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Recent studies show that near-bed turbulence may be better scaled using a mixed velocity scale u*U , where u* is the shear velocity and U denotes the characteristic bulk flow velocity. This implies that the classical scaling consideration for sediment transport rates could be improved by redefining the Shields number based on the mixed velocity scale. To investigate this potential improvement, a series of analyses was performed with laboratory data from 25 sources of sediment discharge studies. The data were carefully selected by minimizing effects of some factors including sidewall, particle gradation, particle density, and fluid viscosity. The findings indicate that the relationship between sediment discharges and flow conditions can be enhanced significantly when the analysis is carried out based on the redefined Shields number. The new relationship does not depend on the type of bedforms, and therefore facilitates the prediction of sediment discharge. Finally, by comparing with previous studies, it shows that the proposed formula markedly improves the prediction of bed-material sediment discharges. [ABSTRACT FROM AUTHOR]

The piano key weir has been successfully used to address discharge deficiencies and site limitations for run-of-river structures and new and existing spillways, serving as an economical alternative to traditional weirs. In addition to upstream sedimentation, weirs may also be exposed to floating driftwood mobilized by hydrologic events. Driftwood accumulation immediately upstream of or on a weir crest can decrease hydraulic performance, potentially increasing upstream flooding and altering hydrodynamic forces on the weir that may cause structural or foundational concerns. It is unclear if floating driftwood accumulation might also influence the sediment self-cleaning characteristics of piano key weirs. Thus, this study used physical experiments to systematically investigate the sediment self-cleaning ability of a Type A piano key weir with driftwood accumulation in a run-of-river setting. The simulated substrate was an alluvial sand bed and the driftwood was naturally shaped. Experiments included combinations of discharge and different driftwood volumes. Results indicate that driftwood accumulations can have a significant influence on the flow field upstream of a piano key weir and positively affect the self-cleaning of sediment. For the conditions considered herein, the cleaning capacity was a function of driftwood volume, accumulation surface area or its extent in the upstream reach, and discharge. Higher flow rates and increased driftwood volume increased and extended upstream scour, resulting in higher sediment transport and self-cleaning without causing a significant backwater or increase in upstream water elevation. With sufficient flow depths at the piano key weir, driftwood would also be anticipated to pass downstream. Based on the results, driftwood accumulation can positively affect the ability of a piano key weir to remove adjacent upstream noncohesive sediment deposits. [ABSTRACT FROM AUTHOR]

Complex interactions between flowing water and bedforms in rivers with sand beds during changing flow rates may result in bedforms, flow resistance, and sediment transport rates that differ on the rising and falling limbs of flow hydrographs. To study this phenomenon, laboratory experiments based on six realistically shaped hydrographs with durations of 1–6 h in 1-h increments were performed in a recirculating flume at the United States Department of Agriculture-Agricultural Research Service (USDA-ARS) National Sedimentation Laboratory. Bed topography, water-surface slope, flow rate, and sediment transport rate were continuously monitored. Counterclockwise hysteresis between discharge and sediment transport rate as well as bedform dimensions were observed for all hydrograph lengths, and the magnitude of hysteresis was quantified. The amount of hysteresis in transport rate, bedform amplitude, and bedform length varied according to the length of the hydrograph, although the relationship was not monotonic. The Engelund–Hansen method was able to approximately predict sediment transport rates based on measured hydraulic parameters. Maximum bedform amplitudes increased with hydrograph length, while bedform lengths increased only minimally beyond the 4-h hydrograph period. [ABSTRACT FROM AUTHOR]

Bioroughness plays an important role in modifying the velocity and sediment flux near the riverbed. It is therefore pertinent to study the influence of benthic fauna on the bed forms. To this end, large-eddy simulations are performed to investigate the influence of the arrays of mounds and their density on the turbulence characteristics in an open-channel flow. The simulated distributions of the time-averaged streamwise velocity and the turbulence intensity are in good agreement with the experimental data. Four numerical simulations are performed with varying streamwise spacings of mounds. Details of the time-averaged and instantaneous flow velocities are analyzed by multiple visualization methods, and the effects of the bioroughness density on the equivalent roughness height and the Darcy–Weisbach friction factor are quantified. The time-averaged flow in the wake of the mounds is characterized by a symmetric pair of vortices. The mounds behave like bluff bodies, increasing the riverbed roughness and heterogeneity in the flow environment. An increase in mound density is to promote the development of secondary currents and to increase the dispersive stress near the bed. The peaks of the Reynolds shear stress distributions decrease in both the streamwise and vertical directions for the high-density case due to a blockage effect. The instantaneous flow features, in the form of various turbulence structures, are generated near the top edge and the wake zone of mounds. The spacing between low-speed streaks decreases with an increase in equivalent roughness height. Multifrequency behavior that is observed is a result of shear layer roll-up from the edges of mounds and the flapping of wake. Finally, two formulas for equivalent roughness height and Darcy–Weisbach friction factor are proposed involving the bioroughness density and height. The findings demonstrate the effects of the bioroughness on the near-bed turbulence characteristics and sediment stability. Practical Applications: Bed forms produced by aquatic benthos are usually of small scale but with a large abundance. We performed a flume experiment as well as large-eddy simulations (a high-fidelity numerical simulation approach) to answer whether these microtopographies are important when studying flow and transport processes near the riverbeds. Four numerical simulations are performed with varying mound density. It was found that the microtopographies created by benthos indeed change the near-bed turbulence characteristics and sediment stability. Specifically at high abundances, benthos-induced bed forms could raise the bed roughness and the friction factor eightfold and threefold, respectively. This suggests the necessity of a modified roughness height formula and a friction coefficient formula based on the benthos abundance, which has been carried out in this study. Thus, engineers who are concerned with the flow and sediment transport in habitats of high-abundance benthos (e.g., wetlands and estuaries) may prefer to use these modified formulas and to develop a more accurate model of sediment transport. [ABSTRACT FROM AUTHOR]

Flash floods induced by high-intensity and short-duration monsoon rainfall can cause severe damage in arid regions. To properly size in-stream infrastructures, such as levees, bridges, and culverts, it is crucial to accurately calculate the peak runoff and sediment load from these flash floods. This case study utilized the Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) model to simulate flash floods and sediment transport in the Lucky Hills watershed located in the Walnut Gulch Experimental Watershed in southern Arizona. The Lucky Hills watershed has two rain gauges and three flumes to measure runoff and sediment load. The HEC-HMS model was used to simulate the three largest precipitation events observed in 2007, 2009, and 2010 with precipitation volumes of 41.66, 46.36, and 37.85 mm, and durations of 126, 99, and 101 min, respectively. The study discussed various methods for simulating rainfall loss, surface and channel flow routing, and soil erosion. Watershed delineations were adopted to evaluate the accuracy of the simulated runoff and sediment concentration. Results showed that the HEC-HMS model can accurately predict surface runoff and sediment concentration, but the threshold value for subbasin size is critical for the model to converge to accurate results. [ABSTRACT FROM AUTHOR]

Tsunamis continue to pose an existential threat to life and infrastructure in many coastal areas around the world. One of the risks associated with tsunamis is the formation of deep scour holes around critical infrastructure and other coastal buildings, compromising their structural integrity and stability. Despite its importance, tsunami-induced scour is still given limited and simplified consideration in design guidelines for coastal structures. To further improve the understanding of tsunami-induced scour processes, and thus provide the basis for safer design of coastal structures, novel large-scale laboratory experiments have been conducted. The experiments featured a unique combination of boundary conditions, including a square coastal structure on a sloping and dry sandy beach. Single broken solitary waves were used to simulate tsunami bores. The spatiotemporal scour development directly at the square column was monitored by a high-resolution camera system, allowing a detailed description of the highly dynamic flow and scour process. Differences in the scour process between the wave runup and drawdown phases are described, and maximum and final scour depths are given as a function of inundation depth, wave height, and distance of the column from the shoreline. The scour process is characterized by several distinct phases of varying intensity and scour rate, the sequence of which varies depending on the location on the sides of the column. It is shown that the drawdown phase has a large influence on the overall scour development, adding up to 58% to the scour depth obtained during the wave runup phase. As a result of significant sediment infilling during the drawdown phase, the maximum scour depths achieved during the drawdown phase are up to twice the final scour depths at the end of a test. This discrepancy between final and maximum scour depths is greater than in previous studies using a flat sediment bed. The results of this study therefore help to interpret scour depths measured during field investigations after a tsunami event and provide a basis for extending design guidelines for coastal structures. [ABSTRACT FROM AUTHOR]

Barrier islands span 10% of the world's coastlines and dominate along the Atlantic and Gulf of Mexico coasts in the USA routinely exposed to storm surge. These islands serve as buffers between the open coast and the mainland, and protect the mainland from storm damage. During storms, significant morphological change can occur, including substantial erosion of the islands' vegetated dunes. In this study, the focus is on the influence of belowground biomass – namely dune plant roots – and dune topographic characteristics on barrier-island erosion during storms. The numerical model XBeach was employed to simulate storm-induced sediment transport. A parameter study was conducted by varying distance from the shoreline to the dune, dune shape, sediment grain size, and sediment mobility as an analogue for biomass. The influence of individual parameters and the collective impact of parameters were analyzed. The results help improve our knowledge of barrier island dynamics and lay a solid groundwork for future investigations. [ABSTRACT FROM AUTHOR]

This study examines the instability of sand waves (dunes and antidunes) from both linear and weakly nonlinear perspectives. The linear and weakly nonlinear analyses use the standard linearization and the center manifold-projection technique, respectively. The mathematical framework includes the depth-averaged continuity and momentum equations, the advection–diffusion equation for suspended sediment concentration, and Exner's equation for bed evolution. The streamline curvature is treated using the Boussinesq approximation. The model considers the departure of the pressure distribution from the hydrostatic law. Both modes of sediment transport, as bedload and as suspended load, are taken into consideration. The perturbations characterize the maximum growth rate for a selected wave number, called the resonant wave number, which is the most favorable wave number for the formation of sand waves. As the flow Froude number and the relative roughness number increase, the dimensionless resonant wave number decreases. The dimensionless amplitude of sand waves increases as the flow Froude number and the relative roughness number increase to achieve a maximum, and subsequently it decreases. The predicted wave number and amplitude of sand waves satisfactorily match the available experimental data. [ABSTRACT FROM AUTHOR]

The progressive reservoir construction and operation have led to nonstationary sediment load response to water discharge and a new equilibrium between sediment supply and transport of a river system. However, an increase in the reservoir impoundments on the nonstationary behaviors could demonstrate significant temporal variability over the river system. In this study, the statistical relationships of annual sediment load with both streamflow and reservoir storage capacity during 1956–2018 are analyzed in the Upper Yangtze River Basin (UYRB), China. The trend and abrupt detection analyses show that the significant departure of annual sediment load to annual water discharge occurs only when the cumulative storage capacity over the course of progressive impoundments reaches a critical value for the cascade reservoirs in UYRB. The critical storage capacity gradually appeared in the 1980s, 1990s, and 2000s, accounting for the sharp increases of small- and medium-sized reservoirs in the tributaries and large-sized reservoirs in the mainstem. Once the critical storage is exceeded, the capacity to trap sediment increases exponentially with the cumulative storage. The significant increase of trapped sediment (e.g., 26%–88% of the free-flow capacity in the tributary and mainstem stations) has led to the shift of sediment-controlling factors from water discharge to the impounded runoff index. The significant reduction of sediment transportation and the altered dominant factors of sediment load could result in the steady erosion of channels and coastlines and adversely impact ecosystems in the downstream Yangtze River. Human activities, especially dam construction, have severely altered the global river system. Dams disrupt the continuity of the river, inevitably altering the natural transport of sediment and reducing sediment loads. Since the 19th century, a large number of reservoirs have been built in the upper reaches of the Yangtze River where water resources are abundant. When the cumulative storage capacity over the course of progressive impoundments reaches a critical value for the cascade reservoirs in the Upper Yangtze River, the capacity for trapping sediment increases exponentially with the cumulative storage. The significant increase in trapped sediment has led to a shift of sediment-controlling factors. The significant reduction of sediment transportation and the altered dominant factors of sediment load could lead to several environmental problems, such as the limited availability of nutrients in the middle and lower reaches, river bed erosion, and the destruction of benthic habitat. The change in the relationship between sediment, water discharge, and reservoir capacity can provide a reference for the environmental management and protection of the Yangtze River. [ABSTRACT FROM AUTHOR]

The concession of the Peruvian Amazon Waterway (PAW) project seeks to strengthen commercial transport between Brazil and Peru by using four rivers: Amazon, Ucayali, Marañón, and Huallaga, in which the Peruvian Ministry of Transportation and Communications (MTC) was in charge of developing the Terms of Reference (TOR). At the end, the TOR did not specify the methodology for the hydrogeomorphic characterization of rivers. The PAW project was designed to meet specific geometric characteristics to ensure navigable conditions: minimum water depth of 2.44 m (8 ft) and channel width ranging from 44 to 56 m, approximately. The zones where the river bottom limits the layout and dimensions of the navigation channel are called shallow zones (SZs). Based on the PAW project's TOR, the EIA-d (Detailed Environmental Impact Assessment) study was carried out using 14 SZs that required dredging 3.87 million m3 of sediments. In 2019, the EDI (Definitive Engineering Study) described 24 SZs with a total dredging volume of 4.39 million m3 of sediments. The number and locations of SZs and the volume of required dredging changed significantly, revealing a lack of understanding of meandering (i.e., Ucayali and Huallaga) and anabranching (i.e., Marañón and Amazon) river processes. This study presents an integral methodology for the hydrogeomorphic characterization of the meandering Huallaga River (the river with 56% of the total estimated volume to dredge) by using remote sensing and field measurements (hydrodynamic, sediment transport, and bathymetry). Geological confinement of river dynamics, together with the presence of anabranching structures, promote stable SZs. In regions where meandering rivers tend to develop ancient and modern geomorphic structures, SZs are temporarily and highly dependent of river dynamics. The present hydrogeomorphic characterization of the Huallaga River will support the Peruvian government to inform their technical guidelines (TOR), especially for infrastructure projects (e.g., PAW) in Amazonian rivers. Several river-related engineering projects lack a broad and comprehensive view to understand a river's spatial and temporal dynamics. One of these projects is the Amazon Waterway (Hidrovía Amazónica in Spanish) which sought to strengthen commercial transport between Brazil and Peru using four rivers: Amazon, Ucayali, Marañón, and Huallaga. The Terms of Reference (TOR) for the Environmental Impact Study (EIA) did not specify the methodology for understanding river dynamics; however, the focus was mainly concentrated on the need to develop dredging activities. During the EIA evaluation stage, more than 400 technical observations were presented, several related to fluvial geomorphology. This article presents a synthesis of how proper river studies should be carried out and how river dynamics is related to aquatic biodiversity, highlighting the importance of understanding rivers before developing infrastructure projects. [ABSTRACT FROM AUTHOR]

Effects of bed material and downstream flow depth on bed evolution in a right-angled channel confluence were studied through laboratory experiments. Velocity field, water surface elevation, and bed profile were measured intermittently until the flow attained equilibrium. Sediment discharges at various locations of the confluence were estimated from bed levels. Six prominent bedforms were identified, and their characteristics were quantified. The sediment discharges from different channels were initially high but decreased to approximately zero as the flow attained equilibrium. Downstream flow depth influenced overall sediment transport in the system, including the main features of bed morphology. Overall erosion in the confluence reduced as the particle size of the bed material increased. In addition, the maximum scour depth occurred at the confluence edge, and, due to the sharp corner of the confluence, its location did not change with time. Results from the present experimental study can help validate numerical models and assist in the design of a right-angled confluence. This study is related to the hydraulics of man-made confluences with mobile beds and can be useful in the following practical applications. More accurate numerical models to predict maximum scour, bed morphodynamics, and sediment transport in man-made confluences can be developed using the detailed bed profile measurements at different time instants presented in this study. Note that available numerical models use sediment discharge formulas (e.g., Meyer-Peter-Muller, Van Rijn), which are for sediment flows in straight channels. The interplay between the three-dimensional flow field, developed bed shear stress, and sediment discharge can be explored. In addition, A man-made confluence with the mobile bed can be designed with findings from the present study. The bedforms corresponding to equilibrium flow depend on discharge ratio, confluence angle, flow conditions at the exit channel, and bed material characteristics. The major bedforms can be incorporated into the confluence geometry. [ABSTRACT FROM AUTHOR]

In this study, the large eddy simulation (LES) under the Eulerian method is used to solve the Navier-Stokes equations for turbulent flow simulation. The Lagrangian point-particle model is applied to track particle trajectories and to calculate the forces exerted by the flow on the particles, and the particle–wall and particle–particle collisions are also accounted for. Nine simulations cases were carried out along the line of previous experiments that considered different bedform regimes, namely, ripples and dunes. The resulting bedload intensity parameter and the simulated bedforms for all the cases agree with the results obtained from the existing classical formulas. The three-dimensionality of sediment transport randomly occurs due to the turbulent flow. Coherent structures are formed as the near-bed low-speed fluid streaks entrain into the mainstream over the stoss-side of the ripples, and the high-speed fluid streaks from the mainstream rush toward the bed over the leeside. As a result, kolk–boil and hairpin vortices develop nearby. Ejection and sweep prevail near the bed, where the particles transport. The phenomenon disappears as the flow intensity increases. The presence of bedload particles also modifies the propagation angle and range of velocity fluctuation, especially in the streamwise direction. To conclude, a logistic regression formula for bedload intensity parameters, accounting for the fluid rotation, deformation, and translation terms that signify the fluid vortical motions, is obtained. It reveals that as long as these three terms are accurately quantified, the bed shear stress and bedload transport rate can be effectively estimated. [ABSTRACT FROM AUTHOR]

Existing knowledge about groyne-induced effects is primarily based on riverine or coastal environments where salinity gradients are absent or limited. However, in estuaries, salinity gradients drive physical processes such as longitudinal and lateral residual flows. The effect of groynes is much more complex because they can modulate channel hydrodynamics and directly affect lateral salinity gradients. In this study, an idealized model is applied to investigate the effects of groyne layouts in estuarine environments, including effects on (1) channel hydrodynamics, (2) lateral water exchange, (3) Coriolis effects, and (4) saltwater intrusion. Model results show that the aspect ratio (the width of groyne fields to the length of groynes) of groyne fields plays an important role. Groynes also induce asymmetry of lateral flows, for example, increasing near-bottom shoal-to-channel flows during low water slack. The aspect ratio has opposite effects on horizontal and vertical components of water exchange. A large aspect ratio strengthens horizontal exchange and weakens density-driven currents. For a large-scale groyne field (several kilometers), Coriolis effects introduce a substantial difference in exchange mechanisms along the north and south banks. A medium range of aspect ratio (2.0–3.0) leads to the strongest saltwater intrusion during both neap and spring tides. Dikes and groynes are common engineering structures in waterways designed to increase along-channel flow velocities and prevent sediment deposition, thereby maintaining navigability. However, especially in estuarine environments, cross-channel flow velocities resulting from these structures may generate sediment transport toward the channel, in contrast to their original intention. Another important impact of these structures is on saltwater intrusion, which is crucial to freshwater resource management. Therefore, this research investigates the groyne-induced effects on cross-channel flows and the saltwater intrusion problem by comparing different layouts of groynes and their consequences. Two important findings may benefit practical applications. First, the interaction between saltwater and freshwater in estuaries enhances cross-channel flows during certain periods, thereby influencing sediment transport. Second, the maximal saltwater intrusion occurs for intermediate width-to-length ratios of the groynes, with lower saltwater intrusion for either very small or very wide groyne fields. [ABSTRACT FROM AUTHOR]

In this study, a robust non-Newtonian numerical model that can simulate coupled mudflow and sediment transport processes is used to model downstream geomorphic changes during a tailings dam failure. The catastrophic breach of the Mount Polley tailings dam is studied to assess the model performance. Spatial and temporal variation of flow properties like density, viscosity, yield stress, sediment size fractions, and friction are considered in the model. A flexible triangular mesh is adopted, and this helps reduce the computational time significantly compared with structured grid with the same average resolution. High resolution multitemporal digital elevation models (DEMs) are used for the detection of geomorphic changes and estimation of erosion and deposition volumes. The results indicate that the final distribution of tailings deposits obtained from the model results is in good agreement with that obtained from topographic analysis. The model predicted the total volume of erosion and deposition with almost 93% accuracy. The tailings characteristics followed the full Bingham model during the propagation. Different runout scenarios obtained from the mudflow-morphodynamic model and a mudflow fixed-bed model are analyzed, and it was concluded that maximum mudflow depth of the flood wave is higher when bed entrainment is considered. Sediment concentration is higher in the wavefront and middle regions and lower in the tail zone. The mudflow fixed-bed model could underestimate the downstream flood arrival time. The results also confirm that the adopted approach can efficiently model the sediment budget in the study site and the volume of tailings and bed materials released to the downstream lake. Tailings dams are used to store waste solids and water generated during mining operations. Failure of such dams can cause serious damage to human settlements and the downstream environment. For public safety, it is necessary to conduct proper risk assessment, and numerical modeling is crucial for estimating flood magnitude, flood path, and flood arrival time. Tailings water is high in sediment concentration and is viscous; thus, its flow characteristics are different from that of pure water. During the collapse of tailings dam, erosion of the downstream terrain occurs, and changes in the downstream channel morphology can be potentially significant. There can also be a change in the total sediment load when additional solids get eroded from the bed layer. The modeling approach used in this study helps to model erosion and deposition patterns and estimate the downstream flood arrival time when transport of sediment in the downstream channel is considered along with the viscous nature of tailings. When conducting risk assessment of tailings dam failures and when planning for evacuation, this mobile-bed modeling approach can be implemented by design engineers and emergency planners. [ABSTRACT FROM AUTHOR]