detecting the near surface redox front in crystalline bedrock using fracture mineral distribution geochemistry and u series disequilibrium oxidizing conditions normally prevail in surface waters and near surface groundwaters but there is usually a change to reducing conditions in groundwater at greater depth dissolved o2 originally present is consumed through biogenic and inorganic reactions along the flow paths fracture minerals participate in these reactions and the fracture mineralogy and geochemistry can be used to trace the redox front an important task in the safety assessment of a potential repository for the disposal of nuclear waste in crystalline bedrock at an approximate depth of 500 m in sweden is to demonstrate that reducing conditions can be maintained for a long period of time oxygen may damage the cu canisters that host nuclear waste additionally in the event of a canister failure oxidizing conditions may increase the mobility of some radionuclides the present study of the near surface redox front is based on mineralogical redox sensitive minerals geochemical redox sensitive elements and u series disequilibrium investigations of mineral coatings along open fractures the fractures have been sampled along drill cores from closely spaced 100 m deep boreholes which were drilled during the site investigation work in the laxemar area south eastern sweden carried out by the swedish nuclear fuel and waste management co skb the distribution of the redox sensitive minerals pyrite and goethite in open fractures shows that the redox front switch from mainly goethite to mainly pyrite in the fractures generally occurs at about 15 20 m depth calcite leaching by recharging water is indicated in the upper 20 30 m and positive ce anomalies suggest oxidation of ce down to 20 m depth the u series radionuclides show disequilibrium in most of the samples indicating mobility of u during the last 1 ma in the upper 20 m u is mainly removed due to oxidation or has experienced complex removal and or deposition at depths of 35 55 m both deposition and removal of u are indicated below 55 m recent deposition of u is generally indicated which suggests removal of u near surface oxidation and deposition of u below the redox front scattered goethite occurrences below the general redox front down to ca 80 m and signs of u removal at 35 55 m mostly correlate with sections of high transmissivity and or high fracture frequencies this shows that highly transmissive fractures are generally required to allow oxygenated groundwaters at depth greater than ca 30 m removal of u oxidation below 55 m within the last 300 ka is not observed although penetration of glacial waters to great depths has been confirmed in the study area their potential o2 load seems to have been reduced near the surface

channel bed steps along nahal yael negev desert israel abstract the height and spacing of channel bed steps formed in boulder and bedrock in the ephermeral channels of the nahal yael watershed show a strong correlation with the channel slope the distance between steps is inversely proportional to slope at slopes of 0 to 20 but then remains fairly constant at higher slopes step height increases proportionally with channel slope these bed steps are similar to those described on perennial channels elsewhere where step formation has been attributed to the effects of high flows which submerge the clasts forming the steps however the ephemeral flows in the tributary channels of the nahal yael system are not sufficient to submerge the clasts forming the boulder steps using rouse s criteria channel bed steps are creating maximum flow resistance at slopes of 12 to 50 along the channels of nahal yael using davies criteria bed steps are creating maximum flow resistance at slopes of 3 to 10 the bed steps also relate to sediment transport nahal yael has a low sediment yield by world standards and the increase in flow resistance due to step formation may serve to increase sediment movement both the boulder and bedrock steps at nahal yael are hypothesized to reflect the scales of flow turbulence along the channels we were unable to evaluate the role of flow volume or boundary roughness in controlling these scales but channel slope appears to exert some control on the scale of turbulence based on the correlation between slope and bed steps the hypotheses developed from field evidence at nahal yael should be testable in flume simulations

hydraulics morphology and energy dissipation in an alpine step pool channel 1 to investigate the relationship between hydraulics and channel morphology in step pool channels we combined three dimensional velocity measurements with an acoustic doppler velocimeter and topographic surveys in a steep step pool channel the rio cordon italy measurements were organized around step pool and tread units and occurred within a range of 36 57 of bankfull discharges as flow moved from steps to their downstream pools in our study reach an average of approximately two thirds of the total energy was dissipated as measured by relative head loss through step pool sequences much of this head loss was achieved by elevation potential energy loss rather than velocity reductions although an overall expected pattern of flow acceleration toward step crests and deceleration in pools was present pool velocities were high especially where upstream step crests were irregular and where residual pool depths were low many steps were porous or leaky with irregular cross channel bed and water surface topography producing high velocity jets and less flow resistance than channel spanning dammed steps longitudinal variations in hydraulics are thus often overshadowed by lateral variations arising from morphologic complexities velocity and turbulence characteristics in the rio cordon show marked differences from data we have collected in a more stable and wood rich channel in the colorado rockies in which ponded steps are more prevalent and pools are slower and more turbulent comparison of these channels illustrates that step pool structure and hydraulics are strongly influenced by flow regime sediment supply lithology time since the last step forming flood and availability of in stream wood

analysis of changes in the step pool morphology and channel profile of a steep mountain stream following a large flood abstract the step pool morphology of steep mountain streams is reorganized during large floods by the removal of steps scouring of the channel bed and the formation of a new step pool sequence by the re deposition of step forming grains in this paper we document these changes caused by the largest recorded flood return period 50 years in the steep mountain stream erlenbach in central switzerland in 2007 by comparing pre flood and post flood surveys of a 650 m long study reach we show that the flood led to statistically significant changes in the distributions of step height and spacing most notably an increase in the variance of both variables organization at the river reach scale is not apparent and the correlations between step properties both pre and post flood are generally weak the flood led to widespread erosion of the river bed locally up to 2 3 m of vertical channel change was observed at least 60 of the steps moved even the largest ones however they remained constrained to the parts of the study reach where landslides on the adjacent hillslopes are most active detailed changes in bed structure were analyzed from the pre and post flood profiles with a statistical scaling approach which identifies the complexity of the spatial organization of sediment in the longitudinal profile multiscaling found in both step and pool sequences reflects the complex spatial structure which is the result of the superposition of steps formed by the history of flood events

the geomorphic significance of step pools in mountain streams abstract this paper develops a conceptual model to define the changing role and significance of step pools as energy dissipators in steep mountain channels although energy dissipation is a significant function of step pools this role changes over time with variations in discharge steps are effective in reducing stream energy at low flows but their effectiveness diminishes with increasing stage accordingly the manner in which energy dissipation occurs also varies with increasing flows spill resistance gives way to a dominance in form and grain resistance the conceptual model for the changing role of step pools is illustrated with data from hydraulic analysis and modeling of step pools in the santa monica mountains of california the model points to the importance of the size of steps in determining their role in energy dissipation and in their interactions with channel hydraulics the model offers a new articulation for the geomorphic significance of step pools in mountain streams and it serves as a useful template for a more complete understanding of step pools over a longer time scale

diagenetic characteristics of low permeability non reservoir chalks from the central north sea abstract the characteristics and diagenetic history of low permeability non reservoir chalk of the central north sea have been investigated to establish how chalk lithologies evolve into rocks capable of acting as a pressure seal the permeability of 25 chalk samples have been measured using the transient pulse decay method and shows that most non reservoir chalks have low permeability typically of tens of nanodarcys irrespective of depth 2000 3900 m clay content facies or stratigraphic formation the samples were further characterised using a number of techniques including petrological analysis insoluble residue measurements and porosimetry petrological studies have revealed that mechanical compaction is a relatively minor process in clean non argillaceous chalks due to a framework of grains that resist compaction chemical compaction is the dominant porosity occluding mechanism in clean chalks principally consisting of overgrowth cementation and grain to grain pressure solution in contrast the diagenetic history of argillaceous chalks is more complex with extensive mechanical compaction occurring in addition to irregular cementation and widespread dissolution the interpretation that the style of diagenesis is controlled by clay content is supported by analysis of pore size distributions as clean chalks show a gradual reduction in mean pore size with depth due to gradual occlusion by cements while argillaceous chalks show constantly small mean pore radii 5 20 nm as a result of early mechanical compaction coupled with dissolution the only sample within the set which did not exhibit low permeability possesses inclined stylolites suggesting that locally stylolites can act as fluid pathways the results show that both clean and argillaceous chalk diagenetic pathways result in low permeability rocks and the diversity of rock types that exhibit low permeability suggests that seals are pervasive throughout the chalk group non reservoir chalks can therefore act as significant barriers to fluid flow trapping hydrocarbons as well as significant pressure seals within the central north sea trapping high pressures beneath the chalk group

the periodic nature of step pool mountain streams rhythmic sequences of steps and pools in steep mountain streams have captured the attention of numerous workers but whether periodicities exist in the occurrence of step pools has not been investigated in a comprehensive way and the implications for such periodicities have not been explored spectral analysis was applied to bed elevation data from streams in the santa monica mountains of southern california to investigate periodicity in the occurrence of step pools results indicate significant step pool periodicities confirmed by independent field measurements that vary from 0 43 channel widths to 2 40 channel widths spectral analysis of channels affected by external influences tends to indicate multiple significant peaks that reflect higher variance in step pool spacing underlying periodicities are nevertheless de tected suggesting that external factors obscure but do not destroy the effects of more general internal mechanisms periodicity in the occurrence of step pools suggests that step pools are bedforms that represent a fundamental mutual adjustment between flow channel morphology and energy expenditure a rhythmic channel thalweg supports the notion that step pools are analogous to meandering in the vertical dimension and implies that because step pool channels are not able to adjust energy expenditure in the plane dimension they instead adjust boundary roughness in the vertical dimension a continuum of rhythmic gravel bedforms that displays the results of such adjustments and that shows how step pools merge into pools and riffles in the stream channel system is discussed and illustrated with empirical data from the santa monica mountains although many differences exist between steep mountain streams and broad alluvial rivers the findings of this study suggest that generalized principles of uniform energy expendi ture may also apply to rhythmic step pool streams these findings are potentially valuable in the management and design of steep channels

pool spacing in forest channels field surveys of stream channels in forested mountain drainage basins in southeast alaska and washington reveal that pool spacing depends on large woody debris lwd loading and channel type slope and width mean pool spacing in pool riffle plane bed and forced pool riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing lwd loading whereas pool spacing in generally steeper step pool channels is independent of lwd loading although plane bed and pool riffle channels occur at similar low lwd loading they exhibit typical pool spacings of greater than 9 and 2 4 channel widths respectively forced pool riffle channels have high lwd loading typical pool spacing of 2 channel widths and slopes that overlap the ranges of free formed pool riffle and plane bed channel types while a forced pool riffle morphology may mask either of these low lwd loading morphologies channel slope provides an indicator of probable morphologic response to wood loss in forced pool riffle reaches at all study sites less than 40 of the lwd pieces force the formation of a pool we also find that channel width strongly influences pool spacing in forest streams with similar debris loading and that reaches flowing through previously clear cut forests have lower lwd loading and hence fewer pools than reaches in pristine forests

substrate influences on step pool sequences in the christopher creek drainage arizona abstract step pool sequences formed in alluvial and bedrock channel segments of the christopher creek drainage in central arizona suggest that bedform geometries may reflect subtle differences in relative substrate resistance and in the ratio of driving forces to resisting forces three alluvial channel segments and two bedrock channel segments along a 72 km2 drainage and a 4 km2 drainage facilitated comparison across different substrates and hydraulic environments as substrate resistance increased or the ratio of hydraulic driving forces and boundary resisting forces decreased channel gradient increased the ratio of step height to length normalized by gradient decreased and pool dimensions normalized by gradient decreased these changes in bedform geometry suggest that as the ratio of driving to resisting forces increases proportionally more flow energy is dissipated within the flow as turbulence and shearing as in the alluvial reaches rather than being applied to erosion of the channel boundaries

burial dissolution of micrite in middle east carbonate reservoirs jurassic cretaceous keys for recognition and timing abstract this paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the middle east three microporous reservoirs were selected the cenomanian mishrif formation in iraq the kimmeridgian tithonian arab d and the barremian kharaib formations both in abu dhabi staining cathodoluminescence and scanning electron microscopy were used for petrographic observations in the selected reservoirs three typical micrite microtextures are distinguished micro rhombic compact anhedral and rounded the chronology of diagenesis based on petrographic observations shows that mineralogical stabilisation through high magnesian calcite replacement by low magnesian calcite led to the formation of micro rhombic micrites non reservoir facies such as seals present compact anhedral micrites correlation between these micrites and high aluminium content suggests alumino silicate minerals clays as a catalyst for pressure dissolution and thus for reservoir degradation through cementation micrites from the upper part of oil reservoirs are dissolved during burial diagenesis dissolution is responsible for rounded shape and for reduction of crystal size it also implies pore diameter and pore throat size enlargement comparison of porosity and permeability with scanning electron microscope data confirms that rounded micrites are better reservoirs than micro rhombic micrites

characteristics of log and clast bed steps in step pool streams of northwestern montana usa the role of log and clast steps was studied in 53 step pool reaches in 32 streams in northwestern montana in each reach 20 consecutive steps were sampled drainage areas ranged from 0 5 to 11 5 km2 mean step spacing was found to be proportional to channel gradient width of the bankfull channel and drainage area mean step height was proportional to the width of the channel no significant difference exists between clast and log steps with regard to step spacing and step height this suggested either that 1 woody debris is mobile in even the smallest channels in the study area or 2 channel flows arrange clast steps around immobile woody debris calculations of theoretical entrainment and transport of sediment suggest that the coarse clasts forming steps are capable of being entrained at bankfull discharge in most channels the presence of immobile log steps composed of living roots in the upstream reaches an increase in the percentage of log steps with gradient and a decrease in this percentage with the width of the bankfull channel and drainage area imply that woody debris is mobile primarily in the downstream reaches calculations of flow resistance suggest that most steps serve to maximize flow resistance these results support the second hypothesis that the height and spacing of clast steps are adjusted around immobile log steps to maintain consistent step characteristics

velocity prediction in high gradient channels summary in 15 mountain stream reaches containing instream wood we characterized velocity and flow resistance at bankfull through low flows these data were 1 used to assess the accuracy of previously published velocity prediction techniques for high gradient channels and 2 were combined with field data from other studies to develop general methodologies for predicting velocity and flow resistance in alluvial and mixed alluvial bedrock channels both with and without step forming instream wood velocity and flow resistance were poorly predicted by variables characterizing grain size and relative grain submergence conversely methods based on detrended standard deviation of bed elevations rz and relative bedform submergence h rz explained up to 84 of the variance in the measured flow resistance coefficients and 97 of the variance in dimensionless velocity with an average velocity of 0 44 m s for the collected measurements velocity was predicted with rms root mean square error as low as 0 071 m s 16 of average when discharge and bedform geometry is known and 0 10 m s 23 when only bedform geometry is known additionally an empirical relationship indicates v u h r z supporting previouslypublished laboratory findings using a field based dataset in complex high gradient channels interactions between instream wood and clasts result in substantially enhanced step heights and flow resistance this compound effect defies description by grain size and relative grain submergence however rz and h rz quantify variability due to both clasts in combination with wood and clasts alone providing relatively accurate predictions for the tested dataset and indicating substantial predictive capabilities in channels where bedforms are the primary source of flow resistance published by elsevier b v

development of channel organization and roughness following sediment pulses in single thread gravel bed rivers large episodic inputs of coarse sediment sediment pulses in forested mountain streams may result in changes in the size and arrangement of bed forms and in channel roughness a conceptual model of channel organization delineates trajectories of response to sediment pulses for many types of gravel bed channels channels exhibited self organizing behavior to various degrees based on channel gradient presence of large in channel wood or other forcing elements the size of the sediment pulse and the number of bed mobilizing flows since disturbance typical channel changes following a sediment pulse were initial decreases in water depth in variability of bed elevations and in the regularity of bed form spacing trajectories of change subsequently showed increased average water depth more variable and complex bed topography and increased uniformity of bed form spacing bed form spacing in streams with abundant forcing elements developed at a shorter spatial scale two to five channel widths than in streams without such forcing mechanisms five to 10 channel widths channel roughness increased as bed forms developed

cerium sequestration and accumulation in fractured crystalline bedrock the role of mn fe hydr oxides and clay minerals abstract this study focuses on the mechanisms of ce sequestration and accumulation in the fracture network of the upper kilometer of the granitoid bedrock of the baltic shield in southeast sweden laxemar area sweden the material includes 81 specimens of bulk secondary mineral precipitates fracture coatings collected on fracture walls identified in 17 drill cores and 66 groundwater samples collected from 21 deep boreholes with equipment designed for retrieval of representative groundwater at controlled depths the concentrations of ce in the fracture coatings although varying considerably 10 90th percentiles 67 438 mg kg 1 were frequently higher than those of the wall rock 10 90th percentiles 70 118 mg kg 1 linear combination fitting analysis of ce l iii edge x ray absorption near edge structure xanes spectra obtained for 19 fracture coatings with relatively high ce concentrations 145 mg kg 1 and a wide range of ce anomaly values revealed that ce iv occurs frequently in the upper 10 m of the fracture network ce iv ce total 0 06 1 00 in 8 out of 11 specimens and is mainly associated with mn oxides modeled as ce oxidatively scavenged by birnessite these features are in line with the strong oxidative and sorptive capacities of mn oxide as demonstrated by previous studies and abundant todorokite and birnessite like mn oxides identified in 3 out of 4 specimens analyzed by mn k edge x ray absorption spectroscopy xas in the upper parts of the fracture network down to 5 m for a specimen with very high ce concentration 1430 mg kg 1 and nasc normalized ce anomaly 3 63 the analysis of ce xanes and mn xas data revealed i a predominance of ce oxide in addition to ce scavenged by mn oxide and ii a large fraction of poorly crystalline hexagonal birnessite and aqueous mn 2 suggesting a recent or on going oxidation of mn 2 in this fracture in addition the ce oxide precipitates on this fracture observed by in situ sem eds contained considerable amounts of mn these spectroscopic and microscopic features led us to suggest that the remarkable accumulation of ce iv in this fracture is a result of repeated formation and dissolution of mn oxides that is formation of mn oxide followed by oxidative scavenging of ce as ce oxide nanoparticles which largely remained during the subsequent reductive dissolution of the mn oxides in addition the xanes data indicate that goethite has the capability to oxidize ce at near neutral ph under our experimental conditions goethite reacted with 0 001 m ce for 48 h in a glove box with o 2

geochemical modelling of the weathering zone of the mina fe u deposit spain a natural analogue for nuclear spent fuel alteration and stability processes in radwaste disposal abstract the mina fe u deposit salamanca spain has been studied in the context of enresa s programme for u mine sites restoration and also as a natural analogue for processes in high level nuclear waste hlnw geological disposal the investigations encompassed an array of geoscience disciplines such as structural geology mineralogy hydrogeology and elemental and isotopic geochemistry and hydrogeochemistry of the site based on the obtained results a conceptual mineralogical and geochemical model was performed integrating the main geochemical processes occurring at the site the interaction between oxidised and slightly acidic water with pyrite pitchblende calcite and dolomite as essential minerals of the u fracture filling mineralisation and hydroxyapatite from the host rock as the main source of p this conceptual model has been tested in a systematic numerical model which includes the main kinetic pyrite and pitchblende dissolution and equilibrium processes carbonate mineral dissolution and goethite schoepite and autunite secondary precipitation the results obtained from the reactive transport model satisfactorily agree with the conceptual model previously established the assumption of the precipitation of coffinite as a secondary mineral in the system cannot be correctly evaluated due to the lack of hydrochemical data from the reducing zone of the site and valid thermodynamic and kinetic data for this hydrated u iv silicate this precipitation can also be hampered by the probable existence of dissolved u iv organic matter and or uranyl carbonate complexes which are thermodynamically stable under the alkaline and reducing conditions that prevail in the reducing zone of the system finally the intense downwards oxic and acidic alteration in the upper part of the system is of no relevance for the performance assessment of a hlnw disposal however the acidic and oxidised conditions are quickly buffered to neutral alkaline and reducing at very shallow depths of relevance for the performance assessment of a hlnw repository even in a natural or artificially perturbed geological environment as mina fe

influence of large woody debris on channel morphology and dynamics in steep boulder rich mountain streams western cascades oregon this study used 20 year records of stream channel change and wood to test hypotheses about the long term influence of large woody debris lwd on channel morphology channel stability and sediment dynamics in a steep boulder rich mountain stream we compared two nearly adjacent reaches of third order mack creek over the period 1978 1997 after virtually all wood was removed from the channel of the lower reach in 1964 we assessed the long term legacy of wood removal using repeated crosssection surveys streamflow data lwd inventory data and detailed mapping and longitudinal profile surveys at each of 11 cross sections in the upper reach and 19 in the lower reach we calculated areas of scour and fill in response to the two largest floods in the record we used quasi likelihood logistic regression models to test the proportion of each reach that experienced change between consecutive surveys over the entire record 1978 1997 as a function of flood return periods the longitudinal profile of the site without lwd was more variable than the reach with lwd at the finest scale f1 m due to a greater frequency of boulder steps but the reach with lwd was more variable at the channel unit scale lwd created steps 1 to 2 5 m high in the wood rich reach accounted for nearly 30 of the total channel fall and created low gradient upstream channel segments one to three channel widths long as a result both reaches have the same average slope about 9 but nearly three times as much of the channel in the wood rich reach had a slope of v5 as in the reach without wood 20 4 of total channel length vs 7 5 of channel length the reach with abundant lwd was less responsive to moderate streamflow events return period f5 years but it responded similarly to peak flows with a return period of about 10 to 25 years although the average magnitude of crosssection changes was the same during the largest flood in the record 25 year return period the reach without lwd experienced scour and coarsening of the bed surface whereas the reach with lwd experienced aggradation upstream of lwd features mack creek may be representative of many steep mountain streams in which channel structure is strongly influenced by nonfluvial processes a legacy of large boulders from glacial or mass movement processes and a legacy of dead wood from ecological processes sediment limited mountain streams with large boulders when deprived of lwd appear to exhibit less morphological variation at the channel unit scale to store less sediment and to release it more readily than those with lwd d 2002 elsevier science b v all rights reserved

large woody debris and flow resistance in step pool channels cascade range washington abstract total flow resistance measured as darcy weisbach f in 20 step pool channels with large woody debris lwd in washington ranged from 5 to 380 during summer low flows step risers in the study streams consist of either 1 large and relatively immobile woody debris bedrock or roots that form fixed or forced steps or 2 smaller and relatively mobile wood or clasts or a mixture of both arranged across the channel by the stream flow resistance in step pool channels may be partitioned into grain form and spill resistance grain resistance is calculated as a function of particle size and form resistance is calculated as large woody debris drag combined grain and form resistance account for less than 10 of the total flow resistance we initially assumed that the substantial remaining portion is spill resistance attributable to steps however measured step characteristics could not explain between reach variations in flow resistance this suggests that other factors may be significant the coefficient of variation of the hydraulic radius explained 43 of the variation in friction factors between streams for example large woody debris generates form resistance on step treads and spill resistance at step risers because the form resistance of step pool channels is relatively minor compared to spill resistance and because wood in steps accentuates spill resistance by increasing step height we suggest that wood in step risers influences channel hydraulics more than wood elsewhere in the channel hence the distribution and function not just abundance of large woody debris is critical in steep step pool channels

characterization and origin of permeability porosity heterogeneity in shallow marine carbonates from core scale to 3d reservoir dimension middle jurassic paris basin france abstract nuclear magnetic resonance nmr stable isotope geochemistry of micro sampled cores nmr well logs and 3d modeling are used to investigate the carbonate permeability porosity heterogeneity along 230 m thick limestones of the paris basin despite the global low porosity and permeability of the limestones two aquifers units with porosity greater than 15 were identified these two aquifers are very different in terms of pore through radii and nmr signal the first one a1 aquifer 1 is a 7 m thick mudstone unit dominated by extended microporosity with pore throat radii of 0 25 m to 0 3 m the second one a2 aquifer 2 is a 15 m thick oolitic grainstone units showing macropores reaching 100 m and pore throat radii of 32 m from core descriptions and wireline logs on 26 wells a 3d static geological model is build the fine tuning of permeability calculations from nmr logs realized along 12 of the wells allows porosity and permeability heterogeneity to be distributed within a 3d model at the reservoir scale area of about 2000 km2 which match the flow behavior illustrated by well tests associated with early meteoric calcite cements and poorly developed burial blocky calcite cements the porous and permeable intervals may be predicted in two stratigraphic and diagenetic considerations firstly the syn sedimentary meteoric dissolution or neomorphism of the initial high magnesium calcite and aragonite particles or clasts into low magnesium calcite particles or cements prevented most mechanical and chemical compaction during the first steps of burial secondly the regional stratigraphic architecture reveals the presence of local permeability barriers which prevented early cretaceous lateral meteoric water circulation and the associated burial calcite cementation

step pool and cascade morphology mosquito creek british columbia a test of four analytical techniques the identification and geometric definition of individual cascade and step pool bedforms are investigated in a steep coarse grained mountain stream mosquito creek by testing four analytical techniques visual identification zero crossing bedform differencing and power spectral analysis the test is the first use of these techniques in a headwater stream and the analysis of two bed profiles showed that visual identification was able to i identify ii determine the geometry of and iii classify the type of individual bedforms better than the other methods the other techniques were not able to differentiate step pools from cascades and the large range of grain sizes and bedform heights hampered their ability to consistently identify stepped bedforms the step pool pronounced channel spanning steps that alternate with channel spanning pools and cascade multi tiered partially channel spanning structures morphology in mosquito creek has formed in the last 20 years as fluvial action has restructured its previously engineered revetment lined planar bed the channel bed exhibits a morphologic regularity that power spectral analysis captured as periodic fluctuations in the bed profiles with mean wavelengths slightly greater than those identified by the other methods further the active reorganization of revetment has formed stepped structures with geometries similar i e height to wavelength ratios to stepped features found in natural mountain streams channel slope partially controlled bedform geometry wavelength and height and bedform height weakly controlled individual step spacing but there was no relation between wavelength and grain size d 90

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

hydrogeochemical evolution and c isotope study of groundwaters from mina fe u deposit salamanca spain implications for processes in radwaste disposal within the framework of the enresa natural analogue programme the u ore deposit of mina fe western spain has been studied as a natural analogue of radioactive spent fuel behaviour after burial in spite of being an extremely perturbed geological environment due to mining activities the main objectives of this project are to determine the hydrogeochemical evolution of the system and identify the main water rock interaction processes that control the physicochemical variables ph and eh of groundwaters including the role played by the organic matter the hydrogeochemical results from 3 consecutive groundwater sampling campaigns separated by a phase of restoration works indicate that the groundwater of the site shows an evolutionary trend from ca so42 acid oxidising waters towards alkaline reducing waters though their evolution with respect to the alkaline and alkaline earth elements is not clear the so42 acid waters are the result of the oxidation processes affecting the primary sulphide rich u mineralisation while the alkaline reducing waters result from the buffer capacity of carbonates from fracture filling materials as the 13c values of dic demonstrate the reducing character of these waters mainly results from the microbiologically mediated partial oxidation of the abundant organic matter existing in the clayey walls of the major faults at the site since other dissolved inorganic redox pairs are insufficient to explain the in situ measured redox potential thus the high content in doc of these waters is also explained since the soil at the site is poorly developed doc dic or both can be responsible for the high u concentration measured in these groundwaters in spite of its reducing character the restoration works performed in the exploitation quarry have also restored the aforementioned evolutionary trend which was the normal evolution of groundwater in the site before mining finally the results are discussed in terms of the geochemical processes expected in a radioactive disposal site after closure

trace metal distribution and mobility in drill cuttings and produced waters from marcellus shale gas extraction uranium arsenic barium development of unconventional shale gas wells can generate significant quantities of drilling waste including trace metal rich black shale from the lateral portion of the drillhole we carried out sequential extractions on 15 samples of dry drilled cuttings and core material from the gas producing middle devonian marcellus shale and surrounding units to identify the host phases and evaluate the mobility of selected trace elements during cuttings disposal maximum whole rock concentrations of uranium u arsenic as and barium ba were 47 90 and 3333 mg kg 1 respectively sequential chemical extractions suggest that although silicate minerals are the primary host for u as much as 20 can be present in carbonate minerals up to 74 of the ba in shale was extracted from exchangeable sites in the shale while as is primarily associated with organic matter and sulfide minerals that could be mobilized by oxidation for comparison u and as concentrations were also measured in 43 produced water samples returned from marcellus shale gas wells low u concentrations in produced water 0 084 3 26 g l 1 are consistent with low oxygen conditions in the wellbore in which u would be in its reduced immobile form arsenic was below detection in all produced water samples which is also consistent with reducing conditions in the wellbore minimizing oxidation of as bearing sulfide minerals geochemical modeling to determine mobility under surface storage and disposal conditions indicates that oxidation and or dissolution of u bearing minerals in drill cuttings would likely be followed by immobilization of u in secondary minerals such as schoepite uranophane and soddyite or uraninite as conditions become more reducing oxidative dissolution of arsenic containing sulfides could release soluble as in arsenate form under oxic acidic conditions the degree to which the as is subsequently immobilized depends on the redox conditions along the landfill flow path the results suggest that proper management of drill cuttings can minimize mobilization of these metals by monitoring and controlling eh ph and dissolved constituents in landfill leachates

step pool evolution in the rio cordon northeastern italy the principle that formative events punctuated by periods of evolution recovery or temporary periods of steady state conditions control the development of the step pool morphology has been applied to the evolution of the rio cordon stream bed the rio cordon is a small catchment 5 km2 within the dolomites wherein hydraulic parameters of floods and the coarse bedload are recorded detailed field surveys of the step pool structures carried out before and after the september 1994 and october 1998 floods have served to illustrate the control on step pool changes by these floods floods were grouped into two categories the first includes ordinary events which are characterized by peak discharges with a return time of one to five years 1 8 5 15 m3 s 1 and by an hourly bedload rate not exceeding 20 m3 h 1 the second refers to exceptional events with a return time of 30 50 years a flood of this latter type occurred on 14 september 1994 with a peak discharge of 10 4 m3 s 1 and average hourly bedload rate of 324 m3 h 1 step pool features were characterized primarily by a steepness parameter c h ls s the evolution of the steepness parameter was measured in the field from 1992 to 1998 the results indicate that maximum resistance conditions are gradually reached at the end of a series of ordinary flood events during this period bed armouring dominate the sediment transport response however following an extraordinary flood and unlimited sediment supply conditions the steepness factor can suddenly decrease as a result of sediment trapped in the pools and a lengthening of step spacing the analogy of step spacing with antidune wavelength and the main destruction and transformation mechanism of the steps are also discussed copyright 2001 john wiley sons ltd

frequency and magnitude of bedload transport in a mountain river magnitude and frequency of bedload transport was examined in the lainbach river bavaria using magnetically tagged particles during the study 18 small to moderate events and one major event occurred about 1 per cent of the flow period exceeded the entrainment threshold and at least once every year about 50 per cent of the tagged particles were mobile themajor event which occurred during the study period resulted in the deposition of a thick layer of sediment in parts of the channel and passive burial of most of the recovered particles the step pool pattern which characterized the study site prior to the large event was obliterated however the channel recovered quickly and has returned to a new step pool pattern the event changed the boundary conditions by increasing the availability of loose sediment and creating higher river bed slopes in reaches between breached check dams as a result movement of individual particles measured for events of both the same magnitude and duration occurring before and after the large event yielded different values for events which occurred after the large event the range and the mean distance of movement were about ten times higher

channel morphology gradient profiles and bed stresses during flood in a step pool channel abstract the stability of a step pool channel is investigated on the basis of measurements during a median annual flood the study reaches range in gradient from 5 to 10 and the flow was 3 m3 s 1 in an 8 m wide channel the estimated shear stress based on the channel gradient was 102 pa which implies that the channel ought to move half meter class boulders calculations of the shear stress using flow resistance equations yield values of about 30 pa these figures imply that the largest boulders in the channel could be moved in fact the channel is stable the effective stress remains well below the threshold to move the large structure forming boulders the steps are stable structures that result from interlocking of the larger stones they are semi randomly located depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks the high variation in step pool wavelength step height residual pool depth and pool length is indicative of the random nature of step location and structure along a reach there is no convincing evidence that special conditions govern their formation

u series in fe u rich fracture fillings from the oxidised cap of the mina fe uranium deposit spain implications for processes in a radwaste repository abstract within the framework of the enresa spain natural analogue programme the u ore deposit of mina fe is being studied as a natural analogue of radioactive spent fuel behaviour in this context the knowledge of the role played by fracture minerals as scavengers of certain analogue elements mainly u and the establishment of the time scale of the rock water interaction processes controlling the uptakes or losses of u in the system are two relevant objectives fracture infill materials from the site have first been mineralogically characterised then the upper part of the u series determined in both bulk samples and u rich leachates obtained by sequential leaching uranium series of the bulk samples indicate that most of the fractures remained as closed systems in the last 1 6 ma while in other fractures water rock interaction processes affecting the upper part of the u series have been identified these processes indicate recent or rapid u accumulation or losses 102 ka or 234u 230th recoil gain the apparently random distribution in depth of fractures where these processes occurred corroborates the different hydraulic behaviour of fractures as a result of their varying degree of sealing uranium concentrations and 234u 238u ars of the leachates obtained with morgan s solution and 6n hcl indicate that minerals dissolved with these reagents u iv vi oxyhydroxides and goethite clays respectively are responsible for the retention of almost all of the u in the bulk samples furthermore the 234u lost by the u minerals dissolved with morgan s solution seems to be fixed onto goethite clay mixtures the intersticial water being the vehicle for the isotopic transfer which in turn is a recent or recent past process

bed morphology and generation of step pool channels flume experiments have been carried out to study the formation processes and the bed morphology of step pool channels from the experiments different step types and step configurations could be distinguished depending on the stream power these step types can be seen as an image of the generation mechanisms of step pool systems these results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution in contrast to a variety of other studies antidunes did not contribute to the generation of the step structures however the data of the presented study fits well into the region of antidune formation proposed by kennedy for sand bed rivers this observation points out that step pool field data located in the kennedy region do not inevitably prove that antidunes played a role in step development it is rather proposed that in kennedy s region of antidune formation there exist hydraulic conditions where the flow resistance is maximized it is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height independently of whether these are antidunes in sand and gravel bed rivers or step pool units in boulder bed streams the considerations of the kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self stabilization by modifying the step pool structure towards a geometry that provides maximum flow resistance and maximum bed stability copyright 2008 john wiley sons ltd

morphological organization of a steep tropical headwater stream the aspect of channel bifurcation abstract channel characterization and instream wood surveys demonstrate the spatial complexity of an ungauged tropical headwater stream hws in the tilaran mountains of costa rica throughout the stream course we characterized 29 individual reaches based on their morphological organization and instream wood occurrences bifurcated reaches were the most numerous and consistently distributed they also had the highest values of wood abundance and loading and contained the majority of unattached instream wood we selected two bifurcated reaches for ultrahigh resolution terrestrial laser scanning tls surveys orthographic tls produced millimeter resolution point cloud data sets from which 2 cm planimetric resolution digital elevation models dems of the channel surface were derived for both reaches cross sectional surveys of these dems using the river bathymetry toolkit rbt produced hydraulic geometries at two flow levels high and low for every meter length of channel reach we input these geometries into a one dimensional analytical model to demonstrate the downstream evolution of mean stream power through varying morphology elements mean stream power variation displays a regular pattern through the vertical modulation of step pool sequences but this regular pattern is lost through channel widening and bifurcation reach geometric data indicates that increasing width to depth ratio is the most notable feature of channel bifurcation and corresponds to a reduction in slope and roughness variation this primary understanding of channel dynamics denotes the complex organization of a tropical hws and promotes further research into tropical hws systems

dissipative analogies of step pool features from rills to mountain streams abstract in this paper the dissipative similarity of step pool units at different spatial scales ranging from rills to streams is analyzed this investigation benefits from the latest theoretical advances in open channel flow resistance high resolution topography from close range photogrammetry applied to rill erosion and the availability of published data from literature on step pool streams at first the integration of a power velocity distribution allowed to obtain a theoretically based expression of darcy weisbach friction factor in which function and exponent of the velocity profile are included then this theoretically deduced flow resistance relationship is calibrated and tested by flow measurements carried out in rill reaches where the step pool units occurred in particular the proposed function is calibrated using the measurements corresponding to 88 rill reaches shaped on a plot having a slope equal to 14 22 and 24 and the calibrated equation is also positively tested with measurements carried out in 48 rill reaches shaped on a 26 sloping plot for the rill flow the developed analysis stated that the friction factor is characterized by estimate errors which are less than or equal to 15 for 86 of cases and less than or equal to 10 for 70 of cases using measurements of flow velocity water depth width and bed slope measurements carried out in 109 reaches of step pool streams this investigation demonstrates that the theoretical flow resistance equation can be applicable to step pool streams carrying out a specific calibration of the function the comparison between the darcy weisbach friction factor values measured in streams and step pool rills demonstrates that in the stream features the friction factor values are on average higher than those related to rills with step pool sequences in conclusion the comparison between rills and streams with step pool units highlights that the same theoretical flow resistance equation can be applied even if a scale effect between the two features is detected

effect of groundwater and sea weathering cycles on the strength of chalk rock from unstable coastal cliffs of nw france abstract the aim of this paper is to evaluate the role of groundwater and sea weathering on the strength of the chalk rocks exposed on the coastline of the english channel in normandy nw france we present a study of the rock strength variations of three representative chalk units lewes chalk seaford chalk and newhaven chalk exposed at various locations on the coastal chalk cliffs the combination of ucs tests and sem observations have been used 1 on dry natural chalk samples 2 on chalk samples at various moisture contents 3 on dry chalk samples submitted to a 10 day cycle of alternating wetting and drying by distilled water and by sea water dry chalk samples show low ucs strength 3 46 4 mpa indicative of very weak rocks when chalk samples are submitted to progressive water wetting they present a decrease of ucs strength and young s modulus of 40 to 50 this behaviour begins at low values of water content within the chalk i e for a degree of water saturation ranging between 10 and 17 when chalk samples are submitted to an artificial weathering cycle with distilled water a decrease in strength is observed whereas the young s modulus increases sem observations indicate the occurrence of microcracks and particle aggregates in the sample when chalk samples are submitted to an artificial weathering with sea water the decrease of ucs strength and young s modulus achieves a minimum sem observations indicate salt crystals within the chalk on the coastal cliffs of nw france weathering processes depend both on chalk lithology which show a range of sensitivity to weathering and on the location of the chalk in the coastal area processes allied to the degree of weathering e g salt crystallisation or fresh water disaggregation differ in the chalk massif on the cliff face and on the shore platform

velocity and flow resistance in step pool streams flow resistance in steep streams with step pool morphology consists of form drag as well as skin friction it therefore alters rapidly as discharge increases and is not expected to show constant manning s n or simple log law behaviour but there is no widely accepted alternative representation we report measurements of at a station hydraulic geometry n and 1 f 0 5 for a range of discharges at each of six field sites and in flume experiments loosely froude scaled to the prototypes velocity and resistance vary strongly with discharge in all cases the flume data extend to formative flows and when rescaled extend the range of the field data but plot in line with them this suggests a single resistance law can describe the full range from trickle to formative flow equations based on the log law with ks step d84 perform unexpectedly well especially when allowance is made for flow blocking and or the ks d84 multiplier is optimised separately for each site the latter suggests step d84 is not a good summary representation of effective roughness but none of a variety of alternative grain size and microtopographic measures performed much better

origin of step pool systems in mountain streams mountain streams very often present a step pool aspect antidune formation alone dispersion and sorting theory or velocity reversal as advocated for the origin of riffle pool sequences do not explain how steps and pools form experiments have been conducted to clarify their origin it was found that the step pool aspect appears after high flows had deformed the bed during the presented tests a self armoring process occurred which resulted in a stabilization of the bed when coarsening of the top layer was little classic antidunes resulted an important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance the substantial increase in resistance was an essential factor in bed stabilization

fluvial geomorphology of a boulder bed debris flow dominated channel in an active tectonic environment abstract numerous streams with a drainage area of a few square kilometers draining the south flank of the santa ynez range in southern california are characterized by steep boulder bed debris flow dominated channels rattlesnake creek is a good example approximately 90 of the stored sediment in the stream valley is stable debris flow deposits found in debris flow terraces and levees adjacent to the channel the remaining 10 of stored sediment resides in the active channel where the channel morphology is dominated by large boulders 1 to 4 m in diameter derived from debris flows or rock fall that along with bedrock outcrops form rock or boulder transverse ribs with frequent step pools the dominant channel pool morphology is the step pool which accounts for most of the pools and approximately 30 of relief drop in elevation through the study reaches less common pool types identified in the study reaches include forced pools that result from convergence of flow with scour between lbres large boulder roughness elements and bedrock outcrop along the channel at near bankfull discharge the lbres and boulders in step pools armor the channel and are primary controls on channel morphology and gradient rock strength is a secondary control on channel morphology location and abundance of pools we hypothesize that step pools when inundated with sand and fine gravel from a disturbance such as wildfire during a flow event fill near bankfull stage and scour on falling stage below bankfull when unit stream power is a maximum over the step thus as with forced pools step pool maintenance depends on a specific pattern of discharge and process the process of scour during falling stage reestablishes step pools during moderate flows following disturbance

the self organization of step pools in mountain streams abstract spontaneous autogenic self organization has been described in numerous geomorphic systems but it has not been investigated in detail with respect to coarse bedforms in general or step pools in particular in this paper we review the spatial organization of step pool systems and present example evidence of step pool development as an autogenic self organization process we then outline the mathematical language for defining spatially divergent self organization and test these ideas using two unique field examples from oregon andrews experimental forest and california baxter creek where step pools developed from planar beds in artificially manipulated channels results show that step pool development is consistent with a spatially divergent self organization phenomenon entropy increases as initially undifferentiated planar channels diverge into steps and pools then declines when a series of steps and pools of consistent size and spacing is established signifying stability in the system the self organization process is accompanied by increasing flow resistance and decreasing slope through increasing the vertical sinuosity of the step pool profile and creation of low or negative gradient pool areas suggesting a minimization of stream power the self adjustment of the step pool bed profile over time represents another manifestation of a general process that results in rhythmic patterns on the surface of earth

the morphologic structure of step pools in mountain streams abstract step pool sequences in streams in the santa monica mountains of southern california impart a characteristic morphologic structure that varies with slope but despite the strong association with slope that suggests a direct process control analysis of 464 step pools in 13 study reaches reveals the relationship to reflect correlation but not necessarily causality the direct control on step height is particle size whereas discharge is the probable causal process determining step wavelength because both particle size and discharge generally change with channel slope downstream a slope variation with step wavelength and height is the apparent indirect geometric result a conceptual model of process form linkages is proposed based on these results whereby the step pool morphology is produced by positive relationships between wavelength and discharge and between height and particle size the model offers new insights for explaining the step pool morphology and it is potentially useful in channeling future efforts toward the appropriate scales and variables

the uranium ore from mina fe salamanca spain as a natural analogue of processes in a spent fuel repository abstract in the frame of the enresa natural analogue programme the uranium ore from the mina fe salamanca spain has been studied as a natural analogue of radioactive spent fuel behaviour this uranium mine is hosted in highly fractured schistose rocks a geological setting that has not been envisaged in the spanish options for radioactive waste burial however some analogies with the processes that might be involved in the evolution of these geological repositories suggested this investigation the pitchblende pyrite carbonate paragenesis has been studied in situ as natural analogue of the nuclear spent fuel behaviour under extremely oxidative dissolution conditions similarly secondary fe oxyhydroxides and clay minerals have also been considered as relevant analogue materials for the retention of uranium and other analogous trace metals a multidisciplinary characterisation of the site has been performed in order to study these processes though the intense mining activities in the site hindered precise determination of the original hydrogeological and hydrochemical features of the investigated zone boa fault zone the mineralogy and geochemistry of fracture fillings mineralisation and associated clayey materials have allowed the geochemical evolution of the system to be established three geochemical zones have been clearly differentiated i the oxidised zone from the surface to approximately 20 m depth ii the redox transition zone from 20 to 50 m depth and iii the reduced zone located below the transition zone the oxidised zone is characterised by the presence of the typical mineral association resulting from the strong acid conditions caused by the total oxidation of pyrite and other sulphides the total oxidation dissolution and leaching of u iv as uranyl sulphate aqueous complexes prevailed in this oxidised zone the redox transition zone is characterised by the coexistence of the primary uranium paragenesis oxidised minerals as well as numerous secondary solid phases as a result of the physico chemical changes in the environment the optimal physico chemical conditions for the coffinitisation of pitchblende and the co precipitation of fe iii u vi took place in this zone in the reduced zone where the primary uranium paragenesis is present we currently find the necessary physico chemical conditions to stabilise pitchblende pyrite and carbonates the physico chemical conditions of the oxidised zone are not relevant to disposal conditions in the transition zone two main geochemical processes take place i the coffinitisation of pitchblende which may be an important process for the stability of spent fuel in reducing conditions and ii the co precipitation of the fe iii and u vi as oxyhydroxides another relevant mechanism for the retention of uranium the physico chemical conditions that prevail below 50 m depth should be sufficient to stabilise a spent nuclear fuel repository in the same way as they have been able to preserve the 34 ma old uranium deposit of the mina fe

distribution of trace elements in fracture fillings from the mina fe uranium deposit spain by sequential leaching implications for the retention processes abstract sequential leaching methods have been used to determine the mineralogical distribution of some trace elements for environmental purposes such as radiological contamination of soils and sediments bioavailability studies and natural analogues of deep geological radwaste disposals in this context a 7 step sequential leaching protocol is applied to fe iii u vi rich fracture filling materials from the oxidised zone of the mina fe u deposit to identify and evaluate the main sinks of natural nuclides and other analogue trace elements since it is crucial in the performance assessment of a nuclear waste repository after a careful characterisation of the samples the analytical data from each leaching step were statistically analysed and then interpreted in light of the mineralogical and geochemical features of the samples precise knowledge of the mineralogical distribution of trace elements by sequential leaching methods is quite complex mainly due to cross contamination throughout the different steps of the experiments thus the results obtained suggest that u is retained as u minerals mainly oxides closely associated with crystalline fe oxyhydroxides though ce and la also form independent compounds such as ce oxides and la nd phosphates they are mainly retained by the amorphous mn oxyhydroxides however the crystalline mn oxyhydroxides are the main sink for ni and crystalline fe oxyhydroxides mainly retain p

characteristic dimensions of the step pool bed configuration an experimental study 1 the dominant dimension of the step pool bed form is its spacing which is determined by the locations at which large step forming grains are deposited we conducted laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps direct observation of step development indicates that deposition of step forming grains was predominantly associated with bed roughness impeding transport or with earlier flow and transport conditions not contemporaneous with step development neither mechanism is likely to occur at regular locations on the bed measured step spacing was found to follow a poisson distribution supporting the conclusion that steps do not develop in preferred or regular locations this contrasts with previous arguments that regular step spacing develops in response to deposition forced at regular locations beneath water surface waves we observe neither the regular distribution nor the associated depositional mechanism although the observed step distribution is well defined with a strong mode that could be interpreted as indicating regular spacing

use of the block punch test to predict the compressive and tensile strengths of rocks this study aimed to explore the applicability of the block punch test in estimating uniaxial compressive and brazilian tensile strengths of granite schist and sandstone from india it was experimentally shown that when predicting uniaxial compressive strength of rocks the block punch test is as useful as the point load test efficacy of the existing empirical equations for predicting uniaxial compressive strength by the block punch index was also evaluated prediction of brazilian tensile strength by block punch index appears to be more precise than by the point load strength performances of the existing strength anisotropy transformation factors in the literature in order to normalize the block punch index in the direction perpendicular to foliations were assessed based on the results of the tests performed on the schistose rocks

flow regimes bed morphology and flow resistance in self formed step pool channels received 3 july 2008 revised 28 december 2008 accepted 13 january 2009 published 25 april 2009 1 we used a mobile bed flume with scaled grain size distribution channel geometry and flow to examine morphology and hydraulics of stepped channels we hypothesized that 1 step geometry and flow resistance differs significantly as a function of the range of grain sizes present 2 a transition from nappe to skimming flow occurs in stepped channels with mobile beds for conditions similar to stepped spillways and 3 the partitioning of flow resistance changes significantly when flow passes from nappe to skimming conditions results support each of these hypotheses and help to illuminate the complexity of v q relationships in stepped channels in which a dramatic decrease in flow resistance and increase in velocity accompany the transition from nappe to skimming flow near step forming events therefore a single flow resistance equation applicable to both ordinary and large floods may not be ideal in stepped channels nonetheless models based on dimensionless velocity and unit discharge appear more robust compared to those based on the darcy weisbach friction factor

alluvial architecture in headwater streams with special emphasis on step pool topography alluvial mountain streams exhibit a range of channel forms pool riffle plane bed step pool and cascades previous work suggested that these forms exist within discrete and progressively steeper slope classes measurements conducted at over 100 sites in west central and central idaho confirm that slope steepens progressively as one moves from pool riffle to plane bed to step pool and finally to cascades median slope for pool riffle topography is 0 0060 for plane beds 0 013 for step pools 0 044 and for cascades 0 068 there is substantial overlap in the slopes associated with these channel forms pool riffle topography was found at slopes between 0 0010 and 0 015 plane beds between 0 0010 and 0 035 step pools between 0 015 and 0 134 and cascades between 0 050 and 0 12 step pools are particularly striking features in headwater streams they are characterized by alternating steep and gentle channel segments the steep segments step risers are transverse accumulations of boulder and cobbles while the gentle segments pools contain finer material step wavelength is best correlated to step height which is in turn best correlated to the median particle size found on step risers this result differs from past studies that have reported channel slope to be the dominant control on step wavelength the presumed geometry and froude number associated with the features under formative conditions are consistent with the existence field for antidunes and by extension with the hypothesis that step pools are formed by antidunes copyright 2000 john wiley sons ltd

formation of riffle pool sequences field evidence for an autogenetic process abstract riffle pool sequences are the characteristic reach scale bedforms of mixed and gravel bedded rivers while most research has been directed to the quantitative description of these sequences and to their maintenance comparatively little attention has been given to the processes responsible for their formation in this paper field results from quasi continuous velocity records obtained with an array of electro magnetic current meters are analysed with respect to a model of macro scale structure in the turbulent velocity field of natural channels this model has been suggested as the most plausible basis for riffle pool formation the results demonstrate the occurrence of a variety of coherent flow structures some of which scale on channel dimensions as predicted by the model but which also suggest that some revision of the association between flow structure and riffle pool formation is necessary it is suggested that riffle pool units are initiated with the generation of roller eddies upstream and downstream from a major flow obstacle corresponding to engineering criteria for scour around bridge piers the obstacle persists long enough to fix the flow pattern and hence account for significant modification of channel form but is ultimately removed as part of the process of bed modification via the extension of scour three distinct stages in the process are involved local scour of a single pool creates deposition downstream which then generates the next downstream flow irregularity a riffle pool sequence is thus created autogenetically as the summation of a sequence of irregularities each unit of which is formed and maintained only by flow dynamics operating at the local scale the process is essentially both deterministic and statistical in character it is further suggested that spatial differences in the near bed turbulence field arising from incipient riffle pool topography themselves create differences in surface sediment entrainment which enhance and maintain the sequence in a form process feedback mechanism similar reasoning might possibly be extended to the entire range of bedform sizes

a review of selected engineering geological characteristics of english chalk abstract chalk is a variable material the properties of which are dependent upon its composition textural features and diagenetic history with the exception of certain horizons in the lower chalk that contain appreciable amounts of clayey material the english chalk is a remarkably pure micritic carbonate rock that generally can be divided into coarse and fine fractions the latter comprises 70 80 of chalk cementation took place more or less contemporaneously with deposition so that the sediment was able to support relatively high overburden pressures hence high values of porosity were retained chalk varies appreciably in density and hardness the harder chalks are the result of diagenetic processes and bioturbation that brought about densification in soft chalks the grains are only bound together at the points of contact by thin films of calcite the latest classification of chalk is based on an assessment of intact dry density discontinuity aperture and discontinuity spacing chalk tends to vary from moderately weak to moderately strong and its strength is significantly reduced on saturation under triaxial loading conditions diagonal shear failure tends to occur at lower confining pressures but at higher confining pressures barrel shaped failure occurs indicating plastic deformation and textural disaggregation similarly at low loading chalk exhibits low volume compressibility but much more significant consolidation occurs if the yield stress is exceeded chalk undergoes dissolution and so solution features are found throughout its outcrop mineworkings in the chalk extend back into the distant past the most ancient being those excavated in the neolithic age for flint several types of workings exist collapse of old mineworkings most of which are unrecorded is difficult to predict the potential for subsidence caused by the collapse of both mineworkings and dissolution features affects development and its occurrence can lead to the abandonment of property or worse the loss of lives

the origin of breccia hosted uranium deposits in carbonaceous metasediments of the iberian peninsula u pb geochronology and stable isotope studies of the fe deposit salamanca province spain the upper proterozoic lower cambrian schist graywacke complex of the iberian peninsula hosts several important uranium deposits which are characterized by their occurrence in fracture and breccia zones and by the mineral association pitchblende coffinite carbonates adularia fe sulfides the fe mine in salamanca province is not only the largest known deposit of this type but also the most important uranium deposit in spain with current reserves in excess of 16 000 metric tons of u 3 o 8 primary mineralization occurs as narrow veins occupying fractures and in cavities as finely laminated sediments showing a variety of geopetal textures three stages of primary mineralization separated by episodes of fracturing and brecciation are recognized the early stage commenced with chloritization of wall rocks in narrow zones adjacent to fractures followed by deposition of an assemblage dominated by ankerite and iron sulfides the second main stage mineralization is the most important for uranium and consists mainly of pitchblende coffinite carbonates iron sulfides and adularia the final stage is characterized by repeated episodic deposition of carbonates iron sulfides pitchblende and coffinite as layered cavity filling material chlorite compositions and fluid inclusion data show that wall rock alteration and ore deposition took place over a temperature range of approximately 280 degrees to less than 60 degrees c probably to subsurficial temperatures in the final stage of mineralization the uranium mineralization has been dated by u pb analyses of pitchblende samples at 34 8 or 1 6 ma indicating formation during the pyrenean phase of the alpine orogeny the u pb data also suggest the possibility of a two stage model for the uranium mineralization with the 34 8 or 1 6 ma event representing remobilization of an earlier pre tertiary mineralization calculated delta 18 o sub h 2 o values of the ore forming fluid range from approximately 14 5 per mil for early stage to near zero per mil for late stage fluid the delta 13 c values vary between 7 3 and 9 6 per mil for early and main stage carbonates and show a sharp progressive decline to 23 6 per mil at the end of the final stage the delta 34 s values of sulfides also decrease from the early stage around 10ppm to later stages down to 51 3ppm the combined geologic evidence suggests deposition of the ore from a hydrothermal system that formed in response to the effects of alpine tectonics on the variscan basement meteoric water descended via steeply dipping faults and as well as undergoing extensive isotope exchange with also leached uranium and other components from the metasediments of the schist graywacke complex particularly carbonaceous slates the fluid was probably expelled toward the surface through the fracture and breccia system by seismic pumping the episodic nature of the mineralization may have been controlled by fault movements that initiated brecciation and release of fluid pressure leading to ore deposition in fractures and cavities

unraveling chalk microtextural properties from indentation tests abstract the petrographical petrophysical and geomechanical properties of the less commonly studied low permeability or tight chalks are presented in this study the latter are relevant as potential unconventional reservoirs or intra reservoir seals tight chalks encompass different lithotypes in which the main factors controlling the petrophysical properties as shown in this study are the non carbonate content and the degree of cementation those parameters strongly modify chalk microtexture and thus its porous network reducing pore sizes hence altering poroperm properties in order to better understand the characteristics of tight chalks an integrated petrographical petrophysical and geomechanical study was carried out on a set of 65 chalk samples from northwestern european outcrops covering a wide range of lithotypes the dataset gathered covers a broad spectrum of values for the determined petrophysical e g porosities from 9 to 45 and geomechanical properties e g strengths from 3 to 60 mpa in the framework of this study indentation tests were performed on the chalk samples this technique proved to be a successful method to quickly estimate rock strength indeed a good linear correlation r2 0 90 has been established between indentation strength and ucs furthermore indentation tests yielded valuable information about the chalk properties both in terms of petrographical cementation clay content petrophysical exponential relationship with porosity and geomechanical in relation to the elastic parameter and plasticity index properties this cheap and easy to operate method is a promising tool to indirectly estimate the mechanical parameters of chalks when core samples are unavailable for laboratory testing in oil field wells

variations in stratigraphic and reservoir properties adjacent to the mid paleocene sequence boundary campo section pyrenees spain abstract the paleocene of the campo section spain has different sedimentological characteristics above and below the mid paleocene unconformity beneath the unconformity sediments are dominated by evaporitic carbonates and collapse breccias above it they are characterized by continental detrital beds alternating with paleosols different subaerial features are observed and correspond to different porosity values pore throat sizes and micrite morphologies unexposed or intertidal facies have low porosity low throat pore size mesoporosity and well preserved rhombic crystals intermediate exposed facies paleosols possess medium porosity medium pore throat size microporosity and mainly micro rhombic crystals finally the facies corresponding to high exposure intensity and to evaporitic original facies presents high porosity permeability large pore throat size and rounded micritic crystals these observations show that the emersion phase caused important dissolution especially when associated with an easily dissolved original lithofacies