local scouring at grade control structures in alluvial mountain rivers 1 scour holes below 73 grade control structures check dams and bed sills in six mountain rivers located in the eastern italian alps have been surveyed the most likely formative water discharge is used to evaluate jet thickness at each structure which along with drop height appears to determine scour hole dimensions as shown by the consistent trends observed for nondimensional plots of both maximum scour depth and length versus the respective drop ratio sediment differences regarding size and lithology apparently play a minor role in determining scour hole dimension measured maximum scour depths are well predicted by a semiempirical equation developed through laboratory results showing an average relative error of 0 13 scour hole geometry is described by several ratios that are thought to represent approximately invariant characteristics a new energy based normalization for scour hole dimensions is proposed as the most suited to evaluate the role of jet geometry and aeration upon scouring efficiency it is noteworthy that the ratio between the maximum scour depth and the total energy available at a drop tends toward an asymptotic value around unity for increasing drop heights

vanadiferous minerals from the nereus deep red sea the chemical analysis by emp eds of 140 samples in cores from six red sea troughs and basins has led to the discovery in the nereus deep of high vanadium concentrations up to 1 3 v2o3 in several bulk samples and to the isolation of a major magnetite phase this vanado magnetite remarkable for its high content of vanadium mean 1 45 v2o3 and its lack of titanium is frequently zoned the v content decreasing toward the outer zones oxides with up to 42 v2o3 have also been found since the origin of the magnetite is clearly authigenic its high vanadium and low titanium concentrations are traced back to the differential hydrothermal leaching of fe ti oxides which occur profusely in basic eruptive rock clasts actually present in some of the overlying seams authors

scales and dynamics of submesoscale coherent vortices formed by deep convection in the northwestern mediterranean sea since 2010 an intense effort in the collection of in situ observations has been carried out in the northwestern mediterranean sea thanks to gliders profiling floats regular cruises and mooring lines this integrated observing system enabled a year to year monitoring of the deep waters formation that occurred in the gulf of lions area during four consecutive winters 2010 2013 vortical structures remnant of wintertime deep vertical mixing events were regularly sampled by the different observing platforms these are submesoscale coherent vortices scvs characterized by a small radius 5 8 km strong depth intensified orbital velocities 10 20 cm s 1 with often a weak surface signature high rossby 0 5 and burger numbers o 0 5 1 anticyclones transport convected waters resulting from intermediate 300 m to deep 2000 m vertical mixing cyclones are characterized by a 500 1000 m thick layer of weakly stratified deep waters or bottom waters that cascaded from the shelf of the gulf of lions in 2012 extending down to the bottom of the ocean at 2500 m the formation of cyclonic eddies seems to be favored by bottom reaching convection occurring during the study period or cascading events reaching the abyssal plain we confirm the prominent role of anticyclonic scvs and shed light on the important role of cyclonic scvs in the spreading of a significant amount 30 of the newly formed deep waters away from the winter mixing areas since they can survive until the following winter they can potentially have a great impact on the mixed layer deepening through a local preconditioning effect

spatial clustering and stress drops of foreshocks of the february 1990 tennyson and weber new zealand earthquakes the ability to distinguish foreshocks from background seismicity is very important in short term earthquake prediction to that end we have looked at spatial clustering using waveform cross correlation and stress drops of foreshocks of two new zealand earthquake sequences that occurred in 1990 the tennyson sequence located in a continental margin type strike slip environment consisted of a group of foreshocks an ml 5 8 mainshock and many aftershocks a cross correlation analysis showed five spatially close clusters of activity prior to the mainshock two were event pairs located within the final aftershock zone two were clusters of four events each located outside the aftershock zone and the fifth was a cluster of eight immediate foreshocks loated within the aftershock zone an analysis of two nearby control regions showed that pairs of identical events were not uncommon but larger clusters were stress drops of three events in the 12 days before the mainshock obtained by deconvolving small events as empirical green s functions were lower than for earlier preshocks and aftershocks source time functions derived from the green s function deconvolution indicated that a unilateral rupture model was more appropriate than a circular source model cross correlation values from the ml 5 9 weber sequence also showed spatial clustering but this was well removed from the mainshock in space and time a control area also showed similar clustering suggesting that it is a normal feature of the seismicity at a convergent margin the weber foreshocks only four in all were not highly correlated for both sequences foreshocks did not correlate with the aftershocks indicating that they occurred in a region of complete coseismic stress relief a stress drop of 1650 bars was obtained for a 44 km deep event that occurred within the upper part of the subducting pacific plate nearby but not related to the tennyson sequence

effect of sill spacing and sediment size grading on scouring at grade control structures sequences of arti cial steps are sometimes used to reproduce the natural step pool morphology of high gradient streams the depth length and shape of the scour holes in gravel bed rivers can be predicted reasonably using recently developed formulae however the properties of the scour holes can sometimes be affected by the distance between structures this effect is called geometrical interference and leads to a reduction of the scour hole compared to its potential size geometrical interference may occur in sequences of arti cial steps in high gradient torrents where structures are sometimes built at distances of a few tens of metres apart but may also apply to natural step pool systems in this paper a series of tests have been conducted to determine the effect of bed sill spacing and sediment grading on the potential erosion by jets forming over the sills a new formula is derived applicable to high gradient streams slope 0 04 which can be applied to the special case of scour holes developed by interfering sills sediment size gradation not accounted for in previous formulae is found to have a signi cant effect on the scour dimensions and is included in the new predictive formula copyright 2004 john wiley sons ltd

affinity and similarity of local scour holes at bed sills 1 excessive erosion of gravel bed rivers represents a hazard which is often controlled by the construction of bed sills shape and volume of equilibrium local scour holes at bed sills in high gradient streams were investigated by analyzing steady flow experimental data scour profiles were normalized by scaling the depths and the longitudinal distance from the upstream sill with the maximum dimensions of the scour hole results exhibited a universal geometrical affinity of the scour geometrical similarity was also found to occur but only in a limited range of physical conditions evidence on the scaling nature of the scour hole shapes has important implications on the prediction of the eroded volume in turn a correct evaluation of the eroded volume is necessary for the optimization of the design of the intersill distance prediction formulae are proposed which link the eroded volume to the geometrical hydraulic and sediment properties

sediment supply and local scouring at bed sills in high gradient streams 1 this paper presents the results of an experimental study about the pattern of local scouring generated at the toe of bed sills which are commonly built in steep gravel bed mountain streams the aim of this study is to evaluate the effect of upstream sediment supply on the scour depth and shape the experiments simulated conditions of a steady upstream sediment supply which had the same grain size composition as the sediment deposit placed on the bed of the flume the geometric characteristics of three scour holes developed under conditions of steady flow discharge and steady upstream sediment supply were measured during 48 different tests this provided a data set containing measurements of 144 scour holes three different sediment feed rates were used and tests were also performed with clear water conditions geometric interference between sills was not observed a natural river gravel with a limited grain size was used the collected data were analyzed using nondimensional groups developed in previous clear water studies it is shown that the imposed sediment transport does not require the introduction of new dimensionless parameters into existing scour depth and length prediction formula the effects of sediment feed are shown to be incorporated in the existing dimensionless parameters the new data set was used to recalibrate existing scour depth formula the influence of jet erosion on scour geometry is also discussed inspection of the data indicates a possible two stage behavior with the threshold between behaviors related to the ratio of the value of the morphological jump created at a scour hole and the flow specific energy at the bed sill

sediment transport through self adjusting bedrock walled waterfall plunge pools many waterfalls have deep plunge pools that are often partially or fully filled with sediment sediment fill may control plunge pool bedrock erosion rates partially determine habitat availability for aquatic organisms and affect sediment routing and debris flow initiation currently there exists no mechanistic model to describe sediment transport through waterfall plunge pools here we develop an analytical model to predict steady state plunge pool depth and sediment transport capacity by combining existing jet theory with sediment transport mechanics our model predicts plunge pool sediment transport capacity increases with increasing river discharge flow velocity and waterfall drop height and decreases with increasing plunge pool depth radius and grain size we tested the model using flume experiments under varying waterfall and plunge pool geometries flow hydraulics and sediment size the model and experiments show that through morphodynamic feedbacks plunge pools aggrade to reach shallower equilibrium pool depths in response to increases in imposed sediment supply our theory for steady state pool depth matches the experiments with an r 2 value of 0 8 with discrepancies likely due to model simplifications of the hydraulics and sediment transport analysis of 75 waterfalls suggests that the water depths in natural plunge pools are strongly influenced by upstream sediment supply and our model provides a mass conserving framework to predict sediment and water storage in waterfall plunge pools for sediment routing habitat assessment and bedrock erosion modeling

numerical simulation of barotropic jets over a sloping bottom comparison to a laboratory model of the northern current 1 in order to study the role of a sloping bottom on the mesoscale variability of the northern current a series of barotropic laboratory experiments was performed on a rotating table setup the present work is devoted to a numerical model which aims to simulate these laboratory conditions in order to further investigate aspects which are difficult to treat in the laboratory the onset of instabilities the influence of the velocity profile the relative position of the jet over the slope and the influence of the jet width on the resulting instabilities are discussed the robustness and the origin of a shelf break topographic rossby wave detected in our westward laboratory flows is also discussed the barotropic model for westward currents such as the northern current with rossby numbers in the range ro 0 1 0 2 predicts wavelengths of 50 75 km similar to those observed in sst images the periods associated with the model instabilities 3 3 3 8 days would be close to a 3 5 day band of mesoscale activity recorded from current meter devices in the gulf of lions which lies out of the traditionally considered baroclinic instability theory other particularities such as the up slope migration of the northern current and the increasing mesoscale activity for increasing jet velocities can also be explained through the present model these results would point out barotropic instability as a factor to consider apart from the traditional idea of baroclinic instability in order to partially explain the northern current mesoscale variability

origin and dynamics of mesoscale eddies in the catalan sea nw mediterranean insight from a numerical model study past observations and satellite sea surface temperature imagery indicate the presence of mesoscale anticyclonic eddies drifting along the catalan coast in september 2001 one of these anticyclonic eddies was surveyed over the shelf break during an oceanographic cruise which permitted the 3 d description of its structure in this work we investigate the origin and dynamics of such catalan eddies using a numerical circulation model of the northwest mediterranean at 3 km resolution driven by high resolution atmospheric analyses and compare model eddies with the observations in the catalan sea we identify two zones of eddy formation in the gulf of lions in front of the city of marseille and at the southeast of coast of roussillon from which anticyclonic eddies are observed to drift toward the catalan sea the hydrology and dynamics of the structures observed in the simulations are characterized sensitivity experiments and energy analysis are performed which allow us to identify the mechanisms associated with their generation properties of the eddy observed during the 2001 cruise at the catalan shelf break are found to compare well with model eddies generated at the southeast of the roussillon coast the model relates the origin of these eddies to the separation of the coastal current downstream from creus cape flow separation is linked to intense downwelling taking place in front of the roussillon coast when strong northwesterly winds events occur

channel changes due to river regulation the case of the piave river italy this paper deals with the channel changes of the piave river in the eastern alps italy which have occurred during this century in response to human interventions in the fluvial system the flow regime and the sediment supply of the river have been considerably altered by hydroelectric dams flow diversions and gravel mining in addition river dynamics have been affected by the construction of streambank protection structures to document these changes a historical analysis was performed using maps and aerial photographs morphological features that were examined included planform configuration channel width braiding index and bed elevation the results indicate that as a consequence of decreases in the flows and sediment supply remarkable channel changes have occurred in the river during this century especially during recent decades the channel has undergone a general narrowing with a decrease in average width to 35 per cent of its initial value while the braiding index has decreased from about 3 to 1 5 in several reaches the planform pattern has changed from braided to wandering the observed trends of channel change suggest that the river has not yet reached a new equilibrium condition and it may therefore be predicted that reductions in width and braiding intensity are likely to continue in the immediate future copyright 1999 john wiley sons ltd

local scour upstream of a slit weir ordinary differential equation based model under steady and unsteady flow conditions abstracttime varying scouring upstream of a sediment release gate during a flood needs to be accurately predicted for reducing sedimentation around a water intake this paper presents an ordinary differential equation based model for computing time variation of scoured volume and maximum scour depth upstream of a slit weir for steady and unsteady flow the model is based on the sediment continuity equation and sediment entrainment volume from a scour hole there are some hypotheses in the model and these hypotheses were examined by carrying out 1 45 runs of scour inception tests 2 36 runs of numerical tests using computational fluid dynamics and 3 33 runs of physical scour tests measuring time variation of scoured topography the model was validated with the scour tests for steady and unsteady flow conducted in the present study in addition to those reported in different research the model was found to agree well with the data of time variation of the scoured volume and maximum scour depth allo

time variation of scour at abutments a semiempirical model is presented to compute the time variation of scour depth in an evolving scour hole at short abutments abutment length flow depth 1 namely the vertical wall 45 wing wall and semicircular in uniform and nonuniform sediments under a clear water scour condition the methodology developed for computing the time variation of scour depth is based on the concept of the conservation of the mass of sediment considering the primary vortex system as the main agent of scouring and assuming a layer by layer scouring process for an equilibrium scour hole the characteristic parameters affecting the nondimensional equilibrium scour depth scour depth abutment length identified based on the physical reasoning and dimensional analysis are excess abutment froude number flow depth abutment length ratio and abutment length sediment diameter ratio experiments were conducted for time variation and equi librium scour depths at different sizes of vertical walls 45 wing walls and semicircular abutments in uniform and nonuniform sediments under limiting clear water scour conditions approaching flow velocity nearly equal to the critical velocity for bed sediments the present model corresponds closely with the data of time variation of scour depth in uniform and nonuniform sediments obtained from the present experiments and reported by different investigators

low frequency waves in the ligurian sea during december 1977 observations of low frequency waves in the ligurian sea in december 1977 are presented from time series of thermal infrared images obtained by satellite noaa 5 the mean wavelength and phase velocity are estimated they are respectively 38 km and 18 cm s 1 these waves are analyzed as large amplitude baroclinic waves fairly good agreement is found with a two layer model

the behavior of jet currents over a continental slope topography with a possible application to the northern current abstract the northern current is a slope current in the northwest mediterranean that shows high mesoscale variability generally associated with meander and eddy formation a barotropic laboratory model of this current is used here to study the role of the bottom topography on the current variability for this purpose a source sink setup in a cylindrical tank placed on a rotating table is used to generate an axisymmetric barotropic current to study inviscid topographic effects experiments are performed over a topographic slope and also over a constant depth setup the latter being used as a reference for the former with the aim of obtaining a fully comprehensive view of the vorticity balance at play the flow may be forced in either azimuthal direction leading to a westward prograde current similar to the northern current or an eastward retrograde current for slow flows eastward and westward currents showed similar patterns dominated by kelvin helmholtz type instabilities for high speed flo

scouring at bed sills as a response to flash floods the temporal development of clear water local scour depth at bed sills in uniform gravel beds is considered experiments are presented on the development of scour holes under unsteady hydraulic conditions with the triangular shaped hydrographs tested being of different durations and different rates of flow variation based on the experimental results and a theoretical framework a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter a flash flood is here defined as an event for which the scour doesn t attain its potential magnitude i e the equilibrium value for the peak hydrograph flow rate this flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream a quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter results show that the ratio between the final scour depth and the potential scour depth

scour below a high vertical drop experimental results on scour below a high vertical drop drop height critical depth 1 in uniform sands and gravels are presented the experimental results are used to describe the effects of important parameters identified from the dimensional analysis on equilibrium scour depth the important observations are that the equilibrium scour depth increases with increase in densimetric froude number whereas the scour depth decreases with increase in sediment size and tailwater depth the time scale of scour depth that follows an exponential law is determined the nondimensional time scale decreases with increase in densimetric froude number

modelling time varying scouring at bed sills in this paper a modelling approach is presented to predict local scour under time varying flow conditions the approach is validated using experimental data of unsteady scour at bed sills the mode

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

erosion by planar turbulent wall jets new scaling laws are presented for the spatial variation of the mean velocity and lateral extent of a two dimensional turbulent wall jet flowing over a fixed rough boundary these scalings are analogous to those derived by wygnanski et al 1992 for the flow of a wall jet over a smooth boundary they reveal that the characteristics of the jet depend weakly upon the roughness length associated with the boundary as confirmed by experimental studies rajaratnam 1967 these laws are used in the development of an analytical framework to model the progressive erosion of an initially flat bed of grains by a turbulent jet the grains are eroded if the shear stress exerted on the grains at the surface of the bed exceeds a critical value which is a function of the physical characteristics of the grains after the wall jet has been flowing for a sufficiently long period the boundary attains a steady state in which the mobilizing forces associated with the jet are insufficient to further erode the boundary the steady state profile is calculated separately by applying critical conditions along the bed surface for the incipient motion of particles these conditions invoke a relationship between the mobilizing force exerted by the jet the weight of the particles and the local gradient of the bed use of the new scaling laws for the downstream variation of the boundary shear stress then permits the calculation of the shape of the steady state scour pit the predicted profiles are in good agreement with the experimental studies on the erosive action of submerged water and air jets on beds of sand and polystyrene particles rajaratnam 1981 the shape of the eroded boundary at intermediate times before the steady state is attained is elucidated by the application of a sediment volume conservation equation this relationship balances the rate of change of the bed elevation with the divergence of the flux of particles in motion the flux of particles in motion is given by a semi empirical function of the amount by which the boundary shear stress exceeds that required for incipient motion hence the conservation equation may be integrated to reveal the transient profiles of the eroded bed there is good agreement between these calculated profiles and experimental observations rajaratnam 1981

general patterns of circulation sediment fluxes and ecology of the palamos la fonera submarine canyon northwestern mediterranean abstract currents particle fluxes and ecology were studied in the palamos submarine canyon also known as the fonera canyon located in the northwestern mediterranean seven mooring arrays equipped with current meters and sediment traps were deployed along the main canyon axis on the canyon walls and on the adjacent slope additionally local and regional hydrographic cruises were carried out current data showed that mean near surface and mid depth currents were oriented along the mean flow direction ne sw although at 400 and 1200 m depth within the canyon current reversals were significant indicating a more closed circulation inside the canyon mean near bottom currents were constrained by the local bathymetry especially at the canyon head the most significant frequency at all levels was the inertial frequency a second frequency of about three days attributed to a topographic wave was observed at all depths suggesting that this wave was probably not trapped near the bottom the current field observed during the most complete survey revealed a meandering pattern with cyclonic vorticity just upstream from and within the canyon the associated vertical velocity ranged between 10 and 20 m day and was constrained to the upper 300 m this latter feature together with other computations suggests that during this survey the meander was not induced by the canyon but by some kind of instability of the mean flow in the canyon suspended sediment concentration downward particle fluxes chlorophyll and particulate c and n were significantly higher up canyon from about 1200 m depth than offshore defining along with the different hydrodynamics two canyon domains one from the canyon head to about 1200 m depth more affected by the canyon confinement and the other deeper than 1200 m depth more controlled by the mean flow and the shelf slope front the higher near bottom downward total mass fluxes were recorded in the canyon axis at 1200 m depth along with sharp turbidity increases and are related to sediment gravity flows during the deployment period the increase in downward particle fluxes occurred by mid november when a severe storm took place on the canyon walls at 1200 m depth suspended sediment concentrations downward particle fluxes chlorophyll and particulate c and n were higher on the southern wall than on the northern wall inversely to the current s energy this could be caused by an upward water supply on the southern canyon wall and or the mean flow interacting with the canyon bathymetry in the swimmers collected by the sediment traps the dominant species was an elasipod holothurian which has not been recorded in other canyons or elsewhere in the mediterranean indicating particular speciation

disturbance caused by bed sills on the slopes of steep streams live bed tests on steep bed slopes are run in a laboratory flume to investigate the slope of equilibrium the tests simulate a general degradation until a new equilibrium is reached subsequently different bed sill settings are tested as a countermeasure against degradation evidence is presented that in some cases the bed slope with sills is milder than the one without sills under the same flow and sediment rates this fact implies that the efficiency of the bed sill system to stop degradation decreases as the sills are placed closer together tests are on the verge of sheet flow and friction factors are heavily dependent on sediment transport reaches in uniform and gradually varied flow behave very differently bed profiles are not straight and grain size is linked to this fact other detailed information on testing equipment experimental program time development and local scour in a total number of 111 tests is given

interaction of a coastal current with a gulf application to the shelf circulation of the gulf of lions in the mediterranean sea abstract the interaction of a coastal current with a shallow shelf is studied using analytical model and numerical simulations first the wave propagations along a steplike bottom topography in a two layer fluid are investigated the authors focus on the case in which the interface between the two layers is deeper than the shelf depth and show analytically that two waves propagate along the shelf break one is represented by a barotropic baroclinic double kelvin wave the other by a baroclinic wave trapped at the shelf break the velocity of the latter is proportional to the difference between the depth of the shelf and the depth of the upper layer off the shelf a primitive equation model was also used to study more realistic configurations 1 with an idealized shallow gulf and 2 with a realistic topography like that of the gulf of lions it is shown that the steady component of the coastal current follows the shelf break a transient cross shelf transport can be induced by a gulf scale gyre associated

sub inertial wave boundary turbulence in the gulf of valencia the bottom boundary layer above sloping topography can be highly turbulent even in deep seas this is demonstrated here using high resolution 1 hz sampling temperature sensors that were moored for 5 months every 0 5 m between 6 5 and 61 m above a 572 m deep seafloor promontory on the continental slope off valencia spain using these data turbulence parameters have been estimated with time and in the vertical values vary over four orders of magnitude they have a dominant local inertial period which is modulated by an about 11 day periodicity associated with variations in a baroclinic unstable boundary current when this current is strong and eastward the upslope phase of inertial wave generates convective turbulence which reaches closest to the bottom and therefore can effect sediment dispersal in late winter equally strong shear induced turbulence in 50 m high kelvin helmholtz k h overturns is forced by 0 2 m s 1 off downslope motions which are preceded by periods of warming of a few 0 01 degrees c before the cooler near bottom water is suddenly flushed over the promontory into the basin such anomalously large k h overturns occurred 6 times in the investigated winter period

analysis of small scale gravel bed topography during armoring 1 in evaluating the resistance of sediment particles to entrainment by the action of the flow in a river the grain geometry is usually characterized using representative sizes this approach has been dictated initially by lack of physical insight but more recently by the lack of analytical tools able to describe the 3 d nature of surface grain organization on water worked sediment beds laboratory experiments are presented where mixed grain size beds were mobilized under a range of hydraulic and sediment input conditions detailed bed topography was measured at various stages statistical tools have been adopted which describe the degree of surface organization on water worked sediment bed surfaces the degree of particle organization and the bed stability can be evaluated in relative terms using the properties of the probability density distribution of the bed surface elevations and in absolute terms using a properly defined 2 d structure function the methods described can be applied directly to natural water worked surfaces given the availability of appropriate bed surface elevation data sets

local scour due to submerged horizontal jet the phenomena of local scour and sediment transport due to a horizontal jet issuing from a sluice and flowing over a rigid apron then on to an erodible bed have been investigated based on experimental data the scour characteristics have been correlated through the development of empirical expressions for the time to reach equilibrium stage the locations of maximum scour depth and peak of dune and the variation of maximum scour depth with time similarity criteria for the predication of development of the scour hole and sediment transport have also been developed and apart from the flow parameters and sediment properties the jet diffusion characteristics play an important role in these processes the empirical relationships developed in this study highlight the necessity of an individual assessment when developing the similarity criteria for a particular flow situation

modeling the intense 2012 2013 dense water formation event in the northwestern mediterranean sea evaluation with an ensemble simulation approach the northwestern mediterranean sea is a well observed ocean deep convection site winter 2012 2013 was an intense and intensely documented dense water formation dwf event we evaluate this dwf event in an ensemble configuration of the regional ocean model nemomed12 we then assess for the first time the impact of ocean intrinsic variability on dwf with a novel perturbed initial state ensemble method finally we identify the main physical mechanisms driving water mass transformations nemomed12 reproduces accurately the deep convection chronology between late january and march its location off the gulf of lions although with a southward shift and its magnitude it fails to reproduce the western mediterranean deep waters salinification and warming consistently with too strong a surface heat loss the ocean intrinsic variability modulates half of the dwf area especially in the open sea where the bathymetry slope is low it modulates marginally 3 5 the integrated dwf rate but its increase with time suggests its impact could be larger at interannual timescales we conclude that ensemble frameworks are necessary to evaluate accurately numerical simulations of dwf each phase of dwf has distinct diapycnal and thermohaline regimes during preconditioning the mediterranean thermohaline circulation is driven by exchanges with the algerian basin during the intense mixing phase surface heat fluxes trigger deep convection and internal mixing largely determines the resulting deep water properties during restratification lateral exchanges and internal mixing are enhanced finally isopycnal mixing was shown to play a large role in water mass transformations during the preconditioning and restratification phases

evolution of scouring process downstream of grade control structures under steady and unsteady flows for many incised channels one of the most common strategies is to install some hard structures such as grade control structures gcss in the riverbed to resist further incision in this study a series of experiments including both steady and unsteady flow conditions were conducted to investigate the scouring process downstream of a gcs three distinct phases including the initial developing and equilibrium phases during the evolution of scour holes were identified in addition a semi empirical method was proposed to predict the equilibrium scour hole profile for the scour countermeasure design in general the comparisons between the experimental and simulated results are reasonably consistent as the studies on temporal variation of the scour depth at gcss caused by floods are limited the effect of flood hydrograph shapes on the scour downstream of gcss without upstream sediment supply was also investigated experimentally in this study based on the dimensional analysis and the concept of superposition a methodology is proposed to simulate the time evolution of the maximum scour depth downstream of a gcs for steady flows moreover the proposed scheme predicts reasonably well the temporal variations of the maximum scour depth for unsteady flows with both single and multiple peak copyright 2012 john wiley sons ltd

time scale for local scour at bridge piers the temporal development of clear water local scour depth at cylindrical bridge piers in uniform sand beds is considered new data are presented and used to quantify the influence of flow duration on the depth of local scour an equilibrium time scale t is defined the data show that both t and the equilibrium scour depth dse are subject to similar influences of flow and sediment parameters as might be expected because they are inherently interdependent a method is given for determination of the time for development of dse for a given pier sediment and approach flow velocity and the concomitant estimation of the scour depth at any stage during development of the equilibrium scour hole the results show that the scour depth after 10 of the time to equilibrium is between about 50 and 80 of the equilibrium scour depth depending on the approach flow velocity of local scour d se is rapidly attained in live bed conditions but rather more slowly in clear water conditions fig 1 clear water scour occurs for mean flow velocities up to the threshold velocity for bed sediment entrainment i e v vc while live bed scour occurs for v vc the maximum equilibrium scour depth dse max occurs at v vc in armored cobble or cohesive sediment bed streams multiple flood events may be required before the maximum clear water scour is reached this may take many years the equilibrium scour depth in live bed con ditions fluctuates due to the effects of bed form migration the dashed lines in fig 1 represent the temporal average scour depth under live bed conditions the diagram also shows the time taken te for the equilibrium scour depth to develop the equilibrium time te is the focus of this paper it increases rapidly with flow velocity under clear water conditions but then decreases rapidly for live bed scour existing equations for depth of local scour at bridge piers give the equilibrium depth and are therefore conservative re garding temporal effects for the live bed conditions that typ ically pertain in floods equilibrium scour depths are appro priate however where clear water scour conditions exist the equilibrium depth of scour may be overly conservative peak flood flows may last only a number of hours or a few days in the field and short floods have insufficient time to generate equilibrium depths for example bridge piers situated on the floodplain may be wet for periods of less than one day during a flood typically clear water conditions pertain at such sites the actual scour may be only a small fraction of the equilib rium scour depth which could take weeks to fully develop johnson and mccuen 1991 developed an analytical model to simulate the temporal process of local scour at piers the model was applied to a hypothetical bridge pier using a gen erated sequence of flood flows over a 75 year period the in dividual storms being of 24 h duration at the end of the 75 year period the scour depth was still increasing

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

internal wave turbulence in the llobregat prodelta nw mediterranean under stratified conditions a mechanism for sediment waves generation abstract an array of 76 high resolution temperature sensors at 0 5 m intervals between 5 and 42 5 m off the bottom was moored near the barcelona harbor buoy in 81 m water depth between october 2013 and april 2014 the mooring was located just seaward of an extensive sediment wave area developed in the llobregat river prodelta with 1 m high crests parallel to the coast and 50 100 m wavelengths in the nw mediterranean the thermal stratification reaches its maximum penetration through the water column in autumn until it is broken by winter convection such a deep stratification affects large scale sub inertial slope currents which are mostly confined to the upper half of the water column by the hampered vertical exchange of frictional turbulence and supports near bottom internal waves between the inertial and buoyancy frequencies observed onshelf propagating frontal bores most likely interact with the sediment waves and contribute to their generation as they are trailed by considerable shear induced turbulence and high frequency internal waves close to the buoyancy frequency that have wavelengths matching those of the sediment waves the bores are either driven by near inertial or 3 7 day periodic sub inertial motions just following a brief period of large convective instability at the end of the offshelf flow phase