seasonal scale nearshore morphological evolution field observations and numerical modeling abstract a coupled waves currents bathymetric evolution model delft 3d is compared with field measurements to test hypotheses regarding the processes responsible for alongshore varying nearshore morphological changes at seasonal time scales a 2001 field experiment along the beaches adjacent to grays harbor washington usa captured the transition between the high energy erosive conditions of winter and the low energy beach building conditions typical of summer the experiment documented shoreline progradation on the order of 10 20 m and on average approximately 70 m of onshore sandbar migration during a four month period significant alongshore variability was observed in the morphological response of the sandbar over a 4 km reach of coast with sandbar movement ranging from 20 m of offshore migration to over 175 m of onshore bar migration the largest seasonal scale onshore migration event observed in a natural setting both observations and model results suggest that in the case investigated here alongshore variations in initial bathymetry are primarily responsible for the observed alongshore variable morphological changes alongshore varying incident hydrodynamic forcing occasionally significant in this region due to a tidal inlet and associated ebb tidal delta was relatively minor during the study period and appears to play an insignificant role in the observed alongshore variability in sandbar behavior at kilometer scale the role of fully three dimensional cell circulation patterns in explaining the observed morphological variability also appears to be minor at least in the case investigated here

geometry flow and sediment transport of alluvial deposits induced by topographically driven flow expansions abstract flow expansions are a fundamental mechanism of sediment deposition here we define flow expansions as self formed alluvial expanding deposits developing over a larger experimental delta that 1 are characterized by unchannelized flow comprising a single sheet flow that covers the entire surface of the expansion and 2 do not interact laterally with still fluid and thus are not related to jets the flow spreading mechanism is transverse topographic curvature rather than mixing with ambient fluid hence we term these expansions topographic expansions to distinguish them from jet related flow expansions in topographic expansions transverse bed convexity drives lateral flow allowing topographic expansions to maintain much higher and more variable opening angles than those of jet related expansions the characteristic depositional body produced by the expansions is a relatively thin tabular sand body with a flat base and slightly convex top comparable to some sheet flood deposits in the stratigraphic record most of the deposition takes place as the topographic expansion develops followed by a longer interval of stable sediment bypass where all measured flow expansions show similar geometries this stable bypass phase is typically terminated by the flow abandoning the deposit or by channelization of the deposit itself after which the cycle can begin again one of the research questions raised by this study is the mechanism by which topographic expansions are able to maintain themselves in an unchannelized condition at flow aspect ratios that should be unstable according to current theories of bar and channel instability

lowstand deltas in the frontier formation powder river basin wyoming implications for sequence stratigraphic models deposits of lowstand deltas formed on the floor of the cretaceous interior seaway of north america are found in the cenomanian lower belle fourche member of the frontier formation central wyoming sandstones located in similar distal basin locations hundreds of kilometers basinward of highstand shoreline deposits form important hydrocarbon reservoirs isolated within marine shales but interpretation of their origin has been highly controversial the distribution geometry and internal facies of these sandstones are documented by an extensive outcrop study and regional subsurface correlations to develop genetic facies models for these deposits this integrated record of lithofacies ichnofacies palynofacies paleocurrent data bedding relationships and isolith maps incorporates observations from nearly 100 measured outcrop sections and about 550 subsurface well logs four episodes of sediment progradation and subsequent transgression each left behind gradually upward coarsening deltaic sandstones that have eroded tops these deltaic sandstones have a lobate to elongate geometry basinward dipping internal clinoform bedding radiating paleocurrents a low to moderate degree of shallow marine burrowing and show variable wave influence and tidal influence on deposition delta plain paralic and nonmarine facies have been eroded from the top of deltaic successions erosion surfaces capping progradational deltaic successions are the only stratal discontinuities that can be mapped regionally and they appear to record transgressive ravinement enhanced over areas of structural uplift compared with lowstand surfaces of erosion which record the bypass of sediments basinward low accommodation during lowstands left little room for sandstones to stack vertically and successive episodes of delta progradation were offset along strike more tide influenced delta deposits formed within shoreline embayments begin page 262 defined by the topography of older wave influenced delta lobes and subtle syndepositional deformation of the basin floor standard sequence stratigraphic terminology is difficult to use in broad lowstand systems like the frontier formation because sandstones do not show simple vertical stacking patterns major stratal discontinuities can form by processes other than lowstand fluvial erosion and minor syndepositional deformation of the basin floor exerts a first order influence on depositional and sediment preservation patterns although many basin distal sandstones have been interpreted to be deposits of offshore bars shelf isolated valley fills and stranded shorelines the frontier formation examples documented here suggest that many of these deposits may be top eroded deltas formed where rivers delivered sediment to lowstand coastlines the external geometry and internal heterogeneities of hydrocarbon reservoirs found in these types of deposits reflect processes active on the low accommodation deltaic shoreline even in cases where subsequent ravinement has significantly truncated the deposits during transgression

barrier formation on an actively prograding delta system the red river delta vietnam abstract an extensive barrier spit system 20 to 30 km long developed on the delta front platform of the ba lat delta within a relatively short period 10 20 years however it was not evident whether the formation was related to river floods typhoons or calm weather conditions or to cross shore or alongshore sediment transport mechanisms therefore a process based numerical model that simulates sediment transport and morphologic changes by waves tides wind and river flow is used to analyze the processes that govern this barrier formation the mechanism that initiates the barrier formation is concluded to be onshore transport by wave asymmetry of sediment previously transported seaward in the buoyant river plume the main mechanism that counteracts the barrier formation is probably destruction of the barrier by river flow during the subsequent transformation from a subaqueous barrier into a sub aerial barrier spit system overwashes and alongshore sediment transport become increasingly important this spit system therefore appears to be dominated by alongshore processes even though its initial formation is more strongly regulated by cross shore processes the importance of cross shore processes results in a minor contribution of typhoons to the initial formation of these barriers typhoons are not very important to destruction of barriers as well because wave energy is gradually dissipated over the gentle offshore profile which in turn results from river plume sedimentation

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

aspects of reservoir geology and production behavior of sirikit oil field thailand an integrated study using well and 3 d seismic data the sirikit oil field lies in the phitsanulok basin one of a series of tertiary rift related structures in central and northern thailand the tectonic history of the area is complex the original extensional half graben was deformed during deposition of the upper reservoir sequence by left lateral strike slip faulting careful interpretation of a three dimensional seismic data set has resulted in the reliable mapping of seismic markers detailed correlation of well logs based on field wide acoustically soft lacustrine claystone markers confirmed a lacustrine deltaic reservoir geologic model derived from core analyses the sirikit deltas are of constructive lobate type with sheetlike mouth bar bay mouth bar reservoir sandstone geometries interpretation of seismic horizon seiscrops amplitude maps of mapped seismic horizons has refined the understanding of the complex local reservoir structure subtle amplitude anomalies interpreted as small displacement faults are visible on horizon seiscrop maps these faults may reduce the drainage area inferred from log correlations the resolution of small dislocations and flexures by seiscrop analysis may apply to well targeting production decline rates are variable in the sirikit wells showing no simple relationship to the perforated reservoir thickness the geologic model explains this behavior in terms of effective drainage area wells perforated in thin sheet sandstones commonly have lower decline rates than those completed in thicker better reservoir quality but isolated channel sandstones

retrograde fluids in granulites stable isotope evidence of fluid migration widespread retrograde alteration assemblages document the migration of mixed h sub 2 o co sub 2 fluids into granulite facies rocks in the adirondack mountains fluid migration is manifest by 1 veins and patchy intergrowths of chlorite plus minus sericite plus minus calcite 2 small veins of calcite many only identifiable by cathodoluminescence and 3 high density co sub 2 rich or mixed h sub 2 o co sub 2 fluid inclusions the distinct and varied textural occurrences of the alteration minerals indicate that fluid rock ratios were low and variable on a local scale stable isotope ratios of c o and s have been determined in retrograde minerals from samples of the marcy anorthosite massif and adjacent granitic gneisses charnockites retrograde calcite in the anorthosite has a relatively small range in both delta sup 18 o sub smow and delta sup 13 c sub pdb 8 6 to 14 9 and minus 4 1 to 0 4 respectively probably indicating that the hydrothermal fluids that precipitated the calcite had exchanged with a variety of crustal lithologies including marbles and orthogneisses and that calcite was precipitated over a relatively narrow temperature interval values of delta sup 34 s sub cdt that range from 2 8 to 8 3 within the anorthosite can also be interpreted to reflect exchange between orthogneisses and metasediments the recognition of retrograde fluid migrationmore is particularly significant in granulite facies terranes because the controversy surrounding the origin of granulites arises in part from differing interpretations of fluid inclusion data specifically the timing of entrapment of high density co sub 2 rich inclusions results indicate that retrograde fluid migration which in some samples may leave only cryptic petrographic evidence is a process capable of producing high density co sub 2 rich fluid inclusions less

sharp based flood dominated mouth bar sands from the burdekin river delta of northeastern australia extending the spectrum of mouth bar facies geometry and stacking patterns abstract distinguishing the deposits of mouth bars from delta distributary channels in the rock record may not be as straightforward as often portrayed because mouth bar deposits can be more variable than usually presumed mouth bars with triangular plan view subaerial geometry are well developed at the mouth of the modern burdekin river of northeastern australia this planform is intermediate between the elongate lozenge shaped mouth bars typical of river dominated deltas and the beach ridge geometries characteristic of wave dominated deltas surface and shallow subsurface bar deposits are predominantly moderately sorted coarse grained sand similar to that of adjacent lower delta plain channel floor sedimentary textures are modified at seaward sides of mouth bars by waves into a foreshore of well sorted fine to medium grained sand and mouth bar sands pass distally and laterally into more mud dominated lithologies the holocene section beneath the lower delta plain is dominated by 5 8 m thick sharp based bodies of coarse grained sand texturally indistinguishable from the modern mouth bar with no vertical grain size trend a slight upward fining trend or in a few cases a coarsening upward trend these sand bodies have low angle seaward dipping internal bedding surfaces and are bounded by surfaces of similar attitude the holocene burdekin delta was and is flood dominated rather than strongly wave influenced as proposed previously and prograded by rapid deposition of mouth bars during river floods most of which last for only a few days mouth bar construction takes place over tens of years the variable vertical grain size profiles of mouth bar deposits suggests formation by both aggradation and progradation and this is supported by the cross sectional geometry of mouth bar clinoform sets imaged geophysically once a mouth bar has become emergent and is stabilized by vegetation a new bar is initiated seaward in this way delta lobes are constructed over 100s to 1000s of years before being abandoned following an avulsion of the trunk river channel to another part of the delta caution is needed in the interpretation of ancient shallow water deltaic successions where sharp based fining upward mouth bar deposits may be confused with distributary channel facies

facies development of atchafalaya delta louisiana a modern bayhead delta early studies of atchafalaya delta focused on large scale characteristics of accretion in atchafalaya bay this study concentrates on sedimentation processes and stratigraphic as well as plan view facies development in a rapidly prograding bayhead delta since initiation of subaqueous delta growth early 1950s the bay has become shallower with accumulation of progressively coarser grained sediment stratigraphic relationships indicate a buildup from the pre 1950 bay bottom through prodelta distal bar distributary mouth bar levee back bar algal flat and channel fill deposits at the present stage of delta development two distinct processes of subaerial growth are recognizable initial growth relates to channel extension bifurcation and development of sinuous overbank channels a later more mature response of lobe fusion and upstream lobe growth becomes dominant as progradation through bifurcation diminishes in older areas of the delta atchafalaya delta growth to 1982 serves as a well documented sedimentary example for understanding bayhead deltas which commonly form in protected environments

geology and stratigraphy of fluvio deltaic deposits in the ivishak formation applications for development of prudhoe bay field alaska significant remaining reserves in prudhoe bay field are confined within deltaic rocks at the base of the triassic ivishak sandstone the initial stratigraphic characterization of the prudhoe bay reservoir was lithostratigraphically based and it depicted this basal reservoir interval as tabular zones between marine shale and overlying coarse grained fluvial sandstones a reassessment of this interval based on cores and genetic stratigraphic correlations depicts en echelon offlapping fluvially dominated deltaic wedges reservoir quality rocks occur in distributary mouth bar distributary channel and fluvial facies associations a paleogeographic reconstruction of one delta lobe includes an alluvial plain crossed by channels of possibly braided or low sinuosity rivers this alluvial plain graded into a delta plain cut by distributary channels that fed distributary mouth bars on a broad delta front river dominance is inferred from the abundance of unidirectional current structures normally graded beds soft sediment deformation and general absence of wave formed tidal and biogenic structures slumping and growth faulting locally replaced coarsening upward deltaic successions with sharp based overthickened mouth bar and distributary channel deposits mudstones deposited following delta lobe abandonment form laterally extensive flow barriers between lobes compartmentalization is most pronounced distally where deltaic sandstones are overlain by and pass laterally into marine shale proximally fluvial and deltaic sandstones are juxtaposed across erosional contacts improving reservoir continuity this stratigraphic interpretation is corroborated by production and surveillance data plus an interference test locally stratigraphy and poor waterflood performance reflect completions in diachronous sandstones that originated in separate deltaic lobes previously poor well performances were attributed to sandstone pinch outs in some cases production can be enhanced with recompletions rather than infill drilling nonconventional wells planned and completed with the benefit of detailed facies association correlations currently are recovering millions of barrels of previously bypassed oil

distributary channels and their impact on sediment dispersal a global analysis of world deltas with details from a natural and an anthropogenic influenced delta demonstrates how distributary channels control the flux of sediment into the coastal ocean the study addresses the range in the number of distributary channels across world deltas using remote sensing techniques a power law relationship is found between the number of distributary channels and the length of river and separately the delta gradient these relationships hold for all types of deltas whether controlled or strongly influenced by waves river discharge tides or ice permafrost or sea ice nature controlled deltas such as the klinaklini delta have distributary channels that act as overflow conduits that become active during flooding events anthropogenic controlled deltas like the po delta have distributary channels that are controlled for flood mitigation or low flow maintenance anthropogenic influences greatly impact the natural rate of delta progradation through changes in sediment supply controlling the position of distributary channels and impacting subsidence from gas and groundwater extraction even with flood controls the po delta traps 16 of the sediment load in its channels that are becoming super elevated at rates of 4 to 10 cm yr with respect to the delta plain a new model is formulated and shown to predict accurately the sediment flux through each channel along with their hydraulic properties deltas with high numbers of distributary channels produce hypopycnal plumes with reduced transport capacity as a result sediment diffuses out of the multi channel deltas as a buoyantly driven plume rather than as a momentum driven jet

the table rocks sandstone a fluvial friction dominated lobate mouth bar sandbody in the westphalian b coal measures ne england abstract westphalian b duckmantian alluvial coal measures along the northumberland coast ne england comprise coal capped coarsening upward crevasse splay sequences of shale siltstone and sandstone interbedded with a number of major distributary channel sandbodies including the table rocks sandstone lithofacies architectural analysis and outcrop geometries divide the table rocks sandstone into flaggy sandstone massive sandstone heterolithic and mudstone facies associations each comprising up to 7 lithofacies types the three sandy facies associations are characterised by lenticular bed geometries on different scales producing a hierarchy of lensoid packages and associated bounding surfaces all showing typical offset stacking patterns 1 lenses represent individual lenticular cross bed sets bounded by 1st order surfaces 2 packages of lenses called lens clusters are bounded by 2nd order surfaces and are the basic architectural building block of the sandy facies associations and 3 vertically stacked lens clusters called amalgamated lens clusters bounded by 3rd order surfaces the table rocks sandbody has a laterally extensive irregular lobate subsurface plan geometry it displays a radial palaeocurrent pattern with 180 dispersion and it forms part of a 14 m thick coarsening upward regressive sequence it is interpreted as a composite lobate crevasse splay delta system that prograded into a shallow interdistributary fresh to brackish water lake up to 14 m deep the shallow lake water fluvial input and extensive development of traction structures such as cross bedding and ripple cross lamination suggests a friction dominated delta in which the four facies associations can be interpreted in terms of discrete elements of the mouth bar environment the flaggy sandstone facies association represents the main axial part of the mouth bar system the erosively based massive sandstone facies association major subaqeous distributary channels the lithologically more variable heterolithic facies association the medial mouth bar and the mudstone facies association the distal mouth bar fringe and prodelta within this environmental setting amalgamated lens clusters are interpreted as small discrete mouth bar sand lobes whose offset imbricate stacking pattern reflects channel spacing and bifurcation the rate of channel shifting or shallow depths and lack of accommodation space thus lens clusters are interpreted as discrete growth elements of the mouth bar sand lobes and lenses as individual bedforms making up these growth elements because of the high rate of channel shifting lack of extensive erosion of the mouth bar lobes and deposition of low discharge fines the lobes retained much of their original depositional geometry thereby providing advantageous gradients for offset deposition and stacking of adjacent sand lobes although the delta complex was maintained by frequent crevassing from the feeder channel and by subsidence due to contemporaneous compaction and or local tectonism it was deeply incised on two occasions by subaqeous channels in response to high magnitude floods or falling lake level

the dynamics of a river bay delta system an analysis has been made to aid our understanding of various phenomena associated with turbulent plane jets that issue from river outlets and discharge into quiescent bays an integrated form of the hydrodynamic equations of flow continuity and momentum balance coupled with advective diffusion mass transport equations has been formulated into a two dimensional spatial domain closed form analytical solutions are obtained with the aid of similarity functions for the velocity and sediment concentration profiles a numerical procedure for estimating the areal and volume deposition of the delta sediments is presented the atchafalaya river delta a newly developing delta in the south central louisiana coast is used to illustrate analytical techniques the results of analytical predictions are compared with the results predicted by other statistical and generic approaches an agreement is found a sensitivity analysis is performed to assess the relative importance of various parameters in the river bay delta system

river mouth bar formation riverbed aggradation and channel migration in the modern huanghe yellow river delta china this paper addresses the recent 1970s 1990s processes of river mouth bar formation riverbed aggradation and distributary migration in the huanghe river mouth area in the light of station based monitoring field measurements and remote sensing interpretation the results show that the morphological changes of the river mouth bar have been closely associated with the largely reduced fluvial discharge and sediment load landforrn development such as bar progradation occurred in two phases i e before and after 1989 which correspond to faster and lower bar growth rates respectively fast riverbed aggradation in the mouth channel was strongly related to river mouth bar progradation during 1976 1996 about 2 8 of the total sediment loads were deposited in the river channel on the upper to middle delta therefore the river water level rose by a few meters from 1984 to 1996 the frequent distributary channel migration which switched the radial channel pattern into the se directed pattern in the mid 1980s was linked with mouth bar formation marine conditions also constrain seaward bar progradation furthermore the history of river mouth bar formation reflects human impacts such as dredging and dyking in order to stabilize the coastal area c 2005 elsevier b v all rights reserved

evolution of atchafalaya lacustrine deltas south central louisiana abstract the entrenched pleistocene holocene mississippi alluvial valley was cut during the last low sea level stand and alluviation of the entrenched valley occurred during the holocene sea level rise regional gulf coast structures salt domes and faults have strongly influenced the morphology of the valley valley paleotopography has controlled depositional patterns and locations of major river courses as well as partially isolated large interdistributary basins within the alluvial deltaic plain interdistributary basin formation is further enhanced by distributary channel progradation switching and abandonment in conjunction with compactional subsidence of the alluvial plain surface geomorphology and sediment cores from the atchafalaya basin louisiana revealed that this basin has alternately accumulated fine grained organic rich sediments and terrigenous clastic dominated meander belt and lacustrine deposits basin subsidence and the presence of an active distributary channel in the basin are major determinants of the lithology of the sedimentary fill prograding lacustrine deltas of the atchafalaya river are the primary mechanism for filling the atchafalaya basin these upward coarsening deltas are relatively thin 3 m but are areally extensive tens to hundreds of kilometers and because they occur so frequently during the infilling process they account for a significant volume of the basin sequence the atchafalaya basin sequence documents lacustrine deltaic episodes dating from the early holocene maringouin and teche deltas to the present atchafalaya delta continued development of the atchafalaya river will rework portions of the backswamp and lacustrine deposits into meander belt deposits upon complete filling of the atchafalaya basin sediment will mostly bypass the basin and become deposited in the marine delta in atchafalaya bay

mechanics of river mouth bar formation implications for the morphodynamics of delta distributary networks 1 in this paper we use observational data and numerical modeling to present a new explanation for the formation of river dominated delta networks observational data from deltas throughout the world show that distributary channel widths depths and lengths decrease nonlinearly with successive bifurcations trends in width and depth are an outcome of hydraulic geometry scaling the trend in channel length is a consequence of delta growth analyses of serial maps show that the positions of bifurcations are the fossilized locations of river mouth bars also called middle ground and distributary mouth bar in front of old delta channel mouths therefore the trend in channel length can be explained through the mechanics of river mouth bar formation and evolution we use delft3d a coupled hydrodynamic and morphodynamic model to simulate the process of river mouth bar formation within an expanding turbulent jet in front of distributary channel mouths our results describe in detail the formation and evolution of a river mouth bar system and demonstrate that the distance to the river mouth bar is proportional to jet momentum flux and inversely proportional to grain size therefore channel length decreases down delta because with each successive bifurcation the jet momentum flux decreases these results can be used to predict the future evolution of river dominated deltas and can be used to aid in hydrocarbon exploration of these depositional environments

permeability characterization of distributary mouth bar sandstones in prudhoe bay field alaska how horizontal cores reduce risk in developing deltaic reservoirs oil production from upstructure drill sites at prudhoe bay field alaska is almost exclusively from fine grained deltaic sandstones distributary channel and distributary mouth bar facies associations in the triassic ivishak formation comprise the pay zones but wells are preferentially completed in the lower permeability distributary mouth bar deposits in an attempt to avoid high gas oil ratio wells thin light oil columns combined with complex stratigraphy and an overlying highly mobile gas cap make planning drilling and completing economic wells challenging to accurately assess lateral trends in permeability within distributary mouth bar sandstones three conventional cores were cut approximately 120 ft 36 m along the 1000 ft 304 m horizontal reach of a recent development well the cored interval consists of seven lithofacies all composed of fine grained sandstone comparison of permeability values to lithofacies demonstrates a striking and consistent trend six lithofacies possess average horizontal permeabilities ranging from 12 to 40 md average horizontal permeability for the seventh lithofacies lithofacies 7 is 129 md porosity and vertical permeability follow similar patterns distributary mouth bars in prudhoe bay field were deposited in fluvially dominated delta lobes in which sediment distribution at river mouths was controlled by friction between the sediment plume and basin bottom during flood stage the best sorted and most permeable sediments i e lithofacies 7 were deposited on the apex or most proximal part of the distributary mouth bars between distributary channels sedimentologic and petrographic data corroborate a strong link between lithofacies and permeability sandstones deposited by unsteady flow conditions e g turbulent begin page 460 scour intermittent ripple migration are likely to be less well sorted and contain more clay and lignitic organic material commonly as drapes and wisps than lithofacies 7 sandstones deposited under more uniform high energy transport conditions small amounts of argillaceous and lignitic laminae serve as nucleation sites for siderite cement precipitation and as catalysts for pressure solution of quartz grains significantly degrading permeability in lithofacies 1 through 6 insights gained from analyzing these cores can reduce the risks associated with well completions in distributary mouth bar sandstones locations targeting distributary mouth bar deposits can be optimized by 1 using existing well data to map time equivalent deltaic facies associations 2 identifying transition zones between distributary channels and distributary mouth bars 3 extrapolating trends of high permeability lithofacies within distributary mouth bars and 4 calculating the well trajectory and length to optimally contact high permeability rock or moderate permeability rock to inhibit gas coning

distributary mouth bar development and role of submarine landslides in delta growth south pass mississippi delta submarine landslides play a major role in the development of distributary mouth bars and are of major importance in transporting sediment from the bar front to deeper water along the mississippi delta front historic maps of the south pass of the mississippi delta show that the bar advanced seaward more than 1 mi 1 6 km between 1867 and 1953 details of the growth of the bar have been elucidated using an elaborate computer modeling program to analyze these historic maps the analysis has shown that the geometry of the bar was controlled by the dynamics of the freshwater plume of river water as it mixed with saline gulf water approximately half the sediment deposited on the bar was moved into deeper water by submarine landslides the underlying causes of bar failure were established during major floods with the deposition of thick blankets of unstable water saturated sediments on the bar front failure occurred one to four years later in response to a variety of triggering mechanisms which either changed the shear strength of the sediment or modified local bottom slope the triggering mechanisms include major storms and hurricanes mudlump activity and possibly increased pore pressures resulting from generation of biogenic gas bar growth and bas nward movement of sediment thus represent a multivariate problem that can be approached by means of a computer analysis of bathymetric data