132420137
Observations on sandbar behaviour along a man‐made curved coast
Sandbars, submerged ridges of sand parallel to the shoreline, affect surfzone circulation, beach topography and beach width. Under time-varying wave forcing, sandbars may migrate onshore and offshore, referred to as two-dimensional (2D) behaviour, and vary in planshape from alongshore uniform ridges to alongshore non-uniform ridges through the growth and decay of three-dimensional (3D) patterns, referred to as 3D behaviour. Although 2D and 3D sandbar behaviour is reasonably well understood along straight coasts, this is not the case for curved coasts, where the curvature can invoke spatial variability in wave forcing. Here, we analyse sandbar behaviour along the ∼3000 m man-made curved coastline of the Sand Engine, Netherlands, and determine the wave conditions governing this behaviour. 2D and 3D behaviour was quantified within a box north and west of the Sand Engine's tip, respectively, using a 2.4-year dataset of daily low-tide video images and a sparser bathymetric dataset. The northern and western sides behaved similarly in terms of 2D behaviour, with seasonal onshore and offshore migration, resulting in a stable position on inter-annual timescales. However, both sandbar geometry and 3D behaviour differed substantially between both sides. The geometric differences (bar shape, bar crest depth and wavelength of 3D patterns) are consistent with computed alongshore differences in breaker height due to refraction. The differences in the timing in growth, decay and morphological coupling of 3D patterns in the sandbar and shoreline are likely related to differences in the local wave angle, imposed by the curved coast. Similar dependency of bar behaviour on local wave height and angle may be expected elsewhere along curved coasts, e.g. shoreline sandwaves, cuspate forelands or embayed beaches. Copyright © 2017 John Wiley & Sons, Ltd.
140690043
Bar/trough generation on a natural beach
Mechanisms for bar/trough generation are examined using velocities measured in the field applied to the Bowen [1980]/Bailard [1981] energetics-based sediment transport model. Measurements consist of a cross-shore array of nine electromagnetic current meters spanning the surf zone and daily bathymetric surveys during a 10-day period during which two storms occurred, when the bathymetry evolved from a three-dimensional terrace to a well-developed linear bar. The model predicts bed and suspended load transport separately based on various velocity moments. The velocities are partitioned into mean currents, low-frequency infragravity and shear instabilities ( 0.05 Hz) to determine the relative importance of various mechanisms to the total transport. Velocity moments are computed over 90-min intervals to resolve tidal fluctuations. Tidal signatures were apparent in all modes of transport. Predicted transport rates are integrated and compared with daily cross-shore bathymetric profiles (averaged over a 400-m length of beach). The suspended load terms were an order of magnitude greater than bed load terms owing to the low fall velocity of the fine-grain sand within the surf zone. Model results for this experiment indicate the dominant mechanism for bar development was sediments mobilized by the strong longshore current and incident short waves within the surf zone and transported offshore by the mean undertow and shoreward transport onshore due to short wave velocity skewness. Using standard coefficients, the model correctly predicted the first-order movement of the bar during storms, but underpredicted trough development, and did not always perform well during mild wave conditions.
203600246
Observations of sand bar evolution on a natural beach
W•ves, currents, •nd the loc•tion of the se•fioor were measured on • b•rred be•ch for •bout 2 months •t nine loc•tions •long • cross-shore tr•nsect extending 255 m from i to 4 m w•ter depth. The se•fioor loc•tion w•s measured nearly continuously, even in the surf zone during storms, with sonar •ltimeters mounted on fixed frames. The crest of • s•nd b•r initially located •bout 60 m from the shoreline moved 130 m offshore (primarily when the offshore significant w•ve height exceeded •bout 2 m), with 1.5 m of erosion near the initial loc•tion •nd 1 m of •ccretion •t the final loc•tion. An energetics-type sediment transport model driven by locally measured near-bottom currents predicts the observed offshore b•r migration, but not the slow onshore migration observed during low-energy w•ve conditions. The predicted offshore b•r migration is driven primarily by cross-shore gradients in predicted suspended sediment transport •ssoci•ted with quasi-steady, near-bottom, offshore flows. These strong (• 50 cm/s) currents, intensified near the b•r crest by w•ve breaking, •re predicted to c•use erosion on the shoreward slope of the b•r •nd deposition on the seaward side. The feedback •moung morphology, w•ves, circulation, •nd sediment transport thus forces offshore b•r migration during storms.
129183732
Nearshore sandbar migration
Field observations suggest that onshore sandbar migration, observed when breaking-wave-driven mean flows are weak, may be related to the skewed fluid accelerations associated with the orbital velocities of nonlinear surface waves. Large accelerations (both increases and decreases in velocity magnitudes), previously suggested to increase sediment suspension, occur under the steep wave faces that immediately precede the maximum onshore-directed orbital velocities. Weaker accelerations occur under the gently sloping rear wave faces that precede the maximum offshore-directed velocities. The timing of strong accelerations relative to onshore flow is hypothesized to produce net onshore sediment transport. The observed acceleration skewness, a measure of the difference in the magnitudes of accelerations under the front and rear wave faces, is maximum near the sandbar crest. The corresponding cross-shore gradients of an acceleration-related onshore sediment transport would cause erosion offshore and accretion onshore of the bar crest, consistent with the observed onshore migration of the bar crest. Furthermore, the observations and numerical simulations of nonlinear shallow water waves show that the region of strongly skewed accelerations moves shoreward with the bar, suggesting that feedback between waves and evolving morphology can result in continuing onshore bar migration.
25011665
Wave-Induced Sediment Transport and Sandbar Migration
Onshore sediment transport and sandbar migration are important to the morphological evolution of beaches but are not well understood. Here, a model that accounts for fluid accelerations in waves predicts the onshore sandbar migration observed on an ocean beach. In both the observations and the model, the location of the maximum acceleration-induced transport moves shoreward with the sandbar, resulting in feedback between waves and morphology that drives the bar shoreward until conditions change. A model that combines the effects of transport by waves and mean currents simulated both onshore and offshore bar migration observed over a 45-day period.
128756118
Three-Dimensional Beach Changes
Beach forms, such as crescentic bars and cuspate shoreline, manifest a three-dimensional structure of rhythmic topography characterized by periodic longshore undulations of both the surf zone bed and the subaerial surface. The three-dimensional structure and the tendency for migration of rhythmic topography cause distinctive modes of beach change. Since crescentic outer bars tend to migrate parallel to shore, profile changes involving an outer-bar system generally reflect two unrelated processes, one arising from net transformation of individual profiles and the other from longshore displacement of a whole rhythmic system in response to currents. Crescentic inner bars exhibit a tendency for predominant shoreward migration during the time of swells. Under this condition, a subaerial beach will initially assume a ridge-and-runnel-type structure and eventually a uniform two-dimensional profile when full accretion is reached. Beach changes involving crescentic inner bars can produce a beach cycle in response to a succession of random storms. A model predicts the most likely cycle length, which agrees with the field observation. Crescentic bars forming between headlands appear to be generated by a standing edge wave continuously excited by an incident seiche of beat frequency, whereas rhythmic topographies on an unbounded coast appear to be caused by sand-wave trains which result from instability of the surf zone bed perturbed by longshore currents and waves.
128549224
Morphodynamic variability of surf zones and beaches: A synthesis
Abstract A synthesis of some results obtained over the period 1979–1982 from a study of beach and surf zone dynamics is presented. The paper deals with the different natural beach states, the process signatures associated with these states, environmental controls on modal beach state, and the temporal variability of beach state and beach profiles. Hydrodynamic processes and the relative contributions of different mechanisms to sediment transport and morphologic change differ dramatically as functions of beach state, that is depending on whether the surf zone and beach are reflective, dissipative or in one of several intermediate states. Depending on beach state, near bottom currents show variations in the relative dominance of motions due to: incident waves, subharmonic oscillations, infragravity oscillations, and mean longshore and rip currents. On reflective beaches, incident waves and subharmonic edge waves are dominant. In highly dissipative surf zones, shoreward decay of incident waves is accompanied by shoreward growth of infragravity energy; in the inner surf zone, currents associated with infragravity standing waves dominate. On intermediate states with pronounced bar-trough (straight or crescentic) topographies, incident wave orbital velocities are generally dominant but significant roles are also played by subharmonic and infragravity standing waves, longshore currents, and rips. The strongest rips and associated feeder currents occur in association with intermediate transverse bar and rip topographies. Long-term consecutive surveys of different beaches with contrasting local environmental conditions provide the data sets for empirical—statistical assessment of beach mobility, direction of change and response to environmental conditions. Conditions of persistently high wave energy combined with abundant and/or fine grained sediment results in maintaining highly dissipative states which exhibit very low mobility. Relatively low mobility is also associated with persistently low-steepness waves acting on coarsegrained beach sediments. In such cases, the modal beach state is reflective. The greatest degree of mobility is associated with intermediate but highly changeable wave conditions, medium grained sediment and a modest or meager sediment supply. Under such conditions, the beach and surf zone tend to alternate among the intermediate states and to exhibit well-developed bar trough and rhythmic topographies. A good association is found between beach state and the environmental parameter Ω = H b ( w s T ) where Hb is breaker height, w s is mean sediment fall velocity and T is wave period. Temporal variability of beach state reflects, in part, the temporal variability and rate of change of Ω, which, in turn depends on deep-water wave climate and nearshore wave modifications.
59145972
The spatial and temporal variability of sand bar morphology
The spatial and temporal variability of nearshore sand bar morphology is quantified using a unique data set spanning 2 years. The data consist of daily time exposure images of incident wave breaking on an open coast sandy beach which may be used to infer bar morphology (Lippmann and Holman, 1989). The morphology in each image is classified into an eight state morphologic scheme in which bars are uniquely defined by four independent criteria. The most frequently observed morphologies are the longshore-periodic (rhythmic) bars, observed in 68% of the data. Linear bars occur under highest wave conditions (H¯s=1.78 m) and are unstable (mean residence time ≈ 2 days). Shore-attached rhythmic bars are the most stable (mean residence time ≈ 11 days) and generally form 5–16 days following peak wave events. Non-rhythmic, three-dimensional bars are very transient (mean residence time ≈ 3 days). Eighty-seven percent of transitions to lower bar types (defined in text) occurred one state at a time, supporting our selection of the ordering of states, and suggesting the suitability of a sequential morphology model. Transitions to higher states occurred under rising wave energy and were evenly spread among the possible higher states, with more substantial changes in morphology resulting from larger wave increases. This suggests that up-state, erosional transitions (based on offshore bar migration) are better described by an equilibrium model where response is better correlated with incident wave energy than with preceding morphological state. Time exposure images were also digitized to yield quantitative estimates of bar crest location as a function of longshore distance. Principal component analysis was used to decompose bar position into two-dimensional (linear) and three-dimensional (longshore variable) components. Cross-shore (linear) bar position ranges ±50 m about the 2-year mean (27 m standard deviation) and dominates bar variability (74.6%). Three-dimensional bar structure accounts for ∼14% of the variance (12 m standard deviation). Changes in incident wave height precede cross-shore bar migration by less than 1 day. Changes in longshore variability are inversely correlated to changing wave conditions, with bar morphology becoming linear rapidly during storms (on time scales of less than 1 day). Evolution to significantly three-dimensional structure typically occurs over 5–7 days following peak wave events.
129667876
Observations and conceptual modelling of morphological coupling in a double sandbar system
Nearshore sandbars, located in <10 m water depth, can contain remarkably periodic alongshore undulations in both cross-shore position and depth. In a double sandbar system, the alongshore spacing of these morphological patterns in the inner sandbar may be identical to those in the outer sandbar. Although this morphological coupling has been observed previously, its frequency and predominance remain unclear. In this paper, we use a 9.3-year dataset of daily low-tide time exposure images from the double-barred beach at Surfers Paradise (Gold Coast, Australia) to analyse the temporal and spatial characteristics of morphological coupling within a double sandbar system. We distinguish five types of morphological coupling between the inner and outer sandbars, of which four coincide with a downstate progression of the outer bar. Coupling is either in-phase (with a landward perturbation of the inner bar facing an outer-bar horn) or out-of-phase (with a seaward perturbation of the inner bar facing an outer-bar horn), where the coupled inner-bar features either consist of rip channels or, predominantly, perturbations of the low-tide terrace. Cross-correlation of the image-derived inner- and outer-bar patterns shows coupling to be a common phenomenon in the double sandbar system studied here, with coupling in 40% of the observations. In contrast to previous observations of sandbar–shoreline coupling at single-barred beaches, in-phase coupling (85% of all coupled bar patterns) predominates over out-of-phase coupling (15%). Based on our observations and bathymetries assimilated from the images for a restricted set of coupling events, we hypothesize that the angle of offshore wave incidence, wave height and depth variations along the outer sandbar determine the type of flow pattern (cell circulations versus meandering currents) above the inner bar and hence steer the type of coupling. Copyright © 2012 John Wiley & Sons, Ltd.
127364147
Sandbar straightening under wind-sea and swell forcing
Abstract Sandbar straightening on natural beaches is known to occur following storm events. However, on a typical south-west Western Australia beach, sandbar straightening is observed under low amplitude wind-sea forcing in summer and remains alongshore-variable following high wave events associated with winter storms. During summer, diurnal variations in wave height and direction are associated with a strong sea breeze cycle. At the peak of the sea breeze the local wind-sea approaches the beach obliquely, driving strong alongshore currents. The occurrence of the seasonal, obliquely-incident wind-sea provides the opportunity for testing the role of the wave angle on sandbar straightening. The location of the bar crest is observed using time exposure video images and the alongshore variability is quantified using the standard deviation of the sandbar position. The role of wave incidence angle on alongshore-variability is highlighted, as the sandbar straightens in the presence of oblique wave forcing associated with summer sea breezes, and remains alongshore-variable with normally incident wave forcing. The transition from alongshore-variable to alongshore-uniform also depends on the antecedent bathymetry, in particular the distance between the shore and the bar crest. When the sandbar is further offshore, alongshore variability persists in the presence of strong sea breezes.
55753757
Morphodynamic response of nearshore bars to a shoreface nourishment
Abstract The autonomous nearshore bar behaviour along the barrier island of Terschelling, The Netherlands, is characterised by the presence of net seaward cyclic migrating sand bars generated near the shoreline. In 1993, a perturbation of the cyclic bar system was introduced by the implementation of a 2 Mm 3 shoreface nourishment supplied to the nearshore bar zone, filling up the trough between the middle and outer bar. The morphodynamic response of the nearshore bars to the nourishment perturbation is investigated using a bathymetric data set with an alongshore extent of 12 km and sampled for 10 years. Bar behaviour is quantified in terms of bar crest position in relation to morphometric parameters such as bar depth, height, width and volume. Along with a pronounced development of a three-dimensional bar system unseen in the autonomous behaviour, the nearshore bars exhibited a 6–7 year arrest in their migrational behaviour during which bar morphology remained stable at immediate pre-nourishment morphometric values. At the subsequent onset of bar movement, bars resumed their migration at a rate predicted by autonomous behaviour in parallel development with morphometric parameters along their predicted trends. It is shown that the observed onshore transport of nourished sediment in the 6–7 year arrest results from a gradual deepening of troughs. Cross-shore sediment transport modelling is used to assess the effect of the nourishment on yearly averaged onshore (short-wave nonlinearity) and offshore (undertow) sediment transport rates. The gradual reappearance of the pre-nourishment bar-trough morphology is shown to engender a normalisation in the cross-shore distribution of sediment transport rates to pre-nourishment rates.
129798515
Morphodynamic response of a two-barred beach to a shoreface nourishment
Abstract The response to a shoreface nourishment of the two-bar system at Noordwijk (the Netherlands) is analyzed based on a daily data set of time-exposure video images collected during about 6 years, complemented with topographic and bathymetric surveys. The 1.7 Mm 3 nourishment, implemented as a 3 km alongshore bump seaward of the outer bar, migrated more than 300 m onshore in 4 years before losing its integrity. Furthermore, the nourishment interrupted the autonomous seaward migration of both bars for the entire duration of the study period and, allaying earlier fears, did not intensify the three-dimensional patterns in the bars, such as the crescentic plan-shape and rip channels. The nourishment did result in clear head effects on both flanks, with the bar becoming discontinuous and the flank section decaying or becoming attached to an offshore-located bar, while the section of bar landward of the nourishment became attached to a landward-located bar. This sequence of morphologies is known as bar switching. Each switching episode took almost one year to complete and can therefore not be ascribed to individual wave events. We suspect that shoreface nourishments enhance the possibility of bar switching by creating alongshore variability in the position and depth of the outer bar and in its cross-shore migration rate and direction. The Noordwijk nourishment did not influence the shoreline position as its trend did not undergo distinctive variations after 1998. Differences in the response of the Noordwijk sandbars to the shoreface nourishment compared with other Dutch nourishments are attributed to the location and size (volume per unit length) of the nourishment with respect to the sandbars and to the median grain size of the nourished material.
130382547
A New Alternative to Saving Our Beaches from Sea-Level Rise: The Sand Engine
ABSTRACT Stive, M.J.F.; de Schipper, M.A.; Luijendijk, A.P.; Aarninkhof, S.G.J.; van Gelder-Maas, C.; van Thiel de Vries, J.S.M.; de Vries, S.; Henriquez, M.; Marx, S., and Ranasinghe, R., 2013. A new alternative to saving our beaches from local sea-level rise: the sand engine. A boldly innovative soft engineering intervention, comprising an unprecedented 21.5 Mm3 sand nourishment known as the Sand Engine, has recently been implemented in the Netherlands. The Sand Engine nourishment is a pilot project to test the efficacy of local mega-nourishments as a counter measure for the anticipated enhanced coastal recession in the 21st century. The proposed concept, a single mega-nourishment, is expected to be more efficient, economical, and environmentally friendly in the long term than traditional beach and shoreface nourishments presently being used to negate coastal recession. Preliminary numerical model results indicate that this local nourishment will result in the widening of the beach along a 10 to 20 km stretch of the coastline and a beach area gain of 200 ha over a 20-year period. First observations show indeed a redistribution of the sand feeding the adjacent coasts, roughly 40% toward the south and 60% toward the north. While the jury is still out on this globally unique intervention, if proven successful, it may well become a global generic solution for combating sea-level-rise driven coastal recession on open coasts.
128992095
Coastal systems and low-lying areas
Since the IPCC Third Assessment Report (TAR), our understanding of the implications of climate change for coastal systems and low-lying areas (henceforth referred to as ‘coasts’) has increased substantially and six important policy-relevant messages have emerged. Coasts are experiencing the adverse consequences of hazards related to climate and sea level (very high confidence). Coasts are highly vulnerable to extreme events, such as storms, which impose substantial costs on coastal societies [6.2.1, 6.2.2, 6.5.2]. Annually, about 120 million people are exposed to tropical cyclone hazards, which killed 250,000 people from 1980 to 2000 [6.5.2]. Through the 20th century, global rise of sea level contributed to increased coastal inundation, erosion and ecosystem losses, but with considerable local and regional variation due to other factors [6.2.5, 6.4.1]. Late 20th century effects of rising temperature include loss of sea ice, thawing of permafrost and associated coastal retreat, and more frequent coral bleaching and mortality [6.2.5]. Coasts will be exposed to increasing risks, including coastal erosion, over coming decades due to climate change and sea-level rise (very high confidence). Anticipated climate-related changes include: an accelerated rise in sea level of up to 0.6 m or more by 2100; a further rise in sea surface temperatures by up to 3°C; an intensification of tropical and extratropical cyclones; larger extreme waves and storm surges; altered precipitation/run-off; and ocean acidification [6.3.2]. These phenomena will vary considerably at regional and local scales, but the impacts are virtually certain to be overwhelmingly negative [6.4, 6.5.3].
131283203
Initial spreading of a mega feeder nourishment : Observations of the Sand Engine pilot project
Sand nourishments are a widely applied technique to increase beach width for recreation or coastal safety. As the size of these nourishments increases, new questions arise on the adaptation of the coastal system after such large unnatural shapes have been implemented. This paper presents the initial morphological evolution after implementation of a mega-nourishment project at the Dutch coast intended to feed the surrounding beaches. In total 21.5 million m3 dredged material was used for two shoreface nourishments and a large sandy peninsula. The Sand Engine peninsula, a highly concentrated nourishment of 17 million m3 of sand in the shape of a sandy hook and protruding 1 km from shore, was measured intensively on a monthly scale in the first 18 months after completion. We examine the rapid bathymetric evolution with concurrent offshore wave forcing to investigate the feeding behaviour of the nourishment to the adjacent coast. Our observations show a large shoreline retreat of O (150 m) along the outer perimeter of the peninsula, with locally up to 300 m retreat. The majority (72%) of the volumetric losses in sediment on the peninsula (1.8 million m3) were compensated by accretion on adjacent coastal sections and dunes, confirming the feeding property of the mega nourishment. Further analyses show that the morphological changes were most pronounced in the first 6 months while the planform curvature reduced and the surf zone slope flattened to pre-nourishment values. In the following 12 months the changes were more moderate. Overall, the feeding property was strongly correlated to incident wave forcing, such that months with high incoming waves resulted in more alongshore spreading. Months with small wave heights resulted in minimal change in sediment distribution alongshore and mostly cross-shore movement of sediment.
55267930
Assessment of beach states at the Sand Motor on the basis of alongshore variability in bathymetry data
Beach state classification, though simple in its concept, is difficult to be distinctly and accurately applied in some real life situations. The issue is further compounded when it is applied to mega-nourishments which, in essence, are morphologically different in terms of an artificially-formed beach versus a naturally-forming beach. Yet, another contributing factor could be due to the difference in geographical locations, with varying wave heights, wave periods and sediment sizes, which result in different permutations of the dimensionless fall-velocity (Ω) values. The six beach state classification scheme presented by Wright and Short (1984) has been the widely-accepted guidelines in the coastal engineering community. Even when references are made to these diagrams (Refer to Figure 2.1), there is an issue of subjectivity as not all coastal researchers may reach the same conclusion when the same image of a particular nearshore morphology is used. This has been found through a questionnaire finding. This paper thus attempts to present a conceptual quantification technique which analyses the alongshore variability along the coast of the Sand Motor as a basis to classify beach state indiscriminately and objectively. (RBB) and Transverse Bar and Rip (TBR), have been observed at the Sand Motor from bathymetric surveys which spanned a monitoring period of two years. It has been found that the success rate of executing the objective beach state classification via the conceptual quantification technique is 94.0% for LBT, 66.6% for RBB and 54.4% for TBR features. Interestingly, LBT has been found to be the persistent and occasional beach state at the updrift and downdrift of the Sand Motor respectively. Furthermore, it has been observed that these occurrences of beach state along the Sand Motor can be divided into three temporal stages namely the formative, the dormant and the developed phases. Stage I has been associated with the formative period of the Sand Motor since its completion in August 2011. It has largely been affected by the formation of a spit and its subsequent dynamics while advancing towards the downdrift of the Sand Motor. Stage II has been a relatively dormant phase where there were no significant changes to the observed beach states. Stage III is considered the developed phase where beach states could be observed along the whole analysed length of the Sand Motor. These three stages have been found to be sensitive to seasonal variations, particularly the increased number of storm events during winter period. The …
131215007
Mesoscale Behaviour of Longshore Bars — Net Onshore or Net Offshore Migration
The aim of this study is to compare long-term (30 yr) bar migration characteristics at two field sites on the Danish North Sea coast. One site displays systematically onshore migrating bars (Skallingen) while the bar migration at the other site (Vejers) is systematically offshore. Using cross-shore profile data and a sediment transport model, the paper discusses the mechanisms that cause divergent longshore bar behaviour at the two sites. The main reason for the difference appears to be larger undertow speeds at Vejers, caused by a steeper outer shoreface and a larger bar relief, leading to breaking wave reformation between bars. INTRODUCTION With the emergence of data sets on long-term profile change in e.g. the Netherlands and on the East coast of the US, it has been realized that multiple bar systems often display a tendency towards a systematic offshore migration of individual bars. Observations of such systematic seaward bar displacements were reported by Kroon (1990) and Wijnberg and Terwindt (1995) off the Dutch coast. Similar offshore migration tendencies were observed at Duck, N.C. (Plant et al., 1999) and in New Zealand (Shand and Bailey, 1999). The characteristics of the offshore bar migration cycle are similar at most sites. Longshore bars are formed close to the shoreline, they oscillate back and forth in the inner surf zone for a few years and then they begin to migrate consistently offshore. In water depths of 4-7 m, the migration cycle slows down and stops, subsequent to which the bars decay and eventually disappear. The return period for the bar migration cycle ranges between 1-13 years for the different study sites (Ruessink and Terwindt, 2000; Ruessink et al., 2003). Coastal Sediments '07 © 2007 ASCE
129304741
Storm-Driven Shoreface Morphodynamics on a Low-Wave Energy Delta: The Role of Nearshore Topography and Shoreline Orientation
Abstract An analysis of storm-driven sedimentary changes on the shoreface of a low–wave-energy cuspate delta on the southern coast of Spain is examined. Bathymetric surveys, ranging from 1- to 40-m depth, were collected 2 days before and 9 days after a high-magnitude Levante storm. The storm lasted only 36 hours, with peak wind gusts of 100 km/h (28 m/s) and maximum wave heights of 7.4 m. Storms of similar magnitude have only been recorded four times in the last 20 years along this coast. The deltaic nearshore and shoreface was divided into three main morphologic and morphodynamic zones based on process-response mechanisms following the storm event. The eastern section, characterised by a steep and concave shoreface, only had minor erosion and deposition as a result of the storm. The area seaward of the river mouth, an intermediate zone where the coastline changes orientation, is characterised by a widening surf zone and an irregular shoreface out to 10-m depth before dipping steeply to 40-m depth. Poststorm changes along this transitional area resulted in the deposition of a thin veneer of sediment over a 3 km2 area from 10- to 40-m depth. The western section, a shallow and wide delta front with a double-bar, a trough, and shore-oblique topography, was modified the most by the storm, with significant upper shoreface erosion and shore-oblique erosional scars in 5–7-m depth. Results of spectral wave modelling of wave-induced force match well with observed morphological change; force is minimal along the east coast, more pronounced along the steep shoreface opposite the river mouth, and at a maximum along the west coast. It is suggested that coastal orientation, shoreface morphology (topography, gradients), and storm parameters (wave and wind direction) generated contrasting wind and wave-induced currents on the delta shoreface, resulting in spatial differences in storm response.
210058007
WAVE POWER AND BEACH-STAGES: A GLOBAL MODEL
A three-dimensional morphodynamic model of sequential beach changes Is presented. The model Is based on variations in breaker wave power generating a predictable sequence of beach conditions. The spectrum of beach conditions from fully eroded-dissipatlve to fully accreted reflective is characterised by ten beach-stages. Using the breaker wave power to beach-stage relationship the model Is applied to explain temporal, spatial and global variations In beach morphodynamlcs.
50081191
Rip currents, mega-cusps, and eroding dunes
Abstract Dune erosion is shown to occur at the embayment of beach mega-cusps O(200 m alongshore) that are associated with rip currents. The beach is the narrowest at the embayment of the mega-cusps allowing the swash of large storm waves coincident with high tides to reach the toe of the dune, to undercut the dune and to cause dune erosion. Field measurements of dune, beach, and rip current morphology are acquired along an 18 km shoreline in southern Monterey Bay, California. This section of the bay consists of a sandy shoreline backed by extensive dunes, rising to heights exceeding 40 m. There is a large increase in wave height going from small wave heights in the shadow of a headland, to the center of the bay where convergence of waves owing to refraction over the Monterey Bay submarine canyon results in larger wave heights. The large alongshore gradient in wave height results in a concomitant alongshore gradient in morphodynamic scale. The strongly refracted waves and narrow bay aperture result in near normal wave incidence, resulting in well-developed, persistent rip currents along the entire shoreline. The alongshore variations of the cuspate shoreline are found significantly correlated with the alongshore variations in rip spacing at 95% confidence. The alongshore variations of the volume of dune erosion are found significantly correlated with alongshore variations of the cuspate shoreline at 95% confidence. Therefore, it is concluded the mega-cusps are associated with rip currents and that the location of dune erosion is associated with the embayment of the mega-cusp.
128411887
Storm-driven changes in rip channel patterns on an embayed beach
This paper introduces a semi-automatic computer algorithm designed to detect rip current channels in video imagery. As a case study, this method is applied to 3.3 years of video data from an embayed beach to demonstrate the link between antecedent surf zone morphology, wave energy and up/down transitions in beach state. An objective measure of rip channel change was developed to define six significant rip reconfiguration events and relate these events to wave energy. Over the period of study no complete resets of the nearshore morphology occurred. The analysis indicates that direct correlation of rip patterns with the instantaneous wave conditions is not a useful way to demonstrate how rips and waves interact. The average wave energy over a period of ten days, combined with storm duration were good indicators of rip channel change, demonstrating that in general, beach morphology responds with a time lag to changes in forcing. Rip channels with a short cross-shore length and narrow alongshore spacing responded faster to changes in wave conditions than rips with a long cross-shore length and wider alongshore spacing. To force changes in the rip morphology, longer rip channels required wave events of higher energy and/or a longer duration. Offshore islands protect the beach under certain wave approach angles, sometimes resulting in a dual-width surf zone, which was narrow at the sheltered end and wide at the exposed end of the beach. The wider surf zone end was characterised by three dominant and persistent rip channels, whereas the narrow surf zone section contained a number of smaller rips which evolved rapidly under wave forcing. Our observations demonstrate the importance of rip channel size in controlling the response time of nearshore morphology.
51992330
Dynamics of single-barred embayed beaches
Abstract The dynamics of single submerged sandbars of two artificial embayed beaches (La Barceloneta and Bogatell, Barcelona, NW Mediterranean) has been studied with a video-recorded data set of 4.3 years. The alongshore-averaged cross-shore migration, the orientation with respect the shoreline and the sinuosity of the barlines have been analyzed and related to wave conditions, alongshore sediment transport and shoreline variability. In general, the submerged bars follow the general cyclic morphological behaviour observed in natural beaches, switching among the four intermediate morphodynamic states, but the studied beaches can be arrested during long periods of low wave conditions. The dominant up-state transition in Barcelona beaches during storms is the transition toward the rhythmic bar and beach state, the complete morphodynamic reset (longshore bar and trough state) only occurring during extreme wave events. The cross-shore migration of bars is dominated by the weekly and interannual components. The interannual component shows an onshore bar migration trend at both beaches, in contrast with the net offshore migration observed in multibarred open beaches. Bar disposition is located progressively seaward in the dominant alongshore transport direction (i.e., oblique with respect to the shoreline). At La Barceloneta beach, shoreline and barline orientations change consistently and a significant correlation between the accumulated alongshore sediment transport and the associated change in bar orientation has been found, suggesting that alongshore transport can play a significant role in barline orientation. Finally, bar sinuosity increases during eastern storms in both beaches. This indicates that the formation of crescentic bars occurs for approximately shore-normal waves. Some of the differences observed in bar morphology and mobility in the two studied beaches are related to their different level of protection with respect to the incident waves (beach indentation).
128551100
The Southern Oscillation Index, wave climate, and beach rotation
Short embayed beaches bounded by headlands are a common feature along the southern and central coastline of New South Wales, Australia. Many of these embayed beaches have experienced severe erosion at their southern end over the last decade. Previous studies have suggested that this erosion may be the result of an oscillatory medium-term phenomenon known as beach rotation. The present study was undertaken with the objectives of: (1) establishing definitive links between the Southern Oscillation Index (SOI), wave climate, and beach rotation, and (2) determining the physical processes governing beach rotation. Data from two similar New South Wales beaches were analysed using time series analysis techniques and image processing techniques (using ARGUS video images) in this study. The results indicate that the northern end of this type of beach accretes during El Nino phases, while the southern end erodes, resulting in a net clockwise rotation of the beach. The opposite occurs during La Nina phases resulting in a net anti-clockwise rotation of the beach. The beach response at the northern and southern ends lags SOI trend shifts by 3 and 17 months, respectively. Waves are predominantly incident from the southeast during both El Nino and La Nina phases. Offshore wave height is positively correlated with the SOI while offshore wave direction is negatively correlated with the SOI. On average, the number of storms per year doubles from El Nino to La Nina. Based on these links between beach width fluctuations, SOI, and incident wave conditions, a conceptual model of beach rotation is presented. The model describes the combinations of cross-shore and longshore sediment transport and hydrodynamic processes that are expected to result in the observed clockwise and anti-clockwise beach rotation during El Nino and La Nina phases, respectively.
129741858
New insights into embayed beach rotation: The importance of wave exposure and cross‐shore processes
Although embayed beach rotation has been viewed and modeled as an alongshore sediment transport process acting on a uniform beach profile, recent research suggests a more complex response whereby alongshore variability in cross-shore sediment fluxes may be more significant. This study utilizes 5 years of fully three-dimensional beach surveys at Narrabeen-Collaroy Beach (SE Australia) to quantify the control of alongshore nonuniform wave exposure and cross-shore processes on embayed beach rotation. Empirical orthogonal function analysis of the alongshore variability in subaerial beach volume/width and berm slope confirms that the dominant mode of subaerial beach variability is an onshore/offshore sediment exchange that is strongly controlled (R > 0.8) by the alongshore gradient in breaker wave height and coincides with a uniform flattening/steepening of the berm slope. A secondary rotation-like signal is observed in both the subaerial beach volume/width data and, significantly, the berm slope. This inverse flattening/steepening of the berm slope between beach extremities is most likely a proxy for differing cross-shore processes within the surf zone between the exposed and sheltered ends of the embayment, particularly with regards to dissipation of storm wave energy by offshore sandbars and beach recovery following storms. Analysis of the corresponding wave data reveals two distinct time scales of wave forcing characteristic of short-term erosion and longer-term recovery processes. A new conceptual model is presented of three differing modes of embayed beach rotation, with the newly identified beach rotation mode controlled by offshore sandbars considered of particular importance at embayments where headland sheltering of oblique waves is pronounced.
128804699
Observations of shoreline-sandbar coupling on an embayed beach
We analyse a seven-year dataset (1999–2005) of shoreline and sandbar variations derived from video observations at the embayed Tairua Beach, New Zealand, to explore sandbar–shoreline coupling and to determine how this coupling is related to alongshore-averaged sandbar–shoreline separation and beach rotation. Our results quantify the coupling between the sandbar and the shoreline which is directly related to the reduced separation between the sandbar and the shoreline. Using a simple predictive shoreline model, we show that this behaviour is related to the energy of the incoming waves. Our observations are obtained from a headland-enclosed beach and show aspects of the morphological state that relate to the existence of headlands that are not considered in the Wright and Short beach state classification. Both the sandbar and shoreline at Tairua Beach rotate relatively quickly (< 1 month) during the winter storm events, when the storms are accompanied by strong alongshore components, gradually returning during the summer months. The sandbar–shoreline coupling length scale is controlled by this rotation, with large wavelengths occurring when the sandbar is strongly rotated.
129239633
Morphodynamics of intermediate beaches: a video imaging and numerical modelling study
Four years of time-exposure images of Palm Beach, Sydney, Australia, are used to: (a) develop a phenomenological scheme based on surf zone properties for the classification of the four intermediate beach states [low tide terrace (LTT), transverse bar rip (TBR), rhythmic bar beach (RBB), and longshore bar trough (LBT)] identified by Wright, L.D., Short, A.D., 1984. Morphodynamic Variability of Surf Zones and Beaches. Marine Geology, 56 93-118; and (b) determine environmental conditions associated with the different beach states. The overall agreement between beach state classifications obtained by visual classifications of images and those predicted by the phenomenological scheme exceeds 90%. Analysis of time series of intermediate beach states and wave data indicates that the nearshore morphology continuously cycles through the four beach states in response to variations in the incident wave energy. The results also indicate that LBT morphology is produced by high-energy wave conditions (erosive), while the progression towards LTT occurs under lower waves (accretive). A depth-averaged morphodynamic model is employed to determine the morphodynamic processes governing the RBB to TBR and TBR to LTT transitions. Model results indicate that the RBB to TBR transition is governed by the erosion of the original longshore bars (in the initial RBB morphology) and the formation and subsequent welding of shoals in the lee of the original longshore bars. Model results also indicate that RBB to TBR transitions can occur under shore normal and oblique wave conditions. The TBR to LTT model simulations indicate that, under obliquely incident waves, progressive erosion of the transverse shoals and deposition of the eroded sediment in rip channels gradually diminishes the longshore variability in morphology, resulting in an LTT morphology.
53975693
Rip spacing and persistence on an embayed beach
[1] Four years of daily time exposure images from an embayed beach were examined to study the spacing, persistence, and location preferences of rips in a natural rip channel system. A total of 5271 rip channels was observed on 782 days. Occurrence statistics showed no evidence of the preferred location pattern associated with standing edge waves trapped in an embayed beach. The histogram of rip spacing, the primary diagnostic observable for most models, was well modeled by a lognormal distribution (mean spacing of 178 m). However, spacings were highly longshore variable (time mean of the standard deviation/longshore mean of rip spacing was 39%), so they are of questionable merit as a diagnostic variable. Storm-driven resets to the longshore uniform condition required by most models occurred only four times per year on average, making rip generation models relevant to only a small fraction of the system behavior. Rip spacings after the 15 observed reset events were uncorrelated with bar crest distance. The lifetime of the 324 individual rip channel trajectories averaged 45.6 days. Rips were equally mobile in both longshore directions, but the coefficient of variation of rip migration rates was large, even for high migration rates. The mean migration rate was well correlated to a proxy for the longshore current (R2 of 0.78). Thus there is no significant evidence that the formation, spacing, and migration of rip channels on this beach can be explained by currently existing simple models. Moreover, the alongshore uniform initial conditions assumed by these models are rare on Palm Beach, making the models generally inapplicable.
56336040
A behavior‐oriented dynamic model for sandbar migration and 2DH evolution
[1] A nonlinear model is developed to study the time‐dependent relationship between the alongshore variability of a sandbar, a(t), and alongshore‐averaged sandbar position, xc(t). Sediment transport equations are derived from energetics‐based formulations. A link between this continuous physical representation and a parametric form describing the migration of sandbars of constant shape is established through a simple transformation of variables. The model is driven by offshore wave conditions. The parametric equations are dynamically coupled such that changes in one term (i.e., xc) drive changes in the other (i.e., a(t)). The model is tested on 566 days of data from Palm Beach, New South Wales, Australia. Using weighted nonlinear least squares to estimate best fit model coefficients, the model explained 49% and 41% of the variance in measured xc and a(t), respectively. Comparisons against a 1‐D horizontal (1DH) version of the model showed significant improvements when the 2DH terms were included (1DH and 2DH Brier skill scores were −0.12 and 0.42, respectively). Onshore bar migration was not predicted in the 1DH model, while the 2DH model correctly predicted onshore migration in the presence of 2DH morphology and allowed the bar to remain closer to shore for a given amount of breaking, providing an important hysteresis to the system. The model is consistent with observations that active bar migration occurs under breaking waves with onshore migration occurring at timescales of days to weeks and increasing 2DH morphology, while offshore migration occurs rapidly under high waves and coincides with a reduction in 2DH morphology. Citation: Splinter, K. D., R. A. Holman, and N. G. Plant (2011), Abehavior‐orienteddynamicmodelforsandbarmigrationand2DH evolution, J. Geophys. Res., 116, C01020, doi:10.1029/2010JC006382.
122858286
The history and technical capabilities of Argus
Over the past 25 years, optical remote sensing has been developed into a very useful tool for sampling the nearshore environment, principally through the use of Argus Stations. However, the capabilities of such systems as scientific tools depend on an understanding of the physics of the camera as an optical sensor (including a detailed knowledge of accuracies and resolution) and of the relationship of optical signals to the geophysical signals they represent. This paper describes the components of Argus Stations with an emphasis on quantitative characterization of the accuracies and resolution of system components. Algorithms for estimation of a range of important nearshore measurements are discussed and their accuracies and ground-truth test results referenced. References to a number of key examples of Argus-based contributions to the scientific literature are cited. Since Argus technology serves as the basis for the CoastView Program, the technical capabilities described below are part of the foundation of CoastView sampling.
140648247
Extracting decadal morphological behaviour from high-resolution, long-term bathymetric surveys along the Holland coast using eigenfunction analysis
Abstract The understanding and prediction of coastal behaviour over a time span of decades and alongshore stretches of tens of kilometres is important in coastal management. The controlling variables for coastal behaviour on these scales are yet unknown. It is hypothesised that regions exhibiting different large-scale coastal behaviour (LSCB-regions) are controlled by different hydrodynamic forcings, or by different sedimentological or morphological constraints, or by some combination of these factors. The objective of this study is to elaborate a method that effectively quantifies large-scale morphological changes from a large bathymetric data set (large temporal and spatial extent combined with a high resolution), such to define LSCB-regions. The method of analysis consists of a combination of the empirical eigenfunction technique and a moving window approach. The method has been applied on a data set of over 14,000 nearshore profiles. The data set consists of yearly soundings of the nearshore profile along a coastal stretch of 115 km length (the Holland coast). The alongshore profile spacing is generally 250 m. All profiles surveyed in the period 1963–1990 are included in the analysis. Application of the method on the Dutch data set reveals that LSCB-regions indeed can be identified. The sizes of the LSCB-regions are 5 km, 15 km, 20 km, 32 km, and 42 km. Differences in behaviour of the ‘secondary’ morphology (i.e. morphology other than the mean profile shape, e.g. nearshore bars) are most pronounced. The change in behaviour between LSCB-regions generally occurs over distances as short as about 2 km. The boundaries between the regions are therefore surprisingly sharp. In general, no clear long-term trends in the development of the shape of the sub-aqueous profile have been observed along the Holland coast. Along certain stretches, trends in cross-shore shifting of the sub-aqueous profile occurred. The average shape of the sub-aqueous profile seems to have no relation with the trend in cross-shore shifting of the shoreline (the +1 m-contour). The (sub)decadal shoreline fluctuations, however, are possibly related to the largescale dynamics of multiple bar systems.
110643617
Practical use of video imagery in nearshore oceanographic field studies
An approach was developed for using video imagery to quantify, in terms of both spatial and temporal dimensions, a number of naturally occurring (nearshore) physical processes. The complete method is presented, including the derivation of the geometrical relationships relating image and ground coordinates, principles to be considered when working with video imagery and the two-step strategy for calibration of the camera model. The techniques are founded on the principles of photogrammetry, account for difficulties inherent in the use of video signals, and have been adapted to allow for flexibility of use in field studies. Examples from field experiments indicate that this approach is both accurate and applicable under the conditions typically experienced when sampling in coastal regions. Several applications of the camera model are discussed, including the measurement of nearshore fluid processes, sand bar length scales, foreshore topography, and drifter motions. Although we have applied this method to the measurement of nearshore processes and morphologic features, these same techniques are transferable to studies in other geophysical settings.
130441723
Quantification of sand bar morphology: A video technique based on wave dissipation
A technique is presented to remotely measure the scales and morphology of natural sand bars based on the preferential dissipation of wind waves and swell over the crests of the bar. Photographic or video images are recorded and statistical uncertainties associated with incident wave height modulations removed by averaging (time exposures). Ground truth testing of the technique was carried out as part of the SUPERDUCK experiment in October 1986. The time exposures generally provided a good mapping of underlying morphology, allowing detection of the bar and determination of cross-shore and longshore length scales. However, during high waves, persistent surface foam obscures the relationship of image intensity to local dissipation (modeled theoretically by dissipation of a random wave field), and an enhancement technique of image differencing must be done to remove the bias. Errors in the estimate of bar crest distance from the shoreline are generally less than 35%, but this value depends on the geometry of the particular bar. Logistic simplicity and quantitative capabilities make this technique very attractive.
129766093
Effect of hydrodynamics and bathymetry on video estimates of nearshore sandbar position
Remote sensing of wave breaker patterns, clearly visible as high-intensity bands in time exposure video images, has become a powerful tool to obtain large-scale (kilometers) and long-term (years) time series of nearshore sandbar position. However, intensity-based bar crest positions xi differ from directly measured positions xb by a time-varying distance Δx, which is of O(10 m) and depends on the offshore wave height H0, the water level η0, and the bathymetry itself. The effect of these parameters on Δx was investigated from simultaneous video observations and bathymetric surveys, obtained in the double-barred system at Egmond aan Zee, Netherlands, and from wave model predictions, assuming that the roller energy represents image intensity. When the wave field over a bar was predicted to be nonsaturated, xi was observed and predicted to move offshore as either η0 decreased or H0 increased. Under saturated conditions, Δx only responded to changes in η0. Additional model investigations showed that an increase in outer bar crest depth, similar to that observed during interannual bar behavior, significantly reduced the Δx variability at the outer bar and increased the Δx variability at the inner bar. Implications of our observational and model findings for studying sandbar position from video imagery are outlined.
140584139
The Performance of Shoreline Detection Models Applied to Video Imagery
Abstract Digital images of the intertidal region were used to map shorelines and the intertidal bathymetry along four geo-morphically and hydrodynamically distinct coastlines in the United States, United Kingdom, The Netherlands, and Australia. Mapping methods, each of which was originally designed to perform well at only one of the four sites, were applied to all four sites, and the results were compared to direct topographic surveys. It was determined that the rms errors of image-derived versus directly surveyed elevations depended on the prevailing hydrodynamic conditions as well as differences in each of the four different mapping methods. Before these differences were accounted for, rms errors ranged from 0.3 to 0.7 m. An empirical correction model that computed local estimates of setup, swash, and surf beat amplitudes reduced errors by about 50%, with residual rms errors ranging between 0.1 and 0.4 m. The model required tuning only one parameter that determined how each method was affected by swash at infragravity and incident wave frequencies. In environments where all methods successfully identify shorelines, the methods can be used somewhat interchangeably. The diversity of methods is advantageous in situations where one or more methods are likely to fail (e.g., lack of color imagery, high degree of alongshore variability). This remote sensing methodology can be applied to the shoreline and intertidal mapping problem across diverse nearshore environments.
18477361
Assessing the Suitability of Video Imaging for Studying the Dynamics of Nearshore Sandbars in Tideless Beaches
Nearshore sandbars, an important natural defense mechanism of the beaches, can be monitored using shore-based video systems. Before studying bar dynamics with video images, we must establish the relationship between the real bar positions and the videoed bar positions (detected by the preferential wave breaking on the shallows). This analysis becomes essential in the two studied tideless beaches of Barcelona due to the critical differences with respect to the sites studied previously. Bogatell beach is terraced (without a trough) in more than 50% of the profiles. There, the videoed barlines are a good proxy of the terrace edge position. In La Barceloneta beach, with dominance of barred profiles, the videoed barlines better represent the bar crest position. On average, the obtained distances between real and videoed bar positions ¿r are 10-15 m, with the videoed barlines located shoreward. Changes in the bathymetric profile shape and the root-mean-squared wave height H rms induce a variability of ¿r of 16 m in La Barceloneta and 13 m in Bogatell. This apparent variability masks the real changes in bar position and should preferably be reduced before further analysis. As a highly significant correlation between ¿r and H rms is detected in the two beaches, the proposed reduction method consists of sampling at a specific range of H rms. This diminishes the variability by 10% to 14 m in La Barceloneta and 11 m in Bogatell. This paper confirms the suitability of using video systems for monitoring bars and terraces in the Barcelona beaches.
55106815
On cross‐shore migration and equilibrium states of nearshore sandbars
[1] The location of submerged sandbars in the nearshore zone changes over time in response to variability in wave conditions. Here, cross-shore sandbar migration is studied with an empirical model consisting of a differential equation relating cross-shore sandbar migration to wave forcing. Model parameters are fitted to data sets containing multiple years of daily-observed positions of five sandbars at three field sites. From the fitted model parameters we determine the cross-shore location of zero-migration (equilibrium location), the stability of this equilibrium, and the rate (response time) at which a sandbar migrates toward or away from the equilibrium location. We find that for breaking waves a sandbar moves toward a stable, wave-height-dependent equilibrium location. For non- to slightly-breaking waves, however, a sandbar moves away from the equilibrium location, implying that the beach profile evolves toward a state without submerged sandbars. The response times can differ up to a factor three for different sandbars at a particular field site, and a factor ten between sandbars at different sites. For breaking waves, response times are found to decrease with increasing wave height from months to days, while for non- to slightly-breaking waves response times exceed several months. In general, response times remain larger than the timescale of variability in wave climate (several days), implying that sandbars spend most of their lifetime out of equilibrium with the wave forcing. The model is able to hindcast the relevant features of cross-shore sandbar migration on timescales of days to several years, and outperforms a baseline prediction of no change for all sandbars. When the equilibrium location is reached by both the hindcasted and actual sandbar, the hindcast error depends on the accuracy of the predicted equilibrium location only, and no longer on the history of accumulated errors. As a result, sandbars that occasionally reach their equilibrium location can accurately be predicted from the wave forcings over their entire lifespan. However, it is difficult to predict the exact rate of the yearly to interannual trends that are observed at two field sites, because the sandbars at these sites never reach the offshore-located equilibrium states associated with high waves.
129700753
Analysis of the scale of errors in nearshore bathymetric data
Most studies of nearshore hydrodynamics, sediment transport, and morphology focus on bathymetric variability within a narrow band of spatial and temporal scales. Typically, these studies rely on bathymetry estimates derived from field observations consisting of discrete samples in space and time with varying degrees of measurement error. Sampling limitations, which result in aliasing, and measurement errors can significantly contaminate variability at resolved scales, and may lead to large errors in the representation of the scales of interest. Using a spectral analysis, interpolation errors were analyzed for three different nearshore bathymetric data sets, each of which targeted a different range of spatial scales. Bathymetric features that were unresolved or poorly resolved (e.g. beach cusps) introduced the potential for contamination in two of the data sets. This contamination was significantly reduced using an appropriate scale-controlled interpolation method, leading to more accurate representations of the actual bathymetry. An additional benefit of using scale-controlled interpolation is that interpolation errors may be estimated independently of actual observations, which allows one to design bathymetric sampling strategies that ensure that dominant scales are either resolved or largely removed. Finally, interpolation errors corresponding to a particular sample design can be used to determine which interpolated values contribute usefully to a band-limited analysis of bathymetric variability.
129378475
Coupled and noncoupled behavior of three‐dimensional morphological patterns in a double sandbar system
[1] Eight weeks of daily (low tide) time exposure images of the Gold Coast (Australia) were used to analyze the development of three-dimensional morphological patterns in a double sandbar system. Under continued low-energy wave conditions following a morphological reset (storm) event, the patterns in both the inner and outer bar evolved, albeit at different spatial and temporal scales, through the accretionary sequence commonly observed at single barred beaches. Continuous wavelet transforms (CWT) of the inner bar and outer bar breaker lines show that after a storm the patterns developed at a wide range of alongshore wavelengths and that variations in wavelength over space and time were common. With the bivariate extension of CWT, continuous cross-wavelet transforms, it is found that the initial inner bar and outer bar three-dimensional patterns were spatially noncoupled. However, over time the inner-bar morphology appeared to couple with that of the outer bar, with the inner bar patterns developing in response to the increasingly three-dimensional, onshore propagating outer bar. The wavelet analysis suggests that the outer bar geometry and the distance between the inner and outer bars are critical parameters governing the morphological evolution of the composite double sandbar system.
55229666
Coupling mechanisms in double sandbar systems. Part 1: Patterns and physical explanation.
Crescentic sandbars and rip channels along wave-dominated sandy beaches are relevant to understand localized beach and dune erosion during storms. In recent years, a paradigm shift from hydrodynamic template models to self-organization mechanisms occurred to explain the formation of these rhythmic features. In double sandbar systems, both the inner- and outer-bar rip channels and crescentic planshapes are now believed to be free instabilities of the nearshore system arising through self-organization mechanisms alone. However, the occasional occurrence of one or two inner-bar rip channels within one outer-bar crescent suggests a forced, morphologically coupled origin. Here we use a nonlinear morphodynamic model to show that alongshore variability in outer-bar depth, and the relative importance of wave breaking versus wave focussing by refraction across the outer bar, is crucial to the inner-bar rip channel development. The coupling patterns simulated by our model are similar to those observed in the field. Morphological coupling requires a template in the morphology (outer-bar geometry) which, through the positive feedback between flow, sediment transport and the evolving morphology (that is, self-organization) enforces the development of coupling patterns. We therefore introduce a novel mechanism that blurs the distinction between self-organization and template mechanisms. This mechanism may also be extended to explain the dynamics of other nearshore patterns, such as beach cusps. The impact of this novel mechanism on the alongshore variability of inner-bar rip channels is investigated in the companion paper. Copyright © 2010 John Wiley and Sons, Ltd.
128726252
Beach Processes and Sedimentation
1. An Introduction to the Study of Beaches. 2. The Geomorphology of Eroding and Accreting Coasts. 3. Beach Morphology and Sediments. 4. The Changing Level of the Sea. 5. The Generation of Waves and their Movement Across the Sea. 6. Wave Breaking and Surf-Zone Processes. 7. Beach Profiles and Cross-Shore Sediment Transport. 8. Wave-Generated Currents in the Nearshore. 9. The Longshore Transport of Sediments on Beaches. 10. Shoreline Planforms and Models to Simulate Their Evolution. 11. Nearshore Morphodynamics. 12. The Protection of Our Coasts. Index.
128403447
State dynamics of a double sandbar system
A 9.3-year dataset of low-tide time-exposure images from Surfers Paradise, Northern Gold Coast, Australia was used to characterise the state dynamics of a double sandbar system. The morphology of the nearshore sandbars was described by means of the sequential bar state classification scheme of Wright and Short [1984. Morphodynamic variability of surf zones and beaches: a synthesis. Marine Geology 56, 93–118]. Besides the two end members (the dissipative (D) and the reflective (R) states) and the four intermediate states (longshore bar and trough (LBT), rhythmic bar and beach (RBB), transverse bar and rip (TBR) and low tide terrace (LTT)), we identified two additional intermediate bar states. The erosive transverse bar and rip (eTBR) state related to the dominant oblique angle of wave incidence at the study site and the rhythmic low tide terrace (rLTT) related to the multiple bar setting. Using the alongshore barline variability and alongshore trough continuity as morphological indicators enabled the objective classification of the inner and outer bar states from the images. The outer bar was mostly in the TBR state and generally advanced sequentially through the states LBT–RBB–TBR–eTBR–LBT, with occasional transitions to the D state. Wave events led to abrupt state transitions of the outer bar, but, in contrast to expectations, did not necessarily correspond to upstate transitions. Instead, upstate (downstate) transitions coincided with angles of wave incidence θ larger (smaller) than 30°. The upstate TBR–eTBR–LBT sequence during high-angle events highlights the role of alongshore currents in bar straightening. The outer bar was found to govern the state of the inner bar to a large extent. Two types of inner bar behaviour were distinguished, based on the outer bar state. For intermediate outer bar states, the alongshore variability of the dominant inner rLTT state (52% in time) mainly related to that of the outer bar, implying some sort of morphological coupling. For dissipative outer bar states, however, the more upstate inner bar frequently separated from the shoreline and persistently developed rip channels as TBR became the most frequent state (60% in time).
128689360
Seasonal accretion and erosion patterns of a microtidal sandy beach
Monthly elevation surveys of a 1.5 km stretch of coast at Noordwijk, the Netherlands, were made (1) to quantify seasonal variability in the cross-shore position of high-, mean- and low-tide contours, beach width and beach volume, and (2) to examine the dependence of the variability on the offshore wave characteristics. Additionally, daily cross-shore positions of contours obtained from video images were used to examine daily beach response to wave characteristics between successive surveys. The more intense and frequent storm events in winter than in summer resulted in a seasonal change from a wide beach with a small volume at the end of winter to a narrow, large-volume beach in summer. The seasonal pattern in beach width resulted from the cross-shore sediment exchange between the supratidal and lower-intertidal part of the beach. In contrast to the coupling between beach behaviour and wave characteristics on a seasonal scale, beach variability between successive surveys was unrelated to day- to week-averages of the preceding offshore wave conditions partly due to the precise timing of a storm relative to the two survey moments. Moreover, daily variability determined from the video images showed that differing antecedent morphology to storms and alongshore variations in beach morphology contributed to spatially and temporally variable beach response.
133446614
Modeling the long-term diffusion and feeding capability of a mega-nourishment
A morphodynamic model based on the wave-driven alongshore sediment transport, including cross-shore transport in a simplified way and neglecting tides, is presented and applied to the Zandmotor mega-nourishment on the Dutch Delfland coast. The model is calibrated with the bathymetric data surveyed from January 2012 to March 2013 using measured offshore wave forcing. The calibrated model reproduces accurately the surveyed evolution of the shoreline and depth contours until March 2015. According to the long-term modeling using different wave climate scenarios based on historical data, for the next 30-yr period, the Zandmotor will display diffusive behavior, asymmetric feeding to the adjacent beaches, and slow migration to the NE. Specifically, the Zandmotor amplitude will have decayed from 960 m to about 350 m with a scatter of only about 40 m associated to climate variability. The modeled coastline diffusivity during the 3-yr period is 0.0021 m2/s, close to the observed value of 0.0022 m2/s. In contrast, the coefficient of the classical one-line diffusion equation is 0.0052 m2/s. Thus, the lifetime prediction, here defined as the time needed to reduce the initial amplitude by a factor 5, would be 90 yr instead of the classical diffusivity prediction of 35 yr. The resulting asymmetric feeding to adjacent beaches produces 100 m seaward shift at the NE section and 80 m seaward shift at the SW section. Looking at the variability associated to the different wave climates, the migration rate and the slight shape asymmetry correlate with the wave power asymmetry (W vs N waves) while the coastline diffusivity correlates with the proportion of high-angle waves, suggesting that the Dutch coast is near the high-angle wave instability threshold.
129186692
Daily to interannual cross-shore sandbar migration: observations from a multiple sandbar system.
Abstract The most conspicuous characteristic of the cross-shore behaviour of nearshore multiple sandbar systems in sea-dominated settings is interannual net offshore migration (NOM), while that of similar systems in swell-dominated environments appears to be the on/offshore migration in response to individual wave events. Here we analyse an approximately 7.5-year data set (July 1999–February 2007) of daily 10-min time-exposure video images of the swell-dominated double-barred northern Gold Coast, Australia and show that events with wave heights exceeding three to four times the annual average can trigger NOM behaviour similar to that observed elsewhere. During four such events, we observed how the outer sandbar migrated some 75–100 m further offshore than usual and then decayed during slow onshore migration in the subsequent low-energy weeks. The next, moderately high wave event caused the ( ≈ 20 – 30 m / day ) offshore migration of the inner bar to become the new outer bar and the onshore birth of a new bar. Between the large wave events both the outer and the inner bars responded primarily to the seasonal signal in wave height, whereby both bars were located further offshore during the higher (lower) energy winter (summer) months. We suspect that bar size (height and width) may control whether a multiple sandbar system exhibits episodic NOM (as observed here) or interannual NOM. In contrast to interannual NOM, episodic NOM is steered by the order of storm events.
55678868
Observations of nearshore crescentic sandbars
[1] The temporal and spatial variability of crescentic sandbars is analyzed with hourly long-term (months) video observations collected at four barred sites and are qualitatively compared to the temporal and spatial variability predicted by hypotheses underpinning existing approaches and models for crescentic bar formation (edge-wave template model, linear stability analysis, and nonlinear models). The observations, coming from the single barred beaches at Duck (North Carolina, USA) and Miyazaki (Kyushu, Japan), and from the double-barred beaches at the northern Gold Coast (Queensland, Australia) and Noordwijk (Netherlands), show that crescentic sandbar wavelength and amplitude variations over space and time are very common. For instance, at any moment in time, the wavelength of the smallest and longest crescentic bar can differ by a factor of 2. Temporal changes in wavelength and amplitude result from merging and splitting of individual crescents, causing the “final” configuration of a crescentic sandbar system to be very different from the initial configuration. The Gold Coast data indicate that these intrinsically nonlinear interactions are an attempt of the crescentic bar system to self-organize into a more uniform pattern, as splitting is usually confined to the longest crescentic bar observed, whereas merging usually combines the smallest crescentic bars into a longer bar. The observed spatial and temporal crescentic bar behavior contrasts qualitatively with behavior predicted from the edge-wave template model and implies that the predictive skill of linear stability models is limited. Nonlinear models are potentially better suited for a comparison against these field observations; several suggestions to improve these models, and hence to facilitate a data-model comparison, are made.
128876635
Impact of the winter 2013–2014 series of severe Western Europe storms on a double-barred sandy coast: Beach and dune erosion and megacusp embayments
Abstract The winter of 2013/2014 was characterized by a striking pattern of temporal and spatial extreme storm wave clustering in Western Europe. The 110-km long Gironde coast, SW France, was exposed to the most energetic wave conditions over the last 18 years. The period was outstanding in terms of the available energy to move sediment and cause large-scale erosion with the 2-month average significant wave height (Hs) exceeding 3.6 m, just below the 0.95 quantile, and 4 distinct 10-year return period storms with Hs > 9 m. These storm waves caused unprecedented beach and dune erosion along the Gironde coast, including severely damaged sea defences at the coastal towns. At the end of the winter, dune erosion scarp height was highly variable alongshore and often exceeded 10 m. Megacusp embayments were observed along the Gironde coast with an average alongshore spacing of 1000 m in the south progressively decreasing to 500 m in the north, with an average cross-shore amplitude of 20 m. While beach megacusps were previously observed to systematically couple to the inner bar along the Gironde coast during low- to moderate-energy wave conditions, severe storm-driven megacusp embayments cutting the dune were found to be enforced and coupled to the outer crescentic bar. A detailed inspection of the 1500 m-long bimonthly topographic surveys of Truc Vert beach shows that in early January 2014 the outstanding shore-normal incident storm swell ‘Hercules’, with Hs and peak wave period Tp peaking at 9.6 m and 22 s, respectively, triggered the formation of a localized megacusp embayment with the erosion scarp height exceeding 6 m in its centre where the dune retreat reached 30 m. The subsequent storms progressively smoothed the megacusp by the end of the winter, mostly through severe erosion of the megacusp horns. Because of the very long period (16 s
9146966
Morphological development of rip channel systems: Normal and near‐normal wave incidence
The process of formation of a rip channel/crescentic bar system on a straight, sandy coast is examined. A short review of earlier studies is presented. A morphodynamic stability model is then formulated. The resulting model includes a comprehensive treatment of shoaling and surf zone hydrodynamics, including wave refraction on depth and currents and waves. The sediment transport is modeled using a total load formula. This model is used to study the formation of rip currents and channels on a straight single-barred coast. It is found that this more comprehensive treatment of the dynamics reveals the basic rip cells predicted in earlier studies for normal incidence. Also as before, cell spacings (λ) scale with shore-to-bar crest distance (X b ), while growth rates decrease. The λ increases with offshore wave height (H) up to a saturation value; increasing H also increases instability. Experiments at off-normal wave incidence ( > 0) introduce obliquity into the evolving bed forms, as expected, and λ increases approximately linearly. the e-folding times also increase with . At normal incidence, λ increases weakly with wave period, but at oblique angles, λ decreases. Tests also reveal the presence of forced circulation cells nearer to the shoreline, which carve out bed forms there. The dynamics of these forced cells is illustrated and discussed along with the associated shoreline perturbation. Transverse bars are also discovered. Their dynamics are discussed. Model predictions are also compared with field observations. The relevance of the present approach to predictions of fully developed beach states is also discussed.
129041095
Quasi-three-dimensional modelling of the morphology of longshore bars
A morphological quasi-three-dimensional (Q3D) area model for barred coasts has been developed. The model combines a two-dimensional depth integrated model for wave-driven currents with a model for undertow circulation currents. The combined model makes a simultaneous simulation of the bar-forming processes associated with the undertow and the horizontal wave-driven circulation currents, which may cause instabilities of the bar and the formation of rip channels. Situations with normal and oblique wave incidence are considered. Compared to the depth integrated approach the Q3D model produces less pronounced alongshore irregularities for obliquely incident waves. For normal incident waves the Q3D model produces a crescentic bar while the depth integrated model predicts almost straight sections of the bar interrupted by rip channels. The sensitivity to variation of wave angle and beach slope is further investigated.
32877011
A mechanism for sandbar straightening by oblique wave incidence
Breaker bars in the surf zone of sandy beaches generally evolve between straight bars parallel to the shore and meandering crescentic bars associated with intense (dangerous) currents flowing seaward through rip channels. Understanding the behavior of such systems is fundamental as they control the entire surf zone dynamics, the shape of the coastline, and the exchange of floating material with the shoreface. Although the mechanisms behind the meandering of an originally straight bar have been studied extensively, a clear physical explanation on the crescentic bar straightening was missing. Recent field observations have highlighted that this morphological reset can be due to wave obliquity. By using a two-dimensional horizontal morphological model, we show that the bar straightening by oblique waves occurs because the rip current is both weakened in intensity and shifted downdrift from the channel deepest section. The technique employed is useful for the study of other types of bed forms.
129558282
The influence of wave direction on the morphological response of a double sandbar system
Nearshore sandbars are often characterized by three-dimensional bed patterns. To analyze the influence of wave direction on the morphological response of a double sandbar system, this paper uses the 2DH nonlinear surf zone model MORFO55. Depending on wave direction, different morphologies emerge as free instabilities. These morphological responses differ in terms of geometry (shape of the alongshore rhythmic patterns) and temporal evolution. Nearly shore-normal waves favor the emergence of crescentic patterns along both the inner and outer bars. These instabilities arise from “bedsurf” coupling, which is the positive feedback between the bed perturbations and the wave-breaking patterns resulting in rip-cell circulations. The system is stable for intermediate wave incidence angles because the longshore current shifts the circulation cells alongshore, inhibiting the growth of initial perturbations. For larger angles, the system is again unstable. Two types of oblique bars emerge. The first type forms between the crest of the inner bar and the shoreline and is oriented down-current. The second type is superimposed on the two bars and is oriented up-current. Although previous studies attribute the formation of oblique bars to the deflection of the longshore current (that is a cross-shore process), we show here that this mechanism contributes to the bar formation but the growth rate is mainly governed by a longshore process. Specifically, it is the positive feedback between the bathymetry and the longshore gradient of the sediment concentration. Finally, interactions between the patterns in the two shore-parallel bars are analyzed. In the observed configurations, the influence is always one way as the inner-bar dynamics never influence those of the outer bar. The outer-bar instabilities cause undulations in the inner bar when the outer-bar instabilities grow faster than those of the inner bar.
205023325
Formation of coastline features by large-scale instabilities induced by high-angle waves
Alongshore sediment transport that is driven by waves is generally assumed to smooth a coastline. This assumption is valid for small angles between the wave crest lines and the shore, as has been demonstrated in shoreline models. But when the angle between the waves and the shoreline is sufficiently large, small perturbations to a straight shoreline will grow. Here we use a numerical model to investigate the implications of this instability mechanism for large-scale morphology over long timescales. Our simulations show growth of coastline perturbations that interact with each other to produce large-scale features that resemble various kinds of natural landforms, including the capes and cuspate forelands observed along the Carolina coast of southeastern North America. Wind and wave data from this area support our hypothesis that such an instability mechanism could be responsible for the formation of shoreline features at spatial scales up to hundreds of kilometres and temporal scales up to millennia.
129739276
Double bar beach dynamics on the high-energy meso-macrotidal French Aquitanian Coast: A review
The French Aquitanian Coast is an approximately 250 km long straight low coast exposed to high energy conditions in a mesomacrotidal setting. Offshore wave conditions are seasonally modulated, predominantly with a WNW incidence, with offshore significant wave height likely to reach 10 m during winter. Truc Vert Beach, representative of most of the Aquitanian Coast beaches, commonly exhibitstwodistinctsandbarsystems.Theinnerbarcangothroughallthestateswithintheintermediateclassificationandusuallyexhibitsa TansverseBarandRipmorphology.Afterafewweeksoflowerenergyconditionsduringsummer,theinnerbarcommonlybecomesaLow TideTerracewithameanwavelengthof400mandameansoutherlymigrationrateofabout2–3m/day.Crescenticbarshavebeenreported in the literature in nontidal to microtidal settings. Long term persistent crescentic patterns are, however, exhibited at a narrow range of wavelength (mean of 700m) by the outer bar at the meso-macrotidal Truc Vert Beach. Most of the time, the outer bar is inactive and stagnates as offshore waves ofHsN3 m are required to induce a significant morphological change. The crescent shape varies from a symmetric shape to a strongly asymmetric shape, likely to be the result of a long period of NW wave conditions. A strong, and rarely observed elsewhere, morphological coupling between the inner and outer bars can sometimes be observed, and may be the result of the combined effectsof the initial presence of a well-developed outer crescentic bar and a long period of shore–normal low energy conditions. A synthesis of all the data available on the area combined with observations on other environments leads to a Truc Vert Beach state model ranging from a modal double bar configuration to an occasional triple bar configuration. This work also identifies knowledge gaps to be explored by further numerical and field studies in tidal double sandbar environments. © 2007 Elsevier B.V. All rights reserved.
128617083
The statistics and kinematics of transverse sand bars on an open coast
Abstract Ten years (1987–1996) of time exposure video images of the nearshore region at Duck, NC were used to study transverse sand bars, bathymetric features of intermediate length scales (10–200 m) oriented oblique or perpendicular to the shoreline. These transverse sand bars extend seaward from both the shoreline (trough transverse bars) and the shore-parallel sand bar (offshore transverse bars). Transverse bars had not previously been observed in an energetic coastal environment such as that at Duck, and their dynamics and role in nearshore processes is unknown. Frequency of occurrence statistics and length scales of the transverse bars were calculated using the video images. Trough and offshore transverse bars appeared a mean of 39 and 73 days per year, respectively. The offshore bars were found to be much larger features than the trough bars, with mean wavelengths (alongshore spacing between consecutive crests) of 79 and 172 m for trough and offshore bars, respectively. Both the trough and offshore bars were found to persist for periods of days to months. The alongshore movement of the bars was measured and compared to estimates of surf zone longshore currents which were calculated from wave height and wave angle data. Both sets of bars were observed to move at rates up to 40 m/day. At times, both trough and offshore bars were observed shifting in the same direction as the current was flowing, and at other times, both sets of bars remained stationary, even under relatively strong longshore currents. Trough bars were also observed moving against the current. An hypothesis, proposed by Barcilon and Lau (1973) [J. Geophys. Res. 78(15): 2656–2664], that the transverse bars were created as a sea bed instability under longshore currents, was tested by comparing the magnitude of estimated surf zone longshore currents with times of formation or presence of transverse bars. There was no evidence to suggest that the bars were formed by this simple longshore current instability mechanism. Instead, it is plausible that the combined effects of waves and currents may drive the formation of these features.
29322226
Characteristics and dynamics of surfzone transverse finger bars
Patches of transverse finger bars have been identified in the surf zone of Noordwijk
beach (Netherlands). They consisted of three to nine elongated accumulations of sand
attached to the low-tide shoreline. The bars extended up to 50 m into the inner surf zone,
had an oblique orientation with respect to the shore-normal, and were quasiregularly
spaced in the alongshore direction.
43497911
Observations and modeling of surf zone transverse finger bars at the Gold Coast, Australia
The occurrence and characteristics of transverse finger bars at Surfers Paradise (Gold Coast, Australia) have been quantified with 4 years of time-exposure video images. These bars are attached to the inner terrace and have an oblique orientation with respect to the coastline. They are observed during 24 % of the study period, in patches up to 15 bars, with an average lifetime of 5 days and a mean wavelength of 32 m. The bars are observed during obliquely incident waves of intermediate heights. Bar crests typically point toward the incoming wave direction, i.e., they are up-current oriented. The most frequent beach state when bars are present (43 % of the time) is a rhythmic low-tide terrace and an undulating outer bar. A morphodynamic model, which describes the feedback between waves, currents, and bed evolution, has been applied to study the mechanisms for finger bar formation. Realistic positive feedback leading to the formation of the observed bars only occurs if the sediment resuspension due to roller-induced turbulence is included. This causes the depth-averaged sediment concentration to decrease in the seaward direction, enhancing the convergence of sediment transport in the offshore-directed flow perturbations that occur over the up-current bars. The longshore current strength also plays an important role; the offshore root-mean-square wave height and angle must be larger than some critical values (0.5 m and 20∘, respectively, at 18-m depth). Model-data comparison indicates that the modeled bar shape characteristics (up-current orientation) and the wave conditions leading to the bar formation agree with data, while the modeled wavelengths and migration rates are larger than the observed ones. The discrepancies might be because in the model we neglect the influence of the large-scale beach configuration.
53356311
High‐angle wave instability and emergent shoreline shapes: 1. Modeling of sand waves, flying spits, and capes
[1] Contrary to traditional findings, the deepwater angle of wave approach strongly affects plan view coastal evolution, giving rise to an antidiffusional “high wave angle” instability for sufficiently oblique deepwater waves (with angles between wave crests and the shoreline trend larger than the value that maximizes alongshore sediment transport, ∼45°). A one-contour-line numerical model shows that a predominance of high-angle waves can cause a shoreline to self-organize into regular, quasiperiodic shapes similar to those found along many natural coasts at scales ranging from kilometers to hundreds of kilometers. The numerical model has been updated from a previous version to include a formulation for the widening of an overly thin barrier by the process of barrier overwash, which is assumed to maintain a minimum barrier width. Systematic analysis shows that the wave climate determines the form of coastal response. For nearly symmetric wave climates (small net alongshore sediment transport), cuspate coasts develop that exhibit increasing relative cross-shore amplitude and pointier tips as the proportion of high-angle waves is increased. For asymmetrical wave climates, shoreline features migrate in the downdrift direction, either as subtle alongshore sand waves or as offshore-extending “flying spits,” depending on the proportion of high-angle waves. Numerical analyses further show that the rate that the alongshore scale of model features increases through merging follows a diffusional temporal scale over several orders of magnitude, a rate that is insensitive to the proportion of high-angle waves. The proportion of high-angle waves determines the offshore versus alongshore aspect ratio of self-organized shoreline undulations.
54961571
Wave‐angle control of delta evolution
[1] Wave-influenced deltas, with large-scale arcuate shapes and demarcated beach ridge complexes, often display an asymmetrical form about their river channel. Here, we use a numerical model to demonstrate that the angles from which waves approach a delta can have a first-order influence upon its plan-view morphologic evolution and sedimentary architecture. The directional spread of incoming waves plays a dominant role over fluvial sediment discharge in controlling the width of an active delta lobe, which in turn affects the characteristic rates of delta progradation. Oblique wave approach (and a consequent net alongshore sediment transport) can lead to the development of morphologic asymmetry about the river in a delta's plan-view form. This plan-form asymmetry can include the development of discrete breaks in shoreline orientation and the appearance of self-organized features arising from shoreline instability along the downdrift delta flank, such as spits and migrating shoreline sand waves—features observed on natural deltas. Somewhat surprisingly, waves approaching preferentially from one direction tend to increase sediment deposition updrift of the river. This ‘morphodynamic groin effect’ occurs when the delta's plan-form aspect ratio is sufficiently large such that the orientation of the shoreline on the downdrift flank is rotated past the angle of maximum alongshore sediment transport, resulting in preferential redirection of fluvial sediment updrift of the river mouth.
130092617
Development of crescentic bars for a periodically perturbed initial bathymetry
[1] The development of crescentic bed patterns starting from an ‘undisturbed’ beach (i.e. one only perturbed so as to simulate background noise and thereby initiate morphological development) is compared with that starting from a beach where monochromatic (and in some cases bichromatic) bed patterns pre-exist, using a fully non-linear model (Morfo55). A wide range of different lengthscales and amplitudes is investigated. The findings suggest that the pre-existence of crescentic bed-forms can influence the subsequent morphological development of this beach significantly, but that this development is related to the undisturbed beach development. Whether a pre-existing bed-form remains under certain forcing conditions depends on the position of the pre-existing lengthscale along the undisturbed linear growth rate curve: If the pre-existing mode shows significant linear growth in the undisturbed scenario, this pre-existing lengthscale remains. Otherwise the pre-existing mode will be replaced by a bed-form with a lengthscale closer to the undisturbed fastest growing mode. A pre-existing lengthscale that is significantly longer than the undisturbed fastest growing mode will be replaced by a higher harmonic of the original lengthscale; for shorter pre-existing lengthscales, this will be the linear fastest growing mode. An increased amplitude of the pre-existing bed-form accelerates the development toward a new stable situation. For small initial amplitudes, the initial response of the system to pre-existing bed-forms could theoretically be described using linearized equations alone. The migration rates of both the pre-existing and newly arising modes correspond to the migration rate of these modes in the undisturbed scenario.
129377570
Nearshore bar crest location quantified from time-averaged X-band radar images
Abstract The remote sensing of the sea surface is commonly regarded as a logistically simple and cost-effective way of obtaining long-term (>years) time series of nearshore sandbar location. In this paper, we demonstrate that, similar to video imagery, time-exposure X-band (∼3 cm wave length) radar images contain alongshore, breaking-related high-intensity bands that can be used to compute the location of the underlying sandbar crests. Analysis of time-exposure X-band images and bathymetric surveys collected at Egmond aan Zee, Netherlands shows that the radar-based bar crest location, defined as the cross-shore location of maximum radar intensity, differs from the in situ surveyed crest location by a distance of O(10 m). This difference, Δxradar, depends linearly on the offshore water level, consistent with model predictions in which the radar intensity is assumed to behave as the energy of the surface roller. The model additionally suggests a dependence of Δxradar on the offshore wave height when the wave field across the bar is non-saturated. In general, such wave conditions did not result in radar images with sufficient contrast to determine the bar crest location and, accordingly, the wave-height dependence was not observed clearly in the field. Because of the dependence of Δxradar on the offshore water level, time series of radar-based bar crest location show artificial variability when the images are collected at different water levels. Various techniques for correcting this variability are discussed.
129770717
A video-based technique for mapping intertidal beach bathymetry
Abstract Measuring the location of the shoreline and monitoring foreshore changes through time are core tasks carried out by coastal engineers for a wide range of research, monitoring and design applications. With the advent of digital imaging technology, shore-based video systems provide continuous and automated data collection, encompassing a much greater range of time and spatial scales than were previously possible using field survey methods. A new video-based technique is presented that utilises full-colour image information, which overcomes problems associated with previous grey-scale methods, which work well at steep (reflective) sites, but are less successful at flatter (dissipative) sites. Identification of the shoreline feature is achieved by the automated clustering of sub-aqueous and sub-aerial pixels in ‘Hue–Saturation–Value’ (HSV) colour space, and applying an objective discriminator function to define their boundary (i.e., ‘shoreline’) within a time-series of consecutive geo-referenced images. The elevation corresponding to the detected shoreline features is calculated on the basis of concurrent tide and wave information, which is incorporated in a model that combines the effects of wave set-up and swash, at both incident and infragravity frequencies. Validation of the technique is achieved by comparison with DGPS survey results, to assess the accuracy of the detection and elevation methods both separately and together. The uncertainties associated with the two sub-components of the model tend to compensate for each other. The mean difference between image-based and surveyed shoreline elevations was less than 15 cm along 85% of the 2-km study region, which corresponded to an horizontal offset of 6 m. The application of the intertidal bathymetry mapping technique in support of CZM objectives is briefly illustrated at two sites in The Netherlands and Australia.