Alluvial Channels Research Articles

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Book Reviews

Developing an expert system for predicting alluvial channel geometry using ANN

Cross section geometry of stable alluvial channels usually is estimated by simple inaccurate empirical equations, because of the complexity of the phenomena and unknown physical processes of regime channels. So, the main purpose of this study is to evaluate the potential of simulating regime channel treatments using artificial neural networks (ANNs). The process of training and testing of this new model is done using a set of available published filed data (371 data numbers). Several statistical and graphical criterions are used to check the accuracy of the model in comparison with previous empirical equations. The multilayer perceptron (MLP) artificial neural network was used to construct the simulation model based on the training data using back-propagation algorithm. The results show a considerably better performance of the ANN model over the available empirical or rational equations. The constructed ANN models can almost perfectly simulate the width, depth and slope of alluvial regime channels, which clearly describes the dominant geometrical parameters of alluvial rivers. The results demonstrate that the ANN can precisely simulate the regime channel geometry, while the empirical, regression or rational equations can’t do this. The presented methodology in this paper is a new approach in establishing alluvial regime channel relations and predicting cross section geometry of alluvial rivers also it can be used to design stable irrigation and water conveyance channels.

Radial Basis Function Network Based Design of Incipient Motion Condition of Alluvial Channels with Seepage

Radial Basis Function Network Based Design of Incipient Motion Condition of Alluvial Channels with SeepageIncipient motion is the critical condition at which bed particles begin to move. Existing relationships for incipient motion prediction do not consider the effect of seepage. Incipient motion design of an alluvial channel affected from seepage requires the information about five basic parameters, i.e., particle sized, water depthy, energy slopeSf, seepage velocityvsand average velocityu.As the process is extremely complex, getting deterministic or analytical form of process phenomena is too difficult. Data mining technique, which is particularly useful in modeling processes about which adequate knowledge of the physics is limited, is presented here as a tool complimentary to model the incipient motion condition of alluvial channel at seepage. This article describes the radial basis function (RBF) network to predict the seepage velocity vs and average velocityubased on experimental data of incipient condition. The prediction capability of model has been found satisfactory and methodology to use the model is also presented. It has been found that model predicts the phenomena very well. With the help of the RBF network, design curves have been presented for designing the alluvial channel when it is affected by seepage.

Changes in hydraulic geometry of the Hwang River below the Hapcheon Re-regulation Dam, South Korea

The changes in hydraulic geometry of the Hwang River downstream of Hapcheon Dam have been investigated since 1982. The study reach is 45 km long from the Hapcheon Re-regulation Dam to the confluence with the Nakdong River, in South Korea. The geographic information system (GIS) analysis of a time series of aerial photographs taken in 1982, 1993 and 2004 showed that the non-vegetated active channel width decreased an average of 152 m (47% width reduction since 1982). The average median bed material size increased from 1.07 mm in 1983 to 5.72 mm in 2003, and the bed slope of the reach decreased from 94 to 85 cm/km from 1983 to 2003. The analysis of aerial photographs and field surveys shows that the 2004 channel width was asymptotically approaching the estimated equilibrium width from the Julien and Wargadalam (Julien, P.Y. and Wargadalam, J., 1995. Alluvial channel geometry: theory and applications. Journal of Hydraulic Engineering, 121 (4), 312–325) hydraulic geometry equations. An exponential model is proposed and is in good agreement with field measurements. The sediment transport model GSTAR-1D shows that the thalweg elevation will reach equilibrium around 2013–2015. The thalweg elevation simulated for 2013 is expected to remain almost identical to the predictions for 2023. The bed elevation changes should affect primarily the 20 km reach immediately downstream of the re-regulation dam.

Metamodel-based design of alluvial channels at incipient motion subjected to seepage

The design of an alluvial channel affected by seepage requires information about five basic parameters: particle size, water depth, energy slope, seepage velocity, and average velocity. The conventional approach to predicting the incipient motion in an alluvial channel cannot be applied in the case of a channel affected by seepage. Metamodelling techniques are nowadays widely used in engineering design to simulate a complex system. Here, a metamodel is described which employs the radial-basis function (RBF) network to predict the seepage velocity and energy slope based on experimental data under incipient motion conditions. It was found that the model fits experimental data very well and provides predictions for the design. With the help of the metamodel generated by the RBF network, design curves based on the RBF metamodel are presented for use in designing an alluvial channel when it is affected by seepage. Citation Kumar, B., Sreenivasulu, G. & Ramakrishna Rao, A. (2010) Metamodel-based design of alluvial channels at incipient motion subjected to seepage. Hydrol. Sci. J. 55(3), 459–466.

Numerical modelling of non-equilibrium graded sediment transport in a curved open channel

The computer code FAST3D has been developed to calculate flow and sediment transport in open channels. In the code, the flow field is calculated by solving the full Reynolds-averaged Navier–Stokes equations with k–ε turbulence model; the bed-load transport is simulated with a non-equilibrium model containing an important parameter, the so-called non-equilibrium adaptation length, which characterizes the distance for sediment to adjust from a non-equilibrium state to an equilibrium state; the bed deformation is obtained from an overall mass-balance equation for sediment transport. The governing equations are solved numerically with a finite volume method on an adaptive, non-staggered grid. The former model assumed uniform bed material. In order to take into account the influence of grain-size distribution of the bed-surface on the evolution of the bed topography and consequently also on the flow field, a sediment transport module has been presently developed by the authors at the Institute of Hydraulic and Water Resources Engineering, Technische Universität München, Germany, for fractional sediment transport using a multiple layer model. This paper presents the numerical results for sediment sorting and the bed deformation in a curved alluvial channel under unsteady-flow conditions according to Yen and Lee (1995). The calculations were compared with data from laboratory measurements. Further, the sensitivity of the simulated results to the non-equilibrium adaptation length is investigated.

A new methodology for the quantitative visualization of coherent flow structures in alluvial channels using multibeam echo‐sounding (MBES)

In order to investigate the interactions between turbulence and suspended sediment transport in natural aqueous environments, we ideally require a technique that allows simultaneous measurement of fluid velocity and sediment concentration for the whole flow field. Here, we report on development of a methodology using the water column acoustic backscatter signal from a multibeam echo sounder to simultaneously quantify flow velocities and sediment concentrations. The application of this new technique is illustrated with reference to flow over the leeside of an alluvial sand dune, which allows, for the first time in a field study, quantitative visualization of large‐scale, whole flow field, turbulent coherent flow structures associated with the dune leeside that are responsible for suspending bed sediment. This methodology holds great potential for use in a wide range of aqueous geophysical flows.

A comparison of average rates of alluvial erosion between the south‐western and south‐eastern United States

AbstractAlluvial channel bed incision and bank widening have been reported in both the south‐western and south‐eastern US throughout the past century. Distinct regional differences in climate and landscape properties likely influence the rate of erosion. This study discusses regional differences in hydraulic driving forces and substrate resistance and tests the hypothesis that regional differences exist in average rates of channel incision, bank erosion, and knickpoint retreat. Specifically, we hypothesize that erosion rates are higher in south‐western US streams and reason that this is because of greater flood magnitudes and limited substrate resistance. A review of the literature documenting incision, bank erosion, and knickpoint retreat, however, indicates that intra‐regional differences are larger than inter‐regional differences and that average rates in the south‐western US are either statistically similar or less than the rates in parts of the south‐eastern US. This could either be a result of strong intra‐regional hydroclimatic and substrate variability or because average erosion rate may not be an appropriate metric for inter‐regional comparisons because of the variability between case studies associated with the field methods to measure erosion, duration of study period, and time since disturbance to the channel. Nevertheless, these findings provide a basis for future evaluations of the relative importance of different controls on driving and resisting forces in these and other landscapes characterized by rapid channel incision and arroyo formation. Copyright © 2010 John Wiley & Sons, Ltd.

Three-dimensional modeling of bank erosion and morphological changes in the Shishou bend of the middle Yangtze River

This paper presents a three-dimensional (3-D) numerical model to simulate morphological changes in alluvial channels due to bank erosion. A method for the simulation of bank erosion is established. This is incorporated into a 3-D mathematical model for turbulent flow and non-uniform, non-equilibrium sediment transport. This model is applied to simulate morphological changes in the Shishou bend of the middle Yangtze River in China, where serious bank erosions occurred during the last two decades. The double-layer sediment structure of the riverbank on the middle Yangtze River is taken into account in the bank erosion module. Both cohesive and non-cohesive bank material in the different layers are considered. The bank erosion module also includes other factors affecting the rate of bank erosion, such as the longitudinal length of failed bank, the thickness of each layer in the double-layer structure, and the erosion-resisting effect of cohesive material from the top layer of failed bank. A locally-adaptive grid system is proposed to efficiently simulate the lateral migration of alluvial channel due to bank erosion. The predictive capability of the 3-D model is examined by laboratory data. Simulated processes of bank erosion agree with field observations in the Shishou bend during the period of October 1996–October 1998, and the bank erosion module plays a significant role in simulating morphological changes of the study reach. In addition, the equivalent channel-forming discharge, which is defined as a constant discharge that can create the same amount of bank erosion in an alluvial channel as that created by natural runoff processes during the same period of time, is proposed to improve calculation efficiency for feasibility studies.

A model for unsteady sediment transport in degrading alluvial channels

The collection of data for unsteady/non-uniform sediment transport rates in a river below a reservoir, which arrests sediment from upstream, is very difficult. The establishment of a model for the determination of these sediment transport rates is equally difficult but important for the safety of a structure below the reservoirs. An indirect technique based on continuity equation for sediment transport has been used to obtain such data for unsteady/non-uniform flow conditions. Based on these data and dimensional analysis, a relationship for determination of sediment transport rate below high capacity reservoirs under unsteady-non-uniform conditions has been proposed which gave very high efficiency when checked with the available data for such conditions. As a consequence, a relationship for extent of degradation has also been established.

The Valley-Fill Deposits of the Kłodnica River (Southern Poland): Environmental Drivers of Facies Changes from the Late Vistulian Through the Holocene

Lithological analyses and radiocarbon dating were used to elucidate the patterns and controls of Late Quaternary valley floor development of the Kłodnica River, the Upper Odra Basin. The research results were discussed with data obtained from valleys of rivers draining piedmont basins and highlands of southern Poland. In consequence, five stages of morpho-sedimentary evolution of the Kłodnica valley were distinguished. In the Late Vistulian a large-scale deposition of channel alluvium took place in the conditions of high river discharges. This sedimentary style probably still existed in the Early Pre-Boreal as long as open grass communities survived in the Kłodnica catchment. The next phase, in the Late Pre-Boreal and Boreal, is characterized by a significant increase in accumulation rate of biochemical facies. The considerable restriction of minerogenic deposition was connected with widespread of forest and gradual limitation of the river discharges. The third stage began at the decline of the Boreal and was defined by decrease of accumulation rate or even biogenic accumulation break. Synchronously, periodic increases of fluvial activity were noticed in the form of cutoffs of meander loops and overbank deposition in oxbows. The beginning of the fourth period took place not earlier than in the Early Sub-Boreal. This stage was distinguished by renewed peat growth/increase in biochemical accumulation rate and periodic increase in alluviation, generally taking place in the conditions of low channel-forming flows. The latest phase (from the Middle Sub-Atlantic till now) is characterized by common initiation of slope deposition and a rapid increase in fluvial sedimentation, especially overbank and tributary fan facies. An increase in minerogenic deposition occurred in response to human impact, which became more significant from the Roman Period and occurred on a large scale from the early Middle Ages. Older settlement phases, including intense settlement from the Hallstatt Period, were not clearly recorded in the Kłodnica valley fill.

Riparian indicators of flow frequency in a tropical montane stream network

Summary Many field indicators have been used to approximate the magnitude and frequency of flows in a variety of streams and rivers, yet due to a scarcity of long-term flow records in tropical mountain streams, little to no work has been done to establish such relationships between field features and the flow regime in these environments. Furthermore, the transition between the active channel of a river and the adjacent flood zone (i.e. bankfull) is an important geomorphologic and ecological boundary, but is rarely identifiable in steep mountain channels that lack alluvial flood plains. This study (a) quantifies relationships between field indicators and flow frequency in alluvial and steepland channels in a tropical mountain stream network and (b) identifies a reference active channel boundary in these channels, based on statistically defined combinations of riparian features, that corresponds to the same flow frequency of the bankfull stage and the effective discharge in adjacent alluvial channels. The relative elevation of transitions in riparian vegetation, soil, and substrate characteristics were first surveyed at nine stream gages in and around the Luquillo Experimental Forest in Northeastern Puerto Rico. The corresponding discharge, flow frequency, and recurrence intervals associated with these features was then determined from long-term 15-min discharge records and a partial duration series analysis. Survey data indicate that mosses and short grasses dominate at a stage often inundated by sub-effective flows. Herbaceous vegetation is associated with intermediate discharges that correspond to the threshold for sediment mobilization. Near-channel woody shrubs and trees establish at elevations along the channel margin inundated by a less frequent discharge that is coincident with the effective discharge of bed load sediment transport. Our data demonstrate that in alluvial channels in the study, both the bankfull stage (as marked by a flood plain) and the channel-forming (effective) discharge are associated with the presence of fine-grained substrate and soil, and tall, mature woody vegetation. In montane reaches that lack a flood plain, a boundary that is characterized by the incipient presence of soil, woody shrubs, and trees corresponds to the same flow frequency as the bankfull discharge of nearby alluvial channels. The reference discharge based on these riparian features in steepland sites has an average exceedance probability between 0.09% and 0.30%, and a recurrence interval between 40 and 90 days. We conclude that flows with similar frequencies influence the establishment of riparian vegetation, soil development, and substrate characteristics along channel margins in similar ways. Thus, these riparian features can be used as an indicator of hydrogeomorphic site conditions to identify active channel boundaries that occur at a constant flow frequency throughout the study stream network.

DEVELOPMENT OF ANALYTICAL MODEL ON DETERMINATION OF STABLE HYDRAULIC GEOMETRY OF ALLUVIAL GRAVEL BED RIVERS

Modeling the hydraulic geometry of Gravel Rivers is of intrest to hydraulic engineers, hydrologists and enviromental scientists who are engaged in predicting the adjustments of an alluvial channel to an altered upstream runoff or sediment supply. The widely used qualitative and regime models, while indicating the general direction of river adjustment, are not able to deal with river response at a quantitative level. On the other hand the existing analytical models do not predict channel geometry to the accuracy required for engineering applications. In this paper a bank stability analysis is incorporated into an analytical procedure for modeling the hydraulic geometry of an alluvial gravel-bed channel. The bank stability criterion accounts for the increased stability of the channel banks due to effect of the bank vegetation and binding of the sediment by root masses. The two key parameters in the bank stability analysis are the median grain diameter of the bank sediment, D50bank and the modified friction angle of the bank sediment, φ΄. A sensitivity analysis of the two parameters indicate that the bank stability can exert a larg influence on the channel geometry. This is supported by testing the theory on published field data.The bank sediment size was assumed to equal the bed sediment size, and the variation of the friction angle, φ΄ ,was investigated. The estimated φ΄ values are seen to increase systematically with the bank vegetation density. The results indicate that increased bank stability induced by the vegetation has a significant influence on channel geometry. The vegetated channels are narrower, deeper and less steep. For well developed bank vegetation, the channel width was found 0.6 time its unvegetated channel width.

Physically based modeling of bedrock incision by abrasion, plucking, and macroabrasion

Many important insights into the dynamic coupling among climate, erosion, and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision. However, many questions regarding incision processes and morphology of bedrock streams still remain unanswered. A more mechanistically based incision model is needed as a component to study landscape evolution. Major bedrock incision processes include (among other mechanisms) abrasion by bed load, plucking, and macroabrasion (a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts). The purpose of this paper is to develop a physically based model of bedrock incision that includes all three processes mentioned above. To build the model, we start by developing a theory of abrasion, plucking, and macroabrasion mechanisms. We then incorporate hydrology, the evaluation of boundary shear stress, capacity transport, an entrainment relation for pluckable particles, a routing model linking in‐stream sediment and hillslopes, a formulation for alluvial channel coverage, a channel width relation, Hack's law, and Exner equation into the model so that we can simulate the evolution of bedrock channels. The model successfully simulates various features of bed elevation profiles of natural bedrock rivers under a variety of input or boundary conditions. The results also illustrate that knickpoints found in bedrock rivers may be autogenic in addition to being driven by base level fall and lithologic changes. This supports the concept that bedrock incision by knickpoint migration may be an integral part of normal incision processes. The model is expected to improve the current understanding of the linkage among physically meaningful input parameters, the physics of incision process, and morphological changes in bedrock streams.

Metamodeling approach to predict friction factor of alluvial channel

The flow resistance of an alluvial channel flow is not only affected by the Reynolds number and the roughness conditions but also the Froude number. Froude number is the most basic parameter in the case of the alluvial channel, thus effect of Froude number on resistance to flow should be considered in the formulation of the friction factor, which is not in the case of present available resistance equations. At present, no generally acceptable quantitative description of the effects of the Froude number on hydraulic resistance has been developed. Metamodeling technique, which is particularly useful in modeling a complex processes or where knowledge of the physics is limited, is presented as a tool complimentary to modeling friction factor in alluvial channels. Present work uses, a radial basis metamodel, which is a type of neural network modeling, to find the effect of Froude number on the flow resistance. Based on the experimental data taken from different sources, it has been found that the predicting capability of the present model is on acceptable level. Present work also tries in formulating an empirical equation for resistance in alluvial channel comprising all the three major parameters, namely, roughness parameter, Froude number and Reynolds number.

On the stable geometry of self-formed alluvial channels: theory and practical applicationThis article is one of a selection of papers in this Special Issue in honour of Professor M. Selim Yalin (1925–2007).

On the basis of previous work by the late Professor M. Selim Yalin and the author, the process of self-formation of alluvial streams and the final (equilibrium or regime) geometry of the self-formed stream are considered in the light of thermodynamic principles, including the first and second laws, and the Gibb’s equation; the stream is treated as an isolated and irreversible system. The present analysis suggests that stream self-formation is guided by the need of the stream to progressively decrease its average flow velocity to accommodate the increase in the entropy of the system with the passage of time. The reduction in flow velocity is achieved by an appropriate alteration of stream slope, cross-sectional geometry, and effective roughness, the regime development being the process of this appropriate alteration. A method is presented for the computation of regime width, depth, and slope. The method rests on the channel formation criterion derived from thermodynamic principles and the expression of regime flow width determined on the basis of zero net cross sediment transport rate at the regime state. The regime channels computed from this method are compared with field and laboratory data from various sources.

From flumes to rivers: Can sediment transport in natural alluvial channels be predicted from observations at the laboratory scale?

Doubt regarding the applicability of laboratory results to alluvial streams has led some to develop sediment transport predictors based solely on field data, and most current sediment transport formulae have typically been calibrated at least partially on field data. This paper examines the transferability of flume results to the field by exploring the extent to which a unified approach to the prediction of (1) flow regime, (2) depth, and (3) total sediment transport can be developed entirely with laboratory data. Relevance vector machine (RVM)‐based probabilistic models were constructed with only laboratory data, and their performances were tested against field data and found to be comparable with or better than currently available methods. Comparison of a laboratory‐trained RVM with a field‐trained RVM suggests that the prediction performances of the two models for unseen field data are not statistically different given the prediction uncertainty. For transferability, the choice of predictor variables is important with successful predictors being characterized by similar probability distribution in the laboratory and field data, e.g., as quantified by the Kullback‐Leibler divergence.

Regional and local controls on the spatial distribution of bedrock reaches in the Upper Guadalupe River, Texas

While studies on gravel mantled and mixed alluvial bedrock rivers have increased in recent decades, few field studies have focused on spatial distributions of bedrock and alluvial reaches and differences between reach types. The objective of this work is to identify the spatial distribution of alluvial and bedrock reaches in the Upper Guadalupe River. We compare reach length, channel and floodplain width, sinuosity, bar length and spacing, bar surface grain size, and slope in alluvial and bedrock reaches to identify whether major differences exist between channel reach types. We find that local disturbances, interaction of the channel and valley sides, variation in lithology, and regional structural control contribute to the distribution of bedrock reaches in the largely alluvial channel. Alluvial and bedrock channel reaches in the Upper Guadalupe River are similar, particularly with respect to the distribution of gravel bars, surface grain size distributions of bars, and channel slope and width. Our observations suggest that the fluvial system has adjusted to changes in base level associated with the Balcones Escarpment Fault Zone by phased incision into alluvial sediment and the underlying bedrock, essentially shifting from a fully alluvial river to a mixed alluvial bedrock river.

Form and growth of bars in a wandering gravel‐bed river

AbstractThe changing form of developing alluvial river bars has rarely been studied in the field, especially in the context of the fixed, compound, mainly alternate gravel bars that are the major morphological feature of the wandering style. Century scale patterns of three‐dimensional growth and development, and the consequent scaling relations of such bars, are examined along the gravel‐bed reach of lower Fraser River, British Columbia, Canada. A retrospective view based on maps and aerial photographs obtained through the twentieth century shows that individual bars have a life history of about 100 years, except in certain, protected positions. A newly formed gravel bar quickly assumes its ultimate thickness and relatively quickly approaches its equilibrium length. Growth continues mainly by lateral accretion of unit bars, consistent with the lateral style of instability of the river. Bar growth is therefore allometric. Mature bars approach equilibrium dimensions and volume that scale with the overall size of the channel. Accordingly, the bars conform with several published criteria for the ultimate dimensions of alternate barforms. Sand bars, observed farther downstream, have notably different morphology. Fraser River presents a typical wandering channel planform, exhibiting elements of both meandered and low‐order braided channels. Hydraulic criteria to which the Fraser bars conform illustrate why this planform develops and persists. The modest rate of bed material transfer along the channel – typical of the wandering type – determines a century‐length time scale for bar development. This time scale is consistent with estimates that have been made for change of the macroform elements that determine the overall geometry of alluvial channels. Copyright © 2009 John Wiley & Sons, Ltd.

Colluvial sediment dynamics in mountain drainage basins

Colluvial sediment dynamics are examined using a 70-year landslide inventory in formerly glaciated mountain drainage basins of coastal British Columbia, Canada. Landslide sediment transfer is documented by identifying landslide types, and by characterizing preferential sites of landslide initiation, delivery, and storage across spatial scales. Data analysis reveals that open-slope landslides delivering material to seasonal or perennial channels and fluvial terraces are the dominant mechanisms of sediment transfer. This finding suggests high instability of the low-order channel network and its ongoing re-organization (degradation) after generalized sediment recharge occurred in the last glacial maximum. In the study period, landslide activity across the landscape has generated net degradation on planar slopes and first-order colluvial channels, whereas unchannelled valleys, higher-order colluvial channels and alluvial channels have accumulated material. The scaling relation of the landslide sediment yield appears to be controlled by the spatial arrangement of the relict glacial macro-forms. Landslide yield is highest in unchannelled topography, decreases at the scale of channel initiation ( A d ~ 0.002 km 2), and remains constant for drainage areas where length scales of cirque/valley walls and hanging valley floors overlap (0.002 < A d < 0.06). Injections of landslide material start declining consistently beyond areas larger than 0.6 km 2 (the scale of relict glacial trough initiation), where fluvial environments become increasingly disconnected from landslide inputs. Cumulative yield indicates that colluvial sediment redistribution is limited to small basins; specifically, 90% of the colluvial load is released at scales smaller than about 0.6 km 2.