spatial organization in the step pool structure of a steep mountain stream vogelbach switzerland 1 spatial organization in the step pool structure of a 1 58 km long section of the vogelbach a steep mountain stream average channel bed slope 0 17 m m in the alpthal basin switzerland is analyzed to evaluate the extent of randomness in the occurrence of steps and to isolate the potential hydraulic and topographic controls on step placement statistics of the observed step sequence are compared with those of random sequences generated by permutation from the observed step height distribution hydraulic and topographic controls on step placement are analyzed from step length and steepness distributions and relations between mean step properties and step height and channel gradient for both observed and random sequences results show that 1 observed step length distributions are statistically significantly different from randomly generated sequences 2 step steepness is significantly different in observed data because of a positive correlation between mean step length and height and it remains fairly constant for all step sizes 3 spatial organization in steps does not extend far beyond the nearest step and 4 the influence of channel gradient on step properties is insignificant and highly variable indicating that hydraulic rather than topographic controls are dominant for step placement in this stream although the vogelbach is a steep stream on the boundary between a cascading and step pool morphology where we would expect randomness to dominate spatial organization was nevertheless detected in many important aspects of the step pool geometry

field derived relationships for flow velocity and resistance in high gradient streams summary we measured velocity and channel geometry in 10 reaches bed gradient 0 08 0 21 of a predominantly step pool channel the rio cordon italy over a range of discharges 3 80 of the bankfull discharge the resulting data were used to compute flow resistance at a station hydraulic geometry relations indicate that in most reaches the exponent describing the rate of velocity increases with discharge was between 0 48 and 0 6 which is within the range of published values for pool riffle channels the rio cordon data are also combined with published hydraulics data from step pool streams to explore non dimensional relationships between velocity and flow resistance and factors including unit discharge channel gradient and step geometry multiple regression analysis of this combined field dataset indicated that dimensionless unit discharge q is the most important independent variable overall in explaining variations in velocity and flow resistance followed by channel slope and the ratio of step height to step length empirical equations are provided both for dimensionless velocity and flow resistance but prediction of the former variable appears more reliable

regional controls on daily to interannual variations of precipitation isotope ratios in southeast china implications for paleomonsoon reconstruction abstract many key paleomonsoon records in east asia rely on past precipitation isotope ratios 18 op as proxies for hydroclimate however the relationship between climate variability and 18 op remains an ongoing debate here we investigate dominant climatic drivers and test actively discussed hypotheses of southeast china 18 op variability over multiple timescales using an 8 yr long daily 18 op record from guangzhou and updated gnip dataset from hong kong our moisture source diagnostic analyses suggest that the primary moisture of guangzhou precipitation essentially comes from the proximal northern south china sea scs convective activities over the primary moisture source regions measured by cumulative precipitation along back trajectory regional precipitation and outgoing longwave radiation play a key role in regulating 18 op variability across different timescales these effects can be related to the direct changes of convection intensity over the scs or the indirect changes associated with the shift of moisture source in consequence 18 op records seasonal monsoon dynamics associated with the intertropical convergence zone migration and the el nino southern oscillation enso further tests of different hypotheses and their links to the enso via the 2015 strong el nino event support the above conclusions taken together these results demonstrate that southeast china 18 op represents a spatial temporally integrated measure of precipitation and convection tracing from the measured site to moisture sources shedding light on the interpretation of paleo isotope data in the monsoon domain

contribution of small glaciers to global sea level observed long term changes in glacier volume and hydrometeorological mass balance models yield data on the transfer of water from glaciers excluding those in greenland and antarctica to the oceans the average observed volume change for the period 1900 to 1961 is scaled to a global average by use of the seasonal amplitude of the mass balance these data are used to calibrate the models to estimate the changing contribution of glaciers to sea level for the period 1884 to 1975 although the error band is large these glaciers appear to account for a third to half of observed rise in sea level approximately that fraction not explained by thermal expansion of the ocean

large values of hydraulic roughness in subglacial conduits during conduit enlargement implications for modeling conduit evolution hydraulic roughness accounts for energy dissipated as heat and should exert an important control on rates of subglacial conduit enlargement by melting few studies however have quantified how subglacial conduit roughness evolves over time or how that evolution affects models of conduit enlargement to address this knowledge gap we calculated values for two roughness parameters the darcy weisbach friction factor f and the manning roughness coefficient n using dye tracing data from a mapped subglacial conduit at rieperbreen svalbard values of f and n calculated from dye traces were compared with values of f and n calculated from commonly used relationships between surface roughness heights and conduit hydraulic diameters roughness values calculated from dye tracing ranged from 75 0 97 for f and from 0 68 0 09 s m 1 3 for n equations that calculate roughness parameters from surface roughness heights underpredicted values of f by as much as a factor of 326 and values of n by a factor of 17 relative to values obtained from the dye tracing study we argue these large underpredictions occur because relative roughness in subglacial conduits during the early stages of conduit enlargement exceeds the 5 range of relative roughness that can be used to directly relate values of f and n to flow depth and surface roughness heights simple conduit hydrological models presented here show how parameterization of roughness impacts models of conduit discharge and enlargement rate we used relationships between conduit relative roughness and values of f and n calculated from our dye tracing study to parameterize a model of conduit enlargement assuming a fixed hydraulic gradient of 0 01 and ignoring creep closure it took conduits 9 25 days to enlarge from a diameter of 0 44 m to 3 m which was 6 7 fold longer than using common roughness parameterizations copyright 2013 john wiley sons ltd

a distributed temperature index ice and snowmelt model including potential direct solar radiation hourly melt and discharge of storglaciaren a small glacier in sweden were computed for two melt seasons applying temperature index methods to a 30 m resolution grid for the melt component the classical degree day method yielded a good simulation of the seasonal patient of discharge but the pronounced melt induced daily discharge cycles were not captured modelled degree day factors calculated for every hour and each gridcell from melt obtained from a distributed energy balance model varied substantially both diurnally and spatially a new distributed temperature index model is suggested attempting to capture both the pronounced diurnal melt cycles and the spatial variations in melt due to the effects of surrounding topography this is accomplished by including a radiation index in terms of potential clear sky direct solar radiation and thus without the need for other data besides air temperature this approach improved considerably the simulation of diurnal discharge fluctuations and yielded a more realistic spatial distribution of melt rates the incorporation of measured global radiation to account for the reduction in direct solar radiation due to cloudiness did not lead to additional improvement in model performance

glacier meltwater and runoff modelling keqicar baqi glacier southwestern tien shan china meltwater and runoff from keqicar baqi glacier a large glacier in the southwestern tien shan northwestern china are simulated using a modified degree day model including potential clear sky direct solar radiation coupled with a linear reservoir model for the period 1 july to 12 september 2003 there is good agreement between modelled and measured meltwater at ablation stakes and between simulated and observed runoff at the glacier terminus a reconstruction of glacier meltwater and runoff from an assumed debris free surface during the study period shows that the effect of the debris layer on glacier meltwater generation crucially affects glacier runoff the model is also used to calculate glacier runoff given the climate scenario resulting from a doubling of co2 as projected by the recm2 regional climate model the projected changes in temperature and precipitation vary from 0 to 2 78c and from 0 to 25 respectively results indicate that glacier runoff increases linearly with temperature over these ranges whether or not the debris layer is taken into consideration the effect of change in temperature is much more noticeable than that for change in precipitation due to the debris layer predominantly covering and insulating the ablation area the response of glacier runoff is less sensitive to temperature increase with a debris covered surface than with a debris free surface glacier runoff is also markedly reduced when a reduced glacier area is prescribed in the 2 78c scenario

comment on step pool streams adjustment to maximum flow resistance by athol d abrahams gang li and joseph f atkinson the writer wishes to congratulate the authors for their very interesting development on step pool streams recent advances on stepped channel flows with fixed bed provide additional information on the topic the discusser will show that the maximum flow resistance condition implies a characteristic flow pattern basically the bed profile of maximum flow resistance on step pool streams is achieved for flow conditions near the transition between nappe and skimming flow

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

hydrodynamics of steep streams with planar coarse grained beds turbulence flow resistance and implications for sediment transport the hydraulics of steep mountain streams differ from lower gradient rivers due to shallow and rough flows energetic subsurface flow and macro scale form drag from immobile boulders and channel and bed forms heightened flow resistance and reduced sediment transport rates in steep streams are commonly attributed to macro scale form drag however little work has explored steep river hydrodynamics in the absence of complex bed geometries here we present theory for the vertical structure of flow velocity in steep streams with planar rough beds that couples surface and subsurface flow we test it against flume experiments using a bed of fixed cobbles over a wide range of bed slopes 0 4 30 experimental flows have a nearly logarithmic velocity profile far above the bed flow velocity decreases less than logarithmically towards the bed and is non zero at the bed surface velocity profiles match theory derived using a hybrid eddy viscosity model in which the mixing length is a function of height above the bed and bed roughness subsurface flow velocities are large 1 m s and follow a modified darcy brinkman forchheimer relation that accounts for channel slope and shear from overlying surface flow near bed turbulent fluctuations decrease for shallow rough flows and scale with the depth averaged flow velocity rather than bed shear velocity flow resistance for rough planar beds closely matches observations in natural steep streams despite the lack of bed or channel forms in the experiments suggesting that macro scale form drag is smaller than commonly assumed in stress partitioning models for sediment transport

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

turbulence at a roughness transition in a depth limited flow over a gravel bed turbulence statistics derived from velocity measurements along a longitudinal transect characterized by a roughness transition are presented the bed morphology along the transect is dominated by composite scales of roughness ranging from an armoured bed in the upstream section to the presence of large protuberant clasts superimposed on the bed in the downstream section an array of four electromagnetic current meters operating at a frequency of 20 hz was used to measure the longitudinal and vertical velocities at various heights above the bed surface and at different positions along the transect emphasis is put on the longitudinal and vertical distributions of turbulent flow statistics such as reynolds shear stress uv cross correlation and quadrant analysis of velocity fluctuations the results indicate that the presence of protuberant clasts modifies the turbulence structure increases turbulence intensity and appears to dominate turbulence generation distinct flow zones are observed in particular a clear distinction is made between i a sublayer below the top of the major roughness elements identified by fluid motions predominantly towards the bed surface and characterized by intense eddying with some vortices or rollers possibly being trapped in this near bed flow region and ii an inner layer located immediately above the major protuberant clasts identified by positive vertical velocities and hence upward directed fluid motion the dominant generative mechanism of flow structure appears to be the shedding of vortices from the lee of obstacles interaction of vortices stretched by shear into the main body of flow downstream from major obstacles dominates the turbulence structure in depth limited flows over coarse and irregular bed surfaces

experimental study on the stability and failure of individual step pool abstract step pools are one of the most common bedforms in mountain streams the stability and failure of which play a significant role for riverbed stability and fluvial processes given this importance flume experiments were performed with a manually constructed step pool model the experiments were carried out with a constant flow rate to study features of step pool stability as well as failure mechanisms the results demonstrate that motion of the keystone grain ks caused 90 of the total failure events the pool reached its maximum depth and either exhibited relative stability for a period before step failure which was called the stable phase or the pool collapsed before its full development the critical scour depth for the pool increased linearly with discharge until the trend was interrupted by step failure variability of the stable phase duration ranged by one order of magnitude whereas variability of pool scour depth was constrained within 50 step adjustment was detected in almost all of the runs with step pool failure and was one or two orders smaller than the diameter of the step stones two discharge regimes for step pool failure were revealed one regime captures threshold conditions and frames possible step pool failure whereas the second regime captures step pool failure conditions and is the discharge of an exceptional event in the transitional stage between the two discharge regimes pool and step adjustment magnitude displayed relatively large variabilities which resulted in feedbacks that extended the duration of step pool stability step adjustment which was a type of structural deformation increased significantly before step failure as a result we consider step deformation as the direct explanation to step pool failure rather than pool scour which displayed relative stability during step deformations in our experiments

role of near bed turbulence structure in bed load transport and bed form mechanics the interactions between turbulence events and sediment motions during bed load transport were studied by means of laser doppler velocimetry and high speed cinematography sweeps u 0 w 0 w 0 which contribute negatively to the bed shear stress and are relatively rare individually move as much sediment as sweeps of comparable magnitude and duration however and much more than bursts u 0 and inward interactions u 0 w 0 when the magnitude of the outward interactions increases relative to the other events therefore the sediment flux increases even though the bed shear stress decreases thus although bed shear stress can be used to estimate bed load transport by flows with well developed boundary layers in which the flow is steady and uniform and the turbulence statistics all scale with the shear velocity it is not accurate for flows with developing boundary layers such as those over sufficiently nonuniform topography or roughness in which significant spatial variations in the magnitudes and durations of the sweeps bursts outward interactions and inward interactions occur these variations produce significant peaks in bed load transport downstream of separation points thus supporting the hypothesis that flow separation plays a significant role in the development of bed forms

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

the linkage between velocity patterns and sediment entrainment in a forced pool and riffle unit a field based project was initiated in order to characterize velocities and sediment entrainment in a forced pool and riffle sequence three dimensional velocities and turbulence intensities were measured with an acoustic doppler velocimeter at 222 different points at three similar flows that averaged approximately 4 35 m 3 s 1 within a large pool riffle unit on north saint vrain creek colorado sediment sorting patterns were observed with the introduction of 500 tracer particles painted according to initial seeding location tracer particles moved sporadically during a 113 day period in response to the annual snowmelt peak flow which reached a maximum level of 14 8 m 3 s 1 velocity data indicate high instantaneous velocities and turbulence levels in the centre of pools patterns of sediment deposition support the notion that stream competence is higher in the pool than the downstream riffle flow convergence around a large channel constriction appears to play a major role in multiple processes that include helical flow development and sediment routing and backwater development with low velocities and turbulence levels above the constriction that may locally limit sediment supply jet flow flow separation vortex scour and turbulence generation enhance scour in the centre of pools ultimately multiple processes appear to play some role in maintenance of this forced pool and the associated riffle copyright 2008 john wiley and sons ltd

field measurements of three dimensional hydraulics in a step pool channel we investigated the effects of morphologic position and discharge on flow structure in a steep 0 10m m mountain channel by collecting three dimensional measurements of time averaged and turbulent velocity components with a sontek flowtracker handheld adv acoustic doppler velocimeter on a 30 m reach of a step pool channel in the colorado rockies velocity profiles were measured at morphologic positions characteristic of steep channels above steps step lips base of steps pools cascades runs and at five different discharges a marked three dimensionality of flow structure was documented in east st louis creek velocities in the streamwise component were the largest contributors to overall velocity vector magnitudes cross stream and vertical components contributed averages of 20 and 15 respectively to overall vector magnitudes turbulence intensities were especially multi dimensional however with large contributions to turbulent kinetic energy from the vertical component of velocity analysis of variance indicated that discharge and morphologic position significantly affected mean streamwise velocities with substantially higher velocities upstream from steps than in pools discharge and morphology effects on cross stream and vertical velocity components however were not significant discharge and morphologic position also significantly affected turbulence intensities for all flow components with the greatest turbulence intensities occurring in pools and at high discharges these results illustrate the strong discharge dependence of hydraulics in step pool channels where relative submergence of bedforms changes rapidly with discharge and the substantial spatial variation in hydraulics created by step pool sequences

the characteristics of turbulence in a shear zone downstream of a channel constriction in a coarse grained forced pool a field based project was initiated to characterize the influence of varying discharge and bed topography along a pool exit slope on the strength of turbulence generated by vortex shedding velocities were measured with an adv downstream of a boulder constriction within a shear zone of a large pool measurements were repeated for four flows that varied from 20 to 50 of bankfull discharge an additional longitudinal profile was conducted along the pool thalweg plots of velocity demonstrate high near bed velocities and turbulence levels in the pools turbulence levels were highest closer to the constriction in the shear zone turbulent kinetic energy decayed in the downstream direction in response to the bed topography and at lower discharges instantaneous velocities are large enough to temporarily lift pebbles at this depth it appears that instantaneous forces created by vortex shedding may play an important role in scouring pools downstream of constrictions the turbulence may also respond to changes in bed topography in a manner that encourages feedbacks among bed topography characteristics of turbulence and sediment transport

variability in the surface temperature and melt extent of the greenland ice sheet from modis 1 satellite derived moderate resolution imaging spectroradiometer modis ice surface temperature ist of the greenland ice sheet shows a positive trend and two major melt events from 2000 to present ist increased by 0 55 0 44 c decade with the greatest increase 0 95 0 44 c decade found in northwestern greenland where coastal temperatures and mass loss are also increasing and outlet glaciers are accelerating ist shows the highest rates of increase during summer 1 35 0 47 c decade and winter 1 30 1 53 c decade followed by spring 0 60 0 98 c decade in contrast a decrease in ist was found in the autumn 1 49 1 20 c decade the ist trends in this work are not statistically significant with the exception of the trend in northwestern greenland major surface melt covering 80 or more of the ice sheet occurred during the 2002 and 2012 melt seasons where clear sky measurements show a maximum melt of 87 and 95 of the ice sheet surface respectively in 2002 most of the extraordinary melt was ephemeral whereas in 2012 the ice sheet not only experienced more total melt but melt was more persistent and the 2012 summer was the warmest in the modis record 6 38 3 98 c our data show that major melt events may not be particularly rare during the present period of ice sheet warming

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

characteristics of velocity profiles along riffle pool sequences and estimates of bed shear stress abstract numerous velocity profiles were measured along a riffle pool sequence for three different flow conditions for each flow condition ranging from baseflow to about 2 3 of bankfull depth ten to fifteen velocity profiles were measured along the same transect the objectives of the study were i to differentiate between components of bed roughness and shear stress attributed to surface friction and large scale bed undulations and ii to investigate the effects of fluctuating discharges on the mean vertical flow structure in pools and riffles no previous attempts have been made to investigate scales of bed roughness from velocity profiles along riffle pool sequences and to determine how near bed velocity gradients change with increasing flow discharge at low flows and as expected riffles are characterized by higher near bed velocity gradients than pools similarly average bed shear stresses as determined from measurements of flow depth and energy gradient indicate a greater resistance and bed shear stress in riffle sections invariably however local shear stresses as determined from near bed velocity gradients in pools and in riffles are much smaller than reach averages estimated from measurements of depth and slope the shape of spatially averaged velocity profiles changes as discharges increases the difference in terms of near bed velocities between pools and riffles decreases as discharge increases which suggests that a velocity reversal may occur at or near bankfull further away from the bed surface velocity differences between pools and riffles do remain significant finally the curvature of the velocity profiles is more significant at high flows and surface roughness is maximized when flow conditions are intermediate perhaps a result of increasing density of small scale sedimentary structures

flow resistance in steep streams an experimental study 1 frequently an assessment of the mean water velocity in a stream is necessary to estimate the discharge associated with a particular flow depth or conversely the mean depth associated with a particular discharge in the absence of a direct measurement of flow velocity a flow resistance approach which establishes the relation between depth and velocity can be applied two approaches have been used in the past traditional approaches based on the use of a resistance coefficient e g darcy weisbach or dimensionless hydraulic geometry approaches to examine if one approach is more appropriate for steep streams data from 31 flume experiments conducted to examine flow resistance in self formed cascade channels were analyzed a dimensionless hydraulic geometry approach developed using at a station data to characterize the q exponent and between site data to characterize the exponent on the channel slope term was more accurate than more traditional approaches the developed relation was similar to the established rational relation v g0 2q0 6s 0 4s0 2 suggesting that the rational relation has merit the approach does not utilize a flow partitioning approach since in steep streams with small relative depths the grains themselves generate form and spill resistance the observation that a single dimensionless hydraulic geometry flow resistance relation can describe measurements across a range of grain sizes and bed slopes 3 21 suggests that steep streams may follow a single scaling relation similar to the regime equations associated with lowland rivers

the role of morphology on the displacement of particles in a step pool river system abstract although studies of sediment transport in steep and coarse grained channels have been more numerous in recent years the dynamics of sediment transport in step pool river systems remain poorly understood this paper investigates displacements of individual clasts through spruce creek quebec canada a classic step pool channel and the effects of the channel morphology on the path length of the clasts passive integrated transponder tags pit were used to track the displacement of 196 individual particles over a range of discharges including the bankfull stage clasts were tracked after five sequences of flood events the results showed that the distance distributions match a two parameter gamma model equal mobility transport occurs for the particle size investigated during each sequence of flood events mean travel distance of the clasts can be estimated from excess stream power and the mobility of the clasts is more than an order of magnitude less than the model reported in riffle pool channels the dominant morphological length scale of the bed also controls the path length of the clasts these results confirm some preliminary observations on sediment transport in step pool channels

water velocity and the nature of critical flow in large rapids on the colorado river utah 1 rapids are an integral part of bedrock controlled rivers influencing aquatic ecology geomorphology and recreational value flow measurements in rapids and high gradient rivers are uncommon because of technical difficulties associated with positioning and operating sufficiently robust instruments in the current study detailed velocity water surface and bathymetric data were collected within rapids on the colorado river in eastern utah with the water surface survey it was found that shoreline based water surface surveys may misrepresent the water surface slope along the centerline of a rapid flow velocities were measured with an adcp and an electronic pitot static tube integrating multiple measurements the adcp returned velocity data from the entire water column even in sections of high water velocity the maximum mean velocity measured with the adcp was 3 7 m s the pitot static tube while capable of only point measurements quantified velocity 0 39 m below the surface the maximum mean velocity measured with the pitot tube was 5 2 m s with instantaneous velocities up to 6 5 m s analysis of the data showed that flow was subcritical throughout all measured rapids with a maximum measured froude number of 0 7 in the largest measured rapids froude numbers were highest at the entrance of a given rapid then decreased below the first breaking waves in the absence of detailed bathymetric and velocity data the froude number in the fastest flowing section of a rapid was estimated from near surface velocity and depth soundings alone

dimensions of standing waves at steps in mountain rivers 1 oscillations of the water surface are common features of flows around critical conditions fr 1 and of both natural and structure induced undular hydraulic jumps the paper describes experimental tests and field data on the dimensions of standing waves created by artificial drops in steep gravel bed rivers the purpose of the research is to analyze what parameters affect the phenomenon to provide semiempirical formulations for wave dimensions and to assess whether previous models for undular hydraulic jumps and antidunes are reasonably predictive the dimensionless wavelength and wave amplitude were correlated to the froude number at the sill the first wavelength appears linearly related to the froude parameter whereas the amplitude of the first wave shows a less defined behavior possibly indicating that a maximum value occurs for sill froude numbers around 1 6 1 7 the corresponding wave steepness i e wave amplitude length results between 0 15 0 16 and this range also characterizes the point at which the dimensionless trough depth below the sill achieves a local maximum probably related to hydrodynamic conditions just before wave breaking the transition between roller and undular jumps at drops is then argued to likely represent a critical stage in step pool streams and the hydraulic and morphological implications of this finding are also discussed with particular emphasis on step stability and step wavelength

evaluation of topographic effects in remotely sensed data abstract remotely sensed data are generally analyzed assuming flat terrain as the spectral signature of the ground depends upon the orientation of the surface slope the topography has to be considered for efficient radiometric corrections in mountainous regions in this paper we present a modelization of the atmospheric downward radiance at the ground level which gives us the relative contributions of the direct solar beam and of the diffused radiation for different atmospheric turbidities and different solar incidence angles lastly we estimate the influence of the topography on these contributions to show up which effects can be neglected

the storage of bed material in mountain stream channels as assessed using optically stimulated luminescence dating abstract a detailed understanding of channel forming and maintenance processes in mountain streams requires some measurement and or prediction of bed load transport and sediment mobility traditional field based measurements of such processes are problematic because of the high formative discharges characteristic of such streams the application of optically stimulated luminescence osl dating is proposed here as a new way of determining actual residency times of fine sediments and consequently validating selected predictions for the entrainment of sediment in these streams model predictions of sediment mobility for selected step pool and plane bed channels in a mountain catchment in south eastern australia are initially calculated using equations of hydraulic competence and the one dimensional hec ras model results indicate that floods exceeding bankfull with recurrence intervals up to 13 years are competent to mobilise the maximum overlying surface grain sizes at both sites osl minimum age model results from 7 samples of well bleached quartz in the fine matrix particles indicate general agreement with selected competence equations the apparent long 100 1400 y burial age of most of the mineral quartz however suggests that competent flows are not able to flush all subsurface fine bed material the depth of maximum bed load exchange flushing was limited to twice the depth of the overlying d 90 grain size application of osl in this study provides important insight into the nature of storage and flushing of matrix material in mountain streams

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

velocity characteristics along a small step pool channel this paper summarizes measurements of velocity along three reaches of a small mountain channel with step pool bedforms a one dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph measurement cross sections were located upstream from a bed step at the step lip downstream from the step and in a uniform gradient run data analyses focused on characteristics of velocity profiles and on correlations between velocity characteristics and the potential control variables bedform type reach gradient and flow depth to test the hypothesis that velocity characteristics are related to channel bedform types anova and ancova tests were performed for the average velocity and coefficient of variation of point velocity data results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge velocity became more variable as stage increased particularly at low gradient reaches with less variable bed roughness velocity profiles suggest that locations immediately downstream from bed steps are dominated by wake turbulence from mid profile shear layers locations immediately upstream from steps at step lips and in runs are dominated by bed generated turbulence adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation whereas favourable pressure gradients at steps are suppressing turbulence the bed generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake generated turbulence and form drag of step pool bedforms copyright 2000 john wiley sons ltd

energy and mass balance of zhadang glacier surface central tibetan plateau climate variables that control the annual cycle of the surface energy and mass balance on zhadang glacier in the central tibetan plateau were examined over a 2 year period using a physically based energy balance model forced by routine meteorological data the modelled results agree with measured values of albedo incoming longwave radiation surface temperature and surface level of the glacier for the whole observation period the radiation component dominated 82 the total surface energy heat fluxes this was followed by turbulent sensible 10 and latent heat 6 fluxes subsurface heat flux represented a very minor proportion 2 of the total heat flux the sensitivity of specific mass balance was examined by perturbations of temperature 1 k relative humidity 20 and precipitation 20 the results indicate that the specific mass balance is more sensitive to changes in precipitation than to other variables the main seasonal variations in the energy balance were in the two radiation components net shortwave radiation and net longwave radiation and these controlled whether surface melting occurred a dramatic difference in summer mass balance between 2010 and 2011 indicates that the glacier surface mass balance was closely related to precipitation seasonality and form proportion of snowfall and rainfall

stability of landslide dams and development of knickpoints the wenchuan earthquake triggered many landslides and numerous avalanches and created 100 odd quake lakes the quake lakes may be removed or preserved the removal strategy was applied to several large landslide dams which were dangerous because massive amounts of water pooled up in the quake lakes the dams could eventually fail under the action of dam outburst flooding potentially endangering the lives of people in the downstream reaches this paper studied the stability of landslide dams and the development of knickpoints by field investigations and experiments and analyzing satellite images the study concluded that if landslide dams were preserved they would develop into knickpoints and act as a primary control of riverbed incision and thus reduce the potential of new landslide the stability of landslide dams depends mainly on the development of the step pool system and stream power of the flood flow if a landslide dam consists of many boulders a step pool system may develop on the spillway channel of the dam which would maximize the resistance consume most of the flow energy and consequently protect the dam from incision the development degree of the step pool system is represented by a parameter s p which was measured with a specially designed instrument a preservation ratio of landslide dams is defined as the ratio of preserved height after flood scouring to the original height of the dam for streams with peak flood discharge lower than 30 m3 s the preservation ratio is linearly proportional to s p for rivers with a peak flood discharge higher than 30 m3 s 30 30 000 m3 s the minimum s p value for stable channel increases with log p in which p is the unit stream power for a landslide dam with a poorly developed step pool system s p is smaller than the minimum value and the outburst flood incises the spillway channel and causes failure of the dam for preserved landslide dams sediment deposits in the quake lakes a landslide dam may develop into a knickpoint if it is stabilized by long term action of the flow large knickpoints can totally change the fluvial processes and river morphology uplift of the qinghai tibetan plateau has caused extensive channel bed incision along almost all rivers for many rivers the incision has been partly controlled by knickpoints upstream reaches of a knickpoint have a new and unchanging base level this brings about a transition from degradation to aggradation and from vertical bed evolution to horizontal fluvial process multiple and unstable channels are prominent in the reaches upstream of the knickpoints if hundreds of landslide dams occurred simultaneously on a reach of a mountain river the potential energy of bank failure and the slope erosion would be greatly reduced and sediment yield from the watershed may be reduced to nearly zero the quake lakes may be preserved long term and become beautiful landscapes streams with long term unfilled quake lakes have good aquatic ecology

sea ice surface temperature retrieval from landsat 8 tirs evaluation of five methods against in situ temperature records and modis ist in arctic region abstract accurate and high resolution sea ice surface temperature ist data is of great importance for arctic climate studies however the validation of high resolution ist data using in situ measurements in polar sea ice regions is lacking this study assesses the accuracy of three split window sw and two single channel sc methods based on landsat 8 thermal infrared imagery at 100 m resolution over arctic sea ice regions the sw methods are proposed by jin et al 2015 sw jin jimenez munoz et al 2014 sw jm and du et al 2015 sw du the sc methods are proposed by jimenez munoz et al 2014 sc jm and barsi et al 2003 2005 sc barsi ist data derived from 58 scenes of the landsat 8 images were compared with coincident in situ ice skin temperatures and near surface air temperatures as measured by a combination of ice mass balance imb buoys snow and ice mass balance array simba buoys and automatic weather stations sw du offers the best accuracy when compared with the skin temperature bias 1 06 k root mean square error rmse 2 08 k and near surface air temperature bias 0 98 k rmse 2 17 k sc barsi ranks second with a bias of 1 55 k and rmse of 2 40 k for the skin temperature as for precision ist from the moderate resolution imaging spectrometer modis has best performance standard deviation std 1 69 k followed by sw du sw jm and sc barsi std 1 80 k 1 82 k and 1 85 k respectively the landsat ist outperforms the modis ist in narrow lead areas owing to its better spatial resolution and sw jm and sc barsi methods agree best with the modis ist in leads and marginal ice zone scenes respectively as all three sw methods are constrained by banding effects with different degrees in a lead scene they are not recommended to be applied on an image scene with severe banding artifacts the small bias 1 26 k and high correlation 0 99 between skin temperature and near surface air temperature prove the capability of using near surface air temperature as a substitute for validating a satellite ist data if skin temperature data are not available

a morpho statistical classification of mountain stream reach types in southeastern australia abstract the concepts of sediment transport capacity q c and sediment supply q s have shown promise in broadly differentiating mountain streams the important role of lithology in determining reach characteristics is also noted but as yet not fully included in existing process domain frameworks this study uses topographic and grain size surveys undertaken over 42 channel reaches with slopes between 0 01 and 0 20 m m to describe mountain stream morphology in southeastern australia reaches with two different lithologies are surveyed to specifically address the role of lithology in resultant channel morphology spatial autocorrelation analysis based on moran s i is used to detect the frequency and occurrence of in stream features such as bars and pools these data are combined with the grain size data to describe eight dominant reach morphologies most channel morphologies display broad characteristics similar to previous accounts of mountain streams although a number of intermediate morphologies affecting subtle variations in form and process where evident there was no significant longitudinal arrangement of the observed morphologies particularly with respect to catchment area and slope this is related to the strongly segmented longitudinal profiles characteristic of these streams where increasing catchment area does not result in a commensurate decrease in slope five of the eight channel morphologies identified are lithology dependent with respect to grain size and shape regime diagrams applied to quantify the physical controls on the different reach morphologies identified different q c q s domains for most morphologies however some lithology dependent morphologies could not be differentiated supporting the need to incorporate the effects of dominant grain shape or sphericity in assigning dimensionless critical shear stress values in determining q s overall the differences in mountain stream reach morphologies described in this study reflect the characteristic segmentation of the longitudinal profiles of these streams and contrasts in the supply and breakdown of parent material

a reconciled estimate of glacier contributions to sea level rise 2003 to 2009 glaciers distinct from the greenland and antarctic ice sheets are losing large amounts of water to the world s oceans however estimates of their contribution to sea level rise disagree we provide a consensus estimate by standardizing existing and creating new mass budget estimates from satellite gravimetry and altimetry and from local glaciological records in many regions local measurements are more negative than satellite based estimates all regions lost mass during 2003 2009 with the largest losses from arctic canada alaska coastal greenland the southern andes and high mountain asia but there was little loss from glaciers in antarctica over this period the global mass budget was 259 28 gigatons per year equivalent to the combined loss from both ice sheets and accounting for 29 13 of the observed sea level rise

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

changes in velocity profiles at roughness transitions in coarse grained channels heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness distinct spatial zones in terms of grain size have been reported for example sand ribbons and bedload sheets the transition from areas of finer to coarser grained surface sediment is often abrupt however the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels this paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistance on the structure of the turbulent boundary layer measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface boundary shear stress and roughness length increase considerably downstream from this transition velocity profiles become concave upwards downstream and upstream sections show significant differences in terms of near bed velocities deceleration downstream of the transition velocity gradient and turbulence intensity of the streamwise velocity component comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity or shear stress associated with the pressure drag produced by large and isolated obstacles

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

geometric adjustment of pools to changes in slope and discharge a flume experiment abstract to characterize the factors controlling pool shape 30 different forced pools were created utilizing a 50 triangular constriction in a 0 5 m wide 6 m long recirculating flume pools were scoured from an initial plane bed of sand with a d50 of 0 25 mm pool depth and length were measured and used as dependent variables in least squares multiple regression analyses discharge channel bed gradient and energy slope were the independent variables additional linear regression analyses were conducted with either pool depth or length and stream power results indicate that both pool depth and length are primarily a function of discharge channel bed and energy slopes are also significantly related to pool length but are not significantly related to pool depth stream power is significantly related to both pool depth and length but r2 values for pool depth versus discharge indicate stronger relations than those between pool depth and stream power observations on the type of geometric adjustment indicate that pools may minimize their rate of energy expenditure primarily through elongation in contrast pool depth appears to be more sensitive to the characteristics of the constrictions that create the forced pools the results suggest that many field studies may suffer from cross correlation problems in particular channel erodibility may exert a more dominant influence on pool geometry than hydraulic controls in many constriction influenced channels

estimating mountain glacier surface temperatures from landsat etm thermal infrared data a case study of qiyi glacier china abstract surface temperatures on mountain glaciers are a direct reflection of the heat budget at the glacier surface which controls the degree of ablation an understanding of surface temperatures is therefore crucial in simulating the evolution of mountain glaciers and in understanding a glacier s response to climate change in situ observations indicate that when the exitance angle equals 75 relative to the nadir angle the emissivity variation at 12 5 m in bare ice is as much as 0 24 however research involving the remote sensing retrieval of mountain glacier surface temperatures does not generally take into account the topography i e the exitance angle and the validation of remote sensing approaches using surficial climate measurements on mountain glaciers is lacking here we utilize the etm multispectral dataset and resampled water vapor content data from mod05 combined with the emissivity model of ice and snow taking the exitance angle into account hori et al 2013 and employ the single channel algorithm developed by jimenez munoz et al 2009 to calculate the remote sensing retrieved surface temperature distribution across qiyi glacier china from july 2012 to september 2013 the remote sensing retrieved temperatures are validated by the 10 min mean measurements of surface temperature from two automatic weather stations at different altitudes on the glacier and the results show good agreement at the two sites with an average bias of 0 96 k and an average root mean square error of 1 5 k of note in summer the influence of thermal emissions from surrounding topography means that the error associated with retrieved temperatures for the glacier tongue and margins might exceed 1 5 k

a distributed surface energy balance model for a small valley glacier i development and testing for haut glacier d arolla valais switzerland this paper describes the development and testing of a distributed surface energy balance model used to calculate rates of surface melting at haut glacier d arolla valais switzerland the model uses a digital elevation model dem of the glacier surface and surrounding topography together with meteorological data collected at a site in front of the glacier to determine hourly or daily totals of the energy balance components and hence of melting over the entire surface of the glacier with a spatial resolutio of 20 m the model can also be used to determine temporal and spatial variations in snow depth snow line position and glacier surface albedo calculations from the model are compared with observations made along the glacier centre line in 1990 and in general the model performs very well the correlation coefficients between calculated and measured snow line elevation albedo and ablation are 0 99 0 85 and 0 81 respectively the main source of error between modelled and measured values of these variables is probably inadequacies in the parameterization of albedo used in the model

bed surface adjustments to spatially variable flow in low relative submergence regimes in mountainous rivers large relatively immobile grains partly control the local and reach averaged flow hydraulics and sediment fluxes when the flow depth is similar to the size of these grains low relative submergence heterogeneous flow structures and plunging flow cause spatial distributions of bed surface elevations textures and sedimentation rates to explore how the bed surface responds to these flow variations we conducted a set of experiments in which we varied the relative submergence of staggered hemispheres simulated large boulders between runs all experiments had the same average sediment transport capacity upstream sediment supply and initial bed thickness and grain size distribution we combined our laboratory measurements with a 3d flow model to obtain the detailed flow structure around the hemispheres the local bed shear stress field displayed substantial variability and controlled the bedload transport rates and direction in which sediment moved the divergence in bed shear stress caused by the hemispheres promoted size selective bedload deposition which formed patches of coarse sediment upstream of the hemisphere sediment deposition caused a decrease in local bed shear stress which combined with the coarser grain size enhanced the stability of this patch the region downstream of the hemispheres was largely controlled by a recirculation zone and had little to no change in grain size bed elevation and bed shear stress the formation development and stability of sediment patches in mountain streams is controlled by the bed shear stress divergence and magnitude and direction of the local bed shear stress field

calculating bed load transport in steep boulder bed channels 1 steep rough channels occupy a large fraction of the total channel length in mountainous regions most sediment mobilized on hillslopes must pass through these streams before reaching lower gradient channels steep channels have wide grain size distributions that are composed of finer more mobile sediment and large rarely mobile grains the large grains can bear a significant portion of the total shear stress and thereby reduce the stress available to move the finer sediment conventional bed load transport equations often overpredict the sediment flux in steep channels by several orders of magnitude we hypothesize that sediment transport equations overpredict the sediment flux because they do not 1 account for the stress borne by rarely mobile grains 2 differentiate between highly and rarely mobile sediment and 3 account for the limited availability of mobile sediment here we modify a conventional bed load transport equation to include these three effects we use measurements of the flow bed properties and sediment flux in a small steep flume to test this equation we supply gravel at a constant rate through fields of regularly spaced immobile spheres and measure the bed coverage by gravel and sphere protrusion the percent of the sphere that protrudes above the gravel deposit for a given sphere spacing the proportion of the bed covered by gravel increases and the sphere protrusion decreases with greater sediment supply thus bed coverage and immobile grain protrusion may serve as proxies for sediment availability in steep rough streams unlike most transport equations that we tested our modified bed load equation predicts sediment fluxes to within an order of magnitude of the measured values our results demonstrate that accurately predicting bed load transport in steep rough streams may require accounting for the effects of local sediment availability coverage by mobile sediment and drag due to rarely mobile particles

integrated research on mountain glaciers current status priorities and future prospects abstract mountain glaciers are sensitive probes of the local climate and thus they present an opportunity and a challenge to interpret climates of the past and to predict future changes furthermore glaciers can constitute hazards including glacier outburst floods changes in the magnitude and timing of runoff in the mountains and adjacent regions and through worldwide loss of glacier ice a global rise in sea level to understand and ultimately to predict the dynamics and nature of climate and associated glacial and hydrological changes requires an integrated approach with communication and collaboration among glaciologists glacial geologists atmospheric scientists geomorphologists geochronologists and tectonists current strategies of research are evolving towards integrating research on mountain glaciers to address key scientific socio economic and political issues given the rapid birth and growth of new technologies and tools with which to study glaciers and glacial landscapes this community stands poised to address many of these challenges in the near future the key challenges that must be met soon include 1 determining the spatial temporal pattern of fluctuations of mountain glaciers from the last glacial cycle through the present 2 relating historical and past fluctuations in glaciers to variability in the primary features of ocean atmospheric circulation 3 identifying important but poorly understood processes controlling the motion and erosion of glaciers 4 developing and expanding the application of numerical models of glaciers 5 modeling the evolution of mountain landscapes in the face of repeated glaciation 6 examining the climate and the balance of energy and mass at the surface of glaciers 7 characterizing the role of intrinsic climate variability on glacier variations and 8 predicting the distribution sizes and nature of glaciers in the future while these ambitious goals are achievable and the research tools exist success will require significant bridging between the existing research communities involved and ambitious integration of research on mountain glaciers

daily air temperature estimation on glacier surfaces in the tibetan plateau using modis lst data the moderate resolution imaging spectroradiometer modis land surface temperature lst data have been widely used for air temperature estimation in mountainous regions where station observations are sparse however the performance of modis lst in high elevation glacierized areas remains unclear this study investigates air temperature estimation in glacierized areas based on ground observations at four glaciers across the tibetan plateau before being used to estimate the air temperature modis lst data are evaluated at two of the glaciers which indicates that modis night time lst is more reliable than modis daytime lst data then linear models based on each of the individual modis lst products from two platforms terra and aqua and two overpasses night time and daytime are built to estimate daily mean minimum and maximum air temperatures in glacierized areas regional glacier surface rgs models mean mean square differences 3 3 3 0 and 4 8 c for daily mean minimum and maximum air temperatures respectively show higher accuracy than local non glacier surface models mean root mean square differences 4 2 4 7 and 5 7 c in addition the rgs models based on modis night time lst perform better to estimate daily mean minimum and maximum air temperatures than using temperature lapse rate derived from local stations

differential bed sedimentology and the maintenance of riffle pool sequences abstract riffle pools are the characteristic reach scale bedforms of gravel and mixed bedded channels of low to moderate slope the purpose of this paper is to clarify several aspects of the sedimentology of riffle pool sequences and to suggest how these both reflect and control riffle pool stability in the presence of spatial differences in the turbulent near bed flow field high frequency velocity measurements obtained using an array of electromagnetic current meters are combined with results from grid and transect sampling of bed sediments from 9 sites on 3 small upland rivers in n exmoor u k riffle sediments are shown to possess greater development of microtopographic structuring than adjacent pools which enhances riffle stability and which may reflect greater turbulent kinetic energy there over most of the flow range data on surface sediment arrangement and turbulent flow characteristics can be combined with different transport threshold equations to explain the maintenance of riffle pool sequences with respect to a particle queuing kinematic wave model without recourse to a competence reversal as conventionally assumed

a comparison of flow intensities in alluvial rivers characteristics and implications for modelling flow processes this paper compares flow intensity data obtained with different instruments from a variety of fluvial environments it examines associations between the root mean square of longitudinal velocity fluctuations flow intensity local mean velocity relative depth and boundary resistance results indicate systematic differences in the behaviour of flow intensity which scale with respect to position in the boundary layer deep sand bedded rivers boundary grain resistance shallow river environments with coarse beds and possibly form resistance shallower sand bedded rivers preliminary approaches to prediction and modelling of variations in flow intensity are suggested based upon linear regression relationships intensity values are also compared with theoretical and empirical limits to the use of taylor s substitution which allows time and frequency properties of a single point velocity time series to be used to yield a flow length scale in general limits are exceeded in all environments for near boundary flow measurements but are met for y d 0 3 in most cases in sand bed rivers and for y d 0 4 in some gravel bed environments 1998 john wiley sons ltd

three dimensional structure of flow at a confluence of river channels with discordant beds this paper presents three dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to 1 describe the three dimensional flow field of a natural junction of channels 2 assess the role of changes in bed morphology occurring during transport effective events on the structure of flow at a confluence and 3 examine how the three dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams three dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four two component electromagnetic current meters six detailed velocity profiles were taken at five cross sections in a wide range of flow conditions the mean field of flow is characterised by 1 the acceleration of flow in the downstream portion of the post confluence channel but by lower velocities upstream in the mixing layer area 2 a stagnation zone at the apex of the junction 3 a zone of flow deviation and strong fluid upwelling close to the avalanche face and at the margin of the tributary mouth bar and 4 reduced velocities over the depositional bar at the downstream junction corner the position and extent of these zones vary with changes in the ratio of momentum flux very high intensity of turbulence peaks up to 50 and turbulent kinetic energy were observed in the mixing layer region distortion of the mixing layer characteristic of flow where bed discordance is present between the two tributary channels was evident from mean and turbulent flow data this field study suggests that the effects of bed discordance on flow sediment transport and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow

effects of bed forms and large protruding grains on near bed flow hydraulics in low relative submergence conditions in mountain rivers bedforms large relatively immobile grains and bed texture and topographic variability can significantly alter local and reach averaged flow characteristics the low relative submergence of large immobile grains causes highly three dimensional flow fields that may not be represented by traditional shear stress flow velocity and turbulence intensity equations to explore the influence of large protruding grains and bedforms on flow properties we conducted a set of experiments in which we varied the relative submergence while holding the sediment transport capacity and upstream sediment supply constant flow and bed measurements were conducted at the beginning and end of each experiment to account for the absence or presence of bedforms respectively detailed information on the flow was obtained by combining our measurements with a 3d numerical model commonly used velocity profile equations only performed well at the reach scale when shallow flow effects and the roughness length of the relatively mobile sediment were considered however at the local scale large deviations from these profiles were observed and simple methods to estimate the spatial distribution of near bed shear stresses are likely to be inaccurate zones of high turbulent kinetic energy occurred near the water surface and were largely controlled by the immobile grains and plunging flow the reach averaged shear stress did not correlate to depth or slope as commonly assumed but instead was controlled by the relative boulder submergence and degree of plunging flow for accurate flow predictions in mountain rivers the effects of bedforms and large boulders must be considered

two dimensional two phase depth integrated model for transients over mobile bed abstractfast geomorphic transients may involve complex scenarios of sediment transport occurring near the bottom as bed load i e saltating sliding and rolling or as suspended load in the upper portion of the flow the two sediment transport modalities may even coexist or alternate each other during the same event especially when the shear stress varies considerably modeling these processes is therefore a challenging task for which the usual representation of the flow as a mixture may result in being unsatisfactory in the present paper a new two phase depth averaged model is presented that accounts for variable sediment concentration in both bed and suspended loads distinct phase velocities are considered for bed load whereas the slip velocity between the two phases is neglected in the suspended load it is shown that the resulting two phase model is hyperbolic and the analytical expression of the eigenvalues is provided the entrainment deposition of sediment between the bottom and the bed

estimation of flow resistance in gravel bedded rivers a physical explanation of the multiplier of roughness length the need to estimate velocity and discharge indirectly in gravel bedded rivers is a commonly encountered problem semilogarithmic friction equations are used to estimate mean velocity using a friction factor obtained from depth and grain size information although such equations have a semi theoretical basis in natural gravel bed channels an empirical constant 6 8 or 3 5 has to be introduced to scale up the characteristic grain size d50 or d84 to represent the effective roughness length in this paper two contrasting approaches are used to suggest that the multiplier of characteristic grain size is attributable to the effect of small scale form resistance reflecting the occurrence of microtopographic bedforms in gravel bedded environments first spatial elevation dependence in short detailed bed profiles from a single gravel bedded river is investigated using semivariogram and zero crossing analyses this leads to objective identification of two discrete scales of bed roughness associated with grain and microtopographic roughness elements second the autocorrelation structure of the three dimensional near bed velocity field is examined to identify regularities associated with eddy shedding and energy losses from larger grains and microtopographic bedforms apart from improving the capacity to determine friction factors for velocity and discharge estimation the findings have implications in general for the initial motion of gravelly bed material

1 min in the life of a river selecting the optimal record length for the measurement of turbulence in fluvial boundary layers in the analysis of instantaneous velocity measurements it is important to consider carefully the properties of the instruments as well as the sampling frequency and the record length although several studies have examined the influence of instrument settings on velocity measurements in rivers little information is available on the effects of the record length on these measurements and on the turbulence parameters that can be extracted from the time series decisions on the record length are often made ad hoc here we quantify the effects of the sampling period on the precision of turbulence statistics e g mean standard deviation skewness reynolds stress quadrant analysis obtained from velocity measurements in fluvial turbulent boundary layers a bootstrapping method is used to determine the standard errors s e of turbulence statistics from 19 time series the time series were sampled in three gravel bed rivers using both electromagnetic current meters ecm and acoustic doppler velocimeters adv the record lengths ranged between 20 and 60 min the sampling frequencies were 20 and 25 hz for the ecms and advs respectively we show that for most turbulence statistics the optimal record length minimum sampling effort to achieve low standard errors ranges between 60 and 90 s we discuss the implication of these record lengths for standard errors estimates and variance losses

geometric scaling of step pools in mountain streams observations and implications understanding the geometric structuring of alluvial step pools is an enduring problem in mountain stream geomorphology many previous studies propose that the key control parameter governing the step pool form is mean bed slope as explored through several examples however we find that slope control formulations are inconsistently demonstrated for broad data sets on this basis we reexamine the bed form architecture of step pools with a new descriptive parameter termed the aspect ratio active stream width divided by step drop height we test with data collected from three different mountain settings for step pools observed over mean bed slopes ranging from 2 to 22 we find that step wavelength and height are well described by when these dimensions are normalized by the step drop height furthermore we observe that varies inversely with mean bed slope context for our findings is provided by recent field and laboratory work which highlights that the cross stream component to the overall flow structure in step pools and scour pools is significant this is important because it expands upon and relates our work to formative models which are based on jet scour and the associated three dimensional flow structure of step pools our work has practical implications for the design of step pool channels because existing approaches are based in large part on structuring artificial systems according to a narrowly defined geometry we observe however that under certain conditions this approach may yield constructed step pools that are unstable as an alternative approach we propose that the geometric relationships developed here can be applied to identify artificial step pool geometries that will be more fully reflective of natural analogs and thus possibly more stable

channel flow competence and sediment transport in upland streams in southeast australia bedload transport data from planebed and step pool reach types are used to determine grain size transport thresholds for selected upland streams in southeast australia morphological differences between the reach types allow the effects of frictional losses from bedforms microtopography and bed packing to be incorporated into the dimensionless critical shear stress value local sediment transport data are also included in a regime model and applied to mountain streams to investigate whether empirical data improve the delineation of reach types on the basis of dimensionless discharge per unit width q and dimensionless bedload transport q b instrumented planebed and step pool sites are not competent to transport surface median grains d50s at bankfull discharge qbf application of a locally parametrized entrainment equation to the full range of reach types in the study area indicates that the majority of cascades cascade pools step pools and planebeds are also not competent at qbf and require a 10 year recurrence interval flood to mobilize their d50s consequently the hydraulic parameters of the regime diagram which assume equilibrium conditions at bankfull are ill suited to these streams and provide a poor basis of channel delineation modifying the diagram to better reflect the dominant transported bedload size equivalent to the d16 of surface sediment made only slight improvements to reach delineation and had greatest effect on the morphologies with smaller surface grain sizes such as forced pool riffles and planebeds likewise the corey shape factor was incorporated into the regime diagram as an objective method for adjusting a base dimensionless critical shear stress c50b to account for lithologically controlled grain shape on bed packing and entrainment however it too provided only minor adjustments to reach type delineation

two methods for firn area and mass balance monitoring of svalbard glaciers with sar satellite images this paper presents two methods for glacier monitoring on svalbard using synthetic aperture radar sar satellite images both methods were developed on glaciers in the kongsfjorden area the first method monitors the firn area extent and the firn line over time by thresholding and filtering the sar image manual detection of the threshold is preferable but using a constant threshold for all images also gives adequate results a retreat of the fire line position is visible especially on kongsvegen corresponding to consecutive years of negative mass balance the second method applies a k means classification to three clusters on the glacier surface the areal extent of the resulting class on the upper part of the glacier correlates remarkably well with the independently measured mass balance of kongsvegen having a correlation coefficient of around 0 89 for the various glaciers this is because the snow from the accumulation area influences the k means classification thus on glaciers where mass balance values are available new mass balance values can be predicted solely from sar images for glaciers where no mass balance is available the area change cannot be calibrated to absolute mass balance values but relative changes can be predicted

comparison of satellite thermochron and air temperatures at summit greenland during the winter of 2008 09 current trends show a rise in arctic surface and air temperatures including over the greenland ice sheet where rising temperatures will contribute to increased sea level rise through increased melt we aim to establish the uncertainties in using satellite derived surface temperature for measuring arctic surface temperature as satellite data are increasingly being used to assess temperature trends to accomplish this satellite derived surface temperature or land surface temperature lst must be validated and limitations of the satellite data must be assessed quantitatively during the 2008 09 boreal winter at summit greenland we employed data from standard us national oceanic and atmospheric administration noaa air temperature instruments button sized temperature sensors called thermochrons and the moderate resolution imaging spectroradiometer modis satellite instrument to 1 assess the accuracy and utility of thermochrons in an ice sheet environment and 2 compare modis derived lsts with thermochron derived surface and air temperatures the thermochron derived air temperatures were very accurate within 0 1 0 3 c of the noaa derived air temperature but thermochron derived surface temperatures were 3 c higher than modis derived lsts though surface temperature is largely determined by air temperature these variables can differ significantly furthermore we show that the winter time mean air temperature adjusted to surface temperature was 11 c higher than the winter time mean modis derived lst this marked difference occurs largely because satellite derived lsts cannot be measured through cloud cover so caution must be exercised in using time series of satellite lst data to study seasonal temperature trends

when does bedload transport begin in steep boulder bed streams the paper presents the results of field measurements of critical conditions for bedload motion conducted in the rio cordon a steep boulder bed stream in the italian alps under conditions of high reynolds numbers and low relative submergence poorly explored before two methods have been used to determine threshold of motion the displacement of marked clasts and the flow competence approach which uses the largest grain size diameter transported by each flood event the high values of confirm the great relevance of non bedload effective shear stresses in step pool streams given by the additional form drag associated with this morphology relative submergence effects on the dimensionless critical shear stress have been quantified by considering the relative submergence ratio rh d84 and the major effect of relative size on the mobility of each particle in steep widely graded bed mixtures has been evaluated finally the dimensionless critical unit discharge has also been adopted in the regression equations as the critical hydraulic parameter because it may represent an easier parameter to use than the critical shear stress for steep rough mountain rivers copyright 2006 john wiley sons ltd

stream bed stabilization using boulder check dams that mimic step pool morphology features in northern italy during the last 10 years the check dams made of boulders have been fully employed for torrent management particularly for high gradient stream stabilization the analysis of this typology is shown in relation to three aspects building features field of use and design according to geomorphologic principles the maso di spinelle torrent located in the province of trento italy north italian alps is an example where a sequence of low check dams made of boulders have been used for bed stabilization the design criteria are taken from the step pool morphology features and the results are encouraging for further applications the artificial step pool grade control structures in the maso di spinelle torrent have been successfully tested by floods events with return periods of about 7 10 and 20 25 years

recent contributions of glaciers and ice caps to sea level rise satellite measurements of earth s gravity field show that the mass loss of glaciers and ice caps contributed to sea level rise by approximately 0 4 millimetres per year between 2003 and 2010 the extent to which mass loss from ice covered land areas contributes to sea level rise is not known accurately in part because previous global estimates use different techniques for glaciers ice caps and ice sheets the gravity recovery and climate experiment grace satellite has provided monthly measurements of the global gravity field since 2002 using grace data jacob et al assess regional mass loss between 2003 and 2010 and conclude that mass loss from ice covered land areas contributed almost 1 5 mm per year to the sea level rise estimated mass loss from the high mountains of asia was negligible in contrast to some other reports glaciers and ice caps gics are important contributors to present day global mean sea level rise1 2 3 4 most previous global mass balance estimates for gics rely on extrapolation of sparse mass balance measurements1 2 4 representing only a small fraction of the gic area leaving their overall contribution to sea level rise unclear here we show that gics excluding the greenland and antarctic peripheral gics lost mass at a rate of 148 30 gt yr 1 from january 2003 to december 2010 contributing 0 41 0 08 mm yr 1 to sea level rise our results are based on a global simultaneous inversion of monthly grace derived satellite gravity fields from which we calculate the mass change over all ice covered regions greater in area than 100 km2 the gic rate for 2003 2010 is about 30 per cent smaller than the previous mass balance estimate that most closely matches our study period2 the high mountains of asia in particular show a mass loss of only 4 20 gt yr 1 for 2003 2010 compared with 47 55 gt yr 1 in previously published estimates2 5 for completeness we also estimate that the greenland and antarctic ice sheets including their peripheral gics contributed 1 06 0 19 mm yr 1 to sea level rise over the same time period the total contribution to sea level rise from all ice covered regions is thus 1 48 0 26 mm 1 which agrees well with independent estimates of sea level rise originating from land ice loss and other terrestrial sources6

detailed documentation of dynamic changes in flow depth and surface velocity during a large flood in a steep mountain stream summary understanding the discharge capacity of channels and changes in hydraulic properties during large storms is essential for prediction of flash floods however such information is limited for steep mountain channels because of their complex nature and the lack of measured data thus we obtained detailed water level and surface velocity data during large floods of a steep mountain channel and documented how complex channel morphology affected water flow during large storms we installed water level and surface velocity sensors at a cascade and at a pool that was 10 m downstream at the aono research forest of the arboricultural research institute of the university of tokyo forests in japan we successfully obtained 1 min interval data for a major storm with total precipitation of 288 mm that fell over 59 h and a maximum rainfall intensity of 25 mm h during the storm height of the water surface from the deepest point of each cross section ranged from 0 35 to 1 57 m and surface velocity ranged from 0 35 to 4 15 m s as expected the changes in flow depth surface velocity and velocity profiles were complex and differed even between the cascade and adjacent pool cross sections dramatic changes in flow conditions first occurred at the cascade when discharge increased to a certain point when water suddenly stagnated at the foot of the cascade and submerged flow might have occurred thereafter the water level increased remarkably but surface velocity and the velocity profile stayed almost constant at the cascade cross section at the downstream pool where most rocks were submerged at a mean water depth of 0 7 m surface velocity suddenly increased dramatically and the velocity profile changed as very slow flow developed in the lower portion of the profile while water levels increased only slightly when the rainfall diminished first the surface velocity markedly declined then the velocity profile returned to its original state at the pool and then submerged flow at the bottom of the cascade ceased these temporally and spatially detailed flow measurements effectively document and characterize flow conditions during a large flood in a steep mountain channel these field data confirm the long held hypothesis that marked changes in flow conditions occur when steps become submerged

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

prediction of sediment transport in step pool channels 1 in mountainous drainage networks sediment mobilized on hillslopes must first pass through steep streams before reaching lower gradient channels the bed of steep channels is typically composed of large relatively immobile boulders and finer more mobile gravel most sediment transport equations overpredict sediment flux in steep streams by several orders of magnitude because they do not account for the stress borne by immobile grains and the limited availability of the more mobile sediment we previously developed and tested in flume experiments a sediment transport equation that accounts for these two effects here we modify the parker 1990 bed load equation to include the resistance borne by steps and selective transport of the relatively mobile sediment using a range of hiding functions we test a number of resistance equations and hiding functions combined with our modified and the original parker equations against measured flow and sediment transport in three steep channels our modified sediment transport equation generally predicts the transported sediment volumes to within an order of magnitude of the measured values whereas the unmodified equations do not the most accurate sediment flux predictions were obtained from using our modified equation combined with a hiding function that calculates highly selective transport of the relatively mobile sediment furthermore this hiding function has a critical shields stress that is similar to those reported for lower gradient channels the effects of the immobile steps on flow and sediment transport are not adequately captured by simply increasing the critical shields stress to values reported in steep streams

local scouring and morphological adjustments in steep channels with check dam sequences this paper describes bed profile and grain size distribution adjustments in a mountain river maso di spinelle river italian alps stabilized by a sequence of boulder check dams the control works were originally designed to simulate the geometry of natural step pool channels where tumbling flow is the dominant hydraulic regime local scouring downstream of 29 drop structures is analysed through the use of nondimensional parameters where maximum scour depth and scour length are normalised to the drop height prior laboratory data reveal a pattern similar to field scours where complex interactions occur between drop height critical flow depth and step spacing the linkage between scour length and depth is also discussed there seems to be a maximum step height for impinging jets that is approximately twice the drop height this maximum may explain the upper limit of the steepness factor found in high gradient step pool streams if such a maximum upper limit is confirmed by further studies this may aid designs of foundation heights for transverse control works in steep channels

morphological effects of local scouring in step pool streams step pool morphology characterizes many high gradient streams in a variety of natural settings but formative processes and evolutionary dynamics are still poorly understood in this paper natural step pool geometry is compared with steep alluvial channels where grade control structures such as check dams and bed sills make the stream profile resemble a natural stepped stream along these channels local scouring due to falling jets forms plunge pools under each structure analogous to natural steps determining the formation of pools in order to test the hypothesis that natural pools are analogous to pools formed below grade control works with respect to their dimensions shape and formative dynamics 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern italian alps pools below grade control works featured a transitional zone between the scour hole and the downstream sloping bed marked by a depositional berm when geometric parameters such as maximum pool depth length and step berm distances are normalized to the jet virtual energy no statistically significant differences were detected between natural and artificial systems these results lend support to an upstream forced cascade model for step pool formation where the energy of falling jets controls the geometry of the pools and is therefore regarded as the most important scaling independent variable copyright 2005 john wiley sons ltd