a linear spectral model of tropical mesoscale systems sensitivity studies abstract a revised version of raymond s wave cisk model is used to study the propagation of mesoscale modes the sensitivity of unstable modes to variations in basic state stability and wind show characteristics of the eastern atlantic during gate as well as parameterized cumulus heating is examined unstable modes exist for mesoscale wavelengths which are sensitive to the details of the cumulus heating parameters as well as the structure of the large scale wind shell in many cases the most unstable mode has a two dimensional structure

planform dynamics of the lower mississippi river this paper presents an analysis of the planform behaviour of the lower mississippi river lmr using a series of maps and hydrographic surveys covering the period 1765 1975 data allow analysis at various time and space scales using fixed and statistically defined reaches both before and after extensive channel modification previous research has interpreted planform change in relation to geomorphological or engineering regime type analyses of channel length and width for the lmr as a single system the analysis here is broadly consistent with these approaches but highlights the importance of meander geometry in the form of the radius of curvature width ratio this neglected factor helps resolve paradoxes relating to observed changes in sediment transport and channel stability when viewed over smaller time and space scales analysis of dynamics using fixed reach boundaries reveals a downstream trend in the pattern of planform behaviour which is closely related to the distribution of valley floor deposits and which also reflects neotectonic influences analysis of changes using statistically determined reach boundaries shows that over shorter time scales meander trains are continually formed and modified over a period of approximately 120 years zones of more or less dynamic behaviour thus move through the lmr the research also provides a context for 20th century engineering interventions to the river these have constrained the magnitude of planform adjustment but also altered the kind of response that is now possible in relation to changes in discharge and sediment load and as a consequence of internal feedbacks within the lmr system copyright 2006 john wiley sons ltd

the formation of longitudinal fronts in a coastal plain estuary longitudinal estuarine fronts aligned parallel to the axis of the estuary are characteristic of the york river their time scale for genesis and decay is of the order of hours they are usually located at the inner edge of the shoals or over the main channel field measurements have shown marked intratidal differences in density and current speeds across this estuary these can be correlated with changes in depth and the strength of the advective processes differential advection between the channel and shoal regions when acting upon a constant longitudinal density gradient is shown to be of sufficient magnitude to generate strong lateral density gradients and thus fronts at particular times within the tidal cycle

interaction of lateral baroclinic forcing and turbulence in an estuary 1 observations of density and velocity in a channel in northern san francisco bay show that the onset of vertical density stratification during flood tides is controlled by the balance between the cross channel baroclinic pressure gradient and vertical mixing due to turbulence profiles of velocity salinity temperature and suspended sediment concentration were measured in transects across suisun cutoff in northern san francisco bay on two days over the 12 5 hour tidal cycle during flood tides an axial density front developed between fresher water flowing from the shallows of grizzly bay into the northern side of suisun cutoff and saltier water flowing up the channel north of the front transverse currents were driven by the lateral salinity gradient with a top to bottom velocity difference greater than 30 cm s south of the front the secondary circulation was weak and along channel velocities were greater than to the north the gradient richardson number shows that stratification was stable north of the front while the water column was turbulently mixed south of the front time series measurements of velocity and salinity demonstrate that the front develops during each tidal cycle in estuaries longitudinal dynamics predict less stratification during flood than ebb tides these data show that stratification can develop during flood tides due to a lateral baroclinic pressure gradient in estuaries with complex bathymetry

tributary connectivity confluence aggradation and network biodiversity in fluvial networks some confluences are associated with tributary driven aggradation where coarse sediment is stored downstream sediment connectivity is interrupted and substantial hydraulic and morphological heterogeneity is generated to the extent that biological diversity is supported by physical diversity it has been proposed that the distribution and frequency of tributary driven aggradation is important for the magnitude and spatial structure of river biodiversity relevant ideas are formulated within the link discontinuity concept and the network dynamics hypothesis but many of the issues raised by these conceptual models have not been systematically evaluated this paper first tests an automated method for predicting the likelihood of tributary driven aggradation in three large drainage networks in the rocky mountain foothills canada the method correctly identified approximately 75 of significant tributary confluences and 97 of insignificant confluences the method is then used to evaluate two hypotheses of the network dynamics hypothesis that linear shaped basins are more likely to show a longitudinal downstream decline in tributary driven aggradation and that larger and more compact basins contain more confluences with a high probability of impact the use of a predictive model that included a measure of tributary basin sediment delivery rather than symmetry ratio alone mediated the outcomes somewhat but as anticipated the number of significant confluences increased with basin size and basin shape was a strong control of the number and distribution of significant confluences doubling basin area led to a 1 9 fold increase in the number of significant confluences and compact basins contained approximately twice as many significant confluences per unit channel length as linear basins in compact basins significant confluences were more widely distributed whereas in linear basins they were concentrated in proximal reaches interesting outstanding issues include the possibility of using spatially distributed sediment routing models to predict tributary driven confluence aggradation and the need to gather ecological data sufficient to properly test for increases in local and network scale biodiversity associated with significant confluences and their network scale controls

application of a linear spectral model to the study of amazonian squall lines during gte able 2b a linear nonhydrostatic spectral model is run with the basic state or large scale vertical profiles of temperature and wind observed prior to convective development along the northern coast of south america during the gte able 2b the model produces unstable modes with mesoscale wavelength and propagation speed comparable to observed amazonian squall lines several tests with different vertical profiles of low level winds lead to the conclusion that a shallow and or weak low level jet either does not produce a scale selection or if it does the selected mode is stationary indicating the absence of a propagating disturbance a 700 mbar jet of 13 m s with a 600 mbar wind speed greater or equal to 10 m s is enough to produce unstable modes with propagating features resembling those of observed amazonian squall lines however a deep layer of moderate winds about 10 m s may produce similar results even in the absence of a low level wind maximum the implications in terms of short term weather forecasting are discussed

reach classifications of the lower mississippi river a classification of the lower mississippi river into geomorphically distinct reaches provides an effective framework within which the influence of numerous morphologic variables can be analyzed a singular realization of the river however requires that a statistical inference be applied to the results which provides a level of assurance to the conclusions a nonparametric method multi response permutation procedures mrpp is employed to evaluate the significance of channel morphology variables within an a priori reach classification based upon sinuosity and valley slope an alternative classification is proffered generated by mrpp which defines homogeneous subpopulations of the river as reaches for the collective response of the channel morphology variables the numeric analyses generate a refinement by further subdivision of the original proposed reach structure within each reach the multivariate response of the system can be studied through space and time within the time period considered here 1880 to 1915 the river channel became progressively wider and shallower these trends are spatially manifest in fewer longer reaches in 1880 and more shorter reaches in 1915 this increased variability is concentrated in the areas affected by the lake county and monroe uplifts

planform channel dynamics along the ningxia inner mongolia reaches of the yellow river from 1958 to 2008 analysis using landsat images and topographic maps channel planform dynamics were analyzed for the ningxia inner mongolia reaches of the yellow river map data were analyzed by means of gis and used to determine the planform characteristics and changes in width sinuosity and shoreline migration rates between 1958 and 2008 the study was based on topographic maps of 1958 and 1967 which had been derived from aerial photos and a field survey and from satellite imagery of 1977 1990 2000 and 2008 the channel planform dynamics were determined by comparing sequential changes in the position of the shorelines in these years sinuosity adjustments were small during the study range 1 34 1 45 the initial sinuosity 1 45 in 1958 gradually decreased to 1 34 in 1990 and then increased to 1 40 in 2008 channel contraction had been the dominant planform process but periodic floods resulted in channel expansion 1958 1990 the river s channel area expanded substantially from 1958 to 1990 by 42 1 then sharply contracted from 1990 to 2000 by 45 8 with no subsequent changes the bank erosion and accretion rates were severe the mean erosion rates ranged from 30 7 to 68 3 m year on the left side and 27 1 to 58 3 m year on the right side the mean accretion rates ranged from 44 4 to 68 3 m year on the left side and 30 5 to 60 4 m year on the right side the mean channel midline shifted by 57 8 m toward the right from 1958 to 2008 the mean channel midline moved 4 8 m to right from 1958 to 1967 54 3 m to the left from 1967 to 1977 44 2 m to the right from 1977 to 1990 64 3 m to the right from 1990 to 2000 and 2 8 m to the right from 2000 to 2008

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

deglacial flood origin of the charleston alluvial fan lower mississippi alluvial valley abstract large magnitude flooding of the mississippi river from proglacial lakes agassiz and superior most likely occurred between 11 300 and 10 900 and 9900 and 9500 yr b p the charleston alluvial fan a depositional remnant of one of these floods is located at the head of a wide alluvial plain near charleston missouri the fan is an elongate convex up sand body 16 24 km composed of medium and fine grained sand at least 8 m thick this sand contrasts with the older coarse grained sand of the braided stream surface to the west and south and younger silty clay of the meandering stream level to the north and east a weakly developed soil separates the underlying braided steam deposits from the alluvial fan a bulk soil radiocarbon date of 10 590 200 yr b p from the contact between the fan and clays of the meandering stream system indicates that the charleston fan was deposited near the end of the early interval of flooding from lake agassiz about 10 900 yr b p if the charleston fan is the last remnant of deglacial flooding in the lower mississippi valley then deposition of significant quantities of sediment from largemagnitude floods between 10 000 and 9500 yr b p did not extend into the lower mississippi valley through thebes gap

connecting the backwater hydraulics of coastal rivers to fluvio deltaic sedimentology and stratigraphy fluvial channels encounter a backwater reach when they approach a standing body of water and recent studies have shown that the transition from normal flow to backwater influenced flow is associated with sediment mass extraction through deposition here we test the hypothesis that systematic changes in the geometry of channel belt deposits and sedimentary architecture occur across this transition using data from the late holocene mississippi southern usa and rhine the netherlands fluvio deltaic systems we use the estimated backwater length and average channel width as characteristic length scales to non dimensionalize the downstream trends in channel belt width for these systems the collapsed data follow similar trends suggesting that the observed variations in channel belt geometry and fluvio deltaic stratigraphy are tied to the location of the backwater transition zone these findings suggest a unifying hydraulic control on fluvio deltaic channel belts and provide a new framework for predicting and understanding the properties of ancient rivers in the coastal zone

spatial autoregressive structure in meander evolution the migration of a river bend depends in part on the high flow velocities that characteristically impinge on its outside bank recent models have treated this in terms of a spatial convolution whereby local bend migration is mathematically a weighted aggregate of up stream curvature and bed topography the convolution model can be tested using river migration data after it is discretized and recast into a finite autoregressive form published isochrones marking former positions of bends on the beatton river canada support the hypothesis that rates of bend migration follow a convolutional relation in addition a comparison of the underlying flow model with published flume experiments involving constant curvature bends illustrates how it predicts the near bank depth averaged velocity associated with a forced bar topography in absence of free bars the autoregressive form of the model is equivalent to a stochastic linear difference equation this allows bend curvature to be treated as a random process cast in the frequency domain the convolution model predicts that big bends tend to grow at the expense of little bends and curvature irregularities in complex trains there exists no tendency for preferential growth of an intermediate bend size the model also predicts the well known shift of maximum migration rates to positions down stream of curvature apexes and implies that the magnitude of this shift increases with decreasing bend size predicted shifts compare well with published measured shifts on the nishnabotna river iowa the sensitivity of the meandering process to initial bend geometries and entrance flow conditions ensures that diverse bend shapes arise along freely migrating rivers independently of factors such as unsteady flow and nonuniform erodibility no single geometrical form serves as an asymptotic evolutionary state for individual bends

lateral circulation generates flood tide stratification and estuarine exchange flow in a curved tidal inlet abstractcross channel transect measurements of microstructure and velocity in a well mixed and curved tidal inlet in the german wadden sea show the occurrence of significant late flood stratification this stratification is found to be a result of lateral straining this study observes a strong single cell lateral circulation which is strongly pronounced at late flood and absent during most of ebb this tidal asymmetry is caused by a systematic interplay between centrifugal forcing and the lateral baroclinic pressure gradient during flood a positive feedback between the terms generates strong lateral circulation whereas during ebb a negative feedback leads to a suppression of the cross channel exchange a theoretical framework based on vorticity is developed which allows lateral and longitudinal circulation to be studied in a consistent way with this framework it is possible to show that the tidal asymmetry of the lateral flow is a major driver of residual longitudinal estuarine circulation here ide

lithologic control of bedrock meander dimensions in the appalachian valley and ridge province a comment braun s finding that the wavelength of bedrock meanders decreases as rock hardness increases is reconciled with hack s and tinkler s finding that meander wavelength is greater in bedrock than alluvial channels in populations of alluvial or bedrock channels bank resistance controls meander wavelength via its influence on channel shape and is negatively correlated with meander wavelength in contrast in mixed populations of alluvial and bedrock channels bank resistance controls meander wavelength through its effect on the recurrence interval of the channel forming discharge and channel size and is positively correlated with meander wavelength

impact of confluence on hydrological and morphological characters of the trunk stream a study on the manu river of north east india tributaries play a significant role in changing flow structure of the main river through their additional discharge a study was conducted at the point of convergence of the river deo with the river manu using simple geospatial technique and field work to justify the hydrological and morphological changes in downstream of the confluence point the spatio temporal change of both the river channels for 10 km length in upstream and downstream of the confluence point was considered moreover field measurements and post field work were carried out to examine the spatial variation of hydrodynamic characteristics like flow velocity depth water discharge wetted perimeter and hydraulic radius of the selected stretches sieving method was applied for grain size analysis of river bed sediment samples the result revealed that both the aggradation and degradation processes were equally active in the upstream segment of the manu river but in downstream segment aggradation exceeded the degradation activity though all the hydrodynamic variables were boosted up in downstream except flow velocity the present research highlighted that the steeper gradient of the r deo had enhanced its competency to transport medium sized grains to the r manu where fine grains were commonly found moreover increased wetted perimeter in downstream specified more friction between channel bed and its flow consequent upon reduced flow velocity with extra sediment load accumulation

mediative adjustment of river dynamics the role of chute channels in tropical sand bed meandering rivers this paper examines processes of chute channel formation in four tropical sand bed meandering rivers the strickland and ok tedi in papua new guinea the beni in bolivia and the lower paraguay on the paraguay argentina border empirical planform analyses highlight an association between meander bend widening and chute initiation that is consistent with recent physics based modelling work gis analyses indicate that bend widening may be driven by a variety of mechanisms including scour and cutbank bench formation at sharply curving bends point bar erosion due to cutbank impingement against cohesive terrace material rapid cutbank erosion at rapidly extending bends and spontaneous mid channel bar formation chute channel initiation is observed to be predominantly associated with two of these widening mechanisms i an imbalance between cutbank erosion and point bar deposition associated with rapid bend extension and ii bank erosion forced by spontaneous mid channel bar development the work extends previous empirical analyses which highlighted the role of bend extension elongation in driving chute initiation with the observation that the frequency of chute initiation increases once bend extension rates and or widening ratios exceed a reach scale threshold a temporal pattern of increased chute initiation frequency on the ok tedi in response to channel steepening and mid channel bar development following the addition of mine tailings mirrors the inter and intra reach spatial patterns of chute initiation frequency on the paraguay strickland and beni rivers where increased stream power and sediment load are associated with increased bend extension and chute initiation rates the process of chute formation is shown to be rate dependent and the threshold values of bend extension and widening ratio for chute initiation are shown to scale with measures of river energy reminiscent of slope ratio thresholds in river avulsion furthermore delft3d simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate such that the process of chute formation may also be rate limiting chute formation is activated iteratively in space and time in response to changes in river energy and sediment load predominantly affecting sites of rapid channel elongation and thereby mediating the river response

alluvial river response to neotectonic deformation in louisiana and mississippi repeat geodetic surveys show uplift of the monroe and wiggins anticlines in louisiana and mississippi there are deformed quaternary terraces which indicate long term deformation in the valleys of the alluvial rivers that cross these structures and there are floodplain and channel convexities that provide evidence of modern deformation in addition the channels show significant variations of morphology sinuosity gradient and depth and behavior appropriate to reaches of increased and decreased valley slope these alluvial rivers are adjusting to modern deformation and their adjustment confirms two geodetic leveling anomalies

mesoscale convective systems 1 mesoscale convective systems mcss have regions of both convective and stratiform precipitation and they develop mesoscale circulations as they mature the upward motion takes the form of a deep layer ascent drawn into the mcs in response to the latent heating and cooling in the convective region the ascending layer overturns as it rises but overall retains a coherent layer structure a middle level layer of inflow enters the stratiform region of the mcs from a direction determined by the large scale flow and descends in response to diabatic cooling at middle to low levels a middle level mesoscale convective vortex mcv develops in the stratiform region prolongs the mcs and may contribute to tropical cyclone development the propagation of an mcs may have a discrete component but may further be influenced by waves and disturbances generated both in response to the mcs and external to the mcs waves of a larger scale may affect the propagation velocity by phase locking with the mcs in a cooperative mode the horizontal scale of an mcs may be limited either by a balance between the formation rate of convective precipitation and dissipation of stratiform precipitation or by the rossby radius of the mcv the vertical redistribution of momentum by an mcs depends on the size of the stratiform region while the net vertical profile of heating of the large scale environment depends on the amount of stratiform rain regional variability of the stratiform rain from mcss affects the large scale circulation s response to mcs heating

the role of advection straining and mixing on the tidal variability of estuarine stratification abstractdata from the hudson river estuary demonstrate that the tidal variations in vertical salinity stratification are not consistent with the patterns associated with along channel tidal straining these observations result from three additional processes not accounted for in the traditional tidal straining model 1 along channel and 2 lateral advection of horizontal gradients in the vertical salinity gradient and 3 tidal asymmetries in the strength of vertical mixing as a result cross sectionally averaged values of the vertical salinity gradient are shown to increase during the flood tide and decrease during the ebb only over a limited portion of the cross section does the observed stratification increase during the ebb and decrease during the flood these observations highlight the three dimensional nature of estuarine flows and demonstrate that lateral circulation provides an alternate mechanism that allows for the exchange of materials between surface and bottom waters even when direct turbu

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

backwater and river plume controls on scour upstream of river mouths implications for fluvio deltaic morphodynamics sediment flux from rivers to oceans is the fundamental driver of fluvio deltaic morphodynamics and continental margin sedimentation yet sediment transport across the river to marine boundary is poorly understood coastal rivers typically are affected by backwater a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume flow deceleration in the backwater zone as well as spreading of the offshore plume should render rivers highly depositional near their mouths leading to sedimentation and eventual elimination of the backwater zone at steady state this reasoning is counter to observations of riverbed scour erosional bed forms and long lived backwater zones near the mouths of some coastal rivers e g mississippi river united states to explain these observations we present a quasi 2 d model of a coupled fluvial backwater and offshore river plume system and apply it to the mississippi river results show that during high discharge events the normal flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface spatial acceleration of flow and erosion of the riverbed as proposed by lane 1957 the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume this points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks

recent geomorphological changes of the middle and lower calore river campania southern italy the geomorphological changes experienced by the middle and lower calore river southern italy between 1957 and 1998 were investigated data derived from field surveys and interpretation of topographic maps and aerial photos were introduced into a geographic information system gis and processed the results showed that the calore river underwent a mean narrowing of 66 with a peak of 86 the channel length increased by approximately 75 m and the sinuosity from 1 311 to 1 314 the extension of fluvial bars reduced by more than 83 while their number increased from 151 to 381 field evidence of riverbed lowering such as river terraces formed in the considered time span and exhumation of foundations of bridges and flood walls were also constantly detected due to these channel adjustments the calore river morphology changed from transitional to single thread these results were coherent with those reported in the pre existing literature about channel adjustments experienced by many rivers during the twentieth century they all were explainable with a reduction of the bedload transport this latter in turn is completely in accordance with the environmental changes that affected the calore river system in the considered period i e damming and or channelization of the main tributaries reduction of liquid discharge due to water withdrawals and sediment mining from the riverbed

geomorphological explanation of the long profile of the lower mississippi river abstract concavity in the long profile of rivers has traditionally been explained through the concept of grade in which the slope declines downstream as a response to changing discharge bed material size and sediment transport applying this concept to particular river systems has however proved problematic the long profile reflects spatially distributed form process feedbacks between all aspects of channel morphology operating at a range of poorly defined time and space scales and in the presence of natural controls in many river systems process form dynamics are further complicated by engineering interventions which add additional extrinsic controls and constrain the range of intrinsic dynamics in this paper the 1974 75 long profile of the lower mississippi river is examined at three scales the regional the reach and the sub reach pool crossing scales a combination of curve fitting zonation algorithms and empirical classification techniques are used to show that although the long profile of the lower mississippi river is concave at the largest scale the profile is characterised by discontinuities shorter trends and zonal variations in the amplitude and wavelength of pool crossing morphology these characteristics are a response to morphological and bed material changes relating to a range of physical geological tectonic tributary input and engineering controls despite its apparent simplicity and correspondence to a graded condition the long profile of the lower mississippi river is actually a complex and scale dependent morphological property at best the concave river profile is therefore a property which emerges from several scales of process form interaction at worst it is no more than an artefact arising from the application of over simplified curve fitting techniques disclosure of the nature of the long profile thus requires the application of a variety of analytical techniques as well as geomorphological explanations which are themselves scale dependent and which consider the interaction of natural processes and the history of engineering intervention

spatial width oscillations in meandering rivers at equilibrium 1 in canaliform rivers channel width at bankfull stage is fairly uniform though at bend apexes it is typically smaller than at crossings conversely in sinuous point bar rivers bankfull width peaks at bend apexes why is there any mechanistic constraint that forces this different behavior we provide an answer to these questions investigating how bankfull width must vary in a sequence of sine generated meanders in order for the constraints of equilibrium constant flow discharge and sediment flux to be satisfied with the help of a 3 d fully nonlinear analytical model of flow and bed topography in meandering rivers with variable width we show that in a meandering channel characterized by a constant longitudinal free surface slope the equilibrium width thus obtained oscillates with a frequency twice the frequency of channel curvature and experiences the maximum width close to inflection points this pattern is typically observed in canaliform rivers we then show that a similar pattern is observed in sinuous point bar rivers provided the hydrodynamic width width of the free surface is replaced by the active width namely the width of the portion of the cross section where transport occurs at formative conditions theoretical results are substantiated by a satisfactory comparison with field observations referring to the mississippi river united states and to the bollin river united kingdom

longitudinal versus lateral estuarine dynamics and their role in tidal stratification patterns in lower south san francisco bay author s hoang olivia abstract the dynamics of shoal channel estuaries require consideration of lateral gradients and transport which can create significant intratidal variability in stratification and circulation when the shoal channel system is strongly coupled by tidal exchange with mudflats marshes or other habitats the gradients driving intratidal stratification variations are expected to intensify to examine this dynamic hydrodynamic data was collected from january 27 2017 february 10 2017 in lower south san francisco bay a small subembayment fringed by extensive shallow vegetated habitats during this deployment salinity variations were captured through instrumentation of 6 stations arrayed longitudinally and laterally allowing for mechanisms of stratification creation and destruction to be calculated directly and compared with observed time variability of stratification at the central station we present observation based calculations of longitudinal straining longitudinal advection lateral straining and lateral advection the time dependence of stratification was observed directly and calculated by summing measured longitudinal and lateral mechanisms we found that the stratification dynamics switch between being longitudinally dominated during the middle of ebb and flood tides to being laterally dominated during the tidal transitions this variability is driven by the interplay between tidally variable lateral density gradients and turbulent mixing relatively constant along estuary density gradients are differentially advected during flood and ebb tides resulting in maximal lateral density gradients around tidal transitions simultaneous decrease in turbulent mixing at slack tides allows lateral density driven exchange to stratify the estuary channel at the slack after flood at the end of ebb barotropic forcing drives negatively buoyant shoal waters towards the channel plain language summary san francisco bay sits within a highly urbanized area the dense population creates large wastewater effluent resulting in high nutrient levels scientists wonder why there have not been annual phytoplankton blooms like observed in other estuaries with lower nutrient levels some have hypothesized it is due to high turbidity levels and tidal breakdown of stratification creating nonideal environments for phytoplankton growth however decadal trends show that the estuary is becoming less turbid and with changes in climate patterns there is potential for persistent stratification we observed development of stratification over the ebb tide and destratification in two distinct events as the tide reverses over the flood tide at the reversal of the tides water in the shoals exchange with the water in the channel creating a pulse of salty water to the channel at the ebb to flood transition and a pulse of fresh water at the flood to the ebb transition destratification occurs in the early flood tide due to a pulse of saline water received from the shoals then due to the advection of less stratified water being pulled to the center channel of the estuary finally stratification is destroyed completely due to longitudinal straining and turbulent mixing