River Bend Research Articles

Influence of deflection angles on flow behaviours in openchannel bends

The deflection angle of a river bend plays an important role on behaviours of the flow within it, and a clear understanding of the angle’s influence is significant in both theoretical study and engineering application. This paper presents a systematic numerical investigation on effects of deflection angles (30°, 60°, 90°, 120°, 150°, and 180°) on flow phenomena and their evolution in open-channel bends using a Re-Normalization Group (RNG) κ-e model and a volume of fluid (VOF) method. The numerical results indicate that the deflection angle is a key factor for flows in bends. It is shown that the maximum transverse slope of water surface occurs at the middle cross section of a bend, and it increases with the deflection angle. Besides a major vortex, or, the primary circulation cell near the channel bottom, a secondary vortex, or, an outer bank cell, may also appear above the former and near the outer bank when the deflection angle is sufficiently large, and it will gradually migrate towards the inner bank and evolve into an inner bank cell. The strength of the secondary circulations increases with the deflection angle. The simulation demonstrates that there is a low-stress zone on the bed near the outer bank and a high-stress zone on the bed near the inner bank, and both of them increase in size with the deflection angle. The maximum of shear stress on the inner bank increases nonlinearly with the angle, and its maximums on the outer bank and on the bed take place when the deflection angle becomes 120°.

Tracer advection in an idealised river bend with groynes

This paper presents numerical simulations of particle advection in the bend of an open channel containing groynes, which is an idealised form of a shallow river bend in a wide river. The flow field is computed using a boundary-fitted solver of the non-orthogonal, curvilinear shallow water equations. The computational grid is generated by solving Poisson-type elliptic partial differential equations using an iterative multi-grid scheme for prescribed boundary coordinates. The shallow water equations are discretised with finite differences in space, and 4th order Runge-Kutta integration in time. Tracers introduced at specific initial locations have their trajectories computed using Lagrangian particle tracking. The numerical shallow flow model is verified by comparison to the analytical solution of fully developed flow in an open channel. The combined shallow flow and Lagrangian particle-tracking model is then used to simulate the advection of tracer particles in a rectangular channel containing a pair of parallel groynes, and tracer particles in a curved open channel containing groynes, of dimensions roughly equivalent to a Danube River bend.

Hydrodynamic Drivers of Juvenile-Salmon Out-Migration in the Sacramento River: Secondary Circulation

The entrances to the two lowest-survival migration routes for juvenile Chinook salmon in the tidal Sacramento River are located in the outside of a river bend where secondary circulation occurs. Three-dimensional simulations are conducted, in the Eulerian and Lagrangian frame, to understand tidal and secondary circulation effects on salmon migration route selection within this river reach. Fish were assumed to behave as neutrally-buoyant particles. Findings show that simulated particle entrainment rates into these routes tend to be larger than those expected from flow entrainment calculations alone, particularly during ebb tides, due to several factors. First, the fraction of the flow diverted to these routes tends to be higher in the shallowest layers, as a result of the secondary circulation that develops in the main river. Second, and supporting previous work done at the study site, the secondary circulation acting upstream also causes the surface-biased salmon distribution to be skewed toward the outside of the bend as they approach the entrance to the migration routes. As a result of these effects, the risk of particles being entrained is maximal near the surface, remaining higher than 50% during the course of a tidal cycle.

Using Time‐Lapse Lidar to Quantify River Bend Evolution on the Meandering Coastal Trinity River, Texas, USA

AbstractTime‐lapse airborne lidar on the coastal Trinity River in Texas, USA, shows profound downstream variability in point bar growth and cut bank erosion resulting from a historically large flood that occurred in 2015. The difference map generated from two surveys covers 55 river bends and captures the transition from quasi‐uniform flow into backwater flow. In the upstream portion of the survey, reach‐averaged deposition is greater than erosion, with high local variability between the two due to the occurrence of bend cutoffs. Point bars shift laterally up to 3 times as much as cut bank retreat in this area. In the backwater zone, cut bank erosion consistently outpaces deposition on point bars and cut bank retreat exceeds the lateral shift of the point bar. Edge‐to‐edge channel width, measured from active scroll bar to outer bank, increases for all bends. This widening is often partially due to point bar widening and movement of the scroll bar away from the channel. The combination of net erosion and limited lateral movement of point bars within the backwater zone implies that during large sustained floods, bank pull instigates channel migration in this area. Farther upstream, net deposition and the high ratio of point bar to cut bank lateral motion imply bar push is more important. Patterns of deposition and erosion within a single bend suggest that channel migration was dominated by downstream bend translation with little bend deformation. Results from this study help to deepen the understanding of coastal river system morphodynamics by connecting changes in the geomorphology to changes in river bend kinematics, hydraulics, and sediment transport.

Experimental Investigation and Numerical Simulation of Morphological Changes in Natural Channel Bend

Sediment transport is an important process in maintaining balance of the form of river. The transport of bed load particles affects the processes of aggradation and degradation of the riverbed significantly. To predict the evolution process of the river morphology, the numerical model is considered as a useful tool. This study developed a two-dimensional (2D) depth-averaged model for the morphological change in the river bend. The flow module is represented by the shallow water equations, and the river morphological changes are represented by the sediment continuity equation. The sediment transport module treats bed load as mixtures of multiple grain-size sediments. A finite difference method was applied to solving the governing equations. The developed model was applied to predict bed-load transport rate on one set of the laboratory experiment. A field study was further applied to demonstrate the capability of the developed model in predicting morphological change in the curved river section in South Korea. The simulation results of the developed model were in good agreement with field data both laboratory experiment and natural channel bend.

Countermeasure of river bend scour using a combination of submerged vanes and riprap

A series of laboratory flume experiments were done in a large-scale 180° bend with non- cohesive sediment to find optimal or effective protection works at a bend. Detailed study of the scour and flow field dynamics with and without protection works was done. Spatially dense, high frequency velocity data were collected and analyzed to describe the pattern and magnitude of three-dimensional (3D) velocity throughout the bend. Characterizing the role of flow field dynamics on the pattern of deposition and erosion through experimental measurements provided valuable data about how such flow features contribute to scour and about the performance of the protection works. From the experimental results, it is revealed that for a perennial river it is not possible to protect from scour either with riprap or with submerged vanes alone. Protection from scour at a bend can be achieved with proper combination of these two works. First, submerged vanes can protect the toe, and, second, riprap can protect the upper part of the slope if it is not damaged through toe erosion. The experiments convincingly demonstrate the efficiency of this bank protection technique.

Responses of Bed Morphology to Vegetation Growth and Flood Discharge at a Sharp River Bend

In this study, we conducted simulations using a two-dimensional, depth-averaged river flow and river morphology model to investigate the effect of vegetation growth and degree of flow discharge on a shallow meandering channel. To consider the effects of these factors, it was assumed that vegetation growth stage is changed by water flow and bed erosion. The non-uniformity of the vegetation growth was induced by the non-uniform and unsteady profile of the water depth due to the irregular shape of the bed elevation and the unsteady flow model reliant on hydrographs to evaluate three types of peak discharges: moderate flow, annual average maximum flow, and extreme flow. To compare the effects of non-uniform growing vegetation, the change in channel patterns was quantified using the Active Braiding Index (ABI), which indicates the average number of channels with flowing water at a cross section and the Bed Relief Index (BRI), which quantifies the degree of irregularity of the cross-sectional shape. Two types of erosion were identified: local erosion (due to increased flow velocity near a vegetation area) and global erosion (due to the discharge approaching peak and the large depth of the channel). This paper demonstrated that the growth of vegetation increases both the ABI and BRI when the peak discharge is lower than the annual average discharge, whereas the growth of vegetation reduces the BRI when the peak discharge is extreme. However, under extreme discharge, the ABI decreases because global erosion is dominant. The conclusions from this study help to deepen the understanding of the interactions between curved river channels and vegetation.

Investigation of Diameter of Riprap around the Bridge Abutment at 180-Degree River Bend

Investigation of Diameter of Riprap around the Bridge Abutment at 180-Degree River Bend

Flood Capacity of Shirakawa River at Tatsudajinnnai Area in Kumamoto Prefecture

In the Shirakawa River, remarkable deformation due to sediment deposition was observed after the river channel improvement work. The sediment deposition was observed at the bend of the river channel. In this research, a model experiment, a quasi-three-dimensional numerical simulation and planer two-dimensional bed variation analysis were conducted in order to evaluate the effect of river bend curvature variation caused by improvement works on channel response and flow capacity. In the model experiment, large sandbars were formed, and the channel become narrower. In the numerical simulation, flood flew over sandbars and gravel deposited on sandbars, which caused decreasing of flow capacity. The gravel deposition was most active when the flow charge was in its peak. There was less inundation at the areas where sediment deposition was occurred.

3D transient numerical flow simulation of groundwater bypass seepage at the dam site of Dongzhuang hydro-junction

Hydrogeological conditions at the Dongzhuang Hydro-Junction site are complex and include underground water divides, a hanging river and hydraulic drops along faults. The site suffers from seepage because of its location in karst terrain. This study addresses dam bypass leakage at the Dongzhuang site and deficiencies in the regional groundwater seepage model. A hydrological conceptual model is generated, as well as a regional numerical simulation of groundwater flow based on analysis of the geology and hydrogeology of the dam site and surrounding area. From the analysis, a detailed 3D model of the groundwater flow field at the dam site is established, in which permeability and precipitation infiltration parameters are corrected using the inversion method. Leakage quantities are calculated for three different conditions: natural state, regular dam impoundment with an impervious curtain, and regular dam impoundment without an impervious curtain. The results show that underground water flows from west to east during all three conditions. The leakage quantity bypassing the dam region without anti-seepage treatment is 220,000m3/d, with seepage concentrated around the dam foundation, dam abutments and the river bend of the reservoir head. Leakage in these areas accounts for 72% of the total leakage in the study area. However, when an impervious curtain is included in the model, the total leakage quantity is reduced to 51,000m3/d and seepage concentrates along the banks of the reservoir. The 3D numerical model of regional groundwater flow is promising in that it can simulate complex hydrogeological conditions, and the accuracy and confidence levels improve when a detailed 3D model of groundwater flow at the dam site is added to the regional model.

Hydrostatic versus nonhydrostatic hydrodynamic modelling of secondary flow in a tortuously meandering river: Application of Delft3D

AbstractGiven the importance of pressure gradients in driving secondary flow, it is worth studying how the modelled flow structures in a natural river bend can be impacted by the assumption of hydrodynamic pressure. In this paper, the performance of hydrostatic versus nonhydrostatic pressure assumption in the three‐dimensional (3D) hydrodynamic modelling of a tortuously meandering river is studied. Both hydrostatic and nonhydrostatic numerical models were developed using Delft3D‐Flow to predict the 3D flow field in a reach of Stillwater Creek in Ottawa, Canada. An acoustic Doppler velocimeter was employed to measure the 3D flow field at a section in a sharp bend of the simulated river at two flow stages. The results of the Delft3D hydrostatic model agreed well with the acoustic Doppler velocimeter measurements: The hydrostatic model predicted reasonably accurately both the streamwise velocity distribution across the section and the magnitude and location of the primary secondary flow cell. The results of the Delft3D nonhydrostatic approximation showed that the model was not conservative and could not accurately generate either the secondary flow or the streamwise velocity distribution. This study illustrated the superior performance of the hydrostatic over nonhydrostatic 3D modelling of the secondary flow using Delft3D. Several possible reasons for unfavourable performance of the nonhydrostatic version of Delft3D are discussed, including the pressure correction technique employed in Delft3D. Considering the uncertainties that may arise in both modelling and field measurements, the 3D hydrostatic Delft3D model was capable of reasonably predicting the river bend flow structures in the studied meandering creek.

Sedimentology of Late Holocene fluvial levee and point-bar deposits from the Cambodian tract of the Mekong River

The study of channel margin deposits is crucial to better understand fluvial systems and has significant social and economic implications. This paper describes sedimentological features of Late Holocene levee and point-bar deposits from five excellent outcrop exposures along the Cambodian tract of the Mekong River. Point-bar deposits show a typical fining-upward trend with low-angle inclined bedding and consist of gravelly sand, sand and mud beds. Levee deposits show a typical coarsening-upward or no clear vertical grain-size trend and consist of mud and very fine sand beds. Channel margin deposits formed in the outer river bend show a simple levee element, whereas inner bend deposits show more complex architecture with combinations of point-bar and levee elements. Water-level fluctuations play a fundamental role in the construction of point-bar and levee architecture; the resulting deposits show evidence of strong river currents with rapid sedimentation alternating with evidence of subaerial exposure with no sedimentation. This combination of sedimentary features can also form in other environments, but is distinctive of channel margin deposits forming in climates with a pronounced seasonality, including large and perennial rivers in tropical savannah climate areas.

Vegetational zonation in a swamp forest, Middle Pennsylvanian, Illinois Basin, U.S.A., indicates niche differentiation in a wetland plant community

A rich fossil flora occurs in the roof shale of the Middle Pennsylvanian (Moscovian) age, Springfield Coal flanking the Galatia channel, a river deposit approximately 1km in width and 150km long, partially contemporaneous with the Springfield peat swamp. Siltstone and sandstone, >30m thick, were deposited in the channel during the final estuarine phase. The river formed under seasonal subhumid climate during early deglaciation as marine waters retreated from the land surface. It existed continuously through glacial maximum, under increasingly humid climate, as a low-sediment or black-water river with peat along its flanks. During early deglaciation, under subhumid climate and sea-level rise, the river became a sediment-laden estuary, drowning the peat swamp and burying vegetation in place under extensive mudflats. This buried flora was studied in three underground mines in southern Illinois, permitting vegetation to be characterized spatially. Three transects, comprising 100 sites, a total linear distance of ~12km, reveal distinct zonation perpendicular to the channel margin. An innermost belt, 0.5 to 2.5km wide, consists almost exclusively of the pteridosperm Neuropteris flexuosa. North of a river bend, three belts succeed the N. flexuosa zone: mixed pteridosperms and marattialean tree ferns; mixed arborescent lycopsids and marattialians; lycopsids with rare marattialians. South and inside the same bend, N. flexuosa is succeeded by mixed lycopsids, particularly Sigillaria, and marattialean ferns. Within the transects, marattialian ferns have wider distribution than other groups, suggesting broader environmental tolerances. Neuropteris flexuosa channel-margin mono-dominance may reflect high sedimentation rates, elevated nutrient conditions, or floating-seed dispersal. Lycopsid dominance in interior areas conforms with tolerance of deep, standing water. The spatial scale examined affects our perception of community assembly. Overall, distinct vegetational zonation suggests niche-assembly; however, within vegetational bands, there is considerable small-scale spatial variation suggesting stochastic properties.

Bed load transport and incipient motion below a large gravel bed river bend

Abstract A new data set of bed load measurements in a cross-section at the exit of a river bend is presented. Data are analyzed to identify processes that contribute to the morphodynamic stability of gravel bed meanders. It is shown that boundary shear stress and bed material texture are strongly coupled, resulting in an almost equal mobility at incipient motion over the bend point bar in relation to channel flow stage. Conversely, for conditions above bankfull an excess of fine sediment towards the inner-bank, likely related to more intense crosswise flux and grain size sorting, results in size selective transport in relation to the local bed material. We suggest that bed armoring and structuring, as well as crosswise sediment flux, add stability to the outer-bank pool, while the point bar is eroded by large floods and restored by moderate flows. Results reveal the strong feedback of processes at different scales promoting stability at bends of gravel bed rivers.

Research on Technique of Pre-judge for Waterlogging of Highway Along River in Mountainous Areas

This paper gives statement about methods that how to recognize if the highway along river is destroyed by water damage, which on the basis of analyzing influence factors of water damage and pre-judge model of highway along river, then analyzes the result combine with application of JiBei mountain area, verifies its scientificity and effectiveness. Subgrade of highway along river in mountains areas suffers from long-term or discontinuous effect of soaking caused by running water. Under soaking effect of discontinuous water flow, running water porous flows into subgrade through slope of subgrade, then unit weight of backfill materials(), cohesive force(C) and internal friction angle() change suddenly, those factors bring down the mechanic strength indexes of subgrade soil, in this case, subgrade softens like ”rubber road” or occurs the lateral movement, then surface of highway cracks, subgrade starts sinking, spring of subgrade frosts boiling. There are two effects exist in washout result. One is water flows past surface of subgrade slope; the other is washing effect to toe of slope, which is caused by swirling flow and vortex resulting from river bend. Establishing scientific, effective pre-judging and identification system becomes more and more significant and important to build scientific pre-judging model which aims at different geographic areas and guides the deployment of prevention work.

Three-dimensional numerical investigation on the influence of geometric shape on flow in river bends

To investigate the influences of river bend geometry on flow structures, three-dimensional simulations were carried out. The influence of aspect ratio, channel sinuousness and curvature ratio were investigated. Simulation results show that with an increasing of channel aspect ratios, the streamwise variation of flow velocities decreases significantly, which is not the case for the spanwise direction. Simulation results also reveal the significant influence of the curvature ratio on the redistribution of primary flows. The streamwise variation of free-surface and bottom-wall velocities was compared and the phase differences between them was identified, which was described as anti-phase and out-of-phase patterns. Deep-water channels with an aspect ratio of 0.15 show significant phase-lag of flow velocities compared with channel geometry, this newly observed phenomenon implies the shift of the peak scouring point in fluvial river meanders. For the first time to our knowledge, the streamwise and spanwise variation of primary and secondary flow strength, as well as their spatial derivatives, are quantitatively provided, which are valuable for theoretical studies. It is also found that the widely adopted assumption that the streamwise velocity component is one order magnitude larger than the spanwise, is only valid for river bends with small curvatures and aspect ratios.

Studi Pengaruh Sudut Belokan Sungai Terhadap Volume Gerusan

Scouring occurs along the streams and generally increases in their bend areas. This occurence threaten the stability of buildings and facilities built around those areas. This study was conducted by experimental model tests in a laboratory using an artificial channel made of sand with diameter of 0,47 mm and the sand layer density of 1,47 gr/cm3. To assess the effect of river bends on the extent of scours, the water flow was simulated on the artificial channel in 9 times i.e.; 3 variations in the angle of river bends (α) and 3 variations in the flow discharge (Q) for each river bend variation. This study revealed that the volume of scours positively correlated to the angle of river bends. The volume of scours decreased more 20% when the angle of river bends decresed from 60º to 30º. The increase of the flow discharge also enlarged the volume of scours more than 100% at the river bends with small angles

Analysis of the curvature of the river bend for regulation purposes

Analysis of the curvature of the river bend for regulation purposes

Earthquake-triggered landslides by the 1718 Tongwei earthquake in Gansu Province, northwest China

The 1718 great Tongwei earthquake in northwest China caused more than 300 large-scale landslides near the epicenter resulting in more than 70,000 deaths. Although previous studies agree that the earthquake-induced landslides in the area are loess-landslides, which slid along ancient terrain surfaces, there is a dearth of evidence to unravel the actual failure mechanisms of these widely-documented events. To clarify their characteristics and mechanisms, the landslides in Pan’an Town, located in Gansu Province, China were rigorously investigated. The field surveys revealed that the landslides occurred in an area comprising loess deposits of several tens of meters underlain by a basal mudstone layer. It was also revealed that instability in the area is strongly influenced by a river valley with large erosion bank slopes, as well as steep mudstone slopes, which are well developed. The majority of the landslides triggered by the Tongwei earthquake were loess-mudstone composite landslides, with the sliding surface in the deep mudstone. Three huge landslides on the northern mountain located at the river bend of Pan’an Town with a total volume of about 6.06 × 108 m3 are the main mass movements during the 1718 catastrophe. In addition, by taking the typical landslide named the Weijiawan landslide as a case study, the internal structure of the earthquake-triggered landslides and their stability were studied. The new findings not only aided the unraveling of the failure mechanisms but also provided insights and knowledge on early recognition of earthquake-triggered landslides in northwest China towns, especially in the surrounding valley, which shares similar geological conditions with landslides studied in this paper.

Block Ramps in Curved Rivers: Morphology Analysis and Prototype Data Supported Design Criteria for Mild Bed Slopes

Eco-friendly river restoration structures are a valid solution for river training projects. Among this structure typology, block ramps have been successfully tested to solve problems related to river sediment control, bed stabilization and energy dissipation. Despite the conspicuous literature dealing with block ramps design in straight rivers, there are no studies analysing the erosive process occurring in the stilling basin downstream of a block ramp in a curved river bend. Therefore, this paper represents the first systematic analysis of their behaviour and of the resulting downstream equilibrium morphology in such geometric configuration. A dedicated model was built to simulate a wide range of hydraulic conditions. Experimental data analysis allowed describing the erosion dynamics occurring in the stilling basin and, at the same time, to derive a useful design relationship by which it is possible to estimate the maximum scour depth. Furthermore, the model results were successfully validated by using field measurements collected in the Porebianka River (Poland). Both field data and laboratory experimental results allowed furnishing a comprehensive description of the scour phenomenon. The proposed relationship represents the first trustable and valid tool for hydraulic design of such structure typology in curved rivers. Copyright © 2016 John Wiley & Sons, Ltd.