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

cross shore mean flow in the surf zone abstract the seaward mean returnflow or undertow in the surf zone which compensates for the shoreward mass flux above the wave trough level is found to be driven by the force imbalance between the wave momentum flux on the one hand and the set up on the other hand in qualitative agreement with the model of dyhr nielsen and sorensen 1970 a model which quantifies this imbalance is shown to yield theoretical results in good accordance with experiments when the proper boundary conditions are accounted for

a morphological stability analysis for a long straight barred coast a morphological stability analysis is carried out for a long straight coast with a longshore bar the situation with oblique wave incidence and a wave driven longshore current is considered the flow and sediment transport are described by a numerical modelling system the models comprise i a wave model with depth refraction shoaling and wave breaking ii a depth integrated model for wave driven currents and iii a sediment transport model for the bed load transport and the suspended load transport in combined waves and current the direction of the sediment transport is taken to be parallel to the depth integrated mean current velocity neglecting the effects of a bed slope and secondary currents an instability is found to develop around the bar crest the instability is periodic in the alongshore direction and tends to form rip channels and to steepen the offshore face of the bar between the rip channels the alongshore wave length of the most unstable perturbation is determined for different combinations of the wave conditions and the geometry of the profile

mtsu recovering seismic moments from tsunameter records we define a new magnitude scale m tsu allowing the quantification of the seismic moment m0 of an earthquake based on recordings of its tsunami in the far field by ocean bottom pressure sensors tsunameters deployed in ocean basins far from continental or island shores which are known to affect profoundly and in a nonlinear fashion the amplitude of the tsunami wave the formula for m tsu m tsu log10 m o 20 log10 x c d tsu c s tsu c o where x is the spectral amplitude of the tsunami c d tsu a distance correction and c s tsu a source correction is directly adapted from the mantle magnitude m m introduced for seismic surface waves by okal and talandier like m m its corrections are fully justified theoretically based on the representation of a tsunami wave as a branch of the earth s normal modes even the locking constant c 0 which may depend on the nature of the recording surface amplitude of the tsunami or overpressure at the ocean floor and its units is predicted theoretically m tsu combines the power of a theoretically developed algorithm with the robustness of a magnitude measurement that does not take into account such parameters as focal geometry and exact depth which may not be available under operational conditions in the framework of tsunami warning we verify the performance of the concept on simulations of the great 1946 aleutian tsunami at two virtual gauges and then apply the algorithm to 24 records of 7 tsunamis at dart tsunameters during the years 1994 2003 we find that m tsu generally recovers the seismic moment m 0 within 0 2 logarithmic units even under unfavorable conditions such as excessive focal depth and refraction of the tsunami wave around continental masses finally we apply the algorithm to the jason satellite trace obtained over the bay of bengal during the 2004 sumatra tsunami after transforming the trace into a time series through a simple ad hoc procedure results are surprisingly good with most estimates of the moment being over 1029 dyn cm and thus identifying the source as an exceptionally large earthquake

on the development of large scale cuspate features on a semi reflective beach carchuna beach southern spain abstract carchuna beach located on the southeast coast of spain has a series of natural permanent horns of time dependent horizontal dimensions which do not appear to be periodically spaced and do not propagate alongshore until at present it has been assumed that the permanency of these forms is related to the existence of bed rocks nevertheless analysis of the configuration of the depth contours and the bed sediment composition does not justify this hypothesis this paper explores the formation of cuspate features by the effects of the wind wave dynamics and their development by nearshore circulation and infragravity waves including edge waves three levels of wave energy flux conditions are considered in all likelihood wave refraction of severe storms on submerged fluvial valleys is the main cause for longshore variation of breaking wave height which seems to be one of the initial conditions necessary for the cuspate to initiate moderate storms enhance the deepening of the embayments whereas mild sea states are able to maintain the features by self organization the possibility of the occurrence of edge waves due to the non linear interaction between the components of a gravity wave spectrum approaching the shore under moderate storm conditions is explored preliminary analysis of video images taken by an argus station seems to confirm this hypothesis

a mechanism for the generation of wave driven rhythmic patterns in the surf zone the coupling between topographic irregularities and wave driven mean water motion in the surf zone is examined this coupling occurs because the topographic perturbations produce excess gradients in the wave radiation stress that cause a steady circulation this circulation in turn creates a sediment transport pattern that can reinforce the bottom disturbance and may thereby lead to the growth of large scale bed forms to investigate this coupling mechanism the linearized stability problem with an originally plane sloping beach and normal wave incidence is solved in two different cases first the breaking line is considered to be fixed and second the perturbations in water depth that produce a displacement of the breaker line are accounted for the first case shows that the basic topography can be unstable with respect to two different modes a giant cusp pattern with shore attached transverse bars that extend across the whole surf zone and a crescentic pattern with alternate shoals and pools at both sides of the breaking line showing a mirroring effect in the second case the varying breaker line may have a strong influence on the circulation this is clear for the giant cusp topography whose growth is totally inhibited in contrast the morphology and the growth of the crescentic pattern remains almost unchanged

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

coupling mechanisms in double sandbar systems part 2 impact on alongshore variability of inner bar rip channels double sandbar systems are common morphological features along sandy wave dominated micro to meso tidal coastlines in the companion paper we demonstrated how various alongshore inner bar rip channel patterns can develop through morphological coupling to an alongshore variable outer bar the simulated coupling patterns are however scarcely observed in the field instead inner bar rip channels more often possess remarkably smaller and more variable alongshore length scales suggesting that coupling mechanisms do not play a substantial role in the overall double sandbar dynamics here we use a numerical model to show that the relative importance of self organization and morphological coupling changes in favour of the latter with an increase in waterdepth variability along the outer bar crest furthermore we find that the typical alongshore variability in inner bar rip channel scale is indicative of a mixture of self organization and morphological coupling rather than self organization alone morphological coupling may thus be more important to understanding and predicting the evolution of inner bar rip channels than previously envisaged copyright 2010 john wiley and sons ltd

probing supraglacial debris on mars 2 crater morphology abstract lobate debris aprons lda lineated valley fill lvf and concentric crater fill ccf on mars interpreted to be debris covered glaciers possess craters with a suite of distinct interior landforms called ring mold craters that have been attributed to the presence of glacial ice at depth or surface modification processes we tested current hypotheses for the formation of ring mold craters by conducting a comprehensive analysis of the size and morphology of 16 457 impact craters 125 m in diameter formed within glacial deposits in deuteronilus mensae two major groups bowl shaped craters and ring mold craters are found with at least nine distinct crater types while there is statistical difference in median diameters between these crater types this difference is relatively small and is within the estimated uncertainty in diameter measurements and may be related to more enhanced erosion of the rims of ring mold craters clear degradation sequences are observed supporting a role for post impact modification in producing at least some of the diversity in crater landforms the spatial density of ring mold craters is also directly correlated with the development of lda lvf and ccf surface textures flow lineations cross cut two ring mold crater types but they maintain their circular planforms in some cases suggesting that the craters initially formed completely within mantling layers deposited after glacial flow had ceased we also find analogous craters in non glacial units glacial ice is therefore not required to form the observed morphologic diversity our observations are most consistent with formation of crater landforms by emplacement and modification of at least two depositional episodes of icy dust i e mantle this mantle was initially tens of meters in thickness to support crater formation and has experienced much downwasting and erosion since emplacement derived crater retention ages of 460 ma for lda lvf and ccf features in the region therefore reflect deposition of mantle units and only give a very minimum age for the formation of lda lvf and ccf

self organization mechanisms for the formation of nearshore crescentic and transverse sand bars the formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow water equations and on a simplified sediment transport parameterization by assuming normally approaching waves and a saturated surf zone rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed two types of bedforms appear one is a crescentic bar pattern centred around the breakpoint and the other herein modelled for the first time is a transverse bar pattern the feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations basically the waves stir up the sediment and keep it in suspension with a certain cross shore distribution of depth averaged concentration then a current flowing with against the gradient of sediment concentration produces erosion deposition it is shown that inside the surf zone these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro morphodynamic input conditions to which of the patterns develops it is suggested that crescentic bar growth would be favoured by high energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment in intermediate conditions mixed patterns may occur

understanding coastal morphodynamic patterns from depth averaged sediment concentration this review highlights the important role of the depth averaged sediment concentration dasc to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern beach cusps surf zone transverse and crescentic bars and shoreface connected sand ridges we present a formulation and methodology based on the knowledge of the dasc which equals the sediment load divided by the water depth that has been successfully used to understand the characteristics of these features these sand bodies relevant for coastal engineering and other disciplines are located in different parts of the coastal zone and are characterized by different spatial and temporal scales but the same technique can be used to understand them since the sand bodies occur in the presence of depth averaged currents the sediment transport approximately equals a sediment load times the current moreover it is assumed that waves essentially mobilize the sediment and the current increases this mobilization and advects the sediment in such conditions knowing the spatial distribution of the dasc and the depth averaged currents induced by the forcing waves wind and pressure gradients over the patterns allows inferring the convergence divergence of sediment transport deposition erosion occurs where the current flows from areas of high to low low to high values of dasc the formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features but the physical mechanisms for their migration their finite amplitude behavior and their decay can also be explored

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

temporal and spatial behaviour of rip channels in a multiple barred coastal system a 15 month data set of daily time averaged video images argus has been analyzed to describe the spatial and temporal variability of the rip channels on a multiple barred coast at noordwijk aan zee the netherlands the landward boundary of the intertidal bars and a proxy of the subtidal bar crest defined as the intertidal and subtidal bar lines respectively were derived from the argus images local seaward directed deviations of the bar lines represent the cross shore and alongshore locations of the rip channels the average intertidal rip spacing was 243 m but the rips were not spaced regularly 0 47 some intertidal rips were observed to fill up during falling tide but the majority remained open the filled intertidal rip channels had more landward positions and migrated more slowly 2 4 versus 4 6 m day in the alongshore direction than the open intertidal rip channels the number and the alongshore migration rate of open intertidal rip channels increased with the preceding wave heights r 0 26 p 0 01 and alongshore component of the offshore wave power r 0 25 p 0 01 respectively the shape of the intertidal bar lines was similar to the subtidal bar line shape suggesting that the intertidal morphology is coupled to the subtidal alongshore variability the phase of two bar lines could vary from in phase 0 to out of phase 180 the phase changes gradually due to different alongshore migration rates of the intertidal and subtidal bar lines copyright 2009 john wiley sons ltd

regularity and randomness in the formation of beach cusps abstract controversy still exists on whether rhythmic features such as beach cusps are the result of the presence of standing edge wave motions in the hydrodynamics or if they are the result of self organising processes the compatibility between the two mechanisms is here investigated through the use of a numerical model simulating the formation and development of beach cusps simulations characterised by different forcing conditions have been performed a series of random simulations each individual run simply differing from the others in the seed used in the random number generator showed the typical self organisation behaviour with the features appearing at different locations and even with slightly different spacing regular series of simulations have been run by changing the wavelength of the template superimposed by the hydrodynamic forcing and the number of cycles when the template was present results surprisingly indicate that even a minimum number of cycles with a regular forcing can deterministically induce the final shoreline configuration furthermore if the forcing template has the same wavelength as the one resulting from purely random simulations growth rates are much faster than those obtained with random conditions implications for the kind of field measurements necessary to discern which of the two mechanisms is responsible for beach cusp formation have also been considered

a mechanism for sandbar straightening by oblique wave incidence 1 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

rip current review rip currents are shore normal narrow seaward flowing currents that originate within surf zone extend seaward of the breaking region rip head and can obtain relatively high velocities within the last decade there have been a significant number of laboratory and field observations within rip current systems an overview of rip current kinematics based on these observations and the scientific advances obtained from these efforts are synthesized rip current flows are partitioned into mean infragravity very low frequency vorticity and tidal contributions and it is found that each contributes significantly to the total data from the laboratory and the field suggest that the rip current strength increases with increasing wave energy and decreasing water depths the maximum mean current occurs inside the surf zone where the maximum forcing is present owing to the dissipation of waves

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

the morphological response of a nearshore double sandbar system to constant wave forcing results of 2dh morphodynamic computations are presented to quantify the temporal evolution of the crescentic patterns emerging in a double nearshore bar system in response to constant wave boundary forcing sixteen different conditions varying both offshore wave height and angle of wave incidence were applied the mean length scales of the emerging irregular crescentic patterns are linearly proportional to the local longshore velocity over the inner and outer bars for similar longshore velocities the length scales of the outer bar are larger than of the inner bar this is explained by accounting for the difference in water depth above the bar crest the variable morphological response times can be explained by including additional bathymetrical parameters the active volume of the bar defined by the breaker index plays an important role in this response time with larger active volumes the bar responds more rapidly to identical boundary conditions also bars with a smaller total volume respond more quickly this faster response is due to the steeper active volume of the bars different initial perturbations resulted in different locations of the emerging features showing that their location is sensitive to the initial bathymetry however the range in length scales and response times due to the different perturbations was significantly smaller than those obtained for the different hydrodynamic conditions based on the present findings we hypothesize that morphological length scales in the field are rarely in equilibrium with the concurrent offshore wave height and angle of incidence owing to the slow response of the sandbars under constant conditions relative to the stochastic nature of natural wave forcing

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

sandbars in motion nearshore surf zones are turbulent places yet despite the apparently chaotic nature of the sea surface in these areas sandbars often show complex patterns with an intricate structure in their perspective stive and reniers discuss recent efforts to understand how these patterns form they highlight the report by hoefel and elgar who consider a new transport mechanism based on flow acceleration within the waves this mechanism should help to improve the predictability of sandbar structures