effects of time reversing array deformation in an ocean wave guide active acoustic time reversal is a technique for focusing sounds recorded in complex unknown environments back to their remote point s of origin it can be accomplished with a transducer array a time reversing array tra that sends and receives sound nearly all prior work on tra performance has involved stationary arrays this letter describes how random array deformation influences tra retrofocusing in shallow ocean environments for harmonic signals randomly drifting array elements degrade tra performance by 20 when the average horizontal wavenumber times the root mean square horizontal element displacement approaches 0 5 tra focusing should be less sensitive to vertical element drift

the significance of channel recharge rates for estimating debris flow magnitude and frequency the quantification of debris flow hazard requires estimates of debris flow frequency and magnitude several methods have been proposed to determine the probable volume of future debris flows from a given basin but most have neglected to account for debris recharge rates over time which may lead to underestimation of debris flow volumes in basins with rare debris flows this paper deals with the determination of debris recharge rates in debris flow channels based on knowledge of debris storage and the elapsed time since the last debris flow data are obtained from coastal british columbia and a relation is obtained across a sample of basins with similar terrain and climatic conditions for rennell sound on the west coast of the queen charlotte islands the power law relation for area normalized recharge rate rt versus elapsed time te was rt 0 23te 0 58 with an explained variance of 75 per cent a difference in recharge rates may exist between creeks in logged and unlogged forested terrain the power function for undisturbed terrain was rt 0 20te 0 49 while the function for logged areas was rt 0 30te 0 77 this result suggests that for the same elapsed time since the last debris flow clearcut gullies tend to recharge at a slower rate than creeks in old growth forest this finding requires verification particularly for longer elapsed times since debris flow but would have important implications for forest resource management in steep coastal terrain this study demonstrates that commonly used encounter probability equations are inappropriate for recharge limited debris flow channels copyright 2005 john wiley sons ltd

source localization with horizontal arrays in shallow water spatial sampling and effective aperture using vertical line arrays matched field processing mfp range depth localization performance in shallow water is often limited by poor sampling of bottom interacting acoustic modes for off broadside sources a horizontal array may offer improved mode sampling and localization performance the relative performance of a horizontal array in mode sampling and localization defines its effective vertical aperture the mfp performance of several horizontal line array apertures is investigated and compared to a full water column vertical array in a simulated range independent shallow water environment it is demonstrated that a horizontal array of sufficient length can equal or exceed the water column vertical array in range depth localization performance expressions are also derived for mode decomposition factors that permit the direct comparison of horizontal and vertical array mode sampling performance horizontal array effective aperture can then be computed without the need of extensive mfp test runs it is shown that the horizontal array effective apertures predicted by these decomposition factors correspond well to actual mfp localization performance for the scenarios considered in this paper thus the decomposition factors are a useful tool for array system design for shallow water mfp localization

a facies model for a quaternary andesitic composite volcano ruapehu new zealand ruapehu composite volcano is a dynamic volcanic sedimentary system characterised by high accumulation rates and by rapid lateral and vertical change in facies four major cone building episodes have occurred over 250 ka from a variety of summit flank and satellite vents eruptive styles include subplinian strombolian phreatomagmatic vulcanian and dome related explosive eruptions and extrusion of lava flows and domes the volcano can be divided into two parts a composite cone of volume 110 km3 surrounded by an equally voluminous ring plain complementary portions of ruapehu s history are preserved in cone forming and ring plain environments cone forming sequences are dominated by sheet and autobrecciated lava flows which seldom reach the ring plain the ring plain is built predominantly from the products of explosive volcanism both the distal primary pyroclastic deposits and the reworked material eroded from the cone much of the material entering the ring plain is transported by lahars either generated directly by eruptions or triggered by the high intensity rain storms which characterise the region ring plain detritus is reworked rapidly by concentrated and hyperconcentrated streams in pulses of rapid aggradation immediately following eruptions and more gradually in the longer intervals between eruptions

explaining the extreme mobility of volcanic ice slurry flows ruapehu volcano new zealand the near invisibility of ice slurry flows in the geological record belies their signifi cant hazard at snow capped volcanoes these four phase flows exhibit extreme rates of volumetric bulking and unusually high mobility mechanisms of their motion are clarified through two examples generated on 25 september 2007 at mount ruapehu new zealand brief explosions through crater lake ejected 5700 m 3 of acidic water that entrained 60 times this volume of snow as it traveled over a snow covered glacier the resulting ice slurry traveled up to 7 7 km height length h l ratio of 0 16 a cogenerated second flow took a more tortuous initial path before riding over the already frozen deposits of the first unit and beyond h l 0 13 for the first time downstream evolution of the kinematic properties of propagating ice slurry fronts could be characterized as well as the longitudinal variation of the physical properties of their resulting deposits the chemistry and composition of the deposits show that during flow vertical percolation of water through the porous ice particle water air mixture generated a basal zone of high internal pore pressure this effect was particularly strong when a thick high density flow front formed which raced ahead of the tail to control runout and consequent hazard

incipient granular mass flows at the base of sediment laden floods and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands extraordinary floods commonly produce thick deposits as a result of the entrainment and transport often at high concentrations of large volumes of clastic detritus this paper describes the sedimentology of a stratified bouldery sand deposit generated by a 20 km3 breakout flood from intracaldera lake taupo following the 1 8 ka taupo eruption the proximal deposit is characterised by a strongly bimodal grain size distribution with downstream fining boulders enclosed in a poorly sorted and weakly bedded coarse sand to gravel matrix this combination of characteristics is inferred to reflect simultaneous deposition of small and large particles by two distinct mechanisms i accretion of the sandy matrix from an incipient basal granular mass flow as the flood lost capacity and ii progressive emplacement of the boulders as the flood lost the competence to roll them along the channel floor the first mechanism is a product of a nonlinear particle concentration gradient in the sediment laden flow which caused it to segregate into a basal incipient granular mass flow that formed the transient depositional system indicated by the lithofacies the second mechanism is a function of the superjacent turbulent turbid flow that acted as the primary transporting system this pairing of transport and depositional systems is likely to characterise many sediment laden aqueous flows with clay mineral concentrations too low to develop non newtonian rheologies such as jokulhlaups and lahars that entrain unweathered pyroclastic material qualitative and quantitative differences in the way bedload is transported in the presence of this basal granular mass flow as compared to simple water make impossible accurate estimates of flow competence based on shields entrainment functions for beds of both uniform and mixed grain size distribution

a model for the analysis of rapid landslide motion across three dimensional terrain a new numerical model for the dynamic analysis of rapid flow slides debris flows and avalanches has been developed the model is an extension of an earlier algorithm and is implemented using a nu

spatial resolution of time reversal arrays in shallow water a series of time reversal experiments was performed in shallow water including a range dependent slope environment time reversal arrays implemented with center frequencies of 445 and 3500 hz achieved sharp focal regions up to ranges of 30 and 13 km respectively in 110 130 m shallow water in this paper resolution expressions are derived using an image method to describe the focal sizes achieved with time reversal arrays in various ocean environments analysis for the measured data indicates that the focal size approaches the diffraction limit of an array for given waveguide conditions i e waveguide geometry and attenuation the measured focal size has implications for the maximum achievable resolution of linear matched field processing which is a computational implementation of the time reversal process

pleistocene holocene transition in benelux in benelux belgium the netherlands and luxembourg considerable progress in late glacial and early postglacial archaeological and environmental research has been made during the last decade a substantial part of this paper discusses new evidence on the chronology of the industries which occupied the southern part of benelux dating the occupations from the sandy north is still problematic long after the glacial maximum during which the area remained uninhabited the south of benelux was visited by magdalenian hunter gatherers while hamburgian settlements were present in the north the significance of the continental creswellian is debated during the allerod interstadial federmesser groups occupied most of the area abandoning it with the onset of the younger dryas the presence of the ahrensburgian during that stadial may have been punctuated new elements were introduced and were maintained during the development of the preboreal early mesolithic which eventually colonised the entire region throughout this sequence changes in material culture and related subsistence activities are evident in view of the major problems concerning the quality of the chrono ecological data this paper can only suggest provisional explanations in terms of human reactions to changing environmental conditions

controls on accumulation of a volcaniclastic fan ruapehu composite volcano new zealand the whangaehu fan is the youngest sedimentary component on the eastern ring plain surrounding ruapehu volcano fan history comprises constructional 830 200 years bp and dissectional 200 years bp phases the constructional phase includes four aggradational periods associated with both syneruptive and inter eruptive behavior all four aggradational periods began when deposition by large lahars changed flow conditions on the fan from channelized to unchannelized subsequent behavior was a function of the rate of sediment influx to the fan the rate of sediment influx in turn was controlled by frequency and magnitude of volcanic eruptions short term climate change and the amount of sediment stored on the volcano flanks fanwide aggradation occurred when rates of sediment influx and deposition on the fan were high enough to maintain unchannelized flow conditions on the fan surface maintenance of an undissected surface required sedimentation from frequent and large lahars that prevented major dissection between events these conditions were best met during major eruptive episodes when high frequency and magnitude eruptions blanketed the volcano flanks with tephra and rates of lahar initiation were high during major eruptive episodes volcanism is the primary control on sedimentation climatic variations do not influence sediment accumulation local aggradation occurred when lahars were too small to maintain unchannelized flow across the entire fan in this case only the major channel system received much sediment following the deposition from the initial lahar this localized aggradation occurred if 1 the sediment reservoir on the flank was large enough for floods to bulk into debris flows and 2 sedimentation events were frequent enough to maintain sediment supply to only some parts of the fan these conditions were met during both minor eruptive and inter eruptive episodes in both cases a large sediment reservoir remained on the volcano flanks from previous major eruptive intervals periods of increased storm activity produced floods that bulked to relatively small debris flows when the sediment reservoir was depleted the fan entered the present dissectional phase syneruptive and noneruptive lahars are mostly channelized and sediment bypasses the fan fan deposits are rapidly reworked this is the present case at ruapehu even though the volcano is in a minor eruptive episode and the climate favors generation of intense storm floods

an empirical method for estimating travel times for wet volcanic mass flows travel times for wet volcanic mass flows debris avalanches and lahars can be forecast as a function of distance from source when the approximate flow rate peak discharge near the source can be estimated beforehand the near source flow rate is primarily a function of initial flow volume which should be possible to estimate to an order of magnitude on the basis of geologic geomorphic and hydrologic factors at a particular volcano least squares best fits to plots of flow front travel time as a function of distance from source provide predictive second degree polynomial equations with high coefficients of determination for four broad size classes of flow based on near source flow rate extremely large flows 1 000 000 m3 s very large flows 10 000 1 000 000 m3 s large flows 1000 10 000 m3 s and moderate flows 100 1000 m3 s a strong nonlinear correlation that exists between initial total flow volume and flow rate for instantaneously generated debris flows can be used to estimate near source flow rates in advance differences in geomorphic controlling factors among different flows in the data sets have relatively little effect on the strong nonlinear correlations between travel time and distance from source differences in flow type may be important especially for extremely large flows but this could not be evaluated here at a given distance away from a volcano travel times can vary by approximately an order of magnitude depending on flow rate the method can provide emergency management officials a means for estimating time windows for evacuation of communities located in hazard zones downstream from potentially hazardous volcanoes

unusual snow slurry lahars from ruapehu volcano new zealand september 1995 the first lahars of the 1995 ruapehu eruptive sequence were generated by explosively ejected crater lake water sediment and juvenile material which incorporated snow and ice to form snow slurry lahars on september 18 small amounts of crater lake water were ejected which mobilized large amounts of snow this was because unstable seasonal snow pack was present in the lahar flow path five days later a larger volume of water was ejected but proportionally less snow was mobilized beyond the volcanic cone the initially homogenized snow slurries segregated into watery flows with a floating snow slurry cap frozen deposits were emplaced as far as 45 km from the source and 1840 m lower in altitude more than 50 of the sediment within the flow was derived from the initial eruption the remainder was entrained before it reached 9 5 km beyond 9 5 km from the source the flows progressively lost their gravel and sand

airborne lidar in support of geomorphological and hydraulic modelling this paper considers the application of airborne laser altimetry lidar to the provision of elevation data at accuracies and spatial densities commensurate with the current generation of high resolution hydraulic models three sets of issues are addressed with reference to a telemac 2d model of a morphologically complex estuary in eastern england first the quality of airborne lidar data is assessed via multiscale calibration against surveyed sections and supplementary control points second image processing techniques are used for i identification of multiple regions of interest within large lidar mosaics ii subregional infilling of voids left by data drop outs iii filtering and subsampling to match topographic information content with model resolution and the density of the computational mesh third the implications of improved terrain data are considered with reference to the estimation of elevations and potential tidal volumes for a number of discrete flood compartments and the modelling of hypothetical inundation scenarios after minor offset correction to ensure registration with local benchmarks the quality of lidar elevation data within a 12 km x 4 kin coverage is found to be consistent with the published specification 0 10 to 0 15 m image processing tools provide an efficient means of managing information content and interfacing this with the gis based functionality of the preprocessing software used in conjunction with the hydraulic model the accuracy and spatial resolution of the lidar data allow the identification of subtle but important topographic variations between adjacent flood compartments in many cases differences in model results obtained using these data relative to previously estimated average flood compartment elevations are small importantly however lidar provides topographic information at an accuracy and resolution close to the present limits of model representation reliable representation of form allows the modeller to concentrate on the physical aspects of model parameterization whilst minimizing the conflation of parameter effects with those of poorly constrained geometric representation copyright c 2003 john wiley sons ltd

volcanic hazard assessment for ruapehu composite volcano taupo volcanic zone new zealand ruapehu is a very active andesitic composite volcano which has erupted five times in the past 10 years historical events have included phreatomagmatic eruptions through a hot crater lake and two dome building episodes ski field facilities road and rail bridges alpine huts and portions of a major hydroelectrical power scheme have been damaged or destroyed by these eruptions destruction of a rail bridge by a lahar in 1953 caused the loss of 151 lives other potential hazards with holocene analogues include strombolian and sub plinian explosive eruptions lava extrusion from summit or flank vents and collapse of portions of the volcano the greatest hazards would result from renewed phreatomagmatic activity in crater lake or collapse of its weak southeastern wall three types of hazard zones can be defined for the phreatomagmatic events inner zones of extreme risk from ballistic blocks and surges outer zones of disruption to services from fall deposits and zones of risk from lahars which consist of tongues down major river valleys ruapehu is prone to destructive lahars because of the presence of 107 m3 of hot acid water in crater lake and because of the surrounding summit glaciers and ice fields the greatest risks at ruapehu are to thousands of skiers on the ski field which crosses a northern lahar path three early warning schemes have been established to deal with the lahar problems collapse of the southeastern confining wall would release much of the lake into an eastern lahar path causing widespread damage this is a long term risk which could only be mitigated by drainage of the lake

on the formation of eruption columns following explosive mixing of magma and surface water when magma vents into the sea or a crater lake the ensuing magma water interaction can affect the style of eruption dramatically if the mass of surface water incorporated into the erupting material is small 108 kg s the height progressively decreases with surface water content this occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air the transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of taal in 1965 and miyake jima in 1983 and for the changes in the eruptive activity at surtsey in 1963 1964 as the access of seawater to the vent became more restricted we also present calculations which suggest that the accretionary lapilli which are often found in wet flow deposits may result from condensation of vapor in both the cold wet collapsing fountains and in the flows themselves

dynamic modelling of entrainment in rapid landslides entrainment of path material is an important feature of many rapid landslides the associated increases in volume and changes in flow character can significantly influence mobility a simple material entrainment algorithm based on the assumption of natural exponential growth with displacement has been incorporated into a new computer model designed to simulate rapid landslide motion across 3d terrain the user controls the growth rate and can also implement a change in rheology at the onset of entrainment a hypothetical example is used to demonstrate the influence of the mass and momentum transfer assumptions a back analysis of the 1999 nomash river landslide is included to show that the simple model is capable of simulating the bulk characteristics of a complex event involving substantial entrainment key words landslides debris flows rock avalanches entrainment erosion dynamic modelling

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

impacts of the 1945 and 1995 1996 ruapehu eruptions new zealand an example of increasing societal vulnerability over the past 50 yr the risk to society from volcanic eruptions has increased sharply due to an increased population more developed and diversified economies and a more technologically advanced infrastructure this fact is demonstrated vividly by the impacts from the two largest eruptions of the twentieth century from the cone volcano of ruapehu which suggest that the vulnerability of key sectors in new zealand society has increased by one to two orders of magnitude over this period both the 1945 and 1995 1996 eruptions included explosive phases that dispersed ash over a wide area of the north island for a period of several months individual ash falls were only a few millimeters thick in communities within 150 km of the volcano and only trace amounts were found in communities farther away the 1995 1996 eruption caused similar physical effects to the 1945 eruption but had considerably greater social and economic impacts the greatest contribution to the cost of the 1995 1996 eruption estimated in excess of 130 million new zealand dollars was the impact on the alpine tourist industry in the central north island essentially nonexistent in 1945 other significant impacts were felt by the rapidly growing aviation and electricity generation sectors the cost of any future eruption of the same magnitude is likely to grow as the vulnerability of our society increases at a rapid rate

depositional record of historic lahars in the upper whangaehu valley mt ruapehu new zealand implications for trigger mechanisms flow dynamics and lahar hazards mt ruapehu in the central north island of new zealand is one of the most lahar prone volcanoes in the world since historic observations began in 1861 ad more than 50 individual lahars have been recorded in the whangaehu valley alone the natural outlet to the summit crater lake these lahars have been triggered by a variety of mechanisms including explosive eruptions that displaced crater lake water over the outlet or ejected it onto the snow clad summit area of the volcano rain remobilisation of tephra deposits on steep slopes displacement over the outlet as a result of syn eruptive changes in lake bathymetry and lake break outs from crater lake following impoundment of excess water behind temporary barriers of tephra and or ice emplaced over the outlet however only 9 lahar deposits can be distinguished in the upper whangaehu valley on sedimentological stratigraphic geomorphic and petrological grounds and these are skewed towards either the largest or the most recent flows in some cases magnitude can be reconstructed from deposit geometry with the largest lahars producing the highest level terraces the coarsest deposits and crossing drainage divides into normally inactive channels this under representation of historic events reflects the low preservation potential of unconsolidated deposits in a steep alpine environment and the overprinting and recycling effect of large magnitude lahars that rework material down to bedrock and effectively reset the stratigraphic record development of magnitude frequency relationships for ruapehu lahars therefore requires the identification of lahar deposits in proximal medial and distal settings in order to ensure that the full range of events is represented

changes in mid and far field human landscape use following the laacher see eruption c 13 000 bp abstract around 13 000 years ago the laacher see volcano east eifel volcanic field rhenish shield erupted cataclysmically the thick tephra blanket in the eruption s near field covered and thereby preserved a large number of archaeological sites ranging from very small to some of the largest and richest sites known from the late glacial in this proximal region there is a striking contrast in the number of sites prior to the eruption and the almost complete lack of sites dating to the period after the event i e to greenland interstadial 1a the preservative function of the tephra cover itself may explain this near field pattern but it is shown here that similar stratigraphic observations can also be made in the laacher see eruption s mid field towards the west and in particular to the north east the aim of this paper is to review the stratigraphic relations of known archaeological sites outside the near field of the laacher see event with a particular focus on the german federal state of hesse and adjacent areas of lower saxony and the thuringian basin here a number of archaeological sites are known where strata of laacher see tephra cap human occupation most often these sites are not reoccupied until much later in prehistory suggesting that the eruption may have led either to a complete or a near complete abandonment of the affected regions or at the very least to a major reorganisation of landscape use

volcanic mixed avalanches a distinct eruption triggered mass flow process at snow clad volcanoes a generally unrecognized type of pyroclastic deposit was produced by rapid avalanches of intimately mixed snow and hot pyroclastic debris during eruptions at mount st helens nevado del ruiz and redoubt volcano between 1982 and 1989 these mixed avalanches traveled as far as 14 km at velocities up to 27 m s involved as much as 10 7 m 3 of rock and ice and left unmelted deposits of single flow units as thick as 5 m during flow downslope heat transfer from hot rocks to snow produced meltwater that partially saturated the mixtures apparently giving these mixed avalanches mobilities equal to or greater than those of dry debris avalanches of similar volume the avalanches studied for this report began as snow avalanches triggered during explosive phases of the eruptions by ballistic bombardment of snow slopes by hot pyroclastic debris erosion by the avalanches incorporated additional snow fragments of glacier ice and other rock debris mixed avalanche deposits were massive very poorly sorted and ungraded to inversely graded prior to melting although rock debris composed as much as 70 wt of the frozen samples the bulk volumetric content of rock debris was only 9 to 36 snow and ice composed between 36 and 72 of the frozen samples with void space making up the remainder of the volume deformation and consolidation after melting of the ice content reduced the deposits to loose porous layers that were only a fraction of their original thickness and that later gave little hint of their origin after melting and desiccation the deposits are highly susceptible to erosion and unlikely to be well preserved in the stratigraphic record

hydrodynamic models of boulder berm deposition the hydrodynamic environment associated with boulder berm deposition at flow expansions during high magnitude flood is considered with respect to the plan geometry of surveyed berms hydraulic computer simulation of a prototype channel expansion and froude scaled physical hydraulic models are used to demonstrate the processes of berm formation all the geometric features observed in the field and temporal scales of berm formation can be reproduced in the laboratory and compared in principle with hydraulic energy diffusers used in engineering practice the experiments indicate that berms form most readily at low froude numbers f 1 5 owing to the deposition of individual bedload clasts and the shearing of transverse bars in the shear flow zone at the edge of the flow separation zone the resultant accretionary berms minimize energy losses and ensure that high flows are transmitted efficiently through channel transitions berm formation may occur with either steady or unsteady supercritical flows f 2 0 through a variety of channel expansions where initial flow already maintains a minimal backwater effect it would appear that formation of berms is not associated with initial flow into dry expansions where no backwater depth is maintained in all cases stable berm formation occurs extremely rapidly and the depositional process is not sensibly affected by steady or unsteady sediment feed rates

sedimentology and flow behavior of a rain triggered lahar mangatoetoenui stream ruapehu volcano new zealand on october 28 1995 heavy rain triggered failure of approximately 3 3 10 5 m 3 of fall deposits on the mangatoetoenui glacier and generated a lahar event that flowed east northeast of ruapehu volcano down the mangatoetoenui stream and eventually entered the tongariro river 19 km downstream the lahar left multiple stacked deposits of at least three major flow surges or pulses which traveled down slope at calculated velocities of as much as 27 m s 1 and calculated maximum peak discharges of as much as 2900 m 3 s 1 the source area of the mangatoetoenui lahar is delineated by a thin furrowed lag of unsorted ash and lapilli margined by steep headwall scarps marking the edges of undisturbed fall deposits failure of fall deposits in the source area triggered debris flows down a north and south branch of the mangatoetoenui stream these two debris flows coalesced and ponded then flowed across seasonal snowpack covering the stream in the proximal zone to 5 km downstream of the source area the deposits of the individual constituent debris flows cannot be distinguished however in the medial zone between 5 and 16 km downstream of the source area and beyond the limit of the snowpack and the junction with perennial streamflow the depositional record of this lahar shows clear evidence for downstream transformation of individual peak flows from debris flow to hyperconcentrated flow transformation began at 5 km from source due to dilution 1 by each peak flow incorporating normal streamflow in the perennial mangatoetoenui stream and tributaries and 2 through deposition of part of each peak flow9s sediment load the hyperconcentrated flow deposits frequently may be subdivided into multiple units which is evidence that the lahar flowed as a series of surges or waves we consider that these were caused by episodic but contemporaneous inputs from a divergent flow from a single source area or by ponding and episodic release of debris within the channel estimated froude numbers for the mangatoetoenui lahar are greater than 3 implying supercritical flow and the development of fluid mechanical instabilities that enhanced these flow surges

evolution of a vent hosted hydrothermal system beneath ruapehu crater lake new zealand a two year chemical monitoring program of ruapehu crater lake shows that it has evolved considerably since the volcano s more active eruptive periods in the early 1970s the present ph 20 c of 0 6 is about one half unit more acid than the baseline values in the 1970s whereas s cl ratios have increased markedly owing in part to declining hcl inputs into the lake but also to absolute increases in so4 levels which now stand at the highest values ever recorded increases in k mg and na mg ratios over the 20 year period are attributed to hydrothermal reaction processes in the vent which are presently causing dissolution of previously formed alteration phases such as natroalunite these observations combined with results of a recent heat budget analysis of the lake have led to the development of hydrothermal convection model for the upper portion of the vent possible vent lake chemical reaction processes between end member reactants have been modelled with the computer code chiller the results are consistent with the view that variations in lake chemistry which are initiated by the introduction of fresh magmatic material into the vent reflect the extent of dissolution reaction progress on the magmatic material and or its alteration products the results also provide insights into the role of such vent processes in the formation of high sulfidation type ore deposits

a general model for mt ruapehu lahars a mathematical model of the motion of lahars is presented lahar flows and travel speeds are calculated using a kinematic wave model which equates gravitational accelerations to frictional losses a chezyor manning type law of friction is assumed in which lahar flow rate is a simple power function of lahar depth multiplied by another simple power of the chanel slope use of the model requires knowledge of essentially only one parameter which appears to be relatively insensitive for flows down a given channel variable channel slope effects are removed by a longitudinal scaling which applies to all flows down a given channel for lahars generated by a single explosive event it is unnecessary to perform numerical calculations to predict lahar flow and travel time but for lahnars produced by multiple sources in which different lahar flows are interacting numerical calculations appear necessary the model is applied to all recorded lahar flows from mt ruapehu and satisfactorily described all lahar flows generated by a single explosive mechanism such flows depend essentially only on total lahar volume the 1968 mt ruapehu lahar generated by a series of smaller eruptive mechanisms was modelled as the interaction of seven point sources of fluid originating from positions mathematically extrapolated up the mountain good agreement was obtained between the predicted times of formation of these 1968 lahars and the times of greatest seismic amplitude

possible erosion mechanisms in snow avalanches snow erosion and entrainment processes in avalanches are classified according to their mechanisms the flow regimes in which they occur and their spatial position within the avalanche simple but process specific models are proposed for erosion by impacts abrasion plowing and blasting on the basis of order of magnitude estimates the first three mechanisms are clearly expected to be important the fourth mechanism stipulates that the compaction of the snowcover ahead of the avalanche leads to the flow of escaping air just in front of the avalanche that may disrupt the snowcover and support formation of a saltation layer the effects of this hypothetical mechanism resemble those of the plowing mechanism all mechanisms depend strongly on the snow properties but with plausible parameter values erosion rates at or above the experimentally found rates are obtained the entrainment rate of an avalanche is most often limited by the shear stress needed to accelerate the eroded snow to avalanche speed

tephra snow and water complex sedimentary responses at an active snow capped stratovolcano ruapehu new zealand a feature of small scale explosive volcanism at stratovolcanoes is the rapid destruction of primary near vent pyroclastic deposits by sedimentary processes a protracted series of explosive eruptions of moderate volume from september 1995 until july 1996 at mount ruapehu in new zealand its largest eruptive episode this century afforded an opportunity to study these remobilisation processes in detail all significant sub plinian eruptions occurred in mid winter forming metre thick tephra accumulations on steep slopes covered with perennial ice and seasonal snow subsequent events demonstrated the variety and complexity of the erosion processes that remobilise primary pyroclasts in such a setting these processes arose from the complex interactions of tephra with snow and ice and liquid water in varying proportions and were very diverse in nature and scale their effectiveness can be gauged from the fact that there is almost no stratigraphic record of any of the 40 eruption episodes recorded in the past 100 years at ruapehu syn eruptive remobilisation processes included the generation of eruption triggered lahars by the ejection of hot water from the crater lake post eruptive interactions mainly remobilised fall deposits from proximal areas and included rain triggered lahars which were among the largest and most hazardous events with the greatest distal impacts

hunter gatherer responses to the changing environment of the moervaart palaeolake nw belgium during the late glacial and early holocene abstract this paper presents new geo archaeological perspectives on the late glacial and early holocene human occupation around a large palaeolake the moervaart palaeolake 25 km 2 intensive fieldwork using invasive and non invasive survey techniques combined with modelling of the palaeotopography and palaeogroundwater and multi proxy palaeoecological analyses have resulted in a detailed reconstruction of the landscape during the final palaeolithic and early mesolithic occupation of the area a major shift in the occupation from the federmesser culture to the early mesolithic was contemporaneous with a sudden and drastic change in the palaeohydrology of the area between ca 13 300 and 13 000 cal bp end of allerod which coincided with a short but abrupt cooling event known as the intra allerod cold period iacp gi 1b it is assumed that this event triggered the sudden drying up of the moervaart palaeolake and surrounding ponds which until then had provided federmesser hunter gatherers with extensive and fertile grounds for hunting gathering and drinking water the population decline which followed this hydrological event was reinforced by the prevailing cold and harsh conditions of the younger dryas and probably lasted until the pre boreal hunter gatherers returned to the area in the boreal now settling along the proximal floodplain regions of a meandering channel which was connected with the southern scheldt river

physical scale modelling of jokulhlaups glacial outburst floods with contrasting hydrograph shapes jokulhlaups glacial outburst floods are hazards in many glaciated environments jokulhlaups generated by different trigger mechanisms have different hydrograph shapes field evidence indicates that jokulhlaups with different hydrograph shapes produce different impacts however little attention has been paid to understanding the controls on jokulhlaups with differing hydrograph shapes and their subsequent impacts historically flume studies have been used to investigate fluvial processes under controlled laboratory conditions nevertheless few flume studies have replicated scaled hydrographs and none have attempted to model field jokulhlaups this study documents the technical procedure by which field prototype jokulhlaups were replicated using physical scale modelling techniques an exponentially rising jokulhlaup characterized by a prolonged rising stage and a rapid falling stage and a linearly rising jokulhlaup characterized by a rapid rising stage and a prolonged falling stage the parameters of water discharge time discharge acceleration and deceleration rate sediment size and sediment discharge were scaled to a 1 20 ratio better sorted sediment more uniform downstream sediment thickness and more gradual aggradation and deposition rates resulted from the prolonged rising stage of the exponentially rising hydrograph more time was available during the prolonged rising stage of the exponentially rising hydrograph for bedload sheets to migrate and for sand ripples to develop rapid rising stage deposition during the linearly rising hydrograph produced thick proximal deposits limited bedload sheet migration massive sand sheets and structureless and poorly sorted sediments during the prolonged falling of the linearly rising hydrograph sufficient time was available to support sand ripple development the study highlights that jokulhlaup hydrograph shape is important for controlling rates of sediment acquisition transport deposition erosion and bedform development the outcomes of these experimental investigations indicate that modelling of high magnitude floods is feasible in a laboratory flume this technical approach has proved very beneficial for developing a greater understanding of flood impacts and the processes that operate during jokulhlaups copyright 2006 john wiley sons ltd

the volcanic history of ruapehu during the past 2 millennia based on the record of tufa trig tephras tufa trig formation comprises a sequence of at least 19 andesitic tephras erupted from mt ruapehu tongariro volcanic centre new zealand tephras of tufa trig formation are the most recent eruptives from ruapehu dated between ca 1850 years b p and the present members of the formation show restricted dispersals principally to the east of mt ruapehu volumes calculated for the most widespread members are all less than 0 1 km3 compared with other mt ruapehu eruptives tufa trig formation tephras represent small eruptions that have contributed little tephra to the ring plain they do however show a greater frequency of eruption with one event occurring on average every 100 years tufa trig formation members tf3 tf18 are black to dark grey vitric coarse ash and lapilli grade tephras which mantle the relief they contain juvenile vitric particles which exhibit varying degrees of vesicularity together with free crystals of pyroxene and feldspar and few lithic fragments several morphological types of vitric pyroclasts are recognised in these tephras the dominant type being of equant blocky morphology with fracture bound surfaces type 1 morphology field characteristics tephra distributions and the morphologies and textures of constituent pyroclasts suggest that these members tf3 tf18 are the products of small volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric water we propose that a source of this water was an ancestral crater lake which formed within the late holocene ca 3000 years b p the morphological compositional and chemical major element characteristics of three tufa trig formation tephras are compared with those of two new tephras erupted from ruapehu volcano during the october 1995 eruptions which comprise part of a newly defined member tf19 of tufa trig formation the comparisons support our interpretation that the majority of the tufa trig formation tephras are primarily the products of hydrovolcanic eruptions other members of the formation tf1 and tf2 are coarse grained scoriaceous tephras and are interpreted to be the products of strombolian events

nanodiamonds and wildfire evidence in the usselo horizon postdate the allerod younger dryas boundary the controversial younger dryas impact hypothesis suggests that at the onset of the younger dryas an extraterrestrial impact over north america caused a global catastrophe the main evidence for this impact after the other markers proved to be neither reproducible nor consistent with an impact is the alleged occurrence of several nanodiamond polymorphs including the proposed presence of lonsdaleite a shock polymorph of diamond we examined the usselo soil horizon at geldrop aalsterhut the netherlands which formed during the allerod early younger dryas and would have captured such impact material our accelerator mass spectrometry radiocarbon dates of 14 individual charcoal particles are internally consistent and show that wildfires occurred well after the proposed impact in addition we present evidence for the occurrence of cubic diamond in glass like carbon no lonsdaleite was found the relation of the cubic nanodiamonds to glass like carbon which is produced during wildfires suggests that these nanodiamonds might have formed after rather than at the onset of the younger dryas our analysis thus provides no support for the younger dryas impact hypothesis

a sealed flint knapping site from the younger dryas in the scheldt valley belgium bridging the gap in human occupation at the pleistocene holocene transition in w europe based on the evidence of a recently excavated sealed site situated at ruien rosalinde in the belgian scheldt valley the response of hunter gatherers to changing climate at the transition from the temperate allerod to the cold younger dryas is discussed radiocarbon dated to the end of the allerod or the very beginning of the younger dryas the site of ruien provides the earliest evidence of a refined lithic technology characterized by the use of a soft stone hammer and the production of straight and regular blade let s from intensively prepared cores with two opposite platforms and sharp striking angles in the course of the younger dryas and early pre boreal this knapping method will become standard all over europe from the tanged point technocomplex in the north to the epi laborian in the south it contrasts sharply with the knapping style of previous lithic traditions such as the late federmesser azilian and bromme technocomplexes which was much less elaborated and mainly oriented towards the knapping of short irregular blades with a hard stone hammer this apparently abrupt technological change was also accompanied by increased raw material procurement networks extending over up to 250 km and a marked microlithisation of the hunting equipment finally the site of ruien is also important as it demonstrates the limited archaeological visibility of younger dryas sites explaining the scarcity of such sites within western europe

changes in whangaehu river lahar characteristics during the 1995 eruption sequence ruapehu volcano new zealand abstract during the 1995 ruapehu eruptive sequence multiple lahars occurred in the whangaehu river which drains ruapehu s crater lake during the earlier phreatic and phreatomagmatic eruptions lahars were generated by expulsion of waters from the lake but once the lake had emptied lahars were formed by remobilisation of seasonal snowpack laden with saturated freshly erupted tephra four types of lahars occurred during the eruptive sequence 1 initial snow slurry lahars composed of granular snow and ice incorporated by eruptively expelled crater lake waters which left behind frozen deposits with 2 5 20 clastic sediment 2 large dilute lahars generated as the volumes of ejected lake water increased and removed much of the readily available snow at least one third of the pre eruption crater lake volume was expelled during one day producing the largest lahars of the series these lahars were hyperconcentrated flows for up to 84 km from source leaving extensive deposits along the channel margins 3 concentrated lahars smaller volume lahars generated as the frequency of eruptions and volumes of expelled water declined these lahars were able to maintain high sediment concentrations measured at 46 52 by volume suspended sediment at 42 km from source their high sediment concentrations were maintained by erosion and incorporation of sand from the deposits of earlier flows which were lining the channel margins 4 remobilised tephra lahars generated following the two largest tephra eruptions of the sequence seasonal snowpack was covered by water saturated tephra warmer spring temperatures and heavy rainfall events caused collapse and remobilisation of snow and tephra producing several lahars in catchments draining eastern ruapehu

unusual ice diamicts emplaced during the december 15 1989 eruption of redoubt volcano alaska abstract ice diamict comprising clasts of glacier ice and subordinate rock debris in a matrix of ice snow grains coarse ash and frozen pore water was deposited during the eruption of redoubt volcano on december 15 1989 rounded clasts of glacier ice and snowpack are as large as 2 5 m clasts of redoubt andesite and basement crystalline rocks reach 1 m and tabular clasts of entrained snowpack are as long as 10 m ice diamict was deposited on both the north and south volcano flanks on redoubt s north flank along the east side of drift piedmont glacier and outwash valley ice diamict accumulated as at least 3 units each 1 5 m thick two ice diamict layers underlie a pumice lithic fall tephra that accumulated on december 15 from 10 15 to 11 45 ast a third ice diamict overlies the pumiceous tephra some of the ice diamicts have a basal ice sandstone layer the north side icy flows reached as far as 14 km laterally over an altitude drop of 2 3 km and covered an area of about 5 7 km 2 on crescent glacier on the south volcano flank a composite ice diamict is locally as thick as 20 m it travelled 4 3 km over an altitude drop of 1 7 km covering about 1 km 2 the much higher mobility of the northside flows was influenced by their much higher water contents than the southside flow s erupting hot juvenile andesite triggered and turbulently mixed with snow avalanches at snow covered glacier heads these flows rapidly entrained more snow firn and ice blocks from the crevassed glacier on the north flank a trailing watery phase of each ice diamict flow swept over and terraced the new icy deposits the last and perhaps each flood reworked valley floor snowpack and swept 35 km downvalley to the sea ice diamict did not form during eruptions after december 15 despite intervening snowfalls these later pyroclastic flows swept mainly over glacier ice rather than snowpack and generated laharic floods rather than snowflows similar flows of mixed ice grains and pyroclastic debris resulted from the november 13 1985 eruption of nevado del ruiz volcano and from eruptions of snowclad mount st helens in 1982 1984 such deposits at snowclad volcanoes are initially broad and geomorphically distinct but they soon become extensively reworked and hard to recognize in the geologic record

the hazards of eruptions through lakes and seawater abstract eruptions through crater lakes or shallow seawater referred to here as subaqueous eruptions present hazards from hydromagmatic explosions such as base surges lahars and tsunamis which may not exist at volcanoes on dry land we have systematically compiled information from eruptions through surface water in order to understand the circumstances under which these hazards occur and what disastrous effects they have caused in the past subaqueous eruptions represent only 8 of all recorded eruptions but have produced about 20 of all fatalities associated with volcanic activity in historical time excluding eruptions that have resulted in about a hundred deaths or less lahars have killed people in the largest number of historical subaqueous eruptions 8 followed by pyroclastic flows excluding base surges 5 tsunamis 4 and base surges 2 subaqueous eruptions have produced lahars primarily on high 1000 m steep sided volcanoes containing small

dynamic interactions between lahars and stream flow a case study from ruapehu volcano new zealand three lahars were sampled in the whan gaehu river on the eastern flank of ruapehu volcano new zealand at 23 5 and 42 km from the source as they were flowing on september 27 and 29 and october 6 1995 the lahars were generated by water explosively ejected from the highly mineralized crater lake we used the chemical contrast between the lahars and resident stream water in their paths to describe a four phase model of a noncohesive lahar wave in a river channel 1 ambient stream water pushed ahead of the lahar in a process of miscible displacement due to hydrodynamic dispersion 2 a zone of mixing between the stream water and the lahar that increases in length with distance from source 3 a remnant of the original lahar least diluted by stream water that decreases in length and dilutes downstream 4 the tail of the lahar surge peak discharge occurs at the end of phase 1 as water is pushed ahead of the lahar peak sediment concentration occurs at the end of phase 2 where debris entrainment by the flow is at its greatest i e in front of the lahar proper deposits record only phases 2 and 3 of the lahar wave phase 1 flow left only a tide line of organic debris and phase 4 deposits were rapidly eroded by later streamflow downstream dilution by stream water eventually caused transformation of phases 2 and 3 of the lahars from debris flow to hyperconcentrated streamflow and then to normal streamflow as the flows became progressively finer grained and more turbulent

final palaeolithic settlements of the campine region ne belgium in their environmental context optical age constraints abstract the sand sequences exposed at the archaeological sites of arendonk korhaan and lommel maatheide offer a unique means to study the possible relation between human occupation patterns and climatic and environmental changes during the weichselian holocene transition in the campine region ne belgium of particular importance is that at these localities the final palaeolithic and early mesolithic settlement phases are stratigraphically separated optically stimulated luminescence osl dating established a chronology for the evolution of the physical landscape around these sites such a framework is essential when trying to understand settlement dynamics in response to environmental forcing thirty samples in total were dated using quartz sar osl the luminescence ages range from 16 2 1 4 ka to 10 6 0 7 ka they indicate that sand deposition at arendonk and lommel was already going on during the final phase of the late pleniglacial and or the onset of the late glacial at arendonk sediments and soils from the early late glacial until the allerod are missing whereas a single interruption in the aeolian activity during the allerod interstadial is reported for lommel landscape stabilisation during the allerod is also suggested by stratigraphical evidence i e the presence of a bleached horizon both at elevated and low lying flat areas and extensive peat growth in wet depressions the bolling interstadial on the other hand has not been recognised in the lithostratigraphic record this corroborates earlier suggestions that the climatic amelioration during the bolling was not accompanied by a general stabilisation of the land surface in the nw european lowlands the continuation of sand transport and deposition across the late pleniglacial late glacial boundary may explain why human societies did not occupy the campine region until the allerod during this interstadial environmental conditions improved sufficiently to attract final palaeolithic federmessergruppen to settle on higher grounds in the area the climate deteriorated at the onset of the late dryas and as a consequence aeolian activity intensified again at arendonk and lommel with at least 0 5 1 m of sand being deposited in a period of 1 ka this may have caused the federmesser to leave the area at both localities no occupation has been attested for the late dryas period until the early holocene when early mesolithic societies settled in the area this study illustrates how luminescence dating can contribute to an improved understanding of the environmental context around archaeological sites in the campine region

a fluid dynamics approach to modelling the 18th march 2007 lahar at mt ruapehu new zealand lahars are water sediment mass flows from a volcanic source they can be triggered by a variety of mechanisms and span a continuum of flow rheology and hydraulic properties even within the same event lahars are extremely powerful landscaping agents and represent a considerable hazard potential however this highly dynamic character and a lack of direct measurements has made modelling lahars difficult this study therefore applies a fluid dynamics model delft3d to analyse the 18th march 2007 dam break lahar at mount ruapehu new zealand the modelled lahar routed through the whangaehu gorge in 30 min crossed the whangaehu fan in 60 min and then over a further 3 h travelled an additional 22 km distance along the whangaehu river to the tangiwai bridge the modelled mean frontal velocity was 6 5 m s 1 along the gorge although peak velocity reached up to 19 6 m s 1 the modelled lahar flow front progressively slowed across the fan but along the river it accelerated from 2 1 3 3 m s 1 calculated peak velocity along the river was 4 5 m s 1 these results generally compare well with gauged records with historical records and with other modelling approaches however discrepancies in frontal velocity and time to peak stage arise due to 1 specifying roughness which arises from slope variations between adjacent computational nodes and which is stage dependant and 2 due to rapid topographic changes that produce frequent hydraulic jumps which are inadequately accommodated in the numerical scheme the overall pattern of discharge attenuation and of relationships between topographic and hydraulic variables is similar to that calculated for lahars on other volcanoes this modelling method could be applied at other similar sites where a likely source hydrograph and high resolution topographic data are available these results have important implications for hazard management at ruapehu and for examining geomorphic and sedimentary impacts of this lahar