reach scale sediment transfers an evaluation of two morphological budgeting approaches this paper compares two approaches used to derive measures of annual sediment transfers within a 1 km long piedmont reach of the gravel bed river coquet in northumberland northern england the techniques utilize i channel planform and cross section surveys based on a theodolite electronic distance measurement edm survey of 21 monumented channel cross sections and channel and gravel bar margins and ii theodolite edm survey generating a series of x y z coordinates from which digital elevation models dems of the reach were constructed calculating the difference between dem surfaces provided a measure of volumetric change between surveys carried out during the spring of 1999 and 2000 the use of kriging in dem generation and differencing permits computation of estimate variances and confidence intervals for sediment transfer error analysis validating the dems using surveyed cross sections indicated a mean error between surveyed and dem generated cross sections of around twice the value of the d50 of the surface sediment in the reach comparison of sediment volumes derived from the two approaches suggests that compared with the dem method monumented cross sections underestimate the magnitude of volumetric changes that occur within the reach the cross section approach relies on a simplistic integration of the volumes whereas dem differencing provides an estimate at a resolution under the control of the analyst furthermore the cross section approach does not permit a reliable estimate of the uncertainty of the volumes calculated in addition the dem methodology based on the morphological unit scale provides an explicit identification of spatial patterns of erosion and deposition a feature that cross section based approaches may fail to include copyright 2003 john wiley sons ltd

stabilization of a gravel channel by large streamside obstructions and bedrock bends jacoby creek northwestern california jacoby creek bed width 12 m bankfull discharge 32 6 m 3 s contains stationary gravel bars that have forms and positions controlled by numerous large streamside obstructions bedrock outcrops large woody debris and rooted bank projections and bedrock bends bank projection width and bar volume measured in 104 channel segments 1 bed width long are significantly cross correlated at lags of 1 3 and 4 indicating the tendency for large obstructions and bends to form bars 3 to 4 bed widths downstream and 1 bed width upstream all of the 18 bars downstream of large obstructions or bends in the study reach were along the obstruction side of the channel or outside bank of the bend most of the pools 85 were next to large obstructions or in bends conversely 92 of large obstructions or bends had pools comparison of the volume of four bars with volumetric bar changes and volume of bedload transported during four high flow events suggests that rates of sediment transport were sufficient to cause major changes in bars during bankfull events the only important channel changes observed in 4 yr however have been associated with the movement of large woody debris and with changes in the angle at which the flow approaches an obstruction a general model is proposed that large obstructions and nonalluvial bends stabilize the form and location of gravel bars bars are stabilized by two related mechanisms 1 large obstructions and bends cause intense quasi steady secondary circulation in scour holes that terminate upstream bars at fixed locations obstruction width channel deflection scour hole width and bed width were measured at 26 obstructions these data show that obstructions wider than approximately one third of the bed form pools spanning the entire channel and thus terminating bars smaller obstructions form scour holes contained within a single bar 2 bars are deposited upstream of large obstructions and sharp bends because of backwater reductions in stream power bars are deposited downstream because flow energy is expended around obstructions and bends and because the flow expands downstream of constrictions that result from large obstructions the formation of bars and pools inherent in many gravel channels can thus be enhanced and fixed in position by flow structures set up around large obstructions and bends formed of resistant materials

large scale dam removal on the elwha river washington usa river channel and floodplain geomorphic change abstract a substantial increase in fluvial sediment supply relative to transport capacity causes complex large magnitude changes in river and floodplain morphology downstream although sedimentary and geomorphic responses to sediment pulses are a fundamental part of landscape evolution few opportunities exist to quantify those processes over field scales we investigated the downstream effects of sediment released during the largest dam removal in history on the elwha river washington usa by measuring changes in riverbed elevation and topography bed sediment grain size and channel planform as two dams were removed in stages over two years as 10 5 million t 7 1 million m3 of sediment was released from two former reservoirs downstream dispersion of a sediment wave caused widespread bed aggradation of 1 m greater where pools filled changed the river from pool riffle to braided morphology and decreased the slope of the lowermost river the newly deposited sediment which was finer than most of the pre dam removal bed formed new bars largely pebble granule and sand material prompting aggradational channel avulsion that increased the channel braiding index by almost 50 as a result of mainstem bed aggradation floodplain channels received flow and accumulated new sediment even during low to moderate flow conditions the river system showed a two to tenfold greater geomorphic response to dam removal in terms of bed elevation change magnitude than it had to a 40 year flood event four years before dam removal two years after dam removal began as the river had started to incise through deposits of the initial sediment wave 1 2 million t of new sediment 10 of the amount released from the two reservoirs was stored along 18 river km of the mainstem channel and 25 km of floodplain channels the elwha river thus was able to transport most of the released sediment to the river mouth the geomorphic alterations and changing bed sediment grain size along the elwha river have important ecological implications affecting aquatic habitat structure benthic fauna salmonid fish spawning and rearing potential and riparian vegetation the response of the river to dam removal represents a unique opportunity to observe and quantify fundamental geomorphic processes associated with a massive sediment influx and also provides important lessons for future river restoration endeavors

transport and entrainment in the red sea outflow plume the red sea outflow exhibits strong seasonal variability in outflow transport due to effects of monsoon winds and seasonal fluctuations in buoyancy forcing as it descends the continental slope in the western gulf of aden it entrains significantly less dense near surface water which itself varies on seasonal time scales high resolution hydrographic and direct velocity data collected during the 2001 red sea outflow experiment redsox are used herein to characterize and quantify the pathways of the red sea outflow water rsow and the associated entrainment of gulf of aden water the outflow transport exhibits a maximum in winter of about 0 29 sv sv 10 6 m 3 s 1 at the exit of the bab el mandeb and approximately doubles to 0 56 sv as it descends into the gulf of aden and entrains ambient water in summer the outflow is much weaker reaching about 0 06 sv at the strait and about 0 18 sv downstream the outflow plume divides into three distinct branches in winter consisting of descending branches along two bathymetrically confined channels the northern and southern channels respectively and an adjusted intrusion layer at shallower depths in the water column estimates of transport of pure red sea outflow water through salt flux conservation show the general partitioning of the outflow between the individual plumes where the northern channel nc accounts for 52 of red sea outflow water the southern channel sc carries 31 and the intrusion layer il the remaining 17 the results also indicate that the transport of red sea outflow water is subject to considerable synoptic temporal variability that is unresolved by the present study

mixing and turbulence in a flooding coastal river abstractan experimental study of turbulence and vertical mixing was carried out in the lower east pearl river mississippi and louisiana during a high discharge period in february and march of 2010 direct estimates were made of the time varying profile of turbulent reynolds stresses x u w through the water column from this both the vertical eddy viscosity km and mean vertical mixing time were calculated and the water column was shown to be vertically well mixed at all times segmenting the stresses into stresses due to river discharge and stresses due to tidal flows shows the coequal nature of each component and highlights the ways they vary in importance during different flow regimes two simple turbulence models are examined and compared to the measured mixing parameters

gravity and convection in a two layer mantle abstract one of the current models of mantle convection assumes that the circulation takes place in two superimposed layers separated by the 700 km discontinuity the present study reports two dimensional numerical experiments carried out to investigate the structural features and gravitational consequences of layered convection the models have two constant viscosity layers heated from below the main variable is the viscosity ratio other free parameters are the layer depths and the over all rayleigh number as the viscosity of the lower layer is increased with respect to that of the upper layer the flow structure shifts gradually from the dominance of viscous coupling of the layers towards thermal coupling viscous coupling prevails when the viscosity ratio is near to one and produces pairs of cells which turn oppositely in the two layers thermal coupling becomes predominant if the viscosity ratio is several hundred or more in a series of models with a depth ratio 1 2 thermal coupling leads to pairs of cells which turn in the same sense in the two layers during the transition between the extremes two or more small rolls appear in the thinner upper layer over one cell of the lower fluid gravity profiles and boundary deformations have been calculated for each model the transfer function admittance of gravity over top surface topography shows a characteristic variation with the viscosity ratio in a series with the depth ratio 1 4 negative values of the admittance are obtained when the lower viscosity is 10 50 times higher than the upper one negative admittance is a typical feature of the subcontinental convection operating behind subduction zones two layer models of this type of convection are able to explain not only the sign of admittance but also the magnitude of observed gravity anomalies

a comparison of past small dam removals in highly sediment impacted systems in the u s abstract the ability to predict the effects of dam removal in highly sediment filled systems is increasingly important as the number of such dam removal cases continues to grow the cost and potential impacts of dam removal are site specific and can vary substantially depending on local conditions of specific concern in sediment impacted removals is the volume and rate of reservoir deposit erosion the complexity and potential accuracy of modeling methods used to forecast the effects of such dam removals vary substantially current methods range from predictions based on simple analysis of pre dam channel geometry to sophisticated data intensive three dimensional numerical models in the work presented here we utilize data collected from past dam removals to develop an additional tool for predicting the rate and volume of sediment deposit erosion through the analysis of sediment discharge deposit removal timeline channel and watershed data in conjunction with post removal monitoring data from a wide range of dam removal projects some significant trends in the evolution of reservoir deposits following dam removal can be seen results indicate that parameters such as median grain size level of cohesion spatial variability of the deposit and removal timeline are among the most influential factors in determining the rate and volume of sediment erosion by comparing local conditions of dams and reservoirs slated for removal with those of past removals we hope that predictions of the rate and volume of sediment deposit erosion can be usefully constrained

direct observations of seasonal exchange through the bab el mandab strait the exchange flow between the red sea and the gulf of aden indian ocean through the bab el mandab strait was measured continuously for 10 months june 1995 march 1996 adcp and temperature salinity chain moorings allow an unprecedented look at the magnitude and seasonal evolution of the inflow layer from the gulf of aden and the high salinity outflow layer from the red sea the timing structure and evolution of the summer season mid depth intrusion of cold low salinity water into the red sea from the gulf of aden is measured for the complete intrusion cycle of 1995 we unexpectedly find the deep outflow still strong in june 1995 with speeds of 0 6 m sec and transport of 0 4 sv 1 sv 106 m sec from july to mid september the deep outflow persists but is attenuated to speeds of 0 2 m sec and transport of 0 05 sv the dominant summer feature the cold low salinity intermediate layer intrusion persists for 3 months occupies 70 of the water column in the strait and carries approximately 1 7 1012 m of cold nutrient rich water into the red sea the winter regime begins in mid september is fully developed by early november and continues to the end of our first observation interval in march 1996 speeds in the lower layer are 0 8 1 0 m sec and 0 4 0 6 m sec in the upper layer at maximum exchange in mid february outflow transport reaches 0 7 sv ubiquitous oscillations in current and salinity at synoptic and intraseasonal periods appear closely related to fluctuations in the along channel wind forcing and perhaps to coastally trapped waves

mixing and entrainment in the red sea outflow plume part i plume structure when the salty and heavy water of the red sea exits from the strait of bab el mandeb it continues downslope into the gulf of aden mainly along two channels the 130 km long northern channel nc is topographically confined and is typically only 5 km wide in it the red sea plume shows unanticipated patterns of vertical structure turbulent mixing and entrainment above the seafloor a 25 120 m thick weakly stratified layer shows little dilution along the channel hence this bottom layer undergoes only weak entrainment in contrast a 35 285 m thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing it is thus the interface that exhibits the bulk of entrainment of the red sea plume in the nc the interfacial layer also carries most of the overall plume transport increasingly so with downstream distance the southern channel sc is wider than the nc and is accessed from the latter by a sill about 33 m above the floor of the nc entrainment into the bottom layer of the sc is diagnosed to be strong near the entry into the sc such that the near bottom density and salinity are smaller in the sc than in the nc at the same distance from bab el mandeb in comparison with winter conditions the authors encountered weaker outflow with shallower equilibration depths during the summer cruise bulk froude numbers computed for the whole plume varied within the range 0 2 1 local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment

channel adjustments following two dam removals in wisconsin 1 we examined channel response following the removal of low head dams on two lowgradient fine to coarse grained rivers in southern wisconsin following removal channels eroded large quantities of fine sediment resulting in deposition 3 5 km downstream at one site baraboo river upstream changes were rapid and included bed degradation minimal bank erosion and sediment deposition on channel margins and new floodplain sand was transported through the former impoundment and temporarily deposited downstream at the second site koshkonong river head cut migration governed channel adjustments a deep narrow channel formed downstream of the headcut with negligible changes upstream of the head cut fluvial changes were summarized in a conceptual channel evolution model that highlighted 1 similarities between adjustments associated with dam removal and other events that lower channel base level and 2 the role of reservoir sediment characteristics particle size cohesion in controlling the rates and mechanisms of sediment movement and channel adjustment index terms 1815 hydrology erosion and sedimentation 1824 hydrology geomorphology 1625 1857 hydrology reservoirs surface keywords dams dam removal channel evolution model

mixing at camarinal sill in the strait of gibraltar during the gibraltar experiment in 1985 1986 observations of shear stratification microstructure and acoustic backscatter revealed a complex mixing regime at camarinal sill in the strait of gibraltar the mixing is forced by strong semidiurnal tidal currents and weaker but more steady baroclinic flows flows are characterized as outflow or inflow nearly equivalent to westward or eastward rising water at gibraltar coincides with outflow at all depths we observed two modes of outflow at the sill in may 1986 the transition layer separating atlantic and mediterranean water was between 0 75 mpa and 1 mpa when we sampled east of the sill near neap tide the transition turned down just before reaching the sill crest where the flow became critical increasing shear soon destabilized the transition producing billows which grew to 30 75 m on the west flank of the sill the overturning generated intense turbulence with peak dissipation rates e exceeding 10 2 w kg 1 this turbulence rapidly thickened the transition from z 30 50 m east of the sill to z 130 150 m within 1 2 km west of the sill from the rate of thickening we estimate the rate of formation of new transitional water during neap tide as 0 8 1 2 105 m3 s 1 this outflow mode did not release eastward propagating internal bores when outflow turned to inflow but the inflow did appear to arrest the deep outflow in may the largest average dissipation rate over and just west of the sill had the same magnitude as estimates of the net energy lost by adjustment of velocity and mass across the sill during outflow other averages are smaller than the energy loss presumably because the limited sampling severely underestimates the average dissipation rate during outflow strong shears were observed to rise to 100 times those in the background internal wave field however probably owing to spatial offsets between the ship s acoustic doppler current profiler and our tethered free fall profiler the only statistically significant correlations were found between overturning scales measured directly and those estimated from e in the same profile we observed the other outflow mode in october 1985 when we profiled over the sill near spring tide and the transition was east of the sill centered near 1 5 mpa after encountering the sill the transition rose sharply and remained within 10 20 m of the surface across the sill plunging steeply downward just past the west edge of the sill this was accompanied by intense turbulence and at least one rebound of the transition we did not observe release of eastward propagating bores when inflow turned to outflow but observations by armi and farmer 1988 in april 1986 found bores released at spring tide in regimes similar to the one we observed in october 1985

observations of stratified turbulent mixing in an estuary neap to spring variations during high river flow abstract observations of turbulent mixing stratification and currents were made with a free falling microstructure profiler and a 600 khz adcp in the hudson river off manhattan in may 1994 the ship was anchored in a relatively straight and uniform channel about 15 m deep over 4 days of intense observations from 19 to 24 may 1994 the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top to bottom salinity difference of 18 to 3 6 while low gradient richardson numbers ri occurred only in the weakly stratified bottom layer on flood during neap tides low ri spanned the water column on the second half of ebb during spring tides turbulent dissipation rates varied roughly inversely to ri during neap conditions turbulent mixing was intense only in the bottom layer on flood where eddy diffusivities k reached more than 10 2 m 2 s 1 while mixing was weak in the centre of the halocline near 5 m during spring tides strong mixing occurred throughout the water column during the latter part of ebb with typical k 5 10 3 m 2 s 1 and typical vertical salt flux of 0 0015 kg m 2 s 1 if it existed in isolation this flux would eliminate the haline stratification on a time scale of 4 h ebb time bands of low ri and intense mixing showed increasing phase lag with increasing distance from the bottom the spring tide ebb time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0 4 pa 4 dyne cm 2 turbulent overturning thorpe scales ranged from below 1 cm in the neap tide halocline to a few metres in the weakly stratified bottom layer on flood the strong spring tide ebb time mixing had thorpe scales of typically 0 5 m throughout the water column ozmidov scales tracked thorpe scales except in the weakly stratified layer above the bottom

role of the lithosphere in mantle convection plate geometry and kinematics generally reflect the mechanical properties of the solid lithosphere rather than those of the fluid mantle underneath and plate formation and subduction account for most of the heat transport from the earth s interior correspondingly mantle convection models must incorporate a stiff but mobile boundary layer like the lithosphere before they can reproduce the main features of mantle convection a relatively easy way to accomplish this in numerical models is to combine a temperature dependent viscosity with an imposed piecewise constant surface velocity boundary condition it is shown how surface heat flux topography gravity and geoid but not plate velocities or stresses can then be derived numerical models confirm that a lithosphere has a first order effect on the underlying flow structure for internally heated models approx imating the case of whole mantle convection the calculated surface signatures are in first order agreement with observations a level of empirical success which hitherto has not been approached by models of mantle convection companion papers exploit the observations more fully to constrain the main features of mantle convection

grain size sediment transport regime and channel slope in alluvial rivers the general relationship between channel morphology and the grain size of sediment in the channel bed is an important but poorly known aspect of alluvial rivers an analysis of an equation for total sediment flux in the limits of suspension bedload and mixed modes of transport indicates distinct steady state regimes of channel morphology such regimes are readily seen in published data for modern alluvial rivers by way of a conventional shields plot or a plot of channel slope as a function of relative grain size d h and the ratio ws u where d and ws are respectively mean diameter and fall speed of bed sediments and h and u are respectively mean depth and friction velocity of the flow with slope and mode of transport in an alluvial river constrained by grain size and channel depth alone estimates of discharge and sediment flux follow directly introduction of the sediment flux relationship into conventional diffusion models for the evolution of an alluvial system provides nominal estimates of

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

convection experiments and the driving mechanism the problem of mantle convection is discussed in light of laboratory and numerical experiments since wegener s time our knowledge of earth behaviour has increased so that we are in a better position to explain continental drift the explanation of this phenomenon must inevitably include mantle convection the problem has three aspects the physical laws the boundary conditions and the material we find that a likely model is high rayleigh number convection in a medium of temperature pressure and stress dependent rheology in this model we expect a strong and heavy upper thermal boundary layer and a fluid and light lower boundary layer the upper one consists of the lithospheric plates which highly organize the large scale circulation while the lower one at the core mantle boundary becomes unstable in the form of diapirs plumes or blobs in a fashion rather independent of the large scale circulation this model has the potential of reconciling the conflicting views of horizontal and vertical tectonics

three dimensional turbulent bottom density currents from a high order nonhydrostatic spectral element model overflows are bottom gravity currents that supply dense water masses generated in high latitude and marginal seas into the general circulation oceanic observations have revealed that mixing of overflows with ambient water masses takes place over small spatial and time scales studies with ocean general circulation models indicate that the strength of the thermohaline circulation is strongly sensitive to representation of overflows in these models in light of these results overflow induced mixing emerges as one of the prominent oceanic processes in this study as a continuation of an effort to develop appropriate process models for overflows nonhydrostatic 3d simulations of bottom gravity are carried out that would complement analysis of dedicated observations and large scale ocean modeling a parallel high order spectral element navier stokes solver is used as the basis of the simulations numerical experiments are conducted in an idealized setting focusing on the startup phase of a dense water mass released at the top of a sloping wedge results from 3d experiments are compared with results from 2d experiments and laboratory experiments based on propagation speed of the density front growth rate of the characteristic head at the leading edge turbulent overturning length scales and entrainment parameters results from 3d experiments are found to be in general agreement with those from laboratory tank experiments in 2d simulations the propagation speed is approximately 20 slower than that of the 3d experiments and the head growth rate is 3 times as large thorpe scales are 1 3 1 5 times as large and the entrainment parameter is up to 2 times as large as those in the 3d experiments the differences between 2d and 3d simulations are entirely due to internal factors associated with the truncation of the navier stokes equations for 2d approximation

anabranching rivers their cause character and classification anabranching rivers consist of multiple channels separated by vegetated semi permanent alluvial islands excised from existing floodplain or formed by within channel or deltaic accretion these rivers occupy a wide range of environments from low to high energy however their existence has never been adequately explained they occur concurrently with other types of channel pattern although specific requirements include a flood dominated flow regime and banks that are resistant to erosion with some systems characterized by mechanisms to block or constrict channels thereby triggering avulsion the fundamental advantage of an anabranching river is that by constructing a semi permanent system of multiple channels it can concentrate stream flow and maximize bed sediment transport work per unit area of the bed under conditions where there is little or no opportunity to increase gradient on the basis of stream energy sediment size and morphological characteristics six types of anabranching river are recognized types 1 3 are lower energy and types 4 6 are higher energy systems type 1 are cohesive sediment rivers commonly termed anastomosing with low w d ratio channels that exhibit little or no lateral migration they are divisible into three subtypes based on vegetative and sedimentary environment type 2 are sand dominated island forming rivers and type 3 are mixed load laterally active meandering rivers type 4 are sand dominated ridge forming rivers characterized by long parallel channel dividing ridges type 5 are gravel dominated laterally active systems that interface between meandering and braiding in mountainous regions type 6 are gravel dominated stable systems that occur as non migrating channels in small relatively steep basins anabranching rivers represent a relatively uncommon but widespread and distinctive group that because of particular sedimentary energy gradient and other hydraulic conditions operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges

stress on the mediterranean outflow plume part i velocity and water property measurements abstract in september 1988 six sections were occupied across the mediterranean outflow plume in the gulf of cadiz within 100 km of the strait of gibraltar vertical profiles of temperature and salinity were collected at ctd stations velocity and temperature profiles were collected with expendable current profilers at a subset of these stations at the channel base the plume undergoes geostrophic adjustment and turns northwest to flow along the continental slope there it decelerates and spreads gradually down the slope as friction slows the current and allows it to cross isobaths within the plume downstream velocity and density increase rapidly in the interfacial layer with depth to the velocity maximum or nose 5 150 m above the bottom below the nose in the bottom layer downstream velocity decreases rapidly toward the bottom but the stratification is weak ekman like veering occurs in the interfacial layer local bottom stresses on the plume are estimated by fitting the near bottom velocity prof

steady state secondary convection beneath lithospheric plates with temperature and pressure dependent viscosity we have constructed a thermomechanical model for upper mantle convection such that the thickness and structure of the lithosphere are determined self consistently by the heat transported by convection in this study of the interaction between the lithosphere and upper mantle strongly temperature and pressure dependent rheologies for both newtonian and non newtonian creep mechanisms are employed for a strictly temperature dependent rheology an insignificant amount of heat less than 12 5 mw m2 can be transported convectively for an interior viscosity 0 1021 pas compatible with postglacial rebound on the other hand for similar values of the interior viscosity steady heat fluxes between 20 and 40 mw m2 are produced by introducing pressure dependence into the rheology for the temperature and pressure dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated once all of the rheological parameters are fixed this finding may have important ramifications for parameterized convection we employ both the single mode mean field approximation and the complete two dimensional equations using finite elements to obtain solutions for the various types of rheologies from evaluating the geophysically relevant observables such as topography free air gravity anomalies surface heat flow and stress fields in the lithosphere we find that the lateral variations of these quantities predicted by a non newtonian rheology are much smaller than those derived from a linear rheology these results suggest that surface variations of geophysical observables are more compatible with a non newtonian rheology in the upper mantle

patterns of sediment slug translation and dispersion following typhoon induced disturbance oyabu creek kyushu japan a six year monitoring programme characterized the migration dispersion patterns of sediment slugs generated following typhoon induced disturbances in 1993 and 1997 along a single thread gravel bed stream oyabu creek on kyushu island japan this laterally con ned creek comprises rif e pool sequences with intervening bedrock outcrops the passage of sediment pulses associated with sediment slug processes re ected and was controlled by the rif e pool structures which provided channel bed roughness the volume of sediment stored along valley oors and the distribution of bedrock outcrops changes to bed material size following major sediment inputs during the disturbance events also exerted an in uence on subsequent sediment slug processes the sequence of rainfall events together with changes to channel bed structure induced different phases in the sediment slug processes the capacity of a reach to store or trap sediment as recorded by the longitudinal structure of the channel varied during these differing phases copyright 2004 john wiley sons ltd

sediment deposition and transport patterns following a reservoir sediment release a reservoir sediment release on the north fork poudre river supplied 7000 m3 of silt to pebble sized sediment to an originally boulder bed channel deposition along the 12 km of channel downstream from the reservoir occurred primarily in pools during the subsequent snowmelt hydrograph sediment was progressively scoured from the upstream and then the downstream pools initial sediment reworking in the pools created a deep narrow thalweg scoured to the original pool bed with additional sediment deposition in lateral eddies continued reworking reduced but did not completely remove these eddy deposits the channel became supply limited with respect to finer grain sized fractions clay to medium sand first at upstream and then at downstream sites and eventually became supply limited with respect to coarser grain sized fractions coarse sand to pebbles bedload transport rates at a site were strongly linked to the depletion of sediment stored in upstream pools magnitude duration and sequence of flows were all important controls on bedload transport and return of the channel to its prerelease state

large scale dam removal on the elwha river washington usa erosion of reservoir sediment abstract base level lowering of reservoirs impounding upstream sediment supply triggers a series of channel evolution steps such as degradation lateral erosion and redeposition that can dramatically alter the reservoir landscape and decouple the relationship between stream power and sediment supply many case studies exist for small dam removals with a few years of sediment storage or dam breaches triggering instantaneous large sediment releases however quantitative information for a controlled drawdown initiating erosion of a large sediment deposit is rare we investigate reservoir sediment response to the phased and concurrent drawdown of two reservoirs on the elwha river washington usa during the largest dam removal in history by measuring changes in reservoir topography and channel morphology as a function of base level lowering river discharge and cohesion after two years the elwha dam was completely removed and three quarters of glines canyon dam were removed reservoir drawdown increments of 3 to 5 m were sufficient to initiate channel degradation and delta progradation across the width of the receding reservoir redistributing decades of accumulated delta sediment throughout the reservoir while the lake still remained the first year of dam removal resulted in up to 5 m of incision through the lake aldwell delta down to the predam surface and in just over 20 m of incision through the lake mills delta in contrast delta progradation resulted in a few meters of deposition in lake aldwell and 2 to 10 m in lake mills on top of prodelta and lakebed deposits in coarse noncohesive sediment a braided channel developed and widened up to tenfold across the entire width of the reservoir the most extensive lateral erosion occurred in noncohesive deposits during multiweek hold periods coinciding with flows greater than the mean annual flow but less than a 2 year flood peak channel widening in more cohesive fine sediments of the prodelta and lakebed was less than half of that in the coarse noncohesive delta sediments dam removal resulted in the erosion and downstream release of 23 of the sediment in lake aldwell 1 12 0 07 million m3 and 37 of the sediment in lake mills 5 95 0 12 million m3 representing nearly four decades of sediment supply from the upstream watershed within a two year time frame a significant portion of the reservoir sediment is expected to remain as sediment terraces within the reservoir landscape but additional erosion is expected after the remainder of the glines canyon dam is removed and during future floods until the river reaches quasi equilibrium after phased dam removal the reservoir landscape consists of a series of sediment terraces of varying heights composed of prograded coarse sediment overlying fine lakebed deposits the predam surface is exposed along the river corridor and abundant 1 to 3 m stumps from pre removal forests create unique morphology where the river interacts with the predam landscape

morphological responses and sediment processes following a typhoon induced dam failure dahan river taiwan the rates and styles of channel adjustments following an abrupt and voluminous sediment pulse are investigated in the context of site and valley characteristics and time varying sediment transport regimes approximately 10 5 x 106 m3 of stored gravel and sand was exposed when barlin dam failed during typhoon weipa in 2007 the dam was located on the dahan river taiwan a system characterized by steep river gradients typhoon and monsoon driven hydrology high episodic sediment supply and highly variable hydraulic conditions topography bulk sediment samples aerial photos and simulated hydraulic conditions are analyzed to investigate temporal and spatial patterns in morphology and likely sediment transport regimes results document the rapid response of the reservoir and downstream channel which occurred primarily through incision and adjustment of channel gradient hydraulic simulations illustrate how the dominant sediment transport regime likely varies between study periods with sediment yield and caliber and with the frequency and duration of high flows collectively results indicate that information on variability in sediment transport regime valley configuration and distance from the dam is needed to explain the rate and pattern of morphological changes across study periods copyright 2013 john wiley sons ltd

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

high resolution velocity and hydrographic observations of the drygalski trough gravity plume during the second cruise of the anslope project the hydrography and velocity fields within a descending gravity plume in the northwestern ross sea shelf area were sampled at high resolution the observations give insights into small scale fluctuations of the three dimensional velocity field in and around the core of a strong gravity current associated with peak velocities close to 2 m s there are two distinct layers of high acoustic backscatter at 300 khz near the seabed associated with increased velocity variance and richardson numbers near 0 25 suggesting that the high backscatter is caused by increased levels of turbulence resulting in increased acoustic bragg scattering c 2008 elsevier ltd all rights reserved

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

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

translation and dispersion of sediment pulses in flume experiments simulating gravel augmentation below dams 1 the effectiveness of gravel augmentation as a river restoration strategy depends on the extent and duration of the topographic and bed texture changes created by the pulse of added sediment previous work has emphasized the strong tendency for natural sediment waves to propagate primarily by dispersion however pulse translation may occur for gravel additions to armored channels downstream of dams where added sediments are finer than the preexisting bed material here we report results of a laboratory investigation in which we created an immobile armored bed and documented the spatial and temporal evolution of the bed topography and bed texture in response to gravel pulses of various volumes and grain sizes the introduced sediment waves evolved by a combination of translation and dispersion with a significant translational component pulse translation and dispersion can be readily discerned on a graph of the time evolution of the downstream cumulative distribution of elevation differences from the preexisting bed topography translation was most evident for smaller volumes of added sediment pulses of finer grained gravel moved through the flume more rapidly resulting in a larger magnitude but shorter duration of bed fining more work is needed to understand the influence of bar pool topography and flow magnitude and duration before the grain size and volume of gravel additions can be selected to achieve optimal patterns of pulse propagation

a tomographic image of mantle structure beneath southern california we determined the variations in seismic structure beneath southern california by using a tomographic method of inversion on teleseismic p delays recorded with the southern california array the algorithm employed was a modified form of an algebraic reconstruction technique art used in medical x ray imaging deconvolution with an empirically estimated point spread function was also used to help in focusing the image the inversion reveals two prominent features beneath the region the first is a thin vertical wedge directly beneath the transverse ranges that is 2 3 faster than the surrounding region this feature deepens to the east attaining a maximum depth of about 250 km beneath the san bernardino mountains the second feature is a major zone of low velocity material that is 2 4 slow under the salton trough rift valley extending to a depth of about 125 km two possible explanations for the spatial association of the transverse ranges with the velocity anomaly below are lithospheric subduction or small scale sublithospheric convection in the region of the big bend of the san andreas fault the low velocity anomaly beneath the salton trough is consistent with convective upwelling there

the dominance of dispersion in the evolution of bed material waves in gravel bed rivers bed material waves are temporary zones of sediment accumulation created by large sediment inputs recent theoretical experimental and field studies examine factors influencing dispersion and translation of bed material waves in quasi uniform gravel bed channels exchanges of sediment between a channel and its floodplain are neglected within these constraints two factors influence relative rates of dispersion and translation 1 interactions between wave topography flow and bed load transport and 2 particle size differences between wave material and original bed material our results indicate that dispersion dominates the evolution of bed material waves in gravel bed channels significant translation requires a low froude number which is uncharacteristic of gravel bed channels and low wave amplitude which for a large wave can be achieved only after substantial dispersion wave material of small particle size can promote translation but it primarily increases bed load transport rate and thereby accelerates wave evolution copyright 2001 john wiley sons ltd

microstructure observations of turbulent mixing in a partially mixed estuary part ii salt flux and stress abstract turbulent mixing in the water column was observed with a microstructure profiler in the hudson river estuary during two cruises in summer and fall 1995 the focus is on the estimation of turbulent salt flux and turbulent stress from measured viscous dissipation rates e and on the tidal and fortnightly variability of these fluxes in estimating eddy viscosity km and eddy diffusivity k the authors follow measurement modeling techniques of busch and osborn while prescribing a variable flux richardson number rf dependent upon the gradient richardson number ri it is argued that a steady state production dissipation balance holds in the turbulent kinetic energy budget all turbulence characteristics varied strongly over semidiurnal tidal cycles and over the fortnightly cycle subject to complications arising from nontidal flows the strongest mixing occurred during flood on neap tides and during ebb on spring tides in the lower part of the water column during floods and spring ebb km

mantle convection plates and hotspots abstract more than 20 years after their discovery plates and hotspots are still awaiting a consistent dynamic model both imply large viscosity variations in the mantle sustained efforts have been targeted on modeling on computer as well as in the laboratory convection in fluids with temperature and pressure dependent viscosity the main results of these studies are reviewed and summarized in a few simple rules several properties of plates such as ridge push or heat flow distribution are well accounted for by convection models with temperature dependent viscosity however the process of subduction is not properly explained by these models one of the simple rules deduced from the study of convection and temperature dependent viscosity states that the viscosity drop across the lower boundary layer adjusts to a low value less than 10 this constrasts strongly with the viscosity variations required by current hotspot formation models which are typically 100 times larger it is suggested that this paradox can be solved if subduction is properly included in the convection models

equilibration and circulation of red sea outflow water in the western gulf of aden hydrographic direct velocity and subsurface float observations from the 2001 red sea outflow experiment redsox are analyzed to investigate the gravitational and dynamical adjustment of the red sea outflow water rsow where it is injected into the open ocean in the western gulf of aden during the winter redsox cruise when outflow transport was large several intermediate depth salinity maxima product waters were formed from various bathymetrically confined branches of the outflow plume ranging in depth from 400 to 800 m and in potential density from 27 0 to 27 5 a result of different mixing intensity along each branch the outflow product waters were not dense enough to sink to the seafloor during either the summer or winter redsox cruises but analysis of previous hydrographic and mooring data and results from a one dimensional plume model suggest that they may be so during wintertime surges of strong outflow currents or about 20 of the time during winter once vertically equilibrated in the gulf of aden the shallowest rsow was strongly influenced by mesoscale eddies that swept it farther into the gulf the deeper rsow was initially more confined by the walls of the tadjura rift but eventually it escaped from the rift and was advected mainly southward along the continental slope there was no evidence of a continuous boundary undercurrent of rsow similar to the mediterranean undercurrent in the gulf of cadiz this is explained by considering 1 the variability in outflow transport and 2 several different criteria for separation of a jet at a sharp corner which indicate that the outflow currents should separate from the boundary where they are injected into the gulf

convection in a two layer mantle with a strongly temperature dependent viscosity recent geochemical and isotopic data support the hypothesis that the earth s mantle convects in two separate layers with an interface at the 650 km seismic discontinuity this paper studies the implications of this hypothesis with regard to the thermal boundary layers that are expected to develop adjacent to the interface between the convecting layers we are particularly concerned with a possible decrease in the mantle viscosity across this interface in order to study the structure of the thermal boundary layers in a fluid with a strongly temperature dependent viscosity we utilize an approximate method developed by howard 1966 if the upper and lower mantle have the same viscosity law we find that stratified convection requires a lower mantle viscosity which is several orders of magnitude lower than the upper mantle viscosity this is not consistent with studies of postglacial rebound it is possible to obtain a viscosity law for the lower mantle that gives a viscosity nearly equal to that of the upper mantle and reasonable temperatures however the required law is not consistent with the currently available experimental data on lower mantle materials therefore we conclude that thermal arguments favor whole mantle convection

the driving mechanism of plate tectonics abstract plate velocities reflect the global mantle circulation which carries the heat from the deep part of the earth to the surface three other measurable parameters put strong constraints on the physical processes involved in mantle dynamics the surface topography the geoid and the lateral density heterogeneities mapped throughout the mantle by seismic tomography using these internal loads and the navier stokes equations a first generation of dynamic earth models have been developed in the last five years they have produced satisfactory predictions for the divergence of the surface velocities and for the geoid these predictions are quite sensitive to the assumed radial viscosity variations the best match with observations has therefore helped to discriminate between various possible viscosity structures a new set of models taking the existence of plates into account has been particularly successful in predicting the plate velocities including their vorticity the driving mechanism of plate tectonics is thus put on a firm physical basis and the computed solutions are quantitatively reasonable

entrainment in shallow rotating gravity currents a modeling study abstract the physics of shallow gravity currents passing through a rotating channel at subcritical froude number is investigated here with a series of idealized numerical experiments it is found that the combined effects of friction and rotation set up a complex transverse circulation that has some crucial implications for the entrainment process a key component of this secondary circulation is a geostrophically balanced transverse jet in the interface that laterally drains fluid from the interface this effect is shown to result in a strong cross channel asymmetry and a spatial separation of the entrainment process drained interfacial fluid is partly replaced by entrained ambient fluid on the deep side of the gravity current whereas the downward mixing of buoyant fluid occurs on the shallow side these results closely corresponding to recent measurements in a shallow channelized gravity current in the western baltic sea illustrate that the description of entrainment as a strictly vertical mixing p

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

management responses to pulses of bedload sediment in rivers abstract rivers can experience sudden pulses of sediment from human and natural erosion processes that can accumulate in the bed abundant studies have examined the sources and dynamics of sediment pulses and problems caused by these pulses particularly flooding avulsions and habitat simplification much less has been written about what managers can do about sediment pulses and that is the purpose of this review the first option for managers is to do nothing and this decision can be informed by many case studies and by theory on the propagation and character of sediment pulses their diffusion translation and celerity doing nothing should be informed by the secondary effects of sediment pulses on channels including widening avulsions and tributary interactions if managers decide that something needs to be done about the sediment they have four options 1 reducing the sediment supply at source 2 trapping sediment in the channel 3 accelerating sediment transport through a reach and 4 directly extracting sediment the most common of these actions is undoubtedly to reduce the supply at source but there are few examples of the consequences of this for sediment pulses there are even fewer examples of trapping accelerating and extracting sediment all of these options have great potential for managing sediment pulses however they also have the potential to trigger incision of tributaries and of the channel behind the passing sediment wave overall the literature equips managers to understand the dynamics of sediment pulses but it does not yet equip them to confidently manage these geomorphic events

short wavelength geoid bathymetry and the convective pattern beneath the pacific ocean summary the geoid and bathymetry data in the pacific ocean are used to shed new light on the structure of upper mantle convection at short wavelengths 200 km the geoid pattern is characterized by parallel lineations lineations however appear less clearly at larger wavelengths 400 or 600 km where the signal due to convective thermal anomalies might be expected the most prominent features among the short wavelength lineations are associated with the well known chains of seamounts and islands other lineations observed in the geoid coincide with computed tracks of other volcanic centres although these tracks are not continuous this suggests that long lived plumes are at the origin of the apparently short chains of seamounts apart from the lineations corresponding to the tracks of classical hotspots there are numerous similar lineations visible on the filtered geoid maps which have the same directions as hotspot tracks these preferential directions and the gravity to bathymetry ratios seem to indicate that the lineations are due to magmas implaced just above or within the lithosphere convective plumes fixed with respect to each other and much more numerous than the classical hotspots could be the source of these magmas a geometrical test of this hypothesis is performed by comparing the lineation pattern with the computed tracks for a set of tentative new hotspots in this way numerous plumes long lived and fixed with respect to each other are found to provide an explanation of the observed small wavelength geoid features

channel response to sediment wave propagation and movement redwood creek california usa redwood creek north coastal californa usa has experienced dramatic changes in channel confguation since the 1950s a series of large floods in 1955 1964 1972 and 1975 combined with the advent of widespread commercial timber harvest and road building resulted in extensive erosion in the basin and contributed high sediment loads to redwood creek since 1975 no peak flows have exceeded a 5 year recurrence interval twenty years of cross sectional survey data document the downstream movement of a sediment wave in the lower 26 km of this gravel bedded river at a rate of 800 to 1600 m a i durng this period of moderately low flows higher transit rates are associated with reaches of higher unit stream power the wave was initially deposited at a site with an abrupt decrease in channel gradient and increase in channel width the amplitude of the wave has attenuated more than i m as it moved downstream and the duration of the wave increased from eight years upstream to more than 20 years downstream channel aggradation and subsequent degradation have been accommodated across the entire channel bed channel width has not decreased significantly after initial channel widening from large 25 year recurrence interval floods three sets of longitudinal surveys of the streambed showed the highest increase in pool depths and frequency in a degrading reach but even the aggrading reach exhbited some pool development through time the aggraded channel bed switched from functioning as a sediment sink to a significant sediment source as the channel adjusted to high sediment loads from 1980 to 1990 sediment eroded from temporary channel storage represented about 25 per cent ofthe total sediment load and 95 per cent of the bedload exported from the basin

mixing in the romanche fracture zone the romanche fracture zone is a major gap in the mid atlantic ridge at the equator which is deep enough to allow significant eastward flows of antarctic bottom water from the brazil basin to the sierra leone and guinea abyssal plains while flowing through the romanche fracture zone bottom water properties are strongly modified due to intense vertical mixing the diapycnal mixing coefficient in the bottom water of the romanche fracture zone is estimated by using the finestructure of ctd profiles the microstructure of high resolution profiler data and by constructing a heat budget from current meter data the finestructure of density profiles is described using the thorpe scaleslt it is shown from microstructure data taken in the bottom water that the ozmidov scale lo is related to lt by the linear relationship lo 5 0 95lt similar to other studies which allows an estimate of the diapycnal mixing coefficient using the osborn relation the thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream eastward of the main sill of the romanche fracture zone in this region a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water estimates of cross isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the romanche fracture zone and the nearby chain fracture zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the romanche fracture zone although the two fracture zones occupy only 0 4 of the area covered by the sierra leone and guinea abyssal plains the diffusive heat fluxes across the 1 4 8c isotherm in the romanche and chain fracture zones are half that found over the abyssal plains across the 1 88c isotherm emphasizing the role of these passages for bottom water property modifications

momentum and energy balance of the mediterranean outflow field data taken in the gulf of cadiz have been analyzed to describe some aspects of the momentum and energy balance of the mediterranean outflow a crucial component of the momentum balance is the total stress entrainment stress and bottom drag which has been estimated from a form of the bernoulli function evaluated from density and current observations for the first 60 km west of the camarinal sill the outflow was confined within a narrow channel on the continental shelf at about 70 km downstream the outflow crossed over the shelf slope break and began to descend the continental slope the buoyancy force increased substantially and the outflow underwent a geostrophic adjustment albeit one heavily influenced by mixing and dissipation the current direction turned 90 degrees to the right at a near inertial rate in this region the estimated geostrophic velocity greatly underestimated the actual current and the estimated curvature rossby number was about 0 5 current speeds were in excess of 1ms 2 1 and the total stress was as large as 5 pa the entrainment stress estimated independently from property fluxes reached a maximum of about 1 pa or considerably smaller than the inferred bottom stress by about 130 km downstream the current was aligned approximately along the local topography the current amplitude and the estimated stress were then much less about 0 3 m s21 and 0 3 pa the entrainment stress was also very small in this region well downstream of the strait this slightly damped geostrophic flow continued on to cape st vincent where the outflow began to separate from the bottom bottom stress thus appears to be a crucial element in the dynamics of the mediterranean outflow allowing or causing the outflow to descend more than a kilometer into the north atlantic in the regions of strongest bottom stress the inferred drag coefficient was about 2 2 12 3 10 23 depending upon which outflow speed is used in the usual quadratic form entrainment stress was small by comparison to the bottom stress but the entrainment effect upon the density anomaly was crucial in eroding the density anomaly of the outflow the observed entrainment rate appears to follow roughly a critical internal froude number function

character and dynamics of the red sea and persian gulf outflows historical hydrographic data and a numerical plume model are used to investigate the initial transformation dynamics and spreading pathways of red sea and persian gulf outflow waters where they enter the indian ocean the annual mean transport of these outflows is relatively small 0 4 sv but they have a major impact on the hydrographic properties of the indian ocean at the thermocline level because of their high salinity they are different from other outflows in that they flow over very shallow sills depth 200 m into a highly stratified upper ocean environment and they are located at relatively low latitudes 12 n and 26 n furthermore the red sea outflow exhibits strong seasonal variability in transport the four main results of this study are as follows first on the basis of observed temperature salinity t s characteristics of the outflow source and product waters we estimate that the red sea and persian gulf outflows are diluted by factors of 2 5 and 4 respectively as they descend from sill depth to their depth of neutral buoyancy the high dilution factor for the persian gulf outflow results from the combined effects of large initial density difference between the outflow source water and oceanic water and low outflow transport second the combination of low latitude and low outflow transport and associated low outflow thickness results in ekman numbers for both outflows that are o 1 this indicates that they should be thought of as frictional density currents modified by rotation rather than geostrophic density currents modified by friction third different mixing histories along the two channels that direct red sea outflow water into the open ocean result in product waters with significantly different densities which probably contributes to the multilayered structure of the red sea product waters in both outflows seasonal variations in source water and oceanic properties have some effect on the t s of the product waters but they have only a minor impact on equilibrium depth fourth product waters from both outflows are advected away from the sill region in narrow boundary currents at least during part of the year at other times the product water appears more in isolated patches

gravity current dynamics and entrainment a process study based on observations in the arkona basin abstract a 19 h time series of dissipation stratification and horizontal velocities has been obtained for a dense gravity current flowing into the arkona basin in the western baltic sea the observations are compared with one dimensional quasi steady theory in which the gravity component in the flow direction is balanced by bottom friction while that in the cross flow direction is balanced by the coriolis force the observations deviate from the theory in that the bottom shear stress is more than 2 times as large as that required to balance the gravity several reasons for this discrepancy are discussed a 1d turbulence model is also compared with the observations profiles of velocity stratification and dissipation rates generally show similar variations with depth as the observations although the observed dissipation rates are somewhat larger than the modeled and the modeled transverse velocities are much larger than the observed subsequently the model is used to investigate the variation of t

a new parameterization for entrainment in overflows abstract dense overflows entrain surrounding waters at specific locations for example sills and constrictions but also along the descent over the continental slope the amount of entrainment dictates the final properties of these overflows and thus is of fundamental importance to the understanding of the formation of deep water masses even when resolving the overflows coarse resolution global circulation and climate models cannot resolve the entrainment processes that are often parameterized a new empirical parameterization is suggested obtained using an oceanic and laboratory dataset which includes two novel aspects first the parameterization depends on both the froude number fr and reynolds number of the flow second it takes into account subcritical fr 1 entrainment a weak but nonzero entrainment can change the final density and consequently the depth and location of important water masses in the open ocean this is especially true when the dense current follows a long path over

influence of coriolis force upon bottom boundary layers in a large scale gravity current experiment implications for evolution of sinuous deep water channel systems oceanic density currents in many deep water channels are strongly influenced by the coriolis force the dynamics of the bottom boundary layer in large geostrophic flows and low rossby number turbidity currents are very important for determining the erosion and deposition of sediment in channelized contourite currents and many large scale turbidity currents however these bottom boundary layers are notoriously difficult to resolve with oceanic field measurements or in previous small scale rotating laboratory experiments we present results from a large 13 m diameter rotating laboratory platform that is able to achieve both stratified and highly turbulent flows in regimes where the rotation is sufficiently rapid that the coriolis force can potentially dominate by resolving the dynamics of the turbulent bottom boundary in straight and sinuous channel sections we find that the coriolis force can overcome centrifugal force to switch the direction of near bed flows in channel bends this occurs for positive rossby numbers less than 0 8 defined as ror

mesoscale eddies in the gulf of aden and their impact on the spreading of red sea outflow water abstract the gulf of aden goa in the northwestern indian ocean is the receiving basin for red sea outflow water rsow one of the world s few high salinity dense overflows but relatively little is known about spreading pathways and transformation of rsow through the gulf here we combine historical data satellite altimetry new synoptic hydrographic surveys and the first in situ direct observations of subsurface currents in the goa to identify the most important processes in the spreading of rsow the new in situ data sets were collected in 2001 2003 as part of the red sea outflow experiment redsox and consist of two ctd ladcp surveys and 49 one year trajectories from acoustically tracked floats released at the depth of rsow the results indicate that the prominent positive and negative sea level anomalies frequently observed in the goa with satellite altimetry are associated with anticyclonic and cyclonic eddies that often reach to at least 1000 m depth i e through the depth range of equilibrated rsow the eddies dominate rsow spreading pathways and help to rapidly mix the outflow water with the background eddies in the central and eastern gulf are basin scale 250 km diameter and have maximum azimuthal speeds of about 30 cm s at the rsow level in the western gulf smaller eddies not detectable with satellite altimetry appear to form as the larger westward propagating eddies impale themselves on the high ridges flanking the tadjura rift both the hydrographic and lagrangian observations show that eddies originating outside the gulf often transport a core of much cooler fresher water from the arabian sea all the way to the western end of the goa where the highest salinity outflow water is found this generates large vertical and horizontal gradients of temperature and salinity setting up favorable conditions for salt fingering and diffusive convection both of these mixing processes were observed to be active in the gulf two new annually appearing anticyclonic eddies are added to the previously identified gulf of aden eddy gae prasad and ikeda 2001 and somali current ring scr fratantoni et al 2006 these are the summer eddy se and the lee eddy le both of which form at the beginning of the summer monsoon when strong southwest winds blowing through socotra passage effectively split the gae into two smaller eddies the se strengthens as it propagates westward deeper in the goa while the lee eddy remains stationary in the lee of socotra island both eddies are strengthened or sustained by ekman convergence associated with negative wind stress curl patches caused by wind jets through or around high orography the annual cycle in the appearance propagation and demise of these new eddies and those described in earlier work is documented to provide a comprehensive view of the most energetic circulation features in the goa the observations contain little evidence of features that have been shown previously to be important in the spreading of mediterranean outflow water mow in the north atlantic namely a wall bounded subsurface jet the mediterranean undercurrent and submesoscale coherent lenses containing a core of mow meddies this is attributed to the fact that the rsow enters the open ocean on a western boundary high background eddy kinetic energy typical of western boundary regimes will tend to shear apart submesoscale eddies and boundary undercurrents even if a submesoscale lens of rsow did form in the goa westward self propagation would transport the eddy and its cargo of outflow water back toward rather than away from its source

stress on the mediterranean outflow plume part ii turbulent dissipation and shear measurements abstract bottom and interfacial stresses on the mediterranean outflow plume are estimated using vertical profiles of turbulent dissipation and velocity collected in the gulf of cadiz turbulent dissipation is high throughout the plume with a local minimum often present near the plume nose depth of maximum downstream velocity bottom stresses are estimated by applying a log layer model to the dissipation measurements the dissipation measurements are also divided by plume scale vertical shear from the horizontal velocity profiles to construct profiles of stress within the plume the mean stress estimates in the bottom layer agree well with those calculated in the log layer from the dissipation measurements alone the bottom layer means are slightly larger than those of the interfacial layer the maximum stresses in each layer are uncertain since they depend on the ill defined shape of the stress profiles within the plume dissipation derived stress estimates in the log layer and those from dissipation

three dimensional models of mantle flow across a low viscosity zone implications for hotspot dynamics the influence of a shallow low viscosity zone on the planform of infinite prandtl number rayleigh benard convection has been investigated at rayleigh numbers ra ranging from 1 5 103 to 3 105 a viscosity contrast of130 has been set between the low and high viscosity layers for ra exceeding about 3 104 and no slip conditions at the top and bottom boundaries a hexagonal convective planform is systematically observed free slip boundary conditions lead to the development of a square convective planform the flow is ascending along the center of the cells and is strongly narrowed in the low viscosity layer a moving top boundary does not destabilize significantly these plumes as long as the drift velocity is lower than the upwelling velocity in the low viscosity layer a higher drift velocity of the upper boundary rapidly induces the formation of convective rolls at ra of 3 105 unsteady flow occurs along the descending currents when the convective cells have the horizontal size predicted by the linear theory this time dependence is prevented when the cells are smaller suggesting that there is a competition between the aspect ratio and the steadiness of the cells such a reduction of the horizontal dimensions of the cells with increasing ra has been observed in large experiments these results show that a moderate viscosity drop at shallow depth in the mantle compatible with the geoid and bathymetric data at hotspot swells induces a marked asymmetry between the upper and lower convective boundary layers this promotes the development of ascending plumes large at depth and very narrow a few tens of kilometers in section below the lithosphere these plumes are very stable in time and space it is inferred that the channelling of the ascending plumes induced by a viscosity drop precludes the development of secondary convection in a top low viscosity layer

mixing and entrainment in the red sea outflow plume part ii turbulence characteristics abstract south of the strait of bab el mandeb saline red sea water flows downslope into the gulf of aden mainly along the narrow 130 km long northern channel nc and the shorter and wider southern channel sc in the nc the red sea plume simultaneously exhibited weak entrainment into a 35 120 m thick weakly stratified bottom layer while a 35 285 m thick interfacial layer above showed signs of vigorous mixing overturns up to 30 m thick and extensive zones of gradient richardson numbers below 1 4 turbulent overturning scales or thorpe scales are extracted from regular ctd profiles and equated to ozmidov scales on this basis interfacial mixing is quantified in terms of estimated turbulent dissipation rates vertical turbulent salt flux and interfacial stress even though these estimates are subject to significant uncertainty they demonstrate the intensity of mixing during strong winter outflow in terms of eddy diffusivities k on the order of 10 2 m2 s 1 the large k occur in strong strati

effects of ambient velocity shear on nonlinear internal wave associated mixing at the columbia river plume front 1 large amplitude nonlinear internal waves nliws are frequently observed propagating away from columbia river tidal plume fronts they are generated because of the deceleration of the frontal bulge during the river influences on shelf ecosystems project cruises the velocity density structure and acoustic backscatter of the plume fronts and frontal nliws were observed using a towed vehicle vessel mounted instrumentation and a vessel x band radar these observations indicate that in the presence of strong ambient velocity shear the nliws with maximum amplitudes occur well below the density interface and at a depth deeper than in the absence of shear this deepening of the nliw shear is associated with intensified vertical mixing below the interface this is consistent with a dynamic analysis of nliws under the influence of sheared ambient flow obtained from a high order kdv model there are two mechanisms responsible for the vertical turbulent mixing intensification the nonlinear interaction between ambient shear and the nliws increases total velocity shear and causes the gradient richardson number rig to decrease below a critical value in certain depth ranges also because of the presence of the ambient shear the maximum nliw velocity shear occurs at a depth below the density interface where there is less stratification

dynamics of rotating shallow gravity currents passing through a channel part i observation of transverse structure abstract a detailed dataset describing a quasi stationary bottom gravity current approximately 10 m thick and 10 km wide passing through a channel like constriction in the western baltic sea is presented the data include full depth synoptic and highly resolved transects of stratification and turbulence parameters as well as detailed velocity transects across the gravity current at different down channel locations the velocity data reveal a persistent transverse circulation creating a characteristic wedge shaped density structure in the interface a strong asymmetry was also found in the interior of the gravity current where the evolution of a dynamically significant transverse density gradient to the right of the down channel flow was observed spectral analysis of the near bottom velocities showed a surprisingly strong contribution to the bottom stress from low frequency motions with periods up to 30 min that are possibly related to internal wave effects cross channel transects of shear microst

numerical investigation of split flows by gravity currents into two layered stratified water bodies the behavior of a two dimensional 2 d gravity current impinging upon a density step in a two layered stratified basin is analyzed using a high resolution reynolds averaged navier stokes model the gravity current splits at the density step and the portion of the buoyancy flux becoming an interflow is largely controlled by the vertical distribution of velocity and density within the gravity current and the magnitude of the density step between the two ambient layers this is in agreement with recent laboratory observations the strongest changes in the ambient density profiles occur as a result of the impingement of supercritical currents with strong density contrasts for which a large portion of the gravity current detaches from the bottom and becomes an interflow we characterize the current partition process in the simulated experiments using the densimetric froude number of the current fr across the density step upstream and downstream when underflows are formed more supercritical currents are observed downstream of the density step compared to upstream fru frd which indicates lower mixing between the current and ambient water after the impingement due to the significant stripping of interfacial material at the density step

large scale dam removal on the elwha river washington usa source to sink sediment budget and synthesis abstract understanding landscape responses to sediment supply changes constitutes a fundamental part of many problems in geomorphology but opportunities to study such processes at field scales are rare the phased removal of two large dams on the elwha river washington exposed 21 3 million m3 or 30 million tonnes t of sediment that had been deposited in the two former reservoirs allowing a comprehensive investigation of watershed and coastal responses to a substantial increase in sediment supply here we provide a source to sink sediment budget of this sediment release during the first two years of the project september 2011 september 2013 and synthesize the geomorphic changes that occurred to downstream fluvial and coastal landforms owing to the phased removal of each dam the release of sediment to the river was a function of the amount of dam structure removed the progradation of reservoir delta sediments exposure of more cohesive lakebed sediment and the hydrologic conditions of the river the greatest downstream geomorphic effects were observed after water bodies of both reservoirs were fully drained and fine silt and clay and coarse sand and gravel sediments were spilling past the former dam sites after both dams were spilling fine and coarse sediments river suspended sediment concentrations were commonly several thousand mg l with 50 sand during moderate and high river flow at the same time a sand and gravel sediment wave dispersed down the river channel filling channel pools and floodplain channels aggrading much of the river channel by 1 m reducing river channel sediment grain sizes by 16 fold and depositing 2 2 million m3 of sand and gravel on the seafloor offshore of the river mouth the total sediment budget during the first two years revealed that the vast majority 90 of the sediment released from the former reservoirs to the river passed through the fluvial system and was discharged to the coastal waters where slightly less than half of the sediment was deposited in the river mouth delta although most of the measured fluvial and coastal deposition was sand sized and coarser 0 063 mm significant mud deposition was observed in and around the mainstem river channel and on the seafloor woody debris ranging from millimeter size particles to old growth trees and stumps was also introduced to fluvial and coastal landforms during the dam removals at the end of our two year study elwha dam was completely removed glines canyon dam had been 75 removed full removal was completed 2014 and 65 of the combined reservoir sediment masses including 8 mt of fine grained and 12 mt of coarse grained sediment remained within the former reservoirs reservoir sediment will continue to be released to the elwha river following our two year study owing to a 16 m base level drop during the final removal of glines canyon dam and to erosion from floods with larger magnitudes than occurred during our study comparisons with a geomorphic synthesis of small dam removals suggest that the rate of sediment erosion as a percent of storage was greater in the elwha river during the first two years of the project than in the other systems comparisons with other pacific northwest dam removals suggest that these steep high energy rivers have enough stream power to export volumes of sediment deposited over several decades in only months to a few years these results should assist with predicting and characterizing landscape responses to future dam removals and other perturbations to fluvial and coastal sediment budgets

observations of turbulence in a partially stratified estuary the authors present a field study of estuarine turbulence in which profiles of reynolds stresses were directly measured using an adcp throughout a 25 h tidal day the dataset that is discussed quantifies turbulent mixing for a water column in northern san francisco bay that experiences a sequence of states that includes a weak ebb and flood that are stratified followed by a strong and eventually unstratified ebb and flood these measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification examination of individual reynolds stress profiles along with profiles of richardson number and turbulent froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects using the measured turbulence production rate p the dissipation rate e is estimated the observed turbulence had values of e nn 2 20 all of the time and e nn 2 200 most of the time suggesting that the observed motions were buoyancy affected turbulence rather than internal waves however at times turbulent froude numbers in much of the upper water column were less than one indicating important stratification effects taken as a whole the data show that stratification affects the turbulent velocity variance q2 most severely that is observed reductions in u9w9 are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes finally the dataset is compared to predictions made using the popular mellor yamada level 2 5 closure these comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions the length scale does not appear to compensate for these errors and as a result similar errors are seen in the eddy viscosity predictions it is hypothesized that the underestimation of q2 is due to an inaccurate parameterization of turbulence self transport from the near bed region to the overlying stratification

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

bank and near bank processes in an incised channel abstract gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion the interaction of these forces control streambank mechanics hydraulic forces exerted by flowing water on in situ bank toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower bank profiles seepage forces exerted on in situ bank material by groundwater downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength data from a study site on goodwin creek ms usa clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore water pressures negative pore water pressures matric suction have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear a stable bank can be transformed into an unstable bank during periods of prolonged rainfall through 1 increase in soil bulk unit specific weight 2 decrease or complete loss of matric suction and therefore apparent cohesion 3 generation of positive pore water pressures and therefore reduction or loss of frictional strength 4 entrainment of in situ and failed material at the bank toe and 5 loss of confining pressure during recession of stormflow hydrographs relatively small frequent flows during the winter have the ability to erode failed bank materials maintain oversteepened unstable bank surfaces and promote prolonged periods of bank retreat channel migration and high yields of fine grained sediment confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe which steepens the bank for example more than 2 m of bank retreat occurred during the study period at the research site on goodwin creek northern mississippi the loss of matric suction negative pore pressures due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability apparent cohesion friction angle soil bulk unit weight and moisture content were measured in situ matric suction was measured continuously in situ with a series of five pressure transducer tensiometers a bank failure algorithm which combines the mohr coulomb approach for saturated conditions and the fredlund modification for unsaturated conditions was developed for layered cohesive streambanks the resulting equation has been used successfully to investigate the role of matric suction positive pore water pressures and confining pressure for layered streambanks composed of cohesive materials

spatial and temporal patterns in fluvial recovery following volcanic eruptions channel response to basin wide sediment loading at mount pinatubo philippines the june 1991 eruption of mount pinatubo philippines was one of the largest volcanic eruptions of the twentieth century emplacing 5 6 km 3 of pyroclastic fl ow material on the fl anks of the volcano the combination of abundant relatively fi ne grained easily erodible material and intense tropical rainfall led to numerous lahars immediately following the eruption even after major lahars ended sediment yields in some basins remained orders of magnitude above pre eruption levels using data collected from 1996 through 2003 we investigated fi ve basins that experienced varying amounts of sediment loading in the 1991 eruption from 1 to 33 of the basin area covered by valley fi lling pyroclasticfl ow deposits from measurements of fl ow and bed characteristics made through time we developed a general conceptual model for channel recovery following basin wide sediment loading initially fi ner grained sediment and pumice are mobilized preferentially through selective transport once the bed is coarse enough for gravel size clasts to interact with one another clast structures develop increasing form roughness and critical shear stress and inhibiting initial clast mobility as sediment inputs continue to decline the channel incises into valley bottom sediments progressively armoring through winnowing at pinatubo incision and armoring occur fi rst as dry season phenomena due to reduced sediment inputs eventually moving to year round low fl ow bed stability observed timing of the onset of in channel biological recovery suggests that reestablishment of channel stability helps catalyze aquatic ecosystem recovery

upper mantle heterogeneities and dynamics the width of geochemical and residual elevation anomalies along the mid ocean ridge system decrease as both the distance between migrating ridge segments and related hotspots and the local geochemical dispersion in the anomalies increase these observations provide further evidence that flows of mantle plumes in the upper mantle are deflected by migrating ridge sinks once they override hotspots

sediment pulse evolution and the role of network structure abstract sediment pulses are triggered through a variety of mechanisms from landslides to land use change how do these pulses move through the fluvial system and how do they evolve in a system with perfect sediment connectivity the erosional response to a perturbation and the resulting signal at the river mouth would match however this rarely occurs many studies have addressed reach scale dynamics of sediment pulses and how they translate or disperse downstream at the watershed scale network structure and storage become more important in modulating the sediment signal here we review the current literature on sediment pulse behavior and then address the role of network structure on maintaining dispersing or transforming sediment pulses in a fluvial system we use a reduced complexity network routing model that simulates the movement of bed material through a river basin this model is run in the greater blue earth river gber basin in minnesota usa first with spatially uniform inputs and then with inputs constrained by a detailed sediment budget once the system reaches equilibrium a sediment pulse is introduced first at a single location and then throughout the system and tracked as it evolves downstream results indicate that pulses able to translate downstream disperse in place upon arriving at over capacity reaches as sediment goes into storage in the gber basin these zones occur just upstream of a knickpoint that is propagating upstream through all mainstem channels as the pulses get caught in these sediment bottlenecks there is a decoupling of the original pulse of sediment and the resulting bed material wave these results show that the network structure both in terms of network geometry and the spatial pattern of transport capacity can play a dominant role in sediment connectivity and should be considered when predicting sediment pulse behavior at the watershed scale

mixing in the stratified interface of the faroe bank channel overflow the role of transverse circulation and internal waves mixing in the stratified interface of the faroe bank channel overflow the role of transverse circulationand internal waves

mixing and spreading of the mediterranean outflow abstract hydrographic and current profiler data taken during the 1988 gulf of cadiz expedition have been analyzed to diagnose the mixing spreading and descent of the mediterranean outflow the s properties and the thickness and width of the outflow were similar to that seen in earlier surveys the transport of pure mediterranean water i e water with s 38 4 psu was estimated to be about 0 4 106 m3 s 1 which is lower than historical estimates most of which were indirect but comparable to other recent estimates made from direct velocity observations the outflow transport estimated at the west end of the strait of gibraltar was about 0 7 106 m3 s 1 of mixed water and the transport increased to about 1 9 106 m3 s 1 within the eastern gulf of cadiz this increase in transport occurred by entrainment of fresher north atlantic central water and the salinity anomaly of the outflow was consequently reduced the velocity weighted salinity decreased to 36 7 psu within 60 km of the strait and decre

rapid channelization and incision into soft bedrock induced by human activity implications from the bachang river in taiwan abstract many taiwanese river channels have been affected by human activity such as engineering works and gravel mining environmental conditions such as high seasonal precipitation and young sedimentary bedrock exaggerate the narrowing and incising of river channels the bachang river is located in southwestern taiwan and the 11 km long channel at its midstream reach has been incised to a maximum depth of approximately 30 m within several decades therefore the morphologic evolution of the bachang river in southwestern taiwan provides an illustration of channel changes in response to human activity the historical data used for this analysis of morphology change include one set of topographic maps and six sets of aerial photographs from the past 100 years the channel changes are analyzed qualitatively and quantitatively and include the channel planform longitudinal profiles channel incision channel width and channel cross sections the results indicate that remarkable channel changes have occurred since the 1980s as a result of human influence the 11 km long study reach transformed from an alluvial type channel to a gorge type channel within several decades the maximum accumulated depth of the channel incision is approximately 30 m with a meter scale annual average incision rate the maximum width has been decreased to approximately one sixth of the original width 448 m the process of channel evolution is divided into four stages and we have concluded that the causes of channel change include six factors from two categories natural factors that include geological conditions hydrological conditions and the process of bedrock erosion and human factors that include gravel mining lateral structures and levees the initiation of channel evolution is triggered by human factors finally we discuss the potential future channel evolution and lessons learned from the case study

effect of sediment pulse grain size on sediment transport rates and bed mobility in gravel bed rivers 1 sediment supply to gravel bed river channels often takes the form of episodic sediment pulses and there is considerable interest in introducing sediment pulses in stream restorations to alter bed surface grain size distributions and bed mobility a series of laboratory experiments was conducted in order to examine how sediment pulse grain size and volume affects the mobility of bed material in gravel bed channels pulses used in the experiments were composed of either the fine tail or the median of the subsurface bed material grain size distribution bed material refers to sediment in the channel prior to the pulse introduction exclusively both types of pulse were finer than the bed material surface median two pulse sizes were used which were either equivalent to the volume of sediment required to cover the entire bed one median subsurface bed material grain diameter deep full unit or 1 4 of this volume 1 4 unit the latter was designed to produce a transitory pulse with the exception of the 1 4 unit coarse pulse introduction of the sediment pulses to the channel caused dramatic increases in the bed load flux the coarse sediment pulses fine the bed surface and coarsen the bed load finer pulses also fine the surface but the bed load fines while the bed material load that excludes pulse material coarsens the greatest effects on the fractional transport occurred during the full unit fine pulse where the pulse covered the greatest bed surface and effectively smoothed the bed increasing near bed velocity and mobilizing the coarse particles overall the coarse pulses were not very effective at mobilizing bed material the large fine pulse mobilized 35 of the bed material surface 35 of its input weight and was most effective at mobilizing the surface however the small fine pulse mobilized 50 of its input weight as it passed through the channel making it the most efficient at mobilizing the bed material

mantle heterogeneities geoid and plate motion a monte carlo inversion seismic tomography in both the upper and the lower mantle as well as subducting oceanic slabs defined by seismicity has been translated into density heterogeneities to generate models of mantle circulation these models can predict both the surface velocities and the geoid which can be compared with plate tectonics and gravity data a given model is specified by 6 parameters related to the viscosities of 3 mantle layers and the absolute amplitudes of density variations in the upper and lower mantle as well as in the slabs the values of these parameters are chosen at random within an acceptable range each model is submitted to an appropriate test comparing observations with predictions the results of the most successful models selected by this monte carlo inversion are displayed they yield preferred mantle viscosity structures exhibiting large variations at depth with a physical interface between upper and lower mantle i e with the possibility for the circulation to penetrate the 650 km discontinuity two classes of viscosity profiles stand out the first one implies a regular increase of the viscosity in the sublithospheric mantle with reasonable values for the density parameters the second one is unexpected in the sense that it predicts a very stiff bottom for the upper mantle it also requires vanishingly small amplitudes for the upper mantle density heterogeneities defined by tomography which would thus have to be of lithological rather than thermal origin with a chemical interface at 650 km the outcome is very similar the same two classes of viscosity structures do yield a satisfactory geoid prediction however only the class of models with a stiff layer at midmantle depths predicts acceptable surface velocities altogether the best models out of some 60 000 which have been tested only explain one third of the geoid and two thirds of the surface divergence for spherical harmonic degrees 1 to 6 nevertheless the main features of these two observed patterns are present in the computed maps and 4 out of 6 correlation coefficients lie close to the 90 confidence level this is true for the geoid as well as for the surface divergence of the displacement velocity however as the internal viscosity structure has been assumed to have spherical symmetry the rotational component of the surface velocities cannot be predicted

bottom layer turbulence in the red sea outflow plume abstract turbulence in the red sea outflow plume in the western gulf of aden was observed with an upward looking five beam 600 khz acoustic doppler current profiler adcp the bottom lander adcp was deployed on the seafloor in two narrow topographically confined outflow channels south of bab el mandeb for periods of 18 40 h at three locations at 376 496 and 772 m depths two deployments were taken during the winter season of maximum outflow from the red sea and two in the summer season of minimum outflow these short term observations exhibit red velocity spectra with high frequency fluctuations of typically a few centimeters per second rms velocity during strong plume flow as well as strong subtidal variations in one winter season event the plume flow was reduced by a factor of 4 over an 18 h time span in variance preserving form velocity spectra show a separation at frequencies of 0 3 3 cycles per hour between low frequency and high frequency signals the latter show significant coherence

modes of mantle convection and the removal of heat from the earth s interior earth thermal histories based on two layer and whole mantle parameterized convection models have been explored to determine the influence of the mode of mantle convection on the content and distribution of mantle radioactive heat sources and on the temperatures and heat transfer properties of the mantle two layer mantle convection thermally insulates the lower mantle and is less efficient at heat removal than whole mantle convection consequently two layer mantle convection requires a lower mantle depleted of radioactive heat sources contrary to the undepleted lower mantles of recent geochemical models whole mantle convection removes 3 times more primordial heat from the earth s interior and 6 times more heat from the core in 4 5 billion years than does two layer convection the average mantle volumetric heat generation rates for both modes of convection are comparable to that of a potassium depleted chondrite thus no potassium is necessary in the core to balance the observed present day surface heat flux however most of the radioactives are concentrated in the upper mantle for two layer convection model geotherms for whole mantle convection compare well with independent estimates of temperature at various depths including the core mantle boundary while the lower mantle is too hot for two layer convection models thermal histories of the upper mantle in two layer models are similar to those of whole mantle models thermal histories for the lower mantle in two layer models are similar to those of the core in whole mantle convection models this is because the lower mantle is thermally insulated by the upper mantle if it convects separately just as the core is thermally insulated by the whole mantle the differences between whole and two layer mantle convection are mostly due to the thermal insulation of the lower mantle and the smaller efficiency of heat removal of the upper mantle compared to that of the whole mantle which is much larger in volume a whole mantle convection model reconciling the need for distinct geochemical reservoirs with the physics of mantle convection is presented

numerical simulation of the red sea outflow using hycom and comparison with redsox observations abstract the outflow of warm salty and dense water from the red sea into the western gulf of aden is numerically simulated using the hybrid coordinate ocean model hycom the pathways of the modeled overflow temperature salinity velocity profiles from stations and across sections and transport estimates are compared to those observed during the 2001 red sea outflow experiment as in nature the simulated outflow is funneled into two narrow channels along the seafloor the results from the three dimensional simulations show a favorable agreement with the observed temperature and salinity profiles along the channels the volume transport of the modeled overflow increases with the increasing distance from the southern exit of the bab el mandeb strait due to entrainment of ambient fluid such that the modeled transport shows a reasonable agreement with that estimated from the observations the initial propagation speed of the outflow is found to be smaller than the estimated interfacial wave speed the

rapid reservoir erosion hyperconcentrated flow and downstream deposition triggered by breaching of 38 m tall condit dam white salmon river washington condit dam on the white salmon river washington a 38 m high dam impounding a large volume 1 8 million m3 of fine grained sediment 60 sand 35 silt and clay and 5 gravel was rapidly breached in october 2011 this unique dam decommissioning produced dramatic upstream and downstream geomorphic responses in the hours and weeks following breaching blasting a 5 m wide hole into the base of the dam resulted in rapid reservoir drawdown abruptly releasing 1 6 million m3 of reservoir water exposing reservoir sediment to erosion and triggering mass failures of the thickly accumulated reservoir sediment within 90 min of breaching the reservoir s water and 10 of its sediment had evacuated at a gauging station 2 3 km downstream flow increased briefly by 400 m3 s 1 during passage of the initial pulse of released reservoir water followed by a highly concentrated flow phase up to 32 sediment by volume as landslide generated slurries from the reservoir moved downstream this hyperconcentrated flow analogous to those following volcanic eruptions or large landslides draped the downstream river with predominantly fine sand during the ensuing weeks suspended sediment concentration declined and sand and gravel bed load derived from continued reservoir erosion aggraded the channel by 1 m at the gauging station after which the river incised back to near its initial elevation at this site within 15 weeks after breaching over 1 million m3 of suspended load is estimated to have passed the gauging station consistent with estimates that 60 of the reservoir s sediment had eroded this dam removal highlights the influence of interactions among reservoir erosion processes sediment composition and style of decommissioning on rate of reservoir erosion and consequent downstream behavior of released sediment

intense mixing of antarctic bottom water in the equatorial atlantic ocean the spreading of antarctic bottom water the densest global scale water mass is highly constrained by ocean floor topography in the atlantic ocean the mid atlantic ridge confines this water mass mainly to the western basins the bottom waters in the eastern basins being renewed by flows through gaps in the ridge1 one such gap is the romanche fracture zone a large offset of the ridge which straddles the equator it has been observed2 that sills within this fracture zone block the passage of waters colder than 0 9 c warmer less dense waters passing over the sills appear to cascade downslope where they are modified by mixing here we present direct measurements which quantify these processes the flow is vertically sheared and exhibits remarkably intense turbulence comparable to that seen at the ocean surface in the presence of winds of l0ms 1 this turbulence mixes the densest waters passing through the fracture zone with the warmer overlying waters so that the coldest waters exiting this region have been warmed by 0 6 c during transit topographic obstructions and turbulent mixing together thus determine the properties of the flows renewing the deepest waters of the atlantic ocean s eastern basins

dynamics of rotating shallow gravity currents passing through a channel part ii analysis abstract the physics of frictional control for channelized rotating gravity currents are analyzed using an extensive dataset including hydrographic current and microstructure measurements from the western baltic sea rotational effects in these gravity currents characterized by ekman numbers of the order of one and subcritical froude numbers induce a complex transverse circulation that strongly affects the internal dynamics the key component of this circulation is a geostrophically balanced transverse jet in the interface that modifies the entrainment process by i laterally draining the interface and ii providing additional interfacial shear comparable to the down channel shear the recirculation of mixed interfacial fluid into the interior distorts the internal density structure of the gravity current and creates a thermal wind shear in the interior that is comparable to the observed shear using a theoretical model this effect is shown to be responsible for the three layer structure of the tr

onset of mantle plumes in the presence of preexisting convection the onset of plumes where broad scale convection already exists is investigated along with the thermal history of the earth geological evidence relating to the onset time of mantle plumes is not definitive no well defined hotspot tracks older than mesozoic have been identified but it is difficult to tell whether this absence is a reliable indication of a late onset of mantle plumes since only continental crust of this age exists using the hotspot track distribution for the pacific the absence of pre mesozoic hotspot tracks on the canadian shield cannot easily be attributed to chance however young continental hotspot tracks are controversial and it is not clear whether tracks such as the monteregian hills white mountain magma series would be recognized as such without considering the contributing evidence of the adjacent oceanic crust and its well defined seamount chains our simple models yield potentially testable predictions the instability of plume formation in a system where broad scale convection already exists depends on horizontal conduit flow in the basal boundary layer toward the plume source geographically plumes are most likely to form where the boundary layer is thickest and where broad scale ascending flow already exists parameterized convection indicates that the heat transport by conduit flow in the boundary layer toward plumes depends linearly on the ratio of effective viscosity for broad scale convection to effective boundary layer viscosity in terms of material properties this ratio is the ratio of viscosity across the boundary layer to about the 0 45 power for the case where the broad scale convection is heated from below the viscosity contrast across the boundary layer depends on the temperature contrast across the layer a late onset of mantle plumes is implied if the temperature contrast at the base of the mantle has increased with time that is if the core has cooled more slowly than the interior of the mantel in one scenario before about 3 b y b p the mantle temperature varied slowly because surface heat flow was nearly balanced by internal radioactivity during this time the mantle was hotter than present and its internal viscosity lower hie temperature contrast at the base of the mantel was thus small after 3 b y internal radioactivity no longer balanced surface heat flow and the mantle cooled about a few hundred degrees celsius as indicated by petrological observations the core cooled slower than the mantle and the temperature contrast across the bottom boundary layer increased with time leading to a late onset of plumes phenomenological inferences are obtained as venus may have cooled more rapidly than the earth its mantle may be too cool for rapid plate tectonics and the gravity field and the convective pattern are dominated by hot upwelling regions

experimental evidence for the effect of hydrographs on sediment pulse dynamics in gravel bedded rivers 1 gravel augmentation is a river restoration technique applied to channels downstream of dams where size selective transport and lack of gravel resupply have created armored relatively immobile channel beds augmentation sediment pulses rely on flow releases to move the material downstream and create conditions conducive to salmon spawning and rearing yet how sediment pulses respond to flow releases is often unknown here we explore how three types of dam releases constant flow small hydrograph and large hydrograph impact sediment transport and pulse behavior translation and dispersion in a channel with forced bar pool morphology we use the term sediment pulse generically to refer to the sediment introduced to the channel the zone of pronounced bed material transport that it causes and the sediment wave that may form in the channel from the additional sediment supply which can include input sediment and bed material in our experiments we held the volume of water released constant which is equivalent to holding the cost of purchasing a water volume constant in a stream restoration project the sediment pulses had the same grain size as the bed material in the channel we found that a constant flow 60 greater than the discharge required to initiate sediment motion caused a mixture of translation and dispersion of the sediment pulse a broad crested hydrograph with a peak flow 2 5 times the discharge required for entrainment caused pulse dispersion while a more peaked hydrograph 3 times the entrainment threshold discharge caused pulse dispersion with some translation the hydrographs produced a well defined clockwise hysteresis effecting sediment transport as is often observed for fine sediment transport and transportlimited gravel bed rivers the results imply a rational basis for design of water releases associated with gravel augmentation that is directly linked to the desired sediment behavior

effects of sediment pulses on channel morphology in a gravel bed river sediment delivery to stream channels in mountainous basins is strongly episodic with large pulses of sediment typically delivered by infrequent landslides and debris flows identifying the role of large but rare sediment delivery events in the evolution of channel morphology and fluvial sediment transport is crucial to an understanding of the development of mountain basins in july 2001 intense rainfall triggered numerous debris flows in a severely burnt watershed in the sapphire mountains of montana ten large debris flow fans were deposited on the valley floor and investigations focused on the channel response to these sediment pulses the channel has aggraded immediately upstream of each fan and braided in reaches immediately downstream channel incision through the fans has created sets of coarse grained terraces the deposition upstream of the pulses consists almost exclusively of fine material resulting in a median bed material size d 50 1 2 orders of magnitude lower than the ambient channel material the volume of sand being transported is so great that these aggrading reaches can extend hundreds of meters upstream of the fans with 1 2 m of sand deposited across the entire valley floor along a 10 km study reach cross section surveys longitudinal profiles and pebble counts chronicle channel response to a punctuated increase in sediment supply and provide insight on the processes of sediment wave dispersal