Search

A potential control of downstream channel width variations on the structure and planform of pool–riffle sequence local bed topography is a key to the dynamics of gravel bed rivers. How established pool–riffle sequences respond to time-varying changes in channel width at specific locations, however, is largely unexplored and challenging to address with field-based study. Here, we report results of a flume experiment aimed at building understanding of how statistically steady pool–riffle sequence profiles adjust to spatially prescribed channel width changes. We find that local bed slopes near steady-state conditions inversely correlate with local downstream width gradients when the upstream sediment supply approximates the estimated transport capacity. This result constrains conditions prior to and following the imposed local width changes. Furthermore, this relationship between local channel bed slope and downstream width gradient is consistent with expectations from scaling theory and a broad set of field-based, numerical, and experimental studies (n=88). However, upstream disruptions to coarse sediment supply through actions such as dam removal can result in a transient flipping of the expected inverse correlation between bed slope and width gradient, collectively highlighting that understanding local conditions is critical before typically implemented spatial averaging schemes can be reliably applied.

DNA double-strand breaks (DSBs) are linked to neurodegeneration and senescence. However, it is not clear how DSB-bearing neurons influence neuroinflammation associated with neurodegeneration. Here, we characterize DSB-bearing neurons from the CK-p25 mouse model of neurodegeneration using single-nucleus, bulk, and spatial transcriptomic techniques. DSB-bearing neurons enter a late-stage DNA damage response marked by nuclear factor κB (NFκB)–activated senescent and antiviral immune pathways. In humans, Alzheimer’s disease pathology is closely associated with immune activation in excitatory neurons. Spatial transcriptomics reveal that regions of CK-p25 brain tissue dense with DSB-bearing neurons harbor signatures of inflammatory microglia, which is ameliorated by NFκB knockdown in neurons. Inhibition of NFκB in DSB-bearing neurons also reduces microglia activation in organotypic mouse brain slice culture. In conclusion, DSBs activate immune pathways in neurons, which in turn adopt a senescence-associated secretory phenotype to elicit microglia activation. These findings highlight a previously unidentified role for neurons in the mechanism of disease-associated neuroinflammation.

This is the peer reviewed version of the following article: [Hassan, M. A., Chartrand, S. M., Radić, V., Ferrer-Boix, C., Buckrell, E., & McDowell, C. (2022). Experiments on the sediment transport along pool-riffle unit. Water Resources Research, 58, e2022WR032796. https://doi.org/10.1029/2022WR032796], which has been published in final form at https://doi.org/10.1029/2022WR032796. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. This study examines channel dynamics and mixed-load bed load transport through a riffle and pool sequence forced by downstream channel width variations within an experimental flume. The experiment consisted of four runs across which we compare and contrast local and spatially-averaged bed surface texture and topography, sediment transport rates and sediment mobility at five locations across a pool-riffle pair. Sediment transport was measured using mini Helley-Smith (HS) samplers and particle tracers seeded in the monitored riffle and pool. In this study, “local” sediment transport rates were highly variable across the five monitoring locations. The lowest sediment transport rate was recorded at the riffle tail whereas the highest rates were measured at the riffle head and the pool centre. The texture of the bed surface and transported load do not explain measured bedload transport trends and depending on how the measurements are aggregated differing interpretations are supported. In general, the bed texture in the pool was finer than the texture in the riffle, however specific grain-size percentile classes derived from pooled population analysis suggests little to no difference between pool and riffle texture. The combined results highlight the importance of acknowledging and applying analysis techniques to better understand the inherent variability of bedload transport within channel reaches where morphology differs, such as pools and riffles.

A potential control of downstream channel width variations on the structure and planform of pool-riffle sequence local bed topography is a key to the dynamics of gravel-bed rivers. How established pool-riffle sequences respond to time-varying shifts in the channel width, however, is largely unexplored and challenging to address with field-based study. Accordingly, here we report results of a flume experiment aimed at building understanding of the response of statistically steady pool-riffle sequences to prescribed channel width adjustments. We find that local bed slopes near steady-state conditions inversely correlate with local width change dynamics, and agree with expectations from scaling theory and a broad set of field-based, numerical and experimental studies (n=88). We also find that coarse sediment release from dam removal can temporally flip the expected inverse correlation, collectively highlighting that local conditions are important for understanding river morphology, and would be overlooked if analysis instead emphasized spatial averaging.

This material may be downloaded for personal use only. Any other use requires prior permission of the American Society of Civil Engineers. This material may be found at https://ascelibrary.org/doi/abs/10.1061/%28ASCE%29HY.1943-7900.0001993 The large pier of an emblematic bridge built in 2008 in the Ebro River (Zaragoza, Spain) obstructs the flow in high floods. Clear-water scour experiments in a scale model were conducted to anticipate maximum local scour depths and design riprap protections. These proved to be effective during a large flood event in 2015, but bed aggradation under the left bridge span and deep scour under the right one, not mirroring the bed deformation observed in the model, raised concerns about the bridge safety. The effects of the protected pier on the changes in the aftermath of the 2015 flood are discussed. It is shown that a large meander upstream generated an imbalance in the spanwise bedload distribution, leading to sedimentation on the left and contraction scour on the right. The paper argues for the need to take into account the effects of large piers on river morphology at the bridge planning phase. The case study shows that using a clear-water model to design the riprap protection is adequate, but more importantly, that the fluvial processes during a flood could only be studied with a live-bed model with geometrical detail of the full river reach, namely, the upstream meander. Thanks to the insightful, helpful comments by the Associate Editor. Thanks to the Ebro Water Authority (Marisa Moreno and Miriam Pardos) and Zaragoza Municipality (Luis Manso) for providing hydrological data and field surveys. We also thank the financial support of the FEDER-COMPETE2020 (POCI) and Portuguese funds (Foundation for Science and Technology, IP) through project PTDC/ECI-EGS/29835/2017—POCI-01-0145-FEDER-029835.

In this research, we explore the alluviation process of an artificial side-channel built in August 2015 and designed to alleviate bank erosion risks on the main river. The study area is located on the lower Gállego River, a meandering-wandering gravel-bed river draining the Southern Pyrenees. This river reach is still dynamic (i.e. chutes, meander migration and other riverine processes are often observed during floods) despite the presence of three dams in the upper catchment, encroachments and extensive mining during the 1960s–80s that led to severe incision. In this research, we focus on the effects of bankfull-like flood events (Qpeak¿=¿354¿m3/s) between February 2016 and February 2017, which alluviated 62% of the side-channel. This research aims at finding the physical mechanisms that explain the alluviation process and at putting them in the general context of the current geomorphic trend of the Gállego River in response to past and ongoing impacts. The causes of the alluviation are studied through collecting and analysing field data, which in turn, are used as input in a novel modelling strategy that includes one-dimensional hydraulic modelling, semi-empirical expressions to obtain the sediment supply entering the side-channel system and a morphodynamic model able to deal with granular sediment mixtures in compound channels. The analyses of the numerical results along with the recent history of impacts and general channel pattern adjustments help us understand about the morphodynamics mechanisms responsible for the alluviation of the side-channel.

Flume experiments were conducted to understand the role of episodic sediment supply on the threshold of motion, streambed state, and stability in gravel bed channels. We demonstrate the importance of bed surface evolution on grain entrainment and hence channel stability. The grain-size distribution (GSD) of the bed surface coarsened quickly under no feed conditions to develop an armored surface. Armor continued to experience particle exchange but remained intact throughout the experiment, and was relatively insensitive to subsequent sediment supply pulses. Bed surface structures (coarse-grain clusters, stone cells, and imbrication) developed during the experiments and continued to respond dynamically to sediment pulses. Thresholds of motion estimated by various methods increased during the experiment and fluctuated in response to changes in sediment supply, though they were poorly predicted either by surface grain-size changes or by surface structure. This reinforces the idea that the threshold for motion is not constant in poorly sorted gravel-bed streams, and it confirms that the critical Shields number in transport models represents a bed state parameter, not a grain mobility number. Changes are controlled by a variety of factors including sediment supply regime, the degree of bed structuring, and the history of bed evolution. Our results highlight the importance of sediment supply regime as a control on bed surface evolution and the channel stabilizing function played by surface structures. The experiment was conducted in the Mountain Channel Hydraulic Experimental Laboratory at the Department of Geography, The University of British Columbia. AH developed the idea and designed the experiments. Claudia vonFlotow, Maria Elgueta-Astaburuaga and Tobias Müller conducted the experiment. MAH, MS, CF-B, JPLJ, JGV, and MC analysed the data and wrote the paper. The research was funded by NSERC Discovery (to M. Hassan) and Canada Foundation for Innovation (to M. Hassan). Matteo Saletti was supported by a Grant from UBC Dean of Arts Office (to M. Hassan). We thank the journal editor, Luca Solari (the journal associate editor), Paul Grams and two anonymous reviewers for suggestions that significantly improved the presentation. Experimental data can be found at http://doi.org/10.5281/zenodo.1227423; images at http://doi.org/10.5281/zenodo.3558356; digital elevation models at https://zenodo.org/record/1478895#.W-ICWPZFxuk, and in the supporting information.

The human pressure upon an alluvial river in the Mediterranean region has changed its riverine and deltaic landscapes. The river has been channelized in the last 70 years while the delta has been retreating for more than a century (a set of data unknown, so far). The paper concentrates on the fluvial component, trying to connect it to the delta evolution. Is the channelization responsible for the delta retreat? We develop a method to compute the actual bed load transport with real information of the past river morphology. The paper compares the computation with very limited measurements, among which are bulk volumes of trapped material at a modern, deep river mouth. The decrease in sediment availability in the last 30 km of the channelized river is deemed responsible for the decrease in the sediment yield to the delta. Moreover, power development and flood frequency should be responsible for a baseline delta retreat during the 20th century. The sediment trapping efficiency of dams is less important than the flow regulation by dams, in the annual sediment yield. Therefore, it is more effective to dismantle channelization than to pass sediment at dams, to provide sand to the beaches.

With the increasing attention on environmental flow management for the maintenance of habitat diversity and ecosystem health of mountain gravel-bed rivers, much interest has been paid to how inter-flood low flow can affect gravel-bed river morphodynamics during subsequent flood events. Previous research has found that antecedent conditioning flow can lead to an increase in the critical shear stress and a reduction in sediment transport rate during a subsequent flood. But how long this effect can last during the flood event has not been fully discussed. In this paper, a series of flume experiments with various durations of conditioning flow are presented to study this problem. Results show that channel morphology adjusts significantly within the first 15 minutes of the conditioning flow, but becomes rather stable during the remainder of the conditioning flow. The implementation of conditioning flow can indeed lead to a reduction of sediment transport rate during the subsequent hydrograph, but such effect is limited only within a relatively short time at the beginning of the hydrograph. This indicates that bed reorganization during the conditioning phase, which induce the stress history effect, is likely to be erased with increasing intensity of flow and sediment transport during the subsequent flood event. This research has been supported by the National Natural Science Foundation of China (grant nos. 52009063, U20A20319, 91747207) and the China Postdoctoral Science Foundation (grant no. 2018M641368).

La pressió humana envers el Llobregat, un riu al·luvial mediterrani, ha canviat els seus paisatges fluvial i deltaic. La reducció d’aportació de sediment groller del riu ha provocat una regressió del delta de fins a 800 m des d’inicis del segle XX, unes dades inèdites fins el moment. L’article es centra en la component fluvial, tractant de lligar-la amb l’evolució del delta. Quines causes són responsables del retrocés del delta? Es desenvolupa un mètode per conèixer el transport sòlid de fons mitjançant informació autèntica de la morfologia del riu en el passat. L’article compara els càlculs amb mesuraments molt limitats, entre els quals hi ha el volum de sediment atrapat a la nova i profunda desembocadura. Els endegaments fets els darrers 70 anys al curs baix del Llobregat pel pas d’infraestructures han disminuït la disponibilitat de sediment en els darrers 30 km del riu, però aquesta no és la causa principal de la reducció de l’aportació de sediment groller cap a la costa. La construcció de nombroses rescloses al curs mig durant el segle XIX i la freqüència de crescudes que produeixen les grans aportacions de sediment serien responsables del retrocés de la línia de la costa del delta des de finals del segle XIX i al llarg del XX. La retenció de sediment als embassaments és menys rellevant que la regulació que aquests exerceixen en el règim de cabals, pel que fa a l’aportació de sediment a la costa. Conseqüentment, per proveir sorra a les platges del delta del Llobregat és més eficient l’enretirada de rescloses i eixamplar la llera desmantellant endegaments que no pas traspassar sediment als grans embassaments.

This is the accepted version of the following article: [Hassan, M. A., Saletti, M., Zhang, C., Ferrer‐Boix, C., Johnson, J. P. L., Müller, T., and von Flotow, C. ( 2020) Co‐evolution of coarse grain structuring and bed roughness in response to episodic sediment supply in an experimental aggrading channel. Earth Surf. Process. Landforms, 45: 948– 961. https://doi.org/10.1002/esp.4788], which has been published in final form at https://onlinelibrary.wiley.com/doi/abs/10.1002/esp.4788 We use flume experiments to better understand how gravel-bed channels maintain bed surface stability in response to pulses of sediment supply. Bed elevations and surface imagery at high spatial resolutions were used to quantify the co-evolution of surface grain-size distribution (GSD), bed roughness statistics, and bed surface structures (clusters, cells and transverse features). Using a new semi-automated method, we identified individual stone structures over a 2 m × 1 m area throughout the experiments. After an initial coarsening, surface GSD and armouring ratio remained nearly stable as sediment pulses caused net bed aggradation. In contrast, individual grain structures continued to form, increase or decrease in size, and disappear throughout the experiments. The response of the bed to sediment pulses depended on the history of surface roughness evolution and bed surface structure development, as these factors changed much more in response to supply perturbations earlier in the experiments compared to later, even as the bed continued to aggrade. We interpret that the dynamic production and destruction of bed surface structures can act as a ‘buffer’ to sediment supply pulses, maintaining a stable bed surface during aggradation with minimal change in grain size or armouring.

Dam removal has been demonstrated as one of the most frequent and effective fluvial restoration actions but at most dam removals, especially of small dams, there has been little geomorphological monitoring. The results of the geomorphological monitoring implemented in two dams in Urumea and Leitzaran Rivers (northern Spain) are presented. The one from Urumea River, originally 3.5 m high, impounded 500 m of river course, was removed instantaneously whereas that in Leitzaran River, was 12.5 m high, impounded 1,500 m of river course, and it is in its second phase of a 4-stage removal process. Changes in channel morphology, sediment size and mobility and river bed morphologies were assessed. The monitoring included different techniques: topographical measurements of the channel, terrestrial laser scanner measurements of river bed and bars, sediment grain size and transport; all of them repeated in four (May, August, November 2011 and May 2012) and five (July and September 2013, April and August 2014 and June 2015 fieldwork campaigns in Urumea and Leitzaran Rivers, respectively. Geomorphic responses of both dam removals are presented, as well as compared between them. Morphological channel adjustments occurred mainly shortly after dam removals, but with differences among the one removed instantaneously, that was immediate, whereas that conducted by stages took longer. Degradational processes were observed upstream of both dams (up to 1.2 m and 4 m in Urumea and Leitzaran Rivers, respectively), but also aggradational processes (pool filling), upstream of Inturia dam (2.85 m at least). Less evident aggradational processes were observed downstream of the dams (up to 0.37 m and 0.50 m in Urumea and Leitzaran Rivers, respectively). Flood events, specially a 100-year one registered during the monitoring period of Mendaraz dam removal, reactivated geomorphological processes as incision and bank erosion, whereas longitudinal profile recovery, grain-size sorting and upstream erosion took longer. This article is protected by copyright. All rights reserved.

Field observations, experiments, and numerical simulations suggest that pool-riffles along gravel bed mountain streams develop due to downstream variations of channel width. Where channels narrow, pools are observed, and at locations of widening, riffles occur. Based on previous work, we hypothesize that the bed profile is coupled to downstream width variations through momentum fluxes imparted to the channel surface, which scale with downstream changes of flow velocity. We address this hypothesis with flume experiments understood through scaling theory. Our experiments produce pool-riffle like structures across average Shields stresses t* that are a factor 1.5–2 above the threshold mobility condition of the experimental grain size distribution. Local topographic responses are coupled to channel width changes, which drive flows to accelerate or decelerate on average, for narrowing and widening, respectively. We develop theory which explains the topography-width-velocity coupling as a ratio of two reinforcing timescales. The first timescale captures the time necessary to do work to the channel bed. The second timescale characterizes the relative time magnitude of momentum transfer from the flowing fluid to the channel bed surface. Riffle-like structures develop where the work and momentum timescales are relatively large, and pools form where the two timescales are relatively small. We show that this result helps to explain local channel bed slopes along pool-riffles for five data sets representing experimental, numerical, and natural cases, which span 2 orders of magnitude of reach-averaged slope. Additional model testing is warranted.

The flow within a sewage screening chamber can be highly complex due to high-flow conditions and the geometrical constraints imposed by the screening chamber itself. This paper presents an application of a two-dimensional depth-averaged shallow flow model for flow simulations in a sewage screening chamber. For this purpose, the shallow flow model is improved by including a simple turbulence model. The improved model is able to reproduce the main features of the flow field, as confirmed by comparing with the measurements resulting from a scale-down laboratory model. The potential of numerical model for analysing the performance of the screening chamber is then demonstrated by testing a new design of including flow dissipating blocks to (i) re-distribute more evenly the flow through different flow passages installed with screens and (ii) reduce maximum mean flow velocity approaching the screens, so as to reduce the risk of damaging the screens during high-flow conditions.

Se presenta la metodología diseñada para el seguimiento geomorfológico del derribo de azudes, así como los resultados obtenidos hasta el momento en dos casos concretos: las presas de Mendaraz (río Urumea) e Inturia (río Leitzaran) en Gipuzkoa. Se han realizado secciones transversales, abundantes mediciones de procesos sobre testigos, análisis de los nuevos depósitos generados y muestreos granulométricos y morfométricos. En el caso de Mendaraz se han registrado rápidas movilizaciones de sedimentos e importantes cambios geomorfológicos aguas arriba y abajo del obstáculo. Este proceso de recuperación de la dinámica fluvial natural y de regularización del cauce fue favorecido y acelerado por la crecida extraordinaria de noviembre de 2011. Además de sus beneficios para el estado ecológico, se ha constatado que el derribo de presas es una medida eficaz para la restauración de la dinámica geomorfológica en cauces fluviales. El seguimiento geomorfológico es una labor fundamental para cuantificar y valorar la dinámica generada a raíz del derribo de la presa y toda su evolución posterior. A methodology for dam removal monitoring and the results of two case studies is presented: Mendaraz dam (Urumea River) and Inturia dam (Leitzaran River), both located in Gipuzkoa. This monitoring is conducted by river survey cross-sections, measurements of processes and granulometrical analysis. Fast sediment erosion and sedimentation together with geomorphological adjustments were detected after dam removal. In Mendaraz an extraordinary flood favored these processes. The recovery of fluvial dynamics shows benefits not only from the ecological point of view but also for the restoration of natural river dynamics. Geomorphological monitoring is a key tool to quantify and assess river evolution and dynamics after dam removal.

Dam removal is commonly used for river restoration. However, there are still some uncertainties associated with dam removal, mainly related to the sediment transport rates released downstream from the deposit that had previously filled the impoundment. This research studies the physical response to dam removal in the antecedent deposit by answering the following questions: (a) how does an initial channel excavated into the deposit evolve, and (b) what is the time distribution of the material released during the early stages of the process. These goals are achieved by an experimental campaign using a poorly sorted mixture of sediment in the antecedent deposit. The research shows that for the given conditions of our experiments, the rate at which the sediment is released depends on the height of the removed dam, the water discharge and the maximum potential volume of sediment to be eroded. This investigation provides new insights of the width evolution when the sediment is composed of a poorly sorted mixture. This evolution is linked to the bed degradation rates: channel narrows during a rapid incisional phase, and subsequently widens when bed degradation rates decrease. Channel width changes propagate upstream as a convection-like perturbation associated with a kinematic wave starting at the location of the antecedent dam. These features are modeled through a new numerical model accounting for mixtures. More specifically, a set of equations has been derived for the variation of bed elevation, channel bottom width, and bed grain-size distribution, which when solved numerically, describe the observed channel processes.

In order to study the morphological evolution of river beds composed of heterogeneous material, the interaction among the different grain sizes must be taken into account. In this paper, these equations are combined with the two-dimensional shallow water equations to describe the flow field. The resulting system of equations can be solved in two ways: (i) in a coupled way, solving flow and sediment equations simultaneously at a given time-step or (ii) in an uncoupled manner by first solving the flow field and using the magnitudes obtained at each time-step to update the channel morphology (bed and surface composition). The coupled strategy is preferable when dealing with strong and quick interactions between the flow field, the bed evolution and the different particle sizes present on the bed surface. A number of numerical difficulties arise from solving the fully coupled system of equations. These problems are reduced by means of a weakly-coupled strategy to numerically estimate the wave celerities containing the information of the bed and the grain sizes present on the bed. Hence, a two-dimensional numerical scheme able to simulate in a self-stable way the unsteady morphological evolution of channels formed by cohesionless grain size mixtures is presented. The coupling technique is simplified without decreasing the number of waves involved in the numerical scheme but by simplifying their definitions. The numerical results are satisfactorily tested with synthetic cases and against experimental data. The participation of C. Ferrer-Boix in this research was made possible in part by support from the Spanish Ministry of Education, programme “Campus de Excelencia Internacional CEI Iberus”. Additionally, this work was partially supported and funded by the Spanish Ministry of Science and Technology under research project CGL2011-28590 and by Diputación General de Aragón, DGA, through FEDER funds.

This text addresses the particular case of motion and causes of motion of granular material as bedload in the fluvial domain. The aim is to perform and overview of key concepts, main achievements and recent advances on the description of the processes involved in erosion, deposition and transport of sediment in open-channels. The theoretical functional relations describing both the initiation of motion and the sediment transport are introduced. The classical problem of the initiation of motion of particles is treated at grain and at reach scales, accounting for the stochastic nature of flow. Concepts of granular kinematics and methods for quantifying the sediment transport rate in rivers are presented. The latter results from the interactions between the flow and the particles on the bed surface. The sediment transport rate, which has been shown to have a stochastic behaviour, is converted to a lumped statistic distribution. Finally, some field and laboratory techniques for measuring sediment transport, accounting for its inherent fluctuations, are introduced.

This paper presents the results of a Gilbert-type delta progradation experiment within an impoundment created by a dam. The delta was composed of a poorly sorted sand-gravel mixture in a bedload-dominated environment. The main goal of the paper is to analyse the sorting process of material within the deposit as the delta progrades towards the dam. Bed profile evolution has been documented and the entire delta has been extensively sampled in order to study sorting processes. Longitudinal and vertical sorting mechanisms are illustrated. What is novel in this investigation is the complete record, within an entire deltaic deposit, of the vertical distribution of streamwise sorting in the absence of suspended load. The data presented herein provide a detailed description of sorting processes in a Gilbert-type delta. The experimental set-up, the water flow and the sediment feed rate chosen determine the evolution of the delta: it initially progrades with little topset aggradation and degrades afterwards. Experimental results fit well with a previously presented empirical sorting model, despite the fact that the experimental conditions used here were well outside the range of those used to derive that model. The relative coarsening of the upper layers of the delta is found to be related to the slow speed at which the delta progrades, the formation of a mobile armour layer and the erosion of the topset towards the end of the run. Furthermore, a strong correlation between the coarsening of the bottom layer of the delta and its front height has been documented and explained: as the delta gets higher, as there is more space to sort sediment, it is more likely that coarse particles failing near the top of the foreset reach the bottom of the foreset. These findings provide new and useful data documenting sediment sorting in granular, bedload-dominated deltas.

Gravel bed rivers commonly exhibit a coarse surface armor resulting from a complex history of interactions between flow and sediment supply. The evolution of the surface texture under single storm events or under steady flow conditions has been studied by a number of researchers. However, the role of successive floods on the surface texture evolution is still poorly understood. An experimental campaign in an 18 m-long 1 m-wide flume has been designed to study these issues. Eight consecutive runs, each one consisting of a low-flow period of variable duration followed by a sudden flood (water pulse) lasting 1.5 h, have been conducted. The total duration of the experiment was 46 h. The initial bed surface was created during a 280 h-long experiment focused on the influence of episodic sediment supply on channel adjustments. Our experiments represent a realistic armored and structured beds found in mountain gravel bed rivers. The armor surface texture persists over the duration of the experiment. The experiment exhibits downstream fining of the bed-surface texture. It was found that sorting processes were affected by the duration of low-flow between flood pulses. Since bed load transport is influenced by sediment sorting, the evolution of bed load transport is impacted by the frequency of the water pulses: short interpulse durations reduce the time over which fine material (transported as bed load) can be winnowed. This, in turn, contributes to declining reduction of the bed load transport over time while the sediment storage increases.

A one-dimensional model that is able to store the stratigraphy emplaced by a prograding delta is validated against experimental results. The laboratory experiment describes the migration of a Gilbert delta on a sloping basement into standing water, i.e., a condition in which the stratigraphy emplaced by the delta front is entirely stored in the deposit. The migration of the delta front and the deposition on the delta top are modeled with total and grain-size-based mass conservation models. The vertical sorting on the delta front is modeled with a lee-face-sorting model as a function of the grain size distribution of the sediment deposited at the brinkpoint, i.e., at the downstream end of the delta top. Notwithstanding the errors associated with the grain-size-specific bedload transport formulation, the comparison between numerical and experimental results shows that the model is able to reasonably describe the progradation of the delta front, the frictional resistances on the delta top, and the overall grain size distribution of the delta top and delta front deposits. Further validation of the model in the case of variable base level is currently in progress to allow for future studies, at field and laboratory scale, on how the delta stratigraphy is affected by different changes of relative base level.

The evolution of the bed surface and bedload transport rates under a sudden increase of the water discharge (pulse) has been relatively unexplored in an experimental setup. Further, no systematic investigation has been performed to analyze the role played by the frequencies at which these pulses occur in the bed adjustments. This contribution presents a novel experimental investigation which aims to shed some light on: (i) how river bed evolves to variable flow conditions under constant sediment feed and (ii) what is the influence of the precedent morphological history in these adjustments. Water pulses of fixed 1.5 h of duration were alternated after periods of low flow of variable duration – from 1 h to 10 h. Runs were performed at a constant sediment feed rate of 7.50 kg/h during both low and high flow periods. Eight consecutive runs were performed in such a way the final configuration of the previous run was the initial condition for the subsequent experiment. Preliminary results show bed structuring (when it is partially destroyed by a sudden increase of the water flow after a long period of time of no feed and when it is reconstructed after a series of pulses) can play an important role in sediment transport regime in gravel bed rivers.

A one-dimensional model that is able to store the stratigraphy emplaced by prograding bedforms with a downstream slip face is validated against experimental results. The laboratory experiment describes the migration of a Gilbert delta on a sloping basement into standing water, i.e. a condition in which the stratigraphy emplaced by the delta front (the lee face) is entirely stored in the deposit. The migration of the delta front and the deposition on the delta top are modeled with a total and a grain size based mass conservation models. The vertical sorting on the delta front is modeled with a lee face sorting model, as a function of the grain size distribution of the sediment deposited at the brinkpoint, i.e. at the downstream end of the delta top. Notwithstanding the errors associated with the grain size specific bedload transport formulation, the comparison between numerical and experimental results shows that the model is able to reasonably describe the progradation of the delta front, the frictional resistances on the delta top, and the overall grain size distribution of the delta top and delta front deposits. Further validation of the model under the case of variable base level is currently in progress to allow for future studies on Gilbert delta progradation at field scale.

Transient scour and fill refers to the general scour and fill in a riverbed due to flood passage. It was given much attention in times of Leopold and Maddock (1953) and Colby (1964) from records of outstanding case studies. Field data from a station in a contracted, long reach of the large sand-bed Pilcomayo River –1192 newly typed in forms– are presented, with sediment concentration in the database up to 60 kg/m 3 , mostly wash load. Water surface rises and falls during a flood event, whereas streambed evolves in reverse (it renders the analogy to an accordion). Streambed scouring accounts for 36% of the change in flow area during floods and data reveal scouring up to 4 m from the streambed elevation when flood starts. Velocity data display large hysteresis loops inexplicable by unsteady flow only. Once streambed starts to fall, river bed scour is self-sustaining. Water depth gradient appears to be a crucial variable in a novel equation for transient scour and fill derived from Exner equation. It turns out that flow non-uniformity is relevant to explain transient scour and fill in contrast to flow unsteadiness, which is not. Quasi-steady gradually varied flow equation solved along the contracted reach is the base for a model that captures well the recorded bed evolution. The model explains most of the transient scour and fill by the downstream expansion, but the non-uniform flow and the bed material suspended load also play a role. The model fails on the days of highest sediment concentrations.

Historical information was used to study river degradation in a case study in NE Spain. The accumulation of good topographical information since the 1940s for the Gállego river (a tributary of the Ebro river, which drains about 4,000 km2 of the southern slopes of the Pyrenees) enables a comparison of longitudinal bed profiles. Historical research has also revealed the volume of gravel mined, which amounts to ≈ 1 million m3 compared to the volume of alluvium lost due to incision in the same period, which is ≈ 2 million m3. This unbalance is explained by a simple model based on a bedload equation and an algorithm to determine whether effective bedload transport is controlled by transport capacity or by the supply of sediment. It follows from the analysis that the incision process has changed the magnitude of the shear stresses on the bottom. This is because, as the river becomes degraded, higher discharges fit into the main channel before spilling.

This chapter reviews the basic principles of sediment transport in streams, channel classification, mountain channel morphology, and associated implications for fish spawning habitats. Local sediment transport processes construct the differing channel morphologies, which are often opportunistically used by spawning fish. Sediment load in streams can be transported in various ways. Dissolved load corresponds to material transported in solution with the fluid, whose concentration depends primarily on the geochemical character of basin geologic materials, and the dissolution of these materials by groundwater flow. Sediment sources in river basins can be distinguished as two types for timescales ranging from individual floods to many flood seasons. Sediment resting on the streambed will move, or become entrained, when the overlying fluid pressure, or shear stress, exceeds some threshold value. Lumped empirical or semi-empirical expressions have been widely used to compute bedload transport rates. Sediment transport and channel morphology are important components of physical habitat for aquatic biota in rivers.