High Bedload Research Articles

What drives major channel widening in mountain rivers during floods? The role of debris floods during a high-magnitude event

Our capability to accurately predict at the local scale the morphological changes in a mountain river affected by a severe flood is currently limited. Given the importance of including geomorphological hazards in flood risk evaluation, some improvements in this direction are required. In fact, on the basis of the controlling factors typically considered in the literature, that are the unit stream power (ωpk) and the confinement index (CI), a relatively broad spectrum of width ratios (WR = ratio between channel width after a flood and channel width before a flood) occurring in response to major floods is commonly observed in streams. We extended previous research focused on parameters that influence the morphological response of a channel to a severe flood by investigating the role played by the type of sediment transport process that occurs in a mountain river. In particular, we explored whether a relationship exists between debris flood (i.e. a condition characterised by extremely high bedload) and intense channel widening. The case study was the Cordevole River (the Dolomites, Italy) and four of its tributaries, which were affected by the severe flooding induced by the Vaia storm in October 2018. WR was determined at the sub-reach scale, and ωpk was calculated with consideration for the flood peak discharge ascertained via hydrological modeling. Despite the significant relationships found among WR, ωpk and CI, 21 sub-reaches (out of 144) of the studied stream network widened intensely reaching a WR higher than approximately 4, often to a substantially larger extent than predicted by statistical models. Such sub-reaches were certainly or probably affected by debris floods, leading us to infer that debris floods can induce widening processes that are more intense than those occurring in response to water flows. Having said that, debris floods do not automatically imply very intense widening. In addition to hydraulic and morphological constraints, the sediment transport processes possibly taking place at a sub-reach of a mountain river should be regarded as another driver of channel modifications during high-magnitude hydrological events.

Alluvial fans and fan deltas in the Paparoa Formation, Greymouth Basin: a new rift model

ABSTRACT Sedimentary facies analysis of conglomerate deposits in the Greymouth Rift Basin has identified the latest Cretaceous to Paleocene alluvial fan and fan delta environments on the northwestern side. The Gravelly Braided River Facies Association is interpreted as high energy, braided river streamflow in a streamflow-dominated alluvial fan environment. The Gravelly Delta Front Facies Association was deposited by high bedload mouth bars and channel avulsion. The Gravelly Prodelta Facies Association is interpreted as high-density turbidity currents, and subaqueous debris flows in a fan delta slope environment. Bedding geometries suggest the fan deltas were Hjulström-types formed on lower angle slopes. The gradual decrease in conglomerate thickness from northwest to southeast indicates that the primary basin bounding fault was located offshore to the northwest, most likely the Cape Foulwind-Canoe Fault Zone. Overall facies distribution and paleoflow directions indicate the Greymouth Basin formed as a half-graben in a purely extensional setting with no strike-slip movement. The presence of contemporaneous sub-basins in the West Coast region suggests they likely experienced similar depositional history to the Greymouth Basin. Our findings agree with previous models from the Taranaki Basin that indicate that rifting was purely extensional, suggesting that the West Coast-Taranaki Rift System recorded primarily orthogonal extension.

Transport characteristics of non-cohesive sediment with different hydrological durations and sediment transport formulas

River sediment transport is an important global environmental issue, for it leads to waterway sedimentation, ecological degradation, and river geomorphologic evolution, its quantification is of the highest importance for river and watershed management. However, it remains unclear what the applicability of different sediment transport formulas in sediment transport process, and what the characteristics of non-cohesive sediment transport processes with different hydrological durations. This study developed a two-dimensional (2D) surface water Flow Model (FM) coupled with a Sediment Transport (ST) model in the MIKE 21 modelling system. The characteristic difference on simulated non-cohesive sediment transport under different sediment concentrations, hydrological durations, and sediment transport formulas were quantified by calculating the bed load, suspended load, and bed level change. Results indicated that the bed load and suspended load using the Engelund and Fredsøe formula were significantly lower than the Van Rijn formula for individual rainfall simulation scenarios. With the increase in peak flow, both suspended load and bed load first increased and then decreased. In the long-term duration simulations, high sediment concentrations were accompanied by higher bed load and lower suspension load for the Engelund and Fredsøe formula, but by lower bed load and suspension load for the Van Rijn formula. The high sediment concentrations resulted in a higher erosion rate, and low sediment concentrations resulted in a higher deposition rate. The two formulas had a high degree of consistency in seasonality and the bed level changes in spring and summer were mainly sedimentary, while in autumn and winter they were mainly erosive. The Engelund and Fredsøe formula produced high riverbed erosion capacity at low discharge, but the Van Rijn formula had high erosion capacity at high discharge.

Architecture of intereruptive and syneruptive facies in an Andean Quaternary palaeovalley: the Huarenchenque Formation, western Argentina

The Huarenchenque Formation is a volcano sedimentary unit deposited to the east of the Plio-Quaternary Andean Magmatic Arc. In order to define depositional settings, two lithofacies associations (fluvial and pyroclastic) were defined. The fluvial facies association is composed of polymictic conglomerates with the predominance of basalt- dominated clasts, coarse- medium-grained conglomeratic sandstones and medium- to coarse-grained sandstones. These deposits occur as stacked or single bodies, display both sheet and channelized geometries, and contain a range of internal sedimentary structures, such as planar, low angle stratification and cross-bedding. This facies association is interpreted as the deposit of a multichannel fluvial system characterized by high bed load, steep gradient and non-cohesive bank materials. Facies and architecture of the fluvial deposits are the result of high bank full discharge related to rapid deglaciation of the Andean Last Glacial Maximum. The pyroclastic facies association is characterized by lapilli and ash tuffs deposited from air fall, pyroclastic density current, and density stratified surge mechanisms. In the Huarenchenque Formation the fluvial and the pyroclastic facies associations show a clear physical separation, suggesting that sedimentation occurred in two distinct (intereruptive and syneruptive) phases. During the long-lived intereruptive phases the sedimentary record corresponds mainly to the deposits of the gravelly braided fluvial system, whereas during syneruptive phases the fluvial valley was almost entirely occupied by primary pyroclastic deposits related to high-explosive episodes of the neighbor Andean strato-volcanoes.
 Although most of the cross-bedded sandstones and conglomerate sandstones are rich in basaltic fragments, some strata are composed almost entirely of pumiceous fragments, while in others there is a marked alternation between “basalt” and “pumiceous” foresets. These attributes reflect the preservation of intrabasinal pyroclastic fragments and allow suggest that: i. explosive volcanic events could be more frequent than reflected by the pyroclastic deposits themselves; ii. syneruptive pyroclastic materials could be eroded (even eliminated) by the fluvial system; iii. contributions of primary pyroclastic material persisted during intereruptive (fluvial-dominated) phases.

Analysis of Flow Resistance Equations in Gravel‐Bed Rivers With Intermittent Regimes: Calabrian fiumare Data Set

AbstractThis study addresses the evaluation of flow resistance in natural gravel‐bed rivers. Through a new data set collected on 136 reaches of 78 gravel‐bed rivers (Calabrian fiumare) in southern Italy, different conventional flow resistance equations to predict mean flow velocity in gravel‐bed rivers were tested in their original form. These equations have shown considerable disagreement with observed data, especially in river reaches characterized by high bed load conditions and for the domains of intermediate‐ and large‐scale roughness. This disagreement produced in almost all the cases an underestimation of the flow resistance, which can be corrected by introducing the Froude number and a particular form of the Shields sediment mobility parameter into the Manning, Chezy, and Darcy‐Weisbach equations. Through analyses carried out both on the whole data set and on its subsets, we propose a semiempirical approach with which, on the one hand, the tractive forces exerted by the flow on the bed are taken into account by considering the ratio between the sediment mobility parameter and its critical value, and on the other hand, water surface distortions are evaluated using the Froude number. This approach has been further validated using a literature‐based data set showing, even in this case, excellent performances. Finally, the literature‐based data set allowed us to improve the performances of the proposed approach in the field of large‐scale roughness. Efficiency tests indicate that the new equations can better reproduce the flow velocity in river reach where conventional flow resistance equations are not able to explain the entire dissipative process.

Bedload Sediment Concentration of Sungai Kahang Downstream Kahang Dam in Johor

Dams construction globally on rivers have generated a significant adjustment to stream hydrology and sediment dynamics. The impoundment of rivers by dam reservoirs will trap the sediments following freely downstream the river channel. Kahang dam is constructed to impound Sungai Kahang (Kahang River) for water supply purposes; this may have adverse effects downstream the dam by starving the river its natural sediments and changing the river flow. This study evaluates the bedload sediments and river discharge downstream the dam by taking field measurements, using the area-velocity and single increment method at three selected gauging locations (St0, St1, St2). The results obtained between January and July 2017 show the highest bedload discharge for all the gauging Stations to be 0.0736 tones/day at St0 and 0.0077 tones/day at St2 while the river flow remains relatively steady between 0.528 m3/s to 2.534 m3/s. The high bedload discharge recorded can be attributed to the anthropogenic activities going on during external construction works going on, as lower bedload concentration values followed immediately after the external construction work was completed.

Transport measurement with a horizontal and a vertical pipe microphone in a mountain stream: taking account of particle saltation

AbstractThe pipe microphone has been shown to be an effective means for monitoring bedload transport in mountain streams. It is commonly installed perpendicular to the flow direction on a stable river bed, such as that of a check dam. Acoustic pulses caused by bedload collisions with the pipe are detected by a microphone. However, bedload particles saltating over the pipe remain undetected. To overcome this disadvantage, we installed a horizontal as well as a vertical pipe microphone in the Ashi‐arai‐dani supercritical channel located in the Hodaka mountain range, Japan. The vertical pipe was installed on the wall of the channel and the horizontal pipe was installed on the channel bed. The acoustic response of the horizontal pipe is expected to be larger than that of the vertical pipe, because the bedload concentration decreases with increasing height above the bed. However, at high amplifications, the peak pulse value from the vertical pipe is higher than that from the horizontal pipe. We explain this observation as follows: under high bedload discharge conditions, the pulses of the horizontal pipe are saturated but those of the vertical pipe are not. We proposed a ratio (Rhv) between the pulses detected by these sensors, and applied this ratio for calibrating the contemporaneous pulses detected by a microphone located immediately upstream of a bedload slot sampler. Indeed the Rhv‐corrected pulses correlated well with the bedload discharge calculated from the sampler, supporting our explanation. We conclude that bedload monitoring using concomitant vertical and horizontal pipe microphones can be used to calibrate centrally located pipe microphones when the bedload concentration is approximately homogeneous laterally across the width of the channel cross‐section, and thereby represent bedload discharges more accurately than with only a single pipe microphone. Copyright © 2017 John Wiley & Sons, Ltd.

Bed-load transport rate based on the entrainment probabilities of sediment grains by rolling and lifting

A function for the bed-load sediment transport rate is derived. This function is obtained by using the entrainment probabilities of the rolling and lifted sediment grains, and by introducing two travel lengths, respectively. The predictions from the new bed-load function agree well with experimental results over the entire experimental range and show significant improvement over the commonly used formula for the bed-load transport rate. The new function shows that, in terms of contributing to the bed-load transport rate, the total entrainment probability of the sediment grains is a weighted summation of those for the lifted and rolling grains, rather than a simple addition of the two. The function is also used to predict the total entrainment probability, saltation length, and the bed layer thickness at a high bed-load transport rate. These predictions all agree well with the experimental results. It is found that, on average, the travel length for the rolling sand grains is about an order of magnitude less than that of the lifted grains.

A PIV-based method to measure spatial gradients in bedload transport over a dune

Traditional sediment transport equations calculate sediment flux from bed shear stress and the equations predict that transport increases nonlinearly with an increase in flow velocity. In a dune field, the dune geometry affects the flow velocity causing accelerating flow over the dune crest and de- and reattachment of the flow downstream of the dune crest. Sediment flux predicted from the reach-averaged bed shear stress gives fairly good results for dune fields, though their simplification is discordant for the complexity of the processes involved. Measurements of the displacement of sand particles over the dune bed were derived from highfrequency image capturing. The two main methods to measure particle velocities from images are particle tracking velocimetry (PTV) and particle image velocimetry (PIV). We compare individual particle tracking with a PIV-based correlation method. The PIV-based method promises to be a more efficient and effective approach to track particle motion. It is more suitable for the conditions of high bedload transport, as present in our experiments. The PIV-based method is based on using images of difference (IoD) and is fully automated and identifies spatial gradients at a support scale in the order of centimetres. Findings align with our general knowledge of accelerating flow over the dune crest. The mean streamwise particle velocity and activity over a dune stoss slope increase. At the scale of 0.026 m the observed particle velocity variability can be explained in the context of general onset and cessation of sediment transport, the effect of the reattachment zone and observed sweep/burst events. By decreasing the streamwise distance between cross-sections, the variations in mean particle velocity induced by superimposed bed defects are distinguished as well. The maximum particle velocity and activity occurred at the same location and consequently the location of the maximum transport over the dune crest was identified. The measurements bridge the gap between individual particle motion studies and (non-local) sediment transport flux measurements.

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.

Bedload pulses in a hydropower affected alpine gravel bed river

This study investigated the sediment resupply and transport dynamics at the Upper Drau River upstream of Lienz (Eastern Tyrol, Austria). Due to a hydropower plant, a 24km long river reach of this alpine gravel bed river is under residual flow conditions, although sediment is still resupplied into the reach through many active torrents and tributaries. As a result, sediment deposition in the residual flow reach intensified, hence increasing maintenance efforts to stabilize this river section and ensure flood protection. In combination with a new sediment management program, a continuous bedload monitoring system was installed 2km downstream of the residual reach in 2001 to support the development of adapted sediment management strategies. The surrogate bedload monitoring system consists of 16 impact plate geophones, installed over a 17m wide cross section. The unprecedented 15-year dataset of high-resolution bedload intensity revealed a complex process of gravel storage and intermittent resupply from the residual reach, allowing the authors a detailed analysis of frequently occurring bedload pulses.These transport features are triggered by increased discharges during floods in the residual reach and created pronounced anticlockwise bedload hysteresis or, with a temporal shift to the event peak, caused distinct shifts in the bedload activity downstream. Bedload pulses produce very high bedload fluxes while in transit, tend to increase bedload flux in the post-event phase, and can alter and reduce the upstream sediment storage leading to a lowering of bedload availability for future pulses. The observed time lags between main discharge events and the arrival of the macro-pulses are correlated with mean water discharge during pulse propagation, thus enabling a prediction of the pulse arrival at the monitoring station solely based on the hydrograph. In combination with the hydrological setup of the reach, the observed bedload pulse time lags allowed an estimation of pulse velocities in the range 0.002−0.05ms−1.

Evaluation of GA-SVR method for modeling bed load transport in gravel-bed rivers

Summary The aim of the present study is to apply Support Vector Regression (SVR) method to predict bed load transport rates for three gravel-bed rivers. Different combinations of hydraulic parameters are used as inputs for modeling bed load transport using four kernel functions of SVR models. Genetic Algorithm (GA) method is applicably administered to determine optimal SVR parameters. The GA-SVR models are developed and tested using the available data sets, and consecutive predicted results are compared in terms of Efficiency Coefficient and Correlation Coefficient. Obtained results show that the GA-SVR models with Exponential Radial Basis Function (ERBF) kernel present higher accuracy than the other applied GA-SVR models. Furthermore, testing data sets are predicted by Einstein and Meyer-Peter and Muller (MPM) formulas. The GA-SVR models demonstrate a better performance compared to the traditional bed load formulas. Finally, high bed load transport values were eliminated from data sets and the models are re-analyzed. The elimination of high bed load transport rates improves prediction accuracy using GA-SVR method.

River Morphology Modeling at the Downstream of Progo River Post Eruption 2010 of Mount Merapi

Mt.Merapi is one of the most active volcanoes in Indonesia. Some of the rivers that the origin is located at Mt.Merapi have a large amount of sediment resources after eruption in October-November 2010. The total volume of sediment is estimated at 130 million m3.The deposited sediment flows to the downstream area as a debris or bed load transport in high density. Few studies considered downstream change along volcanic rivers due to a high density of bed load transport. Using numerical simulation, the impact of high concentration of bed-load transport is applied in Progo River, Yogyakarta, Indonesia. The results show that the high bed-load transport rate increases the mid-channel bar grow rate and the bed degradation near the bank toe. The increase of bed degradation is an important parameter of bank erosion process. Furthermore, the bed morphology on the downstream after 2010 eruption of Mt. Merapi should be considered intensively.

Sediment supply and channel morphology in mountain river systems: 2. Single thread to braided transitions

AbstractDifferences in sediment supply between single‐thread and braided channel types provide a long‐recognized, though difficult to quantify, pattern discrimination. Building on the results from our preceding paper, we present a multiscale assessment of the sediment supply, geomorphology, and sediment transport characteristics of braided, gravel‐bedded reaches in the northern Rocky Mountains, USA. First, we present a quantitative, theoretically based discriminant function that stratifies single‐thread and braided reaches on the basis of variations in bankfull sediment concentration and dimensionless discharge following the work of Millar (2005) and Eaton et al. (2010). This function correctly classifies 50 of the 53 channel types where bed load concentrations are known. Second, while channel pattern transitions are often linked to changes in slope, field studies along Sunlight Creek, Wyoming, show that downstream transitions between single‐thread and braided reaches are instead caused by valley constrictions and changes in the grain size of sediment from tributaries. Finally, we demonstrate that the two‐dimensional variability in flow properties in braided reaches produces locally high values of shear stress and bed load transport. Yet bed load measurements and sediment transport calculations also show that sediment transport rates between adjoining braided and single‐thread reaches may be approximately equal where channels are near the pattern threshold and downstream variations in bank fortitude and channel constriction force pattern transitions. Taken together, these results indicate that high bed load concentrations are fundamental to the braided channel planform and that braided channels likely reflect a quasi‐equilibrium state within watersheds with persistent high sediment supply.

Sediment supply and channel morphology in mountain river systems: 1. Relative importance of lithology, topography, and climate

[1] Quantifying landscape-scale variations in sediment supply to streams and rivers is fundamental to our understanding of both denudational processes and stream channel morpho-dynamics. Previous studies have linked a variety of sediment-supply proxies to climatic, topographic, or geologic factors, but few have connected these directly to the characteristics of fluvial systems draining these landscapes. Here we correlate landscape controls on sediment supply to observed sedimentology and channel patterns through direct measurements of water and sediment fluxes in over 80 drainage basins ranging in area from 1.4 to 35,000 km2 in the northern Rocky Mountains, USA. These data show that the relative sediment supply, defined by the bankfull sediment concentration, is dominated by basin lithology, while exhibiting little correlation to factors such as relief, mean basin slope, and drainage density. Bankfull sediment concentrations (bed load and suspended load) increase as much as 100-fold as basin lithology becomes dominated by softer sedimentary and volcanic rocks, relative to basins with more resistant lithologies. As bed load concentrations increase, stream beds become less armored, and bed load grain size coarsens. At very high sediment concentrations, bed surface, subsurface, and bed load grain sizes converge and a transition from single-thread to to braided channel patterns is commonly observed.

Mechanism Analysis and Criterion for Avoiding Risk of Karst Water Burst Flood Illustrated in Maluqing Tunnel

With 19 groups of data associated with inrush or bursting water from karst cavity, obtained from Yichang-Wanxian Railway Maluqing Tunnel field test, conceptual models of Karst Water Burst Flood (KWBF) was brought forward by statistical analysis and phenomenological induction. KWBF is super-large amount of artesian water together with mud, which statically deposited in karst cavity, suddenly released into tunnel. KWBF has 3 kinds of characteristic: large scale of cavity water, high hydraulic pressure, high bed load content, dynamic supply of ground water, strong destroyed power; no foreboding, paroxysmal and intermittence, antinomy between paroxysmal and arrearage; force majeure, difficulty forecast, etc. Forecasting of Karst Water Burst Flood is conducted with rainfall seeping theory and statistical analysis method, while the base data are obtained from field quantity observation technical such as rainfall, hydrograph measure. Because of pressure dispersion between inside and outside of the cavity, seepage enlarge is evolved into conflux so that deprive groundwater from aquifer aqueduct and fracture, so, the water together with mud release suddenly from karst into tunnel. The key effect factor on KWBF is water count and silted or filling degree of conflux aqueduct. It is the essential numerical criterion that the daily rainfall must be less than 20mm if construction in the tunnel after the karst cavity has been broken through. To evade risk during construction temporary stage and permanent working stage, measure of energy releasing and pressure reducing (ERPR) must be accept. In addition, it was suggested that the key parameter of risk control for karst tunnel is water count and its cumulating velocity, mechanics characteristic of bed load filling in the conflux aqueduct.

Macroinvertebrate grazers, current velocity, and bedload transport rate influence periphytic accrual in a field-scale experimental stream

Periphyton plays an important role in stream ecology, and can be sensitive to macroinvertebrate grazers, near-bed current velocity, and bedload abrasion. We manipulated conditions to examine influences on periphytic accrual in the St. Anthony Falls Laboratory Outdoor StreamLab in Minneapolis, MN, USA. Macroinvertebrate grazers were excluded from 27 of 65 clay tiles using electric pulses. We examined periphytic biomass accrual as a function of grazer presence, sampling run, and near-bed current velocity using ANCOVA. We found significant temporal differences between sampling runs but no significant effect of grazer presence. Along with a strong association between bedload transport rates and mean periphytic biomass, our results suggest that grazers are relatively unimportant in stream systems with high levels of physical disturbance from floods and associated sand bedload. However, the interaction between grazer presence and velocity was marginally significant. Regression analyses showed no relation between velocity and periphyton in the absence of grazers but a negative relation when grazers were present, suggesting that mechanical dislodgement of periphyton by grazers may increase with velocity. We conclude that grazers can have subtle effects on periphyton, particularly in streams with high bedload transport rates.

Inferring bedload transport from stratigraphic successions: examples from Cenozoic and Pleistocene rivers, south central Pyrenees, Spain

Abstract Geologists and geomorphologists have long been concerned with rates of sediment transfer as bedload in gravel-bed rivers, especially as rates of sediment transfer are important factors controlling river aggradation and incision. Bedload transport equations, originally derived for Holocene streams, have been used widely in modern gravel-bed river systems. However, palaeohydraulic reconstructions have received less attention and are generally dismissed as inaccurate since most are estimated to be at least an order of magnitude out. This study focuses on deriving stream power, bedload transport rates and efficiency estimates for Oligo–Miocene and Plio–Pleistocene gravel-bed river deposits from the south central Pyrenees, Spain. The basic data used in the palaeohydraulic calculations are estimates of palaeoslope, palaeovelocity, palaeodepth and the volume of sediment accreted in yearly flood events on gravel bars. Analyses of data from these ancient river systems yield more accurate estimates of relative stream power, bedload transport rates and efficiency parameters. This study illustrates the need for understanding the palaeohydraulics of river systems in order to characterize ancient rivers. Gravel-bed rivers with low sediment supply and high bedload transport rates incise. Conversely, when sediment supply is abundant, bedload transport rates and efficiency are low and the river system aggrades.

Bedload-to-suspended load ratio and rapid bedrock incision from Himalayan landslide-dam lake record

About 5400 cal yr BP, a large landslide formed a > 400-m-tall dam in the upper Marsyandi River, central Nepal. The resulting lacustrine and deltaic deposits stretched > 7 km upstream, reaching a thickness of 120 m. 14C dating of 7 wood fragments reveals that the aggradation and subsequent incision occurred remarkably quickly (∼ 500 yr). Reconstructed volumes of lacustrine (∼ 0.16 km 3) and deltaic (∼ 0.09 km 3) deposits indicate a bedload-to-suspended load ratio of 1:2, considerably higher than the ≤ 1:10 that is commonly assumed. At the downstream end of the landslide dam, the river incised a new channel through ≥ 70 m of Greater Himalayan gneiss, requiring a minimum bedrock incision rate of 13 mm/yr over last 5400 yr. The majority of incision presumably occurred over a fraction of this time, suggesting much higher rates. The high bedload ratio from such an energetic mountain river is a particularly significant addition to our knowledge of sediment flux in orogenic environments.

Sedimentological and geomorphological impacts of the jökulhlaup (glacial outburst flood) in january 2002 at kverkfjöll, northern iceland

Jökulhlaups (glacial outburst floods) are common hazards in many glaciated environments. However, research on the controls on the sedimentological and geomorphological impact of jökulhlaups is rare. Developing a more comprehensive understanding of flood impacts may be useful for hazard identification, prediction and mitigation.This study determines the controls on the sedimentological and geomorphological impact of a jökulhlaup in January 2002 at Kverkfjöll, northern Iceland. This jökulhlaup, caused by geothermal activity, reached a peak discharge of 490 m3s−1 as recorded at a permanent gauging station 40 km downstream from the glacier snout. However, reconstructed peak discharges in the proximal part of the jökulhlaup channel near the glacier snout indicate a peak discharge of 2590 m3s−1. The jökulhlaup hydrograph was characterized by a rapid rising stage and a more gradual falling stage. As a result, sedimentary and geomorphological impacts included poorly sorted, structureless, matrix‐supported deposits; massive sand units; clast‐supported units; ice‐proximal cobbles, rip‐up clasts and kettle‐holes; and steep‐sided kettle‐holes. These features are proposed to be characteristic of rapid rising stage deposition. Additionally, large‐scale gravel bars and bedload sheets prograded and migrated during the rapid rising stage. The development of these bedforms was facilitated by high bedload transport rates, due to high discharge acceleration rates during the rapid rising stage. During the more prolonged falling stage, there was sufficient time for sediment incision and erosion to occur, exhuming cobbles, ice blocks and rip‐up clasts, and creating well‐defined terrace surfaces.This study provides a clearer understanding of hydrological and sedimentological processes and mechanisms operating during jökulhlaups, and helps to identify flood hazards more accurately, which is fundamental for hazard management and minimizing risk.