Morphodynamics of intermediate beaches: a video imaging and numerical modelling study

Ranasinghe, Roshanka; Symonds, Graham; Black, Kerry P.; Holman, Robert A.

doi:10.1016/j.coastaleng.2004.07.018

Cited by 141 publications

(117 citation statements)

References 43 publications

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“…The response time of morphology is much greater than the variation in hydrodynamic forcing so in order for the beach-state to be in equilibrium the wave conditions must be constant for up to several weeks (e.g. Short, 1987;Ranasinghe et al, 2004). Many features of the swash spectra did vary with beach face gradient, which is a broad (and less restrictive) proxy for beach-state (Table 1; Wright et al, 1979;Short, 1979), including the width of the 4  f energy roll-off band ( Figure 5).…”

Section: Accepted M Manuscriptmentioning

confidence: 99%

Spectral signatures for swash on reflective, intermediate and dissipative beaches

et al. 2014

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In this paper we synthesise a large data set gathered from a wide variety of field deployments and integrate it with previously published results to identify the spectral signatures of swash from contrasting beach types. The field data set includes the full range of micro-tidal beach types (reflective, intermediate and dissipative), with beach gradients ranging from approximately 1:6 to 1:60 exposed to offshore significant wave heights of 0.5 m to 3.0 m.The ratio of swash energy in the short-wave (f>0.05 Hz) to long-wave (f<0.05 Hz) frequency bands is found to be significantly different between the three beach types. Swash energy at short-wave frequencies is dominant on reflective and intermediate beaches and swash at long-wave frequencies is dominant on dissipative beaches; consistent with previously reported spectral signatures for the surf zone on these beach types.The available swash spectra were classified using an automated algorithm (CLARA) into five different classes. The ordered classes represent an evolution in the spectrum shape, described by a frequency downshifting of the energy peak from the short-wave into the longwave frequency band and an increase in the long-wave swash energy level compared to a relatively minor variation in the short-wave swash energy level. A universally common feature of spectra from all beach-states was an 4  f energy roll-off in the short-wave frequency band. In contrast to the broadly uniform appearance of the short-wave frequency band, the appearance of the long wave frequency band was highly variable across the beachstates. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT

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Section: Accepted M Manuscriptmentioning

confidence: 99%

Spectral signatures for swash on reflective, intermediate and dissipative beaches

et al. 2014

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“…Invariably, the inner bar system is more three-dimensional than the outer bar system; the former tends to be of the welded and transverse bar type, whereas crescentic and linear bar configurations are more common for the latter (e.g., Price and Ruessink, 2011). Upstate (3D to 2D; offshore migration) and downstate (2D to 3D; onshore migration) transitions occur under increasing and decreasing wave energy conditions, respectively (Short, 1979;Lippmann and Holman, 1990;Ranasinghe et al, 2004;Poate et al, 2014), although longshore currents are also thought to play a role in straightening bar morphology (Price and Ruessink, 2011). There is also morphodynamic feedback between the outer and inner bar system; specifically, the outer bar protects the inner bar from energetic wave actions (Coco et al, 2014), whilst also controlling wave breaker patterns and nearshore current circulation over the inner bar system, even under less energetic wave conditions (Castelle et al, 2010a, b).…”

Section: Introductionmentioning

confidence: 99%

“…This beach state model stimulated a large volume of subsequent research into the key factors controlling beach morphology. Some of these investigations sought to confirm the validity of the model by rigorously analysing more objective and extensive data sets (Lippmann and Holman, 1990;Ranasinghe et al, 2004). Other follow-up studies presented alternative models more suitable for different types of environments; for example, multi-bar beaches (Short and Aagaard, 1993;Short, 1992), low-wave energy settings (Hegge et al, 1996;Jackson et al, 2002) and large tidal environments (Short, 1991;Masselink and Short, 1993;Masselink and Hegge, 1995).…”

Section: Introductionmentioning

confidence: 99%

Role of wave forcing, storms and NAO in outer bar dynamics on a high-energy, macro-tidal beach

et al. 2014

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Outer sand bar dynamics on a high-energy macro-tidal beach were investigated using long-term (multi-year) field datasets of intertidal morphology and offshore bathymetry. Utilising a 15-year time-series of Argus video images, five distinct outer bar types were identified: Mega Rip, Longshore, Crescentic, Crescentic Attached and Welded. The most common classification was Crescentic Attached, with the outer bar oscillations being out of phase with the inner bar oscillations, as would be expected for the shore-normal wave approach at this site. The outer bar had a typical amplitude of 0.5-1 m and a longshore wavelength of 600 m. Changes in outer bar morphology were related to measured and modelled nearshore wave data. However, the outer bar morphology changed over a much longer time scale (monthly-to-annual) than the daily-to-weekly variations in wave height and period. An extended duration of energetic wave action was required to bring about an upstate bar transition to the Longshore or Mega Rip state, where the bar then remained arrested for a significant amount of time, requiring several months of low wave conditions to induce a down state transition through Crescentic to Welded. This slow morphological response is explained by extended relaxation times attributed to the large tidal range at the study site where the outer bar morphology is only active for part of the tidal cycle (several hours around low tide). The configuration and position of the outer bar were related: the more upstate (downstate) bar types being associated with a more offshore (onshore) bar position. The detrended outer bar position was significantly related to a forcing term based on wave power and disequilibrium of the dimensionless fall velocity with offshore (onshore) bar migration occurring when wave conditions were more (less) energetic that the antecedent conditions. The upstate end member (Longshore or Mega Rip) was attained sometime during the winter months for 14 out of the 16 years of monitoring; the outer bar remained attached to the low tide shoreline over the winter 2005/2006 and 2010/2011. These two winters with incomplete upstate cycles were characterised by the lowest winter wave conditions and negative winter North Atlantic Oscillation (NAO) indices, suggesting that the winter NAO is correlated with both beach state and nearshore bar configuration. KEYWORDS

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“…Numerous observations and long-term monitoring of the nearshore zone have revealed the wide range of shapes that nearshore sandbars may attain (e.g. Wright and Short, 1984;Lippmann and Holman, 1990;Van Enckevort et al, 2004;Ranasinghe et al, 2004). Despite each observed sandbar configuration being unique, and the continuous change in shape under the influence of waves and currents, a certain regularity in sandbar morphology has been observed.…”

Section: Introductionmentioning

confidence: 99%

Morphological coupling in multiple sandbar systems – a review

Price

Ruessink

Castelle³

2014

Earth Surf. Dynam.

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Abstract. Subtidal sandbars often exhibit alongshore variable patterns, such as crescentic plan shapes and rip channels. While the initial formation of these patterns is reasonably well understood, the morphodynamic mechanisms underlying their subsequent finite-amplitude behaviour have been examined far less extensively. This behaviour concerns, among other aspects, the coupling of alongshore variable patterns in an inner bar to similar patterns in a more seaward bar, and the destruction of crescentic patterns. This review aims to present the current state of knowledge on the finite-amplitude behaviour of crescentic sandbars, with a focus on morphological coupling in double sandbar systems. In this context we include results from our recent study, based on a combination of remote-sensing observations, numerical modelling and data-model integration. Morphological coupling is an inherent property of double sandbar systems, where the inner bar may attain a type of morphology not found in single bar systems. Coupling is governed by water depth variability along the outer-bar crest and by various wave characteristics, including the offshore wave height and angle of incidence. In recent research, the role of the angle of wave incidence for sandbar morphodynamics has received more attention. Numerical modelling results have demonstrated that the angle of wave incidence is crucial to the flow pattern, sediment transport, and thus the emerging morphology of the coupled inner bar. Moreover, crescentic patterns predominantly vanish under highangle wave conditions, highlighting the role of alongshore currents in straightening sandbars and challenging the traditional conception that crescentic patterns vanish under high-energy, erosive wave conditions only.

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Morphodynamics of intermediate beaches: a video imaging and numerical modelling study

Cited by 141 publications

References 43 publications

Spectral signatures for swash on reflective, intermediate and dissipative beaches

Spectral signatures for swash on reflective, intermediate and dissipative beaches

Role of wave forcing, storms and NAO in outer bar dynamics on a high-energy, macro-tidal beach

Morphological coupling in multiple sandbar systems – a review

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