Rational regime model of alluvial channel morphology and response

A theoretical model is developed for predicting equilibrium alluvial channel form. The concept of greatest relative stability, achieved by maximizing resistance to ?ow in the ?uvial system, is presented as the basis for an optimization condition for alluvial systems. Discharge, sediment supply (quantity and calibre) and valley gradient are accepted as independent governing variates. The model is used to de?ne a dimensionless alluvial state space characterized by aspect ratio (W/d), relative roughness (D/d), and dimensionless shear stress (τ*) or, equivalently, channel slope (S). Each alluvial state exhibits unique values of Froude number and sediment concentration. The range of alluvial states for constant values of relative bank strength (parameterized by an apparent friction angle, ?′) forms a single plane in the state space (W/d, D/d, τ* or S). The scaling relations produced by the model are consistent with laboratory channels exhibiting a range of bank strengths, and with the behaviour of natural channels. Copyright © 2004 John Wiley & Sons, Ltd.     

Development of longitudinal profiles of alluvial channels in response to base-level lowering

Degradation of alluvial channels in cohesive sediments was studied in 15 m and 20 m long flumes with a slope of 0°01 cm/cm. Degradation was initiated by lowering base level to a fixed position, and the development of the longitudinal profile of the channel is analysed through a model formulated as a heat (diffusion) equation. It is based on the equation of sediment continuity, combined with an assumption regarding sediment transport, namely that sediment discharge is linearly proportional to the channel slope. In accordance with the boundary and initial conditions imposed by the experimental setup and procedure, the basic equation is amenable to an analytical solution, which defines bed elevation at any distance and time, as a function of the amount of base-level lowering and a ‘diffusion’ coefficient. Additional problems arising from bank erosion and channel armouring are also treated successfully within the framework of the same model. The results show that in homogeneous alluvial sediments, not subject to armouring, the ultimate result of base-level lowering by a certain amount is degradation all along the channel by the same amount. The main impact of erosion is felt in the early stages after initiation of the process, and mainly near the mouth. The rate of degradation at any station along the channel reaches a peak and then slowly decreases with time, and the peak rate is attenuated with distance from the outlet. The model permits the prediction of intermediate stages of profile development at any distance from the outlet and at different times.     

Adjustment and recovery of unstable alluvial channels: Identification and approaches for engineering management

The management of riverine environments is shown to require a knowledge and awareness of the complex interactions between fluvial and mass-wasting processes, riparian vegetation, and channel form. Identification of the cause of instability rather than the local symptoms, and knowledge of the temporal and spatial aspects of channel adjustment are central to the application of (1) appropriate analyses to estimate future channel changes, (2) appropriate mitigation measures, and (3) the protection of river-crossing structures and adjacent land. Conceptual models of channel evolution and bank-slope development are particularly valuable for interpreting past and present processes, applying appropriate computational techniques to estimate future channel changes, and implementing strategies to mitigate the impacts of processes likely to dominate the channel in the future. Techniques for identification and analysis of channel instability are interdisciplinary and provide a mechanism for estimating changes in channel-bed elevation and channel width with time. Features of channel form and associated riparian vegetation can be used as diagnostic criteria to identify channel processes, the stage of channel evolution and the magnitude and extent of instability. Changes in bed elevation with time can be represented using an exponential function; changes in channel width with time can be calculated using slope stability equations and (or) projection of a temporary angle of stability from a low-angle surface termed the ‘slough line’ that supports re-establishment of woody vegetation. These techniques, in combination with knowledge of the state of channel evolution, can then be used to assess the appropriateness of various mitigation measures to control on-going channel adjustments and to protect river-crossing structures.     

Medium-term effects of two landsliding episodes on channel storage of sediment

Two landsliding episodes between late 1973 and early 1975 delivered about 60000 m³ of sediment to six small deeply incised streams draining a 2·7 km² area. About 4700 m³ of logs in the landslide debris formed major log jams in five streams, which impounded large volumes of landslide-derived sediment. Five years after the landsliding, 42 per cent (25000 m³) of sediment was still in storage behind 35 log jams ranging from 1·4–8·2 m high. The landsliding episodes have produced multi-stepped stream profiles, aggradation of channel reaches up to 150 m long to mean depths between 1·2 and 4·1 m, reductions in gradient, fining of bed material size, and related changes in bedforms and channel width:depth ratios that seem likely to persist for at least several decades. Sediment presently stored behind log jams is equivalent to between 50 and 220 years normal supply of sediment from hillslopes to stream channels. Long-delayed, large magnitude impacts on higher-order channels may occur if sudden failure of log jams is induced by a large storm at some future date.     

River response to channel regulation: Case study of the Raba river,Carpathians, Poland

Processes induced by the channelization of the Raba River in the present century are examined to illustrate the response of a gravel-bed stream following narrowing and straightening of its channel. Up to 3 m of incision has occurred. The change from a slow and relatively steady degradation in the lower reaches to separate degradation events in the higher reaches is attributed to the differential rate of headcut retreat and to the control exerted by mid-channel bars upon the rate of river-energy dissipation. Progressive outwashing of finer grains from bed material has followed the diminishing sediment yield of the basin and the increase in stream power. The ensuing growth in mean grain size and changes in sediment fabric have increased boundary resistance to flow and reduced particle susceptibility to entrainment. Downstream magnification of peak discharges has become increasingly pronounced with the advancing incision. The decrease in flood-plain storage and self-acceleration of flows passing the relatively deep and straight channel has caused flood waves to become progressively more flashy in nature. An increase in channel depth and reduction in gradient caused by downward and backward erosion, as well as bed material coarsening has promoted the re-establishment of an equilibrium. Conversely, flow velocity increases due to flow concentration in the deepened channel. Reduction in grain mobility allows the river to attain a new equilibrium at flow-velocity and stream-power levels higher than before the channelization. Numerous disadvantages of the applied regulation scheme and its failure to reduce flood hazard raise the question of its maintenance. To be successful, any regulation design must take into account changes in sediment supply and flood hydrographs resulting from the simultaneous alterations in basin management.     

A simple engineering model for the seismic site response of alluvial valleys

The seismic site response of alluvial valleys with limited width is studied. The intent of this investigation is to integrate the seismological and engineering perspectives to gain physical insight into the dynamic behaviour of alluvial valleys. A simplified engineering model (frame model) is developed to predict the nonlinear seismic response of symmetrical valleys. The proposed model is one-dimensional and accounts for the limited width of the valley. The frame model identifies the significant vibration modes and their variation in the horizontal and vertical directions. Sensitivity analyses are performed on the valley response to evaluate the effect of the uncertainty in establishing the dynamic soil properties. Response results from the frame model are compared with those calculated using one-dimensional and two-dimensional finite-element models. The proposed frame model response shows good agreement with finite-element model results.     

The response of headwater stream channels to urbanization in the humid tropics

Analysis of the bankfull cross-sections of headwater streams in Ado-Ekiti region of Southwestern Nigeria and their comparison with data from other tropical environments and temperate latitudes reveal that the channel capacities of streams in the humid tropics are relatively smaller than those of temperate regions, averaging 1.51 m² with a coefficient of variation of 87 per cent. This is attributed to the small stream discharge, the predominantly low and highly seasonal flows of the streams, the low shear stress of stream load, and the stabilizing and protective influence of riparian vegetation and surface incrustations. The chanel capacities of the urban streams (mean = 1.13m²) are about 47 per cent smaller than those of the natural streams (mean = 2.12 m²) in the same ecological zone. In terms of hydraulic efficiency, the urban streams also have relatively inefficient cross-sections and larger width/depth ratios than their rural or natural counterparts. Resurveys of seventeen monumented cross-sections reveal that while channel shoulder width increased by only 6 per cent over a one-year period, channel depth and capacity decreased by 16 per cent and 4 per cent respectively; the observed decrease in channel size occurs entirely in the channel depth dimension. Thus the response of stream channels to the urbanization of small headwater catchments in the humid tropics is probably more of vertical accretion of channel bed and reduction in channel capacity rather than the widely-reported anomalous enlargement of urban streams through channel widening. The rapid rate of channel aggradation is attributed to excessive rates of sediment production and delivery to streams in urbanized catchments in the humid tropics, rapid deposition of sediments during small runoff events and on the falling stage of storm hydrographs, and the inability of the streams to evacuate the sediments delivered to them despite the increased discharge and peak flow associated with urbanization. The low competence of the urban streams is attributed to the predominance of low flows, very gentle bed slopes, and most importantly the widespread dumping of refuse into the channels thereby reducing flow velocity and promoting backwater flooding, ponding, and sedimentation. The correlations between drainage basin area, a surrogate for stream discharge, and channel capacity are very strong for the rural watersheds, and the regression analysis indicates a tendency towards a steady-state isometric relationship. Urban channels are, to a large extent, in disequilibrium with the urban hydrological state. However, spatial variations in the degree of urbanization of the catchments, and, therefore in runoff volume and velocity, exercise strong control on channel width, depth, and size. A model of the sequence of stream channel adjustment to the urbanization of small headwater catchments in the humid tropics is presented.     

The effects of deforestation on slope and channel evolution in the tectonically active Darjeeling Himalaya

The main indicators of Quaternary tectonic uplift are the young mountain slopes of the Darjeeling Himalaya, rising straight above the Ganga–Brahmaputra foredeep, fragments of uplifted river terraces and fresh fault scarps. Evidence for the continuation of the uplift includes downcutting of the Tista and other straight rivers in the bedrock, continuing aggradation in the plains and overriding of the metamorphic rocks on the alluvia. Owing to deforestation and extensive land use, the earlier natural tendency of a dominance of channel incision over slope degradation has changed to prevailing aggradation, even in steep valley reaches, caused by intensive slope mass movements and the overloading of the mountain creeks. Aggradation progresses upstream along the rivers dissecting the mountain front.     

A stability criterion inherent in laws governing alluvial channel flow

The stability criterion of maximum flow efficiency (MFE) has previously been found inherent in typical alluvial channel flow relationships, and this study investigates the general nature of this criterion using a wider range of flow resistance and bedload transport formulae. For straight alluvial channels, in which the effect of sediment sorting is insignificant, our detailed mathematical analysis demonstrates that a flow efficiency factor ε occurs generally as the ratio of sediment (bedload) discharge Q_s to stream power Ω (γQS) in the form of . When ε is maximized (i.e. Q_s is maximized or Ω is minimized), maximally efficient straight channel geometries derived from most flow resistance and bedload transport formulae are found compatible with observed bankfull hydraulic geometry relations. This study provides support for the use of the criteria of MFE, maximum sediment transporting capacity and minimum stream power for understanding the operation of alluvial rivers, and also addresses limitations to the direct application of its findings. Copyright © 2002 John Wiley & Sons, Ltd.     

A cellular model of Holocene upland river basin and alluvial fan evolution

The CAESAR (Cellular Automaton Evolutionary Slope And River) model is used to simulate the Holocene development of a small upland catchment (4·2 km²) and the alluvial fan at its base. The model operates at a 3 m grid scale and simulates every flood over the last 9200 years, using a rainfall record reconstructed from peat bog wetness indices and land cover history derived from palynological sources. Model results show that the simulated catchment sediment discharge above the alluvial fan closely follows the climate signal, but with an increase in the amplitude of response after deforestation. The important effects of sediment storage and remobilization are shown, and findings suggest that soil creep rates may be an important control on long term (>1000 years) temperate catchment sediment yield. The simulated alluvial fan shows a complex and episodic behaviour, with frequent avulsions across the fan surface. However, there appears to be no clear link between fan response and climate or land use changes suggesting that Holocene alluvial fan dynamics may be the result of phases of sediment storage and remobilization, or instabilities and thresholds within the fan itself. Copyright © 2002 John Wiley & Sons, Ltd.     

Turbulent mechanisms in open channel sediment-laden flows

The effects of turbulence on water-sediment mixtures is a critical issue in studying sediment-laden flows. The sediment concentrations and particle inertia play a significant role in the effects of turbulence on mixtures. A two-phase mixture turbulence model was applied to investigate the turbulence mechanisms affecting sediment-laden flows. The two-phase mixture turbulence model takes into account the complicated mechanisms arising from interphase transfer of turbulent kinetic energy, particle collisions, and stratification. The turbulence in sediment-laden flows is the result of the interaction of four factors, i.e. the production, dissipation, diffusion, and inter-phase transfer of turbulent kinetic energy of mixtures. The turbulence production and dissipation are two dominant processes which balance the turbulent kinetic energy of mixtures. The turbulence production represents turbulence intensity, while the inter-phase transfer of turbulent kinetic energy denotes the effect of particles on the turbulence of sediment-laden flows. Although, the magnitude of the inter-phase interaction term is much less than that of the turbulence production and dissipation terms, due to an approximate local balance between production and dissipation of the turbulent kinetic energy, even the small order of the inter-phase interaction has a significant impact on the turbulent balance of sediment-laden flows. The presence of particles plays a duel role in the turbulence dissipation of mixtures: both promotion and suppression. An important parameter used to determine the turbulent viscosity of mixtures, which is constant in clear water, is the function of the sediment concentration and particle inertia in sediment-laden flows.     

A study of channel changes in a reach of the North Nashwaaksis stream,New Brunswick,Canada

Repeated surveys of a short channelized reach of the North Nashwaaksis Stream near Fredericton, N.B. over the period 1971-1981 provide the basis for evaluating the type and magnitude of some physical and botanical changes in the study reach. In 1967 a 200 m reach of the stream was shortened by about 10 per cent by cutting a new channel through a pasture. The original channel was protected by a dense growth of shrubs along its banks. The average width of the top of the channel in the channelized reach widened from 11 m to 17 m in response to a major flood in 1973. Rough estimates indicate that as much as 78 per cent of the published suspended-sediment load associated with the flood could reasonably have been derived from the banks in this short channelized reach. During the 1973 flood, a mid-channel bar was formed in the portion of the channel which experienced the most widening. Subsequent to the flood, the mid-channel bar was first colonized by sedges, and then by alders. By 1981, the width of the low flow channel in the channelized reach was about the same as that for the unmodified upstream reaches. This case study provides useful information for those planning channelization projects on small to intermediate size streams in humid temperate areas.     

A Bayesian hierarchical age model for optical dating of single grains of quartz

In optical dating, the last time that a sample of sediment was exposed to sunlight is determined by dividing its equivalent dose (D_e) by the dose rate. For single-grain dating, the sample D_e is based on the statistical analysis of the distribution of D_e values estimated for individual grains, whereas the dose rate is usually determined from measurements of the environmental radioactivity of the bulk sample, together with allowances for radiation sources internal to the grains and cosmic rays. Conventionally, the D_e and dose rate are measured and analysed separately to produce an estimate of the depositional age of a sample, but this approach may result in loss of information because distributions of single-grain D_e values are influenced by several factors. Existing statistical models do not incorporate all the key information contributing to age estimation, such as the pattern and scale of dispersion of single-grain D_e values and dose rates, the associated measurement uncertainties, the effect of natural variability among grains, and the outlier probabilities of D_e and dose rate estimates. Here we propose an empirical Bayesian hierarchical age model (BHAM) for optical dating of quartz samples that incorporates the above information to estimate their depositional ages. The BHAM is based on the implementation of standardised growth curve and L_nT_n methods to integrate information on the full distribution of single-grain D_e values, sources of measurement uncertainty, beta-dose heterogeneity (observed or modelled), and detection of outliers. We present the results of validation tests using data sets of optically stimulated luminescence measurements and dose rates obtained for quartz samples dated previously from Denisova Cave (Russia) and for simulated samples. We conclude that the BHAM represents a robust and flexible approach to dealing with data for single grains of quartz within a Bayesian hierarchical framework and is suitable for application to sediments deposited in a variety of depositional settings.     

Recent channel adjustments in alluvial rivers of Tuscany,central Italy

Drastic channel adjustments have affected the main alluvial rivers of Tuscany (central Italy) during the 20th century. Bed‐level adjustments were identified both by comparing available topographic longitudinal profiles of different years and through field observations. Changes in channel width were investigated by comparing available aerial photographs (1954 and 1993–98). Bed incision represents the dominant type of vertical adjustment, and is generalized along all the fluvial systems investigated. The Arno River system is the most affected by bed‐level lowering (up to 9 m), whereas lower incision (generally less than 2 m) is observed along the rivers of the southern part of the region. Human disturbances appear to be the dominant factors of adjustments: the main phase of vertical change occurred during the period 1945–80, in concomitance with the phase of maximum sediment mining activity at the regional scale. The second dominant type of adjustment that involved most of the rivers in the region consists of a narrowing of the active channel. Based on measurements of channel width conducted on aerial photographs, 38% of the reaches analysed experienced a narrowing greater than 50% of the initial channel width. The largest values of channel narrowing were observed along initially braided or sinuous with alternate bars morphologies in the southern portion of the region. A regional scheme of channel adjustments is derived, based on initial channel morphology and on the amounts of incision and narrowing. Different styles of channel adjustments are described. Rivers that were originally sinuous with alternate bars to braided generally became adjusted by a moderate incision and a moderate to intense narrowing; in contrast, sinuous‐meandering channels mainly adjusted vertically, with a minor amount of narrowing. Copyright © 2003 John Wiley & Sons, Ltd.     

Dynamics of bedload transport in Turkey brook,a coarse-grained alluvial channel

Automatic and continuously recording samplers are deployed in a Hertfordshire gravel-bed stream to show that bedload transport is related to stream power. The pattern is similar to that already established for North American channels but, because the record is so detailed, it is possible to identify the cause of the considerable scatter that is normal in such relationships. A major factor is the occurrence of rhythmic pulses in bedload discharge that are not matched by similar fluctuations in hydraulic variables. It is suggested that these pulses reflect downstream differences in the concentration of mobile particles in a slow-moving traction carpet, and that they may be likened to kinematic waves. The record also reveals that the threshold of sediment transport—always presumed hithero to be associated with incipient motion—is related to the cessation of bedload transport in a river flood. Indeed, the mean value of stream power at the finish of bedload transport is only 20 percent of that prevailing at the moment of incipient sediment motion. Because of this, there is an inevitably poor correlation between actual bedload transport rates and those predicted by bedload equations which rely upon a single traction threshold. These new data show that the general inverse relationship between bedload discharge and water-depth : grain-size ratio proposed by Bagnold (1977, 1980) is not universal. Transport efficiency for this gravel-bed stream is typically 0.05 per cent of available stream power, which compares with 1.6 per cent for a river moving both gravel and sand, and 5 per cent for another channel where bedload is composed predominantly of sand-sized particles. It is argued that coarse and fine-grained alluvial channels may need to be considered separately. By allowing for differences in traction threshold at the beginning and end of bedload events, and by averaging bedload discharge flood by flood in order to smooth out the effect of pulses, it is possible to achieve a reasonably good prediction of average bedload transport rate in terms of stream power.     

A damage model for structures with degrading response

The paper presents a hysteretic damage model for the response simulation of structural components with strength and stiffness deterioration under cyclic loading. The model is based on 1D continuum damage mechanics and relates any 2 work‐conjugate response variables such as force‐displacement, moment‐rotation, or stress‐strain. The strength and stiffness deterioration is described by a continuous damage variable. The formulation uses a criterion based on the hysteretic energy and the maximum or minimum deformation for damage initiation with a cumulative probability distribution function for the damage evolution. A series of structural component response simulations showcase the ability of the model to describe different types of hysteretic behavior. The relation of the model's damage variable to the Park‐Ang damage index is also discussed. Because of its consistent and numerically robust formulation, the model is suitable for the large‐scale seismic response simulation of structural systems with strength and stiffness deterioration.     

Improved simulation of river water and groundwater exchange in an alluvial plain using the SWAT model

Hydrological interaction between surface and subsurface water systems has a significant impact on water quality, ecosystems and biogeochemistry cycling of both systems. Distributed models have been developed to simulate this function, but they require detailed spatial inputs and extensive computation time. The soil and water assessment tool (SWAT) model is a semi‐distributed model that has been successfully applied around the world. However, it has not been able to simulate the two‐way exchanges between surface water and groundwater. In this study, the SWAT‐landscape unit (LU) model – based on a catena method that routes flow across three LUs (the divide, the hillslope and the valley) – was modified and applied in the floodplain of the Garonne River. The modified model was called SWAT‐LUD. Darcy's equation was applied to simulate groundwater flow. The algorithm for surface water‐level simulation during flooding periods was modified, and the influence of flooding on groundwater levels was added to the model. Chloride was chosen as a conservative tracer to test simulated water exchanges. The simulated water exchange quantity from SWAT‐LUD was compared with the output of a two‐dimensional distributed model, surface–subsurface water exchange model. The results showed that simulated groundwater levels in the LU adjoining the river matched the observed data very well. Additionally, SWAT‐LUD model was able to reflect the actual water exchange between the river and the aquifer. It showed that river water discharge has a significant influence on the surface–groundwater exchanges. The main water flow direction in the river/groundwater interface was from groundwater to river; water that flowed in this direction accounted for 65% of the total exchanged water volume. The water mixing occurs mainly during high hydraulic periods. Flooded water was important for the surface–subsurface water exchange process; it accounted for 69% of total water that flowed from the river to the aquifer. The new module also provides the option of simulating pollution transfer occurring at the river/groundwater interface at the catchment scale. Copyright © 2015 John Wiley & Sons, Ltd.     

Dynamic characterization of the migration of a mining pit in an alluvial channel

Research on in-channel sand mining is imperative as it may have a significant impact on channel morphology. Following this quest to quantitatively comprehend the phenomenon, experimental studies were done to investigate the dynamic characteristics of the migration of a mining pit. The evaluation of the migration rate of a mining pit in a physical scale model has found a rise in the migration rate of the pit's upstream edge with increasing discharge. A wavelet analysis applied for analyzing scale-dependent migration of the bed profile of a mining pit also revealed similar findings. Additionally, the wavelet analysis examined the length-scale dependent migration of a mining pit and a decrease in the migration rate has been observed with an increase in the length scale. The plan form of a pit (length-to-width ratio) governs the erosion and deposition processes around the pit. Both physical and statistical approaches show an increase in the migration rate with an increase in the length-to-width ratio of the pit. An empirical formulation has been developed for calculating the migration rate of the upstream edge of a mining pit based on pit geometry (length-to-width ratio), average flow velocity, and critical shear stress of the bed material. The results also show a higher bed load transport rate in the channel subjected to mining as compared to a plain bed channel.     

Bedload transport and temporal variation of non-uniform sediment in a seepage-affected alluvial channel

Understanding bedload transport fluctuations in rivers is crucial for complementing the existing knowledge on sediment transport theory. In this contribution, we use a natural-scale laboratory flume to analyse bedload transport fluctuations in non-uniform sand under normal flow conditions. Based on the significance of downward seepage, we incorporate the seepage effect on bedload transport over a non-uniform sand bed channel. The weight of the dry material was measured, and the volumetric transport rate per unit width (bedload transport rate) was estimated. An important observation is that the bedload transport rate initially rapidly increases with time and reaches a maximum value. Based on experimental data, we propose an empirical expression to estimate temporal bedload transport. In addition, an empirical model for bedload transport is proposed by incorporating downward seepage among other variables. The performance of several existing bedload transport formulae was also taken into account by the experimental datasets.     

The incidence and nature of bedload transport during flood flows in coarse-grained alluvial channels

A continuous record reveals that the incidence of bedload in a coarse-grained river channel changes from flood to flood. Long periods of inactivity encourage the channel bed to consolidate sufficiently so that bedload is largely confined to the recession limb of the next flood-wave. But when floods follow each other closely, the bed material is comparatively loose and offers less resistance to entrainment. In this case, substantial amounts of bedload are generated on the rising limb. This is confirmed by values of bed shear stress or stream power at the threshold of initial motion which can be up to five times the overall mean in the case of isolated floods or those which are the first of the season. This produces a complicated relationship between flow parameters and bedload and explains some of the difficulties in establishing bedload rating curves for coarse-grained channels. Besides this, the threshold of initial motion is shown to occur at levels of bed shear stress three times those at the thresholds of final motion. This adds further confusion to attempts at developing predictive bedload equations and clearly indicates at least one reason why equations currently in use are unsatisfactory. Bedload is shown to be characterized by a series of pulses with a mean periodicity of 1.7 hours. In the absence of migrating bedforms, it is speculated that this well-documented pattern reflects the passage of kinematic waves of particles in a slow-moving traction carpet. The general pattern of bedload, including pulsations, is shown to occur more or less synchronously at different points across the stream channel.