a new methodology for the quantitative visualization of coherent flow structures in alluvial channels using multibeam echo sounding mbes 1 in order to investigate the interactions between turbulence and suspended sediment transport in natural aqueous environments we ideally require a technique that allows simultaneous measurement of fluid velocity and sediment concentration for the whole flow field here we report on development of a methodology using the water column acoustic backscatter signal from a multibeam echo sounder to simultaneously quantify flow velocities and sediment concentrations the application of this new technique is illustrated with reference to flow over the leeside of an alluvial sand dune which allows for the first time in a field study quantitative visualization of large scale whole flow field turbulent coherent flow structures associated with the dune leeside that are responsible for suspending bed sediment this methodology holds great potential for use in a wide range of aqueous geophysical flows

bed load and suspended load contributions to migrating sand dunes in equilibrium dunes dominate the bed of sand rivers and are of central importance in predicting flow roughness and water levels the present study has focused on the details of flow and sediment dynamics along migrating sand dunes in equilibrium using a recently developed acoustic system acoustic concentration and velocity profiler new insights are obtained in the behavior of the bed and the suspended load transport along mobile dunes our data have illustrated that due to the presence of a dense sediment layer close to the bed and migrating secondary bedforms over the stoss side of the dune toward the dune crest the near bed flow and sediment processes are significantly different from the near bed flow and sediment dynamics measured over fixed dunes it was observed that the shape of the total sediment transport distribution along dunes is mainly dominated by the bed load transport although the bed load and the suspended load transport are of the same order of magnitude this means that it was especially the bed load transport that is responsible for the continuous erosion and deposition of sediment along the migrating dunes whereas the bed load is entirely captured in the dune with zero transport at the flow reattachment point a significant part of the suspended load is advected to the downstream dune depending on the flow conditions for the two flow conditions measured the bypass fraction was about 10 for flow with a froude number fr of 0 41 and 27 for flow with froude number of 0 51 this means that respectively 90 for the fr 0 41 flow and 73 for the fr 0 51 flow of the total sediment load that arrived at the dune crests contributed to the migration of the dunes

measuring flow velocity and sediment transport with an acoustic doppler current profiler an acoustic doppler current profiler adcp measures three dimensional velocity profiles within the water column using the doppler shift principle whilst the bottom tracking function and acoustic backscatter can be used to measure bed load velocity and estimate suspended sediment concentration the adcp offers many advantages over traditional single point current meters and sediment samplers including deployment from a moving launch a single instrument for both velocity and sediment transport measurements profiles of three dimensional velocity and suspended sediment and the ability to map an entire flow field limitations of adcps include a large sampling diameter close to the bed coarse measurement of vertical velocity fragmented bottom track records a poor understanding of the relation between bottom tracking and the mechanisms of sediment transport and sensitivity of the acoustic backscatter to particle size

numerical simulation of sand waves in a turbulent open channel flow we develop a coupled hydro morphodynamic numerical model for carrying out large eddy simulation of stratified turbulent flow over a mobile sand bed the method is based on the curvilinear immersed boundary approach of khosronejad et al adv water resour vol 34 2011 pp 829 843 we apply this method to simulate sand wave initiation growth and evolution in a mobile bed laboratory open channel which was studied experimentally by venditti church j geophys res vol 110 2005 f01009 we show that all the major characteristics of the computed sand waves from the early cross hatch and chevron patterns to fully grown three dimensional bedforms are in good agreement with the experimental data both qualitatively and quantitatively our simulations capture the measured temporal evolution of sand wave amplitude wavelength and celerity with good accuracy and also yield three dimensional topologies that are strikingly similar to what was observed in the laboratory we show that near bed sweeps are responsible for initiating the instability of the initially flat sand bed stratification effects which arise due to increased concentration of suspended sediment in the flow also become important at later stages of the bed evolution and need to be taken into account for accurate simulations as bedforms grow in amplitude and wavelength they give rise to energetic coherent structures in the form of horseshoe vortices which transport low momentum near bed fluid and suspended sediment away from the bed giving rise to characteristic boil events at the water surface flow separation off the bedform crestlines is shown to trap sediment in the lee side of the crestlines which coupled with sediment erosion from the accelerating flow over the stoss side provides the mechanism for continuous bedform migration and crestline rearrangement the statistical and spectral properties of the computed sand waves are calculated and shown to be similar to what has been observed in nature and previous numerical simulations furthermore and in agreement with recent experimental findings singh et al water resour res vol 46 2010 pp 1 10 the spectra of the resolved velocity fluctuations above the bed exhibit a distinct spectral gap whose width increases with distance from the bed the spectral gap delineates the spectrum of turbulence from the low frequency range associated with very slowly evolving albeit energetic coherent structures induced by the migrating sand waves overall the numerical simulations reproduce the laboratory observations with good accuracy and elucidate the physical phenomena governing the interaction between the turbulent flow and the developing mobile bed

decadal scale riverbed deformation and sand budget of the last 500 km of the mississippi river insights into natural and river engineering effects on a large alluvial river

dynamics of bedforms in the lower mississippi river abstract the size and roughness characteristics of dunes in the mississippi river are not predicted well by experimental and theoretical relations even though intensive flow measurements were made in the study area although dunes increase in scale with increasing discharge of water and sediment the development of multiple dune sizes and nonuniform flow obscure the relationship of dune geometry to synoptic hydraulic variables some nonuniformity is caused by the development of large bed undulations from kinematic waves that can deform into compound dunes but most of it is related to flow convergence and divergence in pools and riffles varying flow geometry with increasing stage and reach controlled relations between flow and energy loss even though changes of bedform size are not found to lag the flow changes because sand transport is large a considerable volume of sediment is required to initiate and propagate the largest compound dunes this means that they might be profiled in different stages of growth and have a varying effect on the flow during their evolution spatially and temporally

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

hydrodynamic sorting and transport of terrestrially derived organic carbon in sediments of the mississippi and atchafalaya rivers abstract over the course of two years four cruises were conducted at varying levels of discharge in the lower mississippi and atchafalaya rivers mr and ar where grab samples were collected from sand and mud dominated sediments the tetramethylammonium hydroxide tmah thermochemolysis method was used to determine sources of terrestrially derived organic carbon oc in these two sediment types to examine the effects of hydrodynamic sorting on lignin sources in river sediments average lignin concentrations in the lower mr were 1 4 1 1 mg goc 1 at english turn et and 10 4 27 4 mg goc 1 at venice using these concentrations annual lignin fluxes to the gulf of mexico from tidal and estuarine mud remobilization at et and venice were 3 1 2 5 10 5 kg and 11 4 30 0 10 5 kg respectively much of the lignin derived materials in muddy sediments appeared to be derived from non woody grass like sources which should decay more quickly than the woody materials typically found in the sandy deposits the average total oc 1 93 0 47 of english turn sands yields an annual flux of 0 34 0 09 10 9 kg lignin flux in the english turn sands 3 6 2 6 mg gc 1 using the numbers above would be 12 2 9 4 10 5 kg the extensive amounts of sand sized woody materials coffee grinds found in the sandy sediments in both the ar and mr are likely derived from woody plant materials this is the first time it has been demonstrated that sandy sediments in the mr provide an equally important pathway compared to muds for the transport of terrestrially derived organic matter to the northern gulf of mexico using the ar average oc in sand 1 16 0 72 we estimated an annual flux of oc to the shelf of 0 13 0 07 10 9 kg lignin flux for ar sands was estimated to be 12 4 12 1 10 5 kg despite the high error associated with these numbers we observe for the first time that the flux of lignin in sandy sediments in the ar to the northern gulf of mexico is comparable to that found in the mr these results further support the likelihood of grain size related hydrodynamic sorting of terrestrially derived organic carbon in the lower mississippi and atchafalaya rivers suggesting that there is a distinct sandy sediment organic fraction contributed by major rivers to the global carbon cycle

hydrological connectivity in river deltas the first order importance of channel island exchange deltaic systems are composed of distributary channels and interdistributary islands while previous work has focused either on the channels or on the islands here we study the hydrological exchange between channels and islands and point at its important role in delta morphology and ecology we focus our analysis on wax lake delta in coastal louisiana usa and characterize the surface water component of hydrological connectivity through measurements of water discharge and hydraulic tracer propagation we find that deltaic islands are zones of significant water flux as 23 54 of the incoming distributary channel flux enters the islands a calculation of the travel times through a channel island complex shows travel times through the islands to be at least 3 times their channel counterparts a dye release experiment also indicates that travel times in islands are much longer that those within channels as dye remained in the island for the 3 8 day duration of the experiment additionally islands are more sensitive than channels to environmental forces such as tides which cause flow reversal and thus can increase travel times through the islands our work defines the hydrological network of a river delta to include not only the distributary channel network but also the interdistributary islands quantifies the implications of channel island hydrological connectivity to travel times through the system and discusses the relevance of our findings to channel mouth dynamics at the delta front and the potential for denitrification in coastal systems

environmental signal propagation in sedimentary systems across timescales abstract earth surface processes operate across erosionally dominated landscapes and deliver sediment to depositional systems that can be preserved over a range of timescales the geomorphic and stratigraphic products of this source to sink sediment transfer record signals of external environmental forcings as well as internal or autogenic dynamics of the sedimentary system here we evaluate environmental signal propagation across sediment routing systems with emphasis on sediment supply qs as the carrier of up system forcings we review experimental numerical and natural examples of source to sink sediment routing and signal propagation during three timescales 1 historic which includes measurement and monitoring of events and processes of landscape change and deposition during decades to centuries 2 centuries to several millions of years referred to as intermediate timescale and 3 deep time we discuss issues related to autogenic dynamics of sediment transport transient storage and release that can introduce noise lags and or completely mask signals of external environmental forcings we provide a set of conceptual and practical tools for evaluating sediment supply within a source to sink context which can inform interpretations of signals from the sedimentary record these tools include stratigraphic and sediment routing system characterization sediment budgets geochronology detrital mineral analysis e g thermochronology comparative analog approaches and modeling techniques to measure calculate or estimate the magnitude and frequency of external forcings compared to the characteristic response time of the sediment routing systems

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

river discharge influences on particulate organic carbon age structure in the mississippi atchafalaya river system 1 applying ramped pyrolysis radiocarbon analysis to suspended river sediments we generate radiocarbon 14c age spectra for particulate organic carbon poc from the lower mississippi atchafalaya river system mars to better understand a major river system s role in carbon transport ramped pyrolysis 14c analysis generates age distributions of bulk carbon based on thermochemical stability of different organic components our results indicate higher proportions of older material in the poc during higher discharge ages increase throughout the high discharge age spectra indicating that no single component of the poc is responsible for the overall age increases observed instead older material is contributed across the poc age spectrum and unrelated to increased bedload suspension in this comparison of 2 spring discharges less than half of the poc transported during higher discharge is less than 1000 14c years in age constraining of the role of the mars as a flux of atmospheric co2 toward longer term sedimentary sinks in the mississippi delta and the gulf of mexico the results suggest that delta building processes benefit disproportionately from high discharge events carrying larger amounts of sediment because these events involve both a higher proportion of millennially aged carbon from floodplain exchange of poc and a potentially higher proportion of petrogenic carbon 30 530 increase overall an internally consistent picture of po14c age distributions from a major river system emerges as differences in space and time are small compared to the range of ages of poc sources in such a large basin

the role of discharge variability in the formation and preservation of alluvial sediment bodies abstract extant planform based facies models for alluvial deposits are not fully fit for purpose because they over emphasise plan form whereas there is little in the alluvial rock record that is distinctive of any particular planform and because the planform of individual rivers vary in both time and space accordingly existing facies models have limited predictive capability in this paper we explore the role of inter annual peak discharge variability as a possible control on the character of the preserved alluvial record data from a suite of modern rivers for which long term gauging records are available and for which there are published descriptions of subsurface sedimentary architecture are analysed the selected rivers are categorized according to their variance in peak discharge or the coefficient of variation cvqp standard deviation of the annual peak flood discharge over the mean annual peak flood discharge this parameter ranges over the rivers studied between 0 18 and 1 22 allowing classification of rivers as having very low 0 90 annual peak discharge variance deposits of rivers with very low and low peak discharge variability are dominated by cross bedding on various scales and preserve macroform bedding structure allowing the interpretation of bar construction processes rivers with moderate values preserve mostly cross bedding but records of macroform processes are in places muted and considerably modified by reworking rivers with high and very high values of annual peak discharge variability show a wide range of bedding structures commonly including critical and supercritical flow structures abundant in situ trees and transported large woody debris and their deposits contain pedogenically modified mud partings and generally lack macroform structure such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models but is widely developed both in modern and ancient alluvial deposits this high peak variance style is also distinctive of rivers that are undergoing contraction in discharge over time because of the gradual annexation of the channel belt by the establishment of woody vegetation we propose that discharge variability both inter annual peak variation and flashiness may be a more reliable basis for classifying the alluvial rock record than planform and we provide some examples of three classes of alluvial sediment bodies representing low intermediate and high very high discharge variability from the rock record that illustrate this point

modeling flow and sediment transport dynamics in the lowermost mississippi river louisiana usa with an upstream alluvial bedrock transition and a downstream bedrock alluvial transition implications for land building using engineered diversions the lowermost mississippi river defined herein as the river segment downstream of the old river control structure and hydrodynamically influenced by the gulf of mexico extends for approximately 500 km this segment includes a bedrock or more precisely mixed bedrock alluvial reach that is bounded by an upstream alluvial bedrock transition and a downstream bedrock alluvial transition here we present a one dimensional mathematical formulation for the long term evolution of lowland rivers that is able to reproduce the morphodynamics of both the alluvial bedrock and the bedrock alluvial transitions model results show that the magnitude of the alluvial equilibrium bed slope relative to the bedrock surface slope and the depth of bedrock surface relative to the water surface base level strongly influence the mobile bed equilibrium of low sloping river channels using data from the lowermost mississippi river the model is zeroed and validated at field scale by comparing the numerical results with field measurements the model is then applied to predict the influence on the stability of channel bed elevation in response to delta restoration projects in particular the response of the river bed to the implementation of two examples of land building diversions to extract water and sediment from the main channel is studied in this regard our model results show that engineered land building diversions along the lowermost mississippi river are capable of producing equilibrated bed profiles with only modest shoaling or erosion and therefore such diversions are a sustainable strategy for mitigating land loss within the mississippi river delta

eastern mississippi delta late wisconsin unconformity overlying transgressive facies sea level and subsidence abstract analysis of 92 engineering core logs located in the balize sector of the eastern mississippi delta focuses on the late wisconsin unconformity and lithofacies of strata lying immediately below and above this stratigraphic horizon this major sequence boundary is a key feature used to correlate strata across shelf and slope to the basin observations emphasize lithofacies distributions of the latest pleistocene sediments underlying the unconformity the late pleistocene to early holocene transgressive facies and the immediately overlying deltaic deposits maps and a cross section compiled with this information highlight the critical relation between lithofacies distributions and late wisconsin sea level oscillations core analysis reveals that the transgressive facies comprises distinct environments of deposition offshore to onshore findings identify criteria to determine the approximate position of the shoreline at the late wisconsin maximum sea level lowstand and at the extent of early holocene maximum marine inundation in the balize complex mapping of lithologies along the late wisconsin unconformity serves to improve correlation of sediment facies with changes in acoustic response along high resolution seismic profiles an estimate of long term averaged land subsidence suggests lowering of at least 1 mm year near the shelfedge during the past 18 000 years this mapping also refines sea level response models for the mississippi delta including criteria to locate key paleogeographic features such as shelfedges and maximum flooding surfaces in other recent marine deltas and in older stacked deltaic sequences

bed load discharge coefficient using dimensional analysis the bed load discharge coefficient for the case of two dimensional flow over dunes is expressed in terms of bed form geometry and the sand size of the bed material experiments are conducted in a rectangular flume using artifical dunes and a flow visualization method to measure the flow separation length from which the bed load coefficients were determined the results show that the bed load coefficient is affected significantly by sand grain size only when the dunes are flat when dunes are steep the value of the bed load coefficient is affected mainly by the dune steepness application of the bed load coefficient results in some improvement of estimating bed load

observational and numerical particle tracking to examine sediment dynamics in a mississippi river delta diversion abstract river diversions may serve as useful restoration tools along coastal deltas experiencing land loss due to high rates of relative sea level rise and the disruption of natural sediment supply diversions mitigate land loss by serving as new sediment sources for land building areas in basins proximal to river channels however because of the paucity of active diversions little is known about how diversion receiving basins evacuate or retain the sediment required to build new land this study uses observational and numerical particle tracking to investigate the behavior of riverine sand and silt as it enters and passes through the west bay diversion receiving basin located on the lowermost mississippi river delta usa fluorescent sediment tracer was deployed and tracked within the bed sediment over a five month period to identify locations of sediment deposition in the receiving basin and nearby river channel a computational fluid dynamics model with a lagrangian sediment transport module was employed to predict selective pathways for riverine flow and sand and silt particles through the receiving basin observations of the fluorescent tracer provides snapshots of the integrated sediment response to the full range of drivers in the natural system the numerical model results offer a continuous map of sediment advection vectors through the receiving basin in response to river generated currents together these methods provide insight into local and basin wide values of sediment retention as influenced by grain size transport time and basin morphology results show that after two weeks of low mississippi river discharge basin silt retention was approximately 60 but was reduced to 4 at the conclusion of the study riverine sand retention was approximately near 100 at two weeks and 40 over the study period modeled sediment storage was predicted to be greatest at the margins of the primary basin transport pathway this matched the observed dynamics of the silt tracer but did not match the behavior of the sand tracer the degree to which the observational measurements deviate from the model predictions may indicate the relative influence of physical processes other than the mean riverine generated currents such as tides wind generated currents and waves

nature of deformation of sandy bed forms 1 we explore a stochastic component of topographic evolution of sandy river beds and its relationship to bed material flux the behavior of trains of mobile bed forms can be decomposed into two independent constituents translation and deformation translation is the mean downstream migration of the bed at velocity that defines the lagrangian reference frame of the bed deformation is the sum of all changes to the bed s topographic profile measured from within the bed s moving reference frame the occurrence of deformation leads to exponential decorrelation of bed topography that is in dynamic equilibrium with flow conditions for the field and laboratory data sets used correlation decays to 0 5 by the time the bed translates 40 and 360 of the mean bed form length respectively proportions of bed material flux responsible for translation and deformation can be straightforwardly calculated translation flux is measured using the traditional bed form bed load equation deformation flux is determined by excess topographic change scaled by the ratio of horizontal sediment velocity to fall velocity deformation represents the sediment exchanged between bed load and suspended load because deformation is a stochastic process with zero mean the apparent rate of deformation decreases as a function of time interval between bed surveys for the field case deformation accounts for 40 of bed material flux while it is only 1 of the flux in the laboratory

recent decadal growth of the atchafalaya river delta complex effects of variable riverine sediment input and vegetation succession abstract the mississippi river delta plain has experienced substantial wetland loss from subsidence erosion and sea level rise threatening coastal communities and the ecosystems that support them the atchafalaya river the largest distributary of the mississippi river has one of the few prograding delta features along the 200 km deltaic coastline understanding changes in the atchafalaya river delta complex ardc development has critical implications for future prediction and management strategy for the mississippi river delta plain this study was organized to answer two major questions 1 how did development of the ardc respond to fluctuation in riverine sediment supply over the period 1989 2010 and 2 has vegetation succession helped stabilize subaerial land the study quantified annual total suspended sediment yields to the two ardc subdeltas atchafalaya river subdelta arsd and wax lake outlet subdelta wlsd classified delta land cover using satellite imagery over 5 year intervals into three classes barren land vegetation and open water and investigated the relationship of delta land change with sediment yield and vegetation succession over the entire 21 year study period we found a net land gain of 59 km 2 with the arsd accounting for 58 of this gain and wlsd 42 sediment yield to the subdeltas decreased from an average annual of 38 megatonnes mt for arsd and 18 mt for wlsd during 1989 1995 to an average annual of 24 mt for arsd and 17 mt for wlsd during 2004 2010 corresponding to the decrease in riverine suspended sediment concentration concurrently total land growth rate decreased from 2 4 km 2 y 1 to 1 6 km 2 y 1 for arsd and 3 2 km 2 y 1 to 0 6 km 2 y 1 for wlsd however the ardc had a net land loss of 2 1 km 2 during 1999 2004 because of tropical system effects in conjunction with the lack of large river floods defined as discharge 13 800 m 3 s 1 on average more than 60 of newly vegetated land remained vegetated in subsequent years and when compared with barren areas vegetated land was less likely 7 3 vs 32 to be converted to water indicating vegetative stabilization effect however during the period without a major flood vegetation buffering against tropical system erosion was limited this indicates that over the period 1989 to 2010 land growth of the ardc was dictated by large flood events

origin of hysteresis in bed form response to unsteady flows 1 field and laboratory studies indicate that changes in riverbed morphology often lag changes in water discharge this lagged response produces hysteresis in the relationship between water discharge and bed form geometry to understand these phenomena we performed flume experiments to observe the response of a sand bed to step increases and decreases in water discharge for an abrupt rise in discharge we observed that bed forms grew rapidly by collision and merger of bed forms migrating with different celerities growth rate slowed as bed forms approached equilibrium with the higher discharge regime after an abrupt discharge drop bed form decay occurred through formation of smaller secondary bed forms in equilibrium with the lower discharge which cannibalized the original relict features we present a simple model framework to quantitatively predict time scales of bed form adjustment to flow changes based on equilibrium bed form heights lengths and celerities at low and high flows for rising discharge the model assumes that all bed form collisions result in irreversible merger due to a dispersion of initial celerities for falling discharge we derive a diffusion model for the decay of relict high stage features our models predict the form and time scale of experimental bed form adjustments additional experiments applying slow and fast triangular flood waves show that bed form hysteresis occurs only when the time scale of flow change is faster than the modeled and measured bed form adjustment time we show that our predicted adjustment time scales can also be used to predict the occurrence of bed form hysteresis in natural floods

the lowermost mississippi river a mixed bedrock alluvial channel in this study the distribution of channel bed sediment facies in the lowermost mississippi river is analysed using multibeam data complemented by sidescan sonar and compressed high intensity radar pulse seismic data as well as grab and core samples of bed material the channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls the consolidated substratum is made up of latest pleistocene and early holocene fluvio deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends the exposed substratum commonly displays a suite of erosional features including flutes that are quantitatively similar in form to those produced under known laboratory conditions a total of five bed facies are mapped three of which include modern alluvial deposits and two facies that are associated with the relict substratum a radius of curvature analysis applied to the mississippi river centreline demonstrates that the reach scale distribution of channel bed facies is related to river planform from a broader perspective the distribution of channel bed facies is related to channel sinuosity higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment for example the ratio of alluvial cover to substratum is ca 1 5 1 for a 45 km segment of the river that has a sinuosity of 1 76 and this ratio increases to ca 3 1 for a 120 km segment of the river that has a sinuosity of 1 21 the exposed substratum is interpreted as bedrock and given the relative coverage of alluvial sediment in the channel the lowermost mississippi river can be classified as a mixed bedrock alluvial channel the analyses demonstrate that a mixed bedrock alluvial channel boundary can be associated with low gradient and sand bed rivers near their marine outlet

evaluation of sediment diversion design attributes and their impact on the capture efficiency abstractmany riverine systems have been disconnected from their receiving basins by flood protection levees and other engineered systems reconnecting these alluvial rivers with their receiving basins is a viable option to nourish and sustain existing coastal wetland systems as well as to build new land this sediment nourishment can be accomplished through direct dredging and placement or through sediment diversions efficient design of sediment diversions is important to maximize the land building potential this study s objective is to quantitatively identify key design attributes of sediment diversions influencing their ability to capture sediment the outfall channel alignment angle intake invert elevation and diversion size are hypothesized as key parameters the analysis is limited to sediment grain sizes larger than 63 m and has been performed using a validated three dimensional numerical model a time integrated sediment water ratio was used as an indicator to reflect efficiency of sedime

a geomorphic approach to the analysis of bedload and bed morphology of the lower mississippi river near the old river control structure abstract the mississippi river is the ultimate single thread meandering river five hundred kilometers upstream from its mouth about 25 of the river s discharge and sediment load is diverted into the atchafalaya river this diversion is controlled by the old river control structure orcs built by the u s army corps of engineers usace in stages since 1961 to stop the avulsion of the mississippi river into the atchafalaya the effects of orcs on sediment and water discharge and geomorphic change to the lower mississippi river lmr channel are not yet completely understood and require placing the river into a geomorphic context first classifying the channel into similar categories before evaluating change the objectives of this study are to estimate the lmr bedload develop and apply a geomorphic classification of the lmr near the orcs and explore geomorphic change within the classified areas we studied a 115 km long stretch between orcs and baton rouge that is highly impacted by engineering we used six sets of bathymetric multibeam echosounder surveys conducted by the usace multitemporal cartographies and a field survey supported by multibeam echosounder bathymetry acoustic doppler current profiler adcp measurements sediment samples collection and geomorphic observations a three dimensional method for estimating bedload from time elapsed bathymetric surveys was developed and applied on seven sets of time elapsed surveys downstream from orcs from 2010 2011 and 2012 we estimate that the fraction of bedload as a percentage of total sand load between 2003 and 2011 was 13 2 a bedform classification scheme based on bedform height was developed the bed was almost completely mantled by sandy bedforms above the 24 m isoline the studied reach was divided into ten zones according to four geomorphic types based on channel planform geologic controls presence of islands and other morphometric parameters these zones were shown to be physically distinct in terms of bedform location depth to bedform size channel sinuosity channel width to depth ratios and adjustments in the thalweg and channel width from 1948 to 2012 the 1948 to 1975 period displayed a relatively large amount of thalweg aggradation with a spatial average of 1 8 m and an average channel narrowing of 80 m the 1975 to 2012 period had much lower spatial aggradation of 0 5 m and a lower average channel narrowing of 35 m thalweg sinuosity did not adjust very much with an average reduction of 0 02 from 1948 to 2012 and an average of zero adjustment between 1975 and 2012 the shape and spatial extent of a very large anomalous bar near orcs varied at seasonal and decadal timescales and may be influenced by the orcs bed morphology was specific to geomorphic zones which were shown to be physically significant and should be employed in future studies of large rivers

a simple model of bedform migration a model linking subaqueous dune migration to the effective grain related shear stress is calibrated by means of flume data for bedform dimensions and migration rates the effective shear stress is calculated on the basis of a new method assuming a near bed layer above the mean bed level in which the current velocity accelerates towards the bedform crest as a consequence the effective bed shear stress corresponds to the shear stress acting directly on top of the bedform the model operates with the critical shields stress as a function of grain size and predicts the deposition volume per unit time and width of naturally packed bed material on the bedform lee side qbcrest the model is simple built on a rational description of simplified sediment mechanics and its calibration constant can be explained in accordance with estimated values of the physical constants on which it is based predicted values of qbcrest correlate with measured values of bedform height multiplied by bedform migration rate with r2 0 83 copyright 2010 john wiley sons ltd

punctuated sand transport in the lowermost mississippi river 1 measurements of sand flux and water flow in the mississippi river are presented for a portion of the system 35 50 km upstream from the head of its subaerial delta these data are used to provide insight into how nonuniform flow conditions present in the lower reaches of large alluvial rivers affect the timing and magnitude of sand transport near the river outlet field surveys during both low and high water discharge include 1 sequential digital bathymetric maps defining mobile river bottom topography which were used to estimate bed material flux 2 multiple water velocity profiles and 3 multiple suspended sediment profiles collected using a point integrated sampler these data show that total sand transport increases by two orders of magnitude over the measured range in water discharge 11 300 to 38 400 m3 s 1 during low water discharge no sand is measured in suspension and sand discharge via bed form migration is minimal during high water discharge 54 of the sand discharge is measured in suspension while 46 of the sand discharge is part of bed form migration the component of boundary shear stress associated with moving this sediment is estimated using a set of established sediment transport algorithms and values for the total boundary shear stress are predicted by fitting logarithmic velocity functions to the measured profiles the estimates of boundary shear stress using measurements of suspended sand transport bed form transport and downstream oriented velocity profiles are internally consistent moreover the analyses show that boundary shear stress increases by nearly 10 fold over the measured water discharge range we show how this increase in shear stress is consistent with backwater flow arising where the river approaches its outlet the hydrodynamic properties of backwater flow affect the timing and magnitude of sand flux and produce punctuated sand transport through the lowermost mississippi river our field data are used to evaluate the influence of this sand transport style on development of the mixed bedrock alluvial channel for the lowermost mississippi river

observations of dune interactions from dems using through water structure from motion abstract the role of three dimensionality 3d in modulating both flow and sediment transport remains poorly understood 3d bed elevation measurements are difficult to obtain due to irregular dune shapes and submergence using photogrammetric tools for topographic reconstruction has become popular in surface studies yet water refraction makes through water image capturing of submerged dunes still challenging we show that through water images and structure from motion sfm tools can produce accurate digital elevation models dems of a measured dune field with a high level of detail it is a low cost and non intrusive alternative for submerged bed elevation measurements we provide a detailed sfm workflow introduce and discuss the use of camera coordinates and underwater ground control points and processing steps needed to obtain high accuracy dems our method results in dems with mm resolution in controlled laboratory conditions new data for 23 dune field dems from 6 experiments were obtained and are presented the dune field dem time series provide insights on dune interaction processes in our experiments merging and defect creation were most common merging decreases the number of bedforms and defect creation is responsible for more bedforms dunes continue to interact and jostle after achieving equilibrium simple crests are much more frequently observed than irregular sinuous or double crests two dimensional 2d crest shapes were observed although crest shapes were generally of 3d nature observed dune migration rates are not steady and vary in time and space bedform splitting speeds up dune migration whilst neighbouring dunes tend to accelerate or decelerate we identify a cascade of processes and show that 3d dems at high frequency are required to resolve observed rapid deformations and interaction of dunes the identified dune interaction processes are inherent to sediment flux variability whereas the temporal component of the underlying sediment transport processes is still not well understood in relation to average transport

from landform to process morphology and formation of lake bed barchan dunes makgadikgadi botswana abstract a suite of crescentic landforms is visible from remotely sensed imagery within the ntwetwe panpan in the makgadikgadi basin botswana we investigate the most distinct of these landforms using morphometric measurements sedimentary data and optically stimulated luminescence osl signal analysis comparative analysis with previously published barchan morphological data sets suggest the ntwetwe features fall within the spectrum of morphometric parameters found in a range of barchan dunefields from around the world there is currently insufficient comparative morphometric data from sub aqueous dunefields to be able to distinguish the particular formative environment of the dune osl signal analyses however support the hypothesis of grove 1969 grove a t 1969 landforms and climatic change in the kalahari and ngamiland geographical journal 135 191 212 that the last deposition of the sediments within the ntwetwe forms was most likely aeolian in origin luminescence signal analysis is employed to investigate potential transport and bleaching environments of the sediments forming the features but results in this case do not shed further light on the formative conditions of these enigmatic landforms

bedform transport rates for the lowermost mississippi river 1 new methods of data collection and processing are developed to provide quantitative reach scale measurements of bedform transport mass within the tidally influenced mississippi river a multibeam swath profiler was used to collect daily bathymetry over a range of water discharges and bed elevation changes induced by dune migration are measured these values are coupled with bulk physical properties of the bed sediment to constrain mass flux and annual bedform transport is estimated at 2 2 x 10 6 metric tons mt the total annual sand flux from the mississippi river calculated by combining measured bedform transport rates and suspended sediment flux is estimated to be 20 x 10 6 mt survey data also provide information about the spatial distribution of dunes across the channel bottom straight reach segments are commonly mantled by dunes for the entire cross section while bends are typically areas of focused scour devoid of bedforms presumably any sediments associated with migrating dunes are propelled into suspension within bends before redepositing in the subsequent straight reach movement via suspension is therefore an important component for the downriver transport of bed materials in the lower mississippi river

global river slope a new geospatial dataset and global scale analysis abstract a rivers longitudinal gradient i e slope is a key parameter in fluvial hydrology hydraulics and geomorphology it affects a multitude of fluvial variables such as flow velocity and sediment transport limitations in river slope data both its availability and accuracy constrain the fidelity of fluvial modeling particularly at larger or global scales traditional slope calculation algorithms cannot accurately predict river slopes as these are based on cell by cell calculation which is only suitable for hillslopes and small mountainous streams this paper presents a methodology for calculating global river slope and a procedure to upscale it for relatively coarse resolution suitable for global scale modeling the methodology is based on a simple principle of calculating slope from elevation depression over the length of a river segment which is automated to allow global scale calculations version 1 0 of the global river slope glors geospatial dataset is introduced and shown to be a step improvement over a previous product nhdplus for the contiguous united states and compares favorably to observed slope data collected from the literature statistical analysis of earth s continents and large basins highlights interesting spatial trends a semi empirical regression analysis between basin average river slope and other basin scale parameters show that terrain slope accounts for 67 of the variability in basin average river slope with average discharge sediment load and basin temperature contributing additional improvements to global predictions of 3 4 and 3 respectively

suspension of bed material over sand bars in the lower mississippi river and its implications for mississippi delta environmental restoration 1 understanding specific pathways for sand transport in the lower reaches of large rivers including the mississippi is a key for addressing multiple significant geologic problems such as delta building and discharge to the oceans and for environmental restoration efforts in deltaic environments threatened by rising sea levels field studies were performed in the mississippi river 75 100 km upstream of the gulf of mexico outlet in 2010 2011 to examine sand transport phenomena in the tidally affected river channel over a range of discharges methods included mapping bottom morphology multibeam sonar cross sectional and longitudinal measurements of water column velocity and acoustic backscatter suspended sediment sampling and channel bed sampling substantial interaction was observed between the flow conditions in the river boundary shear stress channel bed morphology size and extent of sandy bedforms and bed material sand transport quantity transport mode and spatial distribution a lateral shift was observed in the region of maximum bed material transport from deep to shallow areas of subaqueous sand bars with increasing water discharge bed material was transported both in traction and in suspension at these water discharges and we posit that the downriver flux of sand grains is composed of both locally and drainage basin sourced material with distinct transport pathways and relations to flow conditions we provide suggestions for the optimal design and operation of planned river diversion projects

an empirical model of subcritical bedform migration the ability to predict bedform migration in rivers is critical for estimating bed material load yet there is no relation for predicting bedform migration downstream translation that covers the full range of conditions under which subcritical bedforms develop here the relation between bedform migration rates and transport stage is explored using a field and several flume data sets transport stage is defined as the non dimensional shields stress divided by its value at the threshold for sediment entrainment statistically significant positive correlations between both ripple and dune migration rates and transport stage are found stratification of the data by the flow depth to grain size ratio improved the amount of variability in migration rates that was explained by transport stage to ca 70 as transport stage increases for a given depth to grain size ratio migration rates increase for a given transport stage the migration rate increases as the flow depth to grainsize ratio gets smaller in coarser sediment bedforms move faster than in finer sediment at the same transport stage normalization of dune migration rates by the settling velocity of bed sediment partially collapses the data given the large amount of variability that arises from combining data sets from different sources using different equipment the partial collapse is remarkable and warrants further testing in the laboratory and field

model for calculating bed load transport of sediment a bed load transport model based on the mechanics of sediment moving by saltation yields predicted values of bed load flux as a function of boundary shear stress grain diameter and density the parameters required to calculate bed load transport particle velocity bed load sediment concentration and the height of the bed load layer can all be determined from our model which computes sequences of trajectories of individual saltating grains as well as the concentration of moving grains that the flow can support the latter is related to the momentum the accelerating grains extract from the flow predicted curves of bed load transport vs boundary shear stress agree well with transport data collected by gilbert meyer peter iet al n and the waterways experimental station measured shear stress was corrected for pressure drag when bed forms were present comparison of predicted bed load transport with common bed load equations reveals considerable similarity among the relationships the best agreement with the data is produced by relationships in which the transport rate vanishes as the shear stress approaches the critical shields number as in our model and yalin s equation

hyporheic zone hydrologic science a historical account of its emergence and a prospectus the hyporheic zone defined by shallow subsurface pathways through river beds and banks beginning and ending at the river is an integral and unique component of fluvial systems it hosts myriad hydrologically controlled processes that are potentially coupled in complex ways understanding these processes and the connections between them is critical since these processes are not only important locally but integrate to impact increasingly larger scale biogeochemical functioning of the river corridor up to the river network scale thus the hyporheic zone continues to be a growing research focus for many hydrologists for more than half the history of water resources research this manuscript partly summarizes the historical development of hyporheic zone hydrologic science as gleaned from papers published in water resources research from the birth of the concept of the hyporheic zone as a hydrologic black box sometimes referred to as transient storage zone to its adolescent years of being torn between occasionally competing research perspectives of interrogating the hyporheic zone from a surface or subsurface view to its mature emergence as an interdisciplinary research field that employs the wide array of state of the art tools available to the modern hydrologist the field is vibrant and moving in the right direction of addressing critical fundamental and applied questions with no clear end in sight in its growth there are exciting opportunities for scientists that are able to tightly link the allied fields of geology geomorphology hydrology geochemistry and ecology to tackle the many open problems in hyporheic zone science

mitigating land loss in coastal louisiana by controlled diversion of mississippi river sand the bonnet carre spillway diverts floodwaters from the mississippi river to lake pontchartrain and was opened for 42 days during the 2011 flood according to measurements of the newly deposited sediments at least 31 46 of the river s sand load was diverted into the spillway at this time suggesting that such diversions can help mitigate coastal wetland loss

sand as a stable and sustainable resource for nourishing the mississippi river delta dams have starved the lower mississippi river of sediment over recent decades suggesting that the drowning of the delta is inevitable analysis of the rivers suspended sediment load and morphodynamic modelling suggest that the amount of sand essential for land building has not significantly decreased since dam construction with sand remaining available for several centuries

rapid and widespread response of the lower mississippi river to eustatic forcing during the last glacial interglacial cycle the lower mississippi valley provides an exceptional fi eld example for studying the response of a continental scale alluvial system to upstream and downstream forcing associated with the large orbitally controlled glacialinter glacial cycles of the late quaternary however the lack of a numerical chronology for the widespread pleistocene strata assemblage known as the prairie complex which borders the holocene fl oodplain of the lower mississippi river has so far precluded such an analysis here we apply optically stimulated luminescence osl dating mainly on silt sized quartz from prairie complex strata in total 27 osl ages indicate that the prairie complex consists of multiple allostratigraphic units that formed mainly during marine isotope stages 7 5e and 5a thus the aggradation of the prairie complex is strongly correlated with the sea level highstands of the last two glacialinterglacial cycles fluvial incision during the sea level fall associated with the mis 5a mis 4 transition extended as far inland as 600 km from the present day shoreline testifying to the dominant downstream control of fl stratigraphic architecture in the lower mississippi valley in addition the short reaction time of the lower mississippi river suggests that large fl uvial systems can respond much more rapidly to allogenic forcing than is commonly believed

dunes in the world s big rivers are characterized by low angle lee side slopes and a complex shape dunes form critical agents of bedload transport in all of the world s big rivers and constitute appreciable sources of bed roughness and flow resistance dunes also generate stratification that is the most common depositional feature of ancient riverine sediments however current models of dune dynamics and stratification are conditioned by bedform geometries observed in small rivers and laboratory experiments for these dunes the downstream lee side is often assumed to be simple in shape and sloping at the angle of repose here we show using a unique compilation of high resolution bathymetry from a range of large rivers that dunes are instead characterized predominantly by low angle lee side slopes 10 complex lee side shapes with the steepest portion near the base of the lee side slope and a height that is often only 10 of the local flow depth this radically different shape of river dunes demands that such geometries are incorporated into predictions of flow resistance water levels and flood risk and calls for rethinking of dune scaling relationships when reconstructing palaeoflow depths and a fundamental reappraisal of the character and origin of low angle cross stratification within interpretations of ancient alluvial sediments dunes in the world s big rivers are dominated by lee side slopes with angles of less than 10 according to a bedform analysis of high resolution bathymetric datasets

sediment infilling and geomorphological change of a mud capped raccoon island dredge pit near ship shoal of louisiana shelf abstract to restore degraded barrier shorelines of the mississippi river delta of coastal louisiana sand is dredged from high quality borrow areas on the adjacent continental shelf this dredging process generally creates a topographic low that facilitates the rapid capture of sediments our understanding of sedimentary processes and influences on seabed morphologic evolution is still limited in this study raccoon island dredge pit in a paleo river channel on the louisiana shelf was studied via multiple bathymetric surveys physical parameters include pit geometry and topographic change from post dredging and infilling sediment sources were analyzed raccoon island pit was 100 filled up less than six years after dredging the average infilling rate in the raccoon island pit was 1 10 m year from 2013 to 2018 during the surveying period the pit wall slope decreased by 6 8 within two years the sediment infilling process in the raccoon island pit was likely under the combined influence of topography wind driven currents storm waves and the dynamic atchafalaya river dispersal system the atchafalaya river reached high discharge in 2016 which likely contributed to a surprisingly high sediment infilling rate at raccoon island pit between 2015 and 2018 raccoon island pit is in a mixed texture environment in which the sandy pit was fully filled with mud mud is useful for marsh restoration but is not considered a quality resource for barrier island or beach restoration thus the raccoon island pit is not regarded as a renewable resource for future barrier island restoration

the response of subaqueous dunes to floods in sand and gravel bed reaches of the dutch rhine the branches of the river rhine in the netherlands characterized by a sand gravel bed in the upstream part and a sand bed in the downstream part of the river system show migrating dunes especially during floods in the last 20 years these dunes have been studied extensively high resolution echo sounding measurements of these dunes made with single and multibeam equipment were analysed for three different sections of the rhine river system during several floods this analysis was done to quantify the growth decay and migration rates of the dunes during floods in addition the migrating dunes were used to calculate bedload transport rates with dune tracking the results of dune growth and decay and migration rate are shown to be very different for the various sections during the various floods and these differences are related to differences in grain size of the bed and to differences in the distribution of discharge over the main channel and the floodplain the relations are used to show that the growth and migration rate of dunes and the calculated bedload transport rates during the rising stage of a flood wave can be predicted from the mobility of the bed material with simple power relations

estuarine controls on fine grained sediment storage in the lower mississippi and atchafalaya rivers observations of the lower mississippi river channel in 1999 2003 with side scan and subbottom seismic profilers multibeam bathymetry core radiotracers 7be 137cs 210pb and 234th and water column profiles of fluid and suspended sediment properties are utilized to constrain the timing location and intensity of the seasonal storage of fine grained sediments in the estuarine channel reach below venice louisiana unlike earlier studies that suggested little or no estuarine sedimentation was taking place in the river the present study demonstrates that the river channel above the head of passes seasonally stores as much as 10 of the annual suspended sediment discharge during periods of falling to low mississippi water discharge associated with salt stratification in the channel thalweg mud deposits are up to 3 2 m thick with 7be derived sediment deposition rates of up to 8 9 mm day water column profiling and 234th 7be ratios suggest that 1 particles are transported and settled mainly as flocs and 2 riverine flocs are settling through the strong salt wedge halocline and are protected from re erosion by the salt stratification with onset of the spring freshet and the corresponding seaward retreat of the saltwater wedge the entire volume of fine grained material stored in the lower river except in low freshet years is remobilized and transported to the gulf of mexico in the turbid freshwater plume this tidal hysteresis strongly influences the timing of sediment supply to the ocean high turbidities in the shelf plume during rising water discharge spikes of 1 to 2 week duration are the product of local channel floor remobilization as well as increased supply from the drainage basin pulsed resupply of sediments and the pore water products of early diagenesis potentially have major implications for issues such as continental margin carbon budgets and coastal eutrophication companion surveys conducted in the lowermost 12 km reach of the atchafalaya distributary of the mississippi show no evidence of saltwater wedge intrusion at low discharge or seasonal fine grained sediment storage likely due to the shallow 7 m river mouth sill that impedes the upstream movement of gulf water

response of low angle dunes to variable flow current understanding of bedform dynamics is largely based on field and laboratory observations of bedforms in steady flow environments there are relatively few investigations of bedforms in flows dominated by unsteadiness associated with rapidly changing flows or tides as a consequence the ability to predict bedform response to variable flow is rudimentary using high resolution multibeam bathymetric data this study explores the dynamics of a dune field developed by tidally modulated fluvially dominated flow in the fraser river estuary british columbia canada the dunes were dominantly low lee angle features characteristic of large deep river channels data were collected over a field ca 1 0 km long and 0 5 km wide through a complete diurnal tidal cycle during the rising limb of the hydrograph immediately prior to peak freshet yielding the most comprehensive characterization of low angle dunes ever reported the data show that bedform height and lee angle slope respond to variable flow by declining as the tide ebbs then increasing as the tide rises and the flow velocities decrease bedform lengths do not appear to respond to the changes in velocity caused by the tides changes in the bedform height and lee angle have a counterclockwise hysteresis with mean flow velocity indicating that changes in the bedform geometry lag changes in the flow the data reveal that lee angle slope responds directly to suspended sediment concentration supporting previous speculation that low angle dune morphology is maintained by erosion of the dune stoss and crest at high flow and deposition of that material in the dune trough