Alluvial Channels Research Articles

Spatial–temporal fluvial morphology analysis in the Quelite river: It’s impact on communication systems

Summary During 2008 and 2009 heavy rainfall took place around the Mazatlan County in the Sinaloa state, Mexico, with a return period (Tr) between 50 and 100 years. As a result, the region and its infrastructure, such as the railways and highways (designed for a Tr = 20 years) were severely exposed to floods and, as a consequence damage caused by debris and sediments dragged into the channel. One of the highest levels of damage to the infrastructure was observed in the columns of Quelite River railway’s bridge. This is catastrophic as the railway is very important for trade within the state and also among other states in Mexico and in the USA. In order to understand the impact of the flooding and to avoid the rail system being damaged it is necessary to analyse how significant the changes in the river channel have been. This analysis looks at the definition of the main channel and its floodplain as a result of the sediment variability, not only at the bridge area, but also upstream and downstream. The Quelite River study considers the integration of Geographic Information Systems (GIS) and remote sensing data to map, recognise and assess the spatio-temporal change channel morphology. This increases the effectiveness of using different types of geospatial data with in situ measurements such as hydrological data. Thus, this paper is an assessment of a 20 years study period carried out using historical Landsat images and aerial photographs as well as recent Spot images. A Digital Elevation Model (DEM) of local topography and flow volumes were also used. The results show the Quelite River is an active river with a high suspended sediment load and migration of meanders associated to heavy rainfall. The river also has several deep alluvial floodplain channels which modified the geometry and other morphological characteristics of the channel in the downstream direction. After the identification of the channel changes, their causes and solutions to control, the channel migration and the dynamics structure, a river management plan was projected not only to protect the bridge but also to provide a flood risk awareness in order to reduce the social–economical impact during a flood event.

The structure of turbulent flow in an open channel bend of strong curvature with deformed bed: Insight provided by detached eddy simulation

Results of a detached eddy simulation (DES) are used to better understand the effects of the mean flow three‐dimensionality and secondary currents on turbulence and boundary shear stresses and the mechanisms through which the momentum and Reynolds stresses are redistributed in a strongly curved 193° bend with fixed deformed bed corresponding to the later stages of the erosion and sedimentation process. The ratio between the radius of curvature of the curved reach and the channel width is close to 1.3. The large channel curvature and the point bar induce flow separation near the inner bank and the formation of several strong separated shear layers (SSLs), where production by mean shear dominates. DES shows that in addition to the main cell of cross‐stream circulation developing in the deeper part of the bend, several streamwise‐oriented vortices (SOV) form at the inner bank. DES satisfactorily captures the distribution of the streamwise velocity and streamwise vorticity in relevant cross sections compared to experiment. Comparison with a Reynolds‐averaged Navier‐Stokes (RANS) simulation shows that DES predicts more accurately the velocity redistribution and cross‐stream motions in the channel. This is because RANS significantly underpredicts the circulation and turbulence amplification inside the cores of the SOV vortices. DES is then used to clarify the influence of the SOV vortices and SSLs on the boundary shear stress. DES reveals the presence of several regions of large amplification of the pressure RMS fluctuations near the inner and outer banks, which can locally increase the bed erosion and affect the bank stability in the case of a bend with erodible banks. The mean flow bed shear stress distribution predicted by DES is significantly different than that predicted by RANS, while DES predictions of the mean flow are more accurate. This means that use of eddy‐resolving techniques like DES in mobile bed simulations of flow in curved alluvial channels should result in more accurate predictions of bathymetry.

Experimental and numerical evidence for intrinsic nonmigrating bars in alluvial channels

Alternate bars in straight alluvial channels are migrating or nonmigrating. The currently accepted view is that they are nonmigrating if the width‐to‐depth ratio is at the value of resonance or if the bars are forced by a persistent local perturbation. We carried out 2‐D numerical computations and a long‐duration mobile‐bed flume experiment to investigate this view. We find that nonmigrating bars can also occur in straight channels without resonant width‐to‐depth ratio or steady local perturbation. They appear to be an intrinsic response of the alluvial river bed. This finding bears on explanations for meandering of alluvial rivers, for which nonmigrating bars are seen as a prerequisite. We find, however, that the intrinsic tendency of a straight channel to form meanders usually has a different origin. The identified intrinsic nonmigrating bars can only become the dominant mechanism for incipient meandering if the erodibility of the banks is very low.

1-D numerical simulation of sediment transport in alluvial channel beds

1-D numerical simulation of sediment transport in alluvial channel beds

Locations of channel heads in the semiarid Colorado Front Range, USA

The channel head, defined as the upstream boundary of concentrated water flow and sediment transport between definable banks, represents the transition from hillslope processes to fluvial processes. The ability to delineate the location along a slope at which channels initiate is important for understanding hydrologic and geomorphic processes governing headwater streams. Studies demonstrating an inverse relationship between either contributing drainage area (A) and local valley slope (θ) or basin length (L) and θ for channel heads come primarily from regions with humid climates. Seventy-eight channel heads were mapped in the headwaters of rivers in the semiarid Colorado Front Range. Multiple field sites were chosen to account for variability from aspect and elevation. Surface topographic parameters were measured in the field and analyzed to test the hypothesis that surface processes control channel initiation in this region. Although simple linear regressions indicate a poor inverse relationship between A and L and no relationship between L and θ, multiple regressions indicate that surface topographic parameters explain over half the variability in the location of channel heads. This suggests that surface processes exert an influence on channel initiation, but do not explain as much of the variability as observed in previous studies from wetter regions. A threshold of erosion necessary to initiate a channel was observed at ~10,000m2 for A, although values as high as 600,000m2 were mapped for some channel heads. Variation within the study area correlated with elevation, which is a proxy for differences in volume and type of precipitation; sites at lower elevation with less precipitation, but more intense convective rainfall, tend to have smaller contributing area and basin length. Aspect did not influence surface topographic parameters. We interpret the results to reflect surface and subsurface influences on channel initiation. Nearly 70% of the mapped channel heads coincide with constrictions created by weathered bedrock knobs, suggesting that local factors strongly influence channel initiation once the threshold for contributing area is exceeded. Field-mapped channel head locations plot at or downslope from the inflection point of a regional slope–area curve generated from 10m DEMs, although some extend well downslope into the region assumed to be dissected by alluvial channels. Most actual drainage areas for channel initiation are thus an order of magnitude larger, and plot in a significantly different portion of the slope–area graph, than would result from the widespread practice of assuming channel heads are located at the gradient reversal in such curves.

Geometry of sand-bed channels with seepage

Literature suggests that apart from a significant percentage of water loss from seepage, downward seepage from the channel causes an increase in the mobility of channel bed materials and thus changes the channel stability. Consequently, regime conditions (which provide the relationship among hydraulic parameters) should also be revised by incorporating downward seepage as an additional parameter. In the present work, regime conditions for the prediction of channel geometry in alluvial channels affected by downward seepage have been formulated on the basis of experimental observations.

Assessment of the Geomorphic Effects of Large Floods Using Streamgage Data: the 1951 Floods in Eastern Kansas, USA

Data from 23 U.S. Geological Survey (USGS) streamgages were analyzed to assess the geomorphic effects (short-term change and subsequent recovery) of the record 1951 floods on streams in eastern Kansas. Flood-related, channel-bed elevation change was indicated for 17 gage sites, with substantial deposition at five sites and substantial erosion at two sites. An assessment of post-flood bed elevation recovery was possible for several sites. While recovery to pre-flood channel-bed elevation occurred over a period of months to years at some sites, at other sites recovery was incomplete or absent. Floodrelated channel widening with partial recovery was indicated for one site and possible channel widening was indicated for two sites. It was demonstrated that an analysis of streamgage data is a potentially useful technique for assessing the geomorphic effects of a large flood at a site, provided that the gage has a long period of record and is located on an alluvial channel. In the absence of other lines of evidence, streamgage data can provide an estimate of the direction and magnitude (net) of geomorphic change that otherwise might not be available or attainable.

1-D numerical simulation of sediment transport in alluvial channel beds

ABSTRACT Quantitative estimate of sediment transport processes is important especially in river-control engineering and for water management projects. In this paper the sediment routing of a 1-D model is presented and applied to some study cases. The procedure simulates the variations of bed roughness conditions due to natural sorting and to generation and migration of bed forms. The suspended-load and the bed-load are treated separately.

CALIBRATION AND UNCERTAINTY ANALYSIS OF SWAT MODEL IN A JAPANESE RIVER CATCHMENT

Calibration and uncertainty analysis is necessary to perform the best estimation and uncertainty identification of hydrological models. This paper uses the Soil and Water Assessment Tool-Calibration and Uncertainly Procedures (SWAT-CUP) model to analyze the uncertainty of SWAT model in a Japanese river catchment. The GLUE and SUFI-2 techniques used in this analysis show quite good results with high value of R 2 as 0.98 and 0.95 for monthly simulation. Daily simulation results during calibration and validation are also good with R 2 as 0.86 and 0.80. For uncertainty results, the 95% prediction uncertainty (95PPU) brackets very well with the observation. The p-factors of uncertainty analysis for the calibration and validation periods are 92% and 94%. The calibration result by using GLUE shows better than that by using SUFI-2. However, the processing time of the GLUE approach is longer than SUFI-2 approach when they were run in the SWAT-CUP. The uncertainty analysis indicates that the parameters of effective hydraulic conductivity in main channel alluvium (CH_K2) and base-flow alpha factor for bank storage (ALPHA_BNK) play important roles for calibration and validation of SWAT model.

Effects of bed-load movement on flow resistance over bed forms

The effect of bed-load transport on flow resistance of alluvial channels with undulated bed was experimentally investigated. The experiments were carried out in a tilting flume 250mm wide and 12.5m long with glass-sides of rectangular cross-section and artificial dune shaped floor that was made from Plexi-glass. Steady flow of clear as against sediment-laden water with different flow depths and velocities were studied in the experiments with a fine sand (d 50 = 0.5mm). The results indicate that the transport of fine particles (d 50 = 0.5mm) can decrease the friction factor by 22% and 24% respectively for smooth and rough beds. Increasing the bed-load size (d 50 = 2.84 mm) can decrease the friction factor by 32% and 39% respectively for smooth and rough beds. The decrease in flow resistance is due to filling up of the troughs of dunes. This separation zone is responsible for increasing the flow resistance. On the upstream side of dunes condition is similar to plane bed. Presence of bed-load causes to increase the shear velocity and hence increasing flow resistance. But decreasing in flow resistance is more and it causes to decrease the total flow resistance. Grains saturated the troughs in the bed topography, effectively helping in smoothening of bed irregularities.

Numerical simulation of channel pattern changes Part I: Mathematical model

This paper presents a computational fluid dynamics model for simulation of two- dimensional (2-D) water flow, sediment transport, bank failure processes, and the subsequent channel pattern changes. Effects of secondary currents at channel bends are included in the modified momentum conservation equation of water flow. An improved bank failure model is applied to calculate bank failure due to riverbed erosion, and to simulate lateral migration and planform changes of alluvial channels. The water flow model has been validated using laboratory measurements of flow in consecutive bends designed by the authors, in addition to flume test data from the literature.

Sediment transport constraints on river response to regulation

We demonstrate that alluvial channel response to regulation depends on the characteristic mode of sediment transport in a channel and, thus, for a channel in supply-transport equilibrium, on the caliber of sediment mantling the channel bed, as well as changes in characteristic water discharge and sediment load. We find that regulated, coarse gravel- and cobble-bed alluvial rivers dominated by bedload transport are predicted and observed to preserve the value of a dimensionless ratio of characteristic water discharge, channel slope, width, and size of sedimentary particles mantling the bed associated with the critical conditions required for the onset of significant sediment transport. The covariant geomorphic adjustment of such rivers to regulation is independent of the degree of sediment impoundment, although the nature and rate of changes in any specific characteristic appear to be sensitive to changes in sediment supply. In contrast, covariant geomorphic changes in sand- and fine gravel-bed alluvial rivers dominated by weak suspension transport are predicted and observed to be dependent on the degree of sediment impoundment and commensurate with the change in the sediment concentration of characteristic, formative flows. In both cases, specific geomorphic characteristics of alluvial rivers exhibit varied responses to regulation in different rivers, but across all rivers a systematic covariation in channel slope, width, and grain size is observed that, for a given mode of sediment transport, preserves a characteristic value of the Shields number.

Reduced‐complexity modeling of free bar morphodynamics in alluvial channels

Reduced‐complexity models of river behavior that neglect much of the physics governing fluvial processes have become increasingly popular in recent years. However, previous studies have demonstrated that channel morphology simulated by such models can be both unrealistic and highly sensitive to model grid resolution. A new reduced‐complexity model is presented here and applied to simulate the development and migration of free alternate bars in straight channels. This model incorporates a simple new treatment of lateral flow redistribution driven by topographic steering but neglects the role of momentum conservation, secondary circulation, and factors such as spatial variability in bed roughness. This approach is shown to replicate many of the characteristics of alternate bars observed in individual laboratory experiments conducted by Lanzoni (2000) and to produce results that are in agreement with more generic relationships evident in a larger experimental data set presented previously by Ikeda (1984). Moreover, model results are shown to be independent of grid resolution and are largely consistent with those obtained using more sophisticated models based on the depth‐averaged form of the Navier‐Stokes equations. There remains a need to evaluate and refine model process parameterizations for a wider range of conditions (including those in natural channels) and to examine model behavior in situations involving variable channel width and curvature. However, the results presented here provide the first evidence that such reduced‐complexity models may be able to simulate the vertical and streamwise scaling of free bars in alluvial channels and represent the morphodynamic feedbacks that control their temporal evolution.

Controls on late Quaternary incised‐valley dimension along passive margins evaluated using empirical data

AbstractIncised valleys are canyon‐like features that initially form near the highstand shoreline and evolve over geological time as rivers erode into coastal plains and continental shelves to maintain equilibrium‐gradient profiles in response to sea‐level fall. Most of these valleys flood during sea‐level rise to form estuaries. Incised‐valley morphology strongly controls the rate of creation of sediment accommodation, valley‐fill facies architecture and the preservation potential of coastal lithosomes on continental shelves, and affects coastal physical processes. Nonetheless, little is known about what dictates incised‐valley size and shape and whether these metrics can be used to explain principal formation processes. The main control on alluvial channel morphology over human time scales is discharge; this is based on numerous empirical studies and is well‐constrained because all variables are easily measured at this short time scale. Knowledge of long‐term river evolution over a complete glacio‐eustatic cycle, on the contrary, remains largely conceptual, experimental and based on individual systems because variables that are thought to drive morphological change are not easily quantified. In spite of this difficulty, existing models of incised‐valley formation at the coast suggest that valley evolution is driven largely by downstream forcing mechanisms, highlighting sea‐level and shelf gradient/morphology as the dominant controls on valley incision. Although valleys are cut by rivers, whose channels are a direct reflection of discharge, little empirical data exist in coastal areas to address the degree to which valley evolution is governed by upstream controls. The late Quaternary is the best time period to examine because it provides the most complete sedimentary record and many variables, including sea‐level, tectonics, substrate lithology and drainage network characteristics, are accurately constrained. Here, 38 late Quaternary valleys along the coast of two different passive continental margins are compared, which suggests that valley shape and size are governed primarily by upstream, intrinsic controls such as discharge. Valley width, depth and cross‐sectional area are found to be predictable at the highstand shoreline and are scaled with the size of their drainage basin, which has important implications for estimating sediment discharge to continental shelves and deep water environments during periods of low sea‐level.

Coarse sediment transport in a bedrock channel with complex bed topography

Independent lithologic and structural controls in fluvial bedrock systems interact with coarse sediment transport processes to play a key role in bedrock incision processes such as abrasion. During a 3 year study on the Ocoee River in the Blue Ridge Province of the southern Appalachians, USA, we used painted tracer clasts to measure coarse sediment transport dynamics and address whether different scales of morphologic variability in bedrock channels lead to coarse sediment transport processes that differ from alluvial channels. At the reach scale, folded metasedimentary units are exposed in the channel bed and appear as linear bedrock ribs that vary in amplitude and orientation to flow. Under similar flow conditions for which size‐dependent transport has been observed in alluvial channels (dimensionless Shields stress within 1.5–2.0 times the dimensionless critical shear stress), transport distance was a significant function of grain size where bedrock ribs were longitudinal to flow (Reach 1 and Reach 2). However, transport distance was not size dependent where bedrock ribs were oblique to flow (Reach 3). At different intrareach scales, the variables characterizing the local bedrock topography and sediment architecture were the best predictors of transport distance in all three reaches. Therefore, bed load transport processes may be influenced by bed forms in bedrock streams (bedrock ribs) but at potentially smaller scales than bed forms in alluvial channels. The strong influence of bedrock ribs on coarse sediment transport suggested that coarse sediment transport processes are controlled by different factors in bedrock channels only when bedrock ribs cross the channel at a high angle to the flow.

Simulating Transient Sediment Waves in Aggraded Alluvial Channels by Double-Decomposition Method

By using the double-decomposition (DD) method, this study simulates transient sediment waves caused by aggradation described by a diffusion-type partial differential equation (PDE). The DD method solves the PDE by decomposing the solution function for sediment rate into a summation of M number of components, where M stands for the order of approximation. The solution was approximated by considering only the first three terms. The model satisfactorily simulated laboratory-measured aggradation bed profiles with, on average, a mean absolute error (MAE) of 0.70 cm, a root-mean-square error (RMSE) of 0.84 cm, a mean relative error (MRE) of 1.11%, and R2=0.95. The model performance was also tested by using numerical and error-function solutions. In addition, the results obtained from application of the DD solution to hypothetical field cases were found to be theoretically compatible with what may be observed in natural streams. However, sediment wave fronts in later periods of the simulation time reached equili...

Alluvial fan and wetland interactions: evidence of seasonal slope wetlands from the Silurian of south central Wales, UK

AbstractDistal environs of the Ludlow‐age Trichrug Formation from south central Wales (UK) detail deposition on the outer fringe of an ephemeral debris flow‐dominated alluvial fan. Debrites and subordinate sheetflood deposits are interbedded with sporadic thin sandstone‐dominated heterolithic units deposited in shallow, ephemeral ponds in the axial valley. The latter slope wetland system fringed the permeable alluvial fan deposits, being maintained by streamflow, precipitation and ground water recharge. A prolonged high water table is indicated by low chroma intervals interpreted as the result of gleying. A variety of redoximorphic indicators, including colour mottling and ferricrete concretions, are evidence of iron mobilization and concentration associated with a seasonally fluctuating water table. The slope wetlands were colonized periodically by Skolithos‐generating organisms, most likely to have been arthropods. The widespread occurrence of redoximorphic indicators and ferricrete contrasts markedly with calcrete Vertisols in penecontemporaneous continental deposits (Lower Old Red Sandstone) of the region. It is likely that the alluvial fan‐toe area was the site of a wetland depression, which maintained a high, though fluctuating, water table. In contrast the alluvial channel reaches of the surrounding drainage network were significantly better drained, with floodplains prone to wetting and drying and Vertisol palaeopedogenesis.

Lithological and fluvial controls on the geomorphology of tropical montane stream channels in Puerto Rico

AbstractAn extensive survey and topographic analysis of five watersheds draining the Luquillo Mountains in north‐eastern Puerto Rico was conducted to decouple the relative influences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels. The Luquillo Mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic influence on the stream channels. However, the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment. GIS‐based topographic analysis was used to examine channel profiles, and survey data were used to analyze downstream changes in channel geometry, grain sizes, stream power, and shear stresses. Results indicate that the longitudinal profiles are generally well graded but have concavities that reflect the influence of multiple rock types and colluvial‐alluvial transitions. Non‐fluvial processes, such as landslides, deliver coarse boulder‐sized sediment to the channels and may locally determine channel gradient and geometry. Median grain size is strongly related to drainage area and slope, and coarsens in the headwaters before fining in the downstream reaches; a pattern associated with a mid‐basin transition between colluvial and fluvial processes. Downstream hydraulic geometry relationships between discharge, width and velocity (although not depth) are well developed for all watersheds. Stream power displays a mid‐basin maximum in all basins, although the ratio of stream power to coarse grain size (indicative of hydraulic forcing) increases downstream. Excess dimensionless shear stress at bankfull flow wavers around the threshold for sediment mobility of the median grain size, and does not vary systematically with bankfull discharge; a common characteristic in self‐forming ‘threshold’ alluvial channels. The results suggest that although there is apparent bedrock and lithologic control on local reach‐scale channel morphology, strong fluvial forces acting over time have been sufficient to override boundary resistance and give rise to systematic basin‐scale patterns. Copyright © 2010 John Wiley and Sons, Ltd.

Reformulation of the bed load equation of Meyer‐Peter and Müller in light of the linearity theory for alluvial channel flow

Due to a lack of sufficient theoretical relations, equations for bed load transport in alluvial channel flow have been developed using a variety of empirical assumptions. A linear relationship exhibited in the complex interactions among the three essential components of a river (channel geometry, bed load transport and flow resistance) has led this study to develop a more accurate form of the Meyer‐Peter and Müller (1948, hereafter MPM) relationship, one of the world's most widely used bed load transport equations. This study demonstrates that the application of linearity theory can significantly improve its performance. On the basis of the dataset originally used by MPM, it is shown that the applicable form of the Manning‐Strickler flow resistance relationship can be very satisfactorily determined. The MPM equation is consequently found to be expressible more accurately as Φ = 6[ητb* − 0.047]5/3, where Φ, η and τ*b are the normalized, dimensionless bed load transport rate, bed‐form correction, and flow‐strength parameters, respectively.

Allochthonous native gold in piedmont alluvium in the southern West Siberia

The paper presents the results of our investigations carried out to examine the behavior of native gold in the southern West Siberia during the fluvial transport of an auriferous terrigenous material at the mountain-to-plain transition geomorphological zone. The piedmont alluvium accumulates the small- and, to a lesser extent, fine-grained gold transported from the denudation zones by the main mountain-lowland rivers. The background Au content varies from n to 10n mg/m3. High contents (100n mg/m3 to n g/m3) are recorded in some sectors in the upper zones of channel alluvium (spit and shoal facies). Flatness coefficient of particles is an important characteristic of allochthonous native gold in the piedmont alluvium. This property shows direct correlation with the lateral dimension of particles. Composition is an essential typomorphic property of the small- and fine-grained gold in the studied sediments. This statement is primarily valid for the degree of fineness commonly related to the Ag content in gold. In some cases, a significant indicator role can be played by traces of Hg and Cu. For example, Hg is an important indicator for the gold-mercury mineralization, whereas Cu is an essential indicator in the case of gold mineralization associated with rhodingites and high-temperature Cu-Au mineralization. Abundance of the allochthonous native gold in different-age terrigenous sediments testifies to its high stability during the continental sedimentation.