statistical properties of the surface velocity field in the northern gulf of mexico sampled by glad drifters the grand lagrangian deployment glad used multiscale sampling and gps technology to observe time series of drifter positions with initial drifter separation of o 100 m to o 10 km and nominal 5 min sampling during the summer and fall of 2012 in the northern gulf of mexico histograms of the velocity field and its statistical parameters are non gaussian most are multimodal the dominant periods for the surface velocity field are 1 2 days due to inertial oscillations tides and the sea breeze 5 6 days due to wind forcing and submesoscale eddies 9 10 days and two weeks or longer periods due to wind forcing and mesoscale variability including the period of eddy rotation the temporal e folding scales of a fitted drifter velocity autocorrelation function are bimodal with time scales 0 25 0 50 days and 0 9 1 4 days and are the same order as the temporal e folding scales of observed winds from nearby moored national data buoy center stations the lagrangian integral time scales increase from coastal values of 8 h to offshore values of approximately 2 days with peak values of 3 4 days the velocity variance is large o 1 m2 s2 the surface velocity statistics are more anisotropic and increased dispersion is observed at flow bifurcations horizontal diffusivity estimates are o 103 m2 s in coastal regions with weaker flow to o 105 m2 s in flow bifurcations a strong jet and during the passage of hurricane isaac the gulf of mexico surface velocity statistics sampled by the glad drifters are a strong function of the feature sampled topography and wind forcing

hydraulic and sedimentary processes causing anastomosing morphology of the upper columbia river british columbia canada abstract the upper columbia river british columbia canada shows typical anastomosing morphology multiple interconnected channels that enclose floodbasins and lateral channel stability we analysed field data on hydraulic and sedimentary processes and show that the anastomosing morphology of the upper columbia river is caused by sediment bedload transport inefficiency in combination with very limited potential for lateral bank erosion because of very low specific stream power 2 3 w m 2 and cohesive silty banks in a diagram of channel type in relation to flow energy and median grain size of the bed material data points for the straight upper columbia river channels cluster separately from the data points for braided and meandering channels measurements and calculations indicate that bedload transport in the anastomosing reach of the upper columbia river decreases downstream because of lateral channel stability no lateral storage capacity for bedload is created therefore the surplus of bedload leads to channel bed aggradation which outpaces levee accretion and causes avulsions because of loss of channel flow capacity this avulsion mechanism applies only to the main channel of the system which transports 87 of the water and 90 of the sediment in the cross valley transect studied because of very low sediment transport capacity the morphological evolution of most secondary channels is slow measurements and calculations indicate that much more bedload is sequestered in the relatively steep upper anastomosing reach of the upper columbia river than in the relatively gentle lower anastomosing reach with anastomosing morphology and related processes e g crevassing being best developed in the upper reach this confirms the notion of upstream rather than downstream control of upper columbia river anastomosis

on the generation of african squall lines abstract squall lines sls form an important component of the meteorology of northern africa and in particular contribute substantially to rainfall totals their generation requires the existence of a potentially unstable low level supply of moisture overlain by dry desert air and vertical wind shear beneath the midlevel african easterly jet the instability may be released and an sl initiated by factors such as surface heating topography african waves or surface evaporation the relative importance of each of these factors and the means by which they impact on sl generation is reviewed this is followed by a detailed analysis of one month of satellite imagery and surface data for august 1985 over a portion of central northern africa the novelty of our study lies in the temporal resolution of the satellite imagery which with 21 images per day allows the identification of a large number of short lived sls 4 h duration or less on the southern fringes of the sahara these are likely to contribute

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

absolute transports of mass and temperature for the north atlantic current subpolar front system the flow of subtropical waters carried into the northern north atlantic ocean by the north atlantic current subpolar front system nac spf is an important component of the meridional overturning circulation these waters become colder and denser as they flow through the subpolar region both by mixing with the colder subpolar waters and by atmospheric cooling the relative roles of these two processes remain to be quantified and the mechanisms driving lateral mixing need to be better understood to address those questions a new methodology is developed to estimate the mean absolute transports of mass and heat for the top 1000 dbar in the region of the nac spf for the time period 1993 2000 the transports are obtained by combining historical hydrography with isopycnal rafos float data from the area the mean absolute transport potential field shows an nac spf pipe defined by two bounding transport potential contours this pipe transports 10 0 6 3 5 sv sv 106 m3 s 21 top 1000 dbar from the subtropics into the eastern subpolar north atlantic in contrast to earlier studies the northward flowing nac follows a distinct meandering path with no evidence of permanent branches peeling off the current before reaching the northwest corner as the current enters the northwest corner it loses its tight structure and maybe splits into two or more branches which together constitute the eastward flow along the spf the eastward flow between the northwest corner and the mid atlantic ridge is not as tightly defined because of the meandering and or eddy shedding of the branches constituing the spf as the flow approaches the mid atlantic ridge it converges to cross above the charlie gibbs and faraday fracture zones the mean absolute temperature transport top 1000 dbar by the 10 sv pipe was estimated across 10 transects crossing the nac spf because the mean mass flux is constant in the pipe variations in the mean temperature transports result from lateral exchange and mixing across the pipe s side walls and from air sea fluxes across the surface of the pipe the nac spf current loses 0 18 6 0 05 pw on its transit through the region most of the loss occuring upstream of the northwest corner the heat loss is 10 times the corresponding heat lost to the atmosphere we conclude that cross frontal exchange induced by the steep meanders of the northward flowing nac is the main mechanism by which heat is lost along the current in the region between the tail of the grand banks and the mid atlantic ridge

intermediate and deep currents of the black sea obtained from autonomous profiling floats abstract float trajectories observed at three depths 200 750 and 1550 m in the black sea during a 1 5 yr period offer new insights on the circulation below the permanent pycnocline the float observations for the first time provided direct quantitative evidence for strong currents and a well organized flow structure at intermediate and deep layers in contrast to prior ideas of a rather sluggish deep circulation of the black sea the magnitudes of intermediate and deep currents are typically about 5 10 of the surface currents reaching as much as 5 cm s 1 at 1550 m adjacent to the steep topographic slope on the periphery the combination of float and sea level height altimeter data suggests that a well defined cyclonic circulation extends from the surface to the bottom without reversal in its direction deep currents are steered by the steep topographic slope and well correlated with surface currents at seasonal and longer time scales

can lagrangian tracking simulate tracer spreading in a high resolution ocean general circulation model abstractto model tracer spreading in the ocean lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective diffusive tracer equations online

paleoclimatic implications of late quaternary lacustrine sediments in western nubia sudan abstract owing to the hypercontinental location of western nubia secular fluctuations of climate have been filtered and wet phases can be considered as representative of conditions throughout the southeastern sahara the study area is crossed by the 20 mm isohyet between 9300 and about 4000 yr b p however there were widespread lake and swamp environments with freshwater molluscs ostracods and diatoms and a species rich savanna mammal fauna the center of the west nubian basin approx 18 n an area of about 20 000 km 2 was occupied by a semiaquatic landscape which was situated at the same latitude as paleolake chad from extensive lake carbonates up to about 4 m thick a long term rise of the groudwater table is inferred environments developed that now exist at about latitude 13 n radiocarbon dates from lake sediment sequences cluster between 30 000 and 21 000 yr b p indicating a pleistocene wet phase a gap in radiocarbon dates between 21 000 and 11 000 yr b p signals a phase of hyperaridity similar to the present hyperarid phase with eolian deflation and deposits of sand being the dominant forms of erosion and accumulation

natural levee formation along a large and regulated river the danube in the national park donau auen austria abstract we investigated overbank deposition and natural levee formation along the danube east of vienna based on airborne laserscan data als combined with historical information the analyses of equally spaced cross sections hypsographic curves and point elevations revealed the significance of the distance from the river bank the floodplain width and the floodplain flow situation inflow outflow sections for the spatial variability of sediment deposition deposition was high near the banks natural levee formation in wide floodplain sections and in inflow areas the overall pattern was modified by the presence of side channels and the micro relief of the floodplain from the derived sedimentation rates averaged for the past 120 years over the reach of about 11 0 mm yr 1 at the natural levee and about 0 3 mm yr 1 at the flood protection dyke we can estimate an overbank deposit rate of 416 000 m 3 yr 1 for the entire free flowing reach between river km 1921 and 1880 that is between 18 and 20 of the annual transported suspended load an analysis of historical cross sections showed that there was no natural levee along the riverbanks shortly after the regulation about 100 years ago we thus assume that natural levee formation is a consequence of the danube regulation in the late 19th century under natural conditions overbank deposition was balanced and limited by side erosion and lateral channel migration after stabilization of the banks the overbank deposits were not eroded and distinct natural levees formed these levees will develop and grow further and affect flood protection and floodplain ecology we discuss the interplay between floodplain deposition and lateral bank migration under the concept of dynamic equilibrium

temporal variability of the subarctic front near the charlie gibbs fracture zone we document temporal variability of the northernmost branch of the north atlantic current known as the north subarctic front nsaf for the period 1976 1985 the front s location and temperature salinity t s parameters were determined from 120 repeat transects along three standard sections occupied by russian ships 1 ocean weather station ows c 52 75 n 35 5 w to st john s newfoundland 2 along 35 w from 52 n to 36 5 n and 3 ows c to cape st vincent portugal the nsaf exhibits significant interannual variability of its location shifting 200 km to 300 km in a few months along the ows c st john s section the front reached its southernmost location in 1976 1977 then experienced two 250 to 300 km northward shifts in 1978 1979 and 1980 1981 all of the three shifts peaked in wintertime these events nearly coincide with events of the opposite sign along the ows c cape st vincent section the northernmost location in 1976 1977 and two 200 km southward shifts in 1978 1979 and 1980 1981 the nsaf s behavior is thus coherent near the charlie gibbs fracture zone at the 500 km scale frontal time series associated with the moving front of t and s at the 200 m level for the cold side of the nsaf reveal two events an s200 minimum in the winter of 1976 1977 which might be a manifestation of the great salinity anomaly of the 1970s and a sharp drop of s200 in early 1984 which might signal the arrival of another large salinity anomaly the great salinity anomaly of the 1980s

sofar floats reveal midlatitude intermediate north atlantic general circulation part i a lagrangian descriptive view quasi lagrangian trajectories of 26 sound fixing and ranging sofar floats have been collected near a depth of 700 m in the central north atlantic between 1983 and 1989 aiming at studying the influence of the mid atlantic ridge on the large scale intermediate circulation launched as tight clusters 18 km near neighbor distance on either side of the mid atlantic ridge the floats dispersed quickly over a few months jumping from one mesoscale eddy to the next by and large cyclonic and anticyclonic eddy motions are equipartitioned apparently the mid atlantic ridge remains a barrier even at that shallow depth since only one float from either side drifted across the ridge after a few years floats have circulated through most of the western basin west of the mid atlantic ridge between 30 and 45 n while east of the ridge and south of the azores plateau floats advected east of the great meteor et al seamounts by the azores current wandered more sluggishly on this timescale float dispersion is much more efficient zonally than meridionally an anisotropy mainly seen west of the ridge where floats spread westward over 30 longitude while no float penetrated south of 30 n and only two crossed 45 n northward

statistical analysis of the surface circulation of the california current we use a set of mixed layer drifting buoy trajectories from the california current 20 n 40 n during 1985 1990 to obtain statistically reliable estimates of the mean currents the mean variance field and the geographically varying diffusivity integral timescales and integral space scales typical values for the diffusivity are 1 1 8 7 107 cm2 s 1 while the timescales and space scales are 2 1 7 1 days and 16 59 km respectively the variance field displays a strong westward gradient out to 125 w and diffusivity shows a tendency to decrease toward the southwest part of the domain significant anisotropy is found in the variance field near the coastal boundary and at 30 n 130 w which is the region where the subarctic and northern subtropical fronts approach the california current the antisymmetric component of the diffusivity tensor indicates that cyclonic eddies dominate the mesoscale signature of drifters in this region we seek simple parameterizations to relate the scales of motion of the random velocity field to the diffusivity by testing least squares fits to u02t and u0l where u02 is the velocity variance we found no cases for which these two hypotheses could be distinguished for the meridional component the linear regressions are not successful which suggests that the meridional departure velocities result from a flow regime that is significantly organized by for example waves or coherent structures a subset of the drifters measured temperature along their tracks and we use the resultant data to produce the first direct estimates of the horizontal eddy heat flux divergence based on lagrangian estimates in addition we separately compute the eddy diffusivity parameterization of the eddy heat flux divergence u using our diffusivity estimates and a sea surface temperature climatology the two independent terms agree well which provides a measure of reassurance about the diffusivity estimates the eddy heat flux divergence in the california current is very small 5 w m 2 and does not appear to be significant in the long term heat budget of the upper ocean in this region

human impacts on fluvial systems in the mediterranean region the long history of substantial human impacts on the landscape of the mediterranean region and their effects on fluvial systems is documented these effects have included impacts of deforestation and other land use changes agricultural terracing on a wide scale water transfers and irrigation schemes during the 20th century major changes were made directly to channels through channelisation construction of dams of various sizes and extraction of gravel and indirectly by reforestation these changes have caused a major phase of incision on some rivers runoff and soil erosion have been affected by types of crops and agricultural practices as well as by the varying extent of cultivation and grazing some recent agricultural practices involve wholescale relandscaping of the topography and alteration of surface properties of material the importance of analysing the connectivity within different land units and of the spatial position of human activity within a catchment is illustrated the analysis of connectivity is the key to understanding the variability of impact and the extent of propagation of effects

directly measured mid depth circulation in the northeastern north atlantic ocean the circulation of water masses in the northeastern north atlantic ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes but the ocean circulation at depths below the reach of satellite observations is difficult to measure and only recently have comprehensive direct observations of whole ocean basins been possible here we present quantitative maps of the absolute velocities at two levels in the northeastern north atlantic as obtained from acoustically tracked floats we find that most of the mean flow transported northward by the gulf stream system at the thermocline level about 600 m depth remains within the subpolar region and only relatively little enters the rockall trough or the nordic seas contrary to previous work our data indicate that warm saline water from the mediterranean sea reaches the high latitudes through a combination of narrow slope currents and mixing processes at both depths under investigation currents cross the mid atlantic ridge preferentially over deep gaps in the ridge demonstrating that sea floor topography can constrain even upper ocean circulation patterns

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

evaluation of high resolution sea ice models on the basis of statistical and scaling properties of arctic sea ice drift and deformation sea ice drift and deformation from models are evaluated on the basis of statistical and scaling properties these properties are derived from two observation data sets the radarsat geophysical processor system rgps and buoy trajectories from the international arctic buoy program iabp two simulations obtained with the louvain la neuve ice model lim coupled to a high resolution ocean model and a simulation obtained with the los alamos sea ice model cice were analyzed model ice drift compares well with observations in terms of large scale velocity field and distributions of velocity fluctuations although a significant bias on the mean ice speed is noted on the other hand the statistical properties of ice deformation are not well simulated by the models 1 the distributions of strain rates are incorrect rgps distributions of strain rates are power law tailed i e exhibit wild randomness whereas models distributions remain in the gaussian attraction basin i e exhibit mild randomness 2 the models are unable to reproduce the spatial and temporal correlations of the deformation fields in the observations ice deformation follows spatial and temporal scaling laws that express the heterogeneity and the intermittency of deformation these relations do not appear in simulated ice deformation mean deformation in models is almost scale independent the statistical properties of ice deformation are a signature of the ice mechanical behavior the present work therefore suggests that the mechanical framework currently used by models is inappropriate a different modeling framework based on elastic interactions could improve the representation of the statistical and scaling properties of ice deformation

a statistical description of the mean circulation and eddy variability in the northwestern atlantic using sofar floats abstract velocity data obtained from trajectories of sofar floats launched as part of several experiments carried out in the northwest atlantic during the last two decades are used to give a statistical description of the mean gulf stream and its recirculation the distribution of eddy variability in the region is also presented a mean gulf stream which bifurcates between 55 and 45 w to feed the north atlantic current and a southern recirculation can be clearly identified a weak interior sverdrup flow is also suggested the spatial distribution of eddy variability is consistent with previous descriptions showing a maximum near the gulf stream

natural levee evolution in the rhine meuse delta the netherlands during the first millennium ce this paper presents reconstructions on natural levee development in the rhine meuse delta the netherlands during the first millennium ce covering the full delta plain it is the first study that performs this on a delta scale which allows seeing the delta wide trends on levee forming controls and their feedbacks we mapped the levee morphology and elevation by combining lidar imagery lithological borehole data soil mapping radiocarbon dates archaeological data and gis reconstruction techniques from the detailed levee reconstructions we quantified natural levee dimensions and evaluated the temporal changes therein the dimensions and the changes therein were then linked to external forcings increasing suspended sediment load variable flooding intensity and to natural preconditions e g delta plain width flood basin configuration we show that natural preconditions are an important control on levee shape this is demonstrated for the upper delta where the relatively narrow delta plain combined with strong compartmentation i e the occurrence of many alluvial ridges and enclosed flood basins caused the flood levels to be amplified allowing the natural levees to grow relatively high compartmentation also seems to have stimulated trapping of coarse grained overbank sediments explaining the clear downstream trend in levee width this effect was probably further aided by the clearance of the riparian forests mainly in the upstream and central delta which caused the coarser fraction of the suspended load to be further dispersed into the flood basin leading to wider levees in the first millennium ce several new river courses formed that avoided the areas of natural levee relief of abandoned alluvial ridges on these fossil alluvial ridges the topographical expression gradually reduced because of widespread flood basin trapping of overbank sediment which led to topographic levelling the natural levees that formed during this period along the new courses appear to be relatively high compared to precursor generations in the upper and central delta this is most likely related to the increased suspended sediment supply and intense flooding regime during their formation the hypotheses generated with this new delta wide overview help to better understand the controls in the development of levees which are important elements in river landscapes and in fluvial sedimentary records 2017 elsevier b v

velocity probability density functions for oceanic floats probability density functions pdfs of daily velocities from subsurface floats deployed in the north atlantic and equatorial atlantic oceans are examined in general the pdfs are approximately gaussian for small velocities but with significant exponential tails for large velocities correspondingly the kurtoses of the distributions are greater than three similar pdfs are found in both western and eastern regions above and below 1000 m depth with more significant non gaussianity in the north atlantic than at the equator analogously lagrangian statistics in decaying two dimensional turbulence also display non gaussian velocity pdfs with approximately exponential tails in the limit of large reynolds number

assimilation of drifter observations in primitive equation models of midlatitude ocean circulation 1 motivated by increases in the realism of ogcms and the number of drifting buoys in the ocean observing system a new lagrangian assimilation technique is implemented in an idealized reduced gravity configuration of the layered primitive equation model micom using an extensive set of twin experiments the effectiveness of the lagrangian observation operator and of a dynamical balancing technique for corrected model variables which is based on geostrophy and mass conservation are explored in comparison to a conventional pseudo lagrangian observation operator and an implementation of the kalman filter method the results clearly illustrate that the lagrangian observation operator is superior to the pseudo lagrangian in the parameter range that is relevant for typical oceanic drifter observations and that the simple dynamical balancing technique works well for midlatitude ocean circulation index terms 4263 oceanography general ocean prediction 4255 oceanography general numerical modeling 4594 oceanography physical instruments and techniques 4572 oceanography physical upper ocean processes keywords lagrangian data assimilation data assimilation drifter assimilation ocean prediction ocean modeling

pathways of inflow and dispersion of warm waters in the nordic seas 1 in this study we use 22 acoustically tracked rafos floats to examine the routes and spreading of warm north atlantic waters entering the norwegian sea between iceland and the faroes the majority of floats crossed the iceland faroe ridge at the eastern end where it is deepest they joined the iceland faroe front but rather than continue north with the outer branch of the norwegian atlantic current into the nordic seas most of them jumped over to the inner branch which continues the inflow through the faroe shetland channel northeast over the voring plateau toward the lofoten basin indeed 17 floats whether deployed near iceland or the faroes did so only 2 floats continued north along the outer branch despite the small numbers these results highlight 1 the strong influence of topography on flow patterns 2 the strong crossover of iceland faroes waters to the inner branch and 3 the rapid and structured spreading into the nordic seas

sedimentology and geomorphology of overbank flows on meandering river floodplains abstract along floodplains there occurs an ordered sequence of geomorphic events during floods the following sequence occurs for individual flood events 1 the rise of floodwater and of groundwater 2 the inundation of the floodplain 3a adjustments of the overbank flow pattern to the floodplain environment 3b the flood peak 4 the initial fall of floodwaters and changes in the overbank flow pattern 5 a gradual cessation of the overbank flow 6a infiltration evaporation and a wave like motion of stagnant water and 6b postflood transformation under subaerial conditions field observations support this model for meandering rivers in the polish lowland

eulerian and lagrangian velocity distributions in the north atlantic velocity probability density functions pdfs are calculated using data from subsurface current meters in the western north atlantic ocean the pdfs are weakly but significantly non gaussian they deviate from normality because of an excess of energetic events and there are evidently more such events in the main thermocline than in the deep ocean the pdfs are also compared with those obtained from subsurface floats in the same region the pdfs are statistically indistinguishable so long as the float data are averaged in appropriately sized bins taking too small bins yields overly gaussian float pdfs and taking too large bins yields too non gaussian pdfs with this caveat the lagrangian and eulerian pdfs agree consistent with expectations from theory and previous numerical simulations

morphological changes of gumara river channel over 50 years upper blue nile basin ethiopia summary in response to anthropogenic disturbances alluvial rivers adjust their geometry the alluvial river channels in the upper blue nile basin have been disturbed by human induced factors since a longtime this paper examines channel adjustment along a 38 km stretch of the gumara river which drains towards lake tana and then to the blue nile over a 50 years period agriculture developed rapidly in the catchment and flooding of the alluvial plain has become more frequent in recent times the objectives of this study were to document the changes in channel planform and cross section of the gumara river and to investigate whether the changes could have contributed to the frequent flooding or vice versa two sets of aerial photographs 1957 and 1980 were scanned and then orthorectified recent channel planform information was extracted from spot images of 2006 and google earth channel planform and bed morphology vertical changes were determined for these nearly 50 years period the vertical changes were determined based on aggradation along a permanent structure historic information on river cross sections at a hydrological gauging station and field observations the results indicate that the lower reach of gumara near its mouth has undergone major planform changes a delta with approx 1 12 km 2 of emerged land was created between 1957 and 1980 and an additional 1 km 2 of land has been added between 1980 and 2006 the sinuosity of the river changed only slightly negatively 1 1 i e meandering decreased for the period from 1957 to 1980 and positively 3 0 for the period 1980 2006 comparison of cross sections at the hydrological gauging station showed that the deepest point in the river bed aggraded by 2 91 m for the period 1963 2009 the importance of sediment deposition in the stream and on its banks is related to land degradation in the upper catchment and to artificial rising of lake tana level that creates a backwater effect and sediment deposition in gumara river direct anthropogenic impacts irrigation activities and building of dykes along the river banks have contributed to the huge deposition in the river bed where the abstraction of water for irrigation is intensive seepage water through the banks has contributed to river bank failure in general this study showed that changes to the planform at the mouth of the river and to the riverbed level are substantial moreover the study indicated that the flood carrying capacity of the gumara river channel has diminished in recent times

subsurface valleys and geoarcheology of the eastern sahara revealed by shuttle radar the shuttle imaging radar sir a carried on the space shuttle columbia in november 1981 penetrated the extremely dry selima sand sheet dunes and drift sand of the eastern sahara revealing previously unknown buried valleys geologic structures and possible stone age occupation sites radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand and alluvium filled valleys some nearly as wide as the nile valley and perhaps as old as middle tertiary the now vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat hyperarid region underfit and incised dry wadis many superimposed on the large valleys represent erosion by intermittent running water probably during quaternary pluvials stone age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation the presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present day oases which have been centers of episodic human habitation radar penetration of dry sand and soils varies with the wavelength of the incident signals 24 centimeters for the sir a system incidence angle and the electrical properties of the materials which are largely determined by moisture content the calculated depth of radar penetration of dry sand and granules based on laboratory measurements of the electrical properties of samples from the selima sand sheet is at least 5 meters recent september 1982 field studies in egypt verified sir a signal penetration depths of at least 1 meter in the selima sand sheet and in drift sand and 2 or more meters in sand dunes

oceanic turbulence and stochastic models from subsurface lagrangian data for the northwest atlantic ocean the historical dataset provided by 700 m acoustically tracked floats is analyzed in different regions of the northwestern atlantic ocean the goal is to characterize the main properties of the mesoscale turbulence and to explore lagrangian stochastic models capable of describing them the data analysis is carried out mostly in terms of lagrangian velocity autocovariance and cross covariance functions in the gulf stream recirculation and extension regions the autocovariances and cross covariances exhibit significant oscillatory patterns on time scales comparable to the lagrangian decorrelation time scale they are indicative of sub and superdiffusive behaviors in the mean spreading of water particles the main result of the paper is that the properties of lagrangian data can be considered as a superposition of two different regimes associated with looping and nonlooping trajectories and that both regimes can be parameterized using a simple first order lagrangian stochastic model with spin parameter v the spin couples the zonal and meridional velocity components reproducing the effects of rotating coherent structures such as vortices and mesoscale eddies it is considered as a random parameter whose probability distribution is approximately bimodal reflecting the distribution of loopers finite v and nonloopers zero v this simple model is found to be very effective in reproducing the statistical properties of the data

on variations in static stability along lagrangian trajectories abstract by causing an isopycnal float to rise and sink to neighboring density surfaces one can measure layer thickness along a float trajectory and thus variations in static stability we report here on layer thickness measurements that were obtained from the nearly 100 isopycnal floats that were deployed in the 1993 1995 lagrangian study of the north atlantic current layer thickness variations depend significantly upon location and process in the high velocity core of the meandering north atlantic current layer thickness changes correlate with curvature of the flow stretching in troughs compression in crests but this correlation fails in unsteady flows or outside the high velocity center of the current floats also register significant changes in stratification when they cross between the warm waters in the newfoundland basin and the cold waters from the labrador sea as floats enter and exit what appear to be coherent eddy structures significant but not necessarily predictable changes in layer thickness can occur while the technology to measure layer thickness variations performed as well and reliably as we had hoped for the presence of ubiquitous small scale features in the density field together with the fact that the thickness measurements sampled only o 100 m of the total water column limited our ability to isolate and identify the cause of thickness variations to only relatively energetic features

eddy mean flow decomposition and eddy diffusivity estimates in the tropical pacific ocean 1 methodology 1 eddy diffusivity of the surface velocity field in the tropical pacific ocean was estimated using satellite tracked drifting buoys 1979 through mid 1996 the tropical pacific surface current system is characterized by nonstationarity strong meridional shear and an energetic mesoscale velocity field eddy diffusivity may be defined as the integral of the autocovariance of lagrangian eddy velocities requiring both stationary and homogeneous statistics of the eddy field eddy velocities were obtained by removing a splined mean field to eliminate mean shear from observations binned 1 spatially to group data that have similar dispersion characteristics and 2 temporally to create stationary eddy statistics zonal diffusivity estimates are up to 7 times larger than meridional diffusivity estimates in the high eddy energy regions this anisotropy is associated with the meridional mesoscale wave motion i e by equatorial and tropical instability waves that increases eddy variance but does not lead to a proportional increase in water parcel diffusion because of the coherent character of the trajectory motion at least for initial time lags simple autoregressive models of first and second order are used to describe and classify the resulting eddy statistics an independent confirmation of the diffusivity estimate in the central eastern pacific was obtained by comparing tracer flux divergence computed from a parameterization using diffusivity estimates of our analysis with that from direct eddy reynolds stress flux divergence our results show that diffusivity can be estimated for regions not considered previously either because of sparse data or the complexities of the velocity field

river sedimentation and channel adjustment of the lower yellow river as influenced by low discharges and seasonal channel dry ups fluvial sedimentation can be classified as one of the three types in terms of different morphological units i e vertical overbank deposition vertical deposition on the riverbed and lateral deposition on the riverbank with the example of the lower yellow river the three types of sedimentation have been studied in relation to channel adjustment since 1986 the lower yellow river has entered a new period that is characterized by small discharges with seasonal desiccations dry ups and strong sedimentation has occurred based on the measurement of 36 cross sections along the lower yellow river the rates of the three types of deposition have been calculated and are reported in this study multiple regression has been used to express the relationship between the three rates of deposition and the channel shape adjustment index influenced by the difference in channel sediment and by downstream variations in water discharge and sediment load the indices of channel deposition rate show a clear trend of downstream variation controlled by this the manner of channel adjustment also shows clear trends downstream under the hydrological regime dominated by small discharges the tendencies of adjustment of riverbed slope and bankfull slope are inverse i e the channel slope tends to decrease but the slope at bankfull stage to increase at present the former tendency is dominant thus the sediment carrying capacity of the lower yellow river tends to decrease which intensifies channel sedimentation

lagrangian study of the panama bight and surrounding regions near surface circulation of the panama bight and surrounding regions 0 9 n 73 w 90 w was studied using satellite tracked drifter trajectories from 1979 2004 this region encompasses three major currents showing typical velocities of 30 cm s 1 1 the eastward north equatorial counter current necc 2 the near circular panama bight cyclonic gyre pbcg and 3 the westward south equatorial current sec we do not observe significant modification of the mean surface circulation during el nino southern oscillation events even if the sec is slightly reinforced during relatively warm el nino periods at seasonal scales the circulation is strongly controlled by the activity of the panama wind jet in boreal winter the currents are stronger and an anticyclonic cell is present west of the pbcg this dipole leads to a strong 200 km wide southward current which then disappears during the rest of the year in summer the three major currents have reduced intensity by 30 40 large scale current vorticity shows that the upwelling associated with the pbcg is also 3 4 times stronger in winter than during summer months the kinetic energy is largely dominated by eddy activity and its intensity is double in winter than during summer ageostrophic motions and eddy activity appear to have a substantial impact on the energy spatial distribution in the necc and sec regions lagrangian scales are anisotropic and zonally enhanced in the direction of the mean currents the typical integral time and length scales of these regions are 2 5 days and 50 60 km in the zonal direction and 1 5 days and 25 30 km in the meridional direction lateral eddy diffusivity coefficients are on the order of 11 14 107 cm2 s 1 zonally and 5 6 107 cm2 s 1 meridionally in contrast in the pbcg region the lagrangian characteristics are isotropic with typical timescales of 1 7 days space scales of 30 km and eddy diffusivity coefficients of 6 107 cm2 s 1 in both directions

the north atlantic current and surrounding waters at the crossroads the north atlantic current is a well defined western boundary current that flows north along the east side of the grand banks from 40 to 51 n where it turns sharply to the east and begins its journey across the ocean the current is unique in transporting warm tropical waters to much higher latitudes than any other western boundary current and thus plays a crucial role in ameliorating the climate of the european subcontinent the north atlantic current originates in the gulf stream when the latter curves north around the southeast newfoundland rise a major submarine ridge that stretches se from the grand banks a well defined front delineates the path of the current as long as it flows north as a western boundary current after the current turns east in the north it broadens into a widening band of eastward drift without a sharp or permanent front in the sense of the eastward flowing gulf stream after it separates from cape hatteras the north atlantic current transports more than 40 sv 1 sv 106 m s 1 in the south and about 20 sv by the time it flows east across the mid atlantic ridge the currents along the northward flowing front are quite swift with typical maximum average speeds in the upper 300 m near 1 m s 1 2 knots the current meanders almost as wildly as a snaking river but unlike steep meanders in the gulf stream these meanders appear to be stable and with one exception have not been observed to break off to form pools of warm and or cold waters as frequently occurs in the gulf stream the meanders appear to be induced by major topographic features along the path of the current namely the southeast newfoundland rise the newfoundland seamounts and flemish cap strong recirculations develop on the concave side of the meanders one of these the mann eddy at the first meander crest of the north atlantic current should be regarded as a permanent feature of the north atlantic circulation other meanders also contain recirculations that can persist for months under certain conditions these can merge together to form an extended sw flow recirculation just east of the north atlantic current

a lagrangian based approach for determining trajectories taxonomy and turbulence regimes abstract the use of the ratio between the acceleration and velocity time scales y ta t to separate lagrangian trajectories in homogeneous classes is proposed in fact when analyzing subsurface floats data in the atlantic ocean and surface drifters data in the world s ocean basins it is observed that trajectories having different values of y are characterized by different shapes correlation and dispersal properties in particular trajectories having similar values of the acceleration and velocity time scales clearly show the influence of eddies and are characterized by an oscillating velocity correlation function it is shown here that this trajectory screening is a useful procedure to rationalize the analysis of real lagrangian trajectories and to avoid a mixture of different regimes when averaging quantities the mean statistical quantities computed averaging on quasi homogeneous datasets put in evidence the role of the coherent structures in the dispersion properties both in time and in the ma

isopycnal lagrangian statistics from the north atlantic current rafos float observations one hundred isopycnal floats were tracked on the 27 2 and 27 5 surfaces in the newfoundland basin nfb from july 1993 to july 1995 to study the current structure and exchanges of waters between the subtropical and subpolar gyres the float mapped mean flow consists of weak flows in the nfb and a strong boundary current the north atlantic current nac which separates from the boundary at the northwest corner becoming a diffusive zonal drift the nac meanders are linked to topography and have similar patterns on the two isopycnals despite the fact that the upper layer velocities are twice as fast as the lower layer ones perturbation velocity from the mean is used to compute isopycnal turbulent dispersion and diffusivity this large data set allows us to resolve a narrow mean nac and results in a gaussian turbulence the turbulence approximately follows the classic taylor dispersion theory integral timescales and length scales and turbulent isopycnal diffusivity are of 1 5 2 5 days 20 30 km and 1 7 103 m2 s 1 respectively the timescale increases with depth and decreases with latitude the length scale decreases with depth and longitude and the diffusivity decreases with depth and from nac to nfb compared to previous results from surface drifters and isobaric floats our isopycnal statistics are more isotropic and agree better with the taylor dispersion theory because 1 the mean velocity has a better resolution and 2 the isopycnal floats are better lagrangian followers the diffusivity scales better with the rms velocity and length scale than with the velocity variance and timescale

the north atlantic current based on trajectories of satellite tracked buoys and hydrographic sections both dietrich s and worthington s schemes of the north atlantic current are reconsidered ninety satellite tracked buoys have been analyzed in order to distinguish between these schemes neither could be confirmed the north atlantic current appears as a superposition of a broad west wind drift and a frontal jet in the western part of the north atlantic north of 40 n which becomes independent from the subtropical gyre neither a permanent branching suggested by dietrich nor a confinement of the subpolar gyre to the east of newfoundland per worthington can be found the north atlantic current extends over the entire north atlantic carrying more than 20 sverdrups sv toward the north this northward transport is mainly due to the northward flow around flemish cap and a sudden turn of the north atlantic current east of the mid atlantic ridge the buoy tracks have been used to determine speed and direction in the upper layers 100 m we combine these results with hydrographic data in order to obtain an estimate of the mass transport transport across the mid atlantic ridge is based on recent sections fahrbach et al 1985 while transport in the northeastern north atlantic has been derived from the erika dan sections at 53 5 n and 59 5 n according to these two sections the return flow occurs mainly below 1000 m after water mass conversion in the northern areas the results imply that about 7 sv of atlantic water are transformed into deep water in the area between 53 5 n and 60 n

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

an alternative hypothesis for the origin of the mediterranean salt lens observed off the bahamas in the fall of 1976 abstract a hypothesis is presented that the original salt lens or meddy observed off the bahamas in the fall of 1976 may have been formed not near the mediterranean outflow but instead in the vicinity of the northwest corner 51 n 43 w of the north atlantic current an eddy was observed near the northwest corner by an isopycnal rafos float deployed during the 1993 95 north atlantic current experiment and had nearly identical temperature salinity properties as those of the bahamas lens hydrographic evidence of thick homogeneous layers with similar properties near the northwest corner suggest a possible formation mechanism by which surface eddies containing warm and saline waters are cooled and subducted a plausible scenario is made whereby a northwest corner eddy might be advected southward in the newfoundland basin by the flow around the high pressure ridge east of the north atlantic current and then enter the recirculation gyre immediately south of the gulf stream such an eddy could be advect

arctic sea ice velocity field general circulation and turbulent like fluctuations 1 using buoy trajectories of the iabp data set we analyze the arctic sea ice velocity field as the superposition of a mean field and fluctuations we study how the mean field can be objectively defined using appropriate spatial and temporal averaging scales depending on the season considered 400 km and 5 months for winter i e approximately all the polar winter duration and 200 km and 2 months for summer i e approximately all the polar summer duration the mean velocity field shows a strong intra annual between winter and the following summer as well as interannual variability the fluctuations i e the remaining part of the velocity field after subtracting the mean field are analyzed in terms of diffusion properties although the arctic sea ice cover is a solid we show that the fluctuations follow the same diffusion regimes as the ones predicted for turbulent flows as observed in geophysical fluids like the ocean or the atmosphere we found that the integral time and the diffusivity of sea ice are in the same ranges as those estimated for the ocean i e 1 5 days in winter and 1 3 days in summer and 0 44 103 m2 s for winter and 0 45 103 m2 s in summer respectively however the statistics of the sea ice fluctuating velocity deviate from classical turbulence theory as they show exponential instead of gaussian distributions sea ice velocity and acceleration are intermittent and both are characterized by a multifractal scaling the oceanic and atmospheric dynamic forcing cannot explain solely the statistical properties of sea ice kinematics and dynamics we argue that sea ice dynamic is significantly influenced by the interplay of multiple fractures that are activated intermittently within the ice pack

shuttle imaging radar physical controls on signal penetration and subsurface scattenng in the eastern sahara sir a signal penetration and subsurface backscatter within the upper meter or so of the sediment blanket in the eastern sahara of southern egypt and northern sudan are enhanced both by radar sensor parameters and by the physical and chemical characteristics of eolian and alluvial materials interpretation of sir a images by mccauley et al 1 2 dramatically changed previous concepts of the role that fluvial processes have played over the past 10000 to 30000000 years in shaping this now extremely flat featureless and hyperarid landscape in the present paper we summarize the near surface stratigraphy the electrical properties of materials and the types of radar interfaces found to be responsible for different classes of sir a tonal response the dominant factors related to efficient microwave signal penetration into the sediment blanket include a favorable distribution of particle sizes b extremely low moisture content and c reduced geometric scattering at the sir a frequency 1 3 ghz the depth of signal penetration that results in a recorded backscatter here called radar imaging depth was documented in the field to be a maximum of 1 5 m or 0 25 of the calculated skin depth for the sediment blanket radar imaging depth is estimated to be between 2 and 3 m for active sand dune materials diverse permittivity interfaces and volume scatterers within the shallow subsurface are responsible for most of the observed backscatter not directly attributable to grazing outcrops

eulerian and lagrangian statistics in the south china sea as deduced from surface drifters abstracteulerian and lagrangian statistics of the surface flows over the south china sea scs are investigated based on observations of satellite tracked lagrangian drifters of the surface velocity program although the dataset contains the most recent observations up to 2011 the spatial and temporal distributions of the data are quite inhomogeneous the luzon strait as well as the northern and western parts of the scs are heavily sampled by drifters whereas the internal scs is less sampled the overall eulerian mean circulation derived from all drifter observations resembles the wintertime circulations because of the seasonal sampling bias toward wintertime it is found that the calculation of the diffusivity may be misled by the low frequency temporal variabilities such as annual cycle which could be removed using the gauss markov decomposition in the regions around luzon strait typical values of diffusivity time scale and length scale are 3 7 11 9 107 cm2 s 1 0 8 1 9 days and 16 4 44 4 km r

using vhr multispectral remote sensing and lidar data to determine the geomorphological effects of overbank flow on a floodplain the vistula river poland abstract at the bottoms of river valleys there can be found landforms developed by overbank flow such as crevasse channels and crevasse splays usually cut off from the main river channel by the natural levees humans construct embankments artificial levees which completely divide integrated parts of the river valley and thus gain new areas for agriculture and settlement purposes however during extremely high water stages these areas suffer from flooding very often caused by levee breach the objective of the study is to answer the research question can we use geomorphological analysis of the floodplain to predict extreme flood effects in a large river valley with an artificial levee system the study has been conducted in a reach of the vistula river 60 km downstream from warsaw poland that was affected by catastrophic flood event in may and june 2010 specific emphasis has been put on using very high resolution vhr multispectral remote sensing and lidar light detection and ranging data work is divided into three stages 1 identification of floodplain landforms from palaeofloods on the vhr multispectral satellite imagery 2 outline of the 2010 flood event on the basis of river stage data and acoustic doppler current profiler measurements and a detailed study of its geomorphologic effects on the floodplain on the basis of aerial imagery and lidar data 3 comparison of landforms created in palaeofloods and in the 2010 flood event the results of the study show that geomorphological effects of the recent catastrophic flooding are strikingly similar to palaeoflood landforms developed before the construction of an artificial levee system the main conclusion is that overbank flow in some reaches of the floodplain causes and will cause similar effects as it has done in the past analysis of palaeoflood landforms enables prediction of these effects and can therefore prove useful for flood risk management post flood transformation of palaeoflood landforms by human impact and natural geomorphological processes induces the use of special methods for the identification of the landforms and study of their spatial features vhr multispectral remote sensing enables such analysis in many places of the earth lidar data provide new possibilities for the accurate estimation of overbank deposition and erosion rate on the floodplain

vertically averaged velocities in the north atlantic current from field trials of a lagrangian electric field float abstract a subsurface lagrangian float that utilizes motional induction to calculate vertically averaged velocities was tested in the north atlantic current nac taking advantage of existing cruises and infrastructure the electric field float eff is a rafos float with horizontal electrodes that measures its own velocity by rafos tracking and calculates vertically averaged velocities when merged with the electrode system the observations showed depth averaged velocities that were fast in the core of the nac 0 6 0 9 m s 1 and moderate in adjacent recirculations and eddies 0 3 0 4 m s 1 a float at 850 dbar moved at close to the depth averaged velocity while shallower floats followed surface intensified flow on top of the depth averaged motion integral time scales of depth averaged velocity 1 3 1 6 0 4 d are slightly shorter than time scales of float velocity 1 6 2 0 0 3 d while integral length scales of depth averaged water velocity 35 10 km for u 18 6 km for v are slightly shorter than length scales of float motion 53 12 km for u 28 6 km for v velocity spectra of depth averaged velocity show significant variance at inertial periods quantitative and qualitative validation with multiple independent data sets confirms the accuracy of the instrument and sampling strategy in the nac advancing the limited observational knowledge of depth averaged circulation in subpolar regions

potential controls of alluvial bench deposition and erosion in southern piedmont streams alabama usa abstract benches are bank attached channel deposits occurring at an elevation between the channel bed and top of banks their occurrence in a variety of geologic and hydrologic settings has led to confusion about the mechanisms driving their formation which in turn contributes to difficulty identifying the active floodplain bankfull stage and the determination of environmental flows in some rivers hydrodynamic modeling software river 2d in combination with sediment particle size analysis and total station topographic surveys was used to simulate flow conditions needed to erode and deposit the d84 d50 and d15 particle sizes of concave and lateral benches in two rivers talladega and hillabee creeks in alabama results suggest that bench erosion requires flows at least 150 larger than benchfull stage at the talladega site while the hillabee site experienced erosion at all discharges meeting and exceeding benchfull flow stage likely owing to its overall smaller sediment particle sizes at both sites the presence of vegetation increased the bench area subjected to deposition but somewhat counterintuitively also helped influence the location of erosion by limiting flow vectors in contrast with previous research findings the occurrence of reverse flow was neither sustained nor widespread at either site these findings provide new insight into alluvial benches suggest that the study benches are relatively stable features under the prevailing hydrologic regime and that in some temperate climate settings such as the southern piedmont localized hydraulic controls on bench formation can be superseded in importance by hydrologic flow regime even in the case of concave benches and where flow regulation is not a factor

mean surface circulation and mesoscale turbulent flow characteristics in the eastern south pacific from satellite tracked drifters 1 circulation of the eastern south pacific region 10 34 s 70 100 w was studied using drifting buoy trajectories from 1979 to 2003 three different current regimes were highlighted the south equatorial current flowing westward north of 26 s with surface velocities of 7 8 cm s 1 the south pacific current an eastward current of 2 cm s 1 south of 25 30 s and the chile peru current a northwestward flow of 6 cm s 1 found to the east of 82 w and south of 22 s the velocity variance distribution showed the regional flow to be anisotropic in nature maps of eddy kinetic energy obtained from drifter measurements and satellite altimetry data showed similar patterns with higher energy levels near the south american coast the lagrangian time and length scales of the turbulent flow revealed typical values of 5 9 0 6 days and 40 7 5 4 km in the zonal direction and of 3 5 0 4 days and 29 3 4 km in the meridional direction the estimated diffusivities were 4 9 0 5 107 cm2 s 1 and 2 6 0 3 107 cm2 s 1 in the eastward and northward directions respectively finally we estimated the respective role of horizontal advection and lateral diffusion on the mixed layer heat and salt budgets near the coast the lateral turbulent fluxes in the surface layer were two times larger than the horizontal transports associated with the large scale circulation mean horizontal heat and salt diffusive fluxes were around 25 w m 2 and 50 g m 2 d 1

near surface circulation of the nordic seas as measured by lagrangian drifters in the period june 1991 to august 1993 107 argos tracked drifters drogued to 15 m depth were released in the nordic seas or greenland iceland and norwegian seas the drifter movements revealed the strong and spatially confined current systems along the surface salinity fronts of the iceland faroe frontal zone and of the norwegian coast and along the continental margins and their extensions to the barents sea and spitsbergen the norwegian atlantic current is composed of three distinct streams two continental margin and one coastal branches which join into one single swift mean current west of the lofoten and vesteralen islands where the strongest measured currents are in excess of 110 cm s 1 in addition to the general cyclonic gyre circulation in the nordic seas the drifters indicate smaller cyclonic circulation patterns in all the major subbasins i e the iceland plateau the norwegian the lofoten and the greenland basins no surface signature of the east icelandic current is disclosed by the drifters interpolated and low pass filtered position data were used to construct maps of 15 m depth ensemble mean velocity velocity variability and residence time vigorous eddy fields are dominant in the strong currents and in the lofoten basin eulerian correlations indicate that they tend to propagate to the west in contrast the iceland plateau appears quiescent both in the mean and eddy velocities single particle diffusivities are computed and are found to be in the range 1 7 107 cm2 s 1 the corresponding lagrangian timescale and space scale are 1 3 days and 10 40 km respectively these lagrangian drifter measurements compose the first basin scale accurate near surface velocity data set of the nordic seas

morphology of fluvial levee series along a river under human influence maros river hungary abstract the development and morphometry of fluvial levees reflect the connection between channel and overbank processes which can be altered by various human activities the aims of this study are to investigate the morphology and spatial characteristics of fluvial levees and evaluate the role of some local and catchment scale human activities on their medium term 150 years development this study applies lidar data along a 53 km long reach of the maros river in hungary six fluvial levee types are identified based on the beginning and end of their evolution these levee types were generated by local nineteenth century channel regulation works cutoffs and mid twentieth century channel narrowing which was caused by gravel mining and water impoundment in the upstream sections however other human activities also influenced the development of active fluvial levees because their horizontal evolution could have been limited by embanked flood protection levees or the widening of low lying floodplain benches that were generated by channel narrowing additionally revetment constructions influenced their vertical parameters as higher fluvial levees developed along the fixed banks generally the older active fluvial levees are wider while the younger active levees are narrower with steeper slopes but not always lower on the low lying floodplain levels benches the youngest fluvial levees evolved quite rapidly and consist of coarser material currently only 9 8 to 38 year return period floods could cover the fluvial levees contributing to their evolution this fact and the development of fluvial levee series with two three members reflect a gradual decoupling of the channel from the floodplain

pathways of the north atlantic current from surface drifters and subsurface floats the north atlantic east of the grand banks presents a thermal front from 40 to 50 n between the warm north atlantic current nac and the cold labrador current here we use the trajectories of 84 isopycnal rafos floats deployed on the 27 2 and 27 5 t surface in 1993 1995 and 218 surface drifters deployed by the international ice patrol and the institut fur meereskunde kiel germany between 1978 and 1993 to understand the pathways of the nac the mean flow pattern kinetic energy directional stability and most probable path through 1 by 1 boxes are derived for each of the three surfaces speed and kinetic energy decrease with depth although directional stability is comparable at all levels the axis of the nac which follows the 4000 m isobath along the western boundary is described by mean kinetic energy mke values surpassing 100 cm2 s 2 at the surface or 50 cm2 s 2 on the float surfaces and correspond to regions with directional stabilities in excess of 60 maximum values of eddy kinetic energy are found just offshore of the maximum mke associated with the nac main pathway and decrease rapidly to the east two cyclonic meanders are identified at 44 n 45 w near the newfoundland seamounts and at 46 n 42 w near flemish cap the main difference between the patterns derived for the surface drifters and floats was the greater eastward extent of the cyclonic trough at 44 n and intensity of the mann eddy 42 n 44 w during the float sampling period 1993 1995

late quaternary paleodrainage reconstruction of the maros river alluvial fan abstract the surface of the extensive maros alluvial fan is densely 0 78 km km 2 covered by paleochannels the aim of the present study is to reconstruct the final late quaternary stage of the alluvial fan development by identifying paleodrainage directions and determining morphology and pattern of the paleochannels and the age of their activity on the surface of the alluvial fan 18 meandering braided anastomosing or misfit paleochannel zones were identified the existence of these channels indicates that the main process of the fan development was avulsion rather than lateral channel shift and the overbank floodplain accumulation played a minor role as the paleochannels were not buried the oldest paleodrainages a c shifted towards north from the east west axis of the alluvial fan draining water to the koros basin the similar age 18 7 2 3 ka 14 2 1 4 ka of the paleochannel zones indicates rapid avulsions or the coexistence of paleodrainages in the northern half of the alluvial fan the last large d paleodrainage change occurred 12 4 2 1 ka when the channel turned westward a large meandering channel functioned for a period 12 4 2 1 ka 9 6 1 3 ka and with the coexisting misfit channels these channels drained a considerable amount of water q b 2500 m 3 s in the next development phase e paleodrainages the maros flowed north but after a sharp bend it turned toward the south and it started to form the southern lobe of the alluvial fan these channels existed between 8 5 0 9 ka and 3 5 0 4 ka and had a bankfull discharge of 1000 2000 m 3 s the last f paleodrainage of the maros river existed for a short time 1 9 0 3 ka 1 6 0 3 ka and indicated decreasing discharge q b 1400 m 3 s in the final phase of the alluvial fan evolution the present day course of the river was developed and its discharge decreased further 680 m 3 s

assimilation of drifter observations for the reconstruction of the eulerian circulation field 1 in light of the increasing number of drifting buoys in the ocean and recent advances in the realism of ocean general circulation models toward oceanic forecasting the problem of assimilation of lagrangian observations data in eulerian models is investigated a new and general rigorous approach is developed based on optimal interpolation oi methods which takes into account directly the lagrangian nature of the observations an idealized version of this general formulation is tested in the framework of identical twin experiments using a reduced gravity quasi geostrophic model an extensive study is conducted to quantify the effectiveness of lagrangian data assimilation as a function of the number of drifters the frequency of assimilation and the uncertainties associated with the forcing functions driving the ocean model the performance of the lagrangian assimilation technique is also compared to that of conventional methods of assimilating drifters as moving current meters and assimilation of eulerian data such as fixed point velocities overall the results are very favorable for the assimilation of lagrangian observations to improve the eulerian velocity field in ocean models the results of our assimilation twin experiments imply an optimal sampling frequency for oceanic lagrangian instruments in the range of 20 50 of the lagrangian integral timescale of the flow field index terms 4255 oceanography general numerical modeling 4263 oceanography general ocean prediction 3337 meteorology and atmospheric dynamics numerical modeling and data assimilation keywords lagrangian assimilation drifters lagrangian velocity eulerian velocity numerical model

isopycnal rafos floats as roving hydrographers in the north atlantic current region the properties measured by isopycnal floats can be compiled to provide hydrographic information about the sampled surfaces the height and temperature of two specific volume anomaly surfaces reported by 84 rafos floats deployed in the north atlantic current nac region are compared here with the climatological hydrographic database the mean pathway of the nac and other prominent features of the region are similar in float derived and climatological surfaces the most striking difference between floats and climatology an extended trough in the nac path at 44 n in the float derived surface reflects the sampling time scales the baroclinic transport of the nac estimated from rafos floats using the observed climatological relationship between the potential energy anomaly and the height of the specific volume anomaly surfaces was 42 sv at 42 n and 24 sv at 47 n in reasonable agreement with climatological estimates

accelerated overbank accumulation after nineteenth century river regulation works a case study on the maros river hungary abstract in the nineteenth century the meandering rivers of the carpathian basin were extensively regulated and their large up to 100 km wide floodplains were confined by artificial levees on the narrow 0 5 4 km artificial floodplains confined by these levees overbank sedimentation has become the dominant geomorphological process resulting in rising floodplain levels during the last century the maros river is an extreme example of this process as it carries a significant amount of suspended sediment 4 6 10 6 m 3 y and because its lowland reach was drastically shortened by cutoffs resulting in bed scour and an increased sediment load simultaneously a flood embankment system was constructed reducing the floodplain width by 70 80 the resulting accelerated overbank sediment accumulation increases the flood hazard along the river the aim of this study is to determine the spatial and temporal pattern of overbank sediment accumulation on the artificial floodplain of the lower maros river over the last 150 years and to identify its influencing factors the lateral slope of the protected natural floodplain was projected towards the present day channel allowing the volume m 3 and rate cm y of overbank sedimentation to be calculated using digital terrain modelling dtm changes in the rate of sedimentation were determined by pollen analysis the studied reach was divided into five sections based on the rate of aggradation i the distal surface of the alluvial fan where no sedimentation was measured only incision ii the original front of the alluvial fan 1 0 0 4 cm y iii the recently formed secondary alluvial fan 1 4 0 3 cm y iv the low lying floodplain 0 4 0 2 cm y and v the outlet unit 2 0 0 5 cm y the aggradation rate was slower on elevated forms such as the pre regulation natural levee 0 2 cm y and greater on the low lying floodplain 0 6 cm y the greatest accumulation rate 1 3 2 4 cm y was measured in cutoffs the sedimentation rate was high during the regulation period 1 9 2 4 cm y but later decreased to 0 5 0 9 cm y the rate of aggradation was determined by the width height and slope conditions of the floodplain the impoundment of the main river and the land use in the area

the isopycnal swallow float a simple device for tracking water parcels in the ocean abstract a standard swallow float can be converted into a passive isopycnal follower by the addition of a compressee a spring backed piston in a cylinder designed to give the float the compressibility of seawater thus no archimedian buoyancy is created due to changes in depth in addition the float must have a coefficient of thermal expansion significantly smaller than that of seawater an instrument meeting these requirements ballasted to a chosen isopycnal will approximately follow that isopycnal on time scales longer than the local buoyancy period two preliminary field experiments were conducted in the upper main thermocline near the north wall of the gulf stream during august 1981 a spherical glass float was deployed 200 km downstream from cape hatteras and was tracked for approximately 100 km neutrally buoyant in the oxygen minimum near 380 m the float tracked the t 27 12 0 01 as determined from ctd data during its passage through a mini meander during september 1982 a cylindrical float was tracked for approximately 100 km east of cape hatteras deployed in the gulf stream near the 250 m level the float tracked the t 26 74 0 01 isopyncal surface during a gradual descent and cross stream motion over the 24 hr sampling period thus the floats tracked nearly isopycnally in both field tests clusters of these instruments used in concert with hydrographic surveys have the potential for unique studies of mixing entrainment and topographic effects

on the scaling laws derived from ice beacon trajectories in the southern beaufort sea during the international polar year circumpolar flaw lead study 2007 2008 1 sea ice motion is an important element in mass balance calculations ice thermodynamic modeling ice management plans for industry and ecosystems studies in the historical literature sea ice motion in the beaufort sea was characterized by a predominantly anticyclonic motion during winter months with episodic reversals to cyclonic activity during summer however recent studies have shown an increase in cyclonic activity throughout the annual cycle in this paper we examine circulation in the beaufort sea based on the trajectories of 22 ice beacons launched in the franklin bay area during the international polar year circumpolar flaw lead ipy cfl study during an over wintering experiment in 2007 2008 dispersion characteristics of ice motion show that absolute zonal dispersion follows a t2 scaling law characteristic of advection associated with beaufort gyre circulation whereas absolute meridional dispersion follows a scaling law of t5 4 characteristic of floaters and dispersion in 2 d turbulence temporal autocorrelations of ice velocity fluctuations highlight definitive timescales with values of 1 2 0 7 days in the zonal meridional direction near gaussian behavior is reflected in higher order moments for ice velocity fluctuation probability density functions pdfs non gaussian behavior for absolute displacement pdfs indicates spatial heterogeneity in the ice motion fields atmospheric forcing of sea ice is explored through analysis of daily north american regional reanalysis and in situ wind data where it is shown that ice in the cfl study region travels with an average speed of approximately 0 2 and an average angle of 51 5 to the right of the surface winds during the 2007 2008 winter the results from this analysis further demonstrate seasonality in ice drift to wind ratios and angles that corresponds to stress buoy data indicative of increases in internal ice stress and connectivity due to consolidation of the seasonal ice zone to the coast and perennial ice pack during winter in the beaufort sea region

pathways of cross frontal exchange in the north atlantic current the north atlantic current nac forms part of the boundary between the subtropical and subpolar gyres in the north atlantic ocean the current has topographically controlled stationary meanders that appear to grow and decay a region east of the current in the newfoundland basin contains water of mixed subpolar subtropical properties suggesting that there is exchange across the nac this study considers data from isopycnal rafos floats launched in the nac region from 1993 to 1995 we use the rafos data to define the frontal zone as a pressure range where the jet is most likely to be found this definition requires a latitudinal dependence as the nac shoals to the north floats shallower and deeper than this range are defined to be on the subpolar and subtropical side respectively these definitions are used to estimate mixing that occurs between the current and its surroundings and to estimate the relative quantity of exchange of water parcels between the two gyres only small quantities of mass exchange from one gyre to the other are found but there is a distinct asymmetry leading to a mean flux from the subpolar to subtropical sides we also find that floats spend significant time in the frontal region and are frequently exchanged between fast and slow moving waters particularly at the meander extrema diffusion while in the jet leads to eddy cross frontal exchange which is important for the exchange of properties across the nac

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

scaling properties of sea ice deformation from buoy dispersion analysis 1 a temporal and spatial scaling analysis of arctic sea ice deformation is performed over timescales from 3 h to 3 months and over spatial scales from 300 m to 300 km the deformation is derived from the dispersion of pairs of drifting buoys using the iabp international arctic buoy program buoy data sets this study characterizes the deformation of a very large solid plate the arctic sea ice cover stressed by heterogeneous forcing terms like winds and ocean currents it shows that the sea ice deformation rate depends on the scales of observation following specific space and time scaling laws these scaling properties share similarities with those observed for turbulent fluids especially for the ocean and the atmosphere however in our case the time scaling exponent depends on the spatial scale and the spatial exponent on the temporal scale which implies a time space coupling an analysis of the exponent values shows that arctic sea ice deformation is very heterogeneous and intermittent whatever the scales i e it cannot be considered as viscous like even at very large time and or spatial scales instead it suggests a deformation accommodated by a multiscale fracturing faulting processes

alluvial architecture of the human influenced river rhine the netherlands abstract the alluvial architecture of the embanked rhine distributaries in the netherlands is largely controlled by human interference with fluvial processes embankment straightening and stabilization of the rivers have influenced both in channel and overbank sedimentation embanked river floodplain deposits are lithologically and sedimentologically different from natural not human influenced floodplain deposits after the embankments between 1050 and 1350 ad the dominant process remained lateral within channel accretion however the morphology of the point bars changed in the natural situation the bars had a convex shape as seen from the channel whereas in the embanked situation they are concave as seen from the channel the latter is due to the development of bars along the downstream end of a previous formed bar deposits associated with lateral within channel accretion are typically very fine to medium or coarse sand the infill of residual channels is characterized by the absence of peat after the normalization of the river channel to a fixed width 1850 ad concave bank erosion and development of scroll bars were constricted and overbank deposition became the dominant embanked floodplain forming process thick layers of sandy and clayey deposits were deposited on the embanked floodplain splay like sand lenses are present in the overbank deposits in case of dyke failures a deep scour hole was formed typical deposits associated with dyke breaches are sandy dyke breach deposits and scour hole fills consisting of silty to sandy clay within the embanked floodplain and gyttja within the polder

mean circulation and eddy kinetic energy in the eastern north atlantic sixty two satellite tracked buoys have been released in the northern part of the western and central north atlantic during 1981 and 1982 by the institut fur meereskunde universitat kiel federal republic of germany additionally six buoys deployed in spring 1980 by the bedford institute have been taken over mean values and variances about the mean have been calculated for 3 by 3 areas the mean velocities confirm the large scale gyre circulation as deduced from hydrographie data and numerical models the center of the gyre is located at about 33 n eddy kinetic energy decreases east and southward and is concentrated along the north atlantic current it appears that the north atlantic current polar front is the main source of eddy kinetic energy for the northern north atlantic whereas the mid atlantic ridge is no source of eddy energy eddy kinetic energy along the north atlantic current decreases from about 1000 cm2 s 2 near newfoundland to 300 cm2 s 2 west of scotland a homogeneous pool of low eddy kinetic energy less than 100 cm2 s 2 appears in the central north atlantic east of the mid atlantic ridge the level increases towards the european and african shelf where the mean southward flow also becomes stronger the results concern the upper ocean

characteristics of the near surface currents in the indian ocean as deduced from satellite tracked surface drifters part i pseudo eulerian statistics using the 1985 2013 record of near surface currents from satellite tracked drifters the pseudo eulerian statistics of the near surface circulation in the indian ocean io are analyzed it is found that the distributions of the current velocities and mean kinetic energy mke in the io are extremely inhomogeneous in space and nonstationary in time the most energetic regions with climatologic mean velocity over 50cms 21 and mke over 500cm 2 s 22 are found off the eastern coast of somalia with maxima of over 100cms 21 and 1500cm 2 s 22 and the equatorial io associated with the strong annually reversing somalia current and the twice a year eastward equatorial jets high eddy kinetic energy eke is found in regions of the equatorial io western boundary currents and agulhas return current with a maximum of over 3000cm 2 s 22 off the eastern coast of somalia the lowest eke 500cm 2 s 22 occurs in the south subtropical gyre between 308 and 408s and the central eastern arabian sea annual and semiannual variability is a significant fraction of the total eke off the eastern coast of somalia and in the central eastern equatorial io in general both the mke and eke estimated in the present study are qualitatively in agreement with but quantitatively larger than estimates from previous studies these pseudo eulerian mke and eke fields based on the most extensivedrifterdatasettodate arethemostpreciseinsituestimatestodateandcanbeusedtovalidatesatellite and numerical results

eddy stirring and horizontal diffusivity from argo float observations geographic and depth variability stirring along isopycnals is a significant factor in determining the distribution of tracers within the ocean salinity anomalies on density surfaces from argo float profiles are used to investigate horizontal stirring and estimate eddy mixing lengths eddy mixing length and velocity fluctuations from the ecco2 global state estimate are used to estimate horizontal diffusivity at a 300 km scale in the upper 2000 m with near global coverage diffusivity varies by over two orders of magnitude with latitude longitude and depth in all basins diffusivity is elevated in zonal bands corresponding to strong current regions including western boundary current extension regions the antarctic circumpolar current and equatorial current systems the estimated mixing lengths and diffusivities provide an observationally based data set that can be used to test and constrain predictions and parameterizations of eddy stirring

a synoptic and statistical description of the gulf stream and subtropical gyre using sofar floats abstract trajectories of 17 sofar floats ballasted to nominal depth of 700 and 2000 m and tracked for 15 months have been analyzed to produce synoptic and statistical descriptions of the gulf stream and subtropical gyre sofar floats launched in and near the gulf stream along with positions of the surface gulf stream and satellite tracked buoys demonstrate strong baroclinic shear nevertheless the horizontal patterns of the gulf stream meanders and gulf stream rings are the same at the surface and at 700 and 2000 m depth above the thermocline particle speeds greater than the propagation speed of the meanders cause parcels of water to move with the flow pattern at greater depths the velocities are at most equal to the propagation speed and the deep trajectories are free to pass through the gulf stream or ring strong temporal variability is found near the tail of the grand banks instances of flow both southward into a narrow gulf stream recirculation and northward into the north atlantic current can

why are the meanders of the north atlantic current stable and stationary a simple 1 5 layer reduced gravity model of an f plane jet with a symmetric cross section is employed to explain the stability and stasis of meanders in the north atlantic current the main factors affecting the current s stability are the highly contrasting water masses on both its sides and the current s vertical density structure the stability and stasis of the nac results from 1 the near surface shoaling of the deep isopycnals inshore of the subpolar front and 2 a small internal radius of deformation which is due mainly to the small upper to lower layer density difference in the absence of unstable modes the dominant wavelength of the meanders is determined by the bathymetry so the nac simply follows those contours

historical position of the north atlantic current the path of the north atlantic current nac within the newfoundland basin is determined from the historical hydrographic database 1910 1988 horizontal probability distributions for dynamical and property front locations on specific volume anomaly surfaces are derived from an analysis of all available hydrographic stations in the basin these probability distributions indicate where the fronts were most likely to have been observed in the past they are observed to exhibit a pattern of stationary meanders with troughs located over the southcast newfoundland rise over the newfoundland seamounts and to the east of the flemish cap the width of the nac s envelope varies from 300 km over the southeast newfoundland rise temporal subsamples of the historical data indicate that the path and meander patterns are persistent features for all seasons and for the four decades between 1950 and 1990

fluvial processes morphology and sediment budgets in the coon creek basin wi usa 1975 1993 sediment budget processes in coon creek wisconsin from 1975 to 1993 changed only moderately generally continuing the trends that were evident in 1975 perhaps most importantly sediment yield to the mississippi river appears to be unabated with much of this clearly coming from storage loss especially stream bank erosion vertical accretion continued on the lower new floodplains inset within their meander plains in tributaries and the upper main valley the lower main valley also continued to aggrade but at the rate of about 6 of that occurring in the 1920s and 1930s the major revision from earlier studies is that the upper main valley is a less significant net source of sediment than previously thought and the earlier prognostications of it being a significant net source of sediment in the future may be wrong perturbations have been caused by changes of riparian vegetation and much more importantly by the introduction of fish shelter structures and protected cut banks along the stream system reaching downstream to the end of the upper main valley because these structures do not permit natural stream migration bank erosion and downstream sediment transfer the coon creek basin has lost much of its suitability as a natural laboratory of fluvial processes

warm water pathways transports and transformations in the northwestern north atlantic and their modification by cold air outbreaks 1 this paper presents a numerical study of the warm water pathways transports and water mass transformation in the newfoundland basin region and an investigation of the rectification effects of a series of cold air outbreaks caos on the above processes an initial mean state simulation was compared with observations and showed good agreement then repeated cao events were explicitly included in the surface fluxes and illustrated the following rectification effects the thermal regime of the entire baroclinic layer was impacted as the thermocline deepened and the temperature anomaly was noticeable down to 500 m different mechanisms were responsible for the propagation of the temperature anomaly at different stages during the onset of the cao vertical diffusion propagated the temperature anomaly downward near the surface then vertical advection further propagated the anomaly downward between cao events to about 500 m depth with caos the north atlantic current carried more warm water and its pathway in the northwest corner shifted towards the southeast the volume averaged mean and eddy kinetic energy increased by 30 and 20 respectively and the water mass transformation rate in the newfoundland basin was doubled the gulf stream carried more heat but heat transport to the eastern basin decreased owing to the increase in heat release to the atmosphere

evidence for a large extended paleolake in the eastern sahara as revealed by spaceborne radar lab images recently processed radar images from the two flights of the space shuttle endeavour by the spaceborne imaging radar c x band synthetic aperture radar instrument in 1994 give a clear view of widely distributed sediments of a palaeolake in today s hyperarid eastern sahara nw sudan although a few of the sites of lacustrine sediments were previously examined in the field their extent now visible on the images yields a new perspective on the early to mid holocene wet phase that affected northern africa freshwater wetlands across an area exceeding 15 000 km2 existed between 8800 and 4500 b p owing to a significant increase in rainfall the phenomenon called the west nubian lake constitutes the largest scale hydrographic evidence in the eastern sahara for a northward shift of the tropical rain belt given that the region now receives less than 15 mm of precipitation per year on average the former open water body and its vegetated catchment may have acted as a modifier of the regional climate in connection with the detection of a buried major palaeodrainage system near kufra se libya the west nubian lake confirms the new multifrequency and multipolarization radar imaging as a powerful tool for the reconstruction of arid land palaeohydrology

fluvial geomorphic features of the lower mississippi alluvial valley abstract the lower mississippi valley lmv has been one of the most intensively studied alluvial valleys in the world in terms of it s geological and geomorphic framework and history a brief outline of the history of the major geological and geomorphological investigations of the lmv is provided the results of these investigations are discussed in terms of the fluvial geomorphic framework of the valley and the apparent significant changes in the regime of the mississippi river during the late wisconsinan and holocene stages the lmv occupies the broad deep synclinal trough of the mississippi embayment which extends from cairo illinois to the gulf of mexico in a slightly sinuous north south trend the embayment is filled with a north to south thickening wedge of non marine and marine sediment ranging in age from jurassic to holocene the major landscapes of the lmv may be considered in four regions 1 a narrow active meander belt in a broad valley of late pleistocene valley train in the northern third 2 a wide mosaic of interwoven holocene meander belts in the middle third 3 a relatively narrow valley of the atchafalaya basin bounded on each side by narrow meander belts in the upper part of the lower third and 4 the broad distributary wedge of the deltaic plain in the southernmost region of the valley the valley trains vary in age and landform with the oldest occurring as slightly dissected low ridges and the youngest as broad flats separated by shallow interwoven former braided channels meander belts formed throughout the holocene are comprised of low natural levee ridges flanking abandoned courses and bordered by crescent shaped oxbow lakes and ridge and swale topography in the middle third of the valley meander belts are separated by expansive backswamps of very little relief the deltaic plain is also exceptionally flat interrupted by the low natural levee ridges of the abandoned deltaic distributaries the floodplain of the lmv is a complex mosaic of fluvial features and landscapes within the four landscape regions included in this mosaic are abandoned channels and courses lateral accretion topography of ridges and swales natural levees crevasses and crevasse channels distributary channels backswamps and rimswamps alluvial fans and aprons valley trains braided stream terraces lakes and lacustrine deltas terraces and the alluvial valley bluff changes in the hydraulic regime of the lower mississippi river lmr since the late pleistocene have played a major role in the development of the landscape of the valley the most important regime change was the diminishment of the influence of wisconsinan glaciation in the upper midwest and the resultant evolution of the mississippi river from a broad braided outwash channel to a more narrow but sinuous meandering channel at the end of the pleistocene during the holocene the mississippi river undoubtedly responded to major climatic changes rising sea level tributary stream influence and possibly tectonism diapirism and subsidence through the growth and evolution and abandonment of it s meander belts and deltas

lagrangian turbulence in the adriatic sea as computed from drifter data effects of inhomogeneity and nonstationarity 1 the properties of mesoscale lagrangian turbulence in the adriatic sea are studied from a drifter data set spanning 1990 1999 focusing on the role of inhomogeneity and nonstationarity a preliminary study is performed on the dependence of the turbulent velocity statistics on bin averaging and a preferential bin scale of 0 25 is chosen comparison with independent estimates obtained using an optimized spline technique confirms this choice three main regions are identified where the velocity statistics are approximately homogeneous the two boundary currents west east adriatic current wac eac and the southern central gyre cg the cg region is found to be characterized by symmetric probability density function of velocity approximately exponential autocorrelations and well defined integral quantities such as diffusivity and timescale the boundary regions instead are significantly asymmetric with skewness indicating preferential events in the direction of the mean flow the autocorrelation in the along mean flow direction is characterized by two timescales with a secondary exponential with slow decay time of 11 12 days particularly evident in the eac region seasonal partitioning of the data shows that this secondary scale is especially prominent in the summer fall season possible sampling issues as well as physical explanations for the secondary scale are discussed physical mechanisms include low frequency fluctuations of forcings and mean flow curvature inducing fluctuations in the particle trajectories consequences of the results for transport modeling in the adriatic sea are discussed

eddy kinetic energy in the north atlantic from surface drifters one hundred ten satellite tracked freely drifting buoys measured velocities and trajectories of the near surface currents in the north atlantic mean velocity values and the velocity variance about the mean were calculated for different regions a horizontal map of eddy kinetic energy was prepared on a 2 2 grid between latitudes 20 and 55 n maximum eddy energy 3000 cm2 s 2 coincides with the high speed gulf stream jet where it begins large amplitude meanders near 37 n 67 w a tongue of high eddy energy coincides with the stream s path eastward and around the grand banks into the newfoundland basin where values of 1000 cm2 s 2 are found a weaker tongue extends eastward across the mid atlantic ridge near 45 n a second weak extension reaches southeastward from the stream and crosses the mid atlantic ridge between 30 and 35 n north and south of the stream eddy energy diminishes rapidly reaching an e folding at 300 km from the axis values of 200 cm2 s 2 were observed in the mid gyre region and 100 cm2 s 2 in the eastern north atlantic and north equatorial current although the gross distribution of eddy energy is similar to that determined from ship drift measurements there are significant differences eddy energy from drifters amounts to about twice the value measured by ship drift in the gulf stream and one half the ship drift values in the mid gyre it is suggested that these differences are due to the horizontal averaging of mesoscale motion and the errors in navigation both of which are problems with the ship drift technique

geomorphic change detection using historic maps and dem differencing the temporal dimension of geospatial analysis abstract the ability to develop spatially distributed models of topographic change is presenting new capabilities in geomorphic research high resolution maps of elevation change indicate locations processes and rates of geomorphic change and provide a means of calibrating temporal simulation models methods of geomorphic change detection gcd based on gridded models may be applied to a wide range of time periods by utilizing cartometric remote sensing or ground based topographic survey data to measure volumetric change advantages and limitations of historical dem reconstruction methods are reviewed with a focus on coupling them with subsequent dems to construct dems of difference dod which can be created by subtracting one elevation model from another to map erosion deposition and volumetric change the period of dod analysis can be extended to several decades if accurate historical dems can be generated by extracting topographic data from historical data and selecting areas where geomorphic change has been substantial the challenge is to recognize and minimize uncertainties in data that are particularly elusive with early topographic data this paper reviews potential sources of error in digitized topographic maps and dems although the paper is primarily a review of methods three brief examples are presented at the end to demonstrate gcd using dods constructed from data extending over periods ranging from 70 to 90 years

qualitative and quantitative applications of lidar imagery in fluvial geomorphology the potential for geomorphological mapping and quantitative calculations of light detection and ranging lidar data within fluvial geomorphology was studied for two river catchments within belgium dijle and ambleve which differ in physical settings and floodplain morphology two commercial of the shelf lidar datasets with different specifications horizontal resolution and vertical accuracy were available for parts of the floodplains of both catchments real time kinematic rtk global positioning system gps data were used as ground truth for error calculations qualitative analysis of lidar data allowed the identification of former channel patterns levees colluvial hillslope and fan deposits these results were confirmed by field data topographic surveys and historical maps the pixel resolution proved to be an important factor in the identification of small landforms only features with a width equal to or larger than lidar resolution can be detected this poses limits on the usability of regionally available lidar data which often have a horizontal resolution of several metres the lidar data were also used in a quantitative analysis of channel dynamics in the study area the width of the dijle river channel increased 3 m on average between 1969 and 2003 a sediment budget of channel processes for the period 1969 2003 indicated a total river bank erosion of 16 1 103 m3 and a total within channel deposition of 7 1 103 m3 resulting in a net river erosion of 9 0 103 m3 or c 0 4 mg year 1 per metre river length sequential lidar data can in theory be used to calculate vertical sedimentation rates as long as there is control on the error of the reference levels used copyright 2008 john wiley and sons ltd

a census of eddies observed in north atlantic sofar float data abstract sofar floats that looped in discrete eddies were studied in order to map and describe the distribution and characteristics of eddies in the north atlantic one hundred eighteen individual looping float trajectories loopers were identified each consisting of two or more consecutive loops each looper was interpreted to be in a discrete eddy and its characteristics were estimated from the float trajectory the highest percentage of loopers occured at 700m in the newfoundland basin where roughly half of the float data were in loopers mostly cyclones in the gulf stream region approximately 20 of the float data recorded at 700m were in loopers again mostly cyclones overall 21 of 700m data and 6 of 2000m data were in loopers the fastest swirl speeds 40cm s 1 were in cyclones south of the gulf stream most of which were in gulf stream rings but numerous swift 35cm s 1 anticyclones were found there too swirl velocity decreased with depth to roughly half as swift at 2000m as at 700m for three eddies in the sargasso sea measured simultaneously with floats at these two depths in the western north atlantic the average swirl velocity of cyclones and anticyclones was the same translation velocity of eddies was generally westward to southwestward at a few cm s 1 the mean translation velocity of 39 eddies in the sargasso sea was u bar 2 8 0 4 cm s 1 v bar 0 4 0 4 cm s 1 many of these were located just south of the gulf stream in the region of its recirculation and were probably advected by a westward current there near the gulf stream and along its extension in the newfoundland basin eddies were often advected downstream with speeds up to 15 20cm s 1 and eddy trajectories were often complicated south of 30 n and near the western boundary 700m eddies were advected northwestward and 1300m and 2000m eddies southeastward by boundary currents there numerous energetic anticyclones were observed south of the gulf stream one was tracked for 430 days and its properties well measured the formation of these eddies has not been documented but they are inferred to have formed near and by the gulf stream and to consist of a thick layer of 18 c water lying above a depression in the thermocline analogous anticyclones were observed in the newfoundland basin seaward of the gulf stream extension there

late quaternary floodplain reworking and the preservation of alluvial sedimentary archives in unconfined and confined river valleys in the eastern interior of south africa abstract in this study geomorphological sedimentological and geochronological work was conducted on unconfined and confined reaches of three rivers in the eastern interior of south africa in order to quantify the relative rates of floodplain reworking and alluvial preservation along river courses with variable valley confinement and lithology using optically stimulated luminescence osl dating techniques new chronologies for the schoonspruit and mooi river were created and the existing klip river chronology was expanded the results suggest that floodplains in both unconfined and confined reaches preserve complex spatial and temporal patterns of alluviation although differences in boundary conditions lead to variation in the processes and rates of floodplain construction and reworking on the klip and mooi rivers where local base levels are stable channels in unconfined reaches rework floodplain sediments through slow lateral migration punctuated by local erosion during avulsion events on the schoonspruit where base level has lowered the channel in the unconfined reach is incised the floodplain is abandoned and a large gully has formed in the confined reaches of all three rivers the narrow floodplains are reworked by scour and fill activity and limited lateral migration the osl results suggest that unconfined reaches preserve relatively continuous alluvial records that extend into the pleistocene while the floodplains in the confined reaches preserve relatively discontinuous alluvial records biased toward the late holocene the alluvial geochronologic records in these systems preserve signals of changes in local base level controlled by variation in lithology and incision rather than climate change by defining the processes rates and patterns of floodplain reworking in reaches with different degrees of valley confinement and channel incision the findings contribute to understanding how rivers build modify and preserve alluvial sedimentary archives