Inventory of littoral processes and retrogression on a microtidal beach during a storm season (original) (raw)
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Cross-shore profiles and environmental forcing were used to analyse morphological change of a headland bay beach: Tenby, West Wales (51.66 N; −4.71 W) over a mesoscale timeframe (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013). Beach profile variations were attuned with longer term shoreline change identified by previous research showing southern erosion and northern accretion within the subaerial zone and were statistically significant in both sectors although centrally there was little or no significance. Conversely a statistically significant volume loss was shown at all profile locations within the intertidal zone. There were negative phase relationships between volume changes at the beach extremities, indicative of beach rotation and results were statistically significant (p < 0.01) within both subaerial (R 2 = 0.59) and intertidal (R 2 = 0.70) zones. This was confirmed qualitatively by OPEN ACCESS J. Mar. Sci. Eng. 2015, 3 2 time-series analysis and further cross correlation analysis showed trend reversal timelagged associations between sediment exchanges at either end of the beach. Wave height and storm events displayed summer/winter trends which explained longer term one directional rotation at this location. In line with previous regional research, environmental forcing suggests that imposed changes are influenced by variations in southwesterly wind regimes. Winter storms are generated by Atlantic southwesterly winds and cause a south toward north sediment exchange, while southeasterly conditions that cause a trend reversal are generally limited to the summer period when waves are less energetic. Natural and man-made embayed beaches are a common coastal feature and many experience shoreline changes, jeopardising protective and recreational beach functions. In order to facilitate effective and sustainable coastal zone management strategies, an understanding of the morphological variability of these systems is needed. Therefore, this macrotidal research dealing with rotational processes across the entire intertidal has significance for other macrotidal coastlines, especially with predicted climate change and sea level rise scenarios, to inform local, regional and national shoreline risk management strategies.
Seasonal changes in beach morphology along the sheltered coastline of Perth, Western Australia
Marine Geology, 2001
Seasonal change in beach morphology is traditionally ascribed to a variation in the incident wave energy level with calm conditions in summer resulting in wide beaches with pronounced subaerial berms and energetic conditions in winter causing narrow beaches with nearshore bar morphology. The coastline of Perth, Western Australia, is characterised by a large seasonal variation in the incident wave height and local beaches exhibit a distinct seasonal change in morphology. However, these morphological changes are better explained by a seasonal reversal in the littoral drift direction than by variations in the incident wave energy conditions. In summer, when northward sediment transport prevails due to sea breeze activity, beaches located south of coastal structures, headlands or rocky outcrops become wider due to the accumulation of sediment against the obstacle. These beaches will subsequently erode in winter during storms when the longshore sediment transport is toward the south. In contrast, beaches located north of obstacles will become narrower during summer and wider during winter. The usefulness of the dimensionless fall velocity (where Hb is the breaker height, ws is the sediment fall velocity and T is the wave period) as a predictor of presence/absence of bar morphology and beach type was investigated. It was found that Ω fluctuates around the threshold of bar formation (Ω≈1.5–2) over a variety of time scales (daily, weekly, and seasonally). These temporal variations in Ω in conjunction with the relatively low wave energy level that characterises the coast negates the development of beach and nearshore morphology that is in equilibrium with the hydrodynamic conditions. As a result, bar occurrence and beach type can not be readily predicted using Ω along the Perth coast.
2015
Abstract: Cross-shore profiles and environmental forcing were used to analyse morphological change of a headland bay beach: Tenby, West Wales (51.66 N; −4.71 W) over a mesoscale timeframe (1996–2013). Beach profile variations were attuned with longer term shoreline change identified by previous research showing southern erosion and northern accretion within the subaerial zone and were statistically significant in both sectors although centrally there was little or no significance. Conversely a statistically significant volume loss was shown at all profile locations within the intertidal zone. There were negative phase relationships between volume changes at the beach extremities, indicative of beach rotation and results were statistically significant (p < 0.01) within both subaerial (R2 = 0.59) and intertidal (R2 = 0.70) zones. This was confirmed qualitatively by time-series analysis and
Geomorphology, 2015
Beach classification models are widely used in the literature to describe beach states in response to environmental conditions. These models were essentially developed for sandy barred to barless beaches in micro-to mesotidal environments subject to moderate to high wave energy conditions and have been based on field studies over limited stretches of coast. Here, we further interrogate the performance of the Australian beach classification scheme by analysing beach states and corresponding bar types on a regional scale in a storm-influenced, low wave-energy, microtidal environment, using a large and unique spatial and temporal dataset of supra-and subtidal beach morphology and sedimentology. The 200 km-long coast of the Gulf of Lions in the Mediterranean consists of quasi-continuous sandy beaches with a well-developed double sandbar system. All the reported classical beach states were observed on this coast, from reflective to dissipative, along with two more unusual states: the rock platform-constrained beach state which is associated with bedrock outcrops, and the non-barred dissipative beach state which is more commonly found in large tidal-range settings. LiDAR bathymetry shows that the transitions between beach state zones are marked mainly headlands but transitions also occur progressively along stretches of continuous sandy beach. The longshore distribution of beach states and associated bar types on a regional scale can be related to the variability of hydrodynamic conditions (wave incidence and energy) and sediment characteristics (particle size). However, the influence of these parameters on beach state seems to be largely controlled by the geological context such as the presence of a river mouth, headland or rock platform. Finally, we assessed the ability of the parameter Ω, commonly used to characterise beach states, which combines wave characteristics and sediment fall velocity, to predict the observed beach states and bar types using a very large set of hydrodynamic and sedimentary data. Our results, based on high frequency spatial sampling, show that the fall velocity of the subtidal sediment coupled with wave statistics one month prior the observed beach state strongly improved the predictive power of the parameter Ω.
A Centurial Record of Beach Rotation
Beach rotations are reliant on a bi-directional wave climate and headlands to impede alongshore sediment transport. This manifests itself in localised shoreline retreat or advance but does not lead to long term sediment loss or gain, as beaches often return to initial conditions in response to wave direction shifts and these changes are often seasonal. This paper assesses morphological changes of a headland embayed beach (Tenby, West Wales) over a 180 year period using GIS, cross shore profiles, and wave modelling. Within GIS maps, aerial photographs and direct field measurements identified two significant changes in beach orientation between the periods 1830-1919 and 1919-2009. Analysis of more recent data (1941-2009) showed that a statistically significant (R2 = 64%) negative phase relationship existed between the beach extremities and correlation changes revealed central region rotation. Results were consistent with wave modeling (RCPWave) that showed dominant waves emanate from southwest and cause long term longshore drift from south toward north. Subdominant waves emanating from the southeast cause counter-drift. In the decadal and seasonal term, negative phase relationships indicative of beach rotation were also established. Cross-correlation analysis between beach extremities showed that decadal term rotation occurred at timescales of less than one year. This was verified by seasonal term results, which showed with increased statistical significance that sediment exchange between headlands takes up to two months. Results have implications for coastal zone management and careful examination of these phenomena is required over both seasonal and longer timescales and should be considered in the development of new beach management strategies.
Medium timescale beach rotation; gale climate and offshore island influences
Beach profile surveys, gale climate and atmospheric variations were utilized to assess medium timescale morphological change at South Sands, Tenby, West Wales. Due to beach aspect in relation to offshore islands, gale wave height decreased as wave direction rotated eastwards (r=0.83) and westwards (r=0.88). Similarly, wave heights were in attuned to variations in positive (r=0.68) and negative (r=−0.72) NAO Index, showing a wave height reduction occurred during weakly negative/positive or transitory phases; morphological change was attuned to atmospheric variation at a 2-year timelag. Shelter from offshore islands is given to waves from the predominant southwesterly direction and was confirmed by negligible correlation with South Sands morphology. However, outside the shelter of these offshore islands, correlation was found between south-eastward rotating wave directions (135°–180°) and morphological change, which resulted in southern and central beach erosion and accretion to the north. With a southwesterly rotation (243°–256°) the opposite was true. Beach rotation expressed by volume change within the sub-aerial zone had a negative phased relationship between beach extremities (r=−0.94) and a timelagged association within the intertidal zone (r=0.55). Analyses resulted in the development of two medium timescale rotation models based on incident wave direction and climatic variability. Results have global implications for headland bays in the lee of offshore islands, as well as macro-tidal beach areas; and consequently similar models could inform local, regional and national beach management strategies.
Beach oscillation and rotation: local and regional response at three beaches in southeast Australia
Journal of Coastal Research, 2014
Six years of monthly subaerial surveys across three embayed beaches in southeast Australia located 270 km apart are utilized to compare the response of the beaches at the local and regional scale. The three beaches (Narrabeen, Moruya and Pedro) are exposed to a similar deep water wave climate (H s ~1.5 m, T~10 s), identical tides (spring range 1.6 m) and have similar lengths (~3 km), easterly orientation and medium to fine sand. Over the six years all three beaches had synchronous oscillation and rotation, though at different magnitudes. The lower energy Moruya beach undergoes greater beach oscillation (up to 100 m) and rotation than the higher energy Narrabeen and Pedro. The results highlight the regional scale of synchronous beach response, as well as have implication for our understanding of sediment transport and shoreline stability in relation to beach state, with the lower energy beach having a more dynamic shoreline. Furthermore the six years of data is insufficient to detect longer term trends observed at Narrabeen since 1976.
The effects of storm clustering on beach profile variability
Marine Geology, 2014
Sand and composite sand-gravel beaches show distinctly different morphodynamic responses to natural forcing as a result, primarily, of differences in sediment properties and wave breaking and dissipation characteristics. As the incident wave conditions fluctuate, so the beaches vary in response, affecting their nature and longterm stability. In this paper, beach profile surveys acquired over more than a decade at a sandy beach (Narrabeen Beach, New South Wales, Australia) and a composite sand-gravel beach (Milford-on-Sea, Christchurch Bay, UK) are analysed to compare and contrast cross-shore morphodynamics of the two beach types. The different behavioural characteristics of the two beach types at decadal, inter-annual and intraannual time scales are investigated. Comparisons of beach profiles with Dean's equilibrium profile and Vellinga's erosion profile shows that the Dean's profile satisfactorily represents the time mean profiles of both beach types. Statistical and Empirical Orthogonal Function (EOF) analyses confirm the generally accepted model 3 that the inter-tidal zone is the most morphodynamically active region on a sandy beach whereas the swash zone is the most dynamic region on a mixed sand-gravel beach. The results also imply that during storms composite sand-gravel beaches may destabilise due to cutback of the upper beach while sandy beaches are more likely to be unstable as a result of beach lowering due to sediment transport from the inter-tidal zone to the sub tidal zone during storms. EOF results also show that Milford-on-Sea beach is in a state of steady recession while the Narrabeen Beach shows a cyclic erosion-accretion variability. A multivariate technique (Canonical Correlation Analysis, CCA) shows that on the composite beach a strong correlation exists between incident wave steepness and profile response, which could be attributed to the unsaturated surf zone, whereas on the sandy beach any correlation is much less evident.
Short-Term Beach Rotation Processes in Distinct Headland Bay Beach Systems
2002
This paper investigates morphological changes in headland bay beaches with emphasis on short-term beach rotation processes, elucidating how it is affected by the planform/degree of curvature of the beach, and by the different morphodynamic characteristics of the beach systems monitored. The beaches monitored in the present study were Balneario Camboriu, Brava and Taquaras/Taquarinhas beaches. They have different lengths, degrees of curvature, and levels of exposure to the incident waves, and represent different beach types. Indentation ratio and the SLlCL ratio were measured, and beach profile surveys every 15 days were made in order to measure variations of beach volume and width for each beach. Visual wave and beach observations were recorded daily. Results indicate that morphological changes in headland bay beaches are influenced mainly by beach planform and indentation ratios, presence of rip currents and submerged bars, shoreline length, and beach type. The beach volume and with variations demonstrated that headland bay beaches have defined sectors with different behaviour, as influenced by headland impact on incident waves and longshore currents. Short-term beach rotation is manifested as out of phase variation of beach volume and width between opposite ends of a headland bay beach. Rotation amplitude of about 20 meters was observed at a dissipative beach (Balneario Carnboriu), and on the reflective beach of Taquaras/Taquarinhas. Brava beach did not show clear patterns of short-term beach rotation, but there was a subdivision of the beach into two sectors with different magnitudes of sediment removal and behaviour. The occurrence of short-term beach rotation processes in some of the beaches indicates that, erosive events are often caused by a realignment of the beach shoreline in response to a shift in incident wave direction. In these cases the sediment eroded is not lost from the beach system but deposited elsewhere along the beach, and often returning to the initial location in response to a new shift in wave direction.