Alluvial Channels Research Articles

Coupling slope–area analysis, integral approach and statistic tests to steady-state bedrock river profile analysis

Abstract. Slope–area analysis and the integral approach have both been widely used in stream profile analysis. The former is better at identifying changes in concavity indices but produces stream power parameters with high uncertainties relative to the integral approach. The latter is much better for calculating channel steepness. Limited work has been done to couple the advantages of the two methods and to remedy such drawbacks. Here we show the merit of the log-transformed slope–area plot to determine changes in concavities and then to identify colluvial, bedrock and alluvial channels along river profiles. Via the integral approach, we obtain bedrock channel concavity and steepness with high precision. In addition, we run bivariant linear regression statistic tests for the two methods to examine and eliminate serially correlated residuals because they may bias both the estimated value and the precision of stream power parameters. We finally suggest that the coupled process, integrating the advantages of both slope–area analysis and the integral approach, can be a more robust and capable method for bedrock river profile analysis.

Multiscale characterization of migrating sand wave in mining induced alluvial channel

Sediment transport mechanism is highly influenced by the hydraulic condition of a river. Sand mining from river bed changes the geometry of the bed and influences the flow parameters, and thus can alter the hydraulic condition of a river. Our study is based on laboratory experiments for sand bed channel with a rectangular mining pit. We present both physical and dynamic characteristics of a mining induced channel from the bed elevation data obtained from Seatek instrument. Physical characteristics shows erosion in the entire channel as the flow in the channel is kept higher the flow required for sediment movement. Two distinct water surface slopes occur in the test section centering the mining pit. Shear stress along the test section is determined and observed higher value of downstream shear stress than upstream of the pit. Erosion at the downstream of the pit is more than the upstream and migration of upstream edge occurs. Dynamic analysis of sand wave depending on statistical approach is carried out to investigate the movement of sediment under the influence of mining. Celerity of sand wave increases with increase in flow velocity during the initial time span (1–4h) but in a later time interval (4–7h) it decreases with increase in flow velocity. It has been found that celerity of sediment initially increases with scale size and then decreases, which indicates a higher resistance is being offered by sediment at high scale.

Chute cut-off processes along a small alluvial channel: a case study of Sangra Khal, sub-tributary of Gour Nadi, West Bengal, India

Meandering cutoff is a natural feature of low land alluvial river. It is an inseparable part of it and it is two types, such as neck cutoff and chute cutoff. Among them chute cutoff appears to be complicated and poorly understand. Here, we have observed and tried to describe the mechanisms of chute cutoff within uniform floodplain topography and its locational factors of development. Most of the studies had put their greater emphasis over cutoff formation on larger rivers and they owe it to nick point generation, swale development and embayment formation resulting from overbank flow. Our insitu observations of several years indicate that small agricultural stream with higher return period of over bank flow showing that embayment, nick point formation, weathering, mass wasting and earthworm’s activity are working together to form a chute cutoff. Although their relative degree of dominance in chute formation varies depending on the nature of return period of over bank flow, nature of flood plain and landuse, discharge, vegetation cover, meander properties and channel bank properties.

Bed Load Pickup Rate and Flow Resistance for Turbid Flow on a Movable Plane Bed

This study investigates bed load transport and flow resistance in alluvial channels that contain fine suspended sediment. In all, 123 samples were run in a laboratory flume over uniform bed materials. Different hydraulic conditions were considered in an 8.0 m-long, 0.255 m-wide, and 0.6 m-deep tilting flume. Unit flow discharge ranged from 3.176 × 10−3 to 18.640 × 10−3 m2 s−1, and the slope gradient varied from 2% to 5%. Two sediment-size classes (median diameter, d50, of 1.7 and 3.29 mm) and five suspended load concentrations (10, 20, 30, 40, and 50 g/L) were used. A preliminary analysis showed that the bed load pickup rate increases with an increase in the concentration of suspended particles. Dimensional analysis, which was performed satisfactorily on the bed load transport data, indicated that shear stress is the dominant parameter in bed load pickup. Moreover, the resistance of turbid flow in a channel with an erodible bed was studied. The net results were found to be an increase in the flow resistance over a movable plain bed by increasing suspended load concentration. In other words, increasing suspended particles increased the mobility of bed material and led to a higher friction factor. Special attention was paid to the separation of the friction coefficient between boundary resistance and mobile-bed resistance caused by the mobility of bed materials. The results showed that the damping effect of suspended sediment caused lower boundary resistance with increasing suspended load concentration. On the other hand, suspended particles increased the mobility of bed materials and led to a higher mobile-bed resistance. Analysis indicates that the relative importance of mobile-bed resistance is more than the boundary friction for the high bed load transport rate. Therefore, the damping effect of the suspended sediment on boundary friction can only be observed for low sediment transport or a fixed bed.

Effect of downward seepage on the shape of an alluvial channel

Longitudinal and cross-sectional profiles of alluvial channels are altered by the presence of seepage. However, the available literature suggests that seepage has not been considered in analyses of bank profiles of threshold alluvial channels. In the experimental study reported here, downward seepage was applied to parabolic channels based on tractive force theory in order to determine its effect on the longitudinal and cross-sectional profiles. It was observed that sediment transport in the threshold channels increased when downward seepage was applied. The parabolic shape did not hold, but channels formed a new cross-sectional shape with a flat bed and two curved banks and achieved stability. The reason for the increased sediment transport in a threshold channel after the application of downward seepage was sought by analysing the time-averaged velocities. An empirically derived exponential expression was developed for the bank profiles of threshold alluvial channels affected by seepage. A relationship among hydraulic parameters for a stable channel with downward seepage is also suggested on the basis of the experimental observations.

Effect of downward seepage on turbulent flow characteristics and bed morphology around bridge piers

In this work, experimental investigations have been pursued to analyse the influence of downward seepage on the turbulent characteristics of flow and corresponding changes in vortex structure around circular bridge pier in alluvial channel. Experiments were conducted in sand bed channel with circular piers of different sizes for no seepage, 10% seepage and 20% seepage cases. The measurement of turbulent flow statistics such as velocity and Reynolds stresses is found to be negative within the scour hole at upstream of the pier whereas application of downward seepage retards the reversal of the flow causing a decrement in the velocity and Reynolds stresses. Higher Reynolds shear stress prevails at the downstream side because of the production of wake vortices. Contribution of all bursting events to the total Reynolds shear stress production has been observed to increase with downward seepage. The analysis of integral scale suggest that size of eddies increases with seepage, which is responsible for increase in particle mobility. Initially rate of scouring is more which abatements gradually with expanding time as well as with the increased of downward seepage. Presence of downward seepage reduces the depth and length of vortex and shifts towards downstream side of the pier.

Processes of channel migration of Kundil river in Sadiya, Assam, India.

Channel migration is common in alluvial channels, but the rate of migration and processes involved are varied. The Kundil River is an important tributary of river Lohit, flowing through the alluvial plains of Sadiya in the easternmost part of Assam, India. This paper attempts to investigate the rate, extent and the processes of channel migration for the period 1945-2014. Topographical maps and satellite imageries of different years (1976 to 2014) are used for the study. ERDAS Imagine 9.1 and Arc GIS 9.3 software are used for processing these spatial data. It has been found that there are significant changes in the river course during this period. The bank line shifting of the river has been determined separately for the periods 1945-1988, 1988-2014 and 1945–2014 with fifteen cross sections at an interval of approximately 3 kms. Maximum bank line shifting is found to be 1.88 km (1945-1988). However, the migration is not unidirectional all throughout the channel. The processes involved in lateral shift of the river channel can be grouped into three categories: 1. alteration of flow direction due to cut-off, 2. widening of the channel in response to bar development, 3. progressive shifting of meander bends

HYDRAULIC GEOMETORY EQUATIONS FOR ALLUVIAL CHANNELS AND ITS APPLICATION TO RIVER IMPROVEMENT AND MANAGEMENT

HYDRAULIC GEOMETORY EQUATIONS FOR ALLUVIAL CHANNELS AND ITS APPLICATION TO RIVER IMPROVEMENT AND MANAGEMENT

Autogenic entrenchment patterns and terraces due to coupling with lateral erosion in incising alluvial channels

AbstractThe abandonment of terraces in incising alluvial rivers can be used to infer tectonic and climatic histories. A river incising into alluvium erodes both vertically and laterally as it abandons fill‐cut terraces. We argue that the input of sediment from the valley walls during entrenchment can alter the incision dynamics of a stream by promoting vertical incision over lateral erosion. Using a numerical model, we investigate how valley wall feedbacks may affect incision rates and terrace abandonment as the channel becomes progressively more entrenched in its valley. We postulate that erosion of taller valley walls delivers large pulses of sediment to the incising channel, potentially overwhelming the local sediment transport capacity. Based on field observations, we propose that these pulses of sediment can form talus piles that shield the valley wall from subsequent erosion and potentially force progressive channel narrowing. Our model shows that this positive feedback mechanism can enhance vertical incision relative to 1‐D predictions that ignore lateral erosion. We find that incision is most significantly enhanced when sediment transport rates are low relative to the typical volume of material collapsed from the valley walls. The model also shows a systematic erosion of the youngest terraces when river incision slows down. The autogenic entrenchment due to lateral feedbacks with valley walls should be taken into account in the interpretation of complex‐response terraces.

Probability distribution functions of turbulence in seepage-affected alluvial channel

The present experimental study is carried out on the probability distribution functions (PDFs) of turbulent flow characteristics within near-bed-surface and away-from-bed surfaces for both no seepage and seepage flow. Laboratory experiments were conducted in the plane sand bed for no seepage (NS), 10% seepage (10%S) and 15% seepage (15%) cases. The experimental calculation of the PDFs of turbulent parameters such as Reynolds shear stress, velocity fluctuations, and bursting events is compared with theoretical expression obtained by Gram–Charlier (GC)-based exponential distribution. Experimental observations follow the computed PDF distributions for both no seepage and seepage cases. Jensen-Shannon divergence (JSD) method is used to measure the similarity between theoretical and experimental PDFs. The value of JSD for PDFs of velocity fluctuation lies between 0.0005 to 0.003 while the JSD value for PDFs of Reynolds shear stress varies between 0.001 to 0.006. Even with the application of seepage, the PDF distribution of bursting events, sweeps and ejections are well characterized by the exponential distribution of the GC series, except that a slight deflection of inward and outward interactions is observed which may be due to weaker events. The value of JSD for outward and inward interactions ranges from 0.0013 to 0.032, while the JSD value for sweep and ejection events varies between 0.0001 to 0.0025. The theoretical expression for the PDF of turbulent intensity is developed in the present study, which agrees well with the experimental observations and JSD lies between 0.007 and 0.015. The work presented is potentially applicable to the probability distribution of mobile-bed sediments in seepage-affected alluvial channels typically characterized by the various turbulent parameters. The purpose of PDF estimation from experimental data is that it provides a complete numerical description in the areas of turbulent flow either at a single or finite number of points.

Fluvial morphology and dynamics of the Godavari Khola, southeast Kathmandu, Central Nepal

The Godavari Khola flowing from the south to the north contributes the Hanumante Khola, which is one of important tributaries of the Bagmati River, flowing from the eastern part of the Kathmandu Basin. Recently, the Godavari Khola has been suffering from human encroachments due to rapid urbanization. Studying nature and dynamics of the stream are important works if the stream has to be made less affected and well managed. The present study aims to establish fluvial morphology and stability status of the Godavari Khola. For these purposes, the watershed was analysed for morphometric parameters and planform fluvial morphology, and thirteen representative segments were surveyed from upstream to downstream of the Godavari Khola for recording and analysing sediment properties, stream cross-sections and profiles, and hydraulic parameters, river dynamics and stability.The Godavari River is a fifth order stream, the two upstream segments of which are bedrock channels, and the rest of the segments are alluvial channels. The segments are classified into six kinds such as C4-, C5-, B3-, B4-, E4- and F4-type streams. The upstream segments are of B4-, B3- and ‘F4’-types which show entrenched, steep, gravel to cobble grade streams. The downstream segments are of C4-, C5-, E4- and F4-types showing non-entrenched to low entrenched streams with gentle slopes, high sinuosity and gravel to sand grade bed materials. The stability status of the Godavari Khola shows that the stream segments 1, 2, 11 and 13 lie in the degrading condition whereas the remaining nine segments lie in the aggrading condition. Because of the huge width/depth ratio in majority of the downstream channels, bank erosion is relatively prone in the downstream segments of the Godavari Khola, whereas bed incision is prone in segments 11 and 13.Bulletin of the Department of Geology, Vol. 19, 2016, pp. 15–28

Characteristics of a Mixed Bedrock-Alluvial Channel in a Plateau and Plateau Fringe Region: A Study on the Barakar River of the Chotanagpur Plateau, India

The rivers that bear the properties of both alluvial and bedrock character are considered as bedrock and alluvial mixed character rivers. They are characterised by stable planform, erosion resistant bank, rock outcrop, local variability in channel bed gradient and variable flow regime. The Barakar River in the Chotanagpur plateau and its fringe region of India has the distinctive features of bedrock and gravel mixed river with sinuous and braided nature in the upper and the lower sections of the river, respectively. To investigate and extract the geomorphic features of the bedrock and gravel mixed character of the Barakar River, planform view and behaviour of the sediments were considered. The sinuous bedrock and braided reaches have been analysed using the Sinuosity Index and the Braid-channel Ratio, respectively. The upper sinuous bedrock section is marked by a steep-sided, narrow valley with the presence of incised channel. In the downstream reach of the Maithon Dam, the river channel is a box-shaped bedrock channel with a low form ratio. Potholes and grooves of varying size and shape and inner channel are the main features at the mouth zone. While, in the lower section of the river, the wide and straight braided reach is characterised by higher form ratio and higher gradient than the sinuous reach. The riffle-pool sequence in the braided section is not developed significantly like in the alluvial channel. In the braided reach, the sediment grain size is continuously decreased towards the Maithon Dam and the sediment grains are well sorted. The lithology and hydraulic processes of the river determine the mixed character of the river that favours the fluvial system to maintain its lateral stability.

The Coarse‐Grained Lacustrine Slope Apron Deposits in the Moliqing Area, Yitong Basin, Northeast China

AbstractOverlapping gravity accumulation bodies were formed on the northwestern steep slope of the Shuangyang Formation in the Moliqing fault depression of northeast China. This study analyzed in detail the spatial distribution of the lithofacies and lithofacies associations of these accumulation bodies based on more than 600 m of core sections, and summarized 12 major types of lithofacies and three types of lithofacies associations: (1) the proximal zone consists of gravelly debris flows dominated by alluvial channel conglomerates; (2) the middle zone is dominated by various gravity flow deposits and traction flow deposits; and (3) the distal zone is dominated by mudstones with intercalations of sandy debris and turbidites. Combining with the grain size cumulative probability curves analysis, we determined the transformation of debris flows to sandy debris flows and to turbidity currents in the slope zone of the basin margin, and further proposed a lacustrine slope apron model that is characterized by (1) an inconstant multiple source (line source), (2) an alternation of gravity flow deposits and traction flow deposits dominated by periodical changes in a source flood flow system, and (3) the transformation of sandy debris flow deposits into distal turbidity current deposits. This sedimentary model may be applicable to other fault depressions for predicting reservoir distribution.

Assessing sediment yield in Kalaya gauged watershed (Northern Morocco) using GIS and SWAT model

An efficient design for erosion-control structures of any watershed in the world is entrusted with the delicate forecasting of sediment yields. These outlook yields are usually inferred by extrapolations from past observations. Because runoff, as the transporting vehicle, is more closely correlated with sediment yields than any other variable. So, calibration as well as validation of process-based hydrological models are two major processes while estimating the sediment yield in watershed. The actual survey is fulfilled with the aim of developing a trustworthy hydrologic model simulating stream flow discharge and sediment concentration with least uncertainty among the parameters picked out for calibration so as to verify the effect of the scenarios on the spatial distribution of sediment yield (sediments transported from sub-basins to the main channel during the step of time). Soil and Water Assessment Tool (SWAT, version 2012) model integrated with Geographic Information System (GIS, version 10.1) was used to simulate the stream flow and sediment concentration of Kalaya catchment situated in north of Morocco for the period from 1971 to 1993. Model calibration and validation were performed for monthly time periods using Sequential Uncertainty Fitting 2 (SUFI-2, version 2) within SWAT-CUP using 16 parameters. Our calibration outputs for monthly simulation for the period from 1976 to 1984 showed a good model performance for flow rates with NSE and PBIAS values of 0.76 and −11.80, respectively; also a good model performance for sediment concentration with NSE and PBIAS values of 0.69 and 7.12, respectively. Nonetheless, during validation period (1985–1993) for monthly time step, the NSE and PBIAS values were 0.67 and −14.44, respectively for flow rates and these statistical values were 0.70 and 15.51, respectively for sediment concentration; which also means a good model performance for both. Following calibration, the inclusive effect of each parameter used was ranked using global sensitivity function in SWAT-CUP. From our analysis, the effective hydraulic conductivity in main channel alluvium (CH_K2), USLE support practice factor (USLE_P) and manning's "n" value for the main channel (CH_N2) were found to be the most sensitive parameters during different iterations with different number of simulation but with the same inputs. The least sensitive parameter were found to be different in either cases unlike the most sensitive parameters. As a result, the global evaluated soil erosion rate in the study area varied from 20 to 120t/ha/yr. It was summarized that the entire knowledge of the hydrologic processes happens within the watershed and the consciousness about acceptable meaningful range of the parameters is crucial while developing reliable hydrologic model.

Self‐adjustment in rivers: Evidence for least action as the primary control of alluvial‐channel form and process

AbstractLeast action principle (LAP) in rivers is demonstrated by maximum flow efficiency (MFE) and is the foundation of variational mechanics based on energy and work rather than Newtonian force and momentum. Empirical evidence shows it to be the primary control for the adjustment of alluvial channels. Because most rivers flow with imposed water and sediment loads down valley gradients they have largely inherited, they self‐regulate energy expenditure to match the work they are required to do to remain stable. Overpowered systems develop a variety of channel patterns to expend excess energy and remain stable. Australia offers an opportunity to study low‐energy rivers closely adjusted to very low continental gradients. The anabranching Marshall and single‐thread Plenty Rivers flow down nearly straight channels with average H numbers [ratio between excess bed shear and width/depth (W/D) ratio] close to the optimum of 0.3 for stationary equilibrium. Ridge‐form divisions of the original channel width create anabranches that radically alter W/D ratios relative to bed shear, the same being true for short‐wide islands on the large low‐gradient Yangtze River in China. In contrast, Mount Chambers Creek in Australia's tectonically more active Flinders Ranges is accreting an alluvial fan with unstable distributary channels exhibiting H numbers well below the optimum. LAP also explains profound biases in Earth's stratigraphic record. Because meandering is an energy‐shedding mechanism, sinuous rivers sequester relatively little sediment resulting in all sequences being just a few tens of metres thick. In contrast, low‐energy braided disequilibrium systems can sequester sediment piles over a kilometre in thickness and tens of kilometres wide. LAP provides a new paradigm for river research by identifying the attractor state controlling river channel evolution. It links advances in theoretical physics to fluvial geomorphology, stratigraphy and hydraulic engineering and opens opportunities for diverse investigations in Earth system science. Copyright © 2016 John Wiley & Sons, Ltd.

A philosophy of rivers: Equilibrium states, channel evolution, teleomatic change and least action principle

Until recently no universal agreement as to a philosophical or scientific methodological framework has been proposed to guide the study of fluvial geomorphology. An understanding of river form and process requires an understanding of the principles that govern the behaviour and evolution of alluvial rivers at the most fundamental level. To date, the investigations of such principles have followed four approaches: develop qualitative unifying theories that are usually untested; collect and examine data visually and statistically to define semi-quantitative relationships among variables; apply Newtonian theoretical and empirical mechanics in a reductionist manner; resolve the primary flow equations theoretically by assuming maximum or minimum outputs. Here we recommend not a fifth but an overarching philosophy to embrace all four: clarifying and formalising an understanding of the evolution of river channels and iterative directional changes in the context of least action principle (LAP), the theoretical basis of variational mechanics. LAP is exemplified in rivers in the form of maximum flow efficiency (MFE). A sophisticated understanding of evolution in its broadest sense is essential to understand how rivers adjust towards an optimum state rather than towards some other. Because rivers, as dynamic contemporary systems, flow in valleys that are commonly historical landforms and often tectonically determined, we propose that most of the world's alluvial rivers are over-powered for the work they must do. To remain stable they commonly evolve to expend surplus energy via a variety of dynamic equilibrium forms that will further adjust, where possible, to maximise their stability as much less common MFE forms in stationary equilibrium. This paper: 1. Shows that the theory of evolution is derived from, and applicable to, both the physical and biological sciences; 2. Focusses the development of theory in geomorphology on the development of equilibrium theory; 3. Proposes that river channels, like organisms, evolve teleomatically (progression towards an end-state by following natural laws) and iteratively (one stage forming the basis for the next) towards an optimal end-state; 4. Describes LAP as the methodological basis for understanding the self-adjustment alluvial channels towards MFE. 5. Acknowledges that whereas river channels that form within their unmodified alluvium evolve into optimal minimum-energy systems, exogenic variables, such as riparian or aquatic vegetation, can cause significant variations in resultant river-styles. We specifically attempt to address Luna Leopold's lament in 1994 that no clearly expressed philosophy explains the remarkable self-adjustment of alluvial channels.

Two-Dimensional Bed Variation Models Under Non-equilibrium Conditions in Turbulent Streams

In this paper, a depth-averaged two-dimensional non-equilibrium coupled model has been proposed to determine water surface profiles and bed profiles in alluvial channels and rivers. In this coupled model, a hydrodynamic component was applied to the two-dimensional shallow-water equations to determine hydraulic data. By employing precalculated data, morphodynamic component could determine bed deformation in erodible layer. Moreover, in order to simulate the nature of sediment transport mechanisms more realistic, two turbulence models were used for examining the turbulence parameters. In order to achieve the successful representation of the domain by the meshes, the finite volume method was used to discretize the governing equations in explicit conditions. Furthermore, the unstructured triangular mesh system was developed to solve governing equations by total variation diminishing scheme. In the final step, due to the lack of experimental data to examine the numerical results of proposed model, Flow3D software was employed. The suggested model satisfactorily predicted water surface profiles and bed profiles simulated with Flow3D software.

Floodplain morphology, sedimentology, and development processes of a partially alluvial channel

The floodplain morphology, sediment deposits, and development mechanisms of a partially alluvial, low-moderate energy channel flowing over a mixed gravel/cobble-till bed are investigated and compared to existing ideas of floodplain development. The findings partially support the idea of a floodplain developed through lateral accretion capped with vertically accreted sediments as predicted by the energy-based classification scheme of Nanson and Croke (1992), though oblique accretion and partial channel avulsion are also important. Channel migration consists of limited cross-valley migration and downstream meander translation. Because of low channel sinuosity, well-formed neck cutoffs are rare, and instead the channel cuts headward along the insides of confined or underdeveloped meander bends forming a localized anabranching pattern. The floodplain architecture can be divided into gravel bar and bed deposits (GB), lateral accretion deposits (LA), overbank deposits (FF), and abandoned channel deposits (FF(CH)), which are described with four alluvial facies. Owing to the limited supply of coarse and fine sediment, none of the architectural elements are particularly thick, with total floodplain thickness being <3m. Floodplain development for partially alluvial channels is compared within a new floodplain discrimination framework. Comparisons with common facies models of single-thread, coarse-grained channels show important differences that suggest that the floodplain deposits and formative processes described herein represent a subset of single-thread systems that may be common in partially alluvial channels, particularly in slightly sinuous, coarse-grained channels of low-moderate energy with partly confined floodplains.

Flow characteristics in an alluvial channel covered partially with submerged vegetation

The present study examines the flow characteristics in a stable alluvial channel partially covered with submerged Oryza sativa (rice) stems in a staggered pattern with downward seepage. Measurements were taken in transect across the flume at the vegetated region, interface or junction of vegetated and unvegetated region an unvegetated region to explore the difference in flow characteristics in these regions. The presence of vegetation reduces the velocity, Reynolds stress and turbulent intensities at the downstream vegetated region even with downward seepage which is an important finding for river restoration. An increase in the flow characteristics such as velocity, Reynolds stress and turbulent intensities are observed in the unvegetated region as the flow goes downstream which means that the reduction in the flow characteristics in the vegetated region is diverted towards the unvegetated region. Moment analysis shows that streamwise flux is occurring in the flow direction and vertical flux is occurring in downward direction and increases with increase in seepage percentage. Integral scales also exhibit that with downward seepage, the size of eddies increases. The maximum depth of erosion with downward seepage is more than for the case of no seepage and erosion increases as the flow goes downstream.

Depositional processes in the distal part of a large alluvial fan's feeder channel in Himalayan foothills, India

This paper presents results of detailed facies and architectural element analyses of the sedimentary fill of the Ranibag valley in the Himalayan foothills of north-western India. Forming a buffer zone between the source and sink of the Ganga foreland basin's Gola alluvial fan, the Ranibag valley is filled with coarse gravels comprising five distinct facies. These are clast-supported massive gravel, horizontal-bedded gravel, crudely bedded gravel, sandy gravel and matrix-supported disorganized gravel facies. The specific assemblages and sequences of these facies, defined by fifth- and fourth-order bounding surfaces, belong to mutually associated channel (CH) and gravel bar and bedform (GB) architectural elements. Both the elements have accreted only during floods by combined fluvial and debris-flow processes, but dominantly by the fluvial processes under conditions of high water-to-sediment ratio. Depositional processes in the valley are akin to those of the proximal alluvial fans of the Ganga Basin. Nonetheless, voluminous alluviation of coarser gravels in this extreme distal feeder channel valley has affected the fan processes downstream. It reduced the sediment budget and thus increased water-to-sediment ratio downstream to favour fluvial processes on the Gola fan. Copyright © 2016 John Wiley & Sons, Ltd.