sediment trapping characteristics of a pit trap and the helley smith sampler in a cobble gravel bed river 1 although designed for granule gravels the helley smith hs bed load sampler is often used in streams with much coarser beds to compare magnitudes and grain size distributions of sediment samples collected by a pit trap and a standard hs sampler we conducted 22 sampling events over a wide range of flows during a snowmelt freshet in a stream with a coarse gravel bed each sampling event consisted of three simultaneous measurements one made with a pit trap installed in the bed one with a hs sampler placed on the bed directly beside the pit trap hs i and one with a hs sampler placed on the downstream rim of the pit trap hs p we summed the catches of the pit trap and hs p to estimate actual bed load transport the pit trap and hs i catches were then compared to the summed sample the pit trap yielded a remarkably consistent positively skewed sigmoidal distribution of catch efficiency for all 22 measurements with near 100 efficiency for material larger than 2 8 mm the hs i sampler was more variable in its catch and trapping efficiency exhibiting low trapping efficiency for midrange material 0 71 to 16 mm but high efficiency for finer material the results cast doubt on the accuracy of bed load data sets collected by helley smith samplers in coarse gravel channels

experimental study of incipient motion in mixed size sediment transport rates of five sediments were measured in a laboratory flume three of these sediments had the same mean size the same size distribution shape and different values of grain size distribution standard deviation the critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stress that produced a small dimensionless transport rate the sorting of the sediment mixture had little effect on the critical shear stress of individual fractions once the median size d50 of the mixture and a fraction s relative size di d50 are accounted for our data combined with previously published data show a remarkably consistent relation between the critical shear stress of individual fractions and the fraction s relative grain size despite a broad variation in the available data of mixture sorting grain size distribution shape mean grain size and grain shape all fractions in a size mixture begin moving at close to the same value of bed shear stress during steady state transport conditions this result is apparently true for transport systems where the transport rates of individual fractions are determined solely by the flow and bed sediment recirculating systems as well as for systems where the fractional transport rates are imposed on the system feed systems this equivalence in initial motion results is important because natural transporting systems often show attributes of both types of behavior in an unknown combination

mobility of bed material in harris creek 1 the mobility of bed material is investigated in a small gravel bed river in which the bed is weakly bimodal in texture and relatively stable the bed surface remained intact throughout the freshet and scour was limited to small areas of the bed observations of bedload caught in pit traps and displacement of magnetically tagged particles during two spring freshets permit analysis of the mobility of each size fraction fractional transport rates plot up to three orders of magnitude below the reference transport rate suggested in the literature at low flows no size fraction was fully mobile at intermediate flows shear stress 27 pa sand was fully mobile while larger material remained partially mobile at high flows 43 pa the division between the full and partial mobility regimes occurred around 16 mm which remains finer than the median size of the subsurface material the largest material scarcely moves at mean annual flood magnetic tracers confirmed results obtained from the pit traps three distinct zones of mobility can be defined in fractional transport plots partial mobility full mobility and overpassing suspended critical shear stress for incipient motion varied over an order of magnitude a qualitatively similar result for bimodal sediment was reported by wilcock and mcardell 1993 incipient motion analysis based on the largest grain observed to move yielded an upper envelope implying transport similarity for sizes found on the bed surface and a lower envelope that corresponds with our nonsimilar entrainment criterion

effective discharge in rocky mountain headwater streams summary whereas effective discharge q eff in mountain streams is commonly associated with a moderate flow such as bankfull discharge q bf this study found that the maximum discharge q max and not bankfull discharge is the channel forming or effective flow for gravel transport in plane bed streams where partial bed mobility causes steep gravel transport rating curves q eff may approach bankfull flow in some step pool channels where gravel moves over a static cobble boulder bed our conclusions are based on magnitude frequency analyses conducted at 41 gauged rocky mountain headwater streams because these gauged streams lacked gravel transport data as is typical comparable streams with measured transport rates were used to develop scaling relations for rating curve exponents with stream and watershed characteristics those scaling relations were then used to estimate the steepness of gravel rating curves at the 41 gauged but unsampled sites the measured flow frequency distributions were characterized by two fitted power functions the steepness of the flow frequency distributions and the estimated steepness of gravel transport relations were combined in magnitude frequency analyses to compute q eff

influence of bed patchiness slope grain hiding and form drag on gravel mobilization in very steep streams steep streams are a major portion of channel networks and provide a link to transport sediment from hillslopes to lower gradient rivers despite their importance key unknowns remain perhaps foremost of which is evaluating in steep streams empirical laws for fluvial sediment transport developed for low gradient rivers to address this knowledge gap we painted sediment in situ over 3 years to monitor incipient sediment motion and sediment patch development in five small drainage areas of 0 04 2 km 2 and steep slopes of 5 37 tributaries of elder creek california united states we found that channel beds organized into size sorted sediment patches which displayed active fluvial transport of gravel annually consistent year to year patch median grain sizes partial transport of bed material and significantly higher values of critical shields stress for incipient sediment motion compared to that observed for lower gradient rivers the high critical shields stresses up to 0 5 for the median grain size agree within a factor of 3 to theoretical predictions which account for slope dependent hydraulics grain hiding and sediment patches for grains of approximately the same size as the roughness length scale slope dependent hydraulics and bed patchiness are the dominant controls on critical shields stress values while grain hiding is important for grains larger or smaller than the roughness length scale form drag exists in our monitored tributaries but has a smaller influence than the above effects our field observations show fluvial processes contribute to sediment mobilization in steep channels which are often considered to be dominated by debris flows

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

a two dimensional discrete particle model of gravel bed river systems 1 the formation of bed forms in gravel bed rivers acts as a control on stream ecology and the response of rivers to floods available models do not reproduce the range of observed bed forms and do not consider interactions between the bed and flow hydraulics the model presented here considers a gravel bed river as a complex system in which sediment clasts are represented as discrete elements simple and local rules describe the sediment and flow dynamics using a trimodal sediment distribution irregular forms that scale with particle diameter develop without explicit feedback mechanisms because of the tendency of large particles to roll along the bed surface and collect into chains feedback mechanisms such as imbrication increase the effective entrainment threshold of groups of large particles and increase the stability of these imbricate forms a second type of bed form is associated with saltating grains and emerges where particles are transported at a preferred distance the development and maintenance of larger scale bed forms require feedback between the bed and flow properties by allowing mean velocity to adjust to bed morphology and considering the effect of acceleration on turbulence generation and mean velocity profiles we demonstrate the emergence of forms similar in morphology to gravel sheets dunes and riffle pools the model is best used to complement field based studies and is suitable for testing hypotheses of streambed behavior

bed load transport in an obstruction formed pool in a forest gravelbed stream abstract this paper examines channel dynamics and bed load transport relations through an obstruction forced pool in a forest gravel bed stream by comparing flow conditions sediment mobility and bed morphology among transects at the pool head centre and tail variable sediment supply from within and outside of the channel led to a complex pattern of scour and fill hysteresis despite the large flood magnitude large portions of the bed did not scour scour was observed at three distinct locations two of these were adjacent to large woody debris lwd and the third was along the flow path deflected by a major lwd obstruction bed material texture showed little change in size distribution of either surface or subsurface material suggesting lack of disruption of the pre flood bed fractions larger than the median size of the bed surface material were rarely mobile sediment rating relations were similar although temporal variation within and among stations was relatively high relations between bed load size distribution and discharge were complex showing coarsening with increasing discharge followed by fining as more sand was mobilized at high flow lack of local scour in the pool combined with bed load fining and net fill by relatively fine material implied that the dominant sources of mobile sediment were upstream storage sites and local bank collapse patterns of flow channel dynamics and sediment mobility were strongly affected by a lwd flow obstruction in the pool centre that created turbulent effects thereby enhancing entrainment and transport in a manner similar to scour at bridge piers

fluvial erosion transport equation of landscape evolution models revisited we present a mesoscale erosion deposition model which differs from previous landscape evolution models equations by taking explicitly into account a mass balance equation for the streamflow the geological and hydrological complexity is lumped into two basic fluxes erosion and deposition and two averaged parameters unit width discharge q and stream slope s the model couples the dynamics of streamflow and topography through a sediment transport length function x q which is the average travel distance of a particle in the flow before being trapped on topography this property reflects a time lag between erosion and deposition which allows the streamflow not to be instantaneously at capacity the so called x q model may reduce either to transport limited or to detachment limited erosion modes depending on x but it also may not we show in particular how it does or does not for steady state topographies long term evolution and high frequency base level perturbations apart from the unit width discharge and the settling velocity the x q function depends on a dimensionless number encompassing the way sediment is transported within the streamflow using models of concentration profile through the water column we show the dependency of this dimensionless coefficient on the rouse number we discuss how consistent the x q model framework is with bed load scaling expressions and einstein s conception of sediment motion

multiphysics processes in partially saturated fractured rock experiments and models from yucca mountain 1 the site investigations at yucca mountain nevada have provided us with an outstanding data set one that has significantly advanced our knowledge of multiphysics processes in partially saturated fractured geological media such advancement was made possible foremost by substantial investments in multiyear field experiments that enabled the study of thermally driven multiphysics and testing of numerical models at a large spatial scale the development of coupled process models within the project have resulted in a number of new advanced multiphysics numerical models that are today applied over a wide range of geoscientific research and geoengineering applications using such models the potential impact of thermal hydrological mechanical thm multiphysics processes over the long term e g 10 000 years could be predicted and bounded with some degree of confidence the fact that the rock mass at yucca mountain is intensively fractured enabled continuum models to be used although discontinuum models were also applied and are better suited for analyzing some issues especially those related to predictions of rockfall within open excavations the work showed that in situ tests rather than small scale laboratory experiments alone are essential for determining appropriate input parameters for multiphysics models of fractured rocks especially related to parameters defining how permeability might evolve under changing stress and temperature a significant laboratory test program at yucca mountain also made important contributions to the field of rock mechanics showing a unique relation between porosity and mechanical properties a time dependency of strength that is significant for long term excavation stability a decreasing rock strength with sample size using very large core experiments and a strong temperature dependency of the thermal expansion coefficient for temperatures up to 200 c the analysis of in situ heater experiments showed that fracture closure opening caused by changes in normal stress across fractures was the dominant mechanism for thermally induced changes in intrinsic fracture permeability during rock mass heating cooling and that fracture shear dilation appears to be less significant significant effort was devoted to predicting the long term stability of underground excavations under mechanical strength degradation and seismic loading perhaps one of the most challenging tasks within the project we note that such long term strength degradation is actually an example of a chemically mediated process governed by underlying microscopic stress corrosion and chemical diffusion processes in the yucca mountain project such chemically mediated mechanical changes were considered implicitly through model calibrations against laboratory and in situ heater experiments at temperatures anticipated to be experienced by the rock a possible future research direction would be to simulate such processes mechanistically in a complete coupled thmc framework where c denotes chemical processes

observations of bedload transport in a gravel bed river during high flow using fiber optic dts methods the question how does a streambed change over a minor flood does not have a clear answer due to lack of measurement methods during high flows we investigate bedload transport and disentrainment during a 1 5 year flood by linking field measurements using fiber optic distributed temperature sensing dts cable with sediment transport theory and an existing explicit analytical solution to predict depth of sediment deposition from amplitude and phase changes of the diurnal near bed pore water temperature the method facilitates the study of gravel transport by using near bed temperature time series to estimate rates of sediment deposition continuously over the duration of a high flow event coinciding with bar formation the observations indicate that all gravel and cobble particles present were transported along the riffle at a relatively low shields number for the median particle size and were re deposited on the lee side of the bar at rates that varied over time during a constant flow approximately 1 6 of the bed was predicted to be mobile during the 1 5 year flood indicating that large inactive regions of the bed particularly between riffles persist between years despite field observations of narrow zones of local transport and bar growth on the order of 3 5 times the median particle size in contrast during a 7 year flood approximately 8 55 of the bed was predicted to become mobile indicating that the continuous along stream mobility required to mobilize coarse gravel through long pools and downstream the next riffle is infrequent

analysis on heat transfer and heat loss characteristics of rock cavern thermal energy storage abstract the present study is aimed at demonstrating the feasibility of the rock cavern compared with the above ground tank for the storage of large scale high temperature thermal energy by quantitatively evaluating the heat transfer inside the storage tank and the heat loss characteristics of the surrounding environment as a conceptual model we consider a thermal energy storage tes system coupled with an adiabatic compressed air energy storage plant a caes which utilizes loosely packed bed of rocks as heat storage medium and stores heat of up to 685 c the specifications of the tes model such as the mass flow rate of the heat transfer material and the storage volume were determined through the analysis of the heat transfer in the packed bed using a quasi one dimensional two phase numerical model developed in this study in this procedure the inlet and outlet fluid temperatures and the thermal energy rates to be stored or extracted were examined over 200 consecutive daily cycles to ensure the tes met the requirements for the power generation of the a caes plant then with the determined specifications of the tes a comparative study on the heat loss characteristics of the rock cavern type tes and above ground type tes systems was performed by simulating the operations on a daily basis for a period of 10 years using a three dimensional numerical model the comparison results indicated that the amount of cumulative heat loss in the rock cavern type tes system over the operation period was far smaller than that in the above ground type tes system because of the surrounding rock heating and the consequent reduction in the thermal gradient between the surrounding rock and the storage medium in terms of long term operation the rate of heat loss from the rock cavern type tes system exhibited less sensitive and less dependent behaviors with respect to the insulator performance than that of the above ground type tes system

selective grain entrainment by a current from a bed of mixed sizes a reanalysis the selective entrainment of grains by a current from a bed of mixed sizes and densities is important to grain sorting processes that lead to the formation of placers in sands and to armored beds in gravels existing field and laboratory measurements of selective entrainment depart systematically from the standard threshold curves such as that of shields which are based on experiments with nearly uniform grain sizes entrainment measurements from beds of mixed sizes form trends which obliquely cross the threshold curves for uniform grains the crossing point being roughly at the median diameter of the size distribution due to this crossover the coarser size fractions of the distribution require lower flow stresses for their entrainment than if they formed uniform beds while the finer size fractions require higher stresses than uniform beds with deposits having medians in the range of medium sand through gravel the larger the individual particle within the mixed sizes the greater the flow stress required for its entrainment in this case the smaller grains are preferentially entrained possibly leading to bed armoring in contrast if the median is in the fine to medium sand range the smallest grains in the bed of mixed sizes are the most difficult to entrain due to sheltering by the larger grains where grain density is also involved this sorting can lead to the formation of placers a variety of empirical relationships is considered to quantify the evaluation of selective entrainment from deposits of mixed sizes but no single formulation is satisfactory for all data sets

entrainment displacement and transport of tracer gravels the mass and size distribution of grain entrainment per unit bed area may be measured by replacing a volume of the bed with tracer gravels and observing the mass difference before and after a transport event this measure of spatial entrainment is relevant to any process involving size selective exchange of sediment between transport and bed and may be directly used in calculations of sediment transport rate using an elementary relation for fractional transport components presented here this relation provides a basis for evaluating tracer data collected by different methods and may be used to provide physical insight regarding the expected behaviour of tracer grains the variation with grain size of total displacement length l ti depends on the degree of mobilization of the individual fractions on the bed surface l ti is independent of d i for smaller fully mobile sizes and decreases rapidly with d i for larger fractions in a state of partial transport in which a portion of the surface grains remain immobile through the flow event the boundary between fully and partially mobile grain sizes increases with flow strength these inferences are supported by values of l ti calculated from flume experiments and provide a physical explanation for a summary relation between l ti and d i based on field data 1997 john wiley sons ltd

particle friction angles in steep mountain channels sediment transport rates in steep mountain channels are typically an order of magnitude lower than predicted by models developed for lowland rivers one hypothesis for this observation is that particles are more stable in mountain channels due to particle particle interlocking or bridging across the channel width this hypothesis has yet to be tested however because we lack direct measurements of particle friction angles in steep mountain channels here we address this data gap by directly measuring the minimum force required to dislodge sediment pebbles to boulders and the sediment weight in mountain channels using a handheld force gauge at eight sites in california with reach averaged bed angles ranging from 0 5 to 23 and channel widths ranging from 2 m to 16 m we show that friction angles in natural streams average 68 and are 16 larger than those typically measured in laboratory experiments which is likely due to particle interlocking and burial results also show that larger grains are disproportionately more stable than predicted by existing models and that grains organized into steps are twice as stable as grains outside of steps however the mean particle friction angle does not vary systematically with bed slope these results do not support systematic increases in friction angle in steeper and narrower channels to explain the observed low sediment transport rates in mountain channels instead the spatial pattern and grain size dependence of particle friction angles may indirectly lower transport rates in steep narrow channels by stabilizing large clasts and channel spanning steps which act as momentum sinks due to form drag

partial transport of a sand gravel sediment grains of a single size within a mixed size bed are entrained over a range of flows within this range some grains exposed on the bed surface are active entrained at least once over the duration of a transport event while the remaining surface grains are immobile a condition we define as partial transport we demonstrate the existence and domain of partial transport using observations of grain entrainment on time series of bed photographs of flume experiments with a widely sorted sand gravel mixture the active proportion of the bed surface increases with bed shear stress 0 at a given 0 90 of the active grains are entrained when the cumulative mass transported exceeds approximately 4 times the active mass on the bed mobilization of grains in a size fraction increases from 10 to 90 over a range of 0 of a factor of 2 the bounds of this range increase with grain size di so that at a given 0 sizes over a range of a factor of 4 are in a state of partial transport fractional transport rates are independent of di for fully mobilized fractions and decrease rapidly with di for partially mobile fractions partial transport is associated with substantial transport rates of finer fully mobile sizes limits both the rate and size distribution of grain exchange with the bed subsurface and may be the dominant transport condition in many gravel bed rivers

experiments on the effect of hydrograph characteristics on vertical grain sorting in gravel bed rivers 1 desert ephemeral gravel bed streams typically have bed surfaces that are relatively unarmored compared to the substrate below while gravel bed streams in humid and snowmelt areas typically have well armored surfaces the degree of armoring can be characterized in terms of an armor ratio defined as the ratio of the surface median size to the substrate median size a set of field data shows desert ephemeral gravel bed streams with armor ratios ranging from 0 5 to 2 4 and with an average value of 1 2 the armor ratio of snowmelt fed gravel bed streams in the same set ranges from 2 to 7 with an average value of 3 4 the reason for this difference is sought in terms of differing hydrological characteristics and sediment supply regimes thirteen experiments were conducted to study the formation of armoring under a range of hydrological conditions the experiments have two limiting cases a relatively flat hydrograph that represents conditions produced by continuous snowmelt and a sharply peaked hydrograph that represents conditions associated with flash floods all constant hydrograph experiments developed a well armored structured surface while short asymmetrical hydrographs did not result in substantial vertical sorting all symmetrical hydrographs show some degree of sorting and the sorting tended to become more pronounced with longer duration sediment supply appears to be a first order control on bed surface armoring while the shape of the hydrograph plays a secondary role

temporal variations in bedload transport rates associated with the migration of bedforms temporal variations in bedload transport rates that occur at a variety of timescales even under steady flow conditions are accepted as an inherent component of the bedload transport process rarely however has the cause of such variations been explained clearly we consider three data sets obtained from laboratory experiments that refer to measurements of bedload transport made with continuously recording bedload traps each data set is characterized by a predominant low frequency oscillation on which additional higher frequency oscillations generally are superimposed the period of these oscillations as isolated through the use of spectral analysts ranged between 0 47 and 168 minutes and was associated unequivocally with the migration of bedforms such as ripples dunes and bars the extent to which such oscillatory behaviour may be recognized in a data set depends on the duration of sampling and the length of the sampling time with respect to the period of a given bedform several theoretical probability distribution functions have been developed to describe the frequency distributions of relative bedload transport rates that are associated with the migration of bedforms einstein 1937b hamamori 1962 carey and hubbell 1986 these distribution functions were derived without reference to a sampling interval we present a modification of hamamori s 1962 probability distribution function generated by monte carlo simulation which permits one to specify the sampling interval in relation to the length of a bedform comparisons between the simulated and observed frequency distributions that were undertaken on the basis of the data described herein are good significant at the 90 per cent confidence level finally the implications that temporal variability which is associated with the migration of bedforms have for the accurate determination of bedload transport rates are considered

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

ice cores as an index of global volcanism from 1850 to the present to evaluate an important cause of past climate change climate researchers need reliable estimates of volcanic aerosol loading in the atmosphere previous indices the dust veil index volcanic explosivity index and those by mitchell 1970 sato et al 1993 and khmelevtsov et al unpublished manuscript 1993 all have drawbacks ice core acidity and sulfate records because they contain physical evidence of atmospheric loading are a promising source of information on past volcanic aerosols but these records contain large nonvolcanic signals as well we have compared northern hemisphere nh and southern hemisphere sh annual average versions of five indices with eight nh and six sh ice core records for the period 1850 to the present in an attempt to identify the volcanic signal common to all records the indices are all highly correlated with each other for the nh although the individual ice core records are in general not well correlated with each other or with any of the indices a composite derived from averaging the cores the ice core volcano index ivi shows promise as a new index of volcanic aerosol loading this new index correlates well with the existing non ice core volcanic indices and with high frequency temperature records for the sh the individual ice cores and indices are better correlated the shivi is again highly correlated with all indices and individual ice cores but not with high frequency temperature records for both hemispheres the southern oscillation index shows no significant correlation with the volcanic indices ice cores or the ivi thus providing no evidence for the impact of volcanic eruptions on el nino southern oscillation events

surface based fractional transport rates mobilization thresholds and partial transport of a sand gravel sediment twenty eight coupled observations of flow transport and bed surface grain size distribution were made in a laboratory flume using a wide range of flows and a sediment with a very poorly sorted bimodal grain size distribution these observations permit the transport rates of individual size fractions to be scaled by the proportion of each size immediately available for transport on the bed surface the key to our observations is the use of a sediment in which each size fraction has been painted a different color which permits reliable repeatable and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface at a given flow the fractional transport rates may be divided into two parts a finer grained portion within which fractional transport rates are a function only of their proportion on the bed surface and total transport rate and a coarser grained portion for which fractional transport rates also depend on the proportion of individual grains within a fraction that remain essentially immobile throughout the experimental run we define the latter condition as one of partial transport and observe that the grain size separating partial and fully mobilized transport consistently increases with flow strength complete mobilization of a size fraction occurs at roughly twice the shear stress necessary for incipient motion of that fraction zones of partial and full mobility are quite distinct when fractional transport rates are scaled by the bed surface grain size distribution although a region of partial transport is evident when these data and other experimental and field observations are scaled by the bulk grain size distribution of the sediment bed critical shear stresses for the incipient motion of individual fractions in our experimental sediment vary over an order of magnitude a result strongly in contrast to many earlier observations but consistent with our observations of incipient motion in sediments with bimodal grain size distributions

possible association of stratospheric aerosols and el nino type events to determine whether the 1982 83 el nino event and the eruption of el chichon in april 1982 are related eleven similar eruptions at low latitudes 20 show just the opposite behavior they show significant cooling of the sea surface temperatures in this region especially in the fourth and fifth season after the eruption the association is found to be significant at the 95 level

gcm evaluation of a mechanism for el nino triggering by the el chichon ash cloud the el chichon volcanic eruption in 1982 was immediately followed by the strongest el nino of the century in 1982 83 we investigate a possible mechanism proposed by hirono whereby volcanic eruptions might trigger or enhance el ninos three distributions of volcanic ash aerosol in the troposphere over the eastern pacific ocean were used to force simulations with a modified version of the ncar ccm1 atmospheric general circulation model for the month of april 1982 using observed sea surface temperatures only in the case of mid tropospheric heating is there a strong dynamical reaction in the atmosphere resulting in a weakening of the trade winds in the eastern pacific ocean north of the equator consistent with surface wind observations and with the first part of hirono s theory however the 1982 enso event had started before this wind anomaly and only trade wind collapses in the western equatorial pacific can initiate el ninos the results suggest that while tropospheric aerosols from volcanic eruptions can influence atmospheric circulation the timing and location of the el chichon eruption and the large enso event that followed was a coincidence and that hirono s mechanism was not responsible for the strength or timing of the el nino

modeling of fluid injection induced fault reactivation using coupled fluid flow and mechanical interface model abstract the present study is aimed at developing a numerical model to reproduce coupled hydro mechanical processes associated with fault reactivation by fluid injection in low permeability rock as part of the decovalex 2019 project task b we proposed a modeling approach for simulating the processes using the tough flac simulator and modeled a fault reactivation experiment conducted at mont terri rock laboratory in switzerland the first step of the study involved benchmark calculations considering a simplified fault plane and geometry fluid flow along a fault was modeled using elements of aperture sized thickness on the basis of darcy s law and the cubic law in tough2 whereas the mechanical behavior of a single fault was represented by zero thickness interface elements in flac3d upon which a slip and or separation is allowed a methodology to connect a tough2 volume element to a flac3d interface element was developed for handling the hydro mechanical interactions on the fault during fluid injection two different fault models for describing the evolutions of hydraulic aperture by elastic fracture opening and failure induced aperture increase were considered in the benchmark calculations in the coupling process the changes in geometrical features and hydrological properties induced by mechanical deformation were continuously updated the transient responses of the fault and host rock to stepwise pressurization were examined during the simulation the hydro mechanical behavior including the injection flow rate pressure distribution around the borehole stress conditions and displacements in normal and shear directions were monitored in the surrounding rock and along the fault the results of benchmark calculations suggest that the developed model reasonably represents the hydro mechanical behavior of a fault and the surrounding rock this modeling approach was applied to the fault reactivation experiment of the mont terri rock laboratory in this interpretive modeling a parametric study was conducted to examine the effects of input parameters regarding in situ stress and fault properties on the hydro mechanical responses of the fault to water injection then an optimal parameter set to reproduce the field experiment results was chosen by trial and error the injection flow rate and pressure response during fault reactivation closely matched those obtained at the site which indicates the capability of the model to appropriately capture the progressive pathway evolution during fault reactivation tests at the site the anchor displacements were overestimated by the model but a fair agreement was obtained in terms of the order of magnitude and the variation tendency

el chinchon eruption the dust cloud of the century on passe en revue les differentes methodes d observation du nuage de l eruption d el chichon satellite lidar observation aerienne

breakup and reestablishment of the armour layer in a large gravel bed river below dams the lower ebro changes in armour layer during floods under supply limited conditions are little known this paper describes the breakup and the reestablishment of the bed armour layer in the regulated gravel bed ebro river during a flooding period the study was conducted over a 28 km study reach from 2002 to 2004 the surface subsurface and bed load grain size distribution constitute the bases for the analysis of bed armouring dynamics the results indicate that the magnitude of floods controlled the degree of armouring of the river bed the initial mean armouring ratio was 2 3 with maximum values reaching 4 4 floods in the winter of 2002 2003 q8 caused the breakup of the armour layer in several sections this resulted in the erratic bed load pattern observed during the december 2002 flushing flow and in the increase in bed load transport during successive events most grain size classes were entrained and transported causing river bed incision the mean armouring ratio decreased to 1 9 in contrast during low magnitude floods in 2003 2004 q2 the coarsest fractions 64 mm did not take part in the bed load while finer particles were winnowed thus surface deposits coarsened as a result the armour layer was reestablished i e the mean armouring ratio increased to 2 3 and the supply of subsurface sediment decreased the supply and transport of bed material appear to be in balance in the river reach immediately below the dam in contrast the transport of medium and finer size classes in the downstream reaches was higher than their supply from upstream a phenomenon that progressively reduced their availability in the river bed surface hence the armour layer reworking

sediment storage partial transport and the evolution of an experimental gravel bed under changing sediment supply regimes abstract the adjustment of a gravel bed during cycles of sediment storage was analyzed for a flume experiment designed to study the effects of episodic sediment supply regimes as the bed aggraded and degraded due to changes in sediment feed it developed a complex topography with bedforms at different scales e g pebble clusters bars sediment inputs increased bedload rates and promoted large changes in bed morphology without an external supply the bed armored which increased bed stability counteracted bed degradation and decreased bedload rates fully mobile gravel 2 8 mm was the only fraction that exhibited a deficit over the experiment preferential storage of coarse grains caused the development of a sediment wedge upstream and downstream fining on the bed surface the storage of sand was also significant 60 of feed probably because of infiltration e g in the upstream wedge and hiding effects promoted by bed armor and structures size selective transport caused a cumulative increase in bed slope which was likely to affect sediment transport capacity the large variability in bedload rates for same levels of bed storage indicated that sediment availability and bed state were the main controls on sediment transport regime we provide empirical evidence that cycles of aggradation and degradation caused by changes in sediment feed upstream condition bed composition and topography and the rate and texture of sediment output

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

coupled thermal hydrological mechanical behavior of rock mass surrounding a high temperature thermal energy storage cavern at shallow depth abstract we numerically model the thermal hydrological mechanical thm processes within the rock mass surrounding a cavern used for thermal energy storage tes we consider a cylindrical rock cavern with a height of 50 m and a radius of 10 m storing thermal energy of 350 c as a conceptual tes model and simulate its operation for thirty years at first the insulator performance are not considered for the purpose of investigating the possible coupled thm behavior of the surrounding rock mass then the effects of an insulator are examined for different insulator thicknesses the key concerns are hydro thermal multiphase flow and heat transport in the rock mass around the thermal storage cavern the effect of evaporation of rock mass thermal impact on near the ground surface and the mechanical behavior of the surrounding rock mass it is shown that the rock temperature around the cavern rapidly increases in the early stage and consequently evaporation of groundwater occurs raising the fluid pressure however evaporation and multiphase flow does not have a significant effect on the heat transfer and mechanical behavior in spite of the high temperature 350 c heat source the simulations showed that large scale heat flow around a cavern is expected to be conduction dominated for a reasonable value of rock mass permeability thermal expansion as a result of the heating of the rock mass from the storage cavern leads to a ground surface uplift on the order of a few centimeters and to the development of tensile stress above the storage cavern increasing the potentials for shear and tensile failures after a few years of the operation finally the analysis shows that high tangential stress in proximity of the storage cavern can some shear failure and local damage although large rock wall failure could likely be controlled with appropriate insulators and reinforcement