seismic observations of transient subglacial water flow beneath macayeal ice stream west antarctica 1 new seismic observations of harmonic tremors beneath macayeal ice stream west antarctica are reported each of the two tremor events that we recorded during a six week period had sustained arrival of 3 hz energy for approximately 10 minutes during that time the source location migrated a few kilometers the harmonic nature of the tremors is interpreted as the result of resonance in subglacial water filled cracks and conduits the duration monochromatic nature and movement of the tremor indicate that the source mechanism is likely flow in the subglacial water system resulting from the discharge from a small subglacial lake our results suggest that the subglacial water system produces repeated small outburst floods with possible implications for ice stream dynamics

confined groundwater near the rockhead in igneous rocks in the mid levels area hong kong china abstract it is customary in hong kong to assume that the hydraulic conductivity of weathered igneous rocks decreases with depth or as the rock mass becomes less weathered such a hydraulic conductivity pattern can only lead to an unconfined aquifer this paper presents a case study in the regions in and around the mid levels area in hong kong regarding a possible relatively high hydraulic conductivity k zone and confined groundwater along the rockhead the mid levels area is located at the lower part of the north facing slopes of victoria peak on hong kong island and is prone to landslides although this site has a long history of geotechnical studies because of extensive urban development along the coast and public concern on slope stability hydrogeology of the site remains poorly understood this paper reexamined the hydraulic conductivity data in 7 boreholes conducted in the 1970s and found that 4 of them indicate an increase in k at the rockhead groundwater conditions revealed by tunnel construction at the coast suggest that k close to the rockhead is about 10 times greater than above rockhead a careful analysis of storm response of a piezometer group with tips in different depths indicates that there was an upward flow from the bedrock to the colluvium a field study of two overflow standpipes conducted by the authors showed that the water level can be 0 64 and 3 73 m above the ground surface which illustrates that the deep groundwater is significantly artesian a search of the archived site investigation reports from the government and private companies has led to an identification of about 24 sites with overflow boreholes which suggests that overflow phenomenon is quite common in the study area the paper then concludes that in the study area there is a relatively high k zone along the rockhead and the groundwater in the zone is confined it is recommended that geotechnical engineers should carry out a more careful field study on an overflow borehole because such a borehole indicates a confined groundwater condition important for slope stability study and foundation design

preferential flow paths through soil and rock and their association with landslides this article discusses preferential flow paths through soil and rock and how these contribute to the style and timing of landslides the importance of geological compartmentalization because of the presence of aquicludes such as weathered dykes and faults is emphasized as is channel flow through fracture networks and through natural pipes in soil like material although piping is a contributing factor to some landslides as generally recognized it can also be a sign of inherent distress in a developing landslide where water flow exploits cracks in the dilating rock mass it is suggested that the association might be targeted in ground investigation inactive or low activity pipe systems can be identified by the anomalous presence of alluvial sediments at depth in the geological profile whereas active pipe systems may simply comprise voids that could be misinterpreted as core loss during ground investigation examples of pipe systems encountered during ground investigations are given for various landslides and several case studies are illustrated in this article mitigation measures for preventing landslides triggered by rainfall are discussed together with the difficulties associated with preferential flow paths copyright 2010 john wiley sons ltd

a decade of progress in observing and modelling antarctic subglacial water systems in the decade since the discovery of active antarctic subglacial water systems by detection of subtle surface displacements much progress has been made in our understanding of these dynamic systems here we present some of the key results of observations derived from icesat laser altimetry cryosat 2 radar altimetry operation icebridge airborne laser altimetry satellite image differencing and ground based continuous global positioning system gps experiments deployed in hydrologically active regions these observations provide us with an increased understanding of various lake systems in antarctica whillans mercer ice streams crane glacier recovery ice stream byrd glacier and eastern wilkes land in several cases subglacial water systems are shown to control ice flux through the glacier system for some lake systems we have been able to construct more than a decade of continuous lake activity revealing internal variability on time scales ranging from days to years this variability indicates that continuous accurate time series of altimetry data are critical to understanding these systems on whillans ice stream our results from a 5 year continuous gps record demonstrate that subglacial lake flood events significantly change the regional ice dynamics we also show how models for subglacial water flow have evolved since the availability of observations of lake volume change from regional scale models of water routeing to process models of channels carved into the subglacial sediment instead of the overlying ice we show that progress in understanding the processes governing lake drainage now allows us to create simulated lake volume time series that reproduce time series from satellite observations this transformational decade in antarctic subglacial water research has moved us significantly closer to understanding the processes of water transfer sufficiently for inclusion in continental scale ice sheet models

modis based mosaic of antarctica moa data sets continent wide surface morphology and snow grain size abstract we present digital image mosaics of surface morphology and optical snow grain size for the antarctic continent and surrounding islands assembled from 260 moderate resolution imaging spectroradiometer modis images the products are derived from modis band 1 red light 647 nm and band 2 infrared 857 nm orbit swath data acquired during the 2003 2004 austral summer multiple images of all areas are combined in a data cumulation scheme to improve spatial resolution and increase radiometric content of the mosaics the component images were de striped geo registered and re sampled to the projection grid using the modis swath to grid toolbox ms2gt software a 125 m ground equivalent polar stereographic projection was used identical to previous radar image mosaics and similar to several other continent wide data sets geo location accuracy of the final mosaics is better than 125 m swath acquisition times were limited to the range 05 00 13 30 ut to provide broad but uniform mosaic illumination regions of cloud cover blowing snow and intense forward scattering were masked prior to compositing the moa snow grain size data set image provides the first continent wide semi quantitative map of mean summer snow firn and blue ice grain size optical grain size radius was determined using a normalized difference index derived from modis band 1 and band 2 rad values atmospheric correction adjustments for directional reflectance variation and snow grain size estimation were accomplished by a look up table generated by a combination of published models of snow spectral reflectance and atmosphere radiative transfer validation of the snow grain size image is provided by in situ spectra of snow covered sea ice made in october 2003 we use the new data sets to determine major ice shelf areas and shelf edge coastline and ice grounding line vector files and determine areas of selected blue ice regions these data sets are available for download at http nsidc org data moa

assessing the subglacial lake coverage of antarctica lakes beneath the antarctic ice sheet are known to decrease traction at the ice base and therefore can have a great impact on ice dynamics however the total extent of antarctic subglacial lakes is still unknown we address this issue by combining modeling and remote sensing strategies to predict potential lake locations using the general hydraulic potential equation we are able to reproduce the majority of known lakes as well as predict the existence of many new and so far undetected potential lakes to validate our predictions we analyzed ice penetrating radar profiles from radio echo sounding flights acquired over 1994 2013 in dronning maud land east antarctica and this led to the identification of 31 new subglacial lakes based on these findings we estimate the total number of antarctic subglacial lakes to be 1300 a factor of three higher than the total number of lakes discovered to date we estimate that only 30 of all antarctic subglacial lakes and 65 of the total estimated lake covered area have been discovered and that lakes account for 0 6 of the antarctic ice bed interface

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

ice shelf grounding zone structure from icesat laser altimetry 1 we present a technique for investigating the grounding zone gz of antarctic ice shelves using laser altimetry from the ice cloud and land elevation satellite icesat most surface height variability in the gz is easily resolved by the icesat laser s 65 m footprint and 172 m along track spacing comparisons of repeated tracks sampled at different phases of the ocean tide identify the landward and seaward limits of tide forced ice flexure providing gz location and width information for each track using icesat data in the institute ice stream region of southern ronne ice shelf we demonstrate that the location of the gz based on feature identification in satellite imagery or digital elevation models may be in error by several km our results show that icesat will contribute significantly to improving knowledge of gz structure and to studies requiring accurate gz locations e g ice mass balance calculations and ice sheet ocean modeling

a large deep freshwater lake beneath the ice of central east antarctica in 1974 75 an airborne radio echo survey of ice depths over central east antarctica led to the discovery of a sub ice lake of unknown depth and composition with an area of about 10 000km2 and lying beneath 4km of ice1 in 1993 altimetric data from satellite measurements2 provided independent evidence of the lake s areal extent thus confirming it to be the largest known sub ice lake by an order of magnitude here we analyse new altimetric and radio echo data along with existing seismic data3 to show that the lake is deep mean depth of 125 m or more and fresh and that it has an area that exceeds previous estimates by about 50 dimensions comparable with those of lake ontario we estimate that the residence time of the water in the lake is of the order of tens of thousands of years and that the mean age of water in the lake since deposition as surface ice is about one million years regional ice dynamics can be explained in terms of steady state ice flow along and over the lake

satellite image derived velocity field of an antarctic ice stream the surface velocity of a rapidly moving ice stream has been determined to high accuracy and spatial density with the use of sequential satellite imagery variations of ice velocity are spatially related to surface undulations and transverse velocity variations of up to 30 percent occur such large variations negate the concept of plug flow and call into question earlier mass balance calculations for this and other ice streams where sparse velocity data were used the coregistration of images with the use of the topographic undulations of the ice stream and the measurement of feature displacement with cross correlation of image windows provide significant improvements in the use of satellite imagery for ice flow determination

ice sheet change detection by satellite image differencing abstract differencing of digital satellite image pairs highlights subtle changes in near identical scenes of earth surfaces using the mathematical relationships relevant to photoclinometry we examine the effectiveness of this method for the study of localized ice sheet surface topography changes using numerical experiments we then test these results by differencing images of several regions in west antarctica including some where changes have previously been identified in altimeter profiles the technique works well with coregistered images having low noise high radiometric sensitivity and near identical solar illumination geometry clouds and frosts detract from resolving surface features the etm sensor on landsat 7 ali sensor on eo 1 and modis sensor on the aqua and terra satellite platforms all have potential for detecting localized topographic changes such as shifting dunes surface inflation and deflation features associated with sub glacial lake fill drain events or grounding line changes availability and frequency of modis images favor this sensor for wide application and using it we demonstrate both qualitative identification of changes in topography and quantitative mapping of slope and elevation changes

identification of subglacial lakes using ers 1 radar altimeter radar altimeter data from ers 1 allow detailed topographic mapping of antarctica to 82 degrees s to be carried out revealing several notable surface features among these is the large subglacial lake near vostok which is mapped here in detail the central part of the lake is found to have an exceptionally smooth surface with r m s elevation variations of less than 0 2 m a search for other large antarctic lakes in the data is made based on the smoothness and low gradient of the surface a number of other flat areas are identified with lake locations previously determined from radio echo sounding observations however radar altimeter observations show that a minimum lake size of 20 km is required for a surface above a lake to become flat numerous bowl like features can be seen in the surface topography and these may be associated with intermediate sized subglacial lakes it is determined that high spatial resolution radar altimetery could be used to identify subglacial lakes greater than 10 km in lateral extent flat regions of the ice sheet are particularly useful as they may be used as height reference surfaces to help fix the orbits of radar altimeter satellites

enabling clean access into subglacial lake whillans development and use of the wissard hot water drill system clean hot water drill systems chwdss are used with clean access protocols for the exploration of subglacial lakes and other subglacial aquatic environments e g ice shelf cavities in antarctica a chwds developed for the whillans ice stream subglacial access research drilling wissard project by the science management office at the university of nebraska lincoln unl smo usa was specifically designed for use in west antarctica where the us antarctic program s south pole traverse could assist with logistical support the initial goal was to provide clean access holes through ice up to 1000 m thick following environmental stewardship guidelines however the existing design allows this chwds to be used for ice thicknesses up to 2000 m following modifications to accommodate longer hose lengths in january 2013 the wissard chwds successfully provided for the first time a clean access borehole through 800 m of ice into subglacial lake whillans beneath the west antarctic ice sheet for the deployment of scientific instruments and sampling tools the development and initial use of the wissard chwds required the project team to address a number of constraints while providing contingencies to meet the defined project scope schedule and budget

an improved glacial isostatic adjustment model for the british isles the glacial isostatic adjustment gia of the british isles is of interest due to the constraints that can be provided on key model parameters such as the global meltwater signal local ice sheet history and viscoelastic earth structure a number of recent studies have modelled relative sea level rsl data from this region to constrain model parameters as indicated in these studies the sensitivity of these data to both local and global parameters results in a highly non unique problem this study aims to address this inherent non uniqueness by combining a previously published british irish ice model that is based on the most recent geomorphological data with a new global ice sheet model that provides an accurate prediction of eustatic sea level change in addition constraints from global positioning system gps measurements of present day vertical land motion are considered alongside the entirety of rsl data from both great britain and ireland a model solution is found that provides a high quality fit to both the rsl data and the gps data within the range of earth viscosity values considered the optimal data model fits were achieved with a relatively thin lithosphere 71 km upper mantle viscosities in the range 4 6 1020 pa s and lower mantle viscosities 3 1022 pa s copyright 2011 john wiley sons ltd

a decade of west antarctic subglacial lake interactions from combined icesat and cryosat 2 altimetry we use cryosat 2 interferometric satellite radar altimetry over the mercer and whillans ice streams west antarctica to derive surface elevation changes due to subglacial lake activity at monthly resolution for the period 2010 to 2013 we validate cryosat 2 elevation measurements trends and spatial patterns of change using satellite image differencing and in situ vertical movement from global positioning system gps data two subglacial lake discharge events occur in the same subglacial hydrological catchment within a 9 month period using gps measurements that are spanning the gap between the ice cloud and land elevation satellite and cryosat 2 missions we cross calibrate the two missions to establish the efficacy of cryosat 2 altimetry to measure dynamic changes on the ice sheets

increased flow speed on a large east antarctic outlet glacier caused by subglacial floods large ice streams and outlet glaciers drain greenland and antarctica an observed acceleration of ice velocity in one of these outlet glaciers byrd glacier east antarctica coincides with a large water discharge from two subglacial lakes allowing direct attribution of the change in glacier dynamics to the water drainage network beneath the ice

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

internal soil moisture and piezometric responses to rainfall induced shallow slope failures summary better knowledge regarding internal soil moisture and piezometric responses in the process of rainfall induced shallow slope failures is the key to an effective prediction of the landslide and or debris flow initiation to this end internal soil moisture and piezometric response of 0 7 m deep 1 5 m wide 1 7 m high and 3 94 m long semi infinite sandy slopes rested on a bi linear impermeable bedrock were explored using a chute test facility with artificial rainfall applications the internal response time defined by the inflection point of the soil moisture and piezometric response curves obtained along the soil bedrock interface were closely related to some critical failure states such as the slope toe failure and extensive slope failures it was also found that the response times obtained at the point of abrupt bedrock slope decrease can be used as indicators for the initiation of rainfall induced shallow slope failures an investigation of spatial distributions of soil water content or degrees of saturation s r in the slope at critical failure states shows that the 0 2 m below surface zone remains unsaturated with s r 40 60 regardless of their distances from the toe and the rainfall intensity non uniform distributions of or s r along the soil bedrock interface at critical failure states were always associated with near saturation states s r 80 100 around the point of bedrock slope change or around the transient toe upstream of the slumped mass induced by the retrogressive failure of the slope these observations suggest the important role of the interflow along the soil bedrock interface and the high soil water content or high porewater pressure around the point of bedrock slope deflection in the rainfall induced failure of sandy slopes consisting of shallow impermeable bedrocks the present study proposes an internal response time criterion to substantiate the prediction of rainfall induced shallow slope failures it is believed that the internal response time reflects the overall characteristics of a slope under rainfall infiltration and can be as useful as the conventional meteorology based threshold times the internal response time theory can be generalized via numerical modeling of slope hydrology slope geology and slope stability in the future

evaluation of a numerical model of the british irish ice sheet using relative sea level data implications for the interpretation of trimline observations the glacial isostatic adjustment gia of the british isles is complex due to the interplay between local and non local signals a number of recent studies have modelled the gia response of the british isles using relative sea level data this study extends these previous analyses by using output from a numerical glaciological model as input to a gia model this is a necessary step towards more realistic gia models and although there have been similar studies for the major late pleistocene ice sheets this is the first study to do so for the british isles we test three reconstructions classed as minimal median and maximal in terms of their volume at maximum extent and find it is possible to obtain good data model fits the minimal reconstruction is clearly preferred by the sea level data the ice reconstructions tested were not constrained by geomorphological information of past ice extent lateral and vertical as a consequence the reconstructions extend further than much of this information suggests particularly in terms of ice thickness it is notable however that the reconstructions produce good fits to many sea level data from central mountainous regions e g scottish highlands which lends support to the suggestion that trimlines often used as an constraint on the palaeo ice surface location are in fact features formed at the transition from warm to cold based ice and so mark a minimum constraint on the ice surface altitude based on data model misfits suggestions are made for improving the ice model reconstructions however in many locations the cause of the misfit could be due to non local predominantly fennoscandian ice and so interpretation is not straightforward as a result we suggest that future analyses of this type consider models and observations for both fennoscandia and the british isles copyright 2012 john wiley sons ltd

geotechnical and landslide aspects in weathered granitoid rock masses serre massif southern calabria italy abstract the weathering processes produce important chemical and physical transformations in rock masses producing a decay of their strength and then stability in this study different methodologies were employed to define the physical mechanical properties and the minero petrographical changes at each grade of weathering useful to determine a geological model of the weathered granitoids involved in mass movements the paper focuses on a multidisciplinary research on the weathering profiles of the paleozoic granitoid rocks surrounding the village of fabrizia located in the serre massif southern calabria italy fabrizia lies on an old erosion surface composed of basement rocks made up of weathered granitoids belonging to the serre massif granitoid complex and overlain by thin eluvial colluvial deposits the tectonic uplift of the area combined with mediterranean climatic conditions both active at least since the quaternary are the main causes of the deep weathering of the plutonic rocks the weathered profiles of the granitoid complex have been studied through field observations borehole explorations with geotechnical tests and seismic surveys coupled to minero petrographical analysis and laboratory tests weathered profiles are mainly composed by rock masses varying from completely weathered and residual soil classes v vi to moderately weathered rocks class iii characterized by a progressive increase of geotechnical properties e g seismic wave velocity rock quality designation internal friction angle uniaxial compressive strength with a decrease of weathering the weathered rocks display a wide variety of mass movement landslide types are rockfalls for the fresher rocks class iii translational slides occur between different weathering degrees classes iv and v and rotational slides dominate at the top of the slopes and generally involve the weathering grades from class iv to classes v vi the shallow horizons are composed of thin eluvial colluvial deposits and more weathered granitoids classes v vi that may generate dangerous debris flows or debris avalanches during intense and prolonged rainfall the landslide surface is usually at a depth lower than 2 m in the debris flow type while deeper landslides depth 2 m are rotational and translational types with sliding surfaces occurring mainly in the transition between the classes iv and v the field survey coupled to geotechnical geophysical and minero petrographical data allowed the recognition of the different weathered horizons and relative geotechnical properties useful to the reconstruction of the reliable geological model the model represents an important support for the study of the landslide processes and for a sustainable management of the territory the used approach provided results to set a general reference framework for establishing more precise correlations between mass movement phenomena and weathering processes

experimental investigation of rainfall criteria for shallow slope failures a theory of the internal response based rainfall criterion for predicting slope failure is proposed based on artificial rainfall tests for a sandy slope key elements in a generic model describing shallow slope failure are identified from the tests with extensive soil moisture and piezometric measurements the observed retrogressive slope failure was triggered by a toe failure followed by an abrupt increase in debris discharge the onset of the failure was preceded by a clear transition point on the soil moisture vs time curve at the soil bedrock interface both external and internal responses of slopes to rainfall can be expressed using widely adopted i d rainfall intensity duration plots experimental results from the tests indicate close relationships between the internal based and external based i d envelopes for critical slope states the theory of the internal based i d criterion proposed here will be useful for more accurate slope failure predictions

evidence for subglacial water transport in the west antarctic ice sheet through three dimensional satellite radar interferometry 1 radarsat data from the 1997 antarctic mapping mission are used interferometrically to solve for the 3 dimensional surface ice motion in the interior of the west antarctic ice sheet wais an area of 125 km2 in a tributary of the kamb ice stream slumped vertically downwards by up to 50 cm between september 26 and october 18 1997 areas in the bindschadler ice stream also exhibited comparable upward and downward surface displacements as the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements these results and related lidar observations imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult to observe subglacial environment and its important influence on ice dynamics

the influence of subglacial hydrology on the flow of kamb ice stream west antarctica 1 ice streams on the siple coast west antarctica have a complex history of flow because their basal motion is governed by time varying basal conditions although the mechanical interaction between ice and till is well established very little is known about the potential effect of regionally scaled water transport in a basal water system which has only recently become apparent to investigate the combined effect of hydrological and mechanical processes we developed the hydrology ice and till model in which ice flow is coupled to a coulomb plastic till layer and a basal water system consisting of discrete conduits when the model is applied to kamb ice stream kis results confirm that it is capable of oscillating between fast and stagnant modes of flow we show that when subglacial conduits are disregarded or do not extend to the grounding line the oscillatory behavior of the ice stream is governed by the basal thermal regime when conduits extend to the grounding line the modelled ice stream oscillation period is increased peak speeds are reduced and oscillations may ultimately cease if the volume of water supplied is sufficiently high three different hydrological states characterize the behavioral patterns of ice flow and these states are distinguished by conditions at the grounding line modelled ice stream velocities were found to oscillate with fast and slow periods typically lasting a few hundred years although varying according to hydrological activity our results indicate that kis could reactivate this century given its hydrological setting and 170 years of stagnation

rapid discharge connects antarctic subglacial lakes a hidden subglacial world consisting of more than 140 lakes lies beneath the antarctic ice sheet it is widely assumed that these lakes are a static ecosystem cut off from each other and the rest of the world far from it according to a 16 month study of surface elevation changes in the region changes indicating a rapid discharge of water from one lake to another were detected and it is possible that the entire subglacial drainage basin is flushed through periodically by massive water transfers observation of surface elevation changes over 16 months in an area of the east antarctic ice sheet overlying subglacial lakes suggests a rapid discharge of one lake to at least two other lakes involving a water volume of 1 8 km3 in conflict with previous expectations that subglacial lakes have long residence times and slow circulations the existence of many subglacial lakes1 provides clear evidence for the widespread presence of water beneath the east antarctic ice sheet but the hydrology beneath this ice mass is poorly understood2 such knowledge is critical to understanding ice flow basal water transfer to the ice margin glacial landform development and subglacial lake habitats here we present ice sheet surface elevation changes in central east antarctica that we interpret to represent rapid discharge from a subglacial lake our observations indicate that during a period of 16 months 1 8 km3 of water was transferred over 290 km to at least two other subglacial lakes while viscous deformation of the ice roof above may moderate discharge the intrinsic instability of such a system3 suggests that discharge events are a common mode of basal drainage4 if large lakes such as lake vostok or lake concordia1 are pressurizing it is possible that substantial discharges could reach the coast5 6 our observations conflict with expectations that subglacial lakes have long residence times and slow circulations2 7 8 and we suggest that entire subglacial drainage basins may be flushed periodically the rapid transfer of water between lakes would result in large scale solute and microbe relocation and drainage system contamination from in situ exploration is therefore a distinct risk

an inventory of active subglacial lakes in antarctica detected by icesat 2003 2008 through the detection of surface deformation in response to water movement recent satellite studies have demonstrated the existence of subglacial lakes in antarctica that fill and drain on timescales of months to years these studies however were confined to specific regions of the ice sheet here we present the first comprehensive study of these active lakes for the antarctic ice sheet north of 868 s based on 4 5 years 2003 08 of nasa s ice cloud and land elevation satellite icesat laser altimeter data our analysis has detected 124 lakes that were active during this period and we estimate volume changes for each lake the icesat detected lakes are prevalent in coastal antarctica and are present under most of the largest ice stream catchments lakes sometimes appear to transfer water from one to another but also often exchange water with distributed sources undetectable by icesat suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow thus these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds but also may withdraw water at other times to slow flow

basal shear stress of the ross ice streams from control method inversions 1 we used control method inversions to determine the basal shear stress beneath the ross ice streams where new high resolution velocity data sets have recently become available the inversion algorithm was adapted from an earlier viscous bed algorithm to allow solution for the basal shear stress corresponding to a weak plastic bed we performed several experiments using synthetic data to determine the quality of the inversions these experiments indicate that with high quality surface elevation data e g errors 5 m the inversions are relatively robust with respect to errors in ice flow velocity and bed topography the inversions are consistent with seismic and borehole observations and indicate that the ross ice streams lie atop a bed that is nearly everywhere weak in contrast the tributaries feeding these ice streams overlie alternating patches of strong and weak bed

active lakes of recovery ice stream east antarctica a bedrock controlled subglacial hydrological system a connected system of active subglacial lakes was revealed beneath recovery ice stream east antarctica by icesat laser altimetry here we combine repeat track analysis of icesat 2003 09 operation icebridge laser altimetry and radio echo sounding 2011 and 2012 and modis image differencing 2009 2011 to learn more about the lake activity history the surface and bedrock topographic setting of the lakes and the constraints on water flow through the system we extend the lake activity time series to 2012 for the three lower lakes and capture twomajor lake drainages one lake underwent a large deflation between 2009 and 2011 while another lake which had been continuously filling between 2003 and 2010 started to drain after 2011 most of the active lakes are located in a 1000 km long bedrock trough under the main trunk of recovery ice stream whose base is 1500 2000 m below present day sea level the hydrologic system beneath recovery ice stream is controlled by this unusually pronounced bedrock topography in contrast to most antarctic systems studied to date which are controlled by the ice surface topography hydrologic connections among the lakes appear to be direct and responsive and we reproduce the lake activity using a simple subglacial water model we discuss potential causes of non steady hydrologic behavior in major antarctic catchments

sea level change glacial rebound and mantle viscosity fornorthern europe northwestern europe remains a key region for testing models of glacial isostasy because of the good geological record of crustal response to the glacial unloading since the time of the last glacial maximum models for this rebound and associated sea level change require a detailed knowledge of the ice sheet geometry including the ice thickness through time existing ice sheet reconstructions are strongly model dependent and inversions of sea level data for the mantle response may be a function of the model assumptions thus inverse solutions for the sea level data are sought that include both ice and earth model parameters as unknowns sea level data from fennoscandia the north sea the british isles and the atlantic and english channel coasts have been evaluated and incorporated into the solutions the starting ice sheet for fennoscandia is based on a reconstruction of a model by denton hughes 1981 that is characterized by quasi parabolic cross sections and symmetry about the load centre both global northwestern europe as a whole and regional subsets of the data solutions have been made for earth model parameters and ice height scaling parameters the key results are as follows 1 the response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across scandinavia the north sea and the british isles 2 this response can be adequately modelled by an effective elastic lithosphere of thickness 65 85 km and by an effective upper mantle viscosity from the base of the lithosphere to the 670 km depth seismic discontinuity of about 3 4 1020 pa s the effective lower mantle viscosity is at least an order of magnitude greater 3 the ice thickness over scandinavia at the time of maximum glaciation was only about 2000 m much less than the 3400 m assumed in the denton hughes model 4 the ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin

geomorphology under ice streams moving from form to process ice streams are integral components of an ice sheet s mass balance and directly impact on sea level their flow is governed by processes at the ice bed interface which create landforms that in turn modulate ice stream dynamics through their influence on bed topography and basal shear stresses thus ice stream geomorphology is critical to understanding and modelling ice streams and ice sheet dynamics this paper reviews developments in our understanding of ice stream geomorphology from an historical perspective with a focus on the extent to which studies of modern and palaeo ice streams have converged to take us from a position of near complete ignorance to a detailed understanding of their bed morphology during the 1970s and 1980s our knowledge was limited and largely gleaned from geophysical investigations of modern ice stream beds in antarctica very few palaeo ice streams had been identified with any confidence during the 1990s however glacial geomorphologists began to recognise their distinctive geomorphology which included distinct patterns of highly elongated mega scale glacial lineations ice stream shear margin moraines and major sedimentary depocentres however studying relict features could say little about the time scales over which this geomorphology evolved and under what glaciological conditions this began to be addressed in the early 2000s through continued efforts to scrutinise modern ice stream beds at higher resolution but our current understanding of how landforms relate to processes remains subject to large uncertainties particularly in relation to the mechanisms and time scales of sediment erosion transport and deposition and how these lead to the growth and decay of subglacial bedforms this represents the next key challenge and will require even closer cooperation between glaciology glacial geomorphology sedimentology and numerical modelling together with more sophisticated methods to quantify and analyse the anticipated growth of geomorphological data from beneath active ice streams

structure morphology and water flux of a subglacial drainage system midtdalsbreen norway digital elevation models of the surface and bed of midtdalsbreen norway are used to calculate subglacial hydraulic potential and infer drainage system structure for a series of subglacial water pressure assumptions ranging from atmospheric to ice overburden a distributed degree day model is used to calculate the spatial distribution of melt on the glacier surface throughout a typical summer which is accumulated along the various drainage system structures to calculate water fluxes beneath the glacier and exiting the portals for the different water pressure assumptions in addition 78 dye tracing tests were performed from 33 injection sites and numerous measurements of water discharge were made on the main proglacial streams over several summer melt seasons comparison of the calculated drainage system structures and water fluxes with dye tracing results and measured proglacial stream discharges suggests that the temporally and spatially averaged steady state water pressures beneath the glacier are 70 of ice overburden analysis of the dye return curves together with the calculated subglacial water fluxes shows that the main drainage network on the eastern half of the glacier consists of a hydraulically efficient system of broad low channels average width height ratio 75 the smaller drainage network on the west consists of a hydraulically inefficient distributed system dominated by channels that are exceptionally broad and very low average width height ratio 350 the even smaller central drainage network also consists of a hydraulically inefficient distributed system dominated by channels that are very broad and exceptionally low average width height ratio 450 the channels beneath the western and central glacier must be so broad and low that they can essentially be thought of as a linked cavity system

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

subglacial lake whillans ice penetrating radar and gps observations of a shallow active reservoir beneath a west antarctic ice stream abstract ice penetrating radar and kinematic gps observations from subglacial lake whillans slw west antarctica reveal a shallow lake that is confined by steep basal topographic features radar imaging of slw although indicating wet basal conditions is consistent with a water column depth of only 6 m or less during the near low stand state at the time of the survey kinematic gps profiles reveal that slw is generally defined by a 15 m surface depression centered at s 84 237 w 153 614 this point coincides with the area of lowest hydropotential in the lake basin and also the largest surface elevation range in icesat data therefore this location appears to be an opportune site for subglacial access drilling of this active subglacial lake a distinct basal topographic ridge on the grid south side of the basin is coincident with a strong contrast in relative basal reflectivity 6 db which we interpret as the lake boundary mapped hydropotential calculated assuming hydrostatic equilibrium shows that water enters the lake from the upstream direction and drains downstream we hypothesize that a lake level rise of 5 m plus flexural effects is sufficient to overtop a drainage divide thus slw acts as a temporary storage basin for water beneath whillans ice stream

connected subglacial lake activity on lower mercer and whillans ice streams west antarctica 2003 2008 we examine patterns of localized surface elevation change in lower mercer and whillans ice streams west antarctica which we interpret as subglacial water movement through a system of lakes and channels we detect and measure the lake activity using repeat track laser altimetry from icesat and image differencing from modis image pairs a hydrostatic potential map for the region shows that the lakes are distributed across three distinct hydrologic regimes our analysis shows that within these regimes some of the subglacial lakes appear to be linked with drainage events in one reservoir causing filling and follow on drainage in adjacent lakes we also observe changes near ice raft a in lower whillans ice stream and interpret them as evidence of subglacial water and other changes at the bed the study provides quantitative information about the properties of this complex subglacial hydrologic system and a relatively unstudied component of ice sheet mass balance subglacial drainage across the grounding line

model for the prediction of shear strength with respect to soil suction experimental studies on unsaturated soils are generally costly time consuming and difficult to conduct shear strength data from the research literature suggests that there is a nonlinear increas

field model experiments to determine mechanisms of rainstorm induced shallow landslides in the feiyunjiang river basin china abstract this paper investigates rainfall infiltration processes and the failure mechanisms of slopes in the feiyunjiang river basin field model experiments were conducted on a selected residual soil slope next to the fuao landslide which was about 30 m long and 25 m wide and failed along the interface between soil and bedrock during typhoon megi the results indicated that rainfall infiltration over the slope surface had little impact on pore water pressure and deformation of the slope while the coupled action of slope surface infiltration and preferential infiltration through an artificial ditch at the back of the slope caused rapid saturation near the interface between the residual soil and bedrock resulting in significant displacement along the interface other preferential infiltration channels on slopes in the feiyunjiang river basin include tension cracks water impoundment and intrusive dykes and their weathered layers the conclusions derived from this study are helpful in understanding rainstorm induced soil landslides in the feiyunjiang river basin in china and possibly in other similar slope settings under comparable climate conditions

basal friction of ice stream e west antarctica we use surface velocity derived from sequential landsat imagery and a control method to estimate the basal friction distribution of a major west antarctic ice stream the area averaged basal stress is approximately 1 4 10 4 pa or about 29 of the area averaged driving stress of 4 9 10 4 pa uncertainty of the derived area averaged basal stress is difficult to assess and depends primarily on spatial variation of the flow law rate factor in the constitutive law spatial variation associated with depth averaged temperature variation gives an uncertainty of approximately 10 3 pa approximately 60 of the ice stream has a basal stress magnitude less than 10 4 pa and approximately 30 has less than 10 3 pa these characteristics suggest the presence of a mechanically weak water charged subglacial till small scale sticky spots where basal friction exceeds the area averaged driving stress are scattered irregularly across the subglacial regime and comprise approximately 15 of the ice stream area sticky spots cluster in regions where landsat imagery suggests structural features in the underlying bedrock

ice stream sticky spots a review of their identification and influence beneath contemporary and palaeo ice streams rapidly flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance recent observations have revealed that ice stream flow exhibits considerable variability with relatively rapid changes taking place in speed and direction this spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction known as sticky spots in this paper we provide a detailed review of sticky spot observations from both contemporary and palaeo ice stream beds in order to better understand their nature and influence observations and theoretical considerations reveal four primary causes of stickiness i bedrock bumps ii till free areas iii areas of strong well drained till and iv freeze on of subglacial meltwater these may act together in one location or in isolation and a progressive increase in their distribution could lead to ice stream shut down bedrock bumps are influential under active ice streams where they provide form drag and can create thinner ice which increases the likelihood of basal freeze on increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut down because over long time scales ice stream erosion might be expected to reduce their amplitude the influence of till free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas they are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams if an ice stream operates solely by till deformation it is conceivable that a progressive increase in the exposure of till free areas could lead to shut down through a process of sediment exhaustion areas of strong well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater the collapse of an inefficient cannalised system to a more efficient channelised system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut down in both contemporary and palaeo settings basal freeze on has also been observed and inferred from beneath modern and palaeo ice streams and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream a paucity of data from ice stream sticky spots limits a better understanding of their nature distribution and evolution beneath ice streams future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well preserved palaeo ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification such data will considerably enhance the basal boundary condition in ice stream models which will ultimately refine our predictions of the response of contemporary ice sheets to future changes in climate

characterization of the groundwater response to rainfall on a hillslope with fractured bedrock by creep deformation and its implication for the generation of deep seated landslides on mt wanitsuka kyushu island abstract in this study the hydrogeological response of a hillslope affected by gravitational deformation was analyzed based on the data obtained for two observation boreholes a 10 m borehole representing a relatively superficial section of the hillslope dominated by soil and old landslide deposits and a 40 m borehole representing the bedrock aquifer for this analysis the precipitation events in the area groundwater levels of the two boreholes electrical conductivity and isotopic concentration oxygen and hydrogen of the groundwater in the boreholes and rainfall samples were measured derived from the rainfall data the antecedent precipitation index api with a 6 h half life offers a good correlation with the peaks in the groundwater levels in the bedrock aquifer the characteristics of the groundwater response suggest the existence of a single structure in the bedrock that controls the response of the hillslope the structure serves as a conduit which rapidly drives the rainfall water recharge into the bedrock evidently this process is unrelated to the superficial section represented by the observations in the 10 m borehole the structure is associated with an area of high hydraulic conductivity in the bedrock caused by the gravitational deformation in the hillslope the strong control of this structure in the hillslope s hydrogeological response makes it responsible for the hillslope s stability during high precipitation events this information is highly relevant to areas featuring the generation of several deep seated landslides under heavy rain conditions

identification and control of subglacial water networks under dome a antarctica 1 subglacial water in continental antarctica forms by melting of basal ice due to geothermal or frictional heating subglacial networks transport the water from melting areas and can facilitate sliding by the ice sheet over its bed subglacial water flow is driven mainly by gradients in overburden pressure and bed elevation we identify small median 850 m water bodies within the gamburtsev subglacial mountains in east antarctica organized into long 20 103 km coherent drainage networks using a dense 5 km grid of airborne radar data the individual water bodies are smaller on average than the water bodies contained in existing inventories of antarctic subglacial water and most are smaller than the mean ice thickness of 2 5 km reflecting a focusing of basal water by rugged topography the water system in the gamburtsev subglacial mountains reoccupies a system of alpine overdeepenings created by valley glaciers in the early growth phase of the east antarctic ice sheet the networks follow valley floors either uphill or downhill depending on the gradient of the ice sheet surface in cases where the networks follow valley floors uphill they terminate in or near plumes of freeze on ice indicating source to sink transport within the basal hydrologic system because the ice surface determines drainage direction within the bed constrained network the system is bed routed but surface directed along flow variability in the structure of the freeze on plumes suggests variability in the networks on long 10s of ka timescales possibly indicating changes in the basal thermal state

breaking the seal at grimsvotn iceland of several problems associa ted wit h theoretica l expla nation s of the jokul hla ups which emerge from the outlet gla cie r skeioarar jokull of the ice ca p vatna jokull in sou theast iceland the mechani sm of flood init iation is one that has hi therto defied expl anat ion we provide such an expl ana tion based on a car efu l analysis of the cla ssica l nye ro thlisbe rger model near the subglacial lake grimsvotn the hyd raulic potenti al gradie nt is towards the lake and there is theref ore a catchme nt boun dary under the ice whose location dep ends on the subglacial mel twater drai nage charac terist ics as the con dit ions for a flood approa ch we show that the wat er divi de migra tes towards the lake while at the sa me time the lake pre ssure in creases when the hydra ulic potential gradient to wards the lake is low and the refillin g rate is sl ow the se al will break whe n the cat chment boun dary reaches the lake while the la ke level is st ill below flotati on pressu re whereas if refillin g is rapid flo tation can be achie ved bef ore a flood is initia ted this theory can thus explain why the seal is normally broken when the lake le vel at grimsvotn is still some 60 m below flotation level in addit ion we are abl e to explain why thej okulhla up following the 1996 eruption did not occur until flo tation level was achie ved and we show how the cyeli cit y and magnitude ofjoku lhla ups can be explained wit hin this theory

height changes over subglacial lake vostok east antarctica insights from gnss observations height changes of the ice surface above subglacial lake vostok east antarctica reflect the integral effect of different processes within the subglacial environment and the ice sheet repeated gnss global navigation satellite systems observations on 56 surface markers in the lake vostok region spanning 11 years and continuous gnss observations at vostok station over 5 years are used to determine the vertical firn particle movement vertical marker velocities are derived with an accuracy of 1 cm yr or better repeated measurements of surface height profiles around vostok station using kinematic gnss observations on a snowmobile allow the quantification of surface height changes at 308 crossover points the height change rate was determined at 1 5 mm yr thus indicating a stable ice surface height over the last decade it is concluded that both the local mass balance of the ice and the lake level of the entire lake have been stable throughout the observation period the continuous gnss observations demonstrate that the particle heights vary linearly with time nonlinear height changes do not exceed 1 cm at vostok station and constrain the magnitude of spatiotemporal lake level variations icesat laser altimetry data confirm that the amplitude of the surface deformations over the lake is restricted to a few centimeters assuming the ice sheet to be in steady state over the entire lake estimates for the surface accumulation on basal accretion melt rates and on flux divergence are derived

tunnel valleys concepts and controversies a review abstract although erosion of valleys by subglacial meltwater was first proposed more than 100 years ago the processes that produced these valleys under past ice sheets have remained a topic of vigorous debate the most commonly hypothesized mechanisms have included gradual formation by sediment deformation into the incipient valleys under steady state conditions and subsequent transport by meltwater to the ice margin time transgressive formation along a retreating ice margin by drainage of surface meltwater to the bed or release of impounded meltwater behind a marginal permafrost wedge probably catastrophically and rapid erosion during widespread catastrophic basal sheetflood events recent application of new and enhanced imaging techniques has led to the identification of many new tunnel valleys both on land and offshore throughout the regions glaciated during former ice ages in addition new advances in understanding of subglacial hydrology both from modelling and field instrumentation have helped to constrain the drainage of meltwater from past ice sheet beds this work elucidates the role of subglacial groundwater flow systems in evacuating meltwater from the bed but also shows that groundwater flow alone was not enough to maintain ice bed meltwater pore pressures below the level of ice flotation both modelling and field observations indicate that subglacial conduits oriented parallel to glacial flow lines reorganize groundwater flow systems into basins containing lateral flow toward conduits cut upward into the ice or downward into the bed because of the inability of groundwater flow to account for all the meltwater produced by basal melting other drainage mechanisms involving both non channelized and channelized meltwater flow have been proposed the notion of catastrophic drainage of impounded basal meltwater is still favored by many and the discovery of hundreds of subglacial lakes beneath the antarctic ice sheet and the likelihood of similar lakes beneath pleistocene ice sheets provides a source of water for those advocating catastrophic formation for tunnel valleys proponents of catastrophic drainage consider the valleys to be channels reflecting the width and perhaps depths of flows that formed them tunnel valleys produced by the scandinavian ice sheet have been described and mapped on land as well as on the sea bed valleys and networks of valleys some of which are totally buried have been linked to 3 or more glaciations tunnel valley networks from the last weichselian glaciation in denmark can be related to specific ice margins developed during ice retreat in north america most research has focused on tunnel valleys associated with ice margins formed by the last wisconsin glaciation along the southern margin of the laurentide ice sheet

the use of discrete fracture network simulations in the design of horizontal hillslope drainage networks in fractured rock abstract characteristics of fracture networks are explored in a discrete fracture network framework to provide design guidelines for horizontal drainage networks in fractured rock central to the study is defining how fracture attributes relate to fracture network structure and network scale fluid flow and in turn how flow characteristics of fracture networks influence horizontal drain length and orientation multiple realizations of stochastic fracture networks generated from both synthetic and field specific data sets serve as a basis for understanding physical fracture network structure and resultant global flow and for performing intersection analyses of hillslope drains with flowing fractures study results indicate that the logarithm of the standard deviation of fracture transmissivity log t is the single most important attribute for drainage network design as higher values of log t describe heterogeneous flow patterns where only a small portion of the network conducts a significant quantity of fluid thus recommended drain lengths for intersecting significantly conductive fractures increase with increases in log t fracture trace length orientation and density also play a role albeit secondary to the distribution of transmissivity in defining drain length as a function of drain orientation relative to the mean fracture set orientation the spatially discontinuous nature of fracture networks and the wide range in transmissivity values found in natural fracture networks tend to produce high degrees of variability in computed intersection distances between drains and fractures conducting significant quantities of fluid to account for this variability a conservative approach is recommended where horizontal drain lengths along a pre defined orientation are scaled by discrete fracture network computed intersection distances equal to the upper 95th confidence interval

dynamic distributed drainage implied by the flow evolution of the 1996 1998 adventure trench subglacial lake discharge abstract the transport of subglacial water beneath the east antarctic ice sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities and ultimately global climate periodic outbursts are a critical mechanism in this process recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km 3 traveling some 260 km along the ice bed interface of the adventure subglacial trench between 1996 and 1998 wingham et al 2006 rapid discharge connects antarctic subglacial lakes nature 440 1033 1036 using radar echo sounding data from the adventure subglacial trench region in conjunction with the previously reported satellite observations along with some basic modeling we calculate a mass budget and infer a flow mechanism for the 1996 1998 event the volume released from the source lake exceeded the volume received by the destination lakes by 1 1 km 3 this discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward the downstream release of water from the destination lakes continued until at least 2003 several years after the 1998 cessation of surface subsidence at the source lake by 2003 a total of 1 5 km 3 or nearly 75 of the water released by the source lake had traveled downstream from the destination lakes the temporal evolution of discharge from the outlet can be simulated with the classic ice walled semicircular channel model if and only if the retreat of the source lake shoreline is taken into account further downstream the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable along these reaches mechanical lifting of the ice roof and or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge a distributed system is also consistent with the 3 month delay between water release at the source lake and water arrival at the destination lake observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system ultimately the presence of large subglacial lakes along the flow path of the 1996 1998 adventure subglacial trench flow path delayed the arrival of water to points downstream by approximately 12 months

sticky spots and subglacial lakes under ice streams of the siple coast antarctica locations of subglacial lakes discovered under fast moving west antarctic ice streams tend to be associated with topographic features of the subglacial bed or with areas that have strong variations in basal conditions inversion of ice stream surface velocity indicates that basal conditions under ice streams can be highly variable and that there can be widespread regions where basal traction is high to seek an explanation for why lakes appear to be sited near areas with high basal traction we use numerical models to simulate ice stream dynamics thermodynamics and subglacial water flow we demonstrate that the ice flow over high basal traction areas produces favourable conditions for the ponding of meltwater energy dissipation associated with ice sliding over a region with high basal traction constitutes a water source supplying a lake and ice thickness perturbations induced by ice flow over variable traction create local minima in hydraulic potential variations in thermodynamic processes caused by such ice flow could be responsible for limiting the horizontal extent of the subglacial lakes

confined groundwater zone and slope instability in weathered igneous rocks in hong kong in hong kong slope failure in weathered granite and volcanic rocks has been studied extensively but the impact of hydrogeological conditions on slope stability has received much less attention it is customary in hong kong to assume that the hydraulic conductivity k of weathered igneous rocks decreases with depth or as the rock mass becomes less weathered generally studies of groundwater for determining slope stability treat the saprolite above the rockhead as an aquifer and the rockhead as an impermeable boundary this paper examines direct and indirect aquifer hydraulic information scattered in various sources and demonstrates that such a model may be inappropriate evidence is cited to show that at least in some places a relatively high hydraulic conductivity k zone hkz may exist at depth either in the lower saprolite or at the rockhead the completely decomposed igneous rocks if significantly kaolinized may have low permeability and behave as a confining zone for the hkz the groundwater flow regime in such an hkz may be confined resulting in a higher rise in water pressure in response to rainfall than might otherwise be expected these high pressures may result in a significant reduction in slope stability an examination of the cross sections of some well known landslides in hong kong reveals a chairshaped rockhead profile numerical models coupling saturated and unsaturated subsurface flow are used to investigate the saturation process in the presence of an hkz in slopes in response to typical rainstorms the simplified bishop s method is used to calculate changes in the factor of safety for slopes during such rainstorms the saturation process and pressure distribution in a slope are complicated by the presence of an hkz an hkz that is blocked at the toe represents the worst hydraulic condition for slope stability the paper concludes that although slope stability studies in hong kong have tended to focus on perching within the regolith a critical hydraulic boundary condition may exist locally at the base of the regolith d 2005 elsevier b v all rights reserved

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

an extensive subglacial lake and canyon system in princess elizabeth land east antarctica the subglacial landscape of princess elizabeth land pel in east antarctica is poorly known due to a paucity of ice thickness measurements this is problematic given its importance for understanding ice sheet dynamics and landscape and climate evolution to address this issue we describe the topography beneath the ice sheet by assuming that ice surface expressions in satellite imagery relate to large scale subglacial features we find evidence that a large previously undiscovered subglacial drainage network is hidden beneath the ice sheet in pel we interpret a discrete feature that is 140 20 km in plan form and multiple narrow sinuous features that extend over a distance of 1100 km we hypothesize that these are tectonically controlled and relate to a large subglacial basin containing a deep water lake in the interior of pel linked to a series of long deep canyons the presence of 1 km deep canyons is confirmed at a few localities by radio echo sounding data and drainage analysis suggests that these canyons will direct subglacial meltwater to the coast between the vestfold hills and the west ice shelf

observed velocity fluctuations on a major antarctic ice stream most of the antarctic ice sheet is drained by systems comprising an area of slow sheet flow which converges into a much faster valley glacier or ice stream on reaching the coast this ice then either enters an ice shelf confined by embayments and ice rises or calves off into the ocean the higher speed of ice streams enables them to respond to changes in their environment faster than the slower ice sheet in the west antarctic where the ice sheet moves across bedrock well below sea level the ice streams have the potential to rapidly increase or decrease the ice discharge into the sea1 the timescale for these changes has been placed at a few hundred years if this is true then changes should be detectable by careful measurements on a decadal timescale by comparing recent ice velocity measurements with those collected ten years ago we establish that the ice in the mouth of ice stream b has decelerated by 20 we discuss the possible causes of this deceleration on the basis of our knowledge of the current regional dynamics and the possible ramifications of this deceleration on future ice stream behaviour

three dimensional mapping by cryosat 2 of subglacial lake volume changes we analyze data acquired by the cryosat 2 interferometric radar altimeter and demonstrate its novel capability to track topographic features on the antarctic ice sheet we map the perimeter and depth of a 260 km2 surface depression above an antarctic subglacial lake sgl and in combination with ice cloud and land elevation satellite laser altimetry chart decadal changes in sgl volume during 2007 2008 between 4 9 and 6 4 km3 of water drained from the sgl and peak discharge exceeded 160 m3 s 1 the flood was twice as large as any previously recorded and equivalent to 10 of the meltwater generated annually beneath the ice sheet the ice surface has since uplifted at a rate of 5 6 2 8 m yr 1 our study demonstrates the ability of cryosat 2 to provide detailed maps of ice sheet topography its potential to accurately measure sgl drainage events and the contribution it can make to understanding water flow beneath antarctica key points assessment of novel cryosat 2 interferometric altimetry over land ice demonstration of interferometric capability to track off nadir topography mapping of largest antarctic subglacial lake drainage event observed to date

tidal pacing skipped slips and the slowdown of whillans ice stream antarctica we summarize new observations of the deceleration and stick slip motion of whillans ice stream wis antarctica we refine the location of the large sticky spots that resist motion between slip events the locations of which are controlled by the patterns of subglacial water flow our examination of the long term velocity time series for the ice stream reveals that the decadal scale deceleration is not occurring at a steady rate but varies at the sub decadal timescale this unsteady deceleration modulates the temporal evolution of a broad 50 km across surface elevation bulge forming at the junction between the relatively narrow upstream portion of the ice stream and broad ice plain that constitutes the downstream end of wis comparison of observations from april 2003 and november 2010 reveals significant changes in the tidally modulated stick slip cycle that regulates motion on the ice plain we observe that the timing of slip events has become less regular in response to decreased flow speed in the upstream portions of the ice stream the decreased regularity of slip events has reduced the release of stored elastic strain during slip events increasing the rate of deceleration

the distribution of basal water between antarctic subglacial lakes from radar sounding antarctica s subglacial lakes have two end member geophysical expressions as hydraulically flat radar reflective regions highlighted in ice surface topography and radar sounding profiles definit

dynamic patterns of ice stream flow in a 3 d higher order ice sheet model with plastic bed and simplified hydrology 1 predicting ice sheet mass balance is challenging because of the complex flow of ice streams to address this issue we have coupled a three dimensional higher order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress the model was tested for its sensitivity to regional water availability first with an assumed undrained bed the ice stream oscillates between active and stagnant phases solely as a result of thermodynamic feedbacks occurring at the ice till interface however the velocity amplitude decreases over time as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing ice sheet mode second we assume that the till is able to assimilate water from a hypothetical regional hydrological system this leads to significantly different long term behavior as a continuously oscillating ice stream mode is maintained the extra water incorporated in the till leads to higher velocities triggering stronger thermodynamic feedbacks between the ice and till layer results also suggest that fast flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase similarly till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system underscoring the need to accurately describe the coupling between ice dynamics basal conditions and regional subglacial hydrology in ice sheet models

dynamics of active subglacial lakes in recovery ice stream recovery ice stream has a substantial number of active subglacial lakes that are observed with satellite altimetry to grow and drain over multiple years these lakes store and release water that could be important for controlling the velocity of the ice stream we apply a subglacial hydrology model to analyze lake growth and drainage characteristics together with the simultaneous development of the ice stream hydrological network our outputs produce a good match between modeled lake location and those identified using satellite altimetry for many of the lakes the modeled subglacial system demonstrates development of pressure waves that initiate at the ice stream neck and transit to within 100 km of the terminus these waves alter the hydraulic potential of the ice stream and encourage growth and drainage of the subglacial lakes lake drainage can cause large r channels to develop between basal overdeepenings that persist for multiple years the pressure waves along with lake growth and drainage rates do not identically repeat over multiple years due to basal network development this suggests that the subglacial hydrology of recovery ice stream is influenced by regional drainage development on the scale of hundreds of kilometers rather than local conditions over tens of kilometers

an analytical solution of groundwater response to tidal fluctuation in a leaky confined aquifer an analytical solution is derived to investigate the influence of leakage on tidal response in a coastal leaky confined aquifer system the analytical solution developed here is more general than the traditional solution obtained by ferris 1951 which can be regarded as a special case of the solution presented in this paper this solution is based on a conceptual model under the assumption that the groundwater level in the confined aquifer fluctuates in response to sea tide while that of the overlying unconfined aquifer remains constant this conceptual model is supported by numerous field studies by previous researchers which have demonstrated that the tidal response in an unconfined aquifer may be negligible compared to that in a confined aquifer the leakage has a significant impact on the tidal behavior of the confined aquifer hypothetical studies indicate that both tidal amplitude of groundwater head in the aquifer and the distance over which the aquifer can be disturbed by the sea tide will be considerably reduced because of the existence of leakage this analytical solution is used to investigate the tidal and piezometer data at the chek lap kok airport hong kong special administrative region people s republic of china

modeling 5 years of subglacial lake activity in the macayeal ice stream antarctica catchment through assimilation of icesat laser altimetry subglacial lakes beneath antarctica s fast moving ice streams are known to undergo 1 km 3 volume changes on annual timescales focusing on the macayeal ice stream macis lake system we create a simple model for the response of subglacial water distribution to lake discharge events through assimilation of lake volume changes estimated from ice cloud and land elevation satellite icesat laser altimetry we construct a steady state water transport model in which known subglacial lakes are treated as either sinks or sources depending on the icesat derived filling or draining rates the modeled volume change rates of five large subglacial lakes in the downstream portion of macis are shown to be consistent with observed filling rates if the dynamics of all upstream lakes are considered however the variable filling rate of the northernmost lake suggests the presence of an undetected lake of similar size upstream overall we show that for this fast flowing ice stream most subglacial lakes receive 90 of their water from distant distributed sources throughout the catchment and we confirm that water is transported from regions of net basal melt to regions of net basal freezing our study provides a geophysically based means of validating subglacial water models in antarctica and is a potential way to parameterize subglacial lake discharge events in large scale ice sheet models where adequate data are available

determination of the shear strength parameters of an unsaturated soil using the direct shear test multistage direct shear tests have been performed on saturated and unsaturated specimens of a compacted glacial till a conventional direct shear apparatus was modified in order to use the axis tra

thirteen years of subglacial lake activity in antarctica from multi mission satellite altimetry the ability to detect the surface expression of moving water beneath the antarctic ice sheet by satellite has revealed a dynamic basal environment with implications for regional ice dynamics grounding line stability and fluxes of freshwater and nutrients to the southern ocean knowledge of subglacial activity on timescales important for near term prediction of ice sheet fluctuations decadal to century is limited by the short observational record of nasa s ice cloud and land elevation satellite icesat laser altimetry mission used to generate the last continent wide survey 2003 08 here we use synthetic aperture radar interferometric mode data from esa s cryosat 2 radar altimetry mission 2010 present which samples 45 of the icesat derived subglacial lakes to extend their time series to the end of 2016 the extended time series show that there have been surface height changes at 20 of the 45 lakes since 2008 indicating that some of these features are persistent and potentially cyclic while other features show negligible changes suggesting these may be transient or nonhydrological features continued monitoring of active lakes for both height and velocity changes as well as developing methods for identifying additional lakes is critical to quantifying the full distribution of active subglacial lakes in antarctica

synthesizing multiple remote sensing techniques for subglacial hydrologic mapping application to a lake system beneath macayeal ice stream west antarctica we present an analysis of the active hydrologic system of macayeal ice stream macis west antarctica from a synthesis of multiple remote sensing techniques satellite laser altimetry satellite image differencing and hydrologic potential mapping using a satellite derived dem and a bedrock dem from airborne radio echo sounding combining these techniques augments the information provided by each one individually and allows us to develop a protocol for studying subglacial hydrologic systems in a holistic manner our study reveals five large active subglacial lakes under macis the largest of which undergoes volume changes of at least 1 0 km 3 we discuss the hydrologic properties of this system and present evidence for links between the lakes at least three of the lakes are co located with sticky spots i e regions of high local basal shear stress we also find evidence for surface elevation changes due to ice dynamic effects not just water movement caused by changes in basal resistance lastly we show that satellite radar altimetry is of limited use for monitoring lake activity on fast flowing ice streams with surfaces that undulate on 10 km length scales

the significance of primary volcanic fabrics and clay distribution in landslides in hong kong most rain induced landslides on hong kong s volcanic rocks are shallow and small scale however two major landslides that occurred on hong kong island in 1995 highlight relationships between primary volcanic foliations and kaolin accumulations that may have general significance in determining loci of larger failures in the first case a bedding parallel shear zone was the locus for kaolin concentration in a shallowly dipping tuff this formed a weak planar low permeability layer which acted as the surface of rupture of a translational failure in the second case steeply inclined fabrics eutaxitic foliations and subparallel closely spaced joints occurred within a zone of syn volcanic folding and shearing deep weathering developed along the zone with kaolin concentrations present mainly above the soil rock interface the irregular and variably concave to planar rupture surface was closely associated with kaolin infilled relict joints towards the base of the soil profile kaolin distribution is largely associated with the weathering profile and may have been both physically and chemically redistributed within it halloysite is more abundant than kaolinite in most clay samples at and near both rupture zones and may have developed from kaolinite in response to strain detailed mapping of volcanic fabrics around the major landslides suggests that kaolinized bedding parallel shear zones are uncommon but that deep weathering zones associated with shearing and faulting are common mappable features

mechanism analysis of a landslide in highly weathered volcanic rocks of niushoushan hill in nanjing in this paper the landslide mechanism in highly weathered volcanic rocks is analyzed in niushoushan hill in nanjing this hill is mainly composed of volcanic tuff rocks the gradually weathered rocks reduce the hillslope stability and cause landslides during continuous rainfall the kaolin and montmorillonite clay minerals were tested using an x ray diffractometer the subsequent swelling disintegration and permeable tests indicated the location of soft zones and the poor hydraulic conductivity of the highly weathered tuff hwt the overflow from boreholes and joints exposes a confined aquifer that is caused by rainfall infiltration in the lower moderately weathered tuff mwt which crops out on the top of the hill and is bounded on the interface of the hwt and mwt the electrical resistivity tomography method was carried out for the considered slope to define the boundaries of the discontinuities as a supplement to drilling the displacements of the slope surface and the deep rocks were monitored and the results indicated that sliding had not stopped after a long period of continuous rainfall slope stability analysis considering a water head rise due to rainfall infiltration was carried out using the slide v6 0 computer package the simulation included steady state flow transient seepage associated with rainfall and slope stability variation caused by different infiltration conditions the results indicate that the hwt with low permeability can confine the activity of the lower aquifer and result in a rapid rise of pore pressure in response to rainfall which can lead to slope failure

polythermal glacier hydrology a review 1 the manner by which meltwater drains through a glacier is critical to ice dynamics runoff characteristics and water quality however much of the contemporary knowledge relating to glacier hydrology has been based upon and conditioned by understanding gleaned from temperate valley glaciers globally a significant proportion of glaciers and ice sheets exhibit nontemperate thermal regimes the recent growing concern over the future response of polar glaciers and ice sheets to forecasts of a warming climate and lengthening summer melt season necessitates recognition of the hydrological processes in these nontemperate ice masses it is therefore timely to present an accessible review of the scientific progress in glacial hydrology where nontemperate conditions are dominant this review provides an appraisal of the glaciological literature from nontemperate glaciers examining supraglacial englacial and subglacial environments in sequence and their role in hydrological processes within glacierized catchments in particular the variability and complexity in glacier thermal regimes are discussed illustrating how a unified model of drainage architecture is likely to remain elusive due to structural controls on the presence of water cold ice near glacier surfaces may reduce meltwater flux into the glacier interior but observations suggest that the transient thermal layer of near surface ice holds a hydrological role as a depth limited aquifer englacial flowpaths may arise from the deep incision of supraglacial streams or the propagation of hydrofractures forms which are readily able to handle varied meltwater discharge or act as locations for water storage and result in spatially discrete delivery of water to the subglacial environment the influence of such drainage routes on seasonal meltwater release is explored with reference to summer season upwellings and winter icing formation moreover clear analogies emerge between nontemperate valley glacier and ice sheet hydrology the discussion of which indicates how persistent reassessment of our conceptualization of glacier drainage systems is required there is a clear emphasis that continued integrated endeavors focused on process glaciology at nontemperate glaciers are a scientific imperative to augmenting the existing body of research centered on ice mass hydrology

mapping the grounding zone of the ross ice shelf antarctica using icesat laser altimetry we use laser altimetry from the ice cloud and land elevation satellite icesat to map the grounding zone gz of the ross ice shelf antarctica at 491 locations where icesat tracks cross the grounding line gl ice flexure in the gz occurs as the ice shelf responds to short term sea level changes due primarily to tides icesat repeat track analysis can be used to detect this region of flexure since each repeated pass is acquired at a different tidal phase the technique provides estimates for both the landward limit of flexure and the point where the ice becomes hydrostatically balanced we find that the icesat derived landward limits of tidal flexure are in many places offset by several km and up to 60 km from the gl mapped previously using other satellite methods we discuss the reasons why different mapping methods lead to different gl estimates including instrument limitations variability in the surface topographic structure of the gz and the presence of ice plains we conclude that reliable and accurate mapping of the gl is most likely to be achieved when based on synthesis of several satellite datasets

advances in modelling subglacial lakes and their interaction with the antarctic ice sheet subglacial lakes have long been considered hydraulically isolated water bodies underneath ice sheets this view changed radically with the advent of repeat pass satellite altimetry and the discover

the supply of subglacial meltwater to the grounding line of the siple coast west antarctica recent satellite studies have shown that active subglacial lakes exist under the antarctic ice streams and persist almost to their grounding lines when the lowest lying lakes flood the water crosses the grounding line and enters the sub ice shelf cavity modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice shelf base we examine the spatial and temporal variability in subglacial water supply to the grounding lines of the siple coast ice streams by combining estimates for lake volume change derived from ice cloud and land elevation satellite icesat data with a model for subglacial water transport our results suggest that subglacial outflow tends to concentrate towards six embayments in the siple coast grounding line although mean grounding line outflow is 60m3 s 1 for the entire siple coast maximum local grounding line outflow may temporarily exceed 300 m3 s 1 during the synchronized flooding of multiple lakes in a hydrologic basin variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the ross ice shelf cavity the temporal variability in grounding line outflow results in a net reduction in long term average melt rate but temporary peak melting rates may exceed the long term average by a factor of three

internal soil moisture response to rainfall induced slope failures and debris discharge article i nfo predictions of rainfall induced fast moving mass flow and or debris flows require better knowledge of the mechanism controlling the debris discharge of slopes in debris source areas a series of rainfalltests on 0 32 m deep 0 7 m high 1 35 m wide sandy slopes resting on a bi linear impermeable rigid base was performed soil moisture content and solid discharge measurements were performed to gain insights into the rainfall induced retrogressiveslopefailure thesolid ordebris dischargeisaresultof thewash outof the fluidizedslopetoeby the interflow along the soil bedrock interface characteristics of the failure process for the slopes are represented by mass wasting curves or solid discharge qs vs time t curves which are functions of the rainfall intensity and or the cumulative rainfall the mass wasting curves have inflection points representing transitions from minor toe failures into remarkable retrogressive failures the first inflection point of the soil moisture vs t curve measured at the soil bedrock interface signaling the arrival of the descending wet front may serve as a precursor for predicting the onset of an abrupt solid discharge induced by shallow slope failures the time of peak water content measured at the soil bedrock interface may approximate the time of 5 total solid volume discharge up to the time of 5 of total slope volume discharge a fully saturated state sr 100 was never observed at the 0 2 m below surface zone however it was observed along the soil bedrock interfaceat near toezoneof the slope regardlessof theintensityof rainfallinvestigated retrogressive failures were essentially associated with nonuniformly distributed water content in the slope for both the 0 2 m below surface zone and the soil bedrock interface a more uniform distribution of sr along the full height of the slope was found for slopes subjected to high rainfall intensities of 47 and 65 mm h than that for the slope subjected to a low rainfall intensity of 23 mm h at the inflection point of the qs vs t curve and 5 of total solid volume discharge values of sr at a certain distance from the toe for the soil bedrock interface were higher than those measured at the same distance fromthe toefor the 0 2 m below surface zone indicating the effect of infiltration induced interflowalong the soil bedrock interface and its effects on the fluidization of the slope toe and the retrogressive failure of the slope

a preliminary study on the offshore stratigraphy in hong kong and its hydrogeological implications offshore groundwater reserves commonly sequestered in relict fluvial systems have been reported globally in hong kong submarine terrestrial sediments are the evidence of local fluvial systems during glacial periods and they probably sequester relatively fresh water this study aims to locate these sand deposits an offshore vibrocore in the southwestern waters of hong kong was analysed for its geophysical and elemental properties in order to deduce the palaeoenvironment also marine seismic profiling was adopted at two selected sites to depict the distribution of sand and gravel deposits and to estimate the average thickness of these aquifer materials a four unit sequence was noted with increasing marine influence since 8 ka bp this agrees with the hypothesis that the offshore groundwater reserves are the relics of flow systems during glacial periods the extensive sand deposits in the study areas are often shielded by sufficiently thick marine clay against vertical salinisation allowing fresh groundwater to be preserved the average thickness of these aquifer materials is about 11 m at the study sites if these sand deposits are recharged through onshore outcrops the reserves may be a sustainable water resource

subglacial floods beneath ice sheets subglacial floods jokulhlaups are well documented as occurring beneath present day glaciers and ice caps in addition it is known that massive floods have occurred from ice dammed lakes proximal to the laurentide ice sheet during the last ice age and it has been suggested that at least one such flood below the waning ice sheet was responsible for a dramatic cooling event some 8000 years ago we propose that drainage of lakes from beneath ice sheets will generally occur in a time periodic fashion and that such floods can be of severe magnitude such hydraulic eruptions are likely to have caused severe climatic disturbances in the past and may well do so in the future

causes of sudden short term changes in ice stream surface elevation 1 recent satellite borne observations of antarctica s ice streams show sudden spatially confined surface elevation changes that are interpreted as caused by subglacial water movement using a numerical model of idealized ice stream flow coupled to various simple treatments of subglacial bed conditions we demonstrate that ice stream flow dynamics significantly modulates the surface elevation expression of processes taking place at the ice stream bed this modulation means that observed surface elevation changes do not directly translate to basal elevation changes e g inflation or deflation of subglacial water pockets of equal magnitude and shape thus subglacial water volume change is not directly proportional to the area integral of surface elevation changes model results show that ambiguities in interpretation of surface elevation changes can be overcome with additional measurements such as of surface velocity change and through development of methodology designed to understand transfer of basal change to surface change

in situ rainfall infiltration studies at a hillside in hubei province china field infiltration tests were conducted at a hillside near the ship lock of the three gorges dam in hubei province china the test site consists of residual soil and decomposed granite the infiltration rate is estimated from the in situ tests to be 1 465 2 778 10 6 ms 1 depending on the initial water content the rate at which the infiltration front moves down through the soil matrix within 2 m of the ground surface is estimated to be ca 0 26 m day 1 on average at the end of the in situ tests the matric suction profiles show that the soil below a depth of 80 cm remained unsaturated while the zone above was almost fully saturated this finding was unexpected the site was excavated after the test to examine the abnormal behaviour of the matric suction profiles in the depth a relic joint was identified at a depth of 78 cm at an attitude almost parallel to the slope surface it is surmised that the joint transmitted water laterally and limited further penetration of the wetting front the water in the zone above the joint appeared to be perched this experiment indicates that to describe thoroughly the infiltration process within a weathered jointed granite profile for slope engineering design purposes a model based on the assumption of a uniform porous media is inadequate the model should include the discontinuities this is challenging since it requires field studies to identify the pattern and distribution of the joints the implications of the experimental results on slope stability are discussed the in situ tests provide important information for further studying groundwater seepage under rainfall conditions and a dewatering system design for the slope above the ship lock of the three gorges dam in china 2000 elsevier science b v all rights reserved

case study of slope failure during construction of an open pit mine in indonesia this paper presents back analyses of a failed slope at an open pit lignite mine in muara enim indonesia the slope which was being raised by the dumping of excavated material failed at a dump he