horizontal lamination and the sequence of bed phases and stratification under upper flow regime conditions two forms of horizontal laminae have been produced under upper flow regime conditions in a flume by deposition on a plane bed and on low in phase waves upper plane bed laminae consist of approximately equal numbers of fining upward fu and coarsening upward cu textural laminae heavy minerals when present are typically within or form cu laminae in horizontal laminae formed under low in phase waves fu laminae may predominate and are significantly thicker than cu laminae heavy minerals are associated with the tops of some fu laminae and are absent from cu laminae deposits aggraded on plane beds and low in phase waves both display low angle 13 14 upstream grain imbrication upper plane bed horizontal laminae are attributed to the effects of the burst sweep cycle on local bedload transport bursts form fu laminae and sweeps form cu laminae some fu laminae in in phase wave deposits are thought to form with downstream migration of bed waves on which sand size decreases from trough to crest heavy minerals are concentrated in the tops of these fu laminae due to their enrichment on the crests of the migrating bed waves other fu and cu laminae in in phase wave deposits probably formed by the action of bursts and sweeps as on upper plane beds a sequence of bed phases and stratification formed under upper flow regime conditions is described based on the observations here combined with descriptions from the literature with increasing flow strength the gradual sequence of bed phases is 1 plane bed 2 low relief downstream migrating inphase waves increasing in amplitude and wavelength with increasing flow strength 3 stationary in phase waves 4 upstream migrating including breaking wave antidunes the sequence of stratification formed under these bed phases is 1 upper plane bed horizontal lamination 2 in phase wave horizontal lamination to in phase wave foreset cross laminae 3 in phase wave drape laminae 4 antidune backset cross laminae

thickness of sets of cross strata and planar strata as a function of formative bed wave geometry and migration and aggradation rate cross strata and planar strata formed by the migration of ripples and dunes or low relief bed waves on upper stage plane beds respectively are the most common internal structures in fluvial sands and sandstones the thickness of cross sets or planar laminae at a given location is controlled by 1 aggradation rate relative to downstream migration rate of bed waves and 2 the succession of bed waves of varying height passing the location a quantitative understanding of this relationship is now appearing based on new theory and experimental studies with ripples dunes and upper stage plane beds in many cases the effects of control 1 can be ignored in such cases only the largest bed waves in a population form cross sets or planar laminae the ratio of mean cross set thickness mean height of formative bed waves is less than 0 4 and the ratio of mean cross set thickness mean height of all bed waves is less than 0 5 a simple quantitative model a modified form of the paola borgman model is presented for predicting the height distributions of bed waves ripples dunes low relief bed waves from the thickness distributions of cross sets or planar laminae if the aggradation rate is appreciable more of the smaller bed waves in a population can form cross sets or planar laminae and the preservation ratios increase to 1 in the case of supercritical climbing ripple cross strata in this case it is necessary to determine the angle of climb and length of the bed waves in order to use the model to predict bed wave height distributions

contrasts in morphology and deformation offshore montserrat new insights from the sea calipso marine cruise data 1 during the december 2007 sea calipso experiment we collected seismic reflection profiles offshore of montserrat off the east coast we imaged deep fans of volcaniclastic debris from three volcanoes progressively active from 2 ma to present near shelf sedimentation rates of 8 9 cm ka are approximated following cessation of local volcanic activity the fans were deposited on sediments with apparent dips towards the ese trending montserrat havers fault system mhfs in southern montserrat the mhfs encloses the soufriere hills volcano has elevated crustal blocks at roche s bluff st georges hill and garibaldi hill and extends off the west coast off the west coast the n dip of two faults supports a n dip interpretation for a major component of mhfs the belham valley fault we propose that local deformation is affected by stress redistributions consistent with a right stepping sinistral en echelon fault system but the interplay of transtension and magmatism has resulted in complex and evolving stress regimes

palaeoflood estimates of pleistocene coarse grained river terrace landforms rio almanzora se spain abstract a series of palaeoflood estimation techniques are applied to an inset sequence of middle to late pleistocene river terrace landforms associated with the rio almanzora se spain the study area is a 7 km long transverse reach where 4 terrace levels level 1 highest and oldest level 4 lowest and youngest document some 200 m of incision across an uplifted basement block during the pleistocene for the broader region river terrace aggradation is attributed to increased sediment supply during glacial to interglacial transitions level 1 marine oxygen isotope stages mois 12 11 l2 10 9 l3 8 7 and l4 6 5 within the transverse reach terrace levels 2 3 and 4 are characterised by coarse boulder rich gravels d max 2 5 m organised into 3 5 m high cross beds level 4 is characterised by a series of km scale abandoned meander loops with levels 2 and 3 showing evidence for similar degraded high sinuosity forms the fluvial setting is interpreted as a braided river environment with lateral and longitudinal gravel barforms using field derived sedimentary boulder size etc and morphological width slope etc features flow competence and regime based methods were applied palaeoflood estimates varied from 40 to 2859 m 3 s with mean maximum discharges showing an increase through time l2 278 l3 413 l4 1361 m 3 s palaeoflood estimates were compared to flood events associated with the ephemeral modern rio almanzora catchment 2500 km 2 to establish whether palaeoflood values were reasonable estimations a 46 year 1963 2009 modern flood gauge record shows infrequent flood events with discharges typically 3 s a rare flood event in 1973 5680 m 3 s provides an upper value for comparison with the palaeoflood estimates although estimates appear reasonable they should be considered as minimum values due to 1 some disparities between flow depths derived from the palaeoflood equations 0 7 4 m and field based evidence 3 5 m high cross beds 2 issues related to field sampling of boulders from sections and 3 uncertainties as to the relationship between flood hydrology and transportation of boulders formation of 3 5 m high cross beds and the position of the terrace remnants in the river valley the palaeoflood estimates suggest that relatively large discharges occurred during valley floor aggradation in the glacial interglacial transitions periods characterised by low vegetation cover and high sediment yield discharge increases through time can be explained by the long term drainage evolution of the rio almanzora characterised by an expanding catchment area linked to river capture driven by regional uplift patterns the 3 5 times larger discharge estimates recorded in terrace level 4 are coeval with large and rapid changes in precipitation or elevated seasonal discharge linked to the mois 6 5 glacial interglacial climate transition overall within tectonically active regions affected by river capture fluvial systems could be responding to loss or expansion of catchment areas long term palaeoflood records from inset flights of river terraces could store such signals and may mask complicate any climate palaeoflood relationships

architecture and sedimentation of outwash fans in front of the mýrdalsjokull ice cap iceland in this study we introduce the architecture and style of sedimentation of two outwash fans in front of the myrdalsjokull ice cap central south iceland that broaden different types of ice marginal sedimentation at kotlujokull a southeastern outlet of the ice cap fan formation was monitored from the late 1970s to 2002 thus the style of sedimentation is well understood and the relationship to the depositional architecture easily established in front of slettjokull a northern lobate extension of the ice cap a fan from the 10th century ad is capped by neoglacial till sheets at both sites the fan surfaces were reconstructed in three dimensions ground penetrating radar gpr provided the thickness and internal structure of sediment bodies geological exposures and excavations were used for detailed sedimentological documentation of the outwash deposits in proximal to distal transects the outwash deposits at both kotlujokull and slettjokull are considered to be alluvial fans of the hochsander type i e supraglacially fed as they show all the architectural characteristics of alluvial fans with a semiconical shape a restricted radial length of ca 0 5 1 5 km a plano convex cross profile and a slope gradient ranging between 1 and 5 fan deposits are dominated by planar or low angle cross bedded sand facies interbedded with thin laminae of fine gravel occasionally draped by mud sedimentation occurs during upper flow regime sheetflows associated with antidune migration within an aggrading shallow braided stream network hochsanders are intimately linked to advancing or stationary glaciers with steep ice cored frontal slopes that favor supraglacial over subglacial drainage most important hochsander fans lack any proximal to distal transition in the grain size distribution sediment facies or facies associations because much classification of glacial outwash sediments is based on proximal to distal distinctions it is crucial to consider the depositional model for hochsanders this study also indicates a strong dependence between the altitudes of the buried fan surface and the present day terrain in the forefield of slettjokull illustrating that subsequent to fan formation subglacial landforms merely mask many of the pre existing morphological features less

critical flow in rockbed streams with estimated values for manning s n the writer presented interesting experimental observations the discusser would like to comment on some aspects of the paper regarding critical flow or more accurately near critical flow critical slope calculations and boundary shear stress distributions in undular flows he hopes that the information will assist in refining the author s work

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small

response of coral reefs to climate change expansion and demise of the southernmost pacific coral reef received 21 may 2010 revised 20 june 2010 accepted 6 july 2010 published 3 august 2010 1 coral reefs track sea level and are particularly sensitive to changes in climate reefs are threatened by global warming with many experiencing increased coral bleaching warmer sea surface temperatures might enable reef expansion into mid latitudes here we report multibeam sonar and coring that reveal an extensive relict coral reef around lord howe island which is fringed by the southernmost reef in the pacific ocean the relict reef in water depths of 25 50 m flourished in early holocene and covered an area more than 20 times larger than the modern reef radiocarbon and uranium series dating indicates that corals grew between 9000 and 7000 years ago the reef was subsequently drowned and backstepped to its modern limited extent this relict reef with localised re establishment of corals in the past three millennia could become a substrate for reef expansion in response to warmer temperatures anticipated later this century and beyond if corals are able to recolonise its surface citation woodroffe c d b p brooke m linklater d m kennedy b g jones c buchanan r mleczko q hua and j zhao 2010 response of coral reefs to climate change expansion and demise of the southernmost pacific coral reef geophys res lett 37 l15602 doi 10 1029 2010gl044067

the sedimentary architecture of outburst flood eskers a comparison of ground penetrating radar data from bering glacier alaska and skeiðararjokull iceland we present ground penetrating radar gpr profiles that reveal the sedimentary architecture of an esker deposited during a surge associated outburst flood at the bering glacier alaska the wide up flow end of the esker contains a transition from large backset beds to large foreset beds interpreted to reflect composite macroform development in an enlarged part of the conduit by contrast the narrow down flow portion of the esker is dominated by plane beds interpreted to have been deposited where the conduit was constricted and the flow was faster a previously studied outburst esker at skeidararjokull iceland has a similar morphology and stratigraphic architecture this suggests that outburst floods generate distinct depositional signatures in eskers both in terms of morphology and sedimentary architecture identification of these distinct signatures in ancient eskers will help assess the paleohydraulic conditions under which ancient eskers formed and by extension the nature of meltwater drainage systems beneath the laurentide and eurasian ice sheets

understanding the influence of slope on the threshold of coarse grain motion revisiting critical stream power abstract this paper investigates the slope dependent variation in critical mean bed shear stress for coarse grain motion and evaluates stream power per unit bed area as an alternative threshold parameter explanations for observed slope dependency and existing approaches for predicting the critical stream power per unit bed area are reviewed an analysis of secondary bed load transport datasets is used to examine the strength of associations between stream power per unit bed area mean bed shear stress and mean velocity with bed load transport rate data from an original flume study are combined with secondary data from similar flume experiments to investigate the effect of slope on both critical stream power per unit bed area and critical mean bed shear stress results suggest that stream power per unit bed area is most closely correlated with bed load transport rate and also that critical stream power per unit bed area is less variable with slope than critical mean bed shear stress alternative solutions to approximating critical stream power are explored these include 1 modifying existing expressions for critical stream power to account for higher critical mean bed shear stresses at higher slopes and 2 applying a constant dimensionless critical stream power criterion

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

stabilizing self organized structures in gravel bed stream channels field and experimental observations stable reticulate structures which we call stone cells have been observed in cobble gravel channel beds with low bed material transport rates experiments show that such structures develop simultaneously with the armor layer during an extended period when flows do not exceed the shields threshold by more than 2 times so that bed material transport is low they are constructed by particles moving from less stable positions into more stable configurations against each other intermediate developments include clusters and stone lines they reduce sediment transport by orders of magnitude and are evidently a major stability promoting mechanism in gravel channels the timescale for their development suggests that the boundaries of many gravel bed channels are not in equilibrium with recent competent flows but reflect the history of recent dominant flows

rainfall induced volcanic activity on montserrat 1 dome forming volcanic eruptions cyclically extrude bodies of lava over several months which then become gravitationally unstable and collapse generating pyroclastic flows on 29 july 2001 extreme rainfall over montserrat coincided with a major collapse of the soufriere hills lava dome we present rainfall and seismic records that demonstrate for the first time a relationship between intense rainfall and lava dome collapse with associated pyroclastic flow generation after seven months of little rain and a period of sustained dome growth the onset of intense rain was followed within hours by dome collapse and pyroclastic flows the large scale weather system responsible for the rain was identifiable in satellite images and predicted by meteorological forecasts issued 60 hours prior to the volcanic activity it is suggested that weather prediction of intense rainfall be incorporated with existing geophysical and geochemical measurements to improve warnings of these hazardous events

architectural analysis of a volcaniclastic jokulhlaup deposit southern iceland sedimentary evidence for supercritical flow the 1918 eruption of the glacially capped katla volcano southern iceland generated a violent jokulhlaup or glacial outburst flood inundating a large area of m rdalssandur the proglacial outwash plain where it deposited ca 1 km3 of volcaniclastic sediment the character of the 1918 jokulhlaup is contentious having been variously categorized as a turbulent water flow a hyperconcentrated flow or as a debris flow based on localized outcrop analysis in this study outcrop based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi stationary antidunes and down current migrating antidunes and from regular based bedsets associated with transient chute and pool bedforms all of which are associated with turbulent transcritical to supercritical water flow conditions antidune wavelengths range from 24 to 96 m corresponding to flow velocities of 6 to 12 m sec 1 and average flow depths of 5 to 19 m this range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts architectural analysis of the 1918 jokulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jokulhlaup that could not have been achieved through localized outcrop analysis the observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions the physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations regarding modes of deposition and formative flow hydraulics are to be made

fluvial sediment transport and deposition following the 1991 eruption of mount pinatubo the 1991 eruption of mount pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows bedload sampling in the pasig potrero river one of the most heavily impacted rivers revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth fine grained bed dimensionless bedload transport rates in the pasig potrero river differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments the similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply in arid rivers this is attributed to a flashy hydrograph whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery this work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced d 2002 elsevier science b v all rights reserved

coastal plain sedimentation in the late devonian of southern ireland hummocky cross stratification in fluvial deposits abstract the late devonian toe head formation crops out extensively in southwest ireland and has been considered previously as the topmost old red sandstone terrestrial formation in a conformable non marine to marine sequence the most characteristic lithofacies the flat laminated and inclined parallel laminated sandstones displays a range of structures from flat to gently inclined laminae and includes sets which meet the four criteria of harms et al 1982 for hummocky cross strata associated facies include desiccated mudrocks palaeosols and ripple cross laminated sandstones which lack the flaser linsen dominated bedsets characteristic of the overlying marine strata only non marine fossils are recorded and numerous palynological preparations lack the marine indicators which appear at the top of the formation palaeocurrents show considerable spread with indications of overall easterly transport of sand there is no preferred vertical sequence of lithofacies the balance of evidence strongly favours a non marine environment with the implication that hummocky cross strata as presently defined are not good environmental or even process indicators the fine to very fine sand sizes and possibly the amount of suspended load were important controls on the resultant structures a depositional model of essentially fluvial deposition on a low gradient coastal plain is proposed

interpreting flash flood palaeoflow parameters from antidunes and gravel lenses an example from montserrat west indies the wavelength of stationary water surface waves and their associated antidune bedforms are related to the mean velocity and depth of formative flow in sand bed flume experiments alexander et al 2001 found that lens structures were preserved during antidune growth and change and the dimension of the lenses were empirically related to antidune wavelength and thus could be used to estimate flow velocity and depth this study is the first to compare observations of formative flow conditions and resulting sedimentary structures in a natural setting testing the alexander et al 2001 relationship at a field scale trains of stationary and upstream migrating water surface waves were prevalent during the flash flood in october 2012 in the belham valley montserrat west indies wave positions and wavelengths were assessed at 900 s intervals through the daylight hours of the event within a monitored reach the wave data indicate flow depths up to 1 3 m and velocity up to 3 6 m s 1 sedimentary structures formed by antidune growth and change were preserved in the event deposit these structures include lenses of clast supported gravel and massive sand with varying internal architecture the lenses and associated low angle strata are comparable to sand bed structures formed from stationary and upstream migrating waves in flume experiments confirming the diagnostic value of these structures using mean lens length in the event deposit underestimated peak flow conditions during the flood and implied that the lenses were preserved during waning flow

downstream dilution of a lahar transition from debris flow to hyperconcentrated streamflow nearly instantaneous melting of snow and ice by the march 19 1982 eruption of mount st helens released a 4 106 m3 flood of water from the crater that was converted to a lahar volcanic debris flow through erosion and incorporation of sediment by the time it reached the base of the volcano over the next 81 km that it traveled down the toutle river the flood wave was progressively diluted through several mechanisms a transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater when the total sediment concentration had decreased to about 78 by weight 57 by volume the hyperconcentrated lahar runout flood wave transporting immense quantities of sand in suspension continued to experience progressive downstream dilution although turbulence was significantly dampened by the extremely high suspended load very large standing waves and antidune waves were observed the hyperconcentrated lahar runout flow deposited an unusual faintly stratified coarse sand which locally contained small isolated gravel lenses very similar deposits in the quaternary stratigraphy of mount st helens and other cascades volcanoes suggest that lahars may be more frequent than previously recognized

monitoring and modelling morphological change in a braided gravel bed river using high resolution gps based survey recent research in fluvial geomorphology has emphasized the direct monitoring of channel topography as a useful tool to better understand the interrelationship between river form and process and in particular to estimate bedload transport rates and reach scale sediment budgets until recently however this morphometric approach has been limited to comparison of repeatedly surveyed cross sections with the associated problems of low frequency sampling and an emphasis on cross stream as opposed to downstream morphological dynamics recent advances in analytical photogrammetry have however permitted the acquisition of data at sufficient resolutions to represent macroform 101 m and microform 10 m channel geometry in fully three dimensions while inherently spatially distributed the photogrammetric approach to land surveying is however restricted to observable areas of the channel and is thus of limited use in the subaqueous zone especially if flows are turbid or highly coloured this paper presents an alternative approach to the study of three dimensional morphological dynamics of a divided reach of the gravelly river feshie scotland in which topographic survey of both exposed and submerged areas of the reach was undertaken using the global positioning system gps survey results from two field programmes in 1998 and 1999 are presented in which a geotronics geotracer 2000 real time kinematic gps receiving unit was used to survey a 200 80 m reach rapid radial survey methods were employed permitting the collection of 2000 survey points per day giving a mean density over the 13 ha reach of 1 1 points per square metre a detailed assessment of survey errors and dem quality is presented and a limit of detection of c 10 cm proposed as an upper threshold for the identification of significant change between dems preliminary interpretation of channel dynamics in 1998 99 reveals relatively little change to the overall macroscale reach structure dem differencing techniques however highlight important local changes including the progradation of submerged bar fronts bar head aggradation and scour along major channel thalwegs and the erosion of fines from sloughs copyright 2000 john wiley sons ltd

critical flow constrains flow hydraulics in mobile bed streams a new hypothesis a new hypothesis predicts that in mobile bed river channels interactions between the channel hydraulics and bed configuration prevent the froude number fr from exceeding 1 for more than short distances or periods of time flow conditions in many steep competent streams appear to be close to critical froude numbers of steep slope 0 01 sand bed streams with considerable freedom to adjust boundaries oscillate between 0 7 and 1 3 over 20 to 30 s cycles with an average of 1 0 at the channel thalweg criticalflow in these streams is maintained by the interaction between the mobile bed and free water surface at highfr which results in a cyclical pattern of creation and destruction of bed forms field observations support that a similar mechanism offlow bed form interaction constrains fr 1 in active bed braided gravel rivers step pool streams laboratory rills lahar runout channels and even some bedrock channels empirical and analytical results show that as slope increases competentflows tend to asymptotically approach criticalflow an assumption of criticalflow would dramatically simplify paleohydraulicflow reconstructions and modeling offlow hydraulics in high gradient streams

moving boulders in flash floods and estimating flow conditions using boulders in ancient deposits boulders moving in flash floods cause considerable damage and casualties more and bigger boulders move in flash floods than predicted from published theory the interpretation of flow conditions from the size of large particles within flash flood deposits has until now generally assumed that the velocity or discharge is unchanging in time i e flow is steady or changes instantaneously between periods of constant conditions standard practice is to apply theories developed for steady flow conditions to flash floods which are however inherently very unsteady flows this is likely to lead to overestimates of peak flow velocity or discharge flash floods are characterised by extremely rapid variations in flow that generate significant transient forces in addition to the mean flow drag these transient forces generated by rapid velocity changes are generally ignored in published theories but they are briefly so large that they could initiate the motion of boulders this paper develops a theory for the initiation of boulder movement due to the additional impulsive force generated by unsteady flow and discusses the implications keywords

critical specific stream power in gravel bed rivers experiments with marked pebbles were carried out on different sized rivers of the belgian ardenne catchment areas varying from less than 1 km2 to 2700 km2 specific stream power required to cause bedload movement was evaluated and critical values were obtained three types of relationship between critical specific stream power 0 and grain size d were established the values for 0 in the largest river the ourthe were the lowest and were close to the values obtained for mountainous rivers carrying large boulders in medium sized rivers catchment area between 40 and 500 km2 the critical unit stream power was higher it is likely that it is due to the bedform s greater resistance this resistance would use up some of the energy that can cause movement and transport of bedload the amount of resistance of the bedform can be expressed as bedform shear stress determined by the relationship between grain shear stress that determines movement and transport of the bedload and the total shear stress this ratio varies between 0 4 and 0 5 in the medium sized rivers compared to 0 7 in the ourthe in headwater streams less than 20 km2 there is greater loss of energy due to bedform resistance 0 3 critical specific stream power is higher in this third type of river than in the other two

the characteristics of seismic signals produced by lahars and pyroclastic flows volcan de colima mexico abstract the general characteristics of seismic signals produced by pyroclastic flows generated by either the collapse of a lava dome or an eruptive column and lahars at volcan de colima mexico are discussed the paper concentrates on the 2004 2006 activity associated with and following the extrusion of andesitic block lava in october november 2004 it is shown that the duration of the broad band seismic records of pyroclastic flows lasts a few minutes while the duration of seismic records of lahars continues for tens of minutes or hours the spectra of seismic records produced by pyroclastic flows are characterized by lower peak frequencies around 3 4 hz than for lahars around 6 8 hz this difference in the frequency content together with the difference in the duration of seismic signals allows early diagnostic of the events in real time

alternating braidplain and lacustrine deposition in a strike slip setting the pennsylvanian boss point formation of the cumberland basin maritime canada abstract the boss point formation was deposited in the cumberland basin of southeastern new brunswick and northern nova scotia during the early pennsylvanian the basin is bounded to the northwest and south by strike slip faults active during the pennsylvanian the boss point comprises two main fades associations sandy braidplain bfa and muddy lacustrine lfa the abrupt alternation of which allows 16 depositional megacycles to be defined the lfa forms units 50 m thick that rest sharply but nonerosively on braidplain sandstone in some instances the contact can be traced for over 12 km perpendicular to paleoflow commonly dunes and ripples are preserved in relief beneath lacustrine mudstone implying rapid but very low energy flooding of t e braidplain the bfa comprises multistory channel sandbodies 90 m thick separated from the underlying lfa by sixth order surfaces with up to 15 m of erosional relief the erosion surfaces are commonly overlain by spectacular mudstone breccias and boulders that reflect deep erosion and rotational slumping into channels cut into and sometimes right through lacustrine deposits bfa packages are internally partitioned by fifth order erosion surfaces into individual channel fill units commonly about 5 m thick the bfa is dominated by trough cross bedded fine to lower medium sandstone conglomerate pebbly sandstone and ripple laminated and planar laminated sandstone are minor components rare mud filled abandoned channels suggest a mean width and de th of 160 m and 4 8 m respectively the dominance of trough cross bedding suggests relatively deep channels whose banks may have been maintained by a dense vegetation cover repeated channel avulsion and flood related erosion resulted in a rock record dominated by deep channel and flood stage deposits bartop facies were rarely preserved channel dimensions lateral consistency of facies paucity of muddy channel plugs and regionally consistent paleoflow suggest deposition on a braidplain the distribution of conglomerates indicates a local gravel source area in the caledonia highlands to the northwest but much of the sand is likely to have been sourced far to the southwest the boss point falls into two sharply disjunct paleoflow domains one directed to the north and northeast and the other to the east and southeast separated by the harvey hopewell fault zone the absence of an adequate watershed between the two domains strongly suggests tectonic juxtaposition involving several tens of kilometers of sinistral offset

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

flume study of the effect of relative depth on the incipient motion of coarse uniform sediments an experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0 3 m wide tilting flume the concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport the experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter a revised shields diagram relating critical stress grain reynolds number and depth to grain size ratio has been derived this has implications for the calculation of sediment transport rates design of stable alluvial channels and physical hydraulic modeling

paleohydraulics of chute and pool structures in a paleoproterozoic fluvial sandstone abstract chute and pool structures consist of an inclined surface leading into a pool where a hydraulic jump causes deposition of backset laminae structures fitting this description are present at the base of a very fine grained sandstone bed in the fluvial matinenda formation of northern ontario reconstruction of the paleohydraulic system indicates that a depth of approximately 1 cm and a velocity of approximately 65 cm s formed these bedforms during the initial stage of a flow with increasing depth and decreasing velocity this type of deposit should be most common at the base of sandy beds deposited as emergent bars are overtopped during increasing discharge events

defining conditions for bulking and debulking in lahars through measurements at semeru volcano east java we define the conditions under which bulking entrainment of sediment and pore water and debulking dilution and sedimentation occur in rain triggered volcanic floods lahars two observation sites were installed 510 m apart along the curah lengkong river 11 5 km southeast of semeru s summit this 30 m wide box valley with a gravel and lava base represents a real world flume analogy pore pressure sensors provided stage measurements a broad band seismograph gave insight into sediment content and frictional collisional behavior video cameras were used to measure surface velocities and direct bucket samples were taken eight rainfall induced lahars were recorded lasting 1 3 h with heights of 0 5 2 m peak velocities of 3 7 m s and discharges of 25 250 m3 s flows ranged from typical 40 wt sediment to coarse and dense hyperconcentrated flows 50 60 wt sediment multiple distinct flow packets occurred within the complex lahars and were used to determine internal changes between sites from the multiparameter data set at each site volumetric bulking and wave shortening due to portions of the lahar accelerating toward the flow front are identified initial debulking of lahars between sites may reflect drainage into the dry substrate estimates of discharge and volume at each site lead to the quantification of bulking and debulking by these actively flowing lahars along the channel reach from this we observe that bulking can be localized to certain parts of lahars resulting in intraevent increases in peak discharge that are greater than what would occur if bulking was evenly distributed throughout the flow such data are essential for the development of numerical descriptions and hazard models for mass flows

selective gravel entrainment and the empirical evaluation of flow competence the concept of flow competence is generally employed to evaluate velocities and bed stresses of river floods from the sizes of the largest sediment particles transported for the most part this evaluation has been empirical combining data from a number of separate flood events in different river systems those data are re examined and compared with empirical equations for the selective entrainment of gravel from deposits of mixed sizes it is found that the competence relationships trend counter to those obtained for selective entrainment indicating that the competence evaluations are affected by varying degrees of selective size entrainment individual data sets which have been employed to establish the flow competence relationships either show no trend on their own or yield a trend which runs counter to the competence equation instead being more compatible with the selective entrainment relationships in most instances the empirical competence equations greatly overestimate the hydraulics of flood flows and it is suggested that the better established selective entrainment equations be used for competence evaluations as well empirical equations are available for this purpose relating the dimensionless shields entrainment function or the bed shear stress to the diameter of the largest grain moved and to the median diameter of the deposit as a whole

the azores current system from a meridional section at 24 5 w 1 high spatial resolution hydrographic data including lowered acoustic doppler current profiler ladcp measurements were acquired along a meridional section at 24 5 w in october 2009 the data are analyzed in detail with the purpose of definitively defining and quantifying the zonal azores current system the azores current and azores countercurrent are delimited each extending meridionally for 110 km the azores current is located between 33 5 n and 34 5 n flanked to the north by the azores countercurrent 35 25 36 25 n vertically both currents reach the n 27 975 kg m 3 level 2000 m depth their mass transports ranging across thermocline as well as intermediate layers the azores current transports 13 9 sv 1 sv 106 m3 s 1 109 kg s 1 eastward with its maximum associated with the azores front 33 75 n the azores countercurrent flows below the surface transporting 5 5 sv westward this contributes to a net eastward flow of 8 4 sv across the section at intermediate layers the azores countercurrent transports mixed mediterranean water to the west and the azores current transports mixed sub arctic intermediate water to the east shipboard adcp and satellite derived geostrophic velocity are used to confirm the transports revealed by the hydrographic data

sedimentology of a sandur formed by multiple jokulhlaups kverkfjoll iceland abstract the kverkfjoll sandur in north iceland is the furthest upstream of a suite of fluvial landforms extending for 200 km along the jokulsa a fjollum river incision of the sandur exposes over 3 km of sedimentary sections up to 15 m in height a sandur wide well bedded succession of matrix rich cobble gravel and pebble granule gravel with individual beds 0 2 to 0 5 m thick indicates that the sandur is primarily the product of sandur wide sheet floods with sediment rich hyperconcentrated flows and also some debris flows and channelised turbulent flows this interpretation is evidenced by bedded hyperconcentrated flow deposits occurring as laterally extensive tabular depositional units that dominate the entire sandur reflecting the unconfined nature of the flow clast supported boulder gravel units interpreted as the product of macroturbulent flow occur in relatively narrow but deep channels the sedimentary succession is interpreted as the product of at least six volcanically generated catastrophic outburst floods jokulhlaups during the little ice age the sedimentology of these little ice age flood deposits on a small high gradient sandur contrasts strongly with the deposits of volcanically generated jokulhlaups on large low gradient coastal sandar and sandar associated with retreating glaciers which have been the basis for most previous models of jokulhlaup sedimentation

analysis of antidune migration direction 1 antidunes are bed forms characteristic of upper regime unidirectional flows contrary to bed forms developed in lower regime flows antidunes not only can propagate in the downstream direction but can propagate upstream or remain stationary in this work we analyze the stable hydraulic conditions that determine each of the three antidune movement possibilities and a new theory is developed for distinguishing the occurrence of each of these conditions the theory is developed from an energy balance over a symmetrical antidune for this the bernoulli equation is applied between the antidune crest and antidune trough and the pressure head is corrected to account for the centrifugal forces generated by curvilinear flow manipulation of the fundamental equations produces an antidune mobility dimensionless number fa dependent on the froude number the mean water depth and the antidune wavelength the critical value of the antidune mobility number fa 1 corresponds to the stationary condition while values higher and lower than the critical correspond to downstream and upstream propagation respectively theory predicts that for a given froude number downstream migrating antidunes are formed for higher water depth wavelength ratios than for upstream migrating antidunes likewise by introducing a restriction for the maximum stationary wave height above antidunes theory predicts that downstream migrating antidunes could attain steeper height wavelength ratios than upstream migrating antidunes comparison with published experimental data in literature largely showed agreement between theory and experimental observations

sediment charged flash floods on montserrat the influence of synchronous tephra fall and varying extent of vegetation damage on 20th may 2006 the soufriere hills volcano on the caribbean island of montserrat experienced a large dome collapse and intense rainfall generated flash floods the floods had very high loads of volcanic debris derived both from this and previous eruptions and can thus be classified as lahars the floods reached unusually high water levels and caused substantial geomorphic change in the belham valley detailed rainfall and geomorphological data coupled with the precise timing of events and yewitness accounts have facilitated an assessment of the relative importance of rainfall volume and intensity older volcanic debris pre and syn flood tephra fall and the extent of pre flood vegetation damage for the behavior of this and subsequent sediment laden floods in this setting the change in runoff behavior was controlled by preexisting vegetation damage and synchronous tephra fall and this was critically important in controlling the impact of these flash floods although rainfall intensity and volume have some control on flood occurrence they are not the critical control on flash flood impact on the geomorphology in the belham valley a significant conclusion of this study is that the extreme nature of the flash floods was not caused by extreme rainfall as is commonly believed to be the primary cause of flash floods but rather it was the result of changed runoff behaviour caused by the widespread syn flood tephra deposition and importantly the widespread vegetation damage by volcanic associated acid rain in the preceding weeks

alkalic ocean island basalt type and calc alkalic volcanism in the mexican volcanic belt a case for plume related magmatism and propagating rifting at an active margin the mexican volcanic belt has been traditionally regarded as a classic case of subduction related calc alkalic volcanism however a series of geologic geophysical and petrological arguments makes this simple relationship doubtful a seismic gap beneath the belt a large scale mantle anomaly a graben triple junction domain and the presence of volumetrically important oceanic island basalt oib volcanism throughout the belt suggest a more complex tectonic scenario involving plume and subduction related processes we here propose a model involving the development of a propagating rift opening from west to east in response to plume activity the process started in miocene time within the western sector of the belt guadalajara and gave rise to a graben triple junction and oib type and calc alkalic volcanism extension and volcanism proceeded to the east giving rise to progressively younger ages for the initiation of oib type volcanism 1 miocene in the west e g guadalajara 2 pliocene in the central zone e g michoacan guanajuato and 3 quaternary farther east e g chichinautzin geochemical evidence suggests that part of the modern calc alkalic volcanism e g chichinautzin may be derived from magma mixing between the oib mafic magmas and silicic crust derived magmas however we do not preclude some influence of the subducting slab in the generation of other e g jorullo calc alkalic volcanic rocks our model suggests a currently unrooted upper plume attached to the subcontinental lithosphere which defines a hot zone beneath the mexican volcanic belt

gravel antidunes in the tropical burdekin river queensland australia the geological record is punctuated by the deposits of extreme event phenomena the identification and interpretation of which are hindered by a lack of data on contemporary examples it is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle transporting high velocity currents and therefore their characteristics are poorly documented the deposits of such flows are exposed however in the dry bed of the burdekin river queensland australia following tropical cyclone induced floods long wave length up to 19 m gravel antidunes develop during short days high discharge flows in the upper burdekin river maximum recorded discharge near the study reach over 25 600 m s in february 1927 flood water levels fall quickly metres in a day and flow is diverted away from raised areas of the river bed into subchannels exposing many of the high stage bedforms with little reworking by falling stage currents gravel bedforms were observed on the dry river bed after the moderate flows of february 1994 max 7700 m s and january 1996 max 3200 m s the bedforms had wave lengths in the range 8 19 m amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak discharge flow direction the gravel fabric and size sorting change systematically up the stoss and down the lee faces the antidune deposits form erosive based lenses of sandy gravel with low angle downstream dipping lamination and generally steep upstream dipping a b planes the internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses the erosive based lenses of low angle cross bedded gravel with steep upstream dipping a b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes the antidune gravel lenses are associated with thick to 1 m high angle cross bed sets ancient antidune gravel lenses may be diagnostic of episodic high discharge conditions and particularly when they are associated with high angle cross bedded gravelly sand they may be useful for palaeoenvironmental interpretation

lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows from cyclic steps to humpback dunes abstract the preservation of bedforms related to supercritical flows and hydraulic jumps is commonly considered to be rare in the geologic record although these bedforms are known from a variety of depositional environments this field based study presents a detailed analysis of the sedimentary facies and stacking pattern of deposits of cyclic steps chutes and pools antidunes and humpback dunes from three dimensional outcrops the well exposed middle pleistocene successions from northern germany comprise glacilacustrine ice contact subaqueous fan and glacial lake outburst flood deposits the studied successions give new insights into the depositional architecture of bedforms related to supercritical flows and may serve as an analogue for other high energy depositional environments such as fluvial settings coarse grained deltas or turbidite systems deposits of cyclic steps occur within the glacial lake outburst flood succession and are characterised by lenticular scours infilled by gently to steeply dipping backsets cyclic steps formed due to acceleration and flow thinning when the glacial lake outburst flood spilled over a push moraine ridge these bedforms are commonly laterally and vertically truncated and alternate with deposits of chutes and pools and antidunes the subaqueous fan successions are dominated by laterally extensive sinusoidal waveforms which are interpreted as deposits of aggrading stationary antidunes which require quasi steady flows at the lower limit of the supercritical flow stage and high rates of sedimentation humpback dunes are characterised by downflow divergent cross stratification displaying differentiation into topsets foresets and bottomsets and are interpreted as deposited at the transition from subcritical to supercritical flow conditions or vice versa gradual lateral and vertical transitions between humpback dunes and antidune deposits are very common the absence of planar parallel stratification in all studied successions suggests that the formation of these bedforms is suppressed in flows characterised by hydraulic jumps under highly aggradational conditions the large scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the initial supercritical flows which was strongly affected by the occurrence of hydraulic jumps small scale facies changes and the formation of individual bedforms are interpreted as controlled by fluctuating discharge bed topography and pulsating unstable flows

ice volcano interactions during the 2010 eyjafjallajokull eruption as revealed by airborne imaging radar 1 during the eruption of the ice covered eyjafjallajokull volcano a series of images from an airborne synthetic aperture radar sar were obtained by the icelandic coast guard cloud obscured the summit from view during the first three days of the eruption making the weather independent sar a valuable monitoring resource radar images revealed the development of ice cauldrons in a 200 m thick ice cover within the summit caldera as well as the formation of cauldrons to the immediate south of the caldera additionally radar images were used to document the subglacial and supraglacial passage of floodwater to the north and south of the eruption site the eruption breached the ice surface about four hours after its onset at about 01 30 utc on 14 april 2010 the first sar images obtained between 08 55 and 10 42 utc show signs of limited supraglacial drainage from the eruption site floodwater began to drain from the ice cap almost 5 5 h after the beginning of the eruption implying storage of meltwater at the eruption site due to initially constricted subglacial drainage from the caldera heat transfer rates from magma to ice during early stages of cauldron formation were about 1 mw m 2 in the radial direction and about 4 mw m 2 vertically meltwater release was characterized by accumulation and drainage with most of the volcanic material in the ice cauldrons being drained in hyperconcentrated floods after the third day of the eruption meltwater generation at the eruption site diminished due to an insulating lag of tephra

medano creek colorado a model for upper flow regime fluvial deposition abstract medano creek in colorado provides a unique opportunity to study upper flow regime deposition it is a steep aggrading fluvial system in which antidunes are the dominant bedform medano creek is a straight single channel stream where flow is largely confined to a sequence of upstream migrating topographic steps pools and riffles a stable channel geometry antidunes and upperflow regime plane beds are arranged in an organized pattern on the surface of pool and riffle sequences as a response to spatially varied flow conditions in the creek antidunes form sedimentary structures consisting of low angle backset and foreset beds and subhorizontal laminae grouped into lenses laminae in a lens are often subparallel to the lens bounding surfaces on the downcurrent end and are truncated y the upper bounding surface on the upcurrent end upper flow regime plane bed and rhomboid ripples produce planar laminae through lateral shifting of the active channel and upstream migration of pools and riffles medano creek produces a facies sequence consisting of alternating 10 40 cm thick layers of planar laminae and low angle cross stratified lenticular bedding

formative conditions and sedimentary structures of sandy 3d antidunes an application of the gravel step pool model to fine grained sand in an experimental flume antidunes and their sedimentary structures can be useful in reconstructing paleo hydraulic conditions especially for large discharge events however three dimensional 3d antidunes in sand sized sediments have not yet been studied extensively as compared to either two dimensional 2d antidunes or antidunes in gravel sized sediments in this study we estimated formative conditions of gravel step pool morphologies and applied them to the formation of 3d antidunes over a sand bed formative conditions are expressed in terms of a relationship between the water discharge per unit width and the bed slope flume experiments demonstrated that 3d mound like antidune configurations and their associated internal sedimentary structures could be preserved internal sedimentary structures were characterized by shallow lens like structures whose bases were erosional although gently dipping concave upward lamination was dominant convex upward lamination was occasionally observed the dimensions of lenticular lamina sets can be used to estimate antidune geometry thus if 3d antidunes can be interpreted in the stratigraphic record it is possible to estimate the paleo hydraulic parameters such as water discharge and bed slope more precisely than previously copyright 2010 john wiley sons ltd

indirect velocity measurement from standing waves in rockbed rivers direct measurement of velocities and discharge in turbulent rockbed rivers is dangerous and often impracticable float measurements of surface velocity in sediment free rockbed reaches of rivers are used to test the equation due to kennedy that the mean velocity of a critical wave train is given by v 1 2495 where is the wavelength of the standing waves field data collected for wavelengths between 0 5 m and 7 m estimates surface velocity as about 2 significantly higher than the predicted mean velocity it is concluded that the kennedy equation successfully represents the mean velocity in rockbed rivers which are essentially free from sediment and that measuring the wavelengths of standing wave trains is an accurate and economical method of measuring mean velocity in the field from estimated wave train width and computed critical depth minimum discharge can be estimated for the section

grain fabric of a laboratory antidune the grain fabric of a laboratory antidune composed of coarse to medium grained sand shows a high angle upcurrent imbrication both on the upstream and downstream sides such a grain fabric differs from a normal dune or a megaripple where grain alignment displays an upcurrent imbrication relative to the depositional slope in the foreset lamina and or near parallelism with the slope surface in the toe set lamina recognition of antidunes in ancient rocks is difficult but detailed examination of grain fabric of cross stratification makes it possible to distinguish antidunes or backset bedding from normal dunes or megaripples

bedforms record the flow conditions of the 2011 tohoku oki tsunami on the sendai plain northeast japan abstract this article describes the bedforms formed by the 2011 tohoku oki tsunami on the southern sendai plain ne japan integrated analyses of satellite images field survey data and x ray radiographs revealed the natural shapes of these bedforms the dominant bedforms are dunes and antidunes about 20 cm thick high and relatively flat because of their meter scale wavelengths their morphologies reflect mainly the tsunami run up flow on the flat lowland and the return flow in a narrow steep valley flow depth changed at sites where the level of the ground surface changed along the tsunami flow path causing the flow velocity to fluctuate flow velocity decreased in depressed deep water sections and increased in raised shallow water sections the types and positions of the tsunami deposit bedforms were thus strongly controlled by local ground surface roughness such as raised roadbeds and levees as a result the observed thickness distribution of the tsunami deposits fluctuates it is important to consider the possible existence of bedforms in paleotsunami surveys especially for accurate determination of the thickness distribution of the tsunami deposits because thickness data are essential information for reconstruction of the tsunami inundation limit as well as its flow speed

rainfall induced lahars in the belham valley montserrat west indies rain falling on loose volcanic debris over the soufriere hills volcano montserrat generates hazardous floods in the belham valley these rainfall induced lahars vary greatly in discharge and sediment concentration in space and time they differ from examples documented on other volcanoes in that 1 the eruption has been continuing since july 1995 generating repeated pulses of excess sediment 2 rainfall is the only significant trigger 3 the system is small with short distance to the sea and relatively low altitude at the catchment top repeat mapping and comparison with pre eruption data demonstrate significant geomorphological change with c 120 m shoreline progradation and c 0 4 m a 1 mean aggradation rate in the middle to lower valley the nature of the hazard and area of risk have changed as the valley has aggraded the channel widened and the runoff efficiency increased as a result of rilling and vegetation removal lahars in the belham valley correlate with days when 10 mm rain fell in 24 h with more events triggered in the late rainy season the flows are mainly newtonian but one non newtonian flow event has been demonstrated and is described in detail 20 march 2000 this flow is explained by direct volcanic ash input to the runoff

the fascination of a shallow water theory for the formation of megaflood scale dunes and antidunes abstract exceptional megaflood scale bedforms on earth are commonly associated with the catastrophic draining of glacial lakes in the late pleistocene the widest studied events have been the missoula and altai floods with 300 700 m flow depth 1 20 m bedform height and 10 300 m wavelength nowadays the saint venant equations have succeeded at simulating the catastrophic glacial lake drainage process numerically but we still lack a depth averaged morphodynamic theory able to predict the growth of dunes and antidunes the disparity of spatial scales in megafloods prevents the use of non depth averaged rotational flow equations motivating the present shallow water theory for the formation of megaflood scale bedforms we adopt a non equilibrium sediment transport equation rooted in einstein s pioneering work here we prove that the bed instability triggers to form dunes and antidunes simply by lagging the entrainment term for sediment mass conservation or the bottom shear stress with respect to the depth averaged flow velocity we formalise this result using a linear stability theory that captures the existence regions of dune and antidune in addition to the roll wave instability furthermore in the spirit of kennedy annu rev fluid mech vol 1 1969 pp 147 168 we derive a closed form solution of growth rate and wave speed of the bedform the nondimensional groups controlling the linear instabilities are the froude number r the shields parameter sh and the grain roughness relative to flow depth d subsequently we simulate the drainage of the largest missoula flood numerically to explain the formation of giant antidunes in the camas prairie montana us during the late stage of the megaflood also considered are large fields of gravel dunes in the kuray chuja lake basin altai mountains siberia the simulated hydraulic conditions over bedforms in both basins yield values of the nondimensional parameters that lie in the theoretical region of dunes and antidunes according to the proposed theory and in situ measurements in sandy rivers and flume experiments