concentrated seaward-directed flows that extend from close to the shoreline, through the surf zone, and varying distances beyond. Rip currents are ubiquitous on wave-exposed coasts. Each year they cause hundreds of drowning deaths and tens of thousands of rescues on beaches worldwide and are therefore the leading deadly hazard to recreational beach users. The broad definition above masks considerable natural variability in terms of rip current occurrence in time and space, flow characteristics and behaviour. In particular, surf-zone rip currents have long been perceived as narrow flows extending well beyond the breakers, flushing out the surf zone at a high rate (‘exit flow’ circulation regime), while more recent studies have shown that rip flow patterns can consist of quasi-steady semi-enclosed vortices retaining most of the floating material within the surf zone (‘circulatory flow’ circulation regime). Building upon a growing body of rip current literature involving numerical modelling and theory together with emergence of dense Lagrangian field measurements, we develop a robust rip current type classification that provides a relevant framework to understand the primary morphological and hydrodynamic parameters controlling surf-zone rip current occurrence and dynamics. Three broad categories of rip current types are described based on the dominant controlling forcing mechanism. Each category is further divided into two types owing to different physical driving mechanisms for a total of six fundamentally different rip current types: hydrodynamically-controlled (1) shear instability rips and (2) flash rips, which are transient in both time and space and occur on alongshore-uniform beaches; bathymetrically-controlled (3) channel rips and (4) focused rips, which occur at relatively fixed locations and are driven by hydrodynamic processes forced by natural alongshore variability of the morphology in both the surf zone and inner shelf zone; and boundary-controlled (5) deflection rips and (6) shadow rips, which flow against rigid lateral boundaries such as natural headlands or anthropogenic structures. For each rip current type, flow response to changes in hydrodynamic and morphologic forcing magnitude is examined in regard to velocity modulation and changes in circulation regime, providing key force-response relationships of rip currents. We also demonstrate that in the real world, rip currents form through a mixture of driving mechanisms and the discrete rip types defined in fact form key elements in a wide and complex spectrum of rip currents on natural beaches. It is anticipated that this rip current type classification will serve as a resource for coastal scientists and non-specialists with an interest in the rip current hazard, and as a platform for future rip current studies. Finally, we suggest some important future research directions highlighting the need for coastal and beach safety communities to collaborate in order to improve rip current education and awareness

Role of waves and tides on depth of closure and potential for headland bypassing

No embargo required.© 2018 Depth of closure is a fundamental concept used to define the seaward extent of a morphodynamically active shoreface at a particular temporal scale. The estimation of this limit in relation to the depth in front of the bounding headlands along embayed coastlines allows questioning whether embayments, often deemed closed sediment cells, experience more headland bypassing than expected. Wave-based parameterisations developed for microtidal beaches are most widely used to estimate closure depth; however, a re-evaluation of the concept for shorefaces influenced by geological control (presence of headlands and/or bedrock) and strong tidal currents is appropriate. Here, we use the macrotidal, embayed and high-energy coastline of SW England to identify the ‘active’ nearshore limits with a multi-method approach that includes observations of shoreface morphology and sedimentology, offshore/inshore wave formulations and bed shear stress computations. We identify the basal limit of ‘significant’ (i.e., 0.14 m) morphological change (Depth of Closure; DoC) and a maximum depth of extreme bed activity and sediment transport (Depth of Transport; DoT). Observations of DoC correspond closely to the values predicted by existing formulations based on inshore wave conditions (10–15 m for the study area; relative to mean low water spring water level in this case). The computed DoT, represented by the upper-plane bed transition attained under extreme conditions, exceeds 30 m depth in the study area. The significant implication is that, even though many headlands appear sufficiently prominent to suggest a closed boundary between adjacent embayments, significant wave- and tide-driven sediment transport is likely to occur beyond the headland base during extreme events, especially at low water levels. The maximum depth for significant sediment transport (DoT) was computed across a broad wave-current parameter space, further highlighting that tidal currents can increase this closure depth estimate by ~10 m along macrotidal coastlines, representing a 30% increase compared to tideless settings. This work illustrates the importance of tidal currents in depth of closure calculations and challenges the notion that embayed beaches are generally closed cells, as headland bypassing may be more wide spread than commonly assumed along exposed coastlines globally

Modelling the alongshore variability of optimum rip current escape strategies on a multiple rip-channelled beach

Rip currents are a leading cause of drowning on beaches worldwide. How bathers caught in a rip current should attempt to escape has been a subject of recent debate. A numerical model of human bathers escaping from a rip current flow field is applied to a 2-km long section of the open beach of Biscarrosse, SW France. The study area comprises 4 rip channels that visually appear similar from the beach, but exhibit different morphologies. Simulations are run for 2 representative hazardous summer wave conditions. Results show that small changes in the bar/rip morphology have a large impact on the rip flow field, and in turn on the alongshore variability of the optimal rip current escape strategy. The overall flow regime (dominant surf-zone exits versus dominant recirculation), which is found to be influenced by the alongshore dimensions of the sand bars adjacent to the rip channel, is more important to rip current escape strategy than rip velocity. Flow regime was found to dictate the success of the stay afloat strategy, with greater success for recirculating flow. By comparison, the dominant longshore feeder current and rip-neck orientation determined the best direction to swim parallel toward. For obliquely incident waves, swim parallel downdrift then swim onshore with breaking waves was highly successful and can become a simple safety message for beach safety practitioners to communicate to the general public. However, in SW France where rip spacing is large (∼400 m), surf-zone eddies have large spatial scales of the order of 100+ m, requiring a large distance (100+ m) to swim to reach safety, therefore requiring good swimming ability. This also shows that in addition to rip current intensity, rip flow regime and the depth of adjacent sand bars, rip spacing is important for defining rip current hazard and the best safety message. Our results also indicate that for normal to near-normal wave incidence, rip current hazard and best rip current escape strategy are highly variable alongshore due to subtle differences in bar/rip morphology from one rip system to another. These findings challenge the objective of developing a universal rip current escape strategy message on open rip-channelled beaches exposed to normal to near-normal wave incidence, even for seemingly similar rip channels

Coastal embayment rotation: Response to extreme events and climate control, using full embayment surveys

© 2018 The Authors Barrier beach change in directionally bi-modal wave climates presents an increasing challenge for coastal communities, both in the short-term (storm events), and decadal to centurial time scales (long-term evolution). Predicting and planning for subsequent variations requires understanding of the morphological response to changes in wave energy, along with the atmospheric forces driving the wave climate. In this paper, multi-method topo-bathymetric surveys are used to assess the morphological change of a semi-sheltered gravel barrier (Start Bay, Devon, UK). Total sediment budgets (supra- to sub-tidal), with spatially-varying uncertainty levels, indicate the embayment is closed. One third of total sediment flux occurred in the sub-tidal, establishing the importance of sub-tidal transport for this type of coastline. Our results demonstrate that under the predominance of a given wave direction, rotation first occurs within sub-embayments. Additional sustained and extreme energy levels are then required for full embayment rotation to occur, with significant headland bypassing. In this instance, 6 × 105 m3 of gravel was transported alongshore during a 3-year sustained period of dominant-southerly waves, including a 1:50 year storm season (full-embayment rotation), whilst 3 × 105 m3 was returned during a 2-year period of dominant easterly waves (sub-embayment rotation only). A novel parameter is introduced that predicts beach rotation based on the directional wave balance. In turn, winter wave direction is shown to correlate with a combination of two climate indices. Given adequate predictions of relevant climate indices, these findings constitute the basis of a generalisable method to predict and plan for future beach rotation on similar beaches globally

The Impact of Waves and Tides on Residual Sand Transport on a Sediment‐Poor, Energetic, and Macrotidal Continental Shelf

©2019. The Authors. The energetic, macrotidal shelf off South West England was used to investigate the influence of different tide and wave conditions and their interactions on regional sand transport patterns using a coupled hydrodynamic, wave, and sediment transport model. Residual currents and sediment transport patterns are important for the transport and distribution of littoral and shelf-sea sediments, morphological evolution of the coastal and inner continental shelf zones, and coastal planning. Waves heavily influence sand transport across this macrotidal environment. Median (50% exceedance) waves enhance transport in the tidal direction. Extreme (1% exceedance) waves can reverse the dominant transport path, shift the dominant transport phase from flood to ebb, and activate sand transport below 120-m depth. Wave-tide interactions (encompassing radiation stresses, Stoke's drift, enhanced bottom-friction and bed shear stress, refraction, current-induced Doppler shift, and wave blocking) significantly and nonlinearly enhance sand transport, determined by differencing transport between coupled, wave-only, and tide-only simulations. A new continental shelf classification scheme is presented based on sand transport magnitude due to wave-forcing, tide-forcing, and nonlinear wave-tide interactions. Classification changes between different wave/tide conditions have implications for sand transport direction and distribution across the shelf. Nonlinear interactions dominate sand transport during extreme waves at springs across most of this macrotidal shelf. At neaps, nonlinear interactions drive a significant proportion of sand transport under median and extreme waves despite negligible tide-induced transport. This emphasizes the critical need to consider wave-tide interactions when considering sand transport in energetic environments globally, where previously tides alone or uncoupled waves have been considered

Nearshore sediment pathways and potential sediment budgets in embayed settings over a multi-annual timescale

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