Changes In Bed Morphology Research Articles

THE FEATURES OF THE OKA RIVER LONGITUDINAL PROFILE DEVELOPMENT UNDER THE ANTHROPOGENIC DISTURBANCES OF THE RIVER BED

Based on the analysis of cartographic documents and hydrological data, as well as detailed studies of fl uvial processes in the Upper Oka, carried out in 1990-ies and continued in 2010–2015, it was found that the longitudinal profile of the river was significantly infl uenced by anthropogenic river bed disturbances – exploitation of in-stream mines of alluvial construction materials, concentrated in the middle reaches of the river within 400–800 km from the source. Changes in river bed morphology and sediment deficiency has resulted in the deep erosion. To determine the role of anthropogenic disturbance in the longitudinal profile formation, in accordance with the N.I. Makkaveyev’s approach, the calculation of graded longitudinal profile and its comparison with real one were performed. By calculating the stream-gradient index proposed by J. Hack, the uneven gradient distribution was revealed along the entire river and on separate typical sections. The highest values of the stream-gradient index observed at the site where mechanical modifications are the most – between Kaluga and Kolomna Cities. On this stretch of the river in the late 1930s, the longitudinal profile in most aspects practically coincides with the calculated graded profile. However, mechanical changes of the river bed and the accompanying river bed degradation has led to the fact that along with general elevation decrease, profile became ungraded, and one can expect some deformations aimed at its restoration. As a result, the difficulties in using the resources of the river, which emerged in the early 1990-ies, have increased.

Bridge Pier Scour Evaluation in Meandering Channels

The objectives of this research are to predict and evaluate the effect of releasing high/extreme flow discharges on scour at 13 bridge piers of (3) bridges located at Kafr El-Zayat City. Field data, empirical equations and a two-dimensional model were used. The empirical equations were used to predict the general, contraction and bend scour of the bed's morphological changes along the entire reach of the osetta Branch. The 2-D model was used to predict scour at the bridge piers in the study area considering two scenarios of high river discharges. The expected extending of the scour holes around the main piers was also predicted. The results showed that in case the released discharges were (69.90, 220.00m.m3/day), the total scour around piers evaluated at the (3) bridges were (13.98, 17.74m) for bridge No. (1), (10.33, 13.93m) for bridge No. (2) and (12.64, 15.34m) for bridge No. (3). It is recommended to follow up the geometry of the scour holes after one year of passing extreme high discharges.

Hydrodynamic Equilibrium for Sediment Transport and Bed Response to Wave Motion

An experimental and theoretical identification of hydrodynamic equilibrium for sediment transport and bed response to wave motion are considered. The comparison between calculations and the results of laboratory experiments indicates the linear relation between sediment transport rate and the thickness zm of bed layer in which sediments are in apparent rectilinear motion. This linear relationship allows to use the first order “upwind” numerical scheme of FDM ensuring an accurate solution of equation for changes in bed morphology. However, it is necessary to carry out a decomposition of the sediment transport into transport in onshore direction during wave crest and offshore direction during wave trough. Further, the shape of bed erosion in response to sediment transport coincides with the trapezoid envelope or with part of it, when some sediments still remain within it. Bed erosion area is equal to the one of a rectangle with thickness znm.

Linking bed morphology changes of two sediment mixtures to sediment transport predictions in unsteady flows

AbstractFlume experiments were conducted to measure bed morphology adjustments in sand/gravel and sand/silt sediment mixtures during repeated hydrographs and to link these changes to sediment transport patterns over multiple time scales. Sediment composition and hydrograph flow magnitude greatly influenced channel morphology, which impacted sediment yield, hysteresis, and transport predictions. Bed load yields were larger and more variable for the sand/silt mixture, as gravel in the sand/gravel sediment inhibited grain entrainment, limited bed form growth, and acted to stabilize the bed. Hysteresis patterns varied due to bed form and surface structure adjustments, as well as the stabilizing effect of antecedent low flows. Using half the data set, a dimensionless fractional transport equation was derived based on excess shear stress. Dimensionless reference shear stresses were estimated in two ways: as bulk values from all transport measurements and by applying a separate limb approach in which values were estimated for each limb of each hydrograph. For the other half of the data set, transport predictions with the separate limb approach were more accurate than those from six existing transport equations and the fractional relationship applied with bulk reference shear stresses. Thus, hydrograph limb‐dependent dimensionless reference shear stress links changing bed morphology and sediment transport, providing a parameter to improve transport predictions during individual flood events and in unsteady flow regimes. This approach represents a framework with which to develop site‐specific transport relationships for varying flow regimes, particularly in cases where detailed bed morphology measurements are not feasible and heterogeneous sediment complicates bed structure over time.

Real time measurements of sediment transport and bed morphology during channel altering flow and sediment transport events

Real-time measurements of bed changes over a reach are a missing piece needed to link bed morphology with sediment transport processes during unsteady flows when the bed adjusts quickly to changing transport rates or visual observation of the bed is precluded by fine sediment in the water column. A new technique is presented that provides continuous measurement of sediment movement over the length of a flume. A bedload monitoring system (BLMS) was developed that makes use of pressure pillows under a false flume bottom to measure sediment and water weights over discrete flume channel sections throughout a flow event. This paper details the construction of the BLMS and provides examples of its use in a laboratory setting to reconstruct bed slopes during unsteady flows and to create a real-time record of sediment transport rates across the flume channel bed during a sediment transporting flow.Data gathered from the BLMS compared well against techniques commonly in use in flume studies. When the BLMS was analyzed in conjunction with bed surface DEMs and differenced DEMs, a complete transport and bed adjustment picture was constructed. The difference DEMs provided information on the spatial extent of bed morphology changes. The BLMS supplied the data record necessary to reconstruct sediment transport records through the downstream channel, including locations and time periods of temporary sediment storage and supply. The BLMS makes it possible to construct a continuous record of the spatial distribution of sediment movement through the flume, including areas of temporary aggradation and degradation. Exciting implications of future research that incorporates a BLMS include a more informed management of river systems as a result of improved temporal predictions of sediment movement and the associated changes in channel slope and bed morphology.

Bed morphological changes around a finite patch of vegetation

AbstractFor the appropriate management and restoration of rivers, isolated vegetation is often a practical means for improving stream habitat and ecology. The effect of a finite vegetation patch on flow and bed morphology in an open channel was investigated using laboratory experiments. The patch containing emergent and submerged vegetation was modeled using circular cylinders and located mid‐channel along a side wall. Several configurations of the patch and submergence ratio (i.e. water depth to the height of vegetation), and two flow conditions (i.e. below and above the sediment motion threshold) were considered. For flows below the sediment motion threshold, erosion occurred primarily on the opposite side of the patch and near the leading edge of the patch. The degree of scouring depth observed in both these regions was affected by the submergence ratio and it increased with the non‐dimensional flow blockage (i.e. the product of the patch density and width). In contrast, for flows above the sediment motion threshold, sediment accumulated within and around the patch due to a reduction in bed shear stress, which was strongly influenced by the flow blockage and the obstruction ratio (i.e. the ratio of patch width to channel width). The eroded area observed within the patch was consistent with the interior adjustment region where the deceleration and diversion of flow occurred through the patch. As the flow blockage increased or as the obstruction ratio decreased, the deposition rate within and behind the patch decreased. Furthermore, the deposition rate increased with an increase in the ratio of flow rate through the patch to total flow rate regardless of the submergence ratio. Copyright © 2014 John Wiley & Sons, Ltd.

Bed Scouring During the Release of an Ice Jam

A model is developed for simulating changes in river bed morphology as a result of bed scouring during the release of an ice jam. The model couples a non-hydrostatic hydrodynamic model with the processes of erosion and deposition through a grid expansion technique. The actual movement of bed load is implemented by reconstructing the river bed in piecewise linear elements in order to bypass the limitations of the step-like approximation that the hydrodynamic model uses to capture the bed bathymetry. Initially, an ice jam is modeled as a rigid body of water near the free surface that constricts the flow. The ice jam does not exchange mass or momentum with the stream, but the ice body can have a realistic shape and offer resistance to the flow of water through the constriction. An ice jam release is modeled by suddenly enabling the ice to flow and exchange mass and momentum with the water. The resulting release resembles a dam break wave accelerating and causing flow velocities to rise rapidly. The model is used to simulate the 1984 ice jam in the St. Clair River, which is part of the Huron-Erie Corridor. The jam had a duration of 24 days, and its release was accompanied by high flow velocities. It is speculated that high flow velocities during the release of the jam caused scouring of the river bed. This led to an increase in the river’s conveyance that is partly responsible for the persistence of low water levels in the upper Great Lakes. The simulations confirm that an event similar to the 1984 ice jam will indeed cause scouring of the St. Clair River bed.

Local bed morphological changes due to oriented groins in straight channels

This research investigates the effect of implementing oriented groins on the scour and silting processes in a straight channel. Combined physical and numerical models were used. Twenty-seven (27) runs were conducted in which the geometry of scour and silting associated with model groins was evaluated. Groin models were angled at 60°, 90°, and 120° to the downstream channel side wall with contraction ratios of 0.10, 0.15 and 0.20. The main goals of this paper were to evaluate the effect of the three angles on the scour geometry on minimizing erosion adjacent to the stream banks. Results were analyzed and were graphically presented and the percentages of errors between the obtained results from the used models were reported to define the sufficient compatibility between the used models. Simple formulae were derived to evaluate the scour and silting parameters.

Laboratory observations of flow and sediment transport induced by plunging regular waves

[1] Two parallel experiments involving the evolution and runup induced by plunging regular waves near the shoreline of a sloping bed are considered: (1) a rigid-bed experiment, allowing direct (hot film) measurements of bed shear stresses and (2) a sediment-bed experiment, allowing for the measurement of pore-water pressures as well as observation of sediment suspension and bed morphological changes. Both experiments utilize the same initial bed profile and wave forcing. The experiments show that the mean bed shear stresses experienced onshore of incipient breaking are amplified by nearly a factor of 2 relative to prebreaking conditions, whereas their corresponding turbulent fluctuations are amplified even more strongly, by a factor of 5–6. The plunging processes lead to a series of vortices, whose formation may be explained as the result of shear layer instability. Measurements show that these vortices can significantly enhance peaks in the offshore-directed bed shear stresses. Moreover, near-bed pore pressure measurements indicate that these vortices cause large upward-directed pressure gradients, which in turn produce a corresponding series of suspended sediment plumes shoreward of the initial breaking event. These findings are related to the induced morphological changes over both short and long time scales. The present results are also compared and contrasted with previous experiments utilizing a similar methodology, but involving plunging solitary waves.

Relationships between block quarrying, bed shear stress, and stream power: A physical model of block quarrying of a jointed bedrock channel

We examine interactions among substrate characteristics, hydraulic forces, and erosive processes acting on a jointed, resistant substrate in a flume experiencing supercritical flow. We conducted six experimental runs with no sediment in transport, constant slope, varying discharge, varying channel width, and varying geometry of concrete blocks forming the channel bed. The initial downstream boundary was a vertical knickpoint lip perpendicular to flow. Observed changes in bed morphology occurred primarily by episodic horizontal knickpoint retreat. The knickpoint lip can develop a curve that reflects interlocking of the irregular boundaries between otherwise physically disjointed blocks during rotation of the blocks. Sometimes a block acts like a keystone in an arch and mobilization of the key block facilitates mobilization of the linked blocks on either side, resulting in a large magnitude event. In contrast to previous studies where blocks were vertically plucked, blocks were removed by sliding from the knickpoint. Building on previous work, we develop a theoretical force threshold for sliding block mobilization, which suggests that increased block side length parallel to flow increases block stability, whereas increased side length perpendicular to flow decreases block stability. Stream power was also linked to observed erosion rates as a function of block length, consistent with the idea that block loosening rates play a key role in controlling average erosion rates. Inferred amplification of bed shear stress and pressure at the knickpoint lip, stream power, and joint spacing correlate with the rate of block erosion. The results, although limited in scope and scale, suggest a linear relationship between bed shear stress and stream power with the rate of block erosion. Block sliding, especially at near-vertical knickpoints, can play a key or even dominant role in the morphologic dynamics of the river channel.

AN IMPLICIT 2-D DEPTH-AVERAGED FINITE-VOLUME MODEL OF FLOW AND SEDIMENT TRANSPORT IN COASTAL WATERS

An implicit depth-averaged 2-D finite volume model has been developed to simulate sediment transport and bed morphological changes under actions of currents and waves near coastal inlets. The model computes the depth-averaged 2-D shallow water flow and non-equilibrium transport of total-load sediment, accounting for the effects of wave radiation stresses and turbulent diffusion induced by currents, waves and wave breaking. The model uses a quadtree rectangular mesh to locally refine the mesh around structures of interest or where the topography and/or flow properties change rapidly. The grid nodes are numbered by means of an unstructured index system for more flexibility of mesh generation. The SIMPLEC algorithm is used to handle the coupling of water level and velocity and the Rhie and Chow's (1983) momentum interpolation method is adopted to determine the intercell fluxes on non-staggered grid. Well-developed longshore current and wave setup determined with the reduced 1-D momentum equations are used as the cross-shore boundary conditions. The model has been tested in several laboratory and field cases, showing good performance. In particular, it can use a long time step and is efficient in computation on a PC platform. It has a potential for simulation of long-term coastal morphodynamic processes.

Response of bed morphology and bed material texture to hydrological conditions at an asymmetrical stream confluence

Changes in bed morphology and sedimentology at stream confluences in relation to specific hydrological events have not been documented extensively. Moreover, few, if any, studies have investigated the effect of net hydrological conditions on channel change at a confluence over a period of many years. Repeat cross section surveys and sampling of bed material at a small, asymmetrical stream confluence reveal the short-term effects of hydrological variability on bed morphology and spatial patterns of bed material texture. Survey data over a 15-year period illustrate the long-term response of channel morphology at the confluence to net hydrological conditions. Results confirm that this confluence is a dynamic fluvial environment, involving shifts in the zone of scour and repeated deposition and erosion of a bar complex at the downstream junction corner as the discharge ratio changes in response to hydrological variability in the tributary streams. Persistent faster runoff-response times of the lateral tributary compared to the main stem have resulted in long-term net erosion of the channel bank opposite this tributary. The documented morphological and sedimentological changes are consistent with and can be explained by results of experimental studies and by past investigations at this confluence that have documented changes in patterns of mean flow structure and turbulence in response to variation in the discharge ratio of contributing streams.

Is the critical Shields stress for incipient sediment motion dependent on channel‐bed slope?

Data from laboratory flumes and natural streams show that the critical Shields stress for initial sediment motion increases with channel slope, which indicates that particles of the same size are more stable on steeper slopes. This observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force. Processes that might explain this discrepancy are explored using a simple force‐balance model, including increased drag from channel walls and bed morphology, variable friction angles, grain emergence, flow aeration, and changes to the local flow velocity and turbulent fluctuations. Surprisingly, increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical Shields stress are similar for flume experiments and natural streams, and significant variations in bed morphology in flumes is unlikely. Instead, grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed‐roughness scale to flow depth (i.e., relative roughness). A model for the local velocity within the grain‐roughness layer is proposed based on a 1‐D eddy viscosity with wake mixing. In addition, the magnitude of near‐bed turbulent fluctuations is shown to depend on the depth‐averaged flow velocity and the relative roughness. Extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes.

A 3D non-hydrostatic model to predict flow and sediment transport in loose-bed channel bends

The validation and application of a model to solve for the flow, the sediment transport, and bed morphology changes in open channels is discussed. The non-hydrostatic model solves the three-dimensional (3D) incompressible, Reynolds-Averaged Navier–Stokes (RANS) equations in generalized curvilinear coordinates. The model uses adaptive grids in the vertical direction needed to account for changes in the free surface elevation and bed levels due to erosion/deposition at the bed as the code converges toward steady state (equilibrium conditions). The model is used to predict flow and sediment transport through straight and curved open channels, including the 140? open channel bend studied by Olesen (1985) and the 193? very sharp bend studied by Blanckaert (2002). An estimate of the peak non-dimensional shear stress at the banks is deduced function of the ratio between the radius of curvature of the bend and the channel width

Characterizing uncertainties for quantifying bathymetry change between time-separated multibeam echo-sounder surveys

Changes of bathymetry derived from multibeam sonars are useful for quantifying the effects of many sedimentary, tectonic and volcanic processes, but depth changes also require an assessment of their uncertainty. Here, we outline and illustrate a simple technique that aims both to quantify uncertainties and to help reveal the spatial character of errors. An area of immobile seafloor is mapped in each survey, providing a common ‘benchmark’. Each survey dataset over the benchmark is filtered with a simple moving-averaging window and depth differences between the two surveys are collated to derive a difference histogram. The procedure is repeated using different length-scales of filtering. By plotting the variability of the differences versus the length-scale of the filter, the different effects of spatially uncorrelated and correlated noise can be deduced. The former causes variability to decrease systematically as predicted by the Central Limit Theorem, whereas the remaining variability not predicted by the Central Limit Theorem then represents the effect of spatially correlated noise. Calculations made separately for different beams can reveal whether problems are due to heave, roll, etc., which affect inner and outer beams differently. We show how the results can be applied to create a map of uncertainties, which can be used to remove insignificant data from the bathymetric change map. We illustrate the technique by characterizing changes in nearshore bed morphology over one annual cycle using data from a subtidal bay, bedrock headland and a banner sand bank in the Bristol Channel UK.

Dynamics of a river channel confluence with discordant beds: Flow turbulence, bed load sediment transport, and bed morphology

River channel confluences play a major role in the dynamics of all fluvial systems, and yet our understanding of bed load routing at these sites is very sparse. The dynamics of confluences are a function of the momentum ratio between the combining flows and the three‐dimensional geometry of the junction. Recent experiments have shown that discordance in bed height between the confluent rivers increases turbulence intensity and enhances upwelling of flow within the confluence. However, the significance of these flow characteristics on sediment transport is still unknown. To examine the relations between flow and sediment transport, we have measured near‐bed flow turbulence, bed load transport rates, and changes in bed morphology for eight different flow conditions at a sand bed discordant confluence. Detailed analysis of the near‐bed flow patterns reveals that within the shear layer, low mean flow velocities are combined with the highest values of Reynolds shear stresses and that turbulence generation is associated with intense upward movements of flow. High sediment transport rates are found at the edges of the shear layer region where horizontal‐vertical cross stresses (ρUw′) are high. These patterns match changes in bed morphology where erosion occurs along the shear layer. The relation between the shear layer and sediment transport confirms the role of bed discordance on the dynamics of the confluence. Migration of the shear layer into the confluence, as a result of a change in momentum ratio, modifies local near‐bed flow characteristics, sediment transport rates, and the spatial distribution of deposition and erosion zones.

Residual Flow Structure at a Scour-hole in Bahia Blanca Estuary, Argentina

Abstract Two-dimensional data of mean and sediment flow collected at a natural confluence of channels in the inner Bahia Blanca Estuary are presented. We describe the two-dimensional flow field to assess the role of changes in bed morphology occurring during transport-effective events on the structure of flow at a confluence; and to examine how the flow structure varies with changes in the ratio of momentum flux. At each station, vertical profiles of velocity, salinity, temperature and suspended sediment were obtained during the whole tidal cycle and residual fluxes were obtained. Lagrangean flux is higher than possible fresh water input, denoting an extra output of water which is the flux over the tidal flats along the southern and northern coasts of the channel. The effect of the junction produces a distortion of the vertical profile flow and in the suspended sediment transport. The effect of the particular conformation of the system makes necessary for consider the lateral effect of the tidal flats in ...

One-Dimensional Numerical Model for Nonuniform Sediment Transport under Unsteady Flows in Channel Networks

In this study, the proposed one-dimensional model simulates the nonequilibrium transport of nonuniform total load under unsteady flow conditions in dendritic channel networks with hydraulic structures. The equations of sediment transport, bed changes, and bed-material sorting are solved in a coupling procedure with a direct solution technique, while still decoupled from the flow model. This coupled model for sediment calculation is more stable and less likely to produce negative values for bed-material gradation than the traditional fully decoupled model. The sediment transport capacity is calculated by one of four formulas, which have taken into consideration the hiding and exposure mechanism of nonuniform sediment transport. The fluvial erosion at bank toes and the mass failure of banks are simulated to complement the modeling of bed morphological changes in channels. The tests in several cases show that the present model is capable of predicting sediment transport, bed changes, and bed-material sorting in various situations, with reasonable accuracy and reliability.

Three-dimensional structure of flow at a confluence of river channels with discordant beds

This paper presents three-dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to (1) describe the three-dimensional flow field of a natural junction of channels; (2) assess the role of changes in bed morphology occurring during transport-effective events on the structure of flow at a confluence; and (3) examine how the three-dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams. Three-dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four, two-component electromagnetic current meters. Six detailed velocity profiles were taken at five cross-sections in a wide range of flow conditions. The mean field of flow is characterised by (1) the acceleration of flow in the downstream portion of the post-confluence channel, but by lower velocities upstream in the mixing layer area; (2) a stagnation zone at the apex of the junction; (3) a zone of flow deviation, and strong fluid upwelling, close to the avalanche face and at the margin of the tributary mouth bar; and (4) reduced velocities over the depositional bar at the downstream junction corner. The position and extent of these zones vary with changes in the ratio of momentum flux. Very high intensity of turbulence (peaks up to 50%) and turbulent kinetic energy were observed in the mixing layer region. Distortion of the mixing layer, characteristic of flow where bed discordance is present between the two tributary channels, was evident from mean and turbulent flow data. This field study suggests that the effects of bed discordance on flow, sediment transport, and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow.

Rill erosion and morphological evolution: A simulation model

A mathematical model is advanced to simulate dynamically and spatially varied shallow water flow and soil detachment, transport, and deposition in rills. The model mimics the dynamic process of rill evolution, including variable rates of sediment redistribution along the bed and changes in local bed morphology. The sediment source term in the model uses a point scale, probabilistic relationship based on turbulent flow mechanics and a recently developed sediment transport relationship for rills based on stream power. The interdependent feedback loops between channel bed morphology, local flow hydraulics, and local scour and deposition, within the framework of the full hydrodynamic equations with inertial terms, constitute a mathematical model with the capacity to represent spatial variability and temporal evolution of the rill. Finite elements were applied to numerically solve the hydrodynamic and sediment continuity equations. A series of laboratory flume experiments were performed to evaluate the model. Initial bed slopes were 3, 5, and 7% with step increases of water inflow rates of 7.6, 11.4, and 15.2 L min−1. The soil material used in the flume was a kaolinitic, sandy‐clay loam. The rill model equations were solved for increasingly complex cases of spatial and temporal variabilities. The model followed measured patterns of morphological changes as the rill evolved, which suggests that the feedback loops in the model between erosion, bed morphological changes, and hydraulics were adequate to capture the essence of rill evolution.