catastrophic glacial lake outburst flooding of the patagonian ice sheet abstract reconstructing the magnitude and timing of catastrophic drainage of proglacial lakes during late quaternary deglaciation of palaeo ice sheets is important for understanding rates of landscape change and regional freshwater forcing of ocean currents here we provide the first detailed reconstruction of catastrophic glacial lake outburst flooding from the southern hemisphere focusing on the rio baker catchment in central patagonia firstly we review the evidence for palaeolake drainage events in the baker catchment showing the basin witnessed a number of events during late quaternary deglaciation these floods helped re establish the pacific drainage pathway of the rio baker by 12 6 11 7 ka after millennia blocked by ice during the last glaciation secondly we analyse the landform record and flow hydraulics of a catastrophic flood in patagonia focusing on the final early holocene drainage event of lago general carrera buenos aires using two independent methods a one dimensional hydraulic model and dam break simulation we estimate a minimum peak discharge of 110 000 m3s 1 0 1 sv large enough to form megaflood type landforms using the flow hydrograph we constrain timing of landform genesis eddy bars at the top of the water column 45 65 m above the valley floor were inundated over 2 36 h following the breach while tractive bars formed beneath 20 m of water at sites of valley expansion were flooded for 100 h raised deltas and palaeoshorelines provide geomorphic evidence for palaeolake drainage events across patagonia indicating the likely role that catastrophic floods played in continental scale palaeohydrology and the evolution of fjord ecosystems

the origins of s forms form similarity process analogy and links to high energy subglacial meltwater flows abstract this paper first presents the history of ideas on s form formation with an emphasis on meltwater models the main approach is based on form similarity between modern erosional marks and subglacial s forms form similarity is then extended to process similarity which promotes understanding of s forms three classes of erosional marks are identified obstacle scours basin shaped erosional marks and landforms related to escarpment flow identification is based on form with recognition of elements though it mainly depends on recognition of the whole form in much the same way as we recognize family members or familiar flowers flow conditions related to the various flow features are determined by flow visualization and computational fluid dynamics some s forms are produced in flume models though most are natural features observed in the field examples from beneath past ice sheets are seen in the field and illustrated by air and ground photographs satellite images and dem digital elevation model the results of this work show remarkable similarity between modern erosional marks and ancient subglacial s forms consequently established processes are attributed to ancient forms in turn s forms may be used abductively to interpret subglacial hydrological conditions the lack of waning stage bedforms scoured into primary forms suggests that erosional marks form under abruptly terminating catastrophic flow

physically based modeling of bedrock incision by abrasion plucking and macroabrasion 1 many important insights into the dynamic coupling among climate erosion and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision however many questions regarding incision processes and morphology of bedrock streams still remain unanswered a more mechanistically based incision model is needed as a component to study landscape evolution major bedrock incision processes include among other mechanisms abrasion by bed load plucking and macroabrasion a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts the purpose of this paper is to develop a physically based model of bedrock incision that includes all three processes mentioned above to build the model we start by developing a theory of abrasion plucking and macroabrasion mechanisms we then incorporate hydrology the evaluation of boundary shear stress capacity transport an entrainment relation for pluckable particles a routing model linking in stream sediment and hillslopes a formulation for alluvial channel coverage a channel width relation hack s law and exner equation into the model so that we can simulate the evolution of bedrock channels the model successfully simulates various features of bed elevation profiles of natural bedrock rivers under a variety of input or boundary conditions the results also illustrate that knickpoints found in bedrock rivers may be autogenic in addition to being driven by base level fall and lithologic changes this supports the concept that bedrock incision by knickpoint migration may be an integral part of normal incision processes the model is expected to improve the current understanding of the linkage among physically meaningful input parameters the physics of incision process and morphological changes in bedrock streams

interpretation and downstream correlation of bedrock river terrace treads created from propagating knickpoints 1 derivation of an analytical model for the slope of a strath terrace created following an upstream propagating wave of incision reveals that for detachment limited river incision the exponent on channel slope n governs terrace tread slope terrace elevations can increase upstream n 1 downstream n 1 or can be horizontal n 1 numerical modeling confirms these results for temporally evolving knickpoint geometries and for incision due to sudden base level fall and an increase in rock uplift rate except in the case of bedrock river incision with a slope threshold only exceeded during knickpoint propagation a terrace created from transient headward incision contains no information about the paleo channel gradient gradients in rock uplift rate along channels potentially complicate the interpretation of terraces by altering the primary tilt on strath terraces in particular monotonic gradients in rock uplift can produce terrace treads that are apparently folded because gradients in rock uplift rate are common where terraces are not longitudinally traceable care is warranted in terrace correlation in simple tectonic settings where terraces are longitudinally traceable the slope of a strath terrace created from headward incision may provide a means of estimating the dependency of river incision rate on channel slope terraces in four locations argued to have formed from headward incision are parallel or close to parallel with the active channel implying a slope exponent on river incision rate that is much greater than one or a threshold slope for incision that is only exceeded during knickpoint propagation

the bubble bursts for cavitation in natural rivers laboratory experiments reveal minor role in bedrock erosion the erosion of rock bedded channels generally is considered a slow process caused mainly by abrasion due to bedload or suspended sediments but the mechanisms of rapid erosion remain unclear cavitation is a clear fluid erosive process well known for its effect on engineering structures when water vapour bubbles collapse and the resultant pressure shocks erode the boundary however although the occurrence of cavitation erosion in natural watercourses has long been a matter of debate as yet there are no incontrovertible examples of cavitation damage to natural river beds using flume experiments we show for the first time that only weakly cavitating clear water flows can occur for the range of flow velocities observed in rivers and these do not erode medium hardness rocks after 68 hours during this time period only a very soft rock featured erosional marks due to dissolution thus our results cast significant doubt on the likelihood of identifying cavitation damage in most rivers and provide pointers to those river systems that might be investigated further to identify cavitation erosion copyright 2016 john wiley sons ltd

a mechanistic model for river incision into bedrock by saltating bed load 1 abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium we assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity particle impact velocity and impact frequency depend on saltation trajectories which can be predicted using empirical functions of excess shear stress the model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter which relates rock strength to rock resistance to abrasive wear the model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed

weertman regelation multiple refreezing events and the isotopic evolution of the basal ice layer a simple model is developed to s imulate the isotopic fractionation which accompanies weertman regelation at the bed of temperate based glaciers the fractionation equations of jouzel and souchez 1 982 are applied to multiple refreezing events over measured glacier bed profiles and mass balance is maintained as the basal ice and meltwater produced at one bedrock hummock enter the next s imulation results indicate that undeformed regelation ice layers are on the order of millimetres to centimetres thick often being completely melted at the stoss face of certain hummocks and exceptionally reaching a thickness in excess of 1 0 cm neither the internal morphology nor the isotopic composition of these layers is constant but both vary down glacier in accordance with bedrock configuration a glacier wide fractionation process is identified whereby heavy isotopes are preferentially removed fro m the basal meltwater film and incorporated into the basal ice this process might go some way to explaining the anomalously light isotopic composition measured in base flow waters leaving some glaciers vertical isotope profiles through undeformed basal ice layers are reconstructed and show that significant isotopic excursions can occur at a scale of millimeteres while the range of isotopic compositions within such m ulti layered regelation ice is greater than that which would occur in ice produced by a single refreezing event in circumstances where the regelation system is disrupted by removal of film waters into a network of linked cavities i t is found that the remaining basal ice may be significantly enriched in heavy isotopes relative to the composition of the initial mass inputs to the system heavy isotope enrichment of this m agnitude and consideration of the thickness of the basal ice layers concerned may explain the absence of recorded basal ice samples heavy enough to h ave been formed in equilibrium with subglacial precipitates sampled at one of the sites glacier de tsanfleuron and reported in an earlier paper

paleohydraulic reconstruction of the largest glacial lake missoula draining s glacial lake missoula glm was the source of earth s largest known peak flood discharges onto the channeled scabland northwestern united states the original studies of glm were based on geomorphological mapping of lake sediments and shorelines more recently studies have emphasized the computational fluid dynamics of downstream megaflood routing and characteristics in the channeled scabland and adjacent areas however actual details of the lake draining processes glm have received little scientific attention in this paper we employ two dimensional hydraulic modelling to understand the paleoflow conditions for different scenarios of draining glm at its highest stands values are calculated for three glm drainage hydrographs 2 6 13 17 10 6 m 3 s 1 that previous studies had estimated for outlet flows consistent with regional mapping and with the original interpretation of fluvial geomorphology by other studies the modelling shows that lake silt sequences in the gorges of glm must postdate the most highly energetic outburst events from the lake this is because the flows generated in the lake by megaflood outflows are sufficiently energetic to erode any accumulated silt deposits in contrast the gravels underlying the silts in these zones include boulder sized clasts largescale cross stratification and 70e100 m high bars forms all of which indicate very high energy flood flow conditions the modelling also shows that the paleohydraulic conditions which developed in the lake basin during the largest megaflood outflows would be capable of accounting for the observed bedrock scour at rainbow lake pass various high eddy deposits eddy bars of flood gravel in gorges and the giant ripple marks subaqueous gravel dunes that were classically described in the 1940s previous studies interpreted the glacial lake silt rhythmites and related deposits such as the ninemile section as evidence for numerous fillings and drainings of glm our modelling suggests that these inferred events must have been relatively low in energy and magnitude relative to the earlier outburst s that emplaced the flood gravels subaqueous gravel dunes and scabland like erosion surfaces

a new greenland deep ice core the polar ice sheets are rich sources of information on past atmospheric conditions including paleoclimates a new deep ice core has been drilled in south greenland comparison of the oxygen isotopic profile with that from camp century and with a deep sea foraminifera record indicates that the new core reaches back to about 90 000 years before present in a continuous sequence the details in the wisconsin part of the ice core records seem to be climatically significant and the general trends reveal all of the relevant emiliani stages recorded in deep sea cores the redated camp century record suggests a dramatic termination of the eem sangamon interglacial

feedbacks between erosion and sediment transport in experimental bedrock channels natural bedrock rivers flow in self formed channels and form diverse erosional morphologies the parameters that collectively define channel morphology e g width slope bed roughness bedrock exposure sediment size distribution all influence river incision rates and dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes to explore the mechanics of river incision we conducted laboratory experiments in which the complexities of natural bedrock channels were reduced to a homogenous brittle substrate sand and cement a single sediment size primarily transported as bedload a single erosion mechanism abrasion and sediment starved transport conditions we find that patterns of erosion both create and are sensitive functions of the evolving bed topography because of feedbacks between the turbulent flow field sediment transport and bottom roughness abrasion only occurs where sediment impacts the bed and so positive feedback occurs between the sediment preferentially drawn to topographic lows by gravity and the further erosion of these lows however the spatial focusing of erosion results in tortuous flow paths and erosional forms inner channels scoops potholes which dissipate flow energy this energy dissipation is a negative feedback that reduces sediment transport capacity inhibiting further incision and ultimately leading to channel morphologies adjusted to just transport the imposed sediment load copyright 2007 john wiley sons ltd

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

outburst flood evolution at russell glacier western greenland effects of a bedrock channel cascade with intermediary lakes abstract glacial lake outburst floods have produced a distinctive and widespread quaternary record both onshore and offshore via widespread and intense geomorphological impacts yet these impacts remain poorly understood due to a lack of modern analogues this study therefore makes a systematic quantification of the evolution of a bedrock channelled outburst flood channel topography was obtained from digitised aerial photographs a 5 m grid resolution dem and bathymetric surveys flood inundation was measured in the field from dgps measurements flood evolution was analysed with application of a numerical model novel flood metrics including rate of rise to peak and persistence above a value of hydraulics were calculated the key findings were that outburst floods i that enter intermediary lakes are dramatically attenuated in terms of propagation whilst basin filling proceeds to the outlet level dissipated of energy due to considerable flow recirculation during the rising stage of the flood and after overtopping of the outlet are moderated in terms of peak discharge ii can have kinematic waves that are introduced to a flood via hydraulic ponding iii may develop a partitioning of flow regimes for instantaneous peak discharge bed shear stress and froude number due to time transgressive changes in channel cross section specifically between inner channel and overbank regions furthermore normalised change in cross sectional area and normalised change in channel slope can be used to suggest the relative magnitude of instantaneous peak discharge net erosion along a reach can be related to hydraulic persistence above a marker value and net deposition can be related to a time to peak value reconstructions of quaternary outburst floods should consider the importance of including intermediary lakes modern hazard mitigation studies could usefully note the potential use of reservoirs as an outburst flood alleviation resource

massive ice of the tuktoyaktuk area western arctic coast canada the extensive coastal exposure of massive underground ice at peninsula point southwest of tuktoyaktuk northwest territories is believed to be intrasedimental ice the ice grew beneath a frozen d

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

assessment of hydro morphodynamic modelling and geomorphological impacts of a sediment charged jokulhlaup at solheimajokull iceland understanding of complex flood riverbed interaction processes in large scale field outburst floods remains incomplete not least due to a lack of well constrained field data on hydraulics and sediment transport but also because consensus on an appropriate model framework has yet to be agreed this study presents a novel full 2d hydro morphodynamic model containing both bedload and suspended load capability firstly the model design is presented with an emphasis on its design to simulate rapidly varied sediment laden outburst floods and also the associated geomorphological impacts secondly the model is applied to a large scale field glacier outburst flood or jokulhlaup at solheimajokull iceland for this real world event model scenarios with only water and with inclusion of sediment with different parameter setups were performed results indicated that grain size specifications affected resultant geomorphological changes but that the sensitivity of the simulated riverbed changes to the empirical bedload transport formulae were insignificant notably a positive feedback occurred whereby the jokulhlaup led to significant net erosion of the riverbed producing an increase in flow conveyance capacity of the river channel furthermore bulking effects of sediment entrainment raised the peak discharge progressively downstream as well as the flood volume effects of geomorphological changes on flood water level and flow velocity were significant overall despite the increased computational effort required with inclusion of sediment transport processes this study shows that river morphological changes cannot be ignored for events with significant in channel erosion and deposition such as during outburst floods

on the isotopic composition in δd and δ18o of water and ice during freezing experiments on progressive unidirectional freezing are coi1 ducted to determine rhe evol ution in 00 and 51 bo of successive water samples and ice layers ta ken during the course of freezing r es ults indicate t hal lhi s evoluti on takes pl ace in a bd bibo diagram a long a strai ght line wi th a characteri sti c slope this slope different from that due to the precip itation effec t gives a finger print of the occurrence of a freezing or of a melt ing refreez ing process in the stud ied reservoir residual water was sampled at the beginning of the experiment after 40 80 85 90 and 95 of the material had frozen when the freezing process was complete the ice core was recovered and sectioned the samples covered the freezing ranges from 0 to 10 10 to 20 20 to 50 50 to 60 60 to 70 70 to 80

characterization of subglacial lake vostok as seen from physical and isotope properties of accreted ice deep drilling at the vostok station has reached the surface of subglacial lake vostok lv twice in february 2012 and january 2015 as a result three replicate cores from boreholes 5g 1 5g 2 and 5g 3 became available for detailed and revalidation analyses of the 230 m thickness of the accreted ice down to its contact with water at 3769 m below the surface the study reveals that the concentration of gases in the lake water beneath vostok is unexpectedly low a clear signature of the melt water in the surface layer of the lake which is subject to refreezing on the icy ceiling of lv has been discerned in the three different properties of the accreted ice the ice texture the isotopic and the gas content of the ice these sets of data indicate in concert that poor mixing of the melt and hydrothermal water with the resident lake water and pronounced spatial and or temporal variability of local hydrological conditions are likely to be the characteristics of the southern end of the lake the latter implies that the surface water may be not representative enough to study lv s behaviour and that direct sampling of the lake at different depths is needed in order to move ahead with our understanding of the lake s hydrological regime

evaluating the controls of shear stress sediment supply alluvial cover and channel morphology on experimental bedrock incision rate 1 we explored the dependence of experimental bedrock erosion rate on shear stress bed load sediment flux alluvial bed cover and evolving channel morphology we isolated these variables experimentally by systematically varying gravel sediment flux qs and water discharge qw in a laboratory flume gradually abrading weak concrete bedrock all else held constant we found that 1 erosion rate was insensitive to flume averaged shear stress 2 erosion rate increased linearly with sediment flux 3 erosion rate decreased linearly with the extent of alluvial bed cover and 4 the spatial distribution of bed cover was sensitive to local bed topography but the extent of cover increased with qs qt where qt is flume averaged transport capacity once critical values of bed roughness and sediment flux were exceeded starting from a planar geometry erosion increased bed roughness due to feedbacks between preferential sediment transport through interconnected topographic lows focused erosion along these zones of preferential bed load transport and local shear stresses that depended on the evolving bed morphology finally continued growth of bed roughness was inhibited by imposed variability in discharge and sediment flux due to changes in spatial patterns of alluvial deposition and impact wear erosion was preferentially focused at lower bed elevations when the bed was cover free but was focused at higher bed elevations when static alluvial cover filled topographic lows natural variations in discharge and sediment flux may thus stabilize and limit the growth of roughness in bedrock channels due to the effects of partial bed cover

reconstruction of the largest holocene jokulhlaup within jokulsa a fjollum ne iceland abstract glacial outburst floods jokulhlaups have a significant role for landscape evolution in ne iceland a number of jokulhlaups have routed from the northern margin of vatnajokull during the holocene in this study reconstruction of the largest holocene jokulhlaup along jokulsa a fjollum ne iceland was undertaken using the hec ras hydraulic modelling and hec georas flood mapping techniques with a digital elevation model dem derived from ers insar data and field based wash limit evidence the largest jokulhlaup produced extensive erosional and depositional landforms across an inundated area of 1390 km2 and is calculated to have had a peak discharge of 0 9 106 m3 s 1 power per unit area within this jokulhlaup varied from 6 to 46 000 w m 2 jokulhlaup hydraulics are related to geomorphogical evidence at three key sites in va alda upptyppingar and mo rudalur sub areas in order to explain the abrupt spatial variation of the flood characteristics on a regional scale and to relate erosional and depositional features to spatial variations in jokulhlaup hydraulics these process form relationships of the largest jokulhlaup along the jokulsa a fjollum are compared with large outburst floods elsewhere the largest jokulsa a fjollum jokulhlaup had a factor of 20 times smaller discharge and a factor of 20 times lower power per unit area than altai palaeoflood the largest known flood on the earth

bedrock benches and boulder bars floods in the burdekin gorge of australia the burdekin gorge of northeastern australia lies within the seasonal tropics and is characterized by high discharge variability slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1 200 yrs these floods range in magnitude from 11 000 to 30 000 m 3 s 1 and are characterized by large downstream variations in hydraulics the floods generate high values of velocity boundary shear stress and stream power per unit area downstream fluctuations in these variables help to explain the location of boulder bars high flood levees small scale erosional features in the bedrock and the formation of inner channels boulder bars and flood levees form where shear stress and stream power decrease due to channel widening small scale erosional features such as potholes and troughs are best developed at sites of channel constriction and associated increases in shear stress and stream power the development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure and by flow hydraulics all of the channel features are related to the hydraulics of large floods which therefore are the dominant controls on many aspects of channel morphology in this bedrock gorge

coupled fluid dynamics sediment transport modelling of a crater lake break out lahar mt ruapehu new zealand summary lahars and other sudden onset floods are highly dynamic with temporally and spatially evolving sediment loads numerical models of sudden onset flood behaviour particularly those applied to real world events have tended to neglect sediment transport processes this is a serious though perhaps understandable shortcoming given a lack of field measurements on flow bed interactions the extra computational complexity required and the problem of iteratively updating plastic channel boundaries this paper applies a fluid dynamics model with suspended sediment and bedload transport to calculate flow conditions and rapid landscape change due to an initially dilute lahar from mt ruapehu new zealand we take advantage of an unprecedented dataset of net topographic changes determined from pre and post lahar airborne lidar surveys and time series of hydrological measurements to run and validate the model a sensitivity analysis of key controls on water surface elevation and final sediment thickness illustrates how the flow was supply limited in the upper reaches it was capable of transporting a greater volume and calibre of material than that available modelled patterns of net erosion and deposition can replicate those observed r 2 0 79 and are dominantly controlled by channel gradient and topographic

wisconsin and holocene δ18o variations barnes ice cap canada new 18o measurements on ice samples from barnes ice cap show a relatively large shift from about 23 in the blue ice that makes up the bulk of the ice cap to 38 in a basal layer of clean white ice this strongly suggests that the latter is of pleistocene age thus confirming an earlier interpretation based on less comprehensive data the shift between the pleistocene and holocene ice is attributed in part to the change in climate at the end of the pleistocene and in part to a decrease in elevation of the accumulation area of 700 m beneath the clean white ice there is a layer of dirty white ice with less negative 18o values this ice is believed to have been deposited during a climatically warm period in early to middle wisconsin time in addition however the less negative 18o values in it are in part attributed to fractionation at a time in the past when the basal ice was at the melting point and some of the water produced by pressure melting was lost through a permeable bed

palaeostress analysis of normal faults in granite implications for interpreting riedel shearing related to regional deformation in this study field data from mesoscopic scale brittle structures faults are used to decipher palaeostress and relate it to regional scale kinematics fault slip data from oblique slip normal faults in granite mulgund west dharwar craton southern india are analysed in three steps 1 determination of palaeostress direction using fault slip data 2 kinematic interpretation of fault data 3 fitting the kinematic model to the regional far field stress palaeostress analysis of two sets of faults left lateral and right lateral oblique slip normal faults reveals that both formed under the same compression direction ene wsw it is shown that inferring the faults to be r p r and x shears of a riedel shear system explains their kinematic evolution within the granite the riedel shears developed as a result of sinistral movement along the 305 oriented contact between the granite and the surrounding gneisses riedel shear kinematics is interpreted as being a consequence of deformation partitioning which was favoured by the rheological contrast between the above lithologies during the late stage of a third phase of deformation under 315 135 directed s hmax far field stress

new insights into the mechanics of fluvial bedrock erosion through flume experiments and theory river incision into bedrock drives the topographic evolution of mountainous terrain and may link climate tectonics and topography over geologic time scales despite its importance the mechanics of bedrock erosion are not well understood because channel form river hydraulics sediment transport and erosion mechanics coevolve over relatively long time scales that prevent direct observations and because erosive events occur intermittently and are difficult and dangerous to measure herein we synthesize how flume experiments using erodible bedrock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory we also build on this work by providing new theory for rock resistance to abrasion thresholds for plucking by vertical entrainment sliding and toppling and by assessing bedrock analog materials bedrock erosion experiments in the last 15 years reveal that the efficiency of rock abrasion scales inversely with the square of rock tensile strength sediment supply has a dominant control over bed roughness and abrasion rates suspended sediment is an efficient agent of erosion and feedbacks with channel form and roughness strongly influence erosion rates erodibility comparisons across rock concrete ice and foam indicate that for a given tensile strength abrasion rates are insensitive to elasticity the few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed and the dominance of block sliding and toppling at knickpoints these observations are consistent with new theory for the threshold shields stress to initiate plucking which also suggests that erosion rates in sliding and toppling dominated rivers are likely transport limited major knowledge gaps remain in the processes of erosion via plucking of bedrock blocks where joints are not river bed parallel waterfall erosion by toppling and plunge pool erosion feedbacks between weathering and physical erosion erosional bedforms and morphodynamic feedbacks between channel form and erosion rates despite scaling challenges flume experiments continue to provide much needed tests of existing bedrock erosion theory force development of new theory and yield insight into the mechanics of landscapes

a mechanism of basal ice layer formation involving major ice fabric changes an opportunity to analyse the mechanism for the formation of a basal ice layer was provided by glacier de tsanfleuron in the swiss alps this glacier has some lateral subglacial cavities in which a basal ice layer forms on the bedrock floor and is subsequently incorporated at the base of the glacier petrographic and crystallographic analyses of the different types of ice have provided a means of investigating the successive stages of the process these analyses show major structural changes which provide field confirmation of the deformation mechanisms studied by several authors in laboratory experiments

cover effect in bedrock abrasion a new derivation and its implications for the modeling of bedrock channel morphology the sediment load of a bedrock river plays an important role in the fluvial incision process by providing tools for abrasion the tools effect and by covering and thereby protecting the bed the cover effect we derive a new formulation for the cover effect in which the fraction of exposed bed area falls exponentially with increasing sediment flux or decreasing transport capacity and explore its consequences for the model of bedrock abrasion by saltating bed load erosion rates predicted by the model are higher than those predicted by earlier models in a closed system the maximum erosion rate is predicted to occur when sediment supply is equal to transport capacity for a flat bed by optimizing the channel geometry to minimize the potential energy of the stream and using representative values for both discharge and grain size we derive equations for the geometry of a bedrock river and explore how predictions for width slope and bed cover vary as functions of drainage area rock uplift rate and rock strength the equations predict a dependence of channel width on drainage area similar to the relations using a simple shear stress incision law the slope area relationship is predicted to be concave up in a log log regime with a curvature dependent on uplift rate however this curvature does not deviate sufficiently from a straight line to allow discrimination between models using empirical data dependence of channel width and slope on rock uplift rate can be separated into two domains for low uplift rates channel geometry is largely insensitive to uplift rate due to a threshold effect at high uplift rates there is a power law dependence bed cover is predicted to increase progressively downstream and to increase with increasing uplift rate in our model the width to depth ratio is a function of both tectonic and climatic forcing this indicates that the scaling between channel width and bed slope is neither a unique indicator of tectonic forcing at steady state nor a signature of transience or steady state we conclude that sediment effects need to be taken into account when modeling bedrock channel morphology

dam break outburst flood propagation and transient hydraulics a geosciences perspective summary dam break outburst floods are sudden short lived and high magnitude discharges of water and sediment propagating over an initially dry terrain both field observations by geoscientists and experimental measurements by engineers have produced understanding of outburst floods but neither approach has satisfactorily supplied data that is directly of use to hazard managers this study draws together those two literature groups and provides new experimental data to test hypotheses concerning the effect of a series of external controls on resultant longitudinal flow propagation and transitory hydraulics these external controls form experimental treatments and are i the height of the lock gate raise as a surrogate for dam breach evolution and initial hydrograph ii bed roughness and iii suspended sediment concentration these control both horizontal and vertical fluid motion the latter of which is a key property for controlling aeration bed shear stress and turbulence intensity the experimental treatments show that outburst floods on horizontal beds can be identified to possess three longitudinal flow regimes firstly there is a short acceleration due to the reservoir pressure level i e the depth of impounded water secondly channel flow quickly converges to an inertial regime the third flow regime is viscous and dominated by channel bed friction flow depth increases in the inertial regime then decreases in the viscous regime experiments herein show that the timing of the peaks at a point down channel of each hydraulic quantity varies most notably peak bed shear stress precedes flow velocity which precedes peak flow depth a point of inflexion occurs with distance down channel that is a manifestation of the transition from accelerating to decelerating flows these external controls and the effects of them on longitudinal and vertical motion are analysed statistically and quantitative relationships are derived to underpin the resultant conceptual model this conceptual model should serve as a platform for future probabilistic and process based modelling of outburst floods

deuterium excess in an east antarctic ice core suggests higher relative humidity at the oceanic surface during the last glacial maximum a continuous deuterium excess profile d d 8 18o was obtained from the dome c ice core spanning 32 000 yr it exhibits significant variations over this period in particular d was 4 5 lower around 18 000 yr bp than over the past 7 500 yr interpreted as reflecting higher relative humidity estimated 90 over the oceanic areas providing moisture for antarctic precipitation

rapid formation of a modern bedrock canyon by a single flood event deep river canyons are thought to form slowly over geological time see for example ref 1 cut by moderate flows that reoccur every few years in contrast some of the most spectacular canyons on earth and mars were probably carved rapidly during ancient megaflood events quantification of the flood discharge duration and erosion mechanics that operated during such events is hampered because we lack modern analogues canyon lake gorge texas was carved in 2002 during a single catastrophic flood the event offers a rare opportunity to analyse canyon formation and test palaeo hydraulic reconstruction techniques under known topographic and hydraulic conditions here we use digital topographic models and visible near infrared aerial images from before and after the flood discharge measured during the event field measurements and sediment transport modelling to show that the flood moved metre sized boulders excavated 7 m of limestone and transformed a soil mantled valley into a bedrock canyon in just 3 days we find that canyon morphology is strongly dependent on rock type plucking of limestone blocks produced waterfalls inner channels and bedrock strath terraces whereas abrasion of cemented alluvium sculpted walls plunge pools and streamlined islands canyon formation was so rapid that erosion might have been limited by the ability of the flow to transport sediment we suggest that our results might improve hydraulic reconstructions of similar megafloods on earth and mars

hydrodynamics and geomorphic work of jokulhlaups glacial outburst floods from kverkfjoll volcano iceland jokulhlaups glacial outburst floods occur frequently within most glaciated regions of the world and cause rapid landscape change infrastructure damage and human disturbance the largest jokulhlaups known to have occurred during the holocene within iceland drained from the northern margin of vatnajokull and along the jokulsa a fjollum some of these jokulhlaups originated from kverkfjoll volcano and were routed through anastomosing high gradient and hydraulically rough channels landforms and sediments preserved within these channels permit palaeoflow reconstructions kverkfjoll jokulhlaups were reconstructed using palaeocompetence point measurements slope area one dimensional and depth averaged two dimensional 2d hydrodynamic modelling techniques the increasing complexity of 2d modelling required a range of assumptions but produced information on both spatial and temporal variations in jokulhlaup characteristics the jokulhlaups were volcanically triggered had a linear rise hydrograph and a peak discharge of 50 000 100 000 m3 s 1 which attenuated by 50 75 within 25 km frontal flow velocities were 2 m s 1 but as stage increased velocities reached 5 15m s 1 peak instantaneous shear stress and stream power reached 1 104 n m 2 and 1 105 w m 2 respectively hydraulic parameters can be related to landform groups a hierarchy of landforms is proposed ranging from the highest energy zones erosional gorges scoured bedrock cataracts and spillways to the lowest energy zones of valley fills bars and slackwater deposits fluvial erosion of bedrock occurred in kverkfjallarani above 3 m flow depth 7m s 1 flow velocity 1 102 n m 2 shear stress and 3 102 w m 2 stream power fluvial deposition occurred in kverkfjallarani below 8 m flow depth 11 m s 1 flow velocity 5 102 n m 2 shear stress and 3 103 w m 2 stream power hence erosional and depositional envelopes have considerable overlap probably due to transitional flow phenomena and the influence of upstream effects such as hydraulic ponding and topographic constrictions for example holocene kverkfjoll jokulhlaups achieved geomorphic work comparable to that of other late pleistocene megafloods this work was a result of steep channel gradients topographic channel constrictions and high hydraulic roughness rather than to extreme peak discharges the kverkfjoll jokulhlaups have implications for landscape evolution in north central iceland for water sediment inputs into the north atlantic and for recognizing jokulhlaups in the rock record 2d hydrodynamic modelling is likely to be important for hazard mitigation in similar landscapes and upon other glaciated volcanoes because it only requires an input hydrograph and a digital elevation model to run a model rather than suites of geomorphological evidence and field surveyed valley cross sections for example copyright 2006 john wiley sons ltd

late weichselian glacier outburst floods in north eastern poland landform evidence and palaeohydraulic significance abstract geomorphological evidence of catastrophic glacial floods has been discovered in many formerly glaciated areas of north america in the mountains of siberia and central asia and in northern europe within the area of the last scandinavian ice sheet in the european lowland a wide range of landforms related to glacial meltwater flows have been documented but no unequivocal evidence of cataclysmic glacial outburst floods exist here we discuss a landform signature of late weichselian glacial megafloods in the suwalki lakeland north eastern poland these landforms are oversized ice marginal spillways scabland like features expansion bars megadunes obstacle marks and clusters of kettle holes this unique glacial landscape was probably generated by at least two separate glacial megafloods the older event ca 19 ka ago and the younger event shortly after ca 16 ka ago palaeohydraulic calculations based on the morphometric characteristics of the flood landforms suggest that one of those megafloods had water depth of at least 19 m and flow velocity of around 15 17 ms 1 the flood flow regime was from subcritical to supercritical the calculated maximum flood discharge range was between 0 5 106 and 2 2 106 m3s 1 with the most likely value of around 2 106 m3s 1 these numbers place this megaflood among the largest glacial megafloods worldwide with a possible impact on global climate through rapid release of large volumes of freshwater into the atlantic ocean following a transfer across the european lowland through prominent ice marginal meltwater spillways

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

patterns of bedrock channel erosion on the boso peninsula japan three channels on the boso peninsula japan were examined to assess how cross sectional to reach scale channel morphology relates to substrate variables and how cross sectional to reach scale erosional patterns relate to basin scale longitudinal profile two of the channels shichiri gawa and torii zawa are deeply incised into interbedded sandstone and mudstone whereas the third channel futama gawa is incised into mudstone reach scale channel morphologies include knickpoints chutes and pools plane beds and both transverse and longitudinal bed erosional features step backwater modeling of a september 1996 typhoon related flood flow along each channel indicated that unit stream power is highly variable downstream stream power minima are associated with bends and with coarse clast deposition magnitude of erosional bed features correlates with hydraulics and flow energy expenditure along futama gawa and with variability in substrate resistance along shichiri gawa and torii zawa thickness and ori