Search

Subtidal circulation patterns were studied along an elongated hypersaline system. Month-long data were collected using three acoustic Doppler current profilers. These profilers were moored at depths of around 15 m in three locations: 10 km inland from the mouth of the lagoon, within the central region, and near the head of the lagoon. The variability of temporal and spatial subtidal circulation was explored using Empirical Orthogonal Functions analysis, which explained 93% of the variability through the first three modes. The first mode was influenced by tidal advection, resulting in a tidally-driven residual circulation for elongated channels, with inflows occurring over the shoals and outflows in the deeper channel sections. During the neap-tide cycle, inverse density gradients became relevant, coinciding with the peaks of mode 2 amplitude that accounted for 15% of the total variability. The gravitational circulation exhibited its highest intensity near the head, characterized by a vertically sheared flow with inflow occurring near the surface and outflow underneath. Predominant onshore wind stresses had a significant impact on the subtidal circulation and displayed a strong correlation with the mode 3 amplitude, explaining 11% of the total variability. Wind-driven circulation exhibited a vertically sheared profile, characterized by downwind near-surface flow and upwind flow underneath in the lagoon mouth and head, and a homogeneous downwind profile in the central part of the lagoon. It is concluded that tidal stresses governed the subtidal circulation, followed by density gradients, and wind stresses modified spatial circulation patterns induced by tides and density gradients. This research provides new observational evidence on tidal residuals in long channels and contributes to the understanding of the interactions between the main subtidal circulation drivers in hypersaline lagoons. • Tidal advection governed the subtidal circulation of a hypersaline coastal lagoon. • Density gradients became relevant during neap tide and dominated the circulation near the head. • Wind-driven flow appeared during strong wind conditions and displayed a vertically sheared profile near the mouth. [ABSTRACT FROM AUTHOR]

Wind and current measurements collected between February and March 2017 at three sites in the southern Texas coast were used to determine along-shelf flow variations induced by episodic synoptic-scale cold fronts and to quantify the relevant terms in the inner-shelf along-shelf momentum budget. Inner-shelf flows were wind-driven, but significant along-shelf variations existed between the northern and southern sites. Cold fronts induced downcoast mean, surface and bottom flows along the study site. Strongest downcoast flows occurred in the surface in the northern and southern ends, while these were weaker in the center. Flows averaged over a longer period of time showed the presence of a convergence zone in the central site, off Port Mansfield. Spatial variations of the along-shelf wind stress were the main cause for the flow convergence in the surface layer at the central site. Most intense surface flows occurred in the north (along-shelf wind speeds of > 10 ms−1), less affected by southeasterly (upcoast) Trade winds. Low-energy cold fronts (along-shelf wind speeds of < 10 ms−1) were unable to completely reverse the wind direction towards the south (downcoast) in the southern and central sites, due to the stronger influence of Trade winds. Surface wind stress and pressure gradients were the dominant terms in the along-shelf momentum budget, while the bottom stress and Coriolis terms were considered negligible. The wind-stress term was dominant in the north, while pressure gradients associated with local sea-level responses to the wind forcing were dominant in the southern site. This study highlights the importance of pre-front wind characteristics in spatial variations of the along-shelf flow during cold fronts. [ABSTRACT FROM AUTHOR]

Sediment exchange between the shoreface and the foreshore is investigated through the analysis of sandbar and berm migration cycles at La Misión beach in Baja California, Mexico. In general, nearshore sandbars migrate onshore during low-energy wave conditions merging with the shoreline to become intertidal sandbars that later contribute to berm rebuilding. However, significant alongshore differences were observed in the sandbar welding and berm rebuilding process, which can be attributed to alongshore wave skewness and near-surfzone flow variations. The analysis of topographic and bathymetric surveys collected monthly over a 5-year period revealed larger shoreline retreat and longer recovery times in the southern beach compared to the north. This was related to the inability of sandbars to weld to the intertidal beach during spring and summer, occurring only during extended periods of low-energy waves. In the southern beach, the berm formation process exhibited less dependence on incoming wave energy, persisting during mild-energy winters, and reforming in the presence of sandbars. Hydrodynamic measurements indicated that the southern part of the beach experienced relatively more intense net offshore flows (up to −0.2 ms−1) due to undertow, and possibly related to the presence of persistent rip currents. In contrast, the berm in the northern beach followed a seasonal erosion and rebuilding process associated with sandbar migrations driven by variations in incoming wave energy and cross-shore sediment transport. This was characterized by larger near-bed wave velocity skewness compared to the south. Net flows in the north were predominantly onshore, reaching up to 0.1 ms−1 during lower-energy periods with oblique wave incidence. This study reveals the presence of considerably distinct sandbar and berm migration cycles along a relatively short beach of 2.2 km long. It also highlights how such variability can be related to the morphodynamic feedback with sandbars affecting hydrodynamics, particularly near-bed wave skewness and near-surfzone flow variations, which in turn influence sediment transport and morphological evolution. • Bar merging affects berm rebuilding, influenced by wave skewness and flow variations. • Net onshore flows and higher skewness allow seasonal berm rebuilding. • Net offshore flows and lower skewness kept the bar offshore but the berm recovered. [ABSTRACT FROM AUTHOR]

Abstract Understanding the morphodynamics of surfzone sandbars is very relevant from a sediment transport perspective. Three complete annual sandbar migration cycles were studied on a single-barred beach. The study period included the 2015–2016 El Niño anomaly and the incidence of a highly energetic swell-storm. The sandbar responded directly to the seasonal wave forcing, thus, it migrated offshore during the energetic winter and onshore during the milder summer conditions. Its seasonal migration cycle comprised four stages: (1) generation in autumn (November) at lower than 1 m depth; (2) offshore migration over the winter (until January/February) while H ¯ s > 1.3 m; (3) onshore migration over the early spring (March and April) while H ¯ s < 1.0 m; and during May–June: (4a) subaerial beach welding (unbarred beach) when a dynamic equilibrium reached; or (4b) terrace-bar formation (sandbar flattening) as a consequence of a lack of a dynamic equilibrium. The energetic El Niño winter conditions induced the same amount of offshore sandbar displacement as the individual extreme swell-storm, placing the sandbar at a maximum cross-shore distance of ~190 m, beyond the dynamic equilibrium. Contrary to the expected, a period of mild wave conditions during the 2017 winter favored the landward migration of the sandbar, locating it near its generation point. The duration of mild wave energy conditions and the offshore sandbar location and volume are considered relevant factors that limit the capabilities of the sandbar to reach shallow waters and weld to the subaerial beach before the summer. Highlights • Sandbar formation, offshore and onshore migration and welding or terrace-bar creation occurs over a year. • Similar offshore sandbar displacement was measured after an individual extreme storm than by the end of El Niño winter. • A period of mild wave conditions in winter favoured onshore sandbar migration up to its generation point. [ABSTRACT FROM AUTHOR]

This study investigates the recovery capabilities of a single-barred beach in the Pacific Mexican coast before and after the 2015-2016 El Niño winter. Concurrent hydrodynamic and morphological data collected over a 3-year period (August 2014-2017) were analysed to determine the subaerial-subtidal volumetric exchange and cross-shore subtidal sandbar migrations, in relation to the incident wave forcing. The beach presented a seasonal seaward and landward sandbar migration cycle. The sandbar migrated offshore during the energetic waves between November and February, and onshore during the milder wave period in spring, until welding to the subaerial beach around May. The transfer of sediment towards the subaerial section continued over the summer, reaching a complete recovery by September/October. Prior to El Niño, the subaerial beach successfully recovered by the end of summer 2015 through the landward sandbar migration process. The 2015-2016 energetic winter waves caused a subaerial volume loss of ~ 140 m3 m−1 (from October 2015 to March 2016), more than twice the amount eroded in the other winters, and the sandbar moved further offshore and to deeper depths (3-4 m) than the winter before. In addition, the energetic 2015-2016 winter waves lasted for 2 months longer than in other years, making the 2016 spring shorter. Consequently, during the onshore migration, the sandbar was unable of reaching shallow depths, and a large portion of sand remained in the subtidal beach. The subaerial beach recovered 60 and 65% of the loss in the 2016 and 2017 summers, respectively. It is concluded that the landward migration process of the sandbar during the spring is critical to ensure a full subaerial beach recovery over the mild wave period in summer. The recovery capabilities of the subaerial beach will depend on the cross-shore distance and depth where the sandbar is located, and on the duration of mild wave conditions required for the sandbar to migrate onshore. [ABSTRACT FROM AUTHOR]

This study provides a detailed insight into the seasonal morphological evolution of the subaerial and subtidal sections of an intermediate and mesotidal beach in relation to the nearshore wave conditions. For this purpose, fortnightly − to − monthly topographic and bathymetric measurements were collected on Ensenada Beach (Baja California, Mexico) from November 2012 to February 2016. Wave data were concurrently measured at 20 m water depth, enabling the correlation of the observed morphological changes to the hydrodynamic forcing. The seasonal morphological variability of the beach comprised ± 200,000 m 3 (± 70 m 3 m − 1 ) and resulted primarily from the cross-shore exchange of sediment. The volumetric fluctuations alongshore were up to ± 100,000 m 3 and event-driven, and played an inportant role in the overall sediment balance. In association with the incoming wave conditions, the subaerial and subtidal beach sections experienced a strong seasonal variability. The subaerial beach reached the maximum volumes in the end of summer (September − October) and minimum in winter (January − February) while the opposite response was obtained for the subtidal beach, with the largest and smallest volumes occurring during the same months in winter and summer, respectively. The morphological fluctuations of the upper and lower beach sections were associated with the preceding monthly averaged wave conditions. The subtidal volume variations were highly and positively correlated to the wave height and period, while the subaerial section was highly but negatively correlated to the same variables. The morphological beach response to energetic waves was more rapid than to low-energy conditions. The largest subaerial erosion and subtidal accretion happened in three months (October − January) while the subaerial accretion and subtidal erosion occurred smoothly over nine months (January − October). The total beach volume (subaerial and subtidal) lacked of seasonal variability, and an annual loss of 50,000 m 3 (17 m 3 m − 1 ) occurred from August 2014 to August 2015, indicating that the beach remained fairly stable over that period of time. The largest volumetric variations of 300,000 m 3 (105 m 3 m − 1 ) occurred during the beginning of the 2015–2016 El Niño winter. [ABSTRACT FROM AUTHOR]

Floating raft, red knot, roost, sea level rise, seagrass, shorebirds Here we report on the use of a novel roost site by shorebirds during spring tides, which is the period of the lunar phase when the tidal range is the largest and can therefore serve as a lens through which to understand how shorebirds may be able to adapt to find alternative roost habitats under future sea level rise scenarios. Keywords: floating raft; red knot; roost; sea level rise; seagrass; shorebirds EN floating raft red knot roost sea level rise seagrass shorebirds 1 6 6 09/05/23 20230901 NES 230901 Sea level rise is one of the most unequivocal consequences of climate change, yet one of the least studied in terms of its effects on biodiversity (Nicholls & Cazenave, [9]). [Extracted from the article]

Estimating sediment budget is a challenge that requires understanding the morphological relationship between the subaerial beach and the upper and lower shoreface. Based on a unique dataset of monthly measured topographic and bathymetric measurements, this study describes the morphodynamics of the subaerial and subtidal sections of a highly-dynamic beach over a 5-year period (2015–2020). Typically, the beach presents a longshore bar trough (LBT) state at the end of the winter, driven by net offshore sediment transport. The nearshore morphology evolves into more reflective states (rhythmic bar beach, RBB; transverse bar rip, TBR; and low tide terrace, LTT) driven by onshore sediment transport when the incident wave-energy decreases over the spring and summer. Seasonal variations of the incident wave-energy resulted in large subaerial-subtidal exchanges of sediment (200–300 m3 m−1), but these exchanges were an order of magnitude smaller in yearly scales. Subaerial and upper-shoreface morphological changes varied alongshore. Larger variations existed in the south, influenced by persistent rip channels. Instead, alongshore processes resulting from changes in wave direction may control the morphodynamics in the center and north. The subtidal section in north and center gained significant amount of sand (300 m3 m−1) during high-energy conditions, which contributed to a net annual gain of 60 m3 m−1. The origin of that sediment is currently unknown, it could have bypassed from an adjacent beach, shifted alongshore or inputted from the lower shoreface deeper than the depth of closure. This research shows the presence of very large monthly-to-seasonal total volume fluctuations relative to net annual or multi-annual changes, and highlights that comparatively small additions to the annual sediment budget could potentially contribute to the long-term stability of the beach. • Largest morphological variations in the south influenced by persistent rip channels. • Very large monthly-to-seasonal volume fluctuations relative to net annual changes. • Small additions to the annual budget could contribute to long-term beach stability. [ABSTRACT FROM AUTHOR]

This study contributes to the understanding of the extent at which antecedent hydrodynamic and morphological conditions affect post-storm morphology in response to individual storms and storm clusters. Nearshore wave measurements and concurrent fortnightly to monthly topographies from 2012 to 2020 were used to determine the morphological response of a swell-dominated beach in the Pacific coast of NW Baja California to different storm types. The beach attained an almost nil volumetric balance over the eight-year period, but significant variations existed related to storm incidence. Higher storminess occurred during El Niño years and lower or absence of storminess during years of La Niña phase. The integral wave characteristics of the analyzed swell-dominated storms did not conclusively explain subaerial volume variations. Dissipative morphodynamic conditions prevailed during the analyzed storm events (Ω of 6 to 8) and the wave steepness presented relatively small variations (0.006 to 0.015). The beach quickly reached a storm equilibrium state, preventing subsequent adjustment when further storms in the sequence acted. Moreover, consecutive storms to highly-erosive events caused subaerial beach accretion when low pre-storm volumes (< 50 m3m−1 than average) and mild lower intertidal slopes (tanβ > 0.03) existed. It is concluded that the erosive potential of the swell-driven storms primarily depended on the antecedent sediment availability and near-shoreline slope. • Higher and lower storminess occurred during El Niño and La Niña phases of ENSO, respectively. • Storm equilibrium state is quickly reached, preventing relevant morphological adjustments by subsequent storms. • Antecedent sand volume and beach slope are key controls of the erosion induced by swell-storms of similar wave-steepeness. [ABSTRACT FROM AUTHOR]

Yearly-average along-shelf winds were predominantly upcoast over the last two decades, but large seasonal fluctuations existed. This study provides an example of how the passage of episodic cold fronts generally induce the reversing of along-shelf flows downcoast on the southern Texas coast. A total of 819 cold fronts of 4 days of average duration were identified between 1984 and 2018, and the vertical structure of the flow was analyzed for 38 cold fronts between November 2016 and October 2018 at a location 10 km offshore of Port Aransas. Cold fronts were identified based on local wind speeds and directions and synoptic-scale mean sea-level pressure differences, and then categorized through hierarchical clustering. Theoretical flow expectation for unstratified conditions suggested that the flow response was primarily controlled by the wind forcing during the passage of cold fronts. Detailed analysis on the vertical profile of the flow demonstrated significant time-lag differences, mainly related to the front intensity and duration. Strongest downcoast flows of −0.5 ms−1 encompassing the entire water column (18 m depth) were associated with individual fronts of high intensity (τ y of ∼ −0.15 Nm−2) and long duration (at least 4 days). The most common fronts were individual of low intensity (τ y of ∼ −0.05 Nm−2) and short duration (up to 2 days), and induced relatively weak downcoast currents of up to −0.2 ms−1, and in many cases, these were unable to completely reverse the flow downcoast. This study highlights the relevant role of episodic cold fronts in reversing and intensifying inner-shelf currents on the southern Texas coast, and emphasizes differences in the flow response related to cold front characteristics. • The flow response was primarily controlled by the wind forcing during the passage of cold fronts. • Fronts of 4 days of duration and τ y ∼ −0.15 Nm−2 induced strongest downcoast flows of −0.5 ms−1 in the entire water column. • Most common fronts induced weak downcoast flows of up to −0.2 ms−1, but many were unable to completely reverse the flow. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes the application of Canonical Correlation Analysis (CCA) to derive forcing–response relations between the wave climate and shoreline position on a macrotidal gravel barrier located in the southwest of the U.K., and to develop a tool to determine shoreline positions from wave records. The data sequences comprise wave climate recorded by a nearshore directional wave buoy and video-derived shorelines over a time span of one year and a half. The hydrodynamic conditions are used to determine the probability density function of wave heights and alongshore energy fluxes. These are then related to shoreline change through a CCA analysis. The CCA analysis identifies patterns of behaviour of the wave conditions and the shoreline position, and the relation between both patterns is found to provide useful information about the beach response to wave action. The analysis shows that the movement of sediment is greater at the southern end of the study area and that there is an immediate shoreline response to the wave action. In the case of coastal management it is more often the case that wave forecasts are available on a routine basis. The ability of the CCA to provide useful estimates of shorelines from wave conditions was tested by using measured waves to calculate the corresponding shoreline position from additional data at the end of the sequences. Shoreline positions determined with the CCA agreed well with the measured ones. Thus, the CCA is found to be a useful tool to determine unknown shoreline positions and support effective coastal management if good quality hydrodynamic and morphological data are available to input into the initial set-up of the technique. [Copyright &y& Elsevier]

Eroded sediment can return to the subaerial beach through onshore sandbar migration, thus understanding this process is key for rebuilding coastal morphology. While offshore sandbar migrations are fairly predictable based on wave conditions alone, landward migrations are less predictable and more dependent on sandbar morphometrics. In this study, monthly collected topographic and bathymetric data from August 2014 to September 2018 were analyzed and related to the incoming wave conditions to investigate the control of sandbar location and morphometry on the landward migration process. Four distinct onshore sandbar migration modes were identified, primarily governed by the sandbar crest location and size (volume and height), and the preceding cumulative wave energy. A cross-shore distance of 150 m equivalent to ~3 m depth was defined as a boundary for the fate of the sandbar during the landward migration process. Sandbar welding (SW; Mode I) occurred during low-energy conditions after a moderately energetic winter (cum Pt ≤ 50 kWm−1) when the sandbar (of up to 1 m height and ~ 100 m3m−1) was located shoreward from the cross-shore boundary. A large sandbar (> 100 m3m−1; > 1 m height) that formed offshore from the boundary during a highly-energetic winter (cum Pt >50 kWm−1) turned into a terrace-bar (STT; Mode II) in a subsequent very low-energy period. A terrace-bar of ~100 m3m−1 located near the boundary (~ 3 m depth) became a sandbar (TST; Mode III) during moderately energetic conditions at the beginning of the winter. Sandbar splitting (SS; mode IV) occurred during low-energy conditions when a sandbar of ~100 m3m−1 situated offshore from the boundary; the outer section flattened and followed a net offshore migration cycle while the inner section migrated shoreward. This study highlights the need for considering sandbar morphometric properties in addition to the preceding wave conditions to adequately predict the shoreward migration process of the sandbar. • A sandbar of ≤1 m located at < 3 m depth welds to the shoreline during low energy conditions (Pt max < P ¯ t anual) after a mild winter (Pt max < 10 x P ¯ t anual). • After an energetic winter (Pt max > 10 x P ¯ t anual) a >1 m sandbar at > 3 m depth becomes a terrace-bar during very-low conditions (Pt max < 0.5 x P ¯ t anual). • A ~100 m3m-1 sandbar at > 3 m depth splits into two parts (outer terrace-bar and inner sandbar) during low-energy conditions (Pt max < 2 x P ¯ t anual). [ABSTRACT FROM AUTHOR]