an aerodynamic roughness length map derived from extended martian rock abundance data many boundary layer processes simulated within a mars general circulation model mgcm including the description of the processes controlling dust rising from the martian surface are highly sensitive to the aerodynamic roughness length z0 on the basis of rock size frequency distributions inferred from different martian landing sites and earth analog sites we have first established that lognormal modeled rock size frequency distributions are able to reproduce correctly the observed martian rock populations we have validated the hypothesis that the rock abundance of a given area could be estimated at a first order from its thermophysical properties namely its thermal inertia i and its albedo we have demonstrated the possibility of using rock abundance to estimate the roughness density on mars and to retrieve subsequently the aerodynamic roughness length by using semi empirical relationships based on terrestrial wind tunnel and field measurements by combining our methodology with remote sensing measurements of the thermal emission spectrometer aboard mars global surveyor we have derived a global map of the aeolian aerodynamic roughness length with a 1 8 1 8 resolution over the entire martian surface contrary to what is often assumed the martian aeolian aerodynamic roughness length is spatially highly heterogeneous at the fullest resolution the martian aerodynamic roughness length varies from 10 3 cm to 2 33 cm about 84 of the martian surface seems to be characterized by an aeolian aerodynamic roughness length value lower than 1 cm the spatially uniform value that most of the mgcms simulations have assumed recently since the aerodynamic roughness length z0 is a key parameter in deriving the erosion threshold wind velocities we anticipate a significant impact of our findings on the efficiencies for lifting dust in future mgcms

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

coastal processes and the development of shoreline erosion in 1971 the us army corps of engineers inventoried the stability of the nation s coastlines concluding in their final summary a report on the national shoreline study that of the 135 000 km of coastline some 33 000 km were seriously eroding more recently dolan et al 1983 assembled the existing data on shoreline changes for the united states reaffirming the magnitude of this problem although such inventories have not been undertaken for the entire world s shorelines coastal erosion is undoubtedly a global problem beach erosion is for the most part episodic with the major retreats in the shoreline occurring during unusually intense storms on the east coast of the united states beach erosion and property losses are produced by nor easters and hurricanes on the west coast intense storms move down from the gulf of alaska generating waves that attack the coastline this obvious association of beach erosion with the energy level of storm waves creates an oversimplified impression of the actual physical processes involved in the erosion only recently have we begun to understand and appreciatc the diversity of processes that are actually involved in coastal erosion it is now recognized that long period oscillations may dominate water motions in the nearshore sometimes containing more energy than the storm generated waves these oscillations as well as a variety of nearshore currents rearrange the beach sand to cause highly variable rates of shoreline retreat along even short stretches of beach although the long term rise in sea level has been recognized as an important factor in shoreline

friction wind speeds in dust devils a field study 1 using in situ field measurements we determined surface friction velocities u and aerodynamic roughness heights z0 at the base of 10 dust devils z0 values agree closely with previous studies in this region and range from 0 1 to 1 0 cm the peak u coincide with the largest wind speed measurements within the dust devil and range from 0 9 to 2 4 ms 1 surface friction velocities are sufficient to lift all types of windblown material from dust to pebble sized particles peak u values for dust devils appear to be a linear function of the peak wind speed measured at 2 m height within the dust devil this provides a simple approximation for estimating surface shear stress for dust devils

nearshore oblique sand bars 1 the coupling between hydrodynamics and the evolving topography in the surf zone has been theoretically examined for oblique wave incidence it is shown that positive feedback can lead to the initial growth of several types of rhythmic systems of sand bars the bars can be down current oriented or up current oriented which means that the offshore end of the bar is shifted down current or up current with respect to the shore attachment in the limit of strong current compared to wave orbital motion very oblique down current oriented b ars are obtained with a spacing of several times the surf zone width when wave orbital motions are dominant systems of up current oriented bars and crescentic down current oriented bars appear with spacings of the order of the surf zone width the latter feature consists of alternating shoals and troughs at both sides of the break line with the inner shoals being bar shaped and oblique to the coast the growth e folding time of the bars ranges from a few hours to a few days and it is favored by constant wave conditions the range of model parameters leading to growth corresponds to intermediate beach states in between the fully dissipative and the fully reflective situations preliminary comparison with field observations shows qualitative agreement

observations of rip spacing persistence and mobility at a long straight coastline three years of daily video observations at a long straight beach were analysed to determine the temporal trends and variability of the location spacing persistence and mobility of rips rips were identified at this site on 684 days of the total 947 days 72 occurrence when suitable images were available a median number of 7 rips were observed within the total 2 km study area on days when rips were present no tendency was identified for rips to reoccur in preferred locations alongshore following storm reset events the average alongshore distance between all rips observed was 209 m but with a high standard deviation of 98 m or approximately 47 of the mean rip spacing there was no evidence at this site that rips tended to be regularly spaced alongshore no clear relationship was identified between the number of rips and the prevailing offshore wave conditions including significant wave height peak wave period and incident wave power the majority of rips persisted for 5 or fewer days with an average of 8 days and standard deviation of 9 days rips were stationary 33 of days with migration rates of less than 5 m day observed on 47 of days when rips were present maximum migrations rates of up to 50 m day were observed and generally though not always occurred in the direction consistent with the prevailing offshore swell direction and resulting alongshore current the occurrence of inshore sea breezes may have accounted for the less frequent observations of rip migration against the opposing regional swell direction the results presented at this site complement a recent four year study at a contrasting short and embayed beach detailed in holman et al holman r a symonds g thornton e b and ransinghe r 2006 rip spacing on an embayed beach journal of geophysical research 111 c01006 doi 10 1029 2005 jc002965 17p the specific mechanism s of rip channel infilling at the long straight and littoral drift dominated coastline may have contributed to the observation of shorter rip persistence relative to the embayed site mean rip spacing was observed at both sites to be highly variable irregular and exhibited no identifiable trend associated with the offshore wave conditions this latter observation supports the notion that rips may become rapidly topographically controlled soon after a storm reset event and their location is then primarily determined by the evolving nearshore morphology rather than hydrodynamic forcing this new dataset of daily multi year observations at an open coast site cannot be reconciled with the majority of existing template and instability models for rip formation that predict a relationship between incident wave conditions and regular spacing of rips alongshore rather these observations tend to support emerging theories of rip genesis that predict the irregular and random spacing of rip channels alongshore

air density effects on aeolian sand movement implications for sediment transport and sand control in regions with extreme altitudes or temperatures aeolian sand transport results from interactions between the ground surface and airflow previous research has focused on the effects on sand entrainment and mass transport of surface features and wind velocity but the influence of air density which strongly constrains airflow characteristics and the resulting sand flow has not been widely considered in the present study entrainment saltation characteristics and transport rates were examined at nine experimental sites ranging in elevation from 154 m below sea level aiding lake to 5076 m above sea level tanggula mountain pass on the qinghai tibetan plateau at each site a portable wind tunnel and high speed camera system were set up and the friction wind velocity threshold friction velocity and sand flow structure were observed systematically for a given volumetric airflow lower air density increases the wind velocity low air density also creates a high threshold friction velocity the bagnold wind erosion threshold model remains valid but the value of empirical parameter a decreased with decreasing air density and ranged from 0 10 to 0 07 the smallest values reported in the literature for a given wind velocity increased altitude reduced total sand transport and creeping but the saltation rate and saltation height increased the present results provide insights into the fundamental mechanisms of the initiation and transport of sand by wind in regions with an extreme temperature or altitude for example alpine deserts and low lying lake basins or on other planets including mars these results also provide theoretical support for improved sand control engineering measures the data and empirical equations provided in this paper improve the ability to estimate threshold and transport conditions for wind blown sand

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

orbital change experiments with a mars general circulation model abstract we use a mars general circulation model to examine the effect of orbital changes on the planet s general circulation and climate system experiments are performed for obliquities ranging from 0 to 60 for two different longitudes of perihelion each experiment simulates a full mars year assuming a fixed atmospheric dust distribution and fixed amount of co 2 in the atmosphere cap system we find that global mean surface temperatures and pressures decline with increasing obliquity due to the increasing extent of the winter polar caps the seasonal co 2 cycle and intensity of the solstice circulation amplify considerably with increasing obliquity such that global dust storms are likely at both solstices the most significant feature of the high obliquity solstice circulations is the development of an intense low level jet associated with the return branch of the hadley circulation model surface stresses are used to map regions of preferred dust lifting which are defined in terms of an annual deflation potential for the present obliquity the model predicted regions of high deflation potential are in good agreement with cantor et al s 2001 j geophys res 106 23653 23688 observations which gives us some confidence in the model s ability to predict where lifting might occur when mars orbit parameters are different than they are today in general we find that the dust lifting potential increases sharply with obliquity and is greatest at times of high obliquity when perihelion coincides with northern summer solstice over an obliquity cycle the model global annual deflation potential ranges from several tenths of a millimeter at 0 obliquity to almost 15 mm at 60 obliquity much higher values are possible when the atmosphere is very dusty we find a strong correlation between the deflation potential and surface thermal inertia regions of high deflation potential correspond to regions of high thermal inertia high rock abundance and regions of low deflation potential correspond to regions of low thermal inertia high dust sand abundance furthermore while the regions of preferred lifting high deflation potential expand somewhat with increasing obliquity and dust loading the central parts of tharsis arabia and elysium show no tendency for significant lifting at any obliquity or longitude of perihelion these regions may therefore be very old and represent net long term sinks for atmospheric dust it is the topography of the planet through its influence on surface pressure and wind systems which ultimately determines where dust accumulates finally as was found by fenton and richardson 2001 j geophys res 106 32885 32909 we find no tendency for the development of east southeasterly winds at the pathfinder site for any of our orbital change experiments this suggests that the ancient wind regime discussed by greeley et al 2000 j geophys res 105 1829 1840 was produced by other factors such as polar wander

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

mesoscale linear streaks on mars environments of dust entrainment abstract variable surface albedo features on mars are likely caused by the entrainment and deposition of dust by the wind most discrete markings are associated with topographic forms or with regional slopes that serve to alter the effective wind shear stress on the surface some of the largest variable features here termed mesoscale linear streaks are up to 400 km in length and repeatedly occur in one of the smoothest regions of mars amazonis planitia their orientations and apparent season of variability as observed by viking and mars orbiter cameras indicate linear streak formation by enhanced surface wind stresses during regional or local dust storms and during the initial stages of global dust storms they provide an example of the ability of large scale winds without significant local enhancement to initiate dust motion on mars the sizes and spacing of the linear streaks may be controlled by boundary layer rolls the repetitive formation of these streaks over a span of more than 11 mars years gives one measure of the stability of mars eolian processes

mass movement slope streaks imaged by the mars orbiter camera narrow fan shaped dark streaks on steep martian slopes were originally observed in viking orbiter images but a definitive explanation was not possible because of resolution limitations pictures acquired by the mars orbiter camera moc aboard the mars global surveyor mgs spacecraft show innumerable examples of dark slope streaks distributed widely but not uniformly across the brighter equatorial regions as well as individual details of these features that were not visible in viking orbiter data dark slope streaks as well as much rarer bright slope streaks represent one of the most widespread and easily recognized styles of mass movement currently affecting the martian surface new dark streaks have formed since viking and even during the mgs mission confirming earlier suppositions that higher contrast dark streaks are younger and fade brighten with time the darkest slope streaks represent 10 contrast with surrounding slope materials no small outcrops supplying dark material or bright material for bright streaks have been found at streak apexes digitate downslope ends indicate slope streak formation involves a ground hugging flow subject to deflection by minor topographic obstacles the model we favor explains most dark slope streaks as scars from dust avalanches following oversteepening of air fall deposits this process is analogous to terrestrial avalanches of oversteepened dry loose snow which produce shallow avalanche scars with similar morphologies low angles of internal friction typically 10 30i for terrestrial loess and clay materials suggest that mass movement of low cohesion martian dusty air fall is possible on a wide range of gradients martian gravity presumed low density of the air fall deposits and thin unresolved by moc failed layer depths imply extremely low cohesive strength at time of failure consistent with expectations for an air fall deposit of dust particles as speed increases during a dust avalanche a growing fraction of the avalanching dust particles acquires sufficient kinetic energy to be lost to the atmosphere in suspension limiting the momentum of the descending avalanche front the equilibrium speed where rate of mass lost to the atmosphere is balanced by mass continually entrained as the avalanche front descends decreases with decreasing gradient this mechanism explains observations from moc images indicating slope streaks formed with little reserve kinetic energy for run outs on to valley floors and explains why large distal deposits of displaced material are not found at downslope streak ends the mass movement process of dark and bright slope streak formation through dust avalanches involves renewable sources of dust only leaving underlying slope materials unaffected areas where dark and bright slope streaks currently form and fade in cycles are closely correlated with low thermal inertia and probably represent regions where dust currently is accumulating not just residing

a martian general circulation experiment with large topography abstract a three layer general circulation model used to simulate the martian atmosphere is described and results are presented the model assumes a dust free pure c02 atmosphere and allows for a diurnally varying convective boundary layer smoothed martian topography and albedo variations are incorporated the simulation described is for the period near southern winter solstice season of the viking landings the zonally averaged circulation mass heat and momentum balances and properties of stationary and transient waves are described in some detail and are compared with results of previous simulations of the martian general circulation with related features of the earth s general circulation and with observed characteristics of the martian atmosphere the principal conclusions are the following 1 the simulated zonally averaged circulation is not very sensitive to differences between this model and the earlier general circulation model of leovy and mintz 1969 and compares reasonably well wi

dust coatings on basaltic rocks and implications for thermal infrared spectroscopy of mars 1 thin coatings of atmospherically deposited dust can mask the spectral characteristics of underlying surfaces on mars from the visible to thermal infrared wavelengths making identification of substrate and coating mineralogy difficult from lander and orbiter spectrometer data to study the spectral effects of dust coatings we acquired thermal emission and hemispherical reflectance spectra 5 25 m 2000 400 cm 1 of basaltic andesite coated with different thicknesses of air fall deposited palagonitic soils fine grained ceramic clay powders and terrestrial loess the results show that thin coatings 10 20 m reduce the spectral contrast of the rock substrate substantially consistent with previous work this contrast reduction continues linearly with increasing coating thickness until a saturation thickness is reached after which little further change is observed the saturation thickness of the spectrally flat palagonite coatings is 100 120 m whereas that for coatings with higher spectral contrast is only 50 75 m spectral differences among coated and uncoated samples correlate with measured coating thicknesses in a quadratic manner whereas correlations with estimated surface area coverage are better fit by linear functions linear mixture modeling of coated samples using the rock substrate and coating materials as end members is also consistent with their measured coating thicknesses and areal coverage a comparison of ratios of thermal emission spectrometer tes spectra of dark and bright intracrater and windstreak deposits associated with radau crater suggests that the dark windstreak material may be coated with as much as 90 areal coverage of palagonitic dust the data presented here also will help improve interpretations of upcoming mini tes and thermal emission imaging system themis observations of coated mars surface materials

physical modeling of three dimensional intermediate beach morphodynamics experiments have been performed in a large wave tank in order to study the morphodynamics of rip current systems both accretive and erosive shore normal wave conditions were applied the beach evolving through all the states within the intermediate beach classification under the so called down state accretive and up state erosive morphological transitions results show that any prescribed change in the wave conditions drastically increases the rate at which the morphology changes the surf zone morphology tends toward a steady state when running a given wave climate for a long duration we quantitatively describe a full down state sequence characterized by the progressive evolution of an alongshore uniform bar successively into a crescentic plan shape a bar and rip channel morphology and a terrace from the analysis of a large data set of dense eulerian measurements and bathymetric surveys we depict several feedback mechanisms associated with wave driven rip current circulation wave nonlinearities and the seabed evolution at first a positive feedback mechanism drives a rapid increase in the rate of morphological change beach three dimensionality and rip intensity by the time the sandbar evolves into a bar and rip morphology a negative feedback mechanism characterized by a decaying beach change rate and an increasing beach alongshore uniformity overwhelms the former mechanism an erosive sequence characterized by both an overall offshore bar migration and an increase in beach three dimensionality is also described

role of morphological variability in the evolution of nearshore sandbars abstract computations using a depth averaged morphological process based model of a double nearshore bar system have been used to test the hypothesis that bathymetries with small variability adapt more easily to new hydrodynamic conditions than bathymetries with distinctly imprinted crescentic patterns the computations are used to investigate the assumption that nearshore bathymetries tend to evolve toward a rip channelled pattern matching concurrent constant hydrodynamic forcing if these conditions prevail for an extended period of time in each computation an initially alongshore uniform double barred bathymetry seeded with a small random bed level perturbation was forced by two sequential constant hydrodynamic conditions for each set of conditions four different computations show the effect of a later transition moment and thus more distinctly evolved patterns on the level of adaptation to the second condition after the transition to the second condition different hydrodynamic circulations occur due to differences in the bathymetry at the transition moments depending on how pronounced the existing features were at the moment of transition these circulations either reinforce the existing bathymetric pattern or allow the bathymetry to evolve to a new rip chanelled pattern with a spacing similar to the one that occurs if the second condition had been applied from the start as hydrodynamic conditions generally change more rapidly than the adaptation time which is at least in the order of days it is highly unlikely that observed rip channel distances match length scales expected for concurrent hydrodynamic conditions consistent with field observations e g holman et al 2006 it is therefore concluded that nearshore patterns are formed by a combination of both the antecedent morphology and thus antecedent hydrodynamics and the current local hydrodynamic conditions next to factors like sediment characteristics

generation and nonlinear evolution of shore oblique transverse sand bars the coupling between topography waves and currents in the surf zone may self organize to produce the formation of shore transverse or shore oblique sand bars on an otherwise alongshore uniform beach in the absence of shore parallel bars this has been shown by previous studies of linear stability analysis but is now extended to the finite amplitude regime to this end a nonlinear model coupling wave transformation and breaking a shallow water equations solver sediment transport and bed updating is developed the sediment flux consists of a stirring factor multiplied by the depth averaged current plus a downslope correction it is found that the cross shore profile of the ratio of stirring factor to water depth together with the wave incidence angle primarily determine the shape and the type of bars either transverse or oblique to the shore in the latter case they can open an acute angle against the current up current oriented or with the current down current oriented at the initial stages of development both the intensity of the instability which is responsible for the formation of the bars and the damping due to downslope transport grow at a similar rate with bar amplitude the former being somewhat stronger as bars keep on growing their finite amplitude shape either enhances downslope transport or weakens the instability mechanism so that an equilibrium between both opposing tendencies occurs leading to a final saturated amplitude the overall shape of the saturated bars in plan view is similar to that of the small amplitude ones however the final spacings may be up to a factor of 2 larger and final celerities can also be about a factor of 2 smaller or larger in the case of alongshore migrating bars the asymmetry of the longshore sections the lee being steeper than the stoss is well reproduced complex dynamics with merging and splitting of individual bars sometimes occur finally in the case of shore normal incidence the rip currents in the troughs between the bars are jet like while the onshore return flow is wider and weaker as is observed in nature

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

the evolution of large scale crescentic bars on the northern gulf of mexico coast abstract airborne bathymetric lidar is used to quantify the properties and evolution of kilometer scale crescentic bars in the northern gulf of mexico between 86 0 w and 86 5 w a distance of 50 km repeated lidar surveys in 2004 pre and post hurricane ivan in july 2005 after hurricane dennis and in late 2005 after hurricane katrina in addition to several satellite images show crescentic bar growth over long time scales and destruction over much shorter times morphological changes follow the wright and short classification to some degree although the generation time scales appears much greater than typically reported features with o 1000 m longshore scales develop in months to years rather than days to weeks the reason for this appears to be the order of magnitude difference between the extreme conditions during tropical cyclones which generate the precursor linear bars in relatively deep water and the typical small wave conditions in the gulf when crescentic features develop another unexpected feature shows that it may require more than one direct hurricane landfall to completely remove these large scale crescentic features the crescentic bars examined here form only a small portion of the available dataset work to be reported on shortly will examine bar properties over a 200 km section of coastline

mechanisms controlling the complete accretionary beach state sequence accretionary down state beach sequence is a key element of observed nearshore morphological variability along sandy coasts we present and analyze the first numerical simulation of such a sequence using a process based morphodynamic model that solves the coupling between waves depth integrated currents and sediment transport the simulation evolves from an alongshore uniform barred beach storm profile to an almost featureless shore welded terrace summer profile through the highly alongshore variable detached crescentic bar and transverse bar rip system states a global analysis of the full sequence allows determining the varying contributions of the different hydro sedimentary processes sediment transport driven by orbital velocity skewness is critical to the overall onshore sandbar migration while gravitational down slope sediment transport acts as a damping term inhibiting further channel growth enforced by rip flow circulation accurate morphological diffusivity and inclusion of orbital velocity skewness opens new perspectives in terms of morphodynamic modeling of real beaches

regional sea level variability and trends 1960 2007 a comparison of sea level reconstructions and ocean syntheses several existing statistical and dynamical reconstructions of past regional sea level variability and trends are compared with each other and with tide gauges over the 48 yr period 1960 2007 partially pre dating the satellite altimetry era evaluated statistical reconstructions were built from tide gauge data tgr and dynamical reconstructions from ocean data assimilation oda approaches although most of the tgrs yield global mean time series of sea level with trends deviating within only 0 1 mm yr 1 the spatial anomalies of the trends deviate substantially between the reconstructions over the period predating altimetry in contrast tgrs match observed regional trend patterns fairly well during the satellite altimetry era tgrs match tide gauge data better than oda results however they exhibit less variability in the open ocean compared to altimetric data over the pre altimetry period all reconstructed regional sea level trend patterns deviate substantially from each other in terms of detrended correlations in this earlier period the reconstructions match tide gauges and each other much better in the pacific than in the atlantic an ensemble of all tgr and oda estimates provides some improvements in correlations and trends to both tide gauges and altimetry nevertheless a lack of independent open ocean sea surface height data predating altimetry makes impossible the validation of the ensemble for pre altimetry open ocean sea level trends and variability estimating regional sea level changes prior to altimetry therefore remains an unsolved challenge

martian variable features new insight from the mars express orbiter and the mars exploration rover spirit 1 linear low albedo patterns termed dark wind streaks formed on the floor of gusev crater between september 2003 and february 2004 as seen on high resolution stereo camera images taken on board the mars express orbiter pancam images from the mars exploration rover spirit show that the rover crossed a dark streak during its traverse to bonneville crater spirit microscopic imager data reveal that sand grains within the dark wind streak are relatively free of dust whereas grains outside the streak are mantled with dust during the september 2004 solar conjunction spirit remained in one location from sol 240 to sol 260 comparison of images taken before and after the conjunction shows that patches of soil beneath the rover darkened with respect to the adjacent soils suggesting removal of relatively bright material two mi mosaics taken 18 sols apart of the surface within 0 5 m of the nearest dark patch show that some larger 1 2 mm sand grains moved as far as 0 7 mm these observations support the hypothesis that some dark surface patterns result from the removal and or repositioning of fine grained material by winds exposing a relatively lower albedo substrate such as coarse sand grains other variable features on the gusev floor seen from orbit faded between september 2003 and february 2004 and are interpreted to represent settling of dust from the atmosphere consistent with the accumulation of dust observed on spirit the observation of dark streaks fading with time while some dark streaks were newly formed is consistent with local wind gusts or the passage of dust devils that locally sweep dust from the surface or cause a redistribution of fine grains among larger particles

rock coatings and aeolian abrasion on mars application to the pathfinder landing site rock coatings can be used to constrain the rate of abrasion by wind on mars the susceptibility to abrasion for potential rock coatings on mars salt salt cemented coatings rock varnish and amorphous silica were determined experimentally rock coatings generally abrade more easily than the host rock although amorphous silica is an exception if coatings exist on rocks at the mars pathfinder landing site then the rate of abrasion is extremely low consistent with previous studies of erosion rates alternatively rock varnish or silica coatings could protect the rocks from sandblasting although this may require coating formation under current to recent conditions the pathfinder area may lack efficient abrasive particles and the observed duneforms may be composed of sand size dust aggregates rather than crystalline particles

aeolian behavior of dust in a simulated martian environment the behavior of aeolian dust particles 1 2 microns m in diameter was analyzed in a simulated martian environment three main areas have been investigated 1 characterizing spectral and aeolian properties of martian particles and natural wind blown terrestrial dust in order to identify a suitable surrogate martian dust 2 emplacement of the material in the simulated martian environment and 3 experimental procedures and testing in the martian surface wind tunnel marswit the first phase of the study involved choosing a terrestrial dust that closely resembled martian dust data accumulated from various sources suggest that martian dust is derived from the weathering of basaltic parent material nontronite clay being a good candidate composition a commercial clay carbondale red clay crc was determined to be an appropriate surrogate martian dust using a compressed air dust ejection system an air entrained dust cloud was generated which settled to cover the test section in the marswit this method best replicated the natural acrodynamic settling process presumed to exist on mars low pressure experiments were performed in the marswit facility located at the nasa ames research center moffett field california two separate wind tunnel floors were used for these experiments one provided an aerodynamically smooth surface flow and the other was an aerodynamically rough surface flow initial and momentary particle movement was recorded at friction velocities as low as 2 m s however the dust never reached fluid saltation threshold because individual particles less than 10 m typically do not saltate but pass into suspension an estimated dust flux of 3 7 x 10 7 g cm 2 s was determined for a friction velocity of 2 m s this flux could suspend about 10 000 metric tons of dust per second over the surface of mars the dust behavior on the smooth surface was markedly different than on the rough surface under martian conditions application of the processes described above to the martian environment suggests that on rocky surfaces dust suspension would first occur whereas smooth plains would require higher wind speeds for dust entrainment

morphodynamic modelling of rip channel growth the morphological evolution of a rip channel system due to normally incident waves has been investigated using two different approaches the first approach is a state of the art commercial 2dh coastal area model whereas the second approach is a linear stability analysis the results of the two different approaches have been compared and both models predict rip channel spacings and initial growth rates of similar magnitudes

characteristics of the pathfinder apxs sites implications for the composition of martian rocks and soils an integrated study of spectral elemental abundance and image data from the pathfinder alpha proton x ray spectrometer apxs measurement sites has led to a better understanding of the nature of martian surface materials this rigorous analysis provides a new level of detail that forms the basis for the results reported here and that can be used by future scientists trying to understand rocks and soils on mars each apxs site has been precisely located by analyzing stereo imager for mars pathfinder imp and rover camera images to determine the position of the apxs during deployment rover and imp images are used to assess local geology namely the presence of pebbles clods and aeolian features in soils and surface textures and dust concentrations on rocks imp spectra using the latest calibrations for radiance and reflectance are analyzed at a number of different illumination geometries for each apxs site the three dimensional orientation of the sites has been determined and then used to compute the true sunlight incidence and viewing emission angles where suitable photometric coverage is available reflectances are fit to the hapke irvine function this is then used to adjust spectral reflectances to a normalized illumination geometry common to all rocks and soils comparing all the data sets shows a positive correlation among red blue reflectance ratio so3 content and dust abundance on rocks indicating that rocks are coated with varying amounts of sulfur rich red dust other elements such as silicon and iron are poor tracers for dust because their abundances within soil are similar to those within rock rock apxs targets that are bluer poorer in sulfur and have minimal dust coverage face toward the northeast the direction in which winds may be capable of efficiently removing loose material under current martian conditions the imp spectral properties of the soils show poor correlations with apxs elemental abundances on the basis of currently released apxs data bulk soils at the pathfinder landing site are richer in silica and sulfur than the viking landing sites suggesting a different composition the apxs soils with the most apparent pebbles are the poorest in sio2 indicating that either the pebbles are more mafic than the apxs rocks or the pebble free soil component is inherently enriched in sio2 compared to viking soils the mixture of materials at the apxs sites is a reflection of the overall complexity of martian surface materials a detailed understanding of which should drive the choice of future instruments and missions

morphodynamics of nearshore rhythmic sandbars in a mixed energy environment sw france i mapping beach changes using visible satellite imagery abstract this paper presents a new method to analyze the morphology and migration of shallow water sandbanks based on the retrieval of maps from high resolution spot satellite imagery this approach was applied to the study of intertidal ridge and runnel systems and subtidal crescents that border the southwest coast of france maps were obtained from 16 spot images recorded between 1986 and 2000 ridge and runnel shapes with regard to a reference level were delineated using a watercolor reflectance code parameterized and validated with field data crescent plan shapes which appear on the images due to water transparency or breaking induced foam were directly extracted the spatial maps show that in conformity with field surveys the mean alongshore spacing of intertidal systems and crescents range from 370 146 m variability is indicated by standard deviation to 462 188 m and from 579 200 to 818 214 m respectively several couples of images also show that ridge and runnel systems and crescents move in the longshore drift direction southward by about 2 4 3 1 and 1 m day 1 respectively alongshore migration rates of intertidal systems are confirmed by field surveys whilst crescent dynamics cannot be validated because there is no in situ data available to complete these measurements an analysis of the influence of wave climate on both the shape and movements of these rhythmic sedimentary patterns is proposed in a companion paper

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

aeolian saltation on mars at low wind speeds laboratory experiments indicate that the fluid threshold friction speed u tf required to initiate fully developed aeolian saltation is much higher on mars than on earth a discrepancy exists between mars climate models that do not predict winds this strong and observations that sand sized particles are indeed moving this paper describes how wind friction speeds well below u tf but above the impact threshold u ti required to sustain saltation can initiate sustained saltation on mars but at relatively low flux numerical experiments indicate that a sand grain on mars mobilized sporadically between u ti and u tf will develop over fetch lengths longer than generally available within low pressure wind tunnels trajectories capable of splashing grains that propagate saltation and collectively form a cluster of saltating grains that migrate downwind together the passage of a saltation cluster should leave behind a narrow zone of affected surface grains the cumulative effect of many clusters represents a low flux phenomenon that should produce slow changes to aeolian bedforms over periods in which winds remain close to u ti and never or rarely reach u tf field evidence from small impact ripples along rover traverses is consistent with effects of saltation at these low friction speeds without obvious evidence for events u tf the potential utility of this grain mobility process is that it can operate entirely at more common winds well below u tf and so help explain widespread sand movements observed on mars wherever evidence might be mostly absent for u tf being exceeded

the spatial distribution of rocks on mars abstract the spatial distribution of rocks exposed at the surface of mars has been mapped using viking infrared thermal mapper irtm observations overall there are no regions on the surface of mars at a scale of 1 1 in latitude and longitude that are rock free the modal value of rock abundance is 6 areal coverage with no areas having more than 30 35 rock cover the model developed to determine rock abundance relates the thermal emission in each of the four surface sensing irtm bands to temperature contrasts within the field of view non unit thermal emissivity due to absorption bands in the surface materials and the absorption and scattering of the outgoing energy by atmospheric dust and water ice each of these effects produce characteristic spectral and diurnal signatures allowing them to be separated the temperature contrasts provide a means to determine both the abundance of exposed rocks and the fine component thermal intertia rock abundance alone does not produce the observed variation in bulk thermal inertia of the surface low inertia fine bright surfaces have fewer rocks exposed than do high inertia coarse dark surfaces rock abundance does not correlate well with the rms slope nor reflectivity determined from delay doppler radar measurements there is however a possible correlation between rms slope determined from continuous wave radar observations dual polarization radar measurements which provide the best radar measure of small scale roughness indicate that the tharsis volcanic region is very rough whereas thermal measurements indicate very few rocks and a covering of dust together these observations suggest an approximately 1 m thick mantle of fines covering a very rough subsurface this fine material may be an aeolian deposit of dust storm fallout valles marineris together with several major channels and chaotic terrains have high rock abundances as does the acidalia planitia basin this observed distribution may reflect the initial deposition of coarse material associated with channel formation syrtis major is unique in having a high inertia but a very low rock abundance together with low radar roughness these observations are consistent with mantling of rocks by sand the observed distribution of rocks and fines indicates that aeolian processes both erosional and depositional play a dominant role in shaping the present martian surface

formation and erosion of layered materials geologic and dust cycle history of eastern arabia terra mars 1 eastern arabia terra is mantled in a layer of dust a few centimeters to a meter thick yet contains morphologic features that suggest a history of multiple events of deposition and consolidation of fine grained material and a significant amount of erosion early in martian history this region was affected by volcanic and fluvial activity but has since been dominated by aeolian processes five craters in this region contain interior mound material that ranges in height from 1600 to 2100 m above the crater floor the fluted erosional pattern and the thermal inertia are suggestive of a weakly indurated material and the extensive layering implies that these mounds were formed by a repeated process or processes although these materials primarily occur within craters there are materials outside craters that have similar erosional features and fine laminations suggesting a more extensive deposit the most likely process to form this material is the deposition and cementation of air fall dust and is potentially related to martian obliquity changes the significant amount of erosion of the intracrater mounds unit indicates a dramatic change from a depositional environment to an erosional regime over the past 106 108 years currently dust is accumulating in this region in years with planet encircling dust events but global circulation model results indicate that dust devils may be removing slight amounts of dust from arabia terra these observations suggest that the thickness of the dust mantle may not be currently increasing and may instead be in equilibrium

martian and terrestrial dust devils test of a scaling theory using pathfinder data the mars pathfinder meteorological station recorded wind pressure and temperature fluctuations that have been interpreted as dust devils warm core vortices that form at the bottom of connective plumes we apply a scaling theory renno et al 1998 developed to explain terrestrial dust devil observations to test the validity of this interpretation and to provide a simple physical interpretation of the general characteristics of martian dust devils the theory is based on the thermodynamics of heat engines and predicts the central pressure and the wind speed of the connective vortices our findings are as follows for the best documented candidate event sol 25 observed wind and pressure fluctuations are consistent with those predicted by the model and hence strengthen the interpretation of this case as a dust devil a number of other candidates less well documented however also are consistent with passage of a dust devil over or near the lander temperature fluctuations observed on other sols permit dust devils an order of magnitude larger than the ones measured by the meteorology package the strongest dust devils predicted by our theory have a central pressure deficit of about 50 pa and wind speed of about 60 m s 1 the strongest dust devils are capable of lofting dust and hence support the interpretation of selected pathfinder images as showing the passage of dust devils within sight of the lander

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

wind driven particle mobility on mars insights from mars exploration rover observations at el dorado and surroundings at gusev crater 1 the ripple field known as el dorado was a unique stop on spirit s traverse where dust raising active mafic sand ripples and larger inactive coarse grained ripples interact illuminating several long standing issues of martian dust mobility sand mobility and the origin of transverse aeolian ridges strong regional wind events endured by spirit caused perceptible migration of ripple crests in deposits sse of el dorado erasure of tracks in sandy areas and changes to dust mantling the site localized thermal vortices swept across el dorado leaving paths of reduced dust but without perceptibly damaging nearly cohesionless sandy ripple crests from orbit winds responsible for frequently raising clay sized dust into the atmosphere do not seem to significantly affect dunes composed of more easily entrained sand sized particles a long standing paradox this disparity between dust mobilization and sand mobilization on mars is due largely to two factors 1 dust occurs on the surface as fragile low density sand sized aggregates that are easily entrained and disrupted compared with clay sized air fall particles and 2 induration of regolith is pervasive light toned bed forms investigated at gusev are coarse grained ripples an interpretation we propose for many of the smallest linear light toned bed forms of uncertain origin seen in high resolution orbital images across mars on earth wind can organize bimodal or poorly sorted loose sediment into coarse grained ripples coarse grained ripples could be relatively common on mars because development of durable well sorted sediments analogous to terrestrial aeolian quartz sand deposits is restricted by the lack of free quartz and limited hydraulic sediment processing

distribution and evolution of scalloped terrain in the southern hemisphere mars abstract scalloped depressions are a unique martian surface morphology found in the northern and southern hemisphere latitude dependent dust and ice rich surface mantles these features exhibit a distinct asymmetric north south slope profile characterized by steep pole facing scarps flat floors and gentle equator facing slopes we examined high resolution stereo camera hrsc images of the southern hemisphere to determine their longitudinal distribution which revealed that a majority of scalloped terrain is located in the region of the southern wall of the hellas basin and northern malea planum a detailed map of this area was produced where scallops were found to contour the southern wall of the basin and where the ice rich mantle was seen to be thickest scalloped terrain is concentrated along the topographic highs near the amphitrites and peneus paterae and areal extent and depth decreases with increasing depth into the basin we also examined existing hypothesis for the formation and evolution of scalloped depressions using high resolution imaging science experiment hirise images and data from the thermal emission imaging system infrared themis ir and the thermal emission spectrometer tes our approach provides regional context for the development of scalloped terrains within the southern hemisphere and offers detailed evidence of scallop depressions forming around small cracks presumably caused by thermal contraction morphometric measurements show that scalloped depressions can be as much as 40 m deep with typical depths of between 10 and 20 m our observations of scallop formation and development in the southern hemisphere support a solar insolation model proposed by previous researchers e g morgenstern a hauber e reiss d van gasselt s grosse g schirrmeister l 2007 j geophys res 112 citeid e06010 lefort a russell p s thomas n mcewen a s dundas c m kirk r l 2009a j geophys res 114 e04005 lefort a russell p s thomas n 2009b icarus in press observations made using hirise images suggest that scalloped depressions most likely form from small cracks in the mantle which become larger and deeper through sublimation of interstitial ice from within the mantle sublimation is likely enhanced on equator facing slopes because of increased solar insolation which accounts for the asymmetric slope profile and hemispherical orientation and is demonstrated by themis ir images we suggest that sublimation lag deposits can possibly be removed by dust devils or strong slope winds related to the hellas basin offering an explanation as to why scalloped terrain is so abundant only in this area of the southern hemisphere daytime maximum summer temperatures suggest that sublimation in the study area of malea planum is possible under current conditions if the sublimation lag is removed while it cannot be ruled out that scalloped terrain in malea planum is presently evolving we attribute the extensive distribution to geologically recent obliquity excursions when conditions were more conducive to mesoscale modification of the ice rich mantle

spatial grain size sorting in eolian ripples and estimation of wind conditions on planetary surfaces application to meridiani planum mars the landscape seen by the mars exploration rover mer opportunity at meridiani planum is dominated by eolian wind blown ripples with concentrated surface lags of hematitic spherules and fragments these ripples exhibit profound spatial grain size sorting with well sorted coarse grained crests and poorly sorted generally finer grained troughs these ripples were the most common bed form encountered by opportunity in its traverse from eagle crater to endurance crater field measurements from white sands national monument new mexico show that such coarse grained ripples form by the different transport modes of coarse and fine grain fractions on the basis of our field study and simple theoretical and experimental considerations we show how surface deposits of coarse grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces activation of meridiani planum coarse grained ripples requires a wind velocity of 70 m s at a reference elevation of 1 m above the bed from images by the mars orbiter camera moc of reversing dust streaks we estimate that modern surface winds reach a velocity of at least 40 m s and hence may occasionally activate these ripples the presence of hematite at meridiani planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments however the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site

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

saltation threshold on earth mars and venus new formulations valid for wide ranges of particle diameter and density and gas density are presented for prediction of saltation threshold speed for small particles a low air density wind tunnel was used to extend the range of previous investigations and to separate the effects of reynolds number and interparticle forces of cohesion the new formulations are used to predict saltation threshold for atmospheric conditions on the surface of the earth mars and venus

results of the imager for mars pathfinder windsock experiment the imager for mars pathfinder imp windsock experiment measured wind speeds at three heights within 1 2 m of the martian surface during pathfinder landed operations these wind data allowed direct measurement of near surface wind profiles on mars for the first time including determination of aerodynamic roughness length and wind friction speeds winds were light during periods of windsock imaging but data from the strongest breezes indicate aerodynamic roughness length of 3 cm at the landing site with wind friction speeds reaching 1 m s maximum wind friction speeds were about half of the threshold of motion friction speeds predicted for loose fine grained materials on smooth martian terrain and about one third of the threshold of motion friction speeds predicted for the same size particles over terrain with aerodynamic roughness of 3 cm consistent with this and suggesting that low wind speeds prevailed when the windsock array was not imaged and or no particles were available for aeolian transport no wind related changes to the surface during mission operations have been recognized the aerodynamic roughness length reported here implies that proposed deflation of fine particles around the landing site or activation of duneforms seen by imp and sojourner would require wind speeds 28 m s at the pathfinder top windsock height or 31 m s at the equivalent viking wind sensor height of 1 6 m and wind speeds 45 m s above 10 m these wind speeds would cause rock abrasion if a supply of durable particles were available for saltation previous analyses indicate that the pathfinder landing site probably is rockier and rougher than many other plains units on mars so aerodynamic roughness length elsewhere probably is less than the 3 cm value reported for the pathfinder site

morphodynamic instabilities of planar beaches sensitivity to parameter values and process formulations the initial growth of bed perturbations on planar sloping beaches under the forcing of obliquely incident breaking waves is investigated using a state of the art numerical model this allows for a systematic investigation of the sensitivity of the spatial structures of the bed perturbations and their growth and migration rates to different model formulations and parameterizations if the sediment is only transported in the direction of the net current velocity and sediment stirring is taken proportional to the wave height squared growing up current oriented crescentic bars are found with a preferred spacing of 800 m and a down current migration rate of 10 m d 1 varying the angle of wave incidence drag coefficient and bed slope results in qualitatively similar growing bed forms using an engelund and hansen transport formula very oblique down current oriented bars are obtained that grow in time no preferred wavelength however is found using the bailard transport formula results in growing up current oriented bars with a preferred spacing smaller than 300 m for wave angles smaller than 7 when using either the engelund and hansen or bailard sediment transport formulation it is essential to take the transport in the direction of the wave orbital velocity into account in order to have growing bed perturbations

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

investigation of a self organization model for beach cusp formation and development a recent numerical investigation of self organization werner and fink 1993 suggests that the feedback process between currents and sediment response can result in self organized patterns and can be used to predict beach cusp formation and spacing a similar model based on self organization is tested here in order to understand the processes occurring during beach cusp formation and development to evaluate the sensitivity toward the parameters used and to examine how the model might relate to field observations results obtained confirm the validity of the self organization approach and its capacity to predict beach cusp spacing with values in fair agreement with the available field measurements with most of the input parameters primarily affecting the rate of the process rather than the final spacing however changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior cusp spacing tends to change with time and cusp regularity shows large long term variations cusps are found to be accretionary in the swash zone and in agreement with most observations mean flows are horn divergent over developed topography simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed results indicate that preexisting large amplitude cusps are destroyed if their spacing is substantially different from that expected under self organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach these findings support the hypothesis that self organization is a robust mechanism for beach cusp formation

measurements of dielectric loss factors due to a martian dust analog 1 radar signals can penetrate loose sediments such as dust sand and alluvium to reveal buried geologic structures obscured in visible and infrared images in anticipation of data from instruments such as the marsis and sharad radar sounders and potential future imaging sar and rover mounted gpr instruments measurements have been made to characterize electrical loss factors of a martian dust analog and an iron rich soil this paper presents results from dielectric measurements of carbondale red clay crc and the palagonitic mars jsc 1 soil simulant from 0 2 to 1 3 ghz and from transmission measurements of radar penetration of crc up to 12 ghz attenuations predicted from dielectric measurements are compared to values measured in the experiment and are discussed relative to the frequencies at which imaging radars operate over the frequencies considered loss tangent tan decreases with increasing frequency but attenuation increases due to the influence of wavelength attenuation in crc ranges from 8 to 22 db m over p and l bands and jumps to 36 and 67 db m at c and x bands respectively mars jsc 1 has lower attenuation of 5 db m at p band and 12 db m at l band compared to attenuation measurements of sand both crc and mars jsc 1 cause significantly greater attenuation likely due to their compositions containing iron bearing minerals because martian fine sediments also contain ferric mineral components constraining higher losses of analogs for martian dust and fine sediments is important for predicting the performance of radar instruments operating in orbit or on the surface of mars

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

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

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

morphodynamics of nearshore rhythmic sandbars in a mixed energy environment sw france 2 physical forcing analysis abstract the morphology and migration of rhythmic intertidal ridge and runnel systems and subtidal crescentic bars that border the southwest coast of france were characterized using in situ surveys and maps obtained by remote sensing methods the period from 1986 to 2000 was investigated a total of 35 km of coast was mapped this data set shows several specificities the origin of which are examined in the present report using hydrodynamic data a complete analysis of the influence of wave climate on both the shape and the movements of these rhythmic sedimentary patterns was performed in addition swan and morphodyn coupled numerical models were used to provide quantification of both wave breaking and longshore currents for wave parameters that were representative of the mean values and of the energetic conditions this study demonstrated the short time response of intertidal systems to the wave forcing when the offshore significant wave height h s was lower than 2 5 m regular coastal ridge and runnel systems developed in the intertidal zone and migrated in the longshore drift direction at a rate of 1 7 3 1 m day 1 by contrast the ridge and runnel system morphology abruptly changed when the h s exceeded 2 5 m and after the storm the typical ridge and runnel rhythmic topography was recovered within 5 9 days the crescentic bars which had a convex seaward shape were affected by waves with h s values greater than 3 m slightly less for short waves depending on the wave orientation the crescentic bars moved in the longshore drift direction at a rate that reached 1 m day 1 the data suggested a slight negative correlation between the mean alongshore length of the crescentic bar and the mean h s finally it seemed that increasing the wave obliquity with respect to the coast resulted in the flattening of the crescentic bars thus coupling spot and in situ mapping to hydrodynamic records allow the characterization of coastal morphology and dynamics with time and space samplings that are particularly well adapted to the little studied alongshore morphodynamics this approach should improve the difficult parameterization of morphodynamic models in high energy environments

coupled sandbar patterns and obliquely incident waves 1 in double sandbar systems the alongshore variability in the inner bar often resembles that of the outer bar suggesting that the outer bar acts as a morphological template for the inner bar earlier observations have indicated that this resemblance also termed coupling may take several forms here we apply a nonlinear 2dh morphodynamic model with time invariant forcing to show that the angle of wave incidence is crucial for the alongshore variable morphodynamic evolution of the inner bar for a given crescentic outer bar in contrast to previous modeling efforts of double barred systems which mostly used highly idealized boundary conditions we force our model with realistic hydrodynamics and bathymetrical data derived from video observations at the double barred gold coast australia the results show that for small angles of wave incidence 10 over a crescentic outer bar cell circulation patterns govern the flow at the inner bar giving rise to rip channels that incise the inner bar at the locations of the landward perturbations in the outer bar horns on the other hand for obliquely incident waves 10 20 over a crescentic outer bar the circulatory nature of the flow disappears and gives way to a meandering alongshore current the offshore directed sections of this meandering alongshore current erode the inner bar downdrift of the outer bar horns leading to landward perturbations of the inner bar that are coupled to the outer bar horns an observed coupling type that had not been reproduced as yet oblique wave incidence thus proves to be crucial to the development of this type of sandbar coupling as previously hypothesized from sandbar coupling observations at the gold coast additional simulations including tidal water level variations and bar depth variations demonstrate the robustness of our findings

the martian dust storm of sol 1742 after nearly five earth years on mars the mutch memorial station viking lander 1 finally witnessed a local dust storm that eroded trenches conical piles and other disturbed surfaces in the sample field and near the lander the event called the dust storm of sol 1742 occurred late in the third winter of lander observations between sols 1728 and 1757 analyses of tiny new wind tails and movement of materials indicate that the eroding winds were variable but more northeasterly than those that had previously shaped the surface pebbly residues and movement of 4 5 mm clods suggest drag velocities or friction speeds of the winds were about 2 2 4 0 m s wind speeds at the height of the meteorology boom 1 6 m were probably about 40 50 m s much of the observed erosion could have occurred in a few to several tens of seconds but somewhat longer times are suggested by analogy with the erosion of terrestrial soils most of the erosion occurred where preexisting equilibrium conditions of surface configurations and surface material properties had been altered by the lander during landing and during surface sampler activities but thin layers of bright fine grained dust were also removed and redistributed surfaces where preexisting equilibrium conditions were unaltered appeared to be uneroded

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

mapping surface currents and waves with interferometric synthetic aperture radar in coastal waters observations of wave breaking in swell dominant conditions airborne and spaceborne interferometric synthetic aperture radars insars produce surface velocity measurements at very high spatial resolutions over a large area the data allow construction of the velocity strain field for highlighting ocean surface processes such as wave breaking and rip currents also coherence between signals from two interferometric channels is a descriptor of the correlation condition of the surface roughness that scatters back the radar signals and it is an indication of the ocean surface turbulence wave breaking is a major turbulence source causing surface roughness decorrelation thus the coherence parameter serves as an independent means for detecting wave breaking the results of breaking detection using roughness decorrelation and critical local acceleration are comparable in this paper the breaking fraction in swell dominant mixed seas along a cross shore transect is compared with several steepness parameters characterizing different length scales of surface waves the highest correlation coefficient from 0 90 to 0 99 is between the breaking fraction and windsea mean square slope contributed primarily by short waves this result reinforces the previous field observations showing that the length scales of breaking waves are much shorter than the energetic components near the spectral peak although dominant waves and the associated wave group modulation are important in triggering the breaking process the large spatial coverage of airborne or spaceborne operation further offers the opportunity to investigate evolution of the surface wave spectrum in high spatial subkilometer resolution this capability is very useful for monitoring the coastal wave and current environment

bars bumps and holes models for the generation of complex beach topography in shallow water any two waves of the same frequency are shown to produce complex patterns of drift velocity above the sea bed if the longshore components hi h of the wave numbers of the two waves are different these steady flow patterns exhibit a longshore periodicity of wave number hi h irrespective of whether the waves propagate in the same direction say hi he positive or in opposite directions he negative the interaction of two edge wave modes is examined in detail the drift velocities are calculated and a simple sediment transport model is used to predict the beach topography that would be in equilibrium with these flow patterns as expected crescentic sand bars are produced by the special case of standing edge waves hi h intriguingly for all other cases a combination of complex transverse bars plus meandering or straight offshore bars result patterns that are surprisingly reminiscent of many published descriptions of complex rhythmic topography the extension of the model to three or more waves produces topography that appears to be very irregular although the pattern should repeat over sufficiently long distances along the beach even with only three waves these distances may be very large compared to the scale of the bars

un predictability in rip channel systems 1 predicting the development of rip channel systems on beaches is relevant for beach safety nearshore mixing and dispersion and has been puzzling researchers for decades field observations and computer simulations have focused on predicting the spacing and growth time of rip channels as a function of wave characteristics a satisfactory predictor of rip channel spacing and growth time has not yet been proposed here we show that the lack of predictability of rip channels is an inherent property of the system related to the high sensitivity to the bathymetry prior to pattern development sensitivity to the initial cross shore profile appears to be as important as sensitivity to wave height although we might be able to predict rip channel characteristics the information required by such predictors will need to account for the underlying bathymetry implications arising from this study are that other geomorphic patterns could display similar sensitivity

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