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Nearshore incised valleys are important conduits for the transport of sediment, nutrients, pollutants and organic carbon from the continents to the sea. Therefore, it is essential to understand the autogenic evolution of deltas confined within incised valleys and how such evolution is affected by relative sea‐level rise. To date, limited research has focused on how deltas constrained by incised valleys or other forms of antecedent topography respond to rising sea level. An existing theory of autostratigraphy envisages scenarios in which two‐dimensional or unconfined three‐dimensional fan deltas can experience three evolutionary stages under constant rates of relative sea‐level rise and sediment supply: progradation, autoretreat and post‐autobreak transgression. In this work, an integrated study of geometric numerical models and physical experiments is undertaken to investigate autostratigraphic delta evolution for a variety of incised‐valley geometries, under conditions of constant rates of relative sea‐level rise and sediment supply. Results indicate that interplays of antecedent topography (valley geometries) and sediment mass balance expressed in resultant deltaic geometries can result in autogenic changes in shoreline dynamics and river avulsion frequency on deltas. The following primary findings arise. (i) Compared to valleys with rectangular and trapezoidal cross‐sectional profiles, valleys with triangular cross‐sections tend to contain deltas that experience faster rates of progradation, autoretreat and post‐autobreak transgression under rising sea level, and exhibit a more prominent convex‐seaward shoreline trajectory. (ii) The shoreline trajectory is also related to delta topset geometry, becoming more convex‐seaward under decreasing topset slopes. (iii) River avulsion frequency on deltas with rising sea level varies markedly across valleys with different geometries, even under the same rate of relative sea‐level rise; this is attributed to the difference in temporal evolution of shoreline migration for different valley geometries and the resultant difference in the delta topset aggradation. This study highlights complexities in responses of sedimentary systems under the confinement of different topographic configurations that have hitherto largely been overlooked in sequence‐stratigraphic models. The findings provide insight into future shoreline behaviour and river avulsion hazard on confined deltas, and for decoding the stratigraphic record. [ABSTRACT FROM AUTHOR]

Alluvial rivers aggrade, incise, and adjust their sediment‐transport rates in response to changing sediment and water supply. Fluvial landforms, such as river terraces, and downstream stratigraphic archives may therefore record information about past environmental change. Using a physically based model describing sediment transport and long‐profile evolution of alluvial rivers, we explore how their responses to environmental change depend on distance downstream, forcing timescales, and whether sediment or water supply is varied. We show that amplitudes of aggradation and incision, and therefore the likelihood of terrace formation, are greater upstream and in shorter and/or wetter catchments. Aggradation and incision, and therefore terrace ages, may also lag behind environmental change. How sediment‐transport rates evolve depends strongly on whether water or sediment supply is varied. Diverse responses to environmental change could arise in natural alluvial valleys, controlled by their geometry and hydrology, with important implications for paleo‐environmental interpretations of fluvial archives. Plain Language Summary: Rivers carry sediment from upland regions, where it is produced by weathering and erosion, to low‐lying areas where it is deposited and stored. This process is sensitive to climate: for example, wetter conditions enhance rivers' capacity to transport sediment, causing rivers to cut into their beds, which in turn remobilizes sediment that is transported downstream. Changing climate may also lead to enhanced sediment production and supply to rivers, causing them to accumulate sediment in their beds. Climate cycles can therefore produce alternating periods of sediment accumulation and removal, resulting in ancient floodplains being abandoned and preserved alongside modern rivers ("terraces"), and in variations in the amounts of sediment delivered downstream. Terraces and patterns of sediment accumulation downstream could therefore provide valuable records of past climatic change. We develop a mathematical model that describes the effects of climatic change on river‐bed height and sediment‐transport rates. We show how the amount and timing of river in‐filling and down‐cutting, and the timescales over which they occur, depend on the rate of climatic change, on river length and associated drainage‐basin area, and on the amount of water it carries. These predictions will facilitate more detailed interpretations of terrace records along rivers and of the sedimentary deposits that they leave behind. Key Points: Alluvial rivers respond in diverse ways to cyclical climatic change depending on their geometry and hydrology and on the forcing frequencyTerrace formation is favored upstream and in shorter and wetter catchments and may lag appreciably behind environmental changeVariation in sediment output lags variation in sediment supply but can lead and be amplified with respect to variation in water supply [ABSTRACT FROM AUTHOR]

Detrital 10Be from continental river sands or submarine sediments has been extensively used to determine the average long-term denudation rates of terrestrial catchments, based on the assumption that the rate of cosmogenic nuclide production by the interaction of source rocks with cosmic radiation balances out the loss of these nuclides by surface denudation. However, the 10Be signal recorded in sediments may be affected at the source by the response time of mountainous catchments to high-frequency forcings. In addition, transient sediment storage in piedmonts, alluvial plains and lakes or near the coast may also induce a difference between the erosive signal and its record in the sedimentary sink. Consequently, a significant part of the signal recorded in shallow-water sediments can be lost, as deep marine sediments may simultaneously record a signal coming from newly eroded source rocks along with one coming from the destabilization of previously deposited sediments. In this paper, we use the landscape evolution model Badlands to simulate erosion, deposition and detrital 10Be transfer from a source-to-sink sedimentary system (the Var River catchment, southern French Alps) over the last 100 kyr. We first compare model-based denudation rates with the ones that would be extracted from the 10Be record of local continental sediments (equivalent to river sands) and from sediments deposited offshore over time in order to examine if this record provides an accurate estimate of continental denudation rates. Then, we examine which conditions (precipitation rate, flexure, ice cover) satisfy published measured river incision rates and 10Be concentration in submarine sediments. Our results, based on the Var catchment cosmic ray exposure dating and modelling indicate that, while river sands do accurately estimate the average denudation rate of continental catchments, this is much less the case for deep submarine sediments. We find that deep-sea sediments have a different and often much smoother 10Be signature than continental ones and record a significant time lag with respect to imposed precipitation rate changes, representing the geomorphological response of the margin. A model which allows us to fit both measured 10Be concentration in marine sediments and river incision rates on land involves an increase in precipitation rates from 0.3 to 0.7 m yr -1 after 20 ka, suggesting more intense precipitation starting at the end of the Last Glacial Maximum. [ABSTRACT FROM AUTHOR]

River aggradation or incision at different spatial-temporal scales are governed by tectonics, climate change and surface processes which all adjust the ratio of sediment load to transport capacity of a channel. But how the river responds to differential tectonic and extreme climate events in a catchment is still poorly understood. Here, we address this issue by reconstructing the distribution, ages and sedimentary process of fluvial terraces in a tectonically active area and monsoonal environment in the headwaters of the Yangtze River in the eastern Tibet Plateau. Field observations, topographic analyses and optically stimulated luminescence (OSL) dating reveal a remarkable fluvial aggradation, followed by terraces formations at elevations of 62-55 m (T7), 42-46 m (T6), 38 m (T5), 22-36 m (T4), 18 m (T3), 11 m (T2), 2-6 m (T1) above the present floodplain. Gravelly fluvial accumulation more than 62 m thick has been dated prior to 24-19 ka. It is regarded as a response to cold climate during the Last Glacial Maximum. Subsequently, the strong monsoon precipitation contributed to cycles of rapid incision and lateral erosion, expressed as cut-in-fill terraces. The correlation of terraces suggests that specific tectonic activity controls the spatial scale and geomorphic characteristics of the terraces, while climate fluctuations determine the valley filling, river incision and terrace formation. Debris and colluvial sediments are frequently interbedded in fluvial sediment sequences, illustrating the episodic short-time blocking of the channel around 20 ka. This indicates the potential impact of extreme events on the geomorphic evolution in the rugged terrain. [ABSTRACT FROM AUTHOR]

Waterfall plunge pools experience cycles of sediment aggradation and scour that modulate bedrock erosion, habitat availability, and hazard potential. We calculate sediment flux divergence to evaluate the conditions under which pools deposit and scour sediment by comparing the sediment transport capacities of waterfall plunge pools (Qsc_pool) and their adjacent river reaches (Qsc_river). Results show that pools fill with sediment at low river discharge because the waterfall jet is not strong enough to transport the supplied sediment load out of the pool. As discharge increases, the waterfall jet strengthens, allowing pools to transport sediment at greater rates than in adjacent river reaches. This causes sediment scour from pools and bar building at the downstream pool boundary. While pools may be partially emptied of sediment at modest discharge, floods with recurrence intervals >10 yr are typically required for pools to scour to bedrock. These results allow new constraints on paleodischarge estimates made from sediment deposited in plunge pool bars and suggest that bedrock erosion at waterfalls with plunge pools occurs during larger floods than in river reaches lacking waterfalls. [ABSTRACT FROM AUTHOR]

Formation of alluvial stratigraphy is controlled by autogenic processes that mix their imprints with allogenic forcing. In some alluvial successions, sedimentary cycles have been linked to astronomically‐driven, cyclic climate changes. However, it remains challenging to define how such cyclic allogenic forcing leads to sedimentary cycles when it continuously occurs in concert with autogenic forcing. Accordingly, we evaluate the impact of cyclic and non‐cyclic upstream forcing on alluvial stratigraphy through a process‐based alluvial architecture model, the Karssenberg and Bridge (2008) model (KB08). The KB08 model depicts diffusion‐based sediment transport, erosion and deposition within a network of channel belts and associated floodplains, with river avulsion dependent on lateral floodplain gradient, flood magnitude and frequency, and stochastic components. We find cyclic alluvial stratigraphic patterns to occur when there is cyclicity in the ratio of sediment supply over water discharge (Qs/Qw ratio), in the precondition that the allogenic forcing has sufficiently large amplitudes and long, but not very long, wavelengths, depending on inherent properties of the modelled basin (e.g. basin subsidence, size, and slope). Each alluvial stratigraphic cycle consists of two phases: an aggradation phase characterized by rapid sedimentation due to frequent channel shifting and a non‐deposition phase characterized by channel belt stability and, depending on Qs/Qw amplitudes, incision. Larger Qs/Qw ratio amplitudes contribute to weaker downstream signal shredding by stochastic components in the model. Floodplain topographic differences are found to be compensated by autogenic dynamics at certain compensational timescales in fully autogenic runs, while the presence of allogenic forcing clearly impacts the compensational stacking patterns. [ABSTRACT FROM AUTHOR]

Research on factors affecting sediment regime in glacierized catchments under warming climates is still scarce despite its societal relevance. In particular, coarse bedload transport has never been quantitatively related to water runoff origin (snowmelt vs glacier melt), which provides important information on the role of different sediment sources (glaciers vs hillslopes and channel bed). Drawing on data from multiple spatial and temporal scales in a paradigmatic Alpine glacierized catchment, we show that glacier melt flows play a key role in coarse sediment transport dynamics. Bedload concentration measured during glacier melt flows is up to 6 orders of magnitude larger than during snowmelt. At the catchment scale and within the channel, however, minimal aggradation and degradation was detected over almost a decade. In addition, sedimentation rates at a hydropower weir, inferred from flushing frequency during the last four decades, are tightly associated to summer air temperature and not to precipitation trends, and most of sediment export occurred in July-August. However, sediment flushing frequency has been decreasing since the late 1990s despite very warm summers in the following decades. Collectively, these findings indicate that sediment is dominantly sourced from within glacier-covered areas and that transport rates are thus dictated by seasonal and multi-annual glacial dynamics. As glacier melt flows decrease due to ice mass loss, our results suggest that, for similar basins, a progressive shift from supply-limited (driven by glacier activity) to transport-limited (during rainfall-induced events) sediment transport will occur, disrupting the current near-equilibrium channel conditions. • Hydrograph separation coupled to bedload measurements in a glacierized catchment. • Bedload during glacier melt up to 6 orders of magnitude larger than during snowmelt. • Decadal channel bed equilibrium despite large bedload yields from glaciers. • Sediment export statistically related to summer air temperatures and not to precipitation. • Sediment export peaked in late 1990s despite rising temperatures in 21st century. [ABSTRACT FROM AUTHOR]

The Upper Devonian Duvernay Formation of western Canada is a prolific source rock that in recent years has become an important exploration target for shale gas and oil. We present a sequence stratigraphic model for the Duvernay Formation that characterizes relationships between sea level, basin circulation, lithofacies distribution, and mechanisms for sediment transport and organic-enrichment. The study demonstrates lateral and stratigraphic variability in the character of sequence stratigraphic surfaces and systems tracts and interprets this variability in the context of sea level cycles at varying time scales. Correlation of 24 core descriptions to a network of 759 wells with wireline logs led to the creation of a basin-scale sequence stratigraphic framework. Three 3rd order depositional sequences are identified; the middle sequence straddles a 2nd order maximum flooding surface. Each systems tract shows unique trends in lithofacies, rock properties, and wireline character. Transgressive systems tracts are characterized by the increasingly widespread deposition of clay-poor, siliceous, organic-rich mudstones in the basin, resulting from decreased clastic and carbonate input, concentration of pelagic organic matter and siliceous radiolaria, and low oxygen concentrations in basin center bottom waters. Highstand systems tracts are characterized by sustained radiolarian input and increases in carbonate sediment due to shedding of carbonate detritus during aggradation and progradation of platform margins and reefs. Lowstand systems tracts are more clastic-rich and organic-poor. Near reef complexes, coarse carbonates locally mark the base of the lowstand systems tract. Bottom currents reworked mud- and silt-sized material in toe-of-slope locations and contributed to slope-parallel progradation during regression. • Depositional sequences, parasequences, and bedsets are recognized. • Lowstand: widespread oxygenation, reduced TOC, increased clastics and carbonates • Organic-rich, biosiliceous mudstones widespread during transgression and highstand • Contourite siltstones are interbedded with organic-rich biosiliceous mudstones. • Currents erode platform slope and contribute to slope-parallel progradation. [ABSTRACT FROM AUTHOR]

Abstract: Transient sediment storage and mixing of deposits of various ages during transport across alluvial piedmonts alter the clastic sedimentary record. We quantify buffering and mixing during cycles of aggradation–incision in the north piedmont of the Eastern Tian Shan. We complement existing chronologic data with 20 new luminescence ages and one cosmogenic radionuclide age of terrace abandonment and alluvial aggradation. Over the last 0.5 Myr, the piedmont deeply incised and aggraded many times per 100 kyr. Aggradation is driven by an increased flux of glacial sediment accumulated in the high range and flushed onto the piedmont by greater water discharge at stadial–interstadial transitions. After this sediment is evacuated from the high range, the reduced input sediment flux results in fluvial incision of the piedmont as fast as 9 cm year−1 and to depths up to 330 m. The timing of incision onset is different in each river and does not directly reflect climate forcing but the necessary time for the evacuation of glacial sediment from the high range. A significant fraction of sediments evacuated from the high range is temporarily stored on the piedmont before a later incision phase delivers it to the basin. Coarse sediments arrive in the basin with a lag of at least 7–14 kyrs between the first evacuation from the mountain and later basinward transport. The modern output flux of coarse sediments from the piedmont contains a significant amount of recycled material that was deposited on the piedmont as early as the Middle Pleistocene. Variations in temperature and moisture delivered by the Westerlies are the likely cause of repeated aggradation–incision cycles in the north piedmont instead of monsoonal precipitation. The arrival of the gravel front into the proximal basin is delayed relative to the fine‐grained load and both are separated by a hiatus. This work shows, based on field observations and data, how sedimentary systems respond to climatic perturbations, and how sediment recycling and mixing can ensue. [ABSTRACT FROM AUTHOR]

High-resolution topographic surveys using LiDAR and multibeam sonar can be used to characterize and quantify fluvial change. This study used repeat surveys to explore how topographic change, fluvial processes, sediment budgets, and aggradation and incision rates vary across spatial scales and across two contrasting decadal flow regimes in a regulated gravel/cobble river. A novel method for quantifying digital elevation model uncertainty was developed and applied to a topographic change detection analysis from 2006/2008 to 2014. During this period, which had four modest ~ 3–5 year floods, most sediment was laterally redistributed through bank erosion and channel migration. Erosion primarily occurred in the floodplain (97,000 m 3 ), terraces (80,000 m 3 ), and lateral bars (58,000 m 3 ); while deposition occurred in the adjacent pools (73,000 m 3 ), fast glides (48,000 m 3 ), and runs (36,000 m 3 ). In contrast, significantly higher magnitude and longer duration floods from 1999 to 2006/2008 caused sediment to be displaced longitudinally, with the upstream reaches exporting sediment and the downstream reaches aggrading. The river maintained floodplain connectivity during both periods, despite different processes dominating the type of connectivity. Larger floods promoted overbank scour and avulsion, while smaller floods promoted bank erosion and lateral migration. This study explores and illustrates how the geomorphic response to contrasting flood regimes in a nonuniform river is highly dependent on which landforms are controlling hydraulics. [ABSTRACT FROM AUTHOR]

This paper develops a numerical model for the analysis of sediment transport in irrigated channels. The overland flow equation considers infiltration into the subsurface. The model solves Saint Venant and sediment continuity equations to analyze overland flow and sediment transport, while Richard's equation is used to analyze subsurface flow. Saint Venant and sediment continuity equations are solved using MacCormack-scheme-based finite-difference method and Richard's equation is solved using a mass conservative fully implicit finite-difference method. The model is validated by comparing model-predicted aggradation with experimental observations. The model is used to study the effect of infiltration on aggradation of a channel bed during sediment-laden flows. It was observed that infiltration significantly affects and alters the aggradation phenomenon and aggradation considering infiltration is found to be higher than that without considering infiltration. Among the different unsaturated soil parameters, the saturated hydraulic conductivity and initial dryness of the soil were found to be the main factors affecting the sediment transport in irrigated channels. [ABSTRACT FROM AUTHOR]

Typically, the analysis and design of fluvial flood defence schemes is based on a single N year extreme flow event using a single survey of the river channel and flood plains. Adopting this approach assumes that the channel capacity is identical for all subsequent N year events. If one assumes that the typical design life for a flood defence scheme is of the order of 50 years, then such an approach is flawed because river channel morphology, and hence flood conveyance, may change considerably over this time scale. Therefore, to provide a more robust estimate of future flood inundation, a sensitivity analysis of these changes should be undertaken. This paper proposes a modeling methodology that combines a stochastic model, for estimating streamflow throughout the design period, and a 1D sediment transport model (HEC-RAS), to enable this sensitivity to be included in flood inundation modeling and defence scheme design. The methodology is demonstrated through conceptual implementation to evaluate the change in water surface elevation (WSE) along an alluvial river (River Caldew, England) reach after 50 years of sediment transport. Changes in WSE are assessed when the reach is natural (no flood defences) and modified (with idealized flood defences). Results show that the construction of the flood defence scheme does not alter the overall morphological pattern of the reach but can significantly increase (260%) local aggradation. Additionally, 50 years of morphological change have the potential to increase WSE such that high flows, previously confined within the channel, can overtop the banks and become flood events; and that, the standard freeboard levels of the flood defence scheme may be insufficient to prevent overtopping when morphological change is considered. The method can be considered as a semiquantitative modeling methodology to account for the sediment-related sensitivity of flood risk management; and provides valuable insights into the potential magnitude that this has on future flood inundation. [ABSTRACT FROM AUTHOR]

In mountain environments, the coupling of hillslopes processes with the channel network during extreme events is of great importance for rivers dynamics, as debris flows and landslides are among the most important sources of sediments. The Stolla Creek (40 km2 drainage area, South Tyrol, Italy) is a confined and partly confined mountain channel that was affected by an extreme flood in August 2017, followed by a smaller event in August 2020. The geomorphic effects of the two floods were investigated both in the main channel and over the entire basin with the aim to assess the impacts of the lateral sediment connectivity to the channel response and to the event-scale sediment export. An integrated approach was applied, including radar rainfall estimation, hydrologic-hydraulic analysis, analysis of morphological changes and sediment delivery to the stream network. Hillslope and channel processes were mapped and characterized by using geomorphological analysis of multitemporal orthophotos and Digital Terrain Models. Debris-flow connectivity to the main channel was derived by combining field evidence and GIS-based analysis. The 2017 flood was caused by rainfall with a short duration (6 h) and a rainfall intensity exceeding 45 mm/h. More than 600 debris flows were triggered in the Stolla basin, and the main channel experienced widening (width ratio between 1.3 and 4.9) through bank erosion and overbank deposition. Widening was accompanied by aggradation in the river corridor up to 1.2 m or incision down to −2.2 m. The 2020 flood was characterized by lower rainfall intensity (max 17 mm/h) and a longer duration (48 h), and debris flows were not triggered. The moderate magnitude of the 2020 flood peak did not lead to channel widening, but marked bed incision (up to −1.4 m) occurred in the reaches where aggradation took place during the 2017 event. In both flood events, limited volumes of sediments were exported from the catchment outlet. Overall, our results highlight how structural connectivity at the basin scale determines the potential sediment cascades linking hillslopes to channels but time-varying functional connectivity – driven by hydrological drivers as rainfall intensities and durations – eventually control the actual sediment transport effectiveness both on hillslopes and along the channel. • An integrated geomorphological and hydrological approach is applied to two floods • Basin structural connectivity well describes potential sediment delivery to the main channel • Functional connectivity determined by rainfall characteristics controlled actual sediment supply and transport [ABSTRACT FROM AUTHOR]

The Jinsha River, the upper reaches of the Yangtze River, winds and incises through the eastern margin of the Tibetan Plateau, where Asian monsoon climate prevails. Along the Jinsha River, extensive and thick Quaternary valley fillings and multiple river terraces have been developed. However, their specific relations to climate change and tectonics remain elusive. In this study, we conducted field investigation, chronological studies and numerical modeling in its lower reaches. Our study indicates that the extensive valley fillings in the Wudongde-Menggu segment were formed in cold-arid climate with Optically Stimulated Luminescence (OSL) ages of ~23–19 ka B.P., coinciding with the timing of the Last Glacial Maximum (LGM). We suggest that the post-LGM warming and humidifying, by restrained sediment supply while enhanced discharge, switched the valley from prevailing aggradation into incision. Multiple subsequent fill-cut terraces, postdating the LGM, were formed and nested in the valley fillings. OSL data indicate that sediments of three terraces (T1–T3) were formed at ~10 ka B.P., ~11.5–12.5 ka B.P. and ~13.5 ka B.P., respectively. Our study suggests that these terraces are sensitive records of millennial and sub-millennial cold-arid climate perturbations superimposed over the post-LGM deglaciation. Our modeling highlights a high sensitivity of valley aggradation and incision to cold-arid and warm-wet climate change, as well as a fast valley landscape response in the Jinsha River characterized by high sediment transport influx. We propose that valley landscape and terraces serve as a timely record for climate change in certain rivers with short response time threshold characterized by high sediment transport influx. • Valley filling was driven by the cold-arid climate in the LGM. • Fill-cut terraces record sub-millennial cold-arid climate perturbations. • Modeling supports a sensitive and fast response of Jinsha River to climate change. • The present valley landscape remains in a transient adjustment since the LGM. [ABSTRACT FROM AUTHOR]

• First monitoring of sediment transfer in submarine channel from source to sink. • River input and lobe aggradation approximately balances over decadal timescales. • Most turbidity currents deposit within the proximal part of the channel. • Infrequent large flows flush the channel; transporting sediment onto the lobe. • Flushing events can occur without obvious triggers, and can be internally generated. Submarine channels are the primary conduits for land-derived material, including organic carbon, pollutants, and nutrients, into the deep-sea. The flows (turbidity currents) that traverse these systems can pose hazards to seafloor infrastructure such as cables and pipelines. Here we use a novel combination of repeat seafloor surveys and turbidity current monitoring along a 50 km-long submarine channel in Bute Inlet, British Columbia, and discharge measurements from the main feeding river. These source-to-sink observations provide the most detailed information yet on magnitude-frequency-distance relationships for turbidity currents, and the spatial-temporal patterns of sediment transport within a submarine channel-lobe system. This analysis provides new insights into mass redistribution, and particle residence times in submarine channels, as well as where particles are eventually buried and how that is recorded in the deposits. We observe stepwise sediment transport down the channel, with turbidity currents becoming progressively less frequent with distance. Most flows dissipate and deposit within the proximal (< 11 km) part of the system, whilst longer run-out flows then pick up this sediment, 'shuffling' it further downstream along the channel. This shuffling occurs mainly through upstream migration of knickpoints, which can generate sediment bypass along the channel over timescales of 10–100 yrs. Infrequent large events flush the channel and ultimately transport sediment onto the lobe. These flushing events can occur without obvious triggers, and thus might be internally generated. We then present the first ever sediment budget analysis of an entire submarine channel system, which shows that the river input and lobe aggradation can approximately balance over decadal timescales. We conclude by discussing the implication of this sediment shuffling for seafloor geohazards and particle burial. [ABSTRACT FROM AUTHOR]

A numerical modelling investigation of bed aggradation in open rectangular drainage channels is presented in this paper. A semi-coupled modelling approach for uniform sediments based on 1D-shallow water De Saint Venant - Exner equations was adopted to describe the temporal evolution of the bed in the deposition and transport phenomena associated with aggradation process. Six well-established sediment transport formulas were used to evaluate the sediment discharge over the aggrading bed. Simulations enabled the comparison of results obtained with the selected formulas against a comprehensive set of experimental measurements reported in the literature. Results have shown the adopted model to describe successfully the evolution of sediment bed and water level profiles during the aggradation experiments, with minor differences depending on the used sediment transport relationship. [ABSTRACT FROM PUBLISHER]

It has been proposed that at short timescales of 102–105 yr, climatic variability can explain variations in sediment flux, but in orogens with pronounced climatic gradients rate changes caused by the oscillating efficiency in rainfall, runoff, and/or sediment transport and deposition are still not well-constrained. To explore landscape responses under variable climatic forcing, we evaluate time windows of prevailing sediment aggradation and related paleo-erosion rates from the southern flanks of the Dhauladhar Range in the western Himalaya. We compare past and present 10Be-derived erosion rates of well-dated Late Pleistocene fluvial landforms and modern river sediments and reconstruct the sediment aggradation and incision history based on new luminescence data. Our results document significant variations in erosion rates ranging from 0.1 to 3.4 mm/yr over the Late Pleistocene. We find that, during times of weak monsoon intensity, the moderately steep areas (hillslope angles of 27 ± 13°) erode at lower rates of 0.1–0.4 mm/yr compared to steeper (>40°) crestal regions of the Dhauladhar Range that erode at 0.8−1.3 mm/yr. In contrast, during several millennia of stronger monsoon intensity, both the moderately steep and high slope areas record higher erosion rates (>1-3.4 mm/yr). Lithological clast-count analysis shows that this increase of erosion is focused in the moderately steep areas, where Lesser Himalayan rocks are exposed. Our data thus highlight the highly non-linear response of climatic forcing on landscape evolution and suggest complex depositional processes and sedimentary signals in downstream areas. • Paleo-erosion rates in the Himalaya are modulated by climate forcing. • Erosion rates are higher during episodes of strong monsoon. • Topographic response to climate change is non-linear. • Rapid erosion during short-lived strong monsoon phases result in valley aggradation. [ABSTRACT FROM AUTHOR]

• An efficient 2D shallow water hydro-sediment-morphodynamic model is presented. • Morphodynamic processes under the impact of cascade movable weirs are studied. • Disparate morphological changes may result under different operation schemes. • An alternating operation scheme is proposed to constrain morphological changes. Cascade movable weirs have been increasingly widely used for river landscaping, water storage and ecological wellbeing. The weirs disturb the prior equilibrium in line with fluvial hydrology, sediment transport and geomorphology, and may lead to profound changes in the river, of which, however, the understanding has so far remained poor. Here, the first of its kind, a computational modelling study is presented on the morphodynamic processes under the impact of cascade hydraulic lifting dams, a type of movable weirs, as to the middle Fen River, China. A two-dimensional shallow water hydro-sediment-morphodynamic model is applied to resolve the fluvial processes, which is physically coupled and computationally efficient based on Finite Volume Method, unstructured mesh, local time stepping and parallel computing. Computational results show that the cascade dams alter the flow and sediment transport locally and on the reach scale in a distinct manner under different operation schemes, which in turn incur disparate morphological changes. Collapse of all the dams all year round would lead to net degradation in the study reach, with the floodplains aggrading and the main channel degrading. Operation of all the dams all year round may induce an overall aggradation both on the floodplains and in the main channel. If all the dams collapse in flood seasons and operate in dry seasons (CF), degradation occurs in the main channel whereas the floodplains experience aggradation, with a net degradation. When only the dams in the main channel operate all year round (OM), the floodplains degrade while the main channel aggrades, resulting in a net aggradation in the study reach. Either aggradation or degradation may prevail locally on floodplains, in main channel and individual subreaches. The present findings lead us to propose a complex operation scheme of annually alternating CF and OM to constrain significant morphological changes. [ABSTRACT FROM AUTHOR]

It has been observed that the relation between sediment storage and sediment transport rate in gravel-bed rivers exhibits counter-clockwise hysteresis, or aggradation-degradation cycles. Previous work has attributed these cycles to external changes to the sediment supply rate. We present a set of nine flume experiments that test the hypothesis that aggradation-degradation cycles can occur even under steady sediment supply conditions when the sediment transport rate temporarily nears the sediment supply rate. The experiments are designed to examine the transport-storage relations under a range of conditions with different initial bed condition, sediment supply rate, discharge, and supply material texture. The sediment transport efficiency of the experimental channel was strongly influenced by the existing bed morphology inherited from historical flows. The discharge and grain size distribution of the supply material played a secondary role, with higher discharge conditions and finer sediment feed material producing increased transport efficiencies. The transport-storage relations revealed that the transport rate of the channel changed with the volume of sediment stored and was, therefore, mediated by the channel morphology. Aggradation-degradation cycles were observed even under steady sediment supply and discharge conditions and were mediated by morphologic adjustments within the channel that temporarily led the transport rate to be near equal to the sediment supply rate. In other words, the probability of hysteresis in the transport-storage relation increases when the channel is near a transport equilibrium. The findings demonstrate the complexity associated with developing a unique relation between sediment transport and storage in gravel-bed rivers, and highlights the importance of considering the morphologic evolution and historical bed conditions. • Flume experiments conducted to investigate cycles of aggradation-degradation. • Sediment transport rate variability linked to bed morphology and texture changes. • Transport rate adjusted according to volume of sediment stored. • Aggradation-degradation cycles tend to occur when channel is near equilibrium. [ABSTRACT FROM AUTHOR]

Fluvial, tidal, and combined hydro-morphodynamics interaction in a complex, seasonal, sediment transport regime has been the subject of extensive research. It becomes particularly challenging when there is limited data. The Ganges-Brahmaputra-Meghna (GBM) Delta is one example of a huge system lacking data. Bathymetric data simultaneously covering the rivers and estuaries is hardly present, let alone sequences of bathymetries or a system-wide sediment budget. Hence it is difficult to understand and predict future developments. This research aims to make a sediment budget for the GBM delta with a process-based model. It is a first-ever sediment budget simulation for the GBM system. A process-based morphological model, Delft3D, has been used to reproduce the bathymetric evolution over time and the associated sediment budget. This chapter demonstrates the possibilities for the application of a robust modeling system to assess the morphodynamic evolution and sediment budget and pathways. The Ganges and Jamuna rivers carry sediment load in the ranges of 216–1038 million tonnes/yr and 80–228 million tonnes/yr respectively. The total accumulation in the estuary system is 1150 million tonnes/yr, out of which more than eighty percent of sediment is in suspension. The model results show that about 22% of the total sediment coming into the system is deposited in the floodplains and tidal plains and causes river morphology changes. The rest of the sediment is lost to the pro-delta, to the deep ocean bed, or leaves the domain. The results also indicate that Padma, Gorai, Pussur-Sibsa, Bishkhali, Shahbajpur channel, Lower Meghna, Tentulia Channel, and Arial Khan rivers are mainly in the aggrading phase, whereas, the Ganges, Jamuna, and Baleshwar are in the degrading phase. This particular delta model offers many opportunities to compare with sediment data, where it deals with a poorly surveyed area. • Process-response relation can be checked with Delft-3D.Sediment budget in the GBM or Bengal delta. • Riverbed aggradation and degradation is a sign of dynamic system. • Delta model offers many opportunities. [ABSTRACT FROM AUTHOR]

Many world's large rivers have experienced significant sediment reduction in the past century. However, channel aggradation near the mouth of these rivers may be still occurring or even accelerating, as evidenced by a recent study on the Lowermost Mississippi River. This study aims to investigate whether such a trend exists near the mouth of another large lowland alluvial river, the Atchafalaya River, which enters the Gulf of Mexico via two outlets — the natural mainstem and a man-made straight channel that is shorter and has a much greater slope. The overarching hypothesis is that, despite the reduction in riverine sediment delivery, lowland alluvial rivers such as the Lower Atchafalaya River in their backwater zone aggrade. We collected bathymetric survey records in 1977 and 2006 to analyze riverbed deformation. We utilized stage records at several gauging stations to determine the change in the hydraulic head over time. Geospatial techniques were used to develop channel bed Digital Elevation Models which were used to identify deformation of the 174-km channel bed of the Lower Atchafalaya River. We found a cumulative erosion of 6.34 × 107 m3 and a cumulative deposition of 8.79 × 107 m3, resulting in a net deposition of 2.45 × 107 m3 during the study period. While much of the river including the man-made channel was in a degrading state, the last 35 km of the river's natural mainstem experienced substantial aggradation. On average, riverbed elevation of the Lower Atchafalaya decreased by 1.47 m in the upper 98 km, but increased by 0.96 m in the final 35 km of the river's mainstem to the Gulf of Mexico. The channel erosion in the upstream reach will likely continue with the river discharge projected to increase, and the channel deposition in the downstream mainstem may accelerate because of sea-level rise. • Assessment of channel bed dynamics of a large lowland alluvial river on the Mississippi River Delta • Changes in volume and mass of the channel beds are quantified, and factors affecting them are analyzed. • Channel degradation occurs in the upper reach of the Lower Atchafalaya, mainly affected by flood frequency. • Channel aggradation mainly occurs in the river's final 35 km despite of reduced sediment delivery. [ABSTRACT FROM AUTHOR]

• Annual sediment yields using SEMEP are typically 50–300 ton/km2·yr. • RMSE of model with SEMEP is 53 ton/km2·yr less than that of existing models. • Sediment transport into alluvial rivers is affected by river wetlands and water. • Watershed morphometric characteristics can represent sediment transport. • Geospatial analysis shows urbanization's erosion features, yielding rich sediment. South Korea experiences numerous local sedimentation problems, such as landslides, upland erosion, aggradation and degradation, and flood plain sediment deposition. This has necessitated the development of a reliable and consistent approach for modeling sediment processes in the country. In this study, samples obtained from 35 gauging stations at five alluvial river basins in South Korea were used together with the modified Einstein procedure and series expansion of the modified Einstein procedure to determine the total sediment load at the sampling locations. Using two different methods, the total sediment load of majority of the 35 considered rivers were found to be typically 50–300 ton/km2·yr. A model tree data mining technique was used to develop a model for estimating the specific degradation based on certain meaningful parameters, namely, the 1) elevation at the middle relative area of the hypsometric curve [m], 2) percentage of wetland and water, 3) percentage of urban land, 4) mean annual precipitation [mm], 5) main stream length [km], and 6) watershed form factor [km2/km2]. The root mean square error of the predictions of the proposed model was found to be 55 ton/km2·yr less than those of existing statistical models. Erosion loss maps obtained by the revised universal soil loss equation (RUSLE), satellite images, and aerial photographs were also used to represent the geospatial features affecting erosion and sedimentation. The results of the geospatial analysis indicated that the transport of sediment into the alluvial rivers was affected by the wetlands located near the rivers, and also enabled clear delineation of the unique erosion features of construction sites in the urban areas. In addition, the watershed morphometric characteristics could be used to accurately represent the sediment transport. The proposed data mining methodology promises to facilitate the solution of various erosion and sedimentation problems in South Korea. The geospatial analysis procedure would also enable the understanding of spatially varied erosion and sedimentation processes under different conditions. [ABSTRACT FROM AUTHOR]

• Ephemeral Gully (EG) Erosion is a major erosion concern and needs to be addressed. • Very few models simulate EG erosion. • A thorough review of processes behind EG simulation in existing models is discussed. • Future needs and directions to improve EG simulation processes in models are enumerated. Modeling of ephemeral gully (EG) erosion has lagged that of other soil erosion processes despite its major contribution to watershed sediment losses. Several process and semi-empirical based simulation models have been used to assess the occurrence and location of EGs, magnitude of soil losses from EGs, and degradation, aggradation, and transport of sediment through EGs, but no comprehensive EG model exists. This paper reviews these models and presents a thorough discussion of their background, general formulations, key equations, field assessments, assumptions and limitations. Most current EG models evolved from the original formulations used in CREAMS, with incremental improvements by EGEM, WEPP, AnnAGNPS-REGEM, RUSLER-EphGEE models. These models provide process-based estimation of EG processes and field-scale soil-erosion contributions, but all models, except EphGEE, require users to have a priori knowledge of EG locations and all have significant shortcomings. Several topographic index models are discussed that provide simplistic approaches to locate EGs on the landscape using only topographic features. Process-based threshold index models may provide a more robust simulation of EG location and length, though testing has been limited. EG modeling appears to still be in its infancy, with great opportunities for future research, as discussed herein, to improve the understanding and simulation of EG erosion and transport processes. [ABSTRACT FROM AUTHOR]

The supply of sediment from hillslopes to channels is rarely constant, with discrete events (e.g., landslides) known to transfer large volumes of sediment in geologically-short periods of time, especially in tectonically active areas. Understanding the rates and patterns of subsequent sediment evacuation is important for understanding variability of landscape evolution, as well as mitigating the risk of geohazards associated with bed aggradation and the loss of channel capacity to convey flood waters. Here, we performed a series of controlled laboratory flume experiments to explore the controls on sediment transport after a single sediment input in scenarios with (i) different initial input volumes (4–25 kg) under constant flow conditions (40 l/s), (ii) a constant initial input volume (12 kg) under different flow magnitudes (5–60 l/s), and (iii) a repeat of the input sediment volumes in scenario (i) but with the same volume of water delivered using a ramped hydrograph (0–60–0 l/s). We find the presence of sediment piles impacts the flow hydraulics, with a backwater effect developing upstream of the pile that causes a flow acceleration around the location of the pile. For a given discharge, larger sediment piles have a greater impact on the flow hydraulics, which in turn induces higher rates of sediment transport and erosion of the pile. In all cases, sediment remains at the initial pile input location for the duration of the sediment evacuation, acting to protect the bed from erosion. We highlight the role of geomorphic-hydraulic interactions in controlling the sediment evacuation, and suggest there is an optimal combination of pile size and flow conditions (flow magnitude and hydrograph shape) for accelerated rates of sediment transport, which are important for the short-term and long-term channel dynamics and the landscape evolution variability. • Experiments documenting sediment transport processes following landslide inputs. • Identified the co-evolving interactions between hydraulic and geomorphic processes • Backwater effect around sediment pile induces accelerated transport rates. • Highest sediment transport rates associated with larger piles due to backwater effect. [ABSTRACT FROM AUTHOR]

Rock-avalanches (RAs) are large erosional events which, in a matter of minutes, can deliver acatastrophic volume of debris (≥ 1×106m3) directly to a fluvial system, often dammingmountainous rivers. RAs have a boulder carapace protecting readily transportable sedimentsizes, differing from landslides. Evidence suggests that the point-load sediment delivery ofrock-avalanche deposits (RADs) on fluvial systems can force rivers into a chronicallydisturbed state for up to 104 years. Fieldwork and micro-scale modelling are used to trace RAsediment dispersion in Ram Creek, New Zealand. The data forms a new conceptual model ofriver erosion and depositional response, changes to river planform and sedimentyield, and an effective model timescale of perturbation for both RA and landslidedams.Fieldwork investigation shows large scale aggradation from RA outburst flood sedimentdispersion, buffering vertical bedrock incision. The system is unable to transport the quantityof dam-derived sediments, and will not attain equilibrium before the next major event; it is ina state of persistent disturbance where localised reworking dominates. Lateral bevelling in theslot-type bedrock gorge topography appears to be the dominant form of erosion promotinggorge widening of the weak bedrock. It is theorised that aggradation of RA debris and lateralbevelling may result in the formation of a strath terrace if sediment cover persists,independent of external tectonic and climatic forcing. Normalised steepness and concavityindices of the Ram Creek river profile were compared to four neighbouring streams. TheRAD perturbation to profile metrics is not substantial enough to be discerniblefrom neighbouring streams containing large tectonically induced knickpoints. Thissuggests RA frequency estimates may be underrepresented in landscape evolutionmodels.Micro-scale modelling of the responses to a catastrophic sediment input into Ram Creek;either a RA (including carapace) or a landslide, each show different geomorphic responses.The RA model shows an un-breached carapace acts to trap fluvial sediments, resulting inpartial burial. Further downstream, initial sediment starved incision is followed by a slowerrecovery to pre-RA bed elevations, and continued aggradation of fluvial sediment tovertically displace the profile up towards the RA. In the landslide model, immediate breachchannel formation results in a sediment pulse downstream. Dispersed sediment forcesaggradation, controlling fluvial geomorphology.Fieldwork and modelling both show catastrophic sediment inputs control fluvialplanform, sediment yield, calibre, and storage, and geomorphic forms throughout theaffected fluvial profile. This contradicts the view that rivers in the Southern Alps, areunder-loaded and transport all sediment that is supplied to them. The inability of theriver to remove both the RA and landslide deposits suggests that the river will notrecover before the return period for the next large sediment input, and is a scenariowitnessed across numerous other feeder catchments to larger rivers in New Zealand. Itis suggested here that RA and landslide sediment storage from multiple eventscontrols the geomorphic evolution of smaller mountainous catchments, and RAforced dis-equilibrium may actually be the 'normal&#x2019; state in many mountainousstreams. [ABSTRACT FROM AUTHOR]

Sediment production and transport along sediment-routing systems are thought to vary with prevailing climatic and tectonic conditions. As such, the environmental conditions (climate, tectonics) of the past can potentially be reconstructed from sedimentary deposits. Reconstructions, however, must account for sedimentary signal modification through temporary storage and remobilization along the sediment-routing system. Fluvial fill terraces – the product of alternating phases of channel-bed aggradation and incision – are geomorphic features that result from temporary sediment storage and release. Although a range of field studies, physical experiments, and numerical models have investigated alluvial-channel responses and fill-terrace formation due to changes in environmental boundary conditions, we lack a systematic comparison of differences in channel morphology and terrace morphology formed by different types of perturbations.Here, we show results from seven physical experiments, in which sediment supply, water discharge and base-level elevation were regulated individually. We investigated how different perturbations affected channel morphology, fill-terrace formation, and the transient response of sediment discharge. We found that either a decrease in water discharge or an increase in sediment supply resulted in channel-bed aggradation and channel steepening. In contrast, an increase in water discharge, a decrease in sediment supply, or a drop in base-level triggered channel incision and terrace abandonment. The lag-time between perturbation onset and terrace cutting defines how well channel morphology prior to the perturbation is preserved in the terrace morphology. Lag-times were found to depend on the local channel incision rate and on the synchronicity of incision along the channel. An increase in water discharge caused faster channel incision (and shorter lag-times) compared to a decrease in sediment supply. Whereas incision driven by water- and sediment-discharge changes were initiated nearly simultaneously along the entire channel reach, incision related to base-level drop caused a wave of upstream knickzone migration and diffusion, forming terraces with decreasing ages in the upstream direction. We found that channel conditions prior to perturbation were best preserved by terraces formed following increases in discharge.Morphological adjustments of the channel in response to changes in incoming water or sediment supply also affected sediment discharge at the basin outlet. An increase or decrease in water discharge triggered channel incision or aggradation, recognizable as a temporary increase or decrease, respectively, in sediment discharge at the outlet. Permanent changes in upstream sediment supply were not recognizable at the outlet during the channel adjustment phase due to a negative feedback between the amount of sediment supply and the channel gradient. Changes in upstream sediment supply are thought to become measurable at the outlet only once the channel has adjusted to its new boundary conditions. [ABSTRACT FROM AUTHOR]