Bed-Load Sediment Transport on Large Slopes: Model Formulation and Implementation within a RANS Solver

Standard bed-load sediment-transport formulas are extended using basic mechanical principles to include gravitational influence on large slopes of arbitrary orientation. The resulting sediment fluxes are then incorporated into a morphodynamics model in a general-purpose, three-dimensional, finite-volume, Reynolds-averaged Navier–Stokes (RANS) code. Major features are: (1) the downslope component of weight is combined with the fluid stress to form an effective bed stress (similar to the work of Wu in 2004); (2) the critical effective stress is reduced in proportion to the component of gravity normal to the slope; (3) a simple flux-based model for avalanching is implemented as a numerical means of preventing the local slope from exceeding the angle of repose; (4) an entirely vectorial formulation of bed-load transport is developed to account for arbitrary surface orientation; and (5) methods for reducing numerical instability in the morphodynamics equation are described. Sample computations are shown for scour and accretion in a channel bend and for the movement of sand mounds on erodible and nonerodible bases.     

Evaluation of ADCP Apparent Bed Load Velocity in a Large Sand-Bed River: Moving versus Stationary Boat Conditions

Detailed mapping of bathymetry and apparent bed load velocity using a boat-mounted acoustic Doppler current profiler (ADCP) was carried out along a 388-m section of the lower Missouri River near Columbia, Missouri. Sampling transects (moving boat) were completed at 5- and 20-m spacing along the study section. Stationary (fixed-boat) measurements were made by maintaining constant boat position over a target point where the position of the boat did not deviate more than 3?m in any direction. For each transect and stationary measurement, apparent bed load velocity (vb) was estimated using ADCP bottom tracking data and high precision real-time kinematic (RTK) global positioning system (GPS). The principal objectives of this research are to (1)?determine whether boat motion introduces a bias in apparent bed load velocity measurements; and (2)?evaluate the reliability of ADCP bed velocity measurements for a range of sediment transport environments. Results indicate that both high transport (b>0.6??m/s) and moving-boat conditions (for both high and low transport environments) increase the relative variability in estimates of mean bed velocity. Despite this, the spatially dense single-transect measurements were capable of producing detailed bed velocity maps that correspond closely with the expected pattern of sediment transport over large dunes.     

Assessment of a River Reach for Environmental Fluid Dynamics Studies

Turbulence is the fundamental mechanism governing energy transfer in river flows that was conventionally examined in laboratory flumes. Recently, a trend has been observed for constructing larger scale and outdoor facilities that tend to avoid the problems of upscaling of experimental results. This paper presents the results of an experimental study performed on a river reach used as an environmental field laboratory. The study is focused on the understanding of the spatial arrangement of the flow structure and its dependency on the temporal variability of the flow. Detailed measurements were taken using acoustic Doppler velocimeters and their analysis was completed applying the theory of open-channel flows. The obtained results reveal that the flow structure on the river reach resembles characteristics of a typical three-dimensional open-channel flow. Away from the riverbanks, the flow behaves as a quasi-two-dimensional fully developed turbulent open-channel flow thus providing conditions favorable for field experimental studies of shallow mixing layers and flows over patches of submerged aquatic plants. An interesting observation in the seasonal dynamics of turbulent shear stress patterns was that the height of the roughness layer was reduced in the central part of the flow, though the overall roughness coefficient was increased. At the same time, the structure of the secondary flow near the banks was also substantially altered as the secondary circulations observed at low water levels were replaced by flow separation and internal boundary layers at medium water levels.     

Case Study: Effect of Submerged Aquatic Plants on Turbulence Structure in a Lowland River

Interactions of aquatic plants with turbulent flows in fluvial systems have attracted considerable interest. While there have been recent advances in theories that describe vegetation-flow interactions in idealized laboratory flows, their practical application is still problematic due to limited knowledge of effects caused by heterogeneously (patchy) distributed plants in naturally formed vegetative mosaics in rivers. This paper reports on a study in a lowland river, aimed at quantification and parameterization of vegetation effects on redistribution of mean and turbulent characteristics of the flow and their consequences for hydraulic resistance. The measurements were carried out in summer on a river reach with a patchy mosaic dominated by submerged flexible aquatic plants and repeated at the same water level in early spring before the plants start growing. This design of the study allowed for quantitative evaluation of the effects caused by flow-plants interactions on bulk flow parameters at comparable submergences of riverbed roughness elements (sediment grains and sand bars). The study indicates that symmetrical quasi-two-dimensional open-channel flow structure in unvegetated riverbed was transformed into highly fragmented complex flow pattern spatially arranged by patches and free paths in the mosaic. Despite complexities and three dimensionality of the flow, normalized mean velocity profiles in the patches were satisfactory described by hyperbolic tangent function while flow in the free paths, similarly to unvegetated channel, was in a reasonable agreement with the conventional logarithmic law.     

Comparison of SF6 and Fluorescein as Tracers for Measuring Transport Processes in a Large Tidal River

We present the first large-scale comparison of a fluorescent dye [fluorescein (C20H10O5Na2)] and a gas [sulfur hexafluoride (SF6)] as tracers of advection and longitudinal dispersion from a dual tracer release experiment conducted in the tidal Hudson River. At the beginning of the experiment, 36?kg of fluorescein and ～ 4.3?mol of SF6 were injected into the Hudson River at an averaged depth of 9.5?m, ～ 1?m above the bottom, near Hyde Park, N.Y. After injection, fluorescein distributions were surveyed for 4 days (until it became undetectable) and SF6 distributions were surveyed for 10 days. The dye resolves initial vertical mixing on the day of injection, and then net advection and longitudinal dispersion, whereas SF6 provides information on net advection and longitudinal mixing on larger spatial scales and longer time scales. Quantitative estimates of transport processes (net advection and longitudinal dispersion) calculated from the two methods are consistent for the first three days, and start to deviate on the fourth day when the signal-to-noise ratio of the dye deteriorated.     

Determining Erodibility, Critical Shear Stress, and Allowable Discharge Estimates for Cohesive Channels: Case Study in the Powder River Basin of Wyoming

The continuous discharge of coalbed natural gas-produced (CBNG-produced) water within ephemeral, cohesive channels in the Powder River Basin (PRB) of Wyoming can result in significant erosion. A study was completed to investigate channel stability in an attempt to correlate cohesive soil properties to critical shear stress. An in situ jet device was used to determine critical shear stress (τc) and erodibility (kd); cohesive soil properties were determined following ASTM procedures for 25 reaches. The study sites were comprised of erodible to moderately resistant clays with τc ranging from 0.11?to?15.35?Pa and kd ranging from 0.27?to?2.38?cm3/N?s. A relationship between five cohesive soil characteristics and τc was developed and presented for use in deriving τc for similar sites. Allowable discharges for CBNG-produced water were also derived using τc and the tractive force method. An increase in the allowable discharge was found for channels in which vegetation was maintained. The information from this case study is critical to the development of a conservative methodology to establish allowable discharges while minimizing flow-induced instability.     

Approach for Assessment and Review in a Large Construction Company: Case of Skanska Sweden

Although the need to improve performance and work processes in a company may seem obvious, concrete approaches for this are rare in the construction industry. Admittedly, efforts for embedding improvement processes have been made, but most construction companies are still struggling with concepts, avoiding available tools and techniques for assessment and review. In doing so, they ignore possibilities to identify how they can improve their performance and work processes. In the few cases where tools and techniques have been used, they often fail to address the specific characteristics of the organization. This paper presents how a systematic approach for assessment and review of operations and work processes can be tailored for a large construction company. An in-depth case study carried out at the large construction company Skanska Sweden is the basis for this study. Interaction between research and development work at the company has enabled a deep understanding of the company’s operations, its culture, and its way of working. The approach presented is based on the existing management system at Skanska Sweden and its focus is on creating a structured way to discuss how operations are carried out. The aims are to assess how operations at the local units within the company can be improved and to improve the management system and thereby benefit the organization as a whole.     

A new closure methodology for 1D fully coupled models of mobile-bed alluvial hydraulics: application to silt transport in the Lower Yellow River

A new closure approach involving a common parameter has been incorporated into a 1D fully coupled model of mobile-bed alluvial hydraulics. The objective is to simplify the methodology of 1D river routing models and to improve their accuracy. The common parameter, called control factor m,introduces the concept of Rossiter modes in alluvial hydraulics and represents the interactions between the flow, the sediment transport and the bed morphology. The feasibility of the new closure approach has been established by reproducing numerically the 2002 silt flushing experiment conducted on the Lower Yellow River (LYR) downstream the Xiaolangdi reservoir. From the comparison between the experimental data and the numerical results, a time evolution of the control factor m reproducing the characteristics of the flow has been extracted. This time evolution agrees with analysis conducted previously on other datasets and with data measured during the flush. The results obtained with this time evolution for the hydraulics, the sediment transport and bed adaptation are encouraging but still need improvements and further feeding from complementary experimental data.