comment on are there internal kelvin waves in lake tanganyika by jaya naithani and eric deleersnijder 1 recent numerical modelling efforts have demonstrated the presence of kelvin waves in lake tanganyika naithani and deleersnijder 2004 however it is claimed that these waves should not appear according to classical scaling arguments based on existing classical scaling arguments supported by laboratory and field investigations i will show why they can and apparently do appear in lake tanganyika

rheology of till beneath storglaciaren sweden in order to study in situ the rheology of a deforming subglacial till various instruments were emplaced in till beneath storglaciaren sweden boreholes were used to gain access to the till beneath about 100 m of ice tiltmeters provided an estimate of the shear strain rate in the till two other instruments yielded measures of till strength in addition water pressures were recorded in boreholes and in the till a computer controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag based on a force balance calculation tilt rates varied directly with effective pressure so decreases in water pressure apparently increased the coupling between the glacier and the bed surface speed was either out of phase with tilt or varied independently of tilt thus increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed they were not a result of till deformation till strength varied directly with effective pressure which is consistent with it being a mohr coulomb or frictional material the devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed this implies that the residual strength of the till is insensitive to strain rate thus the appropriate constitutive equation for till rheology may be of the form e e k where k is a constant this is consistent with experimental data reported in the geotechnical literature

a hybrid fuzzy weight of evidence method in landslide susceptibility analysis on the wuyuan area china abstract the present study proposed a hybrid fuzzy weight of evidence model for constructing a landslide susceptibility map in the wuyuan area china where disastrous landslide events have occurred the model combines the knowledge of experts obtained through a fuzzy logic approach and a hybrid weight of evidence method the estimated knowledge based fuzzy membership value of each environmental variable is combined with data based conditional probabilities to derive fuzzy posterior probabilities and landslide susceptibility the developed model was compared with a landslide susceptibility map produced using the data driven weight of evidence method based on 510 landslide and non landslide sites the sites were identified by analyzing airborne imagery field investigation and previous studies landside susceptibility for these sites was analyzed using 10 geo environmental variables slope aspect lithology soil rainfall plan curvature the normalized difference vegetation index distance to roads distance to rivers and distance to faults the resultant hybrid fuzzy weight of evidence method showed high predictive power with the area under the success and predictive curves being 0 770 and 0 746 respectively additional analyses showed that the developed model could work effectively even with limited data

ring shear studies of till deformation coulomb plastic behavior and distributed strain in glacier beds a ring shear device was used to study the factors that control the ultimate steady strength of till at high shear strains tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26 3 and 18 6 for tills containing 4 and 30 clay sized particles respectively other tests at steady normal stresses and variable shear strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate this weakening may be due to small increases in till porosity these results provide no evidence of viscous behavior and suggest that a coulomb plastic idealization is reasonable for till deformation however viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till we hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure during a deformation event transient dilation ofdiscrete shear zones should cause a reduction in internal pore water pressure that should strengthen these zones relative to the surrounding till a process called dilatant hardening consequent changes in shear zone position when integrated over time may yield the observed distributed strain

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

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

cone penetration testing in deltaic soils a multichannel electric cone penetrometer was used to perform a variety of in situ tests in saturated deltaic deposits factors affecting the tip bearing friction sleeve stress and pore water pressures and their interpretation are studied and discussed pore pressure measurements were essential to evaluate the in situ test results the effect of rate of penetration is discussed and the concept of effective bearing introduced as an attempt to interpret cone bearing in undrained or partially drained soil field values of consolidation characteristics from pore pressure decay are compared with traditional laboratory consolidation test results on undisturbed samples the advantages of continuous electric cone logging to evaluate soil stratigraphy are demonstrated field and laboratory test results are presented from a test site where stabilization was used to reduce liquefaction potential of hydraulically placed sand and silt the importance of pore pressure measurements is demonstrated along with examples an

glacier motion dominated by processes deep in underlying till black rapids glacier is a 40 km long surge type glacier in the central alaska range in spring 1997 a wireline drill rig was set up at a location where the measured surface velocities are high and seasonal and annual velocity variations are large the drilling revealed a layer of subglacial till up to 7 m thick that is believed to be water saturated at one location a string of instruments containing three dual axis tiltmeters and one piezometer was successfully introduced into the till the tiltmeters monitored the inclination of the borehole at the ice till interface and at 1 and 2 m into the till for 410 days they showed that no significant deformation occurred in the upper 2 m of the till layer and no significant amount of the basal motion was due to sliding of the ice over the till the measured surface velocity at the drill site is about 60 m a 1 of which 20 30 m a can be accounted for by ice deformation almost the entire amount of basal motion 30 40 m a 1 was taken up at a depth of 2 m in the till possibly in discrete shear layers or as sliding of till over the underlying bedrock we propose that the large scale mobilization of such till layers is a key factor in initiating glacier surges

estimation of hydraulic properties of subglacial till from ploughmeter measurements force variations on a ploughmeter and fluctuations in subglacial water pressure have been measured in the same borehole at storglaciaren sweden to investigate hydraulic properties of the basal till layer a strong inverse correlation of the pressure and force records in conjunction with a significant time lag between the two signals suggests that pore water pressures directly affect the strength of the till variations in subglacial water pressure result in potential gradients across the water till interface at the bottom of the borehole that drive pressure waves downwards through the till layer when the borehole water level is high and back upwards when the water level is low analysis of the propagation velocity of this pressure wave indicates that the hydraulic diffusivity of storglaciaren till is in the range 1 9 3 6 10 6 m 2 s 1 in good agreement with estimates obtained in the laboratory hydraulic conductivity values associated with these diffusivities are between 10 and 10 m s 1 and thus are well within the range of values for other glacial tills

clast collision frequency as an indicator of glacier sliding rate the mechanical interaction between a glacier and subglacial sediment can be observed using an instrumented rod that we refer to as the ubc ploughmeter for clast rich sediments the rate of collision between clasts and a rod dragged through these sediments should be related to the glacier sliding rate by assuming that proglacial measurements of sediment granulometry represent the subglacial granulometry of trapridge glacier yukon territory canada we have used ploughmeter results to obtain an estimated sliding rate of 45 mm d 1 in good agreement with known rates in addition for a subglacial material treated as a solid liquid dispersion having a linear viscous rheology the force of collision experienced by the rod should be proportional to the effective sediment viscosity our estimate of 2 0 x 10 10 pa s agrees well with previously derived values

coupling between a glacier and a soft bed ii model results the relation between the local effective pressure and shear stress on till beneath storglaciaren sweden discussed in iverson and others 1999 provides an empirical basis for studying the processes that control the strength of the ice bed coupling particles in the bed that protrude into the glacier sole support shear stresses that are limited by either ploughing or the traditional sliding mechanisms model calculations based on studies of cone penetration through fine grained sediment and sliding theory agree with the observed relation between shear stress and effective pressure if the water layer at the ice bed interface is assumed to thicken rapidly as the effective pressure approaches zero studies of the hydraulics of linked cavities provide support for this assumption if the mean thickness of the water layer reflects the extent of microcavity development at the interface comparison of the calculated shear stress with the ultimate strength of till suggests that bed deformation limits the shear stress on till beneath storglaciaren only at intermediate effective pressures at very low effective pressures like those inferred at the site of the tiltmeter discussed in iverson and others 1999 and at sufficiently high effective pressures ploughing and sliding should focus motion near the glacier sole a calculation using parameter values appropriate for ice stream b west antarctica suggests that ploughing may occur there at shear stresses not sufficient to deform the bed at depth this conclusion is reinforced by the likelihood that pore pressures in excess of hydrostatic should develop down glacier from ploughing particles thereby weakening the bed near the glacier sole however given the apparent sensitivity of the ice bed coupling to basal conditions that may be highly variable any blanket assumption regarding the flow mechanism of ice masses on soft beds should probably be viewed with skepticism

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

energy partitioning and horizontal dispersion in a stratified rotating lake abstract the response of a stratified rotating basin to the release of a linearly tilted interface is derived this case is compared with a uniformly forced basin in the two limits when the duration of the forcing is much greater than the period of the dominant internal waves and when it is much smaller energy partitioning is studied as a function of the burger number s relative importance of stratification versus rotation showing the dominance of a geostrophic component over the wave field for low s trajectories are integrated numerically revealing the stokes drift of the waves to be always cyclonic transport properties are classified in terms of s and the wedderburn number w relative importance of the disturbance versus stratification the geostrophic flow is the main source of advection but only the waves allow particles to break the barrier to transport between the two geostrophic gyres ultimately leading to stretching and folding for low values of w advection can become chaotic conserva

instruments and methods direct measurement of sliding at the glacier bed sliding at the base of trapridge glacier yukon territory canada was measured using a drag spool we describe this simple and inexpensive instrument as well as its installation and operation from 1990 to 1992 seven sites were instrumented with drag spools at six of the sites basal sliding during the period of observation accounted for 50 70 of the total flow observed at the glacier surface the contribution from ice creep is known to be small so most of the remaining surface motion must be attributed to subglacial sediment deformation for the seventh site the observed sliding rate was 90 of the total flow an indication that the sliding contribution varies spatially across the bed diurnal variations in the response of one of our instruments appear to be correlated to subglacial water pressure fluctuations and are interpreted in terms of changes in sliding velocity rather than the opening and closing of basal cavities

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes

are there internal kelvin waves in lake tanganyika it is generally believed that the earth s rotation has negligible impact on the water circulation in basins which are very narrow or located near the equator however herein evidence is presented of the influence of the earth s rotation on the hydrodynamics of lake tanganyika which is both very narrow width length approximate to 0 08 and located near the equator numerical simulations exhibit small upwellings at the western shores as a result of the thermocline oscillations induced by the southeasterly winds of the dry season these structures tend to propagate cyclonically around the lake similar to internal kelvin waves numerical experiments in which f is varied concludes that internal kelvin waves are present in lake tanganyika it is also evidenced from this study that the internal kelvin waves cannot be anticipated based on classic scaling arguments

basal sliding of ice stream b west antarctica a tethered stake apparatus is used to measure basal sliding in a borehole on ice stream b west antarctica about 300 km upstream east from its grounding line near the head of the ross ice shelf a metal stake emplaced at the top of a layer of unfrozen till underlying the ice is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding the measured sliding motion includes any actual slip across the ice till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface this 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment based on features of the record of apparent sliding alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm in any case it is small compared to the seismically inferred till thickness of 9 m measured apparent sliding averages 69 of the total motion of 1 2 m d 1 over 26 days of observation if a 3 5 day period of slow apparent sliding 8 of the total motion is included in the average the occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c 80 and c 8 of the total motion on a time scale of a few days however it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole if the slow period is therefore omitted the indicated average basal sliding rate is 83 of the total motion in either case basal sliding predominates as the cause of the rapid ice stream motion in the last 2 days of observation the average apparent sliding rate reached 1 17 m d 1 essentially 100 of the motion of the ice stream if till deformation contributes significantly to the ice stream motion the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer these observations if applicable to the west antarctic ice sheet in general pose complications in modeling the rapid ice streaming motion

assessing the paradigm shift deformable glacier beds abstract during the mid 1980s glaciologists recognised that many glaciers and ice masses overlie soft sediments rather than hard bedrock these sediments are typically weaker than glacier ice and may therefore deform this realisation was heralded as a paradigm shift in glaciology since this realisation a wide body of theoretical and field based research has been undertaken to assess the mechanical and hydrological significance of such substrates we review the techniques and results of this work and attempt to assess this paradigm shift in glaciology and in particular its importance for quaternary scientists we suggest areas of research that remain unanswered and identify those in which glacial geology in particular may be able to inform glaciologists

towards a hydrological model for computerized ice sheet simulations ice sheet modelling typically uses grid cells 10km or more on a side so any hydrological and sliding model must average or parameterize processes that vary over shorter distances than this observations and theory suggest that basally produced water remains in a distributed high pressure system unless it encounters low pressure channels fed by surface melt such distributed systems appear to exhibit increasing water storage water transmission and water lubrication of sliding with increasing water pressure a model based on these assumptions successfully simulates some aspects of the non steady response of mountain glaciers to externally forced channel pressure variations it merits testing in ice sheet modelling

the seasonal evolution of wind internal wave resonance in lake kinneret data from a thermistor chain and wind sensor collected over an annual stratification cycle in lake kinneret israel during 2000 were used to investigate the seasonal evolution of wind internal wave resonance internal wave periods determined from an analytical model were compared with observations and we show that resonance during 2000 occurred during three distinct times of the year at the onset of stratification march during the heating phase june and during the cooling phase november in all cases resonance was between the wind and the dominant radial azimuthal and vertical mode one cyclonic kelvin wave previously observed in lake kinneret internal waves are a ubiquitous feature of lakes existing because of wind forcing acting on a stratified water column where the stratification is generally due to temperature wind applied to the surface can cause a surface setup of water at the downwind end this pressure force is balanced by a tilting of the metalimnion in the opposite sense that is downward at the downwind end spigel and imberger 1980 when the wind forcing relaxes the water surface oscillates a barotropic wave as does the metalimnion a baroclinic wave the two motions can effectively be considered to be decoupled monismith 1985 because they form orthogonal solutions to the wave equation it is the internal baroclinic waves that are of interest in the present study because they provide much of the energy and motion for biogeochemical processes in lakes in large lakes the picture becomes complicated because the effects of the earth s rotation begin to influence the frequency and structure of the internal waves internal gravity waves in large lakes take the form of cyclonic kelvin and or poincare or anticyclonic poincare waves antenucci et al 2000 antenucci and imberger 2001 where the wave crests rotate around the perimeter of the lake basin antenucci and imberger 2001 showed that the frequency as well as the ratio of potential kinetic energy of internal gravity waves in approximately circular or elliptic lakes was dependent on the aspect ratio radial mode azimuthal mode and burger number the burger number is defined asc lf where c is the phase speed of internal waves in a nonrotating system l is some length scale characterizing the basin width and f is the coriolis parameter for a small burger number rotation dominates over gravity and waves approach the oceanic case known as inertial oscillations in which the motion is dominated by the kinetic energy signal for a large acknowledgments we acknowledge the field support and technical staff from the kinneret limnological laboratory kll for providing assistance particularly tzahi rosenberg for maintaining the lake station and helping in the transfer of data from kll to the centre for water research and peter yeates who provided the time series of the lake number

estimating rates of basal motion and internal ice deformation from continuous tilt measurements over a two year period continuous measurements of temporal changes in tilt conducted with a string of tilt meters in a borehole on unteraargletscher bernese alps switzerland have been used to estimate the basal motion component this estimation is based on a comparison of the measurements with synthetic tilt curves computed using a parameterization of a simplified flow field the best agreement is found for a ratio of basal motion to forward motion due to ice deformation slip ratio equal to about 1 2 measured tilt curves exhibit a number of different transient features while an overall increase in tilt angle is observed at every tilt meter location two of the sensors recorded anomalous tilt behaviour these anomalies are characterized by sudden and drastic variations in tilt a particularly intriguing example of such short term tilt variations was recorded with a tilt meter positioned 40 m above the bed during the 1997 summer melt season

mechanical and hydraulic properties of till beneath bakaninbreen svalbard original article can be found at http www igsoc org journal copyright igs doi 10 3189 172756501781832304 full text of this article is not available in the uhra

high resolution measurements of spatial and temporal variations in surface velocities of unteraargletscher bernese alps switzerland data from automated in situ measurements of surface displacements on unteraargletscher bernese alps switzerland made a few times a day from late june until late october 1998 are presented two short term speed up events were observed and partial evidence for a third motion event was seen short term motion events are thus not limited to the start of the melting season strong correlation between the timing of the motion events and periods of heavy rainfall in conjunction with the large size of the area of the glacier which becomes activated during the events both point towards an external triggering mechanism in the time periods between two motion events surface velocities do not vary significantly with time after a motion event surface velocities do not return to previous values suggesting that the events have a lasting effect on the structure of the subglacial drainage system

black box modeling of the subglacial water system measurements of water pressure beneath trapridge glacier yukon territory canada yield the following generalizations about subglacial conditions in the studied region 1 even over short distances the subglacial water system is highly heterogeneous 2 the subglacial water system consists of at least two distinct components which we refer to as the connected and unconnected water systems 3 regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system 4 large spatial pressure gradients can exist within the unconnected water system and between the connected and unconnected systems 5 rapid pressure variations can occur in the unconnected water system 6 pressure variations in the unconnected water system do not match those in the connected system and can in fact be strongly anticorrelated with pressure variations in the connected system if the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing the input output relation between forcing and response can be efficiently represented as a low order nonlinear ordinary differential equation the response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of 1 7 hours and 7 4 hours that we believe are associated with process rates for substrate compression and pore water diffusion respectively

measurements of ice deformation at the confluence area of unteraargletscher bernese alps switzerland an experimental study of ice deformation at the confluence area of unteraargletscher bernese alps switzerland has been carried out surface velocities were measured by repeatedly surveying stakes and with the use of remote sensing methods the variation of the vertical strain rates with depth was measured in boreholes the confluence center line is subjected to longitudinal horizontal extension which is exceeded in magnitude by a concomitant transverse compression vertical strain rates change from positive extension at the surface to negative compression in the lowest layers of the glacier

rapid soft bed sliding of the puget glacial lobe the puget lobe of the late pleis inferred to be lodgment till based on the tocene cordilleran ice sheet advanced into presence of oriented stones oriented stria western washington over a thick sequence of tions on stones overconsolidation and unlithified sediments the basal drag due to deformation features sliding over this surface was calculated by 1 this paper investigates the rapid motion of idealizing the sediment bed as a rigid planar the puget lobe over its bed of deformable sedi surface scattered with roughness elements cor ment studies of large vanished glaciers com responding to individual sedimentary particles plement basal motion studies of modern glaci and 2 taking the minimum reconstructed slid ers in which the glacier geometry and velocity ing velocity to be 500 m yr 1 the calculated are well constrained but the characteristics of basal drag minimized by choices of parameters the bed can only be inferred from observations is 400 kpa much higher than the reconstructed over very limited areas kamb et al 1985 gravitational driving stress of 40 kpa indi seismic measurements blankenship et al 1986 cating that a rigid bed and low water pressure or modelling lingle and brown 1987 obser are not consistent with the glacierfs rapid vations on glacier beds recently exposed by motion high subglacial water pressure aver glacial retreat can provide information about aging 90 of the ice overburden 10 mpa is conditions during a somewhat greater time span inferred from overconsolidation of subglacial but are restricted to the marginal environ clays the occurrence and deformation of ment in addition inferences about the bed of water deposited sediments within the till and modern glaciers are limited by the short dura the requirement to drain large quantities of tion and seasonality of the observations a water from the interface suggest that water notable benefit of studying the beds of pieis pressure reached 99 of the ice overburden tocene glaciers is that they record basal ploughing of till by ice entrained clasts per conditions for intervals spanning entire glaci vasive shearing and finally ice bed separa ations and accessible far from the glacier tion become possible as water pressure margins increases at the low inferred effective nor after a brief summary of data available from mal stress the basal drag is reduced from the previous reconstructions of the puget lobe the rigid bed case by water layers which decouple basal conditions of the lobe are explored along the smallest particles from the glacier sole two complementary lines we first present cal and ploughing which reduces the resistance culations of basal drag due to glacier sliding offered by the largest particles the limited for different models of till bed roughness that shear strain observed in much of the till indicate that rapid sliding requires very low implies pervasive shearing did not contribute effective stresses and extensive ice bed significantly to ice motion basal motion was decoupling or ploughing second we summarize confined to the ice bed interface or to dis geological and geotechnical data that indicate tinct faults within the substrate

seasonal evolution of the basin scale internal wave field in a large stratified lake the response of the water column to varying conditions of stratification and wind forcing was investigated in lake kinneret israel using data collected from thermistor chains and acoustic doppler current profilers during 1997 and 1998 the strong daily sea breeze was found to generate a vertical mode 1 internal kelvin wave and basin scale internal poincarewaves of vertical modes 1 2 and 3 the kelvin wave the dominant component of the internal wave field was responsible for alongshore velocities in the nearshore regions in the upwind nearshore regions velocities were dominated by the forced response to the wind and were cross shore in nature in the lake interior the kelvin wave effect on the horizontal velocity field was minor compared to the higher vertical mode poincarewaves the kelvin wave is shown to exist in resonant and nonforced states with the wind whereas the vertical mode 1 poincarewave energy remained relatively constant despite large variability in the forcing condi tions the energy in the higher mode poincarewaves varied greatly both on daily and seasonal timescales the results demonstrate that the wind energy forces multiple basin scale internal wave modes and that prior motion in the water column must be considered when determining the subsequent internal wave response in periodically forced systems lake kinneret israel has long been an important regional resource providing a substantial supply of drinking water as well as supporting a commercial fishery the monomictic lake is forced daily during the summer by a powerful sea breeze with stratification and wind patterns showing little variation from year to year these factors along with a fa vorable bathymetric pattern combine to make lake kinneret an ideal place for the field investigation of large scale inter nal waves and related phenomena such as the benthic bound ary layer horizontal transport and vertical mixing processes in addition to providing substantial insight into the local pro cesses and hence influencing local management study of the lake also provides fundamental knowledge of the dynam ics of enclosed basins on a geophysical scale during summer the internal rossby radius in lake kin neret is typically one half the basin width hence large scale motion in the lake will be affected by the earth s rotation and basin scale internal waves may be of topographic ross by kelvin or poincareform topographic rossby waves are sub inertial frequency oscillations that exist only in rotating fluids with variable bathymetry mysak et al 1985 stocker and hutter 1987 and these waves result from changes in depth affecting the relative vorticity of fluid elements in the absence of vertical boundaries wavelike solutions to the lin ear inviscid equations of motion for a flat bottomed region of homogeneous fluid with a constant rate of rotation are 1

hydraulic and mechanical properties of glacial sediments beneath unteraargletscher switzerland implications for glacier basal motion the force on a ploughmeter and subglacial water pressure have been measured in the same borehole at unteraargletscher switzerland in order to investigate ice sediment coupling and the motion at the base of a soft bedded glacier a strong inverse correlation of the recorded pressure and force fluctuations in conjunction with a significant time lag between the two signals suggests that pore water pressures directly affect the strength of the subglacial sediment the lag is interpreted to reflect the time required for the water pressure wave to propagate through the pores of the sediment to the depth of the ploughmeter analysis of the propagation velocity of this pressure wave yielded an estimate of the hydraulic diffusivity a key parameter necessary to characterize transient pore water flow furthermore the inferred inverse relationship between pore water pressure and sediment strength implies that coulomb plastic deformation is an appropriate rheological model for the sediment underlying unteraargletscher however the sediment strength as derived from the ploughmeter data was found to be one order of magnitude smaller than that calculated for a coulomb frictional material using the water pressure measurements this significant discrepancy might result from pore water pressures in excess of hydrostatic down glacier from the ploughmeter as the ploughmeter is dragged through the sediment sediment is compressed if the rate of this compression is large relative to the rate at which pore water can drain away excess pore water pressures will develop that have the potential to weaken the sediment the same process could lead to highly fluid sediment down glacier from clasts that protrude into the glacier sole and thus would otherwise provide the roughness to couple the glacier to its bed iverson 1999 rapidly sliding glaciers overlying sediments might therefore move predominantly by ploughing which tends to focus basal motion near the glacier sole rather than at depth in the bed

residual strength of clays in landslides folded strata and the laboratory the post peak drop in drained shear strength of an overconsolidated clay may be considered as taking place in two stages first at relatively small displacements the strength decreases to the fully softened or critical state value owing to an increase in water content dilatancy second after much larger displacements the strength falls to the residual value owing to reorientation of platy clay minerals parallel to the direction of shearing if the clay fraction is less than about 25 the second stage scarcely comes into operation the clay behaves much like a sand or silt with angles of residual shearing resistance typically greater than 20 degrees conversely when the clay fraction is about 50 residual strength is controlled almost entirely by sliding friction of the clay minerals and further increase in clay fraction has little effect the angles of residual shearing resistance of the three most commonly occurring clay minerals are approximately 15 degrees for kaolinite 10 degrees for illite or clay mica and 5 degrees for montmorillonite when the clay fraction lies between 25 and 50 there is a transitional type of behaviour residual strength being dependent on the percentage of clay particles as well as on their nature the post peak drop in strength of a normally consolidated clay is due only to particle reorientation measurements of strength on natural shear surfaces agree within practical limits of variation with values derived from back analysis of reactivated landslides this field residual strength can be recovered by multiple reversal shear box tests on cut plane samples but in high clay fraction materials it is typically somewhat higher than the strength measured in ring shear tests residual strength is little affected by variation in the slow rates of displacement encountered in reactivated landslides and in the usual laboratory tests but at rates faster than about 100 mm min qualitative changes take place in the pattern of behaviour a substantial gain in strength is followed with increasing displacement by a fall to a minimum value in clays and low clay fraction silts this minimum is not less than the slow or static residual but in clayey silts with clay fractions around 15 25 according to tests currently in progress the minimum can be as low as one half of the static value author trrl

review of subglacial hydro mechanical coupling trapridge glacier yukon territory canada abstract the interaction of basal processes with the subglacial drainage system is a critical issue in understanding glacier dynamics since the recognition that many glaciers and ice masses overlie soft sediments rather than hard bedrock much research has been undertaken to investigate how mechanical and hydrological conditions of a deformable substrate control the coupling at the ice bed interface and thus affect fast ice flow and glacier surging in research undertaken on trapridge glacier a small surge type glacier in the st elias mountains yukon territory canada we have combined extensive field investigations using novel measurement techniques and theoretical modelling to study hydro mechanical coupling processes measurements of subglacial water pressure indicate that the basal water system can be dramatically inhomogeneous both spatially and temporally since ice bed coupling is strongly influenced by subglacial water pressure the stresses at the bed are also markedly heterogeneous and are expected to form a patchwork distribution which mimics the pressure distribution of the basal water system this heterogeneity in the stress field at the ice bed interface introduces a pronounced variability to the basal motion mechanics as such basal sliding and subglacial sediment deformation are not steady and continuous processes instead the variability of the subglacial water system leads to a spatial and temporal interplay of increased ice bed coupling at low water pressures at one site or time with ice bed decoupling during rising water pressures at other sites or times thus on the one hand there is downglacier shear deformation of the bed and accumulation of elastic strain in ice and sediment while on the other hand there is enhanced slip sliding of the glacier and upglacier shear motion of the bed due to an elastic relaxation of the sediment