three dimensional cfd modeling of self forming meandering channel a three dimensional cfd model was used to compute the formation of the meandering pattern in an initially straight alluvial channel the numerical model was based on the finite volume method using an unstructured grid with dominantly hexahedral cells the k e model was used to predict turbulence and the simple method was used to compute the pressure the sediment transport was computed as bed load in addition to solving the convection diffusion equation for suspended sediment transport the bed changes were calculated and the grid was altered during the computation as channel erosion and deposition caused wetting and drying the model was tested by comparing with results from physical model studies carried out at colorado state univ fort collins colo the results showed successfully the replication of many of the meander characteristics including secondary currents cross sectional profiles meander planform meander wavelength downstream meander migration and chute formation

numerical modelling of downstream migrating antidunes a numerical model is presented that compute the geometrical dimensions and movement of downstream migrating antidunes the model solves the navier stokes equations together with the k epsilon turbulence model to find the water flow field over the bedforms a two dimensional width averaged grid is used the bed elevation changes are computed by solving the convection diffusion equation for suspended sediments and bed load together with the engelund hansen sediment transport formula the free surface is computed with an algorithm based on water continuity in the surface cells non orthogonal adaptive grids were used moving vertically with the computed location of the bed and the free water surface the numerical model was tested on data from a physical model study where regular downstream migrating antidunes had been observed the numerical model started out with a flat bed and the trains of antidunes formed over time many of the physical processes observed in earlier studies were replicated by the numerical model four dune parameters were computed in the current tests the antidune wavelength height and celerity together with the average water depth the antidune wavelengths were best predicted with an accuracy of 3 8 compared with the measurements the antidune heights were computed with a deviation of 11 25 compared with an empirical formula the water depths over the antidunes were predicted with an accuracy of 3 9 related to the measured values the average antidune celerity was the parameter with largest deviation for the coarsest grid it was overpredicted with 37

variations in natural levee morphology in anastomosed channel flood plain complexes abstract natural levees are common features of alluvial river systems yet their origin and evolution are poorly understood in this paper we present morphologic and sedimentologic data from two anastomosed rivers and offer a hypothesis of natural levee growth in these systems based on mechanisms of sediment transport in settings where floodbasins fill at the same rate as the channel levees form by turbulent diffusion of suspended sediment away from a high velocity thread into a floodbasin of relatively stagnant water the theory of diffusive transport suggests that these levees should be narrow steep and display abrupt decreases in grain size due to rapid decreases in carrying capacity with distance into the floodbasin in environments where an appreciable water surface slope is maintained between the main channel and the floodbasin levees form by advection of sediment out of the channel and into the floodbasin advective transport theory indicates that these levees should be broad and gently sloped with grain sizes gradually decreasing away from the main channel natural levees occurring in the anastomosed reach of the upper columbia river in se british columbia display significantly different morphology from levees in the cumberland marshes region of the lower saskatchewan river in east central saskatchewan at the upper columbia site the rise in stage of the floodbasin water nearly keeps up with that of the channel water because of good communication between channel and floodbasins through crevasses this inhibits the establishment of a water surface slope and the bulk of the escaping sediment is deposited close to the channel in contrast the cumberland marshes region is characterized by wide and volumetrically large floodbasins these conditions keep floodbasin water surface elevations relatively low and maintain an appreciable elevation head between the channel and its floodbasin fostering levee growth by advective transport

numerical modeling of bed deformation in laboratory channels a depth average model using a finite volume method with boundary fitted grids has been developed to calculate bed deformation in alluvial channels the model system consists of an unsteady hydrodynamic module a sediment transport module and a bed deformation module the hydrodynamic module is based on the two dimensional shallow water equations the sediment transport module is comprised of semiempirical models of suspended load and nonequilibrium bedload the bed deformation module is based on the mass balance for sediment the secondary flow transport effects are taken into account by adjusting the dimensionless diffusivity coefficient in the depth average version of the k e turbulence model a quasi three dimensional flow approach is used to simulate the effect of secondary flows due to channel curvature on bed load transport the effects of bed slope on the rate and direction of bed load transport are also taken into account the developed model has been validated by computing the scour hole and the

grain size controls on the morphology and internal geometry of river dominated deltas predictions of a delta s morphology facies and stratigraphy are typically derived from its relative wave tide and river energies with sediment type playing a lesser role here we test the hypothesis that all other factors being equal the topset of a relatively noncohesive sandy delta will have more active distributaries a less rugose shoreline morphology less topographic variation in its topset and less variability in foreset dip directions than a highly cohesive muddy delta as a consequence its stratigraphy will have greater clinoform dip magnitudes and clinoform concavity a greater percentage of channel facies and less rugose sand bodies than a highly cohesive muddy delta nine self formed deltas having different sediment grain sizes and critical shear stresses required for re entrainment of mud are simulated using deflt3d a 2d flow and sediment transport model model results indicate that sand dominated deltas are more fan shaped while mud dominated deltas are more birdsfoot in planform because the sand dominated deltas have more active distributaries and a smaller variance of topset elevations and thereby experience a more equitable distribution of sediment to their perimeters this results in a larger proportion of channel facies in sand dominated deltas and more uniformly distributed clinoform dip directions steeper dips and greater clinoform concavity these conclusions are consistent with data collected from the goose river delta a coarse grained fan delta prograding into goose bay labrador canada a reinterpretation of the kf 1 parasequence set of the cretaceous last chance delta a unit of the ferron sandstone near emery utah usa uses ferron grain size data clinoform dip data clinoform concavity and variance of dip directions to hindcast the delta s planform the kf 1 last chance delta is predicted to have been more like a fan delta in planform than a birdsfoot delta

stratigraphic variability due to uncertainty in model boundary conditions a case study of the new jersey shelf over the last 40 000 years abstract prediction of stratigraphy by numerical sedimentary models is influenced by a series of geological boundary conditions paleo bathymetry sediment supply sea level ocean storm climate and tectonics these boundary conditions evolve non linearly over time and have considerable uncertainty this study explores the impact of the uncertainty of complex boundary conditions on stratigraphic prediction two numerical models are sequentially run the climate driven hydrological model hydrotrend generates a time series of discharge and sediment load at the river mouth which then feed into the process based stratigraphic model sedflux 2d sedflux disperses sediment in the fluvio marine domain the bedload fluxes drive floodplain and mouthbar deposition while suspended sediment is dispersed in the ocean through hypopycnal plumes seafloor sediment is reworked by ocean storms failures and subsequent transport as sediment gravity flows sedflux records the thickness of the deposited sediment and the grain size preserved over time along a longitudinal profile one base case simulation and 20 sensitivity tests of the new jersey shallow margin over the last 40 000 yr serve to explore the impact of uncertain boundary conditions initially the shelf is exposed and arctic conditions prevail in the drainage area paleo temperature and precipitation estimates differ significantly between different community climate system model realizations impacting river sediment flux predictions subsequently high sediment supply from the rivers draining the melting laurentide ice sheet has a dominant impact on the shelf stratigraphy drainage basin characteristics are strongly controlled by the hard to define details of the laurentide ice sheet dynamics over the last 10 000 yr the shelf becomes sediment starved and storm dominated but the paleo storm conditions are uncertain and thus require a wide range of values shallow seismic data have been used to validate the large scale deposited thicknesses of the sedflux prediction the base case sedflux prediction for the sedimentary architecture of the new jersey shelf is consistent with the acoustically observed distribution of major sediment wedges due to the strong forcing of the relatively well constrained sea level changes the range in thickness predictions over the sensitivity tests is well within the lateral variations reconstructed from the observed seismic data sedflux predictions and a data set of 654 seafloor samples taken on the new jersey shelf between 50 and 150 m water depth shows a subtle fining towards deeper water sedflux systematically overpredicts the component of fine sediments at the seafloor likely because the model does not predict shell hash paleo storm climate is found to have a high uncertainty and strongly impacts the predicted grain size distributions we advocate a modeling approach which couples a series of sensitivity experiments to a predicted base case experiment in that way the stratigraphic variability caused by uncertainty in the boundary conditions is evident for later users

intratidal variations in stratification and mixing in the hudson estuary this study represents a detailed examination of intratidal variations in boundary layer structure in a relatively narrow straight section of the hudson river estuary during high river discharge the presence of active mixing is inferred from estimates of the richardson number the maximum height above bed to which mixing occurs is comparable during flood and ebb however the regions of mixing differ in density structure during the flood active mixing is confined to a well mixed near bed layer whereas during the ebb active mixing occurs throughout regions of significant stratification tidal straining maintains stratification during the ebb while it promotes the growth of a uniform bottom mixed layer during the flood in general maximum and minimum near bed stratification occur during late ebb and flood respectively reflecting the dominant role played by tidal straining in determining intratidal variation vertical stretching was also found to be an important term in the stratification balance during the ebb

anatomy of turbulence in thermally stratified lakes turbulence within the thermocline of two thermally stratified lakes lake biwa japan and lake kinneret israel was investigated using a portable flux profiler this instrument provided high resolution profiles of temperature conductivity and two components of velocity within a measuring volume of approximately 8 mm 3 each data profile was segmented into statistically stationary segments a range of properties of the turbulence including direct estimates of the vertical mass flux were then calculated for each segment it was found that turbulence in the thermocline was generally patchy but within a patch dissipation levels were relatively high the turbulent motions were found to be fine grained with small thorpe scales and a large skewness of the distribution of the displacement scale this is distinct to that found in the ocean where shear instability produces large overturns the measurement showed that the net vertical mass flux in the thermocline was negligible and less than that predicted by osborn 1980 this is explained by noting that the net buoyancy flux consisted of two opposing parts a down gradient irreversible flux due to turbulent mixing and an up gradient reversible flux due to restratification that often cancel each other within a segment the low measured net buoyancy flux within the interior of the lakes suggested that other processes such as gravitational adjustments and benthic boundary layer processes should be responsible for the basin averaged vertical transport in these lakes

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

the contribution of salt fingers to vertical mixing in the north atlantic tracer release experiment abstract the north atlantic tracer release experiment natre was performed in an area moderately favorable to salt fingers however the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent this is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems despite the lack of a staircase optical shadowgraph profiles revealed that small scale tilted laminae previously observed in a salt finger staircase were abundant at the natre site using microstructure observations the strength of salt finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy the dissipation ratio by examining the dissipation ratio in a parameter space of density ratio r and richardson number ri the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred while the model for turbulence descr

controls on natural levee development in the columbia river british columbia canada natural levees of the columbia river near golden british columbia were investigated to identify the mechanisms that control levee development and morphology topographic profiles of 12 levee pairs were surveyed and measurements of water surface elevation flow velocity flow direction and turbidity were obtained during an average magnitude flood 1ae2 years recurrence interval sedimentation rates and grain size distributions were measured from sediment traps placed along levee to floodbasin transects results show that water and sediment exchange between the channel and floodbasin was mainly by advection during flooding local floodbasins behave more as efficient water pathways than water storage features resulting in down valley floodbasin flows capable of limiting basinward growth of levees levee shape results primarily from two independent factors 1 maximum channel water stage which limits levee height and 2 floodbasin hydraulics which control width in the columbia river the competence of floodbasin flows results in relatively narrow and steep levees natural levees grow under two general conditions of deposition as governed by flood stage elevation relative to levee crest elevation front loading and back loading during large floods when crests are inundated front loading preferentially aggrades the proximal portions of levees with sediment directly from the channel thus increasing levee slope during average or below average floods when many levee crests are not overtopped back loading preferentially aggrades the distal levee areas and floodbasin floor reducing levee slope in the study area a balance between front and back loading sustains these narrow and steep levee shapes for long periods reflecting an equilibrium between hydraulic regime floodplain morphology and deposition

morphology and sedimentology of deltas in fjord and fjord valley lakes abstract sediment supply depositional processes and the associated morphology and sedimentology of deltas in a fjord and several fjord lakes have been studied two deltas in lakes in eastern norway receive sediments from fluvial erosion of quaternary glacial deposits and are only to a slight extent influenced by modern glaciers the other deltas are situated in western norway in the drainage area of the jostedalsbreen ice cap the meltwater rivers draining this glacier are characterized by a high rate of sediment transport in a wide range of grain sizes the main modes of the grain size distribution curves occur in the silt sand and cobble fractions the sediment transport and the water discharge of the rivers have been measured during the same periods the sedimentation rate of different grain fractions and their downstream variation in the frontal area of the deltas were determined by sediment traps the morphology of the deltas have been investigated with echo sounding differences in downstream sedimentation rates of the various deltas are related to dynamic and geometrical properties of the systems the effect of different factors on delta morphology and sedimentology is discussed

estimation of runoff critical shear stress for soil erosion from soil shear strength abstract critical shear stress is an important soil parameter governing detachment by runoff which appears in numerous erosion models soil shear strength has been often presented as the best soil property to predict critical shear stress however only a few studies have documented the relationship between soil shear strength and critical shear stress and the results obtained have sometimes seemed contradictory this is why we have put together all available data about the relationship between critical shear stress and soil shear strength in order to analyse and discuss its significance and its generality results showed that contradictions between different studies mainly arise because total shear stress is a poor predictor of detachment on rough soils when grain shear stress is used rather than total shear stress a significant relationship between saturated soil shear strength and critical shear stress does exist soil shear strength may thus be used to predict critical grain shear stress however it is recalled that in general only total shear stress is predicted by erosion models which makes detachment predictions difficult on rough natural soils

x band sea surface coherence time inferred from bistatic sar interferometry coherence time plays an important role in ocean surface observation by means of synthetic aperture radar sar for x band the autocorrelation function acf of the backscatter from the sea surface has been predicted using models up to now there exist only few experimental data and an empirical analysis of the sea surface coherence time based on a larger amount of data is missing in this paper we show how space based bistatic sar along track interferometry is used for the first time to measure the acf we achieve this by statistical analysis of the interferometric scene coherence as a function of the along track interferometric time lag instead of indirect estimation from the doppler bandwidth we use many data acquisitions of opportunity for that purpose thereby exploiting the diversity of along track baselines between the two sar sensors of the tandem x satellite formation the acf is related to the temporal decorrelation which is not directly observable we extract this component by quantifying and compensating for all other sources of decorrelation that contribute to the observed coherence

what is a fluvial levee abstract fluvial levees are elevated partitions between channels and floodplains because of their character and position levees may provide critical controls on and insights into geomorphic processes that determine the distribution of water and sediment within river systems few studies have analysed the character distribution sedimentology and processes that form levees in modern depositional environments characterisation of levee deposits from the mississippi river continues to form the basis for most levee interpretations from the rock record this discussion paper assesses the reliability of interpretations of levee deposits in numerous examples from the rock record and their associated inferences for river style this uncertainty reflects the lack of definitive sedimentological attributes for levee deposits their limited preservation potential and the fact that levee identification in the rock record is inhibited by the reliance on geometric descriptors or indirect associations between channel and floodplain facies given these concerns it is suggested that geomorphologists and sedimentologists need to recognise the limitations of our present knowledge of levees and work towards a more systematic understanding of these significant fluvial landforms in the full spectrum of modern and ancient river settings

overbank sediment deposition patterns for straight and meandering flume channels experiments with the 10 m flood channel facility at hr wallingford uk indicate a fundamental dependency of the overbank deposition pattern of channel suspended sediments on channel planform two experiments 100 and 140 l s 1 in a 1 95 m wide straight channel showed deposition concentrated in a berm along the channel bank little sediment was transferred further onto the floodplain for the larger flow the berm formed further from the channel a single experiment 103 l s 1 with a 1 31 m wide meandering channel showed deposition across the entire floodplain tongue between successive meanders maximum deposition occurred on the downstream side of the meander just past the bend apex these generalized flume results complement the real world but site specific data of field studies copyright 2002 john wiley sons ltd

multichannel analysis and suppression of sea clutter for airborne microwave radar systems one efficient method to detect slow targets with low signal to clutter plus noise ratios is space time adaptive processing stap to apply stap to maritime radar systems the multichannel properties of sea clutter have to be known this paper investigates these characteristics for airborne microwave radar systems to detect vessels in the open sea it is shown that the motion of sea echoes and different sea scattering types cause different multichannel properties of sea clutter compared with land clutter this paper provides a mathematical basis to describe multichannel sea clutter and predict stap performance over water to confirm the derived properties several real multichannel sea data sets for different swell directions and sea states were acquired with the airborne radar system phased array multifunctional imaging radar pamir and simulations were conducted this paper demonstrates the reproducibility of real sea clutter multichannel characteristics for different sea conditions with the help of calculations and simulations which are proposed in this analysis

topographically induced drag and mixing at a small bank on the continental shelf recent turbulence measurements over a small bank on the continental shelf off oregon reveal a previously undetected site for intense mixing of the coastal ocean the flow is hydraulically controlled and turbulence diffusivities over the bank are more than 100 times greater than estimates made on the shelf away from topography the total drag exerted by the bank on the flow field is a combination of bottom friction plus form drag analogous to mountain drag and is comparable to the coriolis force this drag is sufficient to decelerate the flow over the bank in a matter of hours

modelling hydrodynamics in the rio parana argentina an evaluation and inter comparison of reduced complexity and physics based models applied to a large sand bed river abstract depth averaged velocities and unit discharges within a 30 km reach of one of the world s largest rivers the rio parana argentina were simulated using three hydrodynamic models with different process representations a reduced complexity rc model that neglects most of the physics governing fluid flow a two dimensional model based on the shallow water equations and a three dimensional model based on the reynolds averaged navier stokes equations flow characteristics simulated using all three models were compared with data obtained by acoustic doppler current profiler surveys at four cross sections within the study reach this analysis demonstrates that surprisingly the performance of the rc model is generally equal to and in some instances better than that of the physics based models in terms of the statistical agreement between simulated and measured flow properties in addition in contrast to previous applications of rc models the present study demonstrates that the rc model can successfully predict measured flow velocities the strong performance of the rc model reflects in part the simplicity of the depth averaged mean flow patterns within the study reach and the dominant role of channel scale topographic features in controlling the flow dynamics moreover the very low water surface slopes that typify large sand bed rivers enable flow depths to be estimated reliably in the rc model using a simple fixed lid planar water surface approximation this approach overcomes a major problem encountered in the application of rc models in environments characterised by shallow flows and steep bed gradients the rc model is four orders of magnitude faster than the physics based models when performing steady state hydrodynamic calculations however the iterative nature of the rc model calculations implies a reduction in computational efficiency relative to some other rc models a further implication of this is that if used to simulate channel morphodynamics the present rc model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations these observations illustrate the trade off between model realism and efficiency that is a key consideration in rc modelling moreover this outcome highlights a need to rethink the use of rc morphodynamic models in fluvial geomorphology and to move away from existing grid based approaches such as the popular cellular automata ca models that remain essentially reductionist in nature in the case of the world s largest sand bed rivers this might be achieved by implementing the rc model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales

elevation adjustments of paired natural levees during flooding of the saskatchewan river natural levees control the exchange of water between an alluvial channel and its floodplain but little is known about the spatial distribution and evolution of levee heights the summer 2005 flood of the saskatchewan river cumberland marshes east central saskatchewan inundated large areas of floodplain for up to seven weeks forming prominent new deposits on natural levees along main stem channels measurements of flood deposit thickness and crest heights of 61 levee pairs show that the thickest deposits occur on the lower pre flood levee in 80 of the sites though no clear relationship exists between deposit thickness and magnitude of height difference only 16 of the pairs displayed thicker deposits on the higher levee half of which occurred at sites where relatively clear floodbasin waters re entered turbid channels during general flooding difference in crest elevation e between paired levees is approximately log normally distributed both before and after the flood though with different mean values supplemental observations from tank experiments indicate that during near bankfull flows temporally and spatially variable deposition and erosion occur on levees due to backwater effects associated with nearby channel bars and irregular rises of the channel bed forced by channel extension during floods preferential deposition in lows tends to even out crest heights copyright 2009 john wiley sons ltd

modelling free forming meander evolution in a laboratory channel using three dimensional computational fluid dynamics abstract the present study investigated the use of computational fluid dynamics cfd in predicting the formation development and migration of free forming meander bends the three dimensional cfd model computed water flow and sediment transport in alluvial channels and predicted vertical and horizontal bed changes different algorithms and parameters were tested to provide an insight into the application range of cfd when modelling free forming meander formation the computational domain was discretized by an unstructured grid a control volume method was used for the discretization of the navier stokes equations for the flow calculation and of the convection diffusion equation for the sediment transport calculation turbulence was modelled by the k e turbulence model the simulation was started from an initially straight grid with neither sediment feed nor any perturbation at the inflow boundary the model computed the river bed evolution over a real time period of 3 d results were compared with laboratory experiments and showed that the cfd model can predict many of the characteristics of the alluvial meander formation and migration however some limitations and uncertainties exist that have to be clarified in future investigations

sediment transport modeling review current and future developments the use of computational models for solving sediment transport and fate problems is relatively recent compared with the use of physical models several considerations govern the choice between physical and computational models namely the nature of the problem that needs to be solved the available resources and the overall cost associated with the problem solution in some specific problems a combination of physical and computational models can be used to obtain a better understanding of the processes under investigation de vries 1973 using computational hydrodynamic sediment transport models in general involves the numerical solution of one or more of the governing differential equations of continuity momentum and energy of fluid along with the differential equation for sediment continuity an advantage of computational models is that they can be adapted to different physical domains more easily than physical models which are typically constructed to represent site specific conditions another advantage of computational models is that they are not subject to distortion effects of physical models when a solution can be obtained for the same flow conditions identical reynolds and froude numbers same length scale in the three directions etc as those present in the field with the rapid developments in numerical methods for fluid mechanics computational modeling has become an attractive tool for studying flow sediment transport and associated pollutant fate processes in such different environments as rivers lakes and coastal areas representative processes in these environments include bed aggradation and degradation bank failure local scour around structures formation of river bends fining coarsening and armoring of streambeds transport of point source and nonpoint

river width adjustment i processes and mechanisms in 1993 a task committee tc of the asce was established to study the hydraulics bank mechanics and modeling of width adjustment in alluvial channels the work of the tc in reviewing width adjustment processes and mechanisms is reported in this paper a companion paper presents the findings of the tc with regard to width adjustment modeling this paper first establishes the geomorphic context within which width adjustments occur and it demonstrates that width adjustment may take place over a wide range of scales in time and space in the past engineering analyses of channel width have tended to concentrate on prediction of the equilibrium width for stable channels most commonly the regime extremal hypothesis and rational mechanistic approaches are used and these are reviewed herein more recently attention has switched to channels that are adjusting their morphology either due to natural instability or in response to changes in watershed land use river regulation or channel engineering chara

estimating ocean coherence time using dual baseline interferometric synthetic aperture radar a new technique for measuring the coherence time of the ocean surface at radar wavelengths has been developed and tested this technique requires an interferometric synthetic aperture radar system with at least two unique baselines along the direction of platform motion the coherence time of the surface may be presented as a high resolution coherence time map this technique was tested using the jpl airsar along track interferometer measurements of the ocean coherence at the l band were made at high spatial resolution under a variety of conditions a new operating technique that results in the required two baselines is described some parametric analysis is performed pertaining to the design of such a system a representative interferometric data set acquired at the strait of messina in 1991 is presented to illustrate the technique the interferometric phase data reveal and measure tidal currents flowing through the strait while the coherence time image shows considerable structure not visible in the backscatter or phase image coherence times in and around the strait were measured to be of the order of 0 1 s at the l band which is in agreement with previous estimates and measurements

internal tide dissipation mixing and vertical nitrate flux at the shelf edge of ne new zealand an internal tide on the ne shelf of new zealand was observed with a combination of moored temperature loggers and current meters and vertical profiling with a microstructure probe internal tide energy flux across the shelf edge was calculated to be 400 w m with considerable variability driven by the passage of a storm through the region energy associated with the internal tide was significantly greater than the energy of the barotropic tide or of inertial shear dissipation of the internal tide calculated from the energy loss between two of the moorings was estimated to be 15 10 mw m 2 the associated vertical eddy diffusivity was 4 3 x 10 4 m 2 s 1 the microstructure observations indicated internal tide driven vertical diffusivities at the nitracline of 7 x 10 4 m 2 s 1 the observations of vertical eddy diffusivities are combined with measurements of the vertical nitrate gradient to suggest that mixing driven by the internal tide is the dominant mechanism for driving diapycnal nutrient supply during summer the calculated flux of about 12 mmol n m 2 d 1 into the photic zone is suggested to drive significant new subsurface production throughout the summer amounting to a possible contribution to annual new production on the shelf of 100 g c m 2

natural levee deposition during the 2005 flood of the saskatchewan river abstract in summer 2005 a controlled flood of the saskatchewan river east central saskatchewan canada resulted in general floodplain inundation and extensive natural levee deposition along a 60 km reach extending from 40 km below the e b campbell dam to cumberland lake levee crests along channel banks were inundated for up to 7 weeks in some areas of the floodplain new deposits on levee crests varied from 0 to 70 cm in thickness displaying large variations both along reach and in opposing sites across channels mean grain sizes mainly silt and very fine sand likewise varied considerably among sample sites pre and post flood surveys of channel cross sections along the flooded reach permitted assessments of relationships between channel area changes and patterns of levee sedimentation in this system in which virtually all new flood sediment was derived by channel scour results show that both net deposition and net erosion occurred within the channel cross sections but that on average net channel enlargement of 4 2 prevailed over the entire survey reach when weighted by cross section size over the 60 km flooded reach zones of thick levee deposition occur at or just downstream of two areas of major channel enlargement and an intermediate zone of thin levee deposits is associated with an intermediate area of net channel aggradation this bimodal distribution of flood deposit thickness is inferred to have resulted from differences in sediment supply produced locally by the different extents of channel perimeter erosion two other factors 1 position of interfacial zones between clear floodbasin water and turbid channel water and 2 difference in pre flood levee heights contributed to the poor correspondence in thickness and grain size between opposing levees at some sites additional features of the new levee deposits including increases in transverse slopes abrupt basinward fining and paucity of deposition in distal areas due to clear floodbasin waters are characteristic of strong front loading that results when suspended sediment production is restricted to channel erosion processes

overbank sand deposition in relation to transport volumes during large magnitude floods in the dutch sand bed rhine river system using aerial photographs and field measurements sandy overbank deposits formed by the large magnitude floods of 1993 94 and 1995 were quantified along two branches of the dutch rhine river system the waal 1993 94 and 1995 and the ijssel 1995 these deposits were laid down intermittently all along the length of these rivers on the top and landward slope of the natural levees and covered about 4 per cent of the embanked floodplain on the waal and about 1 per cent on the ijssel the overbanks and transport mechanism is basically convective by nature the spatial variability of overbank sedimentation points to the important role played by helicoidal currents in determining overbank deposition the presence of embankments and training works appears to influence the sand transport to and morphological development of the floodplains along the dutch rhine river system overbank deposition volumes about equal present estimates of sand transport during a large magnitude flood it appears that studies on sand transport in the dutch rhine carried out so far underestimate sand transport during floods 1998 john wiley sons ltd

ocean wave imaging using an airborne single pass across track interferometric sar an airborne single pass across track interferometric synthetic aperture radar insar is used to image ocean waves a theoretical model explaining the imaging mechanisms is developed and simulations of the interferogram as well as the conventional sar intensity image are presented for given ocean wave spectra distortions of digital elevation models dem derived from insar data are explained by the motion of the sea surface a monte carlo method based on forward simulations is used to estimate variance spectra of the distorted elevation models it is shown that a straightforward estimation of wave height using the distorted insar elevation model is in good agreement with true wave height for low amplitude swell with about 10 error depending on propagation direction and coherence time however severe underestimation of wave height is found for wind seas propagating in flight direction forward simulations show that the distorted insar dem is less dependent an the model chosen for the real aperture radar mechanism than conventional sar images data acquired during an experiment over the north sea by a high precision insar system are compared with simulations

theoretical evaluation of several possible along track insar modes of terrasar x for ocean current measurements the german satellite terrasar x scheduled for launch in late 2006 will permit high resolution ocean current measurements by along track interferometric sar along track insar in various experimental modes of operation using different sections of its x band sar antenna array with a total length of 4 8 m as individual receive antennas depending on antenna and receive chain settings effective insar time lags of about 0 17 to 0 29 ms can be realized in combination with different noise levels single look resolutions swath widths and incidence angles we give an overview of the characteristics of the possible insar modes and evaluate their suitability for current measurements on the basis of simulated data products our results indicate that the quality of interferometric stripmap data from terrasar x will be clearly superior to the quality of the existing data acquired over the dutch coast during the shuttle radar topography mission accurate current retrievals can be expected at effective spatial resolutions on the order of 500 m however in modes using a multiplexed single receive chain the effective swath width of stripmap data will be limited to only 15 km while dual receive chain operation offers a swath width of 30 km for stripmap data and promises a reasonable data quality even for scansar data with a maximum swath width of 100 km finally we consider fundamental relations between along track baseline instrument noise and resulting insar phase noise to discuss the potential for current measuring performance improvements of terrasar x follow on satellites

sedimentology floods of record research on two bolivian rivers provides explanations of how and when they flood it also gives pointers for interpreting earth s history as recorded by the sediments left behind by flood waters

surface water subduction during a downwelling event in a semienclosed bay the ria de vigo is a bay strongly influenced by upwelling downwelling cycles along the adjacent coast of nw iberia moored and ship board observations during september 2006 showed that subduction initially associated with an estuarine circulation strengthened when a strong downwelling circulation resulting from northward wind over the coastal ocean was generated in the outer ria causing ambient waters to be advected outward in the lower layer incoming surface waters confined the estuarine circulation to the shallow interior and displaced isopleths downward through the water column at 10 m d 1 as the estuarine circulation retreated inward strong flow convergence developed between middle and inner ria in the layer above 15 m while divergence developed beneath the convergence increased through the period of downwelling favorable wind at a rate consistent with the observed isopleth displacement velocities the coefficient of turbulent diffusion kt from a microstructure profiler indicated that mixing was strong in the estuarine circulation and subsequently in the downwelling zone where localized instabilities and temperature salinity inversions were observed during the downwelling concentrations of phytoplankton including potentially harmful species increased especially in the middle and inner ria as a result of inward advection subduction and the ability of the dinoflagellates to maintain their position in the water column by swimming in the course of the 5 day event the water mass of all but the innermost ria was flushed completely and replaced by waters originating in the coastally trapped poleward flow along the atlantic coastline this article is protected by copyright all rights reserved

cross shore distribution of longshore sediment transport comparison between predictive formulas and field measurements abstract the skill of six well known formulas developed for calculating the longshore sediment transport rate was evaluated in the present study formulas proposed by bijker bijker e w 1967 some considerations about scales for coastal models with movable bed delft hydraulics laboratory publication 50 delft the netherlands journal of the waterways harbors and coastal engineering division 97 4 1971 687 engelund hansen engelund f hansen e 1967 a monograph on sediment transport in alluvial streams teknisk forlag copenhagen denmark ackers white journal of hydraulics division 99 1 1973 2041 bailard inman journal of geophysical research 86 c3 1981 2035 van rijn journal of hydraulic division 110 10 1984 1431 110 11 1984 1613 110 12 1984 1733 and watanabe watanabe a 1992 total rate and distribution of longshore sand transport proceedings of the 23rd coastal engineering conference asce 2528 2541 were investigated because they are commonly employed in engineering studies to calculate the time averaged net sediment transport rate in the surf zone the predictive capability of these six formulas was examined by comparison to detailed high quality data on hydrodynamics and sediment transport from duck nc collected during the duck85 superduck and sandyduck field data collection projects measured hydrodynamics were employed as much as possible to reduce uncertainties in the calculations and all formulas were applied with standard coefficient values without calibration to the data sets overall the van rijn formula was found to yield the most reliable predictions over the range of swell and storm conditions covered by the field data set the engelund hansen formula worked reasonably well although with large scatter for the storm cases whereas the bailard inman formula systematically overestimated the swell cases and underestimated the storm cases the formulas by watanabe and ackers white produced satisfactory results for most cases although the former overestimated the transport rates for swell cases and the latter yielded considerable scatter for storm cases finally the bijker formula systematically overestimated the transport rates for all cases it should be pointed out that the coefficient values in most of the employed formulas were based primarily on data from the laboratory or from the river environment thus re calibration of the coefficient values by reference to field data from the surf zone is expected to improve their predictive capability although the limited amount of high quality field data available at present makes it difficult to obtain values that would be applicable to a wide range of wave and beach conditions

interferometric radar measurement of ocean surface currents we describe a new method of measuring surface currents using an interferometric synthetic aperture radar an airborne implementation has been tested over the san francisco bay near the time of maximum tidal flow resulting in a map of the east west component of the current only the line of sight component of velocity is measured by this technique where the signal to noise ratio was strongest statistical fluctuations of less than 4 cm s 1 were observed for ocean patches of 60 60 m

three dimensional modeling of sediment transport in a narrow 90 channel bend a three dimensional numerical model was used for calculating the velocity and bed level changes over time in a 90 bended channel the numerical model solved the reynolds averaged navier stokes equations in three dimensions to compute the water flow and used the finite volume method as the discretization scheme the k e model predicted the turbulence and the simple method computed the pressure the suspended sediment transport was calculated by solving the convection diffusion equation and the bed load transport quantity was determined with an empirical formula the model was enhanced with relations for the movement of sediment particles on steep side slopes in river bends located on a transversally sloping bed a sediment particle has a lower critical shear stress than on a flat bed also the direction of its movement deviates from the direction of the shear stress near the bed these phenomenona are considered to play an important role in the morphodynamic process in sharp channel bends the calculat

observations of mixing near the sides of a deep lake in winter the dissipation rate of turbulent kinetic energy per unit mass vertical eddy diffusivity kz and the rate of dissipation of temperature variance x are estimated over the sloping sides of lake geneva both far from and near the bed using temperature data from horizontal tracks of a submarine during periods of winter cooling the estimated values are about one order of magnitude greater near the slope than those distant from it the mean dissipation rate per unit mass averaged over the epilimnion varies between o 10 210 m 2 s 23 in calm weather to o 10 28 m 2 s 23 in winds of 8 m s 21 for surface buoyancy fluxes of 6 3 10

three dimensional numerical modelling of levee depositions in a scandinavian freshwater delta abstract a three dimensional numerical model was used to compute the water flow and the sediment deposition for a 100 year flood in the delta of lake oyeren in norway the model solved the reynolds averaged navier stokes rans equations with the k e turbulence model on a three dimensional unstructured grid the sediment transport was computed solving the convection diffusion equation for sediment concentration using an empirical formula for the bed pickup rate the results were tested against sedimentation data obtained from field measurements in a qualitative way both the mapped sedimentation patterns on the delta islands and the computed ones showed the typical levee like structures along the deltaic channels a series of sensitivity tests showed the important role of uncertainties in the bed roughness parameter the incoming sediment concentration the grain size distribution on the channel bed and the vegetation parameters the presence of vegetation strongly affected the position and intensity of the sediment depositions and its proper parameterization plays a key role for the modelling according to the computations the levee deposition during the 1995 flood appeared as a process where fine sand mainly originating from the channel bed was brought into suspension and deposited on the floodplain where the flow velocity was lower and the flow became overloaded with sediment an inspection of the simulated flow patterns and velocity vectors suggested that advective sediment transfer out of the channel and into the floodplain probably was the dominating process for the levee depositions

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

tidal asymmetry in an estuarine pycnocline depth and thickness tidal variations in estuarine stratification are revealed by the depth and thickness of the density interface the depth of the interface may be predicted using an inviscid two layer model that combines baroclinic estuarine circulation with barotropic tidal currents helfrich 1995 here we present results from a two layer model modified to include the effects of bottom friction and interfacial mixing modeled layer thickness and speed compare favorably with prior analytic studies farmer and armi 1986 pratt 1986 we use a bulk richardson number criterion to estimate the thickness of the pycnocline from two layer model results the predicted pycnocline depth and thickness compare remarkably well with observations we also investigate the effects of changing bottom friction and barotropic currents on the pycnocline thickness

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

hydraulic erosion of cohesive riverbanks abstract this study identifies and assesses the controls on hydraulic erosion of cohesive riverbanks on a 600 m reach of an urban ephemeral stream with active bank erosion we examined hydraulic bank erosion by separating estimated bank shear stress into four properties magnitude duration event peak and variability the values of these independent variables were used as a bank erosion context at three transects stepwise regression showed that the event peak maximum peak of excess shear stress best predicts cohesive bank erosion at the two transects with moderate critical shear stresses 1 93 4 08 n m 2 while the variability all peaks of excess shear stress best predicts erosion at the transect with low critical shear stress 0 95 n m 2 these results suggest that the amount of hydraulic erosion of cohesive riverbanks is dictated by flow peak intensities finally the results of this study were combined with results from previous bank erosion studies to produce a conceptual model for estimating bank erosion rates based on their silt clay content

bed topography and sediment sorting in channel bend with unsteady flow bed topography and transverse sediment sorting in an alluvial channel bend were investigated under unsteady flow conditions with nonuniform sediment five experiments each having the same initial sediment size gradation but a different inflow hydrograph were done in a 180 channel bend with a constant radius of curvature in addition bed elevations across various sections of the bend were measured and bed surface sediment were sampled at the peak and end of the hydrograph in each experiment experimental results indicated that the characteristics of the hydrograph have prominent influences on bed topography and transverse sediment sorting those cases with a higher ramping rate of the hydrograph have greater deposition heights near the inner bank and larger scour depths near the outer bank furthermore the sediment is finer near the inner bank and coarser near the outer bank in cases with a higher ramping rate the bed deformation and sediment size variation correlated well with the unsteady flow par

anatomy and dynamics of a floodplain powder river montana u s a abstract centimeter scale measurements on several powder river floodplains provide insights into the nature of overbank depositional processes that created the floodplains during a 20 year period after a major flood in 1978 rising stages initially entered across a sill at the downriver end of the floodplains later as stages continued to rise water entered the floodplains through distinct low saddles along natural levees the annual maximum depth of water over the levee crest averaged 0 19 m from 1983 through 1996 and the estimated flow velocities were approximately 0 15 m s 1 water ponded in the floodplain trough a topographic low between the natural levee and the pre flood riverbank and mud settled as thin layers of nearly constant thickness mud layers alternated with sand layers which were relatively thick near the channel together these beds created a distinctive natural levee in some locations individual flood deposits began as a thin mud layer that gradually coarsened upwards to medium grained sand coarsening upwards sequences form initially as mud because only the uppermost layers of water in the channel supply the first overbank flows which are rich in mud but starved of sand at successively higher stages fine sands and then medium sands increase in concentration in the floodwater and are deposited as fine and medium sand layers overlying the initial mud layer theoretical predictions from mathematical models of sediment transport by advection and diffusion indicate that these processes acting alone are unlikely to create the observed sand layers of nearly uniform thickness that extend across much of the floodplain we infer that other transport processes notably bedload transport must be important along powder river even with the centimeter scale measurements of floodplain deposits daily hydraulic data and precise annual surface topographic surveys we were unable to determine any clear correspondence between the gauged flow record of overbank floods and the depositional layers mapped in the floodplain these results provide a detailed example of floodplain deposits and depositional processes that should prove useful for interpreting natural levee deposits in a variety of geologic settings

diapycnal diffusivity inferred from scalar microstructure measurements near the new england shelf slope front abstract conductivity microstructure was used to estimate the diapycnal thermal eddy diffusivity kt near the new england shelf slope front in early august 1997 two datasets were collected with a towed vehicle one involved several horizontal tows in and above a warm salty layer near the seafloor and the other was from a tow yo transect that sampled most of the water column in the bottom layer kt derived from microstructure is a factor of about 5 smaller than estimates derived from tracer dispersion at the same density level and the diffusivity decreases sharply as the buoyancy frequency n increases kt n 3 1 with several assumptions this behavior is consistent with laboratory results for shear driven entrainment across a density interface the bottom layer cools as it moves up the shelf mainly due to diapycnal mixing and a simplified temperature budget of the layer yields a diffusivity of 3 10 6 m2 s 1 which is between the values derived from microstructure and tracer dispersion in the tow

current measurements by sar along track interferometry from a space shuttle we present one of the first studies on ocean current retrievals from interferometric synthetic aperture radar insar data acquired during the shuttle radar topography mission srtm in february 2000 the insar system of srtm was designed for high resolution topographic mapping of the earth s land surfaces using two sar antennas on a space shuttle with a cross track separation of 60 m an additional along track antenna separation of 7 m resulted in an effective time lag of about 0 5 ms between the two images which could theoretically be exploited for target velocity retrievals however the feasibility of ocean current measurements with srtm has been questionable since the time lag was much shorter than the theoretical optimum about 3 ms at x band and the signal to noise ratio over water was quite low nevertheless some x band insar images of coastal areas exhibit clear signatures of tidal flow patterns as an example we discuss an image of the dutch wadden sea we convert the insar data into a line of sight current field which is then compared with results of the numerical circulation model kustwad for tidal phases close to the conditions at the time of the srtm overpass we obtain correlation coefficients of up to 0 6 and rms differences on the order of 0 2 m s furthermore we find that srtm resolves current variations down to spatial scales on the order of 1 km this is consistent with predictions of a numerical insar imaging model remaining differences between srtm and kustwad derived currents can be attributed mainly to residual motion errors in the srtm data as well as to a limited representation of the conditions at the time of the srtm overpass in the available kustwad results

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

three dimensional cfd modeling of morphological bed changes in the danube river 1 the bed changes in a section of the river danube were computed using a 3 d computational fluid dynamics model a time series of discharges during the flood in 2002 was used the results compared reasonably well with regular bed level surveys before and after the flood the danube river section was 6 km long and located between vienna and the austrian slovakian border the fully three dimensional numerical model solved the navier stokes equations using the k epsilon turbulence closure nonuniform sediment transport was computed using the formulas of wu et al 2000b considering hiding exposure algorithms both bed deformation and sorting processes were calculated a number of parameter sensitivity tests were carried out on roughness values parameters in the sediment transport capacity formula parameters in the hiding exposure formulas critical shields number and variations in the formulas for the effect of a sloping bed additionally the sediment inflow to the section was varied together with investigations of physical parameters such as vegetation and groyne structures the results were most sensitive to the shields number for critical movement of the sediments an error analysis was performed to give a quantitative assessment of the parameter sensitivity tests

stratification and turbulence in subtidal channels through intertidal mudflats 1 field observations in san francisco bay of subtidal channels that drain through intertidal mudflats indicate that substantial periodic stratification develops in very shallow flows with distinct asymmetries between flood and ebb conditions the greatest variability in salinity and stratification occurs during the wet winter months very strong longitudinal salinity gradients develop across the intertidal zone between salty subtidal water and fresher water draining into the marsh upstream tidal straining of the longitudinal gradient creates the periodic stratification stratifying through ebbs and destratifying through floods the stratification can be very strong in very shallow flows with n2 0 1 s 2 in about 0 5 m water depth analysis of the data with dimensionless numbers shows that at times the stratification is strong enough to suppress turbulence and mixing the asymmetry in stratification between ebbs and floods results in asymmetries in eddy viscosity and eddy diffusivity with lower values during the stratified ebbs because of the intertidal elevation of the system the strong longitudinal salinity gradient is regenerated each time the mudflats and marsh drain the system approaches a tidally periodic steady state where the flow dynamics are dominated by the sharp front of salty water that advects into the intertidal zone on the rising tide and exits the system by each lower low water the field data are supported by a numerical model with simple mudflat channel bathymetry both the field and numerical work indicate that the shallow low energy flows in mudflat channels transition between strongly stratified suppressed turbulence and relatively unstratified active turbulence each tidal cycle

turbulence in an estuarine embayment observations from beatrix bay new zealand 1 this study characterizes turbulence in a stratified estuarine embayment using measurements of temperature gradient microstructure the water column was generally strongly stratified with only moderate tidal forcing dissipation levels and turbulence length scales in the surface mixing layer the pycnocline the fluid interior and the benthic boundary layer were measured and compared with expectations based on previous work in different systems parameters important for modeling including the measured dissipation ratio d and the gradient richardson number rig were calculated the observations highlight a number of points 1 the lack of strong tidal flows means that buoyancy effects in estuarine embayments are more important than in the estuary proper 2 the combination of reduced tidal mixing and interacting thermohaline stratification suggests the importance in certain situations of diffusive convection 3 despite the shallow depths 40 m a number of hydrodynamic regimes vertically adjacent to one another exist resulting in a highly variable distribution of turbulence properties

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

behavior of meandering overbank channels with graded sand beds measurements of velocity distributions depth variation and sediment transport have been made under bankfull and overbank flow conditions in meandering channels with a graded sand bed using the large scale u k flood channel facility the overbank conditions depend upon the relative strength of opposing secondary circulation cells generated by shear at the channel crossover and centrifugal forces around the meander bend generally the shear generated secondary flow either reversed or weakened the centrifugal circulation around the next downstream bend this led to considerable modification of the main channel bed morphology which in turn altered flow distributions measurements of the lateral distribution of bed load were made using a scale helley smith sampler this demonstrated that the bed load was generally concentrated within a limited width of the channel and tended to take the shortest route through the meanders comparisons of observed and calculated bed material load gives an indication of how secondary circulation around meanders under both bankfull and overbank conditions affects the predictive performance of formulas derived for predominantly one dimensional flow

improved ocean surface velocity precision using multi channel sar this paper investigates new approaches to estimating the motion of the dynamic ocean surface using a multi channel synthetic aperture radar msar with m phase centers arranged in an along track configuration the objective of this paper is to determine the processing methods that produce the finest velocity resolution an issue that arises due to the finite coherence time of radar backscatter produced by the sea surface the investigation is carried out both theoretically using synthesized data produced from a modeled msar covariance matrix as well as experimentally using images collected with a 16 channel system three processing methods are considered linear regression along with a multi baseline phase progression estimation of the velocity centroid and coherent averaging of the shortest baseline interferograms both the theoretical and experimental results indicate that simple averaging of the shortest baseline interferograms often produces the best velocity precision

sensitivity of bottom stress and bottom roughness estimates to density stratification eckernforde bay southern baltic sea thermohaline density stratification may significantly alter the classic near bottom logarithmic velocity profile in many weak to moderately energetic partially mixed estuaries results from eckernforde bay suggest fits to log profiles which neglect thermohaline stratification may lead to overestimates of bottom stress and roughness of the order of 130 and 600 respectively measurements of velocity obtained at four heights within 1 m of the seabed are input to theoretical models for velocity shear derived via dimensional arguments for the overlap layer previous investigators applying dimensional arguments to thermohaline stratification in estuaries have assumed buoyancy flux to be independent of height within the overlap layer this may be a poor assumption since there is no significant source or sink of thermohaline buoyancy at the sediment water interface in this paper dimensional arguments which do not assume constant buoyancy flux are used to reduce estimates of the drag coefficient and bottom roughness to below the unreasonably high values predicted by simple log profiles formulations assuming very weak and relatively strong stratification are applied and estimates of buoyancy frequency derived from fits to velocity profiles are compared with independent estimates of stratification estimates of bottom stress and roughness derived from velocity profiles are also found to be sensitive to fluid acceleration uncertainties in instrument settling and limitations in current meter accuracy but these latter effects appear secondary in eckernforde bay to the impacts of thermohaline stratification

floodplain development in an engineered setting engineered flood bypasses or simplified conveyance floodplains are natural laboratories in which to observe floodplain development and therefore present an opportunity to assess delivery to and sedimentation within a specific class of floodplain the effects of floods in the sacramento river basin were investigated by analyzing hydrograph characteristics estimating event based sediment discharges and reach erosion deposition through its bypass system and observing sedimentation patterns with field data sediment routing for a large iconic flood suggests high rates of sedimentation in major bypasses which is corroborated by data for one bypass area from sedimentation pads floodplain cores and sediment removal reporting from a government agency these indicate a consistent spatial pattern of high sediment accumulation both upstream and downstream of lateral flow diversions and negligible sedimentation in a hydraulic shadow directly downstream of a diversion weir the pads located downstream of the shadow recorded several centimeters of deposition during a moderate flood in 2006 increasing downstream to a peak of 10 cm thick and thinning rapidly thereafter flood deposits in the sediment cores agree with this spatial pattern containing discrete sedimentation layers from preceding floods that increase in thickness with distance downstream of the bypass entrance to several decimeters thick at the peak and then thin downstream these patterns suggest that a quasi natural physical process of levee construction by advective overbank transport and deposition of sediment is operating the results improve understanding of the evolution of bypass flood control structures the transport and deposition of sediment within these environments and the evolution of one class of natural levee systems copyright 2008 john wiley sons ltd

mapping surface currents and waves with interferometric synthetic aperture radar in coastal waters observations of wave breaking in swell dominant conditions airborne and spaceborne interferometric synthetic aperture radars insars produce surface velocity measurements at very high spatial resolutions over a large area the data allow construction of the velocity strain field for highlighting ocean surface processes such as wave breaking and rip currents also coherence between signals from two interferometric channels is a descriptor of the correlation condition of the surface roughness that scatters back the radar signals and it is an indication of the ocean surface turbulence wave breaking is a major turbulence source causing surface roughness decorrelation thus the coherence parameter serves as an independent means for detecting wave breaking the results of breaking detection using roughness decorrelation and critical local acceleration are comparable in this paper the breaking fraction in swell dominant mixed seas along a cross shore transect is compared with several steepness parameters characterizing different length scales of surface waves the highest correlation coefficient from 0 90 to 0 99 is between the breaking fraction and windsea mean square slope contributed primarily by short waves this result reinforces the previous field observations showing that the length scales of breaking waves are much shorter than the energetic components near the spectral peak although dominant waves and the associated wave group modulation are important in triggering the breaking process the large spatial coverage of airborne or spaceborne operation further offers the opportunity to investigate evolution of the surface wave spectrum in high spatial subkilometer resolution this capability is very useful for monitoring the coastal wave and current environment

episodic sediment accumulation on amazonian flood plains influenced by el nino southern oscillation continental scale rivers with a sandy bed sequester a significant proportion of their sediment load in flood plains the spatial extent and depths of such deposits have been described and flood plain accumulation has been determined at decadal timescales but it has not been possible to identify discrete events or to resolve deposition on near annual timescales here we analyse 210 pb activity profiles from sediment cores taken in the pristine beni and mamore river basins which together comprise 720 000 km2 of the amazon basin to investigate sediment accumulation patterns in the andean amazonian foreland we find that in most locations sediment stratigraphy is dominated by discrete packages of sediments of uniform age which are typically 20 80 cm thick with system wide recurrence intervals of about 8 yr indicating relatively rare episodic deposition events ocean temperature and stream flow records link these episodic events to rapidly rising floods associated with la nina events which debouch extraordinary volumes of sediments from the andes we conclude that transient processes driven by the el nino southern oscillation cycle control the formation of the bolivian flood plains and modulate downstream delivery of sediments as well as associated carbon nutrients and pollutants to the amazon main stem

three dimensional modeling of nonuniform sediment transport in an s shaped channel a three dimensional numerical model was applied to compute uniform and nonuniform sediment transport and bed deformation in an s shaped laboratory channel located at the university of innsbruck where detailed measurements of the velocity field and bed elevation changes were made the channel had two bends a trapezoidal cross section and a slope of s 0 005 gravel with a mean diameter of 4 2 mm was used as movable bed material and for sediment feeding wu s formula for multiple grain sizes was compared with van rijn s formula using one grain size fairly good agreement was found between the computed and measured bed elevations for both approaches whereas wu s formula could further improve the numerical results looking at the physics of the erosion pattern the computed scour areas were located slightly more downstream than what was observed in the physical model the current study also includes several parameter tests grid distribution in vertical lateral and longitudinal direction time step numb

microstructure observations of turbulent mixing in a partially mixed estuary part i dissipation rate variations of turbulent mixing in the water column and in the benthic boundary layer were observed with a microstructure profiler in the hudson river estuary during two cruises in summer and fall of 1995 variability patterns of stratification shear richardson number ri and turbulent dissipation rates e were similar to those of earlier observations with strong turbulence in the weakly stratified bottom layer weak turbulence in the halocline during neap tides and low ri and high e spanning the water column during spring ebbs depthintegrated turbulent dissipation rates re dz approximately equaled the work done by the tidal pressure gradient force adjusted for the change in tidal kinetic energy alternatively a suitable scaling for re dz is also provided by the product of bottom shear stress and depth average velocity t by a relationship that fails at slack tide however at heights above bottom of z 0 3 m and again excepting slack tides observed e were highly correlated with a law of the wall dissipation rate eb 5 kz ratios e eb were close to 1 here the friction 3 u

coupled changes in sand grain size and sand transport driven by changes in the upstream supply of sand in the colorado river relative importance of changes in bed sand grain size and bed sand area sand transport in the colorado river in marble and grand canyons was naturally limited by the upstream supply of sand prior to the 1963 closure of glen canyon dam the river exhibited the following four effects of sand supply limitation 1 hysteresis in sediment concentration 2 hysteresis in sediment grain size coupled to the hysteresis in sediment concentration 3 production of inversely graded flood deposits and 4 development or modification of a lag between the time of a flood peak and the time of either maximum or minimum depending on reach geometry bed elevation construction and operation of the dam has enhanced the degree to which the first two of these four effects are evident and has not affected the degree to which the last two effects of sand supply limitation are evident in the colorado river in marble and grand canyons the first three of the effects involve coupled changes in suspended sand concentration and grain size that are controlled by changes in the upstream supply of sand during tributary floods sand on the bed of the colorado river fines this causes the suspended sand to fine and the suspended sand concentration to increase even when the discharge of water remains constant subsequently the bed is winnowed of finer sand the suspended sand coarsens and the suspended sand concentration decreases independently of discharge also associated with these changes in sand supply are changes in the fraction of the bed that is covered by sand thus suspended sand concentration in the colorado river is likely regulated by both changes in the bed sand grain size and changes in the bed sand area a physically based flow and suspended sediment transport model is developed tested and applied to data from the colorado river to evaluate the relative importance of changes in the bed sand grain size and changes in the bed sand area in regulating suspended sand concentration although the model was developed using approximations for steady uniform flow and other simplifications that are not met in the colorado river the results nevertheless support the idea that changes in bed sand grain size are much more important than changes in bed sand area in regulating the concentration of suspended sand published by elsevier b v

discriminating between pore filling load and bed structure load a new porosity based method exemplified for the river rhine sediments contained in the river bed do not necessarily contribute to morphological change the finest part of the sediment mixture often fills the pores between the larger grains and can be removed without causing a drop in bed level the discrimination between pore filling load and bed structure load therefore is of practical importance for morphological predictions in this study a new method is proposed to estimate the cut off grain size that forms the boundary between pore filling load and bed structure load the method evaluates the pore structure of the river bed geometrically only detailed grain size distributions of the river bed are required as input to the method a preliminary validation shows that the calculated porosity and cut off size values agree well with experimental data application of the new cut off size method to the river rhine demonstrates that the estimated cut off size decreases in a downstream direction from about 2 to 0 05 mm covariant with the downstream fining of bed sediments grain size fractions that are pore filling load in the upstream part of the river thus gradually become bed structure load in the downstream part the estimated mass percentage of pore filling load in the river bed ranges from 0 in areas with a unimodal river bed to about 22 in reaches with a bimodal sand gravel bed the estimated bed porosity varies between 0 15 and 0 35 which is considerably less than the often used standard value of 0 40 the predicted cut off size between pore filling load and bed structure load dc p is fundamentally different from the cut off size between wash load and bed material load dc w irrespective of the method used to determine dc p or dc w dc w values are in the order of 10 1 mm and mainly dependent on the flow characteristics whereas dc p values are generally much larger about 100 mm in gravel bed rivers and dependent on the bed composition knowledge of dc w is important for the prediction of the total sediment transport in a river including suspended fines that do not interact with the bed whereas knowledge of dc p helps to improve morphological predictions especially if spatial variations in dc p are taken into account an alternative to using a spatially variable value of dc p in morphological models is to use a spatially variable bed porosity which can also be predicted with the new method in addition to the morphological benefits the new method also has sedimentological applications the possibility to determine quickly whether a sediment mixture is clast supported or matrix supported may help to better understand downstream fining trends sediment entrainment thresholds and variations in hydraulic conductivity