Bedform transport rates for the lowermost Mississippi River (original) (raw)
Related papers
Punctuated sand transport in the lowermost Mississippi River
Journal of Geophysical Research, 2011
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 m 3 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. Citation: Nittrouer, J. A., D. Mohrig, and M. Allison (2011), Punctuated sand transport in the lowermost Mississippi River,
Journal of Geophysical Research: Earth Surface, 2013
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.
2017
This report compiles and makes readily available data and supporting information contained in past U.S. Army Corps of Engineers reports on the sizes and size distributions of sediment forming the bed of the Lower Mississippi River. Based on these sources, the report establishes how particle size distributions and the widely used, statistically representative sizes (D50, D16, and D84) of the bed material vary with distance downstream and through time. Preliminary analyses are performed to identify downstream and time trends in bed material characteristics at a variety of scales, and establish the sensitivity of bed material transport rates and annual loads calculated using the Hydraulic Engineering Center-River Analysis System/Sediment Impact Analysis Method (HEC-RAS/SIAM), to local and short-term variability in bed sediment characteristics. The report closes by considering the overall conclusions and wider applications of its findings. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.
Flow over bedforms in a large sand-bed river: A field investigation
Journal of Hydraulic Research, 2008
An experimental field study of flows over bedforms was conducted on the Missouri River near St. Charles, Missouri. Detailed velocity data were collected under two different flow conditions along bedforms in this sand-bed river. The large river-scale data reflect flow characteristics similar to those of laboratory-scale flows, with flow separation occurring downstream of the bedform crest and flow reattachment on the stoss side of the next downstream bedform. Wave-like responses of the flow to the bedforms were detected, with the velocity decreasing throughout the flow depth over bedform troughs, and the velocity increasing over bedform crests. Local and spatially averaged velocity distributions were logarithmic for both datasets. The reach-wise spatially averaged vertical-velocity profile from the standard velocity-defect model was evaluated. The vertically averaged mean flow velocities for the velocity-defect model were within 5% of the measured values and estimated spatially averaged point velocities were within 10% for the upper 90% of the flow depth. The velocity-defect model, neglecting the wake function, was evaluated and found to estimate the vertically averaged mean velocity within 1% of the measured values. RÉSUMÉ Une étude expérimentale en nature des écoulements sur des formes de lit a été conduite sur le fleuve Missouri près de St. Charles, Missouri. Des données détaillées de vitesse ont été rassemblées dans deux conditions différentes d'écoulement le long des formes du lit dans ce fleuve sableux. Les données à l'échelle des grands fleuves reflètent des caractéristiques d'écoulement semblables à celles des écoulements en laboratoire, avec la séparation d'écoulement se produisant en aval de la crête de dune et le recollement de l'écoulement sur la face de la dune suivante Des réponses de sillage de l'écoulement aux dunes ont été détectées, avec la vitesse diminuant dans tout le tirant d'eau au-dessus des creux, et la vitesse augmentant au dessus des crêtes. Les distributions des vitesses moyennes locales et spatiales étaient logarithmiques pour les deux ensembles de données. L'écart du profil des vitesses moyennes dans l'espace ramené au modèle standard de défaut de vitesse a été évalué. Les vitesses moyennes verticales ramenées à une moyenne d'écoulement pour le modèle de défaut de vitesse étaient à moins de 5% des valeurs mesurées et les vitesses moyennes dans l'espace étaient à moins de 10% pour les 90% du tirant d'eau supérieur. Le modèle de défaut de vitesse, négligeant la fonction de sillage, a été évalué pour estimer la vitesse moyenne sur la verticale à moins de 1% des valeurs mesurées.
Geometric and Statistical Characteristics of Bed Forms in the Lower Mississippi River
Coastal Sediments '07, 2007
Bed forms in alluvial systems are three dimensional. The most common bed forms are: ripples, dunes and bars. Under certain flows, different bed forms may be superimposed on each other, e.g. dunes superimposed on bars or ripples superimposed on dunes. The irregularity in the bed of alluvial streams caused by the existence of bed forms produces additional flow resistance, which affects the hydrodynamics and sediment transport properties of the system. The geometry of bed forms in large alluvial rivers such as the Lower Mississippi River exhibits both coherent and random characteristics. Average wave lengths and heights alone cannot adequately describe these bed forms. A spectral approach was used in this study to capture the periodicity and frequency distributions of the observed bed forms along a study reach in the Lower Mississippi River extending from river mile zero (Head of Passes) to river mile 234 (near Baton Rouge, LA).
Influence of Dunes on Channel‐Scale Flow and Sediment Transport in a Sand Bed Braided River
Journal of Geophysical Research: Earth Surface, 2020
Current understanding of the role that dunes play in controlling bar and channel‐scale processes and river morphodynamics is incomplete. We present results from a combined numerical modeling and field monitoring study that isolates the impact of dunes on depth‐averaged and near‐bed flow structure, with implications for morphodynamic modeling. Numerical simulations were conducted using the three‐dimensional computational fluid dynamics code OpenFOAM to quantify the time‐averaged flow structure within a 400 m × 100 m channel using digital elevation models (DEMs) for which (i) dunes and bars were present within the model and (ii) only bar‐scale topographic features were resolved (dunes were removed). Comparison of these two simulations shows that dunes enhance lateral flows and reduce velocities over bar tops by as much as 30%. Dunes influence the direction of modeled sediment transport at spatial scales larger than individual bedforms due to their effect on topographic steering of the...
Sedimentology, 1993
The geometry and kinematics of river dunes were studied in a reach of the Calamus River, Nebraska. During day-long surveys, dune height, length, steepness, migration rate, creation and destruction were measured concurrently with bedload transport rate, flow depth, flow velocity and bed shear stress. Within a survey, individual dune heights, lengths and migration rates were highly variable, associated with their three-dimensional geometry and changes in their shape through time. Notwithstanding this variability, there were discernible changes in mean dune height, length and migration rate in response to changing discharge over several days. Changes in mean dune height and length lagged only slightly behind changes in discharge. Therefore, during periods of both steady and unsteady flow, mean dune lengths were quite close to equilibrium values predicted by theoretical models. Mean dune steepnesses were also similar to predicted equilibrium values, except during high, falling flows when the steepness was above that predicted.Variations in mean dune height and length with discharge are similar to those predicted by theory under conditions of low mean dune excursion and discharge variation with a short high water period and long low water period. However, the calculated rates of change of height of individual dunes vary considerably from those measured. Rates of dune creation and destruction were unrelated to discharge variations, contrary to previous results. Instead, creations and destructions were apparently the result of local variations in bed shear stress and sediment transport rate.Observed changes in dune height during unsteady flows agree with theory fairly well at low bed shear stresses, but not at higher bed shear stresses when suspended sediment transport is significant.
Water Resources Research, 2000
The Colorado River in Marble and Grand Canyons displays evidence of annual supply limitation with respect to sand both prior to [Topping et al., this issue] and after the closure of Glen Canyon Dam in 1963. Systematic changes in bed elevation and systematic coupled changes in suspended-sand concentration and grain size result from this supply limitation. During floods, sand supply limitation either causes or modifies a lag between the time of maximum discharge and the time of either maximum or minimum (depending on reach geometry) bed elevation. If, at a cross section where the bed aggrades with increasing flow, the maximum bed elevation is observed to lead the peak or the receding limb of a flood, then this observed response of the bed is due to sand supply limitation. Sand supply limitation also leads to the systematic evolution of sand grain size (both on the bed and in suspension) in the Colorado River. Sand input during a tributary flood travels down the Colorado River as an elongating sediment wave, with the finest sizes (because of their lower settling velocities) traveling the fastest. As the fine front of a sediment wave arrives at a given location, the bed fines and suspended-sand concentrations increase in response to the enhanced upstream supply of finer sand. Then, as the front of the sediment wave passes that location, the bed is winnowed and suspended-sand concentrations decrease in response to the depletion of the upstream supply of finer sand. The grain-size effects of depletion of the upstream sand supply are most obvious during periods of higher dam releases (e.g., the 1996 flood experiment and the 1997 test flow). Because of substantial changes in the grain-size distribution of the bed, stable relationships between the discharge of water and sand-transport rates (i.e., stable sand rating curves) are precluded. Sand budgets in a supply-limited river like the Colorado River can only be constructed through inclusion of the physical processes that couple changes in bed-sediment grain size to changes in sand-transport rates. flume experiments. As discussed by Parker and Wilcock [1993], these experiments typically fall into two categories: (1) those using sediment-recirculating flumes (in which the water and sediment are reintroduced at the upstream end of the flume at the same rate that they leave the downstream end) and (2) those using sediment-feed flumes (in which the sediment is supplied at the upstream end of the flume in a manner decoupled from the rate at which the sediment leaves the downstream end). In sediment-recirculating flumes the sediment supply at the upstream end of the flume equals the sediment export at the downstream end of the flume. Thus these flumes are like rivers in which the upstream supply of sediment is in equilibrium with the upstream supply of water. In these experiments, no substantial change occurs in the grain size of the sediment on the bed of the flume [e.g., . In sediment-feed flumes, however, the upstream supply of sediment is decoupled fro m the upstream supply of water. In these experiments the grain-size distribution of the bed sediment and the sediment-transport rate are free to change substantially as a function of the interaction between the rates of sediment feed and downstream transport (as described by )