flow and sediment transport induced by tide propagation 2 the wavy bottom case 1 a model is formulated to describe the flow field locally induced by the propagation of a tidal wave and to evaluate the sediment transport the three dimensional reynolds averaged momentum equations are considered and coriolis terms are retained since they affect the tide propagation turbulent reynolds stresses are described by means of the boussinesq assumption and the eddy viscosity is assumed to be an assigned function of the distance from the sea bed the effects of wind waves on sediment transport are taken into account and both the bed load and the suspended load are computed a comparison of the obtained results with laboratory data and field measurements shows that the model provides a reliable and accurate description of both the velocity field and sediment transport and does not require high computational costs

landward fining from multiple sources in a sand sheet deposited by the 1929 grand banks tsunami newfoundland abstract a sandy deposit from the 1929 grand banks tsunami in newfoundland contains sediment from two distinct sources one from an inferred gravel shoreline close to the deposit and one from a sandy dune some 200 m seaward of the deposit the deposit ranges from 0 to 15 cm thick and is composed of a bimodal mix of fine and coarse sand we took approximately 100 core samples of this deposit in an attempt to characterize lateral grain size trends within the sand although the coarse fraction does fine with distance inland the fine fraction does not change size over the study area and the aggregate grain size changes in no systematic way we interpret this deposit to represent the mixture of material picked up at the bar with material picked up at the gravel shoreline the bar material does not fine in part because it is already fairly well sorted but also because it is far from its source the shoreline material on the other hand is poorly sorted so that the tsunami took only those grains it was capable of moving and deposited them near their source we estimated the size of the tsunami by determining the flow depth flow velocity combinations required to advect sand from the bar to the back of the deposit and by estimating the shear velocity required for motion of the largest grain we found during our survey this modeling indicates an average flow depth of about 2 5 2 8 m over the area at a flow velocity of 1 9 2 2 m s this estimate compares well with eyewitness accounts of a maximum flow depth of 7 m at the shoreline if our estimate represents an average over the whole study area

the morphodynamics of tidal sand waves a model overview this paper reviews recent theoretical studies of sand waves which are rhythmic large scale bedforms observed in the continental shelf far from the near shore region emphasis is given to the investigations carried out in the framework of the eu research project humor first the results of linear morphodynamic stability analyses are described which allow to understand the initial behavior of the sand waves hence indications on the physical processes controlling the appearance and development of sand waves are obtained along with quantitative predictions of the wavelength of sand waves and of their migration speed then nonlinear models are described which are used to predict the equilibrium profile of sand waves and their interaction with human interventions like sand extraction or the construction of pipelines finally we discuss an analytical model which describes how the sand wave instability behaves when it is triggered locally this leads to the generation growth and expansion of a so called sand wave packet

nearshore tsunami inundation model validation toward sediment transport applications model predictions from a numerical model delft3d based on the nonlinear shallow water equations are compared with analytical results and laboratory observations from seven tsunami like benchmark experiments and with field observations from the 26 december 2004 indian ocean tsunami the model accurately predicts the magnitude and timing of the measured water levels and flow velocities as well as the magnitude of the maximum inundation distance and run up for both breaking and non breaking waves the shock capturing numerical scheme employed describes well the total decrease in wave height due to breaking but does not reproduce the observed shoaling near the break point the maximum water levels observed onshore near kuala meurisi sumatra following the 26 december 2004 tsunami are well predicted given the uncertainty in the model setup the good agreement between the model predictions and the analytical results and observations demonstrates that the numerical solution and wetting and drying methods employed are appropriate for modeling tsunami inundation for breaking and non breaking long waves extension of the model to include sediment transport may be appropriate for long non breaking tsunami waves using available sediment transport formulations the sediment deposit thickness at kuala meurisi is predicted generally within a factor of 2

tsunamigenic incisions produced by the december 2004 earthquake along the coasts of thailand indonesia and sri lanka field observations and satellite images indicate that tsunami waves exhibit specific patterns during flooding and recession forming characteristic incisions in the coastal landscape to study these incisions we analyze high resolution remote sensing images of the coastline of indonesia thailand and sri lanka impacted by the tsunami of december 26th 2004 the analysis sheds light on the different mechanisms by which currents scour incisions during the flooding and receding phases of a tsunami during flooding the high velocity flow indents the levees of existing tidal channels and bays leaving short flood scours the receding water then dissects the coastline with equally spaced return channels widening toward the coast

the loess of north central china geotechnical properties and their relation to slope stability abstract loess is an aeolian deposit consisting of predominantly silt sized quartz particles and containing variable amounts of clay sized minerals loess is generally classified as a water softening material because upon wetting the loess fabric rapidly weakens or collapses the strain hardening of malan loess and the brittle failure of lishi and wucheng loess are in strong contrast to the failure behaviour of these loess deposits in a remoulded state from tests carried out on samples with varying moisture contents using a modified bromhead ring shear apparatus it was found that the effective apparent cohesion gradually increases and the effective internal friction angle decreases with an increase in moisture content when the moisture content reaches a material specific threshold the effective cohesion decreases rapidly and the effective internal friction angle stabilises at a residual value the frequent failure of loess slopes in the western part of the chinese loess plateau is closely related to progressive weathering along zones in these slopes which causes a dramatic decrease in strength from the peak strength condition progressive weathering is common in the loess slopes in the western part of the chinese loess plateau during the process shear strength reduction along potential slip surfaces may be achieved by leaching of readily soluble salts destruction of cementation bonds and redistribution of particles localized collapse of the loess fabric causes internal deformation and consequently peak strength conditions are concentrated on a progressively smaller area of the failure plane therefore the mode of failure of loess slopes is generally determined by brittle failure of the undisturbed and unweathered central parts of the slopes it is important that both the weathered and unweathered strength of the loessmaterials in this area be established in order to analyze the stability of existing loessslopes many of which are steepand lie directly above domestic and industrial urban areas

hydrodynamics and sediment dynamics of north sea sand waves and sand banks and discussion seabed drag coefficients have been measured at a site within the norfolk banks and at a site within the sand wave field in the southern north sea using pressure sensors and moored current meters at the sand banks site a seabed tripod measuring turbulent flows within 1 m of the bed was also used the results are generally in agreement with values used in numerical models at the sand banks site the drag coefficient increases with wind conditions but at the sand waves site there is a reduction during the highest wave conditions attributed to the drag reduction caused by sand resuspension from the bed this result suggests that sediment effects must be considered if wave current interaction is included in numerical models of the region the drag coefficient for reversing tidal flows over the asymmetric sand waves is found to be larger for flow towards the steeper face suggesting a small form drag component studies of sand movement used bedform mapping fluorescent sand tracing and photography of migrating ripples there is evidence for the early stages of formation of a new bank between broken bank and well bank at the sand waves site ripple migration is found to be a useful estimator of bedload transport under conditions of negligible suspension over the sand banks a definite correlation of surface water properties notably the turbidity with the topography of the banks was observed the mechanism for this surface effect is not known but it is consistent with the observation of features seen in satellite visible light images which outline the shapes of the banks

spatially varying environmental properties controlling observed sand wave morphology sand wave morphology and dynamics on continental shelves vary substantially and we hypothesize that these spatial variations depend on local bed properties and hydrodynamic characteristics to date process based modeling studies have not been able to simulate realistic equilibrium sand wave heights and empirical studies are mostly limited to case studies in order to explain the spatial variation in the morphology of equilibrium sand waves on continental shelves with processes and local bed conditions a large scale investigation is required in this paper we use high resolution multibeam echo soundings hydrodynamic models and databases and sedimentary data for the analysis of respectively sand wave shape characteristics and the comparison to hydrodynamic and sedimentary characteristics for the netherlands continental shelf the results are quantified lengths heights and asymmetry of all sand waves in the dutch part of the north sea furthermore we show that the mode of sediment transport bed load or suspended is a dominant factor in explaining sand wavelength height and asymmetry full results of shape characteristics of all sand waves on the netherlands continental shelf together with the tidal velocity water depth surface wave height and median grain size are provided in a repository with this paper http doi org 10 4121 uuid 0d7e016d 2182 46ea bc19 cdfda5c20308 these results are highly valuable for applied offshore engineering projects and to modelers for validating their morphodynamic model results

hydrodynamic implications of textural trends in sand deposits of the 2004 tsunami in sri lanka abstract field observations and sediment samples at a coastal plain setting in southeastern sri lanka were used to document the erosional and depositional impacts of the 2004 indian ocean tsunami and to interpret the hydrodynamic processes that produced an extensive sand sheet deposit tsunami deposit thicknesses ranged from 6 to 22 cm with thickness being controlled partly by antecedent topography the deposit was composed of coarse to medium sand organized into plane parallel laminae and a few laminasets vertical textural trends showed an overall but non systematic upward fining and upward thinning of depositional units with an upward increase in heavy mineral laminations at some locations repeated patterns in the vertical textural trends upward fining upward coarsening uniform were used to subdivide and correlate the deposit into five hydro textural stratigraphic units the depositional units were linked to hydrodynamic processes and upcurrent conditions such as rates of sediment supply and composition of the sediment sources vertical changes in grain size distributions recorded the depositional phases associated with flow acceleration initial unsteady pulsating flow relatively stable and uniform flow flow deceleration slack water and return flow or flow redirection study results suggest that vertical textural trends from multiple cross shore sections can be used to interpret complex tsunami flow histories but at the location examined interpretation of the lateral textural trends did not provide a basis for identifying the correct sediment transport pathways because flow near the landward boundary was multidirectional

revisiting the linear theory of sand dune formation a linear stability theory for dune and antidune formation is presented that implements a rotational two dimensional flow model as in previous linear theories the phase lag between sediment transport and bed elevation remains the main mechanism driving instability however it is shown that this phase lag varies significantly in a neighbourhood of the bed moreover since the layer in which sediments are moving has a finite though small thickness it is assumed that the perturbations of the fluid stress driving bedload transport should be evaluated at the top of the layer itself it is shown that such an apparently minor modification of the classical approach alters remarkably the balance between stabilizing and destabilizing effects that drives the instability process this also allows a clarification of the debated role of bedslope on the formation of dunes and antidunes following the above ideas antidunes are shown to form in the absence of suspended sediment load and without any effect associated with sediment inertia the present analysis ultimately allows a successful unification of the theories of dune and antidune formation

observations and predictions of run up for a significant range of offshore wave conditions and foreshore slopes run up observations are compared to semiempirical formulations and predictions of an existing numerical model based on the depth averaged one dimensional nonlinear shallow water equations with bore like breaking wave dissipation and quadratic bottom friction the numerical model is initialized with time series of sea surface elevation and cross shore velocity observed in 80 cm mean water depth approximately 50 m offshore of the mean shoreline on a gently sloping beach and in 175 cm water depth 100 m offshore of the shoreline on a steep concave beach run up was measured with a stack of resistance wires at elevations 5 10 15 20 and 25 cm above and parallel to the beach face at sea swell frequencies nominally 0 05 f 0 18 hz run up energy is limited by surf zone dissipation of shoreward propagating waves so that increasing the offshore wave height above a threshold value does not substantially increase the predicted or observed sea swell run up excursions e g run up is saturated existing semiempirical saturation formulations are most consistent with the observations and numerical model predictions of run up excursions nearest the bed in contrast at infragravity frequencies 0 004 f 0 05 hz where surf zone dissipation is relatively weak and reflection from the beach face is strong e g saturation formulas are not applicable the run up excursions increase approximately linearly with increasing offshore wave height the numerical model also accurately predicts that the tongue like shape of the run up results in sensitivity of run up measurements to wire elevation for instance run up excursions and mean vertical superelevation above the offshore still water level increase with decreasing wire elevation and continuous thinning of the run up tongue during the wave uprush can result in large phase differences between run up excursions measured at different wire elevations numerical model simulations suggest that run up measured more than a few centimeters above the bed cannot be used to infer even the sign of the fluid velocities in the run up tongue

hydrodynamics and sediment transport in the swash zone a review and perspectives the dominant hydrodynamic forcing and resulting sediment transport mechanisms in the swash zone are reviewed combined with a discussion of future measurement and modelling requirements the importance of swash zone processes in the overall behaviour of the nearshore littoral zone are identified with the paper subsequently focusing on aspects that directly relate to sediment transport on beaches the review considers analytical and numerical modelling work and both field and laboratory data the hydrodynamics of the swash zone are first discussed with reference to the surf zone boundary swash characteristics forcing mechanisms and shoreline oscillations subsequent sections consider more micro scale processes such as the internal flow kinematics turbulence the bed boundary layer and infiltration exfiltration through permeable beds the second part of the paper outlines the fundamental mechanics of sediment transport in the swash zone followed by a more detailed discussion of sediment transport modes induced by the dominant hydrodynamic conditions modelling concepts are reviewed and the strengths and weaknesses of different approaches identified a final summary and future modelling perspectives conclude the paper

the role of suspended load transport in the occurrence of tidal sand waves tidal sand waves are dynamic bed patterns which are formed by the complex interaction between hydrodynamics sediment transport and geomorphology field data from the southern north sea reveal that sand waves are absent where suspended load transport is the dominant transport mode in order to understand the mechanisms responsible for the absence of sand waves we study the influence of suspended load transport on the formation of tidal sand waves with a numerical process based geomorphological model delft3d model simulations are presented in which the vertical eddy viscosity and sediment diffusivity are both spatially and temporally variable k e turbulence model first it is shown that the preferred wavelength of sand waves for a relatively large grain size increases by the inclusion of suspended sediment while for a relatively small grain size the flat bed is stable and no sand waves evolve second it is shown that suspended load transport causes the suppression of long sand waves resulting in a finite range of wavelengths that experience growth finally by varying flow velocity amplitude and grain size critical conditions for sand wave formation are found i e conditions for which sand waves are marginally generated

tidal sand wave formation influence of graded suspended sediment transport 1 the influence of graded suspended sediment transport on the formation of tidal sand waves is investigated hereto in the framework of a linear stability analysis we examine the time development of small amplitude perturbations superimposed to a flat heterogeneous seabed in a shallow tidal sea dominated by one tidal constituent the heterogeneous bottom material is modeled by a bimodal sediment mixture and the transport of sediment close to the bottom as well as in suspension is considered the active layer concept is employed and hiding exposure effects are taken into account the mechanisms giving rise to the sediment sorting process along the bed forms are found to be unaffected by the fractional transport of sediment in suspension on the other hand the formation of sand waves is quantitatively substantially affected by the graded suspended sediment transport and the wavelength of the most amplified mode is found to depend strongly on the characteristics of the sediment mixture in particular the graded suspended sediment transport largely affects the contribution to the amplification rate related to the sorting process this leads to the growth of bed forms characterized by a smaller wavelength compared with that predicted for a uniform sediment when the mean grain size of the sediment mixture is relatively coarse on the other hand longer bed forms appear when the sediment mixture is characterized by a relatively fine mean grain size moreover a destabilizing effect is identified in the former case while the flat bed configuration is stabilized in the latter case a comparison of the theoretical predictions with field observations seems to support the model even though it cannot be considered conclusive

modeling sand wave characteristics on the belgian continental shelf and in the calais dover strait 1 the capability of the model of besio et al 2006 to predict the main geometrical characteristics crest orientation wavelength of tidal sand waves is tested by comparing the theoretical predictions with field data in particular the field observations carried out by mouchet 1990 and van lancker et al 2005 along the continental shelf of belgium are used additional comparisons are carried out against the field measurements described by le bot 2001 and le bot and trenteseaux 2004 which were carried out in an adjacent region attention is focused on the prediction of the wavelength of the bottom forms indeed the capability of a linear stability analysis to predict the occurrence of sand waves has been already tested by hulscher and van den brink 2001 and more recently by van der veen et al 2006 the obtained results show that the theoretical predictions fairly agree with field observations even though some of the comparisons suggest that the accuracy of the predictions depends on the accurate evaluation of the local current and sediment characteristics

large scale slope failure involving triassic and middle miocene salt and shale in the gulf of cadiz atlantic iberian margin sheets of salt and ductile shale advancing beyond the thrust front of the gibraltar arc iberian moroccan atlantic continen tal margin triggered downslope movements of huge allochth onous masses these allochthons represent the cadiz nappe which detached from the gibraltar arc along low angle normal faults and migrated downslope from the iberian and moroccan continental margins towards the atlantic ocean extensional tectonics initiated upslope salt withdrawal and downslope diapirism during large scale westward mass wasting from the shelf and upper slope low angle salt and shale detachments bound by lateral ramps link extensional structures in the shelf to folding thrusting and sheets of salt and shale in the gulf of cadiz from backstripping analyses carried out on the depo centres of the growth fault related basins on the shelf we infer two episodes of rapid subsidence related to extensional collapses these were from late tortonian to late messinian 200 400 m myr 1 and from early pliocene to late pliocene 100 150 m myr 1 the extensional events that induced salt movements also affected basement deformation and were probably associated with the westward advance of frontal thrusts of the gibraltar arc as a result of the convergence between africa and eurasia the complexities of salt and or shale tectonics in the gulf of cadiz result from a combination of the deformations seen at convergent and passive continental margins

tidal induced large scale regular bed form patterns in a three dimensional shallow water model the three dimensional model presented in this paper is used to study how tidal currents form wave like bottom patterns inclusion of vertical flow structure turns out to be necessary to describe the formation or absence of all known large scale regular bottom features the tide and topography are treated as a coupled system and the potential free instabilities in this system are investigated a linear stability analysis is used to study the dynamics of bed forms this yields information on the role of physical mechanisms such information cannot be obtained by direct numerical nonlinear simulation the basic state represents a horizontal elliptical flow veering in vertical direction over a flat bottom it is shown that in such a system modes can grow which have characteristics of both tidal sand banks and sand waves the model predictions are compared with observations and show good agreement

the influence of storms on sand wave evolution a nonlinear idealized modeling approach we present a new 2dv nonlinear process based morphodynamic model to investigate the effects of storms specifically wind driven flow and wind waves on finite amplitude tidal sand wave evolution simulations are performed on periodic domains of two lengths i on a 350 m domain comparable to the wavelength of observed sand waves we study the evolution toward equilibrium shapes and ii on a 4 km domain we study the evolution from a randomly perturbed seabed our model results demonstrate that both wind driven flow and wind waves reduce sand wave height and tend to increase wavelength wind driven flow breaks the tidal symmetry resulting in horizontal sand wave asymmetry and migration waves alone do not induce migration but can enhance migration induced by for example tidal asymmetry and wind driven flow on the 350 m domain we further find that migration rates decrease with increasing sand wave height however in an irregular sand wave field large sand waves tend to overtake the smaller ones suggesting a complicated interaction among neighboring bed forms the above results concern steady state storm conditions however since storms occur on an intermittent basis we also simulated a synthetic storm climate consisting of alternating short periods of storm conditions and long periods of fair weather conditions simulations reveal a dynamic equilibrium with sand wave heights significantly below those obtained for tide only conditions also for relatively short storm duration our work identifies mechanisms that explain why sand wave heights are generally overpredicted by numerical models that do not include storm processes

suspended sediment transport mechanisms in high energy swash abstract measurements of suspended sediment concentration at three heights above the bed and cross shore velocity at a single height were obtained using miniature optical backscatter sensors and a miniature electromagnetic current meter from the swash zone of a high energy macrotidal dissipative beach time series from relatively low h s 0 8 m and high h s 2 2 m energy conditions were chosen for analysis possible onshore and offshore sediment transport mechanisms are identified as a sudden offshore to onshore velocity transition and turbulence in the swash front leading to subsequent onshore advection by the uprush and b low frequency high velocity backwashes exceeding a certain threshold for sediment suspension these mechanisms are related to the velocity field through skewness and asymmetry an increase in negative infragravity skewness in high energy conditions suggests a potential shift towards offshore net transport time averaged vertical profiles of cross shore suspended sediment flux were calculated from the velocity and concentration time series these show a tendency towards offshore transport in high energy conditions particularly at low levels in the water column the actual sediment fluxes are believed to be more offshore than those calculated one reason for this discrepancy is the inability of the instruments to detect swash activity at very low levels in the water column

horizontal and vertical sediment sorting in tidal sand waves modeling the finite amplitude stage the bed of coastal seas displays a large number of rhythmic bed features of which sand waves are relevant to study from an engineering perspective sediments tend to be well sorted over these bed forms which is so far poorly understood in terms of modeling of finite amplitude sand waves using delft3d we employ bed stratigraphy and consider four different grain size classes which are normally distributed on the phi scale the standard deviation sortedness is then varied whereas the geometric mean grain size is kept constant the results show that typically the crests of sand waves are coarser than the troughs residual flow causes net sedimentation on the lee side of the crest and consequently the general sorting pattern is distorted since larger grains experience a larger settling velocity they are deposited on the upper lee slope whereas the smaller grains are found on the lower lee slope due to sand wave migration also the internal structure of the sand wave is revealed which follows the same sedimentation pattern as the lee slope surface these results qualitatively agree with sorting patterns observed offshore the sorting processes lead to longer wavelengths and lower wave heights as a function of standard deviation this relates to the dampening effect of suspended sediment transport for fine grains finally it appears that the modeled wave heights fall in the same range as observations in the north sea these results are valuable for e g predicting the morphological response after engineering activities and determining suitable aggregates for sand extraction

run up of tsunamis and long waves in terms of surf similarity abstract in this paper we review and re examine the classical analytical solutions for run up of periodic long waves on an infinitely long slope as well as on a finite slope attached to a flat bottom both cases provide simple expressions for the maximum run up and the associated flow velocity in terms of the surf similarity parameter and the amplitude to depth ratio determined at some offshore location we use the analytical expressions to analyze the impact of tsunamis on beaches and relate the discussion to the recent indian ocean tsunami from december 26 2004 an important conclusion is that extreme run up combined with extreme flow velocities occurs for surf similarity parameters of the order 3 6 and for typical tsunami wave periods this requires relatively mild beach slopes next we compare the theoretical solutions to measured run up of breaking and non breaking irregular waves on steep impermeable slopes for the non breaking waves the theoretical curves turn out to be superior to state of the art empirical estimates finally we compare the theoretical solutions with numerical results obtained with a high order boussinesq type method and generally obtain an excellent agreement

2004 indian ocean tsunami inflow and outflow at phuket thailand at bangtao beach phuket the 2004 indian ocean tsunami produced a repeated sequence in which rapid inflow of turbulent water was followed by ponding and then by gradual outflow photographs and eyewitness accounts show an initial withdrawal followed by series of inflows the tsunami left behind a sand sheet as much as 25 cm thick that contains parallel inclined landward and seaward lamina in addition to the normal grading commonly reported from tsunami deposits the sheet contains evidence for two times of vigorous inflow each of these is marked by mud rip ups medium to coarse sand that grades upward to fine landward inclined laminae and a sharp basal contact the top of the sand sheet when observed in the first days after the tsunami abounded in current dune and ripple bedforms of mostly landward orientation the tsunami s first positive wave left noonshore sedimentary record in this pitting area the second wave deposited sand that is much less extensive and slightly finer than that of the third wave the deposit of both these waves contains multiple fining upward sequences possibly due to multiple surges in one wave train the depth averaged flow velocity estimated from thickness and grain size are in the range 7 21 m s whereas a near bottom threshold velocity calculated from bedforms reveals the order of magnitude difference from 1 74 to 1 03 m s 2007 elsevier b v all rights reserved

stability of loess abstract lutenegger a j and hallberg g r 1988 stability of loess eng geol 25 247 261 the natural stability of loess soils can be related to fundamental geotechnical properties such as atterberg limits water content and void ratio field observations of unstable conditions in loess deposits in the upper midwest u s a show relationships between instability and the in situ moisture content and the liquidity index of the loess unstable loess can attain natural moisture contents equal to or greater than its liquid limit implications of these observations for applied engineering works are described

3d seismic analysis of circular evaporite dissolution structures eastern mediterranean buried circular collapse structures above a tabular evaporitic body are recorded by recently acquired 3d seismic data on the levant basin and continental margin offshore israel eastern mediterranean the structures formed during the pliocene as buried messinian late miocene evaporites underwent extensive dissolution in a submarine deep water setting three dimensional seismic analysis is used to describe the detailed morphology of the structures and the associated overburden allowing the reconstruction of their origin and development it is proposed that evaporite dissolution led to the collapse of the weakly lithified overburden and this deformed with a series of concentric extensional faults from the structural analysis of the overburden the estimated maximum duration of the dissolution event is 0 75 1 ma the mechanism proposed for the creation of the circular collapse structures is subjacent dissolution of the more soluble evaporites in the messinian evaporites as a result of focused vertical fluid flow at the base of the evaporitic series rapid release of overpressured fluids as for example during an earthquake is thought to have initiated the focused fluid flow which impinged on the evaporitic seal to the point where dissolution occurred creating the localized circular collapse structures in the overburden

modelling the effect of suspended load transport and tidal asymmetry on the equilibrium tidal sand wave height abstract tidal sand waves are rhythmic bed forms found in shallow sandy coastal seas reaching heights up to ten meters and migration rates of several meters per year because of their dynamic behaviour unravelling the physical processes behind the growth of these bed forms is of particular interest to science and offshore industries various modelling efforts have given a good description of the initial stages of sand wave formation by adopting a linear stability analysis on the coupled system of water movement and the sandy seabed however the physical processes causing sand waves to grow towards equilibrium are far from understood we adopt a numerical shallow water model delft3d to study the growth of sand waves towards a stable equilibrium it is shown that both suspended load transport and tidal asymmetry reduce the equilibrium sand wave height a residual current results in asymmetrical bed forms that migrate in the direction of the residual current the combination of suspended load transport and tidal asymmetry results in predicted equilibrium wave heights comparable to wave heights found in the field

traces of tsunami preserved in inter tidal lacustrine and marsh deposits some examples from northeast japan the japan sea earthquake of may 26 1983 triggered a large scale tsunami that surged over the coast of ne japan and caused considerable damage the tsunami resulted in peculiar sedimentary processes on the coast depending on the hydraulic scale of the tsunami three kinds of sedimentary processes were recognized each of which left unique deposits such deposits are expected to remain in sedimentary sequences preserving remarkable sedimentary facies borehole samples drilled at different localities in coastal regions have been examined to detect traces of ancient tsunamis sedimentological and geochemical studies of the samples reveal that layered deposits similar to those developed by the tsunami of the japan sea earthquake are common in the drilled cores estimated ages of these deposits are exactly equivalent to the dates of ancient tsunamis documented in historical records these results show that the invasion of tsunamis is recorded in coastal depositional sequences from analyses of long borehol

stratification effects by cohesive and noncohesive sediment this study focuses on stratification effects induced by the interaction between suspensions of fine grained cohesive and noncohesive sediment and the turbulent flow in estuarine and coastal environments a concise literature review reveals that this interaction may cause an appreciable modification of the vertical profiles of velocity vertical eddy viscosity diffusivity and reynolds stresses this interaction is further studied with a one dimensional vertical 1dv numerical model which includes the standard k turbulence model with buoyancy destruction terms application of this model to the experimental results by coleman 1981 shows that the measured changes in the velocity profile can be explained entirely by sediment induced buoyancy effects it is argued that capacity for cohesive sediment is cc e a fluid mud layer is formed upon the deposition of the cohesive sediment flocs contrary to the case with noncohesive sediment where the depositing grains immediately form a rigid bed thus a two layer fluid system develops causing significant damping of the vertical mixing processes decreasing the carrying capacity further this positive feed back results in a catastrophic collapse of the turbulence field and the concentration profile a scaling law for this saturation behavior is derived from classical stratified flow theory this law is sustained with a series of numerical experiments with the 1dv model it also predicts the suspended sediment concentrations observed in the yellow river to the right order of magnitude it is concluded that sediment induced buoyancy yields appreciable stratification effects at already moderate suspended sediment concentrations in the case of cohesive sediment these stratification effects can result in a catastrophic collapse of the turbulent flow field and the vertical profile of suspended sediment concentration

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

coastal sedimentation associated with the december 26 2004 tsunami in lhok nga west banda aceh sumatra indonesia abstract with wave heights of 30 m and runups reaching 50 m a s l and 6 km inland the december 26 2004 tsunami in sumatra was one of the largest tsunamis in recorded human history in this paper we present a description of the event in the lhok nga bay west coast of banda aceh and an interpretation of the tsunami sand deposits mostly based upon grain size analysis the 3 5 km long transect of lampuuk displays two landward fining thinning and sorting sequences the thickest cross sections of the first sequence of deposits 0 1 5 km inland suggest a deposition by three consecutive runups inflows and a final backwash return flow or outflow from 1 5 to 3 5 km inland the record is dominated by the second and highest wave 15 30 m normally graded couplets or triplets of layers were used to identify the runup of each wave the topmost layers interpreted as the backwash deposition describe a seaward sequence of increasing mean grain size and decreasing degree of sorting the local effects of the topography could be identified thickest deposits in the topographic lows 50 80 cm great spatial variations in thickness and upper laminated texture when the sedimentation was limited by steep slopes landward coarsening and very poor sorting at the wave breaking point bimodal grain size distributions reflecting different sources of sediments finally by coupling the longitudinal and vertical trends of the 26 cross sections we propose a model of sediment transport and deposition in a large tsunami wave

a field study of turbulence and sediment dynamics over subaqueous dunes with flow separation this study examines flow turbulence and sand suspension over large dunes in canoe pass a distributary channel of the fraser river delta canada dune morphology is characterized by a symmetrical shape and steep leeside slopes over 30 velocity was measured with an electromagnetic current meter and suspended sand concentration with four optical backscatter obs probes the general patterns of time averaged velocity and sand suspension are consistent with previous studies including an increase in mean velocity and decrease in turbulence intensity and sand concentration with height above the bed reversed flow with high turbulence intensity and high sand concentrations in the leeside flow separation zone and an increase in near bed velocity and sand concentration along the stoss side of the dune frequency spectra of near bed velocity and obs records from leeside separation zones are composed of two distinct frequencies providing field confirmation of scale relations based on flume experiments the low frequency spectral signal probably results from wake flapping and the high frequency signal from vortex shedding the wake flapping frequency predominates outside the separation zone and is linked to turbulent structures that suspend sand predictions from a depth scale strouhal law show good agreement with measured wake flapping frequencies cross correlations of obs records reveal that turbulent sand suspension structures advect downstream at 23 25 above the horizontal these advection angles are similar to coherent flow structures measured in flumes and to sand suspension structures visualized over large dunes in the field

implications of passive salt diapir kinematics for reservoir segmentation by radial and concentric faults abstract reservoirs associated with salt diapirs that are circular or elliptical in plan view can be segmented by radial faults concentric faults both or neither explanations for this variation are offered based on integration of fault kinematics published studies and seismic examples strata around diapirs can be divided into three structural domains with characteristic fault styles 1 roof zone between the present day depositional surface and crest of a buried inactive diapir this zone is characterised by doming radial faults and occasionally conic graben doming is usually caused by differential compaction and increases with depth 2 a steeply dipping sheath of strata surrounds the diapir out to a distance of less than one diapir radius recording near surface diapir growth in poorly lithified sediments these contain early gravity slumps and late concentric extensional faults regional layer bound polygonal fault systems form radial faults within approximately one diapir radius the only significant radial faulting mechanism below the roof zone 3 diapir root zone of strata whose structural architecture records the diapirs initiating mechanism typically rim synclines or graben that detach on the salt source layer concentric faults at radial distances of a kilometre or more have been found only in root zone rim synclines and sheared sheath strata these models and observations can assist seismic interpretation and structural model building in areas of poor geophysical imaging for instance below overhanging salt

2004 indian ocean tsunami flow velocity measurements from survivor videos 1 the tsunami of 26th december 2004 severely affected banda aceh along the north tip of sumatra indonesia at a distance of 250 km from the epicenter of the magnitude 9 0 earthquake this tsunami flow velocity analysis focused on two survivor videos recorded within banda aceh more than 3 km from the open ocean the exact locations of the tsunami eyewitness video recordings were revisited by the survey team between february 22 and 25 2005 to record camera calibration ground control points the motion of the camera during the recordings was determined the individual video images were rectified with a direct linear transformation dlt assuming a planar water surface at the level finally a cross correlation based particle image velocimetry piv analysis was applied to the rectified video images to determine instantaneous tsunami flow velocity fields the measured tsunami flow velocities were within the range of 2 to 5 m s

settling velocity of sediments at high concentrations new data on the settling velocity of artificial sediments and natural sands at high concentrations are presented the data are compared with a widely used semiempirical richardson and zaki equation trans inst chem eng 32 1954 35 which gives an accurate measure of the reduction in velocity as a function of concentration and an experimentally determined empirical power n here a simple method of determining n is presented using standard equations for the clear water settling velocity and the seepage flow within fixed sediment beds the resulting values for n are compared against values derived from new and existing laboratory data for beach and filter sands for sands the appropriate values of n are found to differ significantly from those suggested by richardson and zaki for spheres and are typically larger corresponding to a greater reduction in settling velocity at high concentrations for fine and medium sands at concentrations of order 0 4 the hindered settling velocity reduces to about 70 of that expected using values of n derived for spheres at concentrations of order 0 15 the hindered settling velocity reduces to less than half of the settling velocity in clear water these reduced settling velocities have important implications for sediment transport modelling close to and within sheet flow layers and in the swash zone

power law scaling and probabilistic forecasting of tsunami runup heights a power law scaling relationship describes tsunami runup heights at ten locations in japan knowledge of the scaling law for tsunamis can be the basis for probabilistic forecasting of the size and number of future events and for estimating probabilities of extremely large events using tsunami runup data archived by the u s national geophysical data center we study ten locations where the tsunami record spans at least one order of magnitude in runup height and the temporal record extends back several decades a power law or upper truncated power law describes the cumulative frequency size distribution of tsunami runup heights at all ten locations where the record is sufficient to examine shorter time intervals within the record the scaling relationship for the shorter time intervals is consistent with the scaling relationship for the entire record the scaling relationship is used to determine recurrence intervals for tsunami runup heights at each location in addition to the tsunami record used to determine the scaling relationship at some of the locations a record of large events 5 m extends back several centuries we find that the recurrence intervals of these large events are consistent with the frequency predicted from the more recent record for tsunami prone locations where a scaling relationship is determined the predicted recurrence intervals may be useful for planning by coastal engineers and emergency management agencies

laboratory experiments of tsunami runup on a circular island laboratory experiments of a 7 2 m diameter conical island were conducted to study three dimensional tsunami runup the 62 5 cm tall island had 1 on 4 side slopes and was positioned in the center of a 30 m wide by 25 m long flat bottom basin solitary waves with height to depth ratios ranging from 0 05 to 0 20 and source lengths ranging from 0 30 to 7 14 island diameters were tested in water depths of 32 and 42 cm twenty seven capacitance wave gages were used to measure surface wave elevations at incident and four radial transects on the island slope maximum vertical runup measurements were made at 20 locations around the perimeter of the island using rod and transit a new runup gage was located on the back or lee side of the island to record runup time series

on the formation of sand waves and sand banks a fully three dimensional model is proposed for the generation of tidal sand waves and sand banks from small bottom perturbations of a flat seabed subject to tidal currents the model predicts the conditions leading to the appearance of both tidal sand waves and sand banks and determines their main geometrical characteristics a finite wavelength of both sand waves and sand banks is found around the critical conditions thus opening the possibility of performing a weakly nonlinear stability analysis able to predict the equilibrium amplitude of the bottom forms as shown by previous works on the subject the sand wave crests turn out to be orthogonal to the direction of the main tidal current the present results also show that in the northern hemisphere sand bank crests are clockwise or counter clockwise rotated with respect to the main tidal current depending on the counter clockwise or clockwise rotation of the velocity vector induced by the tide only for unidirectional or quasi unidirectional tidal currents are sand banks always counter clockwise rotated the predictions of the model are supported by comparisons with field data finally the mechanisms leading to the appearance of sand waves and sand banks are discussed in the light of the model findings in particular it is shown that the growth of sand banks is not only induced by the depth averaged residual circulation which is present around the bedforms and is parallel to the crests of the bottom forms a steady drift of the sediment from the troughs towards the crests is also driven by the steady velocity component orthogonal to the crests which is present close to the bottom and can be quantified only by a three dimensional model while the former mechanism appears to trigger the formation of counter clockwise sand banks only the latter mechanism can give rise to both counter clockwise and clockwise rotated sand banks

process based modeling of tsunami inundation and sediment transport 1 the infrequent and unpredictable nature of tsunamis precludes the use of field experiments to measure the hydrodynamic and sediment transport processes that occur instead these processes are often approximated from laboratory numerical and theoretical studies or inferred from observations of the resultant sediment deposits here delft3d a three dimensional numerical model is used to simulate the inundation and sediment transport of a tsunami similar in magnitude to the 26 december 2004 indian ocean tsunami over one measured and three idealized morphologies the model is first shown to match well the observations taken at kuala meurisi sumatra and then used to examine in detail the processes that occur during the tsunami the model predicts that at a given cross shore location the onshore flow accelerates rapidly to a maximum as the wavefront passes and then gradually decelerates before reversing direction and flowing offshore the onshore flow does not tend to zero everywhere at maximum inundation but instead flow reversal occurs near the shoreline even as the wavefront continues to inundate landward while some sediment is eroded by the passing wavefront the suspension of sandy sediment is dominated by the long duration high velocity backwash that occurs along the beach face and offshore of the shoreline some of the sediment suspended during backwash is advected shoreward by the subsequent wave creating large spatial gradients in the suspended sediment concentrations which may not be in equilibrium with the local hydrodynamics the inundation and transport of sediment during a tsunami can be affected by complexities in the morphological profile and interactions between multiple waves and many of the hydrodynamic and sediment transport processes predicted here are similar to analogous processes previously observed in the swash zone

unified view of sediment transport by currents and waves i initiation of motion bed roughness and bed load transport attention is given to the properties of sediment beds over the full range of conditions silts to gravel in particular the effect of fine silt on the bed composition and on initiation of motion critical conditions is discussed high quality bed load transport data sets are identified and analyzed showing that the bed load transport in the sand range is related to velocity to power 2 5 the bed load transport is not much affected by particle size the prediction of bed roughness is addressed and the prediction of bed load transport in steady river flow is extended to coastal flow applying an intrawave approach simplified bed load transport formulas are presented which can be used to obtain a quick estimate of bed load transport in river and coastal flows it is shown that the sediment transport of fine silts to coarse sand can be described in a unified model framework using fairly simple expressions the proposed model is fully predictive in the sense that only the basic hydrodynamic parameters dep

flow velocity and sediment transport in the swash zone of a steep beach detailed measurements of flow velocity and total sediment load were obtained in the swash zone on a steep beach swash motion was measured using ducted impeller flow meters and capacitance water level probes during wave uprush the onshore flow increased almost instantaneously from zero to its maximum velocity after the arrival of the leading edge of the swash lens and subsequently decreased gradually to zero for the remainder of the uprush during backwash the offshore flow increased steadily from zero to its maximum towards the end of the backwash and dropped rapidly to zero as the beach fell dry the duration of backwash was typically longer than that of uprush and maximum water depth on the beach was attained just prior to the end of the uprush the total sediment load was measured for 35 individual wave uprush events using a sediment trap the amount of sediment transported by a single uprush was typically two to three orders of magnitude greater than the net transport per swash cycle difference between uprush and backwash inferred from surveys of beach profile change the measured immersed weight total load transport rate displayed a strong relationship with the time averaged velocity cubed which is consistent with equations for both bedload transport and total load transport under sheet flow conditions the bagnold 1963 1966 bedload transport model was tested against our field data and yielded ib ku3tu tan tan where ib is the immersed weight of bedload transported during the entire uprush kg m 1 k is a coefficient kg m 4 s2 u is the time averaged flow velocity for the uprush m s 1 tu is the uprush duration s is the friction angle of the sediment and is the beach slope the empirically determined value for the coefficient k was 1 37 0 17

unified view of sediment transport by currents and waves iv application of morphodynamic model the tr2004 transport formulations for clay silt and sand as proposed in parts 1 and 2 have been implemented in morphodynamic models to predict bed level changes these models have been verified using various laboratory and field data cases concerning coastal flow in offshore and near shore zones furthermore the model has been applied to two complicated sediment environments concerning the flow around a spurdike in a river and the tidal flow of cohesive sediments in the yangtze estuary in china overall it is concluded that the morphodynamic models using default settings performs reasonably well the applied scaling factors of the sediment transport model are in the generally accepted range of 0 5 2

development of subaqueous barchanoid shaped dunes due to lateral grain size variability in a tidal inlet channel of the danish wadden sea 1 high resolution bathymetry at centimeter scale accuracy acquired with a multibeam echo sounder system revealed the existence of barchanoid shaped large i e length 10 100 m to very large i e length 100 m dunes in the gradyb tidal inlet channel in the danish wadden sea the development of these dunes is due to an increase in dune celerity from 12 m yr in the center of the channel to around 30 m yr at the sides this increase in dune celerity can be explained by the fact that dune heights decrease from 3 1 m in the center of the channel to 1 4 m at the sides as a smaller sediment volume has to be moved per unit time for equal dune celerity water depth is uniform across the channel likewise high resolution acoustic doppler current profiler measurements across the channel showed a uniform distribution of both ebb and flood flow thus no correlation between dune dimensions and water depth or flow velocity was established instead high accuracy bed sampling along the crests of the dunes showed a decrease in mean grain size from 0 63 mm in the center to 0 36 mm at the sides of the channel the decrease in dune height is ascribed to this decrease in grain size given that flow depth and flow velocity are uniform across the channel the lateral decrease in grain size is suggested to result from sorting effects by secondary currents directed from the center toward the sides of the channel in the trough lee side region of the barchanoid shaped dunes

development of subaqueous barchanoid shaped dunes due to lateral grain size variability in a tidal inlet channel of the danish wadden sea development of barchanoid shaped dunes

reconstruction of tsunami inland propagation on december 26 2004 in banda aceh indonesia through field investigations this paper presents the results from an extensive field data collection effort following the december 26 2004 earthquake and tsunami in banda aceh sumatra the data were collected under the auspices of tsunarisque a joint french indonesian program dedicated to tsunami research and hazard mitigation which has been active since before the 2004 event in total data from three months of field investigations are presented which detail important aspects of the tsunami inundation dynamics in banda aceh these include measurements of runup tsunami wave heights flow depths flow directions event chronology and building damage patterns the result is a series of detailed inundation maps of the northern and western coasts of sumatra including banda aceh and lhok nga

evolution of the nw margin of the north permian basin uk north sea mapping of depositional zones within the zechstein supergroup using semi regional 2d and 3dseismic data constrains the geological evolution of the northwestern margin of the north permian basinin uk offshore quadrants 14 15 19 20 and 21 four depositional zones are identified on the basis of mappable seismic facies which are assumed to vary with halite content well control characterizes the most reflective seismic facies as an interbedded carbonate and anhydrite shelf succession which grades laterally through clastic rich sequences to the least reflective a thick halite basinal equivalent the trendsof the depositional zones illustrate the geometry of the north permian basin and may have been locally controlled by basement lineaments within the north permian basin a n s trending axis devil s hole high characterized by thick reflective facies separates the central north sea from the forth approaches basin basement fault blocks control facies type and thickness along the northwest margin of the north permian basin in the moray firth area permian tilt of the zechstein shelf related to regional flexure played a role in precipitating gravity driven tectonics within the seismically reflective facies on the east flank of the devil s hole high this permian phase of salt tectonics resulted in numerous intra zechstein structures which can be interpreted as rafted and folded zones of interbedded halite carbonate sulphate and clastics the rafts initially slid downdip towards the main halite basin then their evolution became dominated by differential subsidence of the interbeds which forced salt layers into adjacent passive diapirs this episode predates deposition of the triassic smith bank shales which rest upon the zechstein with local angular unconformity the spatial distribution of zechstein facies influenced the tectonostratigraphy of the triassic jurassic and cretaceous systems boundaries between zechstein facies correspond to the limit of triassic halokinesis on the west central shelf and the limit of detachment of jurassic and lower cretaceous thin skinned tectonics in moray firth half graben

introduction to special section on marine sand wave and river dune dynamics marine sand waves and river dunes are rhythmic patterns formed at the interface between a sandy bed and turbulent flows driven by tidal and river currents their origin development and dynamics are far from being fully understood to enhance the discussion on this topic we organized the international conference marine sand wave and river dune dynamics marid in april 2004 in enschede netherlands scientific papers that originated from the marid conference are presented in this special section this introduction places these papers in the context of recent progress

swash on a gently sloping beach waves observed in the inner surf and swash zones of a fine grained gently sloping beach are modeled accurately with the nonlinear shallow water equations the model is initialized with observations from pressure and current sensors collocated about 50 m from the mean shoreline in about i m depth and model predictions are compared to pressure fluctuations measured at five shoreward locations and to run up run up was measured with a vertical stack of five wires supported parallel to and above the beach face at elevations of 5 10 15 20 and 25 cm each 60 m long run up wire yields time series of the most shoreward location where the water depth exceeds the wire elevation as noted previously run up measurements are sensitive to the wire elevation owing to thin run up tongues not measured by the more elevated wires as the wire elevation increases the measured mean run up location moves seaward low frequency infragravity energy decreases and higher frequency sea swell energy increases these trends as well as the variation of wave spectra and shapes e g wave skewhess across the inner surf zone are well predicted by the numerical model

swash zone sediment suspension and transport and the importance of bore generated turbulence a study of swash zone sediment transport was conducted at gleneden beach oregon during february 25 28 1994 the data collected included suspended sediment concentration ssc sea surface elevation and velocity initially 4 and 8 cm above the bed at three cross shore locations within the swash zone spanning high tides ensemble averages of 6 9 and 12 s duration swash events showed that the uprush suspension was high concentrated in the leading edge and nearly vertically uniform above the lower 1 2 cm of the water column shortly after the sensors were inundated by run up the sediment rapidly settled out of the water during flow reversal the ssc was small but increased again in the backwash backwash vertical profiles were markedly different from uprush profiles with much of the suspension being confined to very near the bed where strong vertical gradients in ssc existed these marked differences show that the backwash is not simply the reverse of the uprush implying significant differences in the underlying fluid dynamics and sediment transport mechanisms backwash sediment suspension increased with flow duration however ensemble averaged ssc profiles of varying duration showed that the backwash concentrations were not consistent at the same temporal phases which suggests that water depth in addition to flow duration may be a controlling factor strong cross shore gradients in ssc suggest that bore derived turbulence may affect local sediment transport specifically our data show this bore generated turbulence turbulent kinetic energy directly influences local sediment suspension hence standard bed shear bagnold type sediment transport models may no longer be valid in the vicinity of the bore in the vicinity of the bore a higher correlation between bore generated turbulence and suspended sediment transport was found than between a bagnold type formulation and suspended sediment transport

comparison between predicted and observed sand waves and sand banks in the north sea for the first time a prediction model of regular morphological patterns on the seabed was tested against observations of sand wave and sand bank occurrence in the entire north sea the model which originates from first physical principles predicts this occurrence via two dimensionless parameters on the basis of the water depth the tidal velocity amplitude the level of zero intercept above the seabed z0 and a viscosity variation parameter alt epsilon the latter two quantities were varied in a number of predictions for the entire north sea whereas for the first two local values were used the range of realistic values of alt epsilon and z0 was large enough to let these two parameters distinguish between the possible combinations of bed forms as is shown in the shallower southern bight of the north sea the results were more sensitive to variations in z0 than in alt epsilon a slightly more detailed approach focused on sand waves only in the southern north sea and estimated local values for z0 using depth information quantification of the results showed that the model was able to predict the contours of the sand wave patches but it could not account for the absence of the bed features within this area the type of bed deposit partly explains the smaller scale variation the work confirms the validity of the theoretical bed form prediction model and verifies the hypothesis that the large scale seabed features are formed as free instabilities of tide topography interactions

tsunamis as geomorphic crises lessons from the december 26 2004 tsunami in lhok nga west banda aceh sumatra indonesia abstract large tsunamis are major geomorphic crises since they imply extensive erosion sediment transport and deposition in a few minutes and over hundreds of kilometres of coast nevertheless little is known about their geomorphologic imprints the december 26 2004 tsunami in sumatra indonesia was one of the largest and deadliest tsunamis in recorded human history we present a description of the coastal erosion and boulder deposition induced by the 2004 tsunami in the lhok nga bay located to the west of banda aceh northwest sumatra the geomorphological impact of the tsunami is evidenced by beach erosion some beaches have almost disappeared destruction of sand barriers protecting the lagoons or at river mouths numerous erosion escarpments typically in the order of 0 5 1 5 m when capped by soil and more than 2 m in dunes bank erosion in the river beds the retreat along the main river is in the order of 5 15 m with local retreats exceeding 30 m large scars typically 20 50 cm deep on slopes dislodgement of blocks along fractures and structural ramps on cliffs the upper limit of erosion appears as a continuous trimline at 20 30 m a s l locally reaching 50 m the erosional imprints of the tsunami extend to 500 m from the shoreline and exceed 2 km along riverbeds the overall coastal retreat from lampuuk to leupung was 60 m 550 000 m 2 and locally exceeded 150 m over 276 000 m 3 of coastal sediments were eroded by the tsunami along the 9 2 km of sandy coast the mean erosion rate of the beaches was 30 m 3 m of coast and locally exceeded 80 m 3 m the most eroded coasts were tangent to the tsunami wave train which was coming from the southwest the fringing reefs were not efficient in reducing the erosional impact of the tsunami the 220 boulders measured range from 0 3 to 7 2 m large typically 0 7 1 5 m with weights from over 50 kg up to 85 t we found one boulder less than 1 m large at 1 km from the coastline but all the others were transported less than 450 m 30 m and the moderate tsunamis height