on the mechanism of wavelength selection of self organized shoreline sand waves sandy shorelines exposed to very oblique wave incidence can be unstable and develop self organized shoreline sand waves different types of models predict the formation of these sand waves with an initially dominant alongshore wavelength in the range 1 10 km which is quite common in nature here we investigate the physical reasons for such wavelength selection with the use of a linear stability model the existence of a minimum wavelength for sand wave growth is explained by an interplay of three physical effects a largest relative to the local shoreline wave angle at the downdrift flank of the sand wave b wave energy concentration at the updrift flank due to less refractive energy dispersion and c wave energy concentration slightly downdrift of the crest due to refractive focusing for small wavelengths effects a and c dominate and cause decay while for larger wavelengths effect b becomes dominant and causes growth however the alongshore gradients in sediment transport decrease for increasing wavelength making the growth rate diminish there is therefore a growth rate maximum giving a dominant wavelength lm in contrast with previous studies we show that lm scales with 0 0 is the wavelength of the offshore waves and is the mean shoreface slope from shore to the wave base an estimate of the order of magnitude of the distance waves travel to undergo appreciable transformation our model investigations show that the proportionality constant between lm and 0 is typically in the range 0 1 0 4 depending mainly on the wave incidence angle

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

high angle wave instability and emergent shoreline shapes 2 wave climate analysis and comparisons to nature 1 recent research has revealed that the plan view evolution of a coast due to gradients in alongshore sediment transport is highly dependant upon the angles at which waves approach the shore giving rise to an instability in shoreline shape that can generate different types of naturally occurring coastal landforms including capes flying spits and alongshore sand waves this instability merely requires that alongshore sediment flux is maximized for a given deepwater wave angle a maximum that occurs between 35 and 50 for several common alongshore sediment transport formulae here we introduce metrics that sum over records of wave data to quantify the long term stability of wave climates and to investigate how wave climates change along a coast for long point a flying spit on the north shore of lake erie canada wave climate metrics suggest that unstable waves have shaped the spit and furthermore that smaller scale alongshore sand waves occur along the spit at the same locations where the wave climate becomes unstable a shoreline aligned along the trend of the carolina capes united states would be dominated by high angle waves numerical simulations driven by a comparable wave climate develop a similarly shaped cuspate coast local wave climates along these simulated capes and the carolina capes show similar trends shoreline reorientation and shadowing from neighboring capes causes most of the coast to experience locally stable wave climates despite regional instability

probabilistic design of dunes an example from the netherlands abstract dunes act as primary sea defences in some regions over the world and they have then the same function as common sea dikes they prevent flooding and protect people against drowning e g in the netherlands in other regions dunes form only the transition zone between sea and land and are sometimes part of important holiday resorts in those cases high investment costs have been made frequently in building houses and hotels close to the edge of the sea under storm surge conditions however dunes can be damaged by the sea knowing the amount of retreat of the dune s toe is in both cases important a design method for dunes primarily directed at the problem of the loss of life safety has been developed in the netherlands in recent years due to the large number of factors determining dune erosion a probabilistic design method was developed

formation of coastline features by large scale instabilities induced by high angle waves along shore 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

large scale dynamics of sandy coastlines diffusivity and instability 1 the dynamics of small amplitude perturbations of an otherwise rectilinear coastline due to the wave driven alongshore sediment transport is examined at large time and length scales years and kilometers a linear stability analysis is performed by using an extended one line shoreline model with two main improvements 1 the curvature of the coastline features is accounted for and 2 the coastline features are assumed to extend offshore as a bathymetric perturbation up to a finite distance for high incidence angles instability is found in accordance with ashton et al 2001 however it is seen that instability is inhibited by high waves with long periods and gently sloping shorefaces so that in this case the coastline may be stable for any angle similarly there is no instability if the bathymetric perturbation is confined very close to the coast it is found that the traditional linearized one line model larson et al 1987 tends to overpredict the coastline diffusivity the overprediction is small for the conditions leading to a stable coastline and for moderate incidence angles but can be very dramatic for the conditions favoring instability an interesting finding is that high angle waves instability has a dominant wavelength at the linear regime which is in the order of 4 15 km one to two orders of magnitude larger than the length scale of surf zone rhythmic features intriguingly this is roughly the same range of the wavelength of some observed shoreline sand waves and in particular those observed along the dutch coast a model application to this coast is presented

wave beach and dune interactions in southeastern australia abstract a morphodynamic classification of surfzones beaches and dunes of the microtidal low to high energy southeast australian coast is presented the first section a waves and beaches briefly deals with the transformation of deep water wave energy as it crosses the shelf nearshore and surfzone depending on the deep water wave characteristics and shelf and inshore morphology resultant breaker wave energy may be high moderate or low and the accompanying beach surfzone morphodynamic state in fine to medium sand beaches either dissipative intermediate or reflective dissipative beaches have wide surfzones with shore parallel bar s and channel s and predominantly shore normal circulation intermediate beaches are characterised by rip circulation crescentic transverse bars and megacusps and reflective beaches by a barless surfzone and steep cusped or bermed beach each beach form has a characteristic level of beach stability zone of sediment storage and mode of beach and dune erosion landward aeolian sediment transport of swash deposited sand section b is dependent on the subaerial beach topography and the aerodynamic flow regime across that topography characteristic profile shapes are ascribed to each beach type dissipative intermediate and reflective aeolian sand transport rates are potentially highest on dissipative beaches moderate on intermediate beaches and lowest on reflective beaches these rates determine the potential size of foredunes which are correspondingly largest on dissipative beaches and smallest on reflective beaches the combination of mode and frequency of beach dune erosion rates of aeolian sand transport and foredune volume and morphology provide an explanation of the nature and morphology of landward occurring large scale dune systems dissipative beaches are frequently characterised by large scale transgressive dune sheets intermediate by a trend from large scale parabolic dune systems high wave energy to small scale blowouts low wave energy and reflective beaches by minimal dune development

sand waves along the dutch coast sand waves defined as longshore wave like movements of the shoreline measured in a horizontal plane are described along several stretches of the shoreline of the netherlands they have a celerity in the order of 50 200 m yr a period of 50 150 years and an amplitude of 30 500 m they are found along the whole dutch shoreline analysis of sand waves shows that the assumed effect of groins might be very small along this shoreline there are indications that groins did not have any long term effect on coastal regression

the effects of beach width variability on coastal dune development at decadal scales abstract a cellular automata model is used to evaluate over decadal time spans the effect of beach width changes on the development of coastal dune systems the model includes the effects of aeolian transport hydrodynamic erosion and accretion groundwater and vegetation growth simulations using fixed and mobile beach widths scenarios were carried out for a 90 year period unlimited and limited sediment supply conditions were regulated by groundwater depth the final topographies were compared based on morphological characteristics such as dunefoot position and volume increase results show that there is a preferential cross shore position where the foredune tends to be built which is a function of beach width and sediment supply for narrow beaches foredunes tend to develop at higher elevations than for wide beaches due to differences in wave dissipation whereas dune volume is controlled by hydrodynamic erosion and dune recovery potential by sediment supply furthermore if sediment supply is limited the effect of beach width on dune volume only appears for beach widths greater than 300 m suggesting that limitation in supply can dominate dune growth on regular beaches whereas on wide systems such as sand flats and spits beach width size dominates these results suggest that for a decadal scale beach width controls the space available for dune formation thus the position of the most seaward dune but the effect of beach width on dune volume can be overruled by other supply limiting conditions such as groundwater depth

dunefoot dynamics along the dutch coast the dynamics of the dunefoot along a 160 km portion of the dutch coast has been investigated based on a data set of annual surveys dating back to as early as 1850 the linearly detrended or residual dunefoot positions comprise an alongshore uniform and an alongshore non uniform component the former is expressed as 10 to 15 m of landward retreat along extensive 10 km stretches of coast during years with severe storm surges and as up to 5 m of seaward advance during years without significant storm activity the latter alongshore non uniform component is organized in sandwave like patterns which may have a longevity of decades to up to the duration of the entire data set 150 years their wavelengths vary along the coast from 3 5 to 10 km migration rates are 0 200 m a 1 dunefoot sandwaves are shown to be the shoreward extensions of similar sandwave patterns in the beach position the non uniform dunefoot behaviour constitutes at least 80 per cent of the total residual dunefoot dynamics implying that along the dutch coast residual dunefoot variability is controlled by temporal and spatial variability in beach characteristics and not by storm induced uniform erosion various potential mechanisms causing beach sandwaves are discussed copyright 2002 john wiley sons ltd

storminess and high tide beach change stanwell park australia 1943 1978 abstract coastal storms have been considered significant agents in transporting sediment modifying morphology and causing recent beach erosion along the new south wales coast the concomitance of storms warmer sea surface temperatures and poleward movement of the hadley cell was linked to beach erosion on stanwell park beach between 1943 and 1978 this result was defined using an accurately constructed compilation of coastal storms and a precisely measured time series of high tide positions taken from 105 oblique photographs the two data sets are amongst the best of their kind in the world indices of storm magnitude representing cumulative significant wave height were constructed at quarterly intervals and for the year prior to each photograph the quarterly and cumulative storm indices explain 9 9 and 16 6 respectively of the high tide variance at stanwell park each unit increase in the latter index equivalent to an additional 1 m in storm wave height shifts the high tide position shoreward by 0 47 m this is equivalent to the effect produced by a 1 cm rise in sea level while storminess is a crucial variable controlling high tide position on this beach its role cannot be separated easily from that produced by rainfall sea levels sea surface temperatures and atmospheric circulation along the east australian coast and across the pacific ocean over 40 of the high tide data variance on stanwell park beach can be accounted for in decreasing order of importance by rainfall storminess tropical pacific ocean air circulation and sea level these results have implications for other beaches in that single factors such as increased storminess or rising sea levels may not necessarily be the best factors for explaining recent worldwide beach erosion

coastline sand waves on a low energy beach at el puntal spit spain abstract coastline sand waves have been observed at el puntal spit located on the north coast of spain the spit has been monitored by an argus video system since 2003 and the formation and destruction of sand waves has been observed coastline data from the video images are analyzed by means of principal components analysis obtaining a mean sand wave length of 125 150 m and a maximum amplitude of 15 m it is also observed that sand waves reach their maximum amplitude at about 15 days no propagation of these sand waves is noticed during the approximately two month long events analyzed sand wave formation and evolution are examined in relation with the prevailing local wave conditions during that period incident waves at the west end of the spit approach from the east northeast with a very high angle with respect to the shoreline field observations suggest that sand waves may result from an instability in alongshore sediment transport caused by moderate energy waves with a high angle incidence

migration and sedimentology of longshore sandwaves long point lake erie canada this paper describes the formation migration and sedimentology of sandwaves along the distal end of long point a 40 km long spit in lake erie some seven to nine sandwaves occur in a zone over the last 14 km of the spit they are characteristically 50 100 m wide at the downdrift end range in length from 350 m to 1500 m and migrate alongshore at rates that are typically 100 300 m year 1 measurements over a 7 year period show two forms of alongshore sandwave migration 1 a migratory jump and 2 downdrift accretion the migratory jump is commonly 200 500 m year 1 and results from the onshore migration and welding of an inner nearshore bar to the downdrift end of the sandwave this in turn leads to emergence of the bar over a distance of several hundred metres downdrift of the sandwave and isolation of the trough landward of the bar infilling of the trough abstracts large volumes of sediment from the local sediment transport system and may affect movement of the sandwave in the following year or movement of the next sandwave downdrift downdrift accretion commonly results in migration of 50 150 m year 1 and results from the refraction of waves around the distal end of the sandwave and episodic accretion of small swash bars this mechanism occurs less frequently and appears to reflect a local condition of lower sediment abundance often triggered by a large migratory jump in the previous year the process of bar emergence and infilling produces a distinct suite of sedimentary structures associated with the infilling of the landward trough and building of the sandwave berm the initial shoreline perturbation that generates the sandwave results from onshore migration and welding of inner nearshore bars and the development and growth of the sandwaves is promoted by refraction of highly oblique waves

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

mechanisms associated with the erosion of sand dune cliffs magilligan northern ireland a fifteen year history of coastal dune recession at magilligan northern ireland has revealed both time and space contrasts in processes and morphology since 1968 storm frequency has increased while dune retreat reached a peak 5 4 m in 1978 1979 three types of dune eroding events are noted resulting from particular wave wind tide combinations the spatial variability in dune scarping is associated with vegetation and soil development and slope failures range from granular avalanches to retrogressive rotational slumps

on bar growth and decay during interannual net offshore migration abstract multiple bar systems often show a cyclic net offshore directed migration with return periods on the order of years generally a bar is generated near the shoreline grows in height and width while migrating offshore before finally decaying at the seaward limit of the surf zone based on a three year hindcast of a bar cycle at noordwijk netherlands and on additional synthetic runs using a wave averaged cross shore process model this study identifies the dominant mechanisms that govern the bar amplitude growth and decay during net interannual offshore migration the bar amplitude response is particularly sensitive to the water depth above the bar crest h xb and the angle of wave incidence these variables largely control the amount of waves breaking on the bar and the strength and cross shore distribution of the associated longshore current the longshore current has its maximum landward of the bar crest inducing additional stirring of sediment on the landward bar slope and trough the enhanced sediment concentration in the trough region shifts the cross shore transports peak landward of the bar crest forcing bar amplitude growth during offshore migration for increased h xb values wave breaking becomes less frequent consequently reducing the influence of the longshore current on sediment stirring therefore the resulting dominance of the cross shore current results in a sediment transport peak at the bar crest causing bar amplitude decay all four types of bar response viz all combinations of onshore offshore migration and bar amplitude growth decay can occur for a single wave height and wave period combination depending on h xb and additional hindcast runs in which the wave direction was assumed time invariant confirmed that h xb and largely control the transient bar amplitude response

shoreline instability under low angle wave incidence 1 the growth of megacusps as shoreline instabilities is investigated by examining the coupling between wave transformation in the shoaling zone longshore transport in the surf zone cross shore transport and morphological evolution this coupling is known to drive a potential positive feedback in case of very oblique wave incidence leading to an unstable shoreline and the consequent formation of shoreline sand waves here using a linear stability model based on the one line concept we demonstrate that such instabilities can also develop in cases of low angle or shore normal incidence under certain conditions small enough wave height and or large enough beach slope the wavelength and growth timescales are much smaller than those of high angle wave instabilities and are nearly in the range of those of surf zone rhythmic bars o 102 103 m and o 1 10 days the feedback mechanism is based on 1 wave refraction by a shoal defined as a cross shore extension of the shoreline perturbation leading to wave convergence shoreward of it 2 longshore sediment flux convergence between the shoal and the shoreline resulting in megacusp formation and 3 cross shore sediment flux from the surf to the shoaling zone feeding the shoal even though the present model is based on a crude representation of nearshore dynamics a comparison of model results with existing depth averaged two dimensional model output and laboratory experiments suggests that the instability mechanism is plausible additional work is required to fully assess whether and under which conditions this mechanism exists in nature

a remote sensing technique for the identification of aeolian fetch distance this paper describes a new method for assessing aeolian fetch distances in beach dune systems the remote sensing technique has advantages over currently used proxy measures of fetch distance such as those based on tide gauges and beach profiles the method uses a digital camera global positioning system surveying and a geographical information system software package to produce a rectified image of the beach surface from this direct measurements of the wet dry beach boundary can be combined with wind direction measurements to determine fetch distance the method offers an improvement on current approaches to sediment transport estimates a 28 day study on magilligan strand northern ireland showed that the commonly used high water mark approach overestimated fetch distance by 30 compared with the technique reported here

wave driven alongshore sediment transport and stability of the dutch coastline abstract the wave driven alongshore sediment transport is commonly supposed to smooth out the irregularities on the coastline however it has been shown that waves approaching the coast with a high angle with respect the shore normal can reverse that tendency and cause the rectilinear coast to be unstable ashton a murray a b arnault o 2001 formation of coastline features by large scale instabilities induced by high angle waves nature 414 296 300 falques a calvete d 2005 large scale dynamics of sandy coastlines diffusivity and instability j geophys res 110 doi 10 1029 2004jc002587 the extended one line coastline model presented in the latter paper is here applied to investigate the stability of the dutch coast the main aim is testing the hypothesis that the shoreline sand waves observed along this coast could be generated by such an instability it is found that the dutch coast has potential for instability this is most prominent on the holland coast followed by the delta coast and is very weak on the wadden coast whether the instability actually occurs or not depends on the cross shore bathymetric profile of the shoreline waves under the sensible assumption that the bathymetric perturbation is just a shift of the equilibrium beach profile the dutch coast is stable in this case the mean annual coastline diffusivity is evaluated and it is found to be typically about 0 010 0 015 m 2 s 1 that is roughly smaller by a factor 2 than that predicted by the traditional one line model however the dutch coast may be unstable with respect to coastline waves with a maximum bathymetric signal at a few hundred meters from the coast this is shown in one case where the shoals associated with the sand wave are inside the surf zone during moderate storm waves thus the sand waves could result from the cross shore redistribution of the sand associated with an alongshore series of shoals and bed depressions generated by the alongshore transport in the surf zone while the generation or not of such shoreline waves by this instability strongly depends on their profile its propagation once they have been created is less sensitive and is well reproduced by the present model it is explained why the propagation is to the ne along the delta and wadden coasts why it is faster on the latter and why on the holland coast there is no clear propagation direction

shoreline evolution of the holland coast on a decadal scale the decadal evolution of the dunefoot position along 120 km of the holland coast is analysed firstly a new definition of dunefoot is introduced based on hydrodynamic and morphologic considerations which is not affected by local and instantaneous processes occurring immediately before the bathymetric survey the dunefoot evolution over decadal scales indicates the existence of spatial and temporal oscillations in the shoreline position with magnitudes of 2 3 km length and a periodicity of 4 15 years two main controlling factors of this behaviour are identified 1 influence of sub aqueous bar systems and 2 changes in the storm wave conditions reaching the coast although the precise controlling processes of the relation between the dunefoot and the subaqueous profile still remain unclarified we introduce the concept that the development of a morphological bar cycle requires a fixed amount of time integrated forcing that is proportional to the cumulative effect of storm waves beach mobility along the holland coast on decadal scales 10 20 m is similar or lower than mobility introduced by storms or by seasonal cycles however it is important to consider these changes for the possible implications on the local vulnerability of the coast to extreme events copyright 1999 john wiley sons ltd

analysis of observed two and three dimensional nearshore bar behaviour abstract shore parallel sandbar systems often exhibit considerable quasi regular alongshore variations such as crescentic plan shapes therefore morphological change in such systems commonly consists of two and three dimensional variability the former is related to overall on offshore bar migration whereas the latter is induced by the horizontal amplitude growth migration or length scale change of the quasi regular topography in this paper we used a six week data set of bathymetric surveys and video images of breaking induced foam to quantify short term days weeks two and three dimensional variability in the bar crest position of the double barred beach at egmond aan zee netherlands the alongshore uniform response was computed as the mean of each bar crest line whereas the alongshore non uniform behaviour was characterised by the amplitude around each mean a more sophisticated description of the inner bar data set was provided by complex empirical orthogonal function ceof analysis the first complex mode containing about 85 of the variance in the data set corresponded to the amplitude growth and longshore migration of an approximately 600 m long alongshore non uniformity the longshore migration rate varied between 0 and 150 m day and was found to be well related to the longshore component of the offshore wave energy flux the second complex mode explained about 10 of the variance and largely described the alongshore averaged cross shore bar migration the ceof results suggest that short term variability in bar crest position is largely due to changes in the quasi regular topography and not to alongshore uniform on offshore oriented behaviour

beach systems of the central netherlands coast processes morphology and structural impacts in a storm driven multi bar system abstract the 124 km long central netherlands coast consists of a sand barrier system fronted by beach and surf zone containing 2 to 3 bars the beach and surf zone are the product of wind waves generated in the north sea interacting with the medium to fine shoreface sands in a micro tidal environment the inner bar is usually attached to the beach as a ridge and runnel cut by drains and rips the second and third bars are highly rhythmic and are characterised respectively by transverse bars and rips and rhythmic to longshore bar and trough bar types structural impacts influence 48 km of shore with harbour moles inducing lower wave energy and shoreline progradation a 4 5 km long dyke replaces part of the beach but apparently has had little impact on mesoscale bar dynamics while 158 groynes in three fields have produced more intermediate rip driven surf zones natural beach processes however dominate the entire coast with the hierarchical bar morphology related to cross shore wave breaking the equilibrium beach concept of wright et al 1987 was used to empirically predict beach bar type the results explain both the hierarchy of bar type and the temporal variation in beach bar type finally a multi bar beach model for the coast is presented which builds upon the single bar model of wright and short 1984

self organized kilometer scale shoreline sand wave generation sensitivity to model and physical parameters the instability mechanisms for self organized kilometer scale shoreline sand waves have been extensively explored by modeling however while the assumed bathymetric perturbation associated with the sand wave controls the feedback between morphology and waves its effect on the instability onset has not been explored in addition no systematic investigation of the effect of the physical parameters has been done yet using a linear stability model we investigate the effect of wave conditions cross shore profile closure depth and two perturbation shapes p1 cross shore bathymetric profile shift and p2 bed level perturbation linearly decreasing offshore for a p1 perturbation no instability occurs below an absolute critical angle c0 40 50 for a p2 perturbation there is no absolute critical angle sand waves can develop also for low angle waves in fact the bathymetric perturbation shape plays a key role in low angle wave instability such instability only develops if the curvature of the depth contours offshore the breaking zone is larger than the shoreline one this can occur for the p2 perturbation but not for p1 the analysis of bathymetric data suggests that both curvature configurations could exist in nature for both perturbation types large wave angle small wave period and large closure depth strongly favor instability the cross shore profile has almost no effect with a p1 perturbation whereas large surf zone slope and gently sloping shoreface strongly enhance instability under low angle waves for a p2 perturbation finally predictive statistical models are set up to identify sites prone to exhibit either a critical angle close to c0 or low angle wave instability

morphology formation and migration of longshore sandwaves long point lake erie canada this paper describes the morphology and dynamics of features termed here longshore sandwaves which are developed on a large barrier spit in the great lakes the sandwaves are local beach protuberances with alongshore lengths of 500 2500 m and maximum widths at the downdrift end of 50 90 m which migrate alongshore in the direction of net drift at 150 300 m yr 1 they have a lifespan of 10 yrs or more and their spatial and temporal scales are an order of magnitude greater than the rhythmic topography associated with the nearshore bars and rip cells which are also characteristic of the littoral zone the sandwaves are initiated by the onshore movement and welding of inner nearshore bars during nonstorm periods in areas of local sediment abundance the alongshore movement of the sandwaves occurs through downdrift extension which is initiated by attachment of a landward migrating nearshore bar to the downdrift end of the sandwave and subsequent infilling of the trough landward of the bar the presence and movement of the sandwaves results in a complex spatial temporal pattern of beach and dune erosion and accretion with accretion occurring opposite the sandwave and erosion focused in the embayments between sandwaves

longshore sand waves at southampton beach new york observation and numerical simulation of their movement abstract this paper describes measurements and numerical simulations of the movement of longshore sand waves observed at southampton beach long island new york these large morphologic features are created by periodic opening of a small inlet and subsequent welding of its ebb shoal to the beach longshore sand waves are wave like forms that maintain identity while moving along the shore and represent a simple type of collective sediment movement or largescale coastal behavior despite being described as early as 1939 few measurements of longshore sand waves have been made and their dynamic behavior is not well understood eleven longshore sand waves were identified along southampton beach and have an average length of 0 75 km and an amplitude of 40 m migration speed was greatest under winter wave action and reached an average 1 09 km year 1 and yearly average speed including summer calms was a much lower 0 35 km year 1 during a 16 month observation period the longshore sand waves did not disperse during their steady migration westward in the direction of predominant wave incidence and longer waves moved faster than those with smaller wavelengths a numerical model of shoreline change including both standard particulate longshore sand transport and a component describing migration of a longshore sand wave is developed in which the velocity of the wave is related to the calculated longshore discharge of water the model shows wave refraction to be an important mechanism contributing to the longevity of longshore sand waves acting to retard the otherwise expected diffusion model calculations of the migration of a single longshore sand wave agree quantitatively with the observations

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

earth s middle age this research was funded through natural environment research council grant ne j021822 1 and the apc was paid through the rcuk oa block grant

relationship between foredune profile morphology and aeolian and marine dynamics a conceptual model abstract foredunes are highly dynamic coastal features that are intrinsically connected with their adjacent beaches correlations among parameters representative of aeolian and marine dynamics and the size and location of foredunes were carried out the results indicate that foredune steady state location depends mainly on marine dynamics given that foredune toe level was positively correlated with the total water level of 10 year return period which guarantees that the foredune is only eroded by waves occasionally and therefore will have time to recover and return to its long term morphological state foredune steady state size was characterized by its volume which was positively correlated with aeolian sediment drift potential and exponentially related to the total water level with a 10 year return period aeolian sediment drift potential is indicative of the total amount of sand that potentially reaches the foredune and therefore the foredune size is directly related to it higher total water level with a 10 year return period leads to a foredune located further from the shoreline and at a higher altitude higher toe level as mentioned above consequently the beach is wider and the presence of moisture is expected to be lower this wider and dryer beach allows the aeolian sediment transport to approximate to its maximum value increasing its potential to build a bigger foredune data confirmed that the total water level with a 10 year return period and the foredune volume are exponentially related the correlations found show that foredune steady state size is related to both aeolian sediment drift potential and marine dynamics based on these results a long term conceptual model is proposed to explain the different morphological characteristics of the steady state shape of foredunes as a function of the joint action of marine and wind dynamics

evidence for late holocene earthquakes on the utsalady point fault northern puget lowland washington trenches across the utsalady point fault in the northern puget lowland of washington reveal evidence of at least one and probably two late holocene earthquakes the teeka and duffers trenches were located along a 1 4 km long 1 to 4 m high northwest trending southwest facing topographic scarp recognized from airborne laser swath mapping glaciomarine drift exposed in the trenches reveals evidence of about 95 to 150 cm of vertical and 200 to 220 cm of left lateral slip in the teeka trench radiocarbon ages from a buried soil a horizon and overlying slope colluvium along with the historical record of earthquakes suggest that this faulting occurred 100 to 400 calendar years b p a d 1550 to 1850 in the duffers trench 370 to 450 cm of vertical separation is accommodated by faulting 210 cm and folding 160 to 240 cm with probable but undetermined amounts of lateral slip stratigraphic relations and radiocarbon ages from buried soil colluvium and fissure fill in the hanging wall suggest the deformation at duffers is most likely from two earthquakes that occurred between 100 to 500 and 1100 to 2200 calendar years b p but deformation during a single earthquake is also possible for the two earthquake hypothesis deformation at teeka trench in the first event involved folding but not faulting regional relations suggest that the earthquake s were m 6 7 and that offshore rupture may have produced tsunamis based on this investigation and related recent studies the maximum recurrence interval for large ground rupturing crustal fault earthquakes in the puget lowland is about 400 to 600 years or less

modeling large scale shoreline sand waves under oblique wave incidence waves develop with wavelengths between 2 and 5 km they migrate downdrift at about 0 5 km yr and they reach amplitudes up to 120 m within 13 years larger wave obliquity higher waves and shorter wave periods strengthen the shoreline instability cross shore transport is essential for the instability and faster cross shore dynamics leads to a faster growth of the sand waves simulations with variable wave incidence angles alternating between 60 and 30 show that a large proportion of high angle waves is required for spontaneous sand wave formation at least 80 insight is provided into the physical mechanism behind high angle wave instability and the occurrence of a optimal length scale for sand wave growth the generic model results are consistent with existing observations of shoreline sand waves in particular with those along the southwest coast of africa

coastline undulations on the west coast of denmark offshore extent relation to breaker bars and transported sediment volume abstract this paper deals with field measurements of undulations of the bottom bathymetry along an otherwise straight coast at the danish west coast two bathymetric datasets and two time series of wave measurements are used in order to determine the following properties the offshore extent of shoreline undulations the amount of sediment transported alongshore in the shoreline undulations the relationship between the shoreline undulations and longshore bars and the relationship between the morphology and the hydrodynamics in one of the data sets the shoreline undulations are well correlated with undulations on the depth contours between 5 m and 2 m relative to mean sea level in the other data set only undulations on the depth contours between 1 m and 1 m are well correlated with the shoreline undulations the main difference in the wave climate between the two locations is the orientation of the shoreline such that the wave incidence angle is more oblique on the section of shoreline with well correlated depth contours suggesting that the well correlated depth contours are a result of the oblique wave incidence

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

sand dispersion from an ephemeral river delta on the central california coast abstract an ephemeral river delta on the wave dominated central california coast was studied to determine how and when river sand was incorporated into the longshore transport regime the delta at the mouth of the san lorenzo river was built in january 1982 by a 30 year flood records of river flow sediment load and channel volume were used to estimate the river sediment yield repeated topographic surveys of the delta and surrounding beaches confirmed the estimates of river yield and traced sand movement from the river mouth directional wave data confirmed the direction of longshore sand movement and identified sites of erosion and accretion due to divergence in the longshore transport most sand delivered to the delta in flood was deposited seaward of the surfzone about a quarter of this sand was returned shoreward each summer by swell for a time wave refraction across the submerged delta trapped the river sand and littoral drift from upcoast on the delta limiting the sand supply to the beaches downcoast as storm waves broadened the delta refraction lessened and the river sand began to disperse alongshore longshore dispersion was assisted by seasonal cross shore movements directly through a net longshore creep and indirectly through the return of sand from offshore to the surfzone transport the overall pattern of river sand dispersion was that of a low amplitude sandwave

holderness coast erosion and the significance of ords a sample ord a low section of beach characteristic of the holderness coast is examined as it moved southwards between 1977 and 1983 and its significant role in coast erosion is demonstrated the reduction in beach level at the cliff foot by up to 3 9 m enabled most hwn tides to reach it as compared with only some hws tides along the inter ord beach and the volume of till eroded from the cliffs increased by eight times to an annual mean of 72m3m 1 the holderness tills are shown to be composed of 31 per cent sand and coarser sediment which is the sediment range of the beaches where an ord is sited the massive injection of beach sediment goes to form the ord s most prominent constructional feature the lower beach ridge which extends southwards from the centre of the ord analysis of 12 months observer wave data collected at withernsea in 1969 70 indicates that a net southward sand movement of 144 000 m3 occurred comparison of this rate with beach sediment input rates along the whole coast backed by till cliffs suggests a sediment deficit at the northern end and a surplus towards the southern end this conclusion is supported by an overall increase in beach sediment volume southwards from barmston within this longshore sediment transport system the ords migrate southwards from their point of origin in the barmston skipsea area without losing their identity until reaching the tip of spurn head