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The transport of coarse sediment during floods often exhibits hysteresis patterns from changes in flow fields, channel geometry, bedforms, or sediment supply conditions. Flume experiments that simulate hydrographs tend to confirm that hysteresis is a consequence of the progressive organization of surface sediments in terms of grain protrusion, imbrication, orientation, and roughness. Hysteretic patterns are also highly dependent on the kind of sediment supply conditions, and the type of simulated hydrograph. A factor that has not been investigated extensively is the effect of the timing and sequencing of floods on bedload transport. Depending on its magnitude and duration, each flood leaves the channel bed in a different condition, which influences the bedload transport of the next event, representing the river bed's memory of past floods, which can determine future responses to natural disturbances. In this study, I investigated the effects of different sequences of events, i.e., the flood history, on sediment transport through a series of flume experiments that simulated three types of stepped and symmetrical hydrographs (ranging from short-duration/high-magnitude to long-duration/low-magnitude events) under sediment recirculation conditions. Hydrographs were simulated as a sequence of the same event, and with events in different sequences, in order to explore the effects of different antecedent conditions on sediment transport. The results show that a previous event decreases the rates of sediment transported by a certain hydrograph by around 40% if a high-magnitude event precedes another one and around 70% if a low-magnitude event precedes another event of similar magnitude. A low-magnitude event does not affect the rate of sediment transported by a subsequent high-magnitude flood, but a high-magnitude event reduces the sediment transported by a subsequent long-duration/low-magnitude event. [ABSTRACT FROM AUTHOR]

The post-glacial adjustment of streams can be described as a battle between two geomorphic giants: the magnitude and resistance of the glacial imprint and the competence of subsequent fluvial flows. In northern Fennoscandia, which deglaciated ~9500 y BP, the outcome of this battle provides both insight into poorly studied high-latitude regions and a glimpse into the trajectory of currently deglaciating regions. The glacial signature of eroded bedrock and deposited till in this region have organized tributary stream networks into three distinct process domains: lakes, slow-flowing reaches (S0<0.1% in peat/ fine sediment), and rapids (S0: 0.1-7% with gravel to boulder beds). Thus, these post-glacial streams have two characteristics that complicate a process-based understanding of channel form and sediment transport: (1) they are typically semi-alluvial, in that they contain coarse glacial legacy sediment, and (2) mainstem lakes buffer sediment and water fluxes. Here, I present research to constrain controls on channel geometry and sediment transport in semi-alluvial rapids at three scales: (1) the particle-scale, quantifying sediment transport with a field tracer study, (2) the reach-scale, determining the potential for bedforms or sediment clusters through a flume experiment, and (3) the catchment scale, examining downstream variations in width using GIS-based analyses.Sediment particle dynamics within five semi-alluvial rapids (S0: 2.1-7%) were quantified using RFID-tagged tracer clasts. After a large snowmelt flood (>Q50), median transport distances for D10- to D50-sized clasts were <0.1 m, with no clear relationship between slope or shear stress and transport distance. Rather, as previous studies in boulder-bed mountain streams show, boulder protrusion and grain resistance likely play a greater role, increasing critical shear stress and reducing sediment transport. Based on a flume study mimicking reach-scale conditions of semi-alluvial rapids, it is clear that large clasts (>D84) are basically immobile and do not adjust to form step-pool features. But patterns of deposition and scour around boulders exist, with more deposition upstream of boulders at higher flows. Most geomorphic work is done at low and intermediate flows (Q2-Q10), with little additional transport at Q50 flows, which has a relatively low magnitude due to lake buffers. To determine whether channel width has adjusted to contemporary flow regimes at the catchment scale, downstream variation in bankfull width was analyzed in two medium-sized catchments. Regressions of width vs. drainage area (derived from 2m DEMs) have low explanatory power (R2: 0.05-0.5); coefficients (α) are low (<1) and regression exponents (β) show wide variation (0.26-0.75). Instead, spatial variability in till deposits and peat formation likely control channel width. While these semi-alluvial streams in stream-lake catchments are technically in equilibrium, as they are not aggrading or degrading, their stable channel geometry does not reflect the contemporary landscape sediment yield or hydroclimatic regime. Past studies have shown that bedrock river gorges in northern Sweden formed during deglaciation with little Holocene erosion. Similarly, tributary channels with coarse glacial sediment and mainstem lakes are frozen in time, reflecting catchment- and reach-scale morphologies shaped during high flows at deglaciation, with contemporary adjustment occurring only at the particle to sediment-cluster scales. [ABSTRACT FROM AUTHOR]

The morphology and dynamics of the 1500 km-long Guianas coast, South America, are strongly influenced by mud supplied by the Amazon River. A fraction of this mud (20% of nearly 1 billion tons annually) is organized into large banks that migrate along the coast under the influence of waves and currents, separated by 'inter-bank' zones. 'Bank' zones significantly dissipate the wave energy transmitted shoreward, whereas inter-bank zones are commonly characterized by a relatively mud-free shoreface, and wave energy is thus more efficiently transmitted to the shore. The alongshore alternation of shifting bank and inter-bank zones is strongly modulated by water discharge from the local rivers, which depends essentially on catchment size. The hydrology and sediment fluxes of many of these rivers, which drain the crystalline rocks of the Guiana Shield between the Amazon and the Orinoco River deltas, are still largely unknown. Even the catchment size of several of these rivers is not known with certainty. The relationship between discharge from these Guiana Shield rivers and the Amazon mud-bank belts results in a variety of estuarine morphological and dynamic configurations. Further estuarine diversity is engendered by lithology. A preliminary classification of 15 estuaries, based on plan morphology and ongoing analyses of estuarine hydrodynamics, suggests four basic types: (1) estuaries fixed by headlands of Precambrian basement rocks (Mahury, Cayenne, and Kourou Rivers); these are associated with small catchments and found only in French Guiana, where the basement crops out along the coast; (2) estuaries with mouths diverted westward (the direction of regional alongshore sediment transport) by prograded mud capes and spits, reflecting a significant influence of multi-decadal to multi-millennial coastal accretion of Amazon-derived mud (Cassipore, Uaça, Rivers in Amapa, Brazil; Oyapock between Brazil and French Guyana; Approuague, Sinnamary and Mana Rivers in French Guyana; Suriname and Coppename Rivers in Suriname; Berbice River in Guyana); (3) estuaries oriented normal to the coast and associated with relatively large-discharge rivers (Maroni River between French Guiana and Suriname; Corentyne River between Suriname and Guyana; (4) a prograded estuary mouth with a transitional morphology towards a delta (Essequibo River in Guyana, the largest river on the Guianas coast after the Amazon and the Orinoco). On-going monitoring of the sediment fluxes and hydrodynamics of a type 1 (Mahury) and a type 3 estuary (Maroni), both of which are fixed (lithologically for type 1, and hydrodynamically for type 3), and akin, morphologically, to normal trumpet- or funnel-shaped estuaries, shows a tropical seasonal regime but which is strongly influenced by Amazon mud during the low-discharge dry season. This mud influence from the distant Amazon constitutes a unique aspect of the river mouths on the Guianas coast. [ABSTRACT FROM AUTHOR]

The response of high-latitude rivers to global warming will be manifested through changes in discharge magnitudes, sediment transport and channel morphology. These are ultimately controlled by the ice characteristics of the system, which will be affected by future reductions in the duration of the frozen period, earlier ice break-up, changes to the distribution of seasonally frozen ground, and increasing freeze-thaw cycles. Studies examining the effects of the cold winter season (e.g., freezing temperatures, river ice, ice-covered flow) on channel changes and seasonal feedbacks of sediment transport remain limited in comparison to those undertaken on temperate and tropical river systems. Before being able to predict the future changes in river ice development and its impacts on seasonal sediment transport of high-latitude rivers, present river ice cover growth processes are first needed to be examined more in-depth. Therefore the aims are to investigate, 1) whether the existing ice growth thickness and layering models are good in predicting reality at present, and 2) what are the impacts of hydro-climatically varying years on river ice development in a subarctic meandering river. The ice thicknesses have been measured manually in cross-sections from a meander bend during the mid-winters of 2014-18. Different ice growth equations are applied for estimating the river ice growth during those years of varying hydro-climatic conditions (i.e. from mild to severe winters). These modelled results are tested and verified against the field observations. Air temperature (including freeze-thaw days), precipitation, ice thickness, snow depth, flow velocity, water depth and bathymetry data are available. The preliminary results determined based on the data gathered at Pulmanki River are presented. Based on the results, further studies of the impacts of river ice on hydro- and morphodynamics can be performed for present and future hydro-climatic conditions. [ABSTRACT FROM AUTHOR]

Single coarse grain scale measurements relate directly to the study of sediment transport in mountainous streams, primarily because these streams are characterised by a mixed and variable riverbed bed topography (ranging from "bare bedrock" to "fully alluvial"). Quantifying the force regime that controls the transport of a single pebble in situ has been proven historically difficult. Most of the measurement techniques are measuring indirect cross-averaged or point (non-inertial to the grain) transport metrics and using those to describe or predict grain motion (probabilistically or deterministically). The introduction of inertial Micro Mechanical and Electrical Sensors (MEMS) sensors (sensor-assemblies that comprise of micro-accelerometers, gyroscopes and compasses) to this problem, triggered the idea of a "smart-pebble": a mobile stone-like assembly that can measure directly the forces (and consequently metrics such as grain velocities, positions, kinetic energies etc.) of sediment transport exactly as experienced by single grains (from their inertial frame). Today, almost ten years after this idea was introduced in the literature, we know that there are accuracy limitations on the use of the MEMS suitable for monitoring fluvial transport (mainly relevant to unrestricted calculations of grain position, tracking) but we also know that there is a potential to calculate directly single pebble dynamics for short time scales and after consistent calibration and analysis. In this presentation, in-situ measurements from two purpose specific smart pebbles will be presented, deployed in three different mountain river stretches. The first dataset (river Calder, Scotland) is the collection of pre-entrainment pebble vibrations (20-minute periods), in low flow conditions. The second dataset is the measurement of seven complete single pebble transport events (entrainment- translation-deposition), derived from an alpine river (Erlenbach, Austria) during the snowmelt season (April 2018). Finally, the third dataset relates single pebble transport events with coherent ADCP measurements, both collected in the upstream region of river Dee (Scotland) during moderate/high flooding conditions. The three datasets are connected using an "exceedance over threshold" analysis, which is formalised after the re-working of an, experimentally tested, fundamental Newton-Euler model. [ABSTRACT FROM AUTHOR]

Rivers are the most effective agent of erosion on earth, transporting massive amounts of detrital and dissolved matter into depositional basins. Therefore, the relationship between denudation of continents and the pathways of sediment transport by large rivers has been extensively examined (e.g., sediment flux gauging, basin-wide erosion rates, recognizing sediment source and mixing). However, due to the complex nature of sediment storage and transport dynamics in large-scale fluvial systems, our understanding of the seemingly simple relationship between erosion and deposition in large rivers is limited.In large-scale fluvial systems, where sediment is stored for extended periods, stable cosmogenic nuclides are a useful tool for quantifying the time sediment spends exposed during fluvial transport. We examined changes in cosmogenic Ne concentrations along an ~450 km section in the lower basin of the modern Colorado River. Sediments along the river show no clear trend between Ne concentration and distance down the river. Nevertheless, the variability in Ne concentrations can be translated to the maximal time the sediment was exposed during its transport down the lower Colorado River (~120 kyr).Applying these understandings to an ancient river, we collected Lower Cretaceous fluvial sediments from 5 locations along an ~400 km transect in the general downstream direction of the Lower Cretaceous Kurnub River. This river flowed across the passive margin of northern Gondwana into the Tethys Sea. Both variability and maximum duration of exposure (up to ~130 kyr) of sediments in the Kurnub River are similar to the Colorado River.These results, combined with the residence time of sediments in large modern rivers deduced from U-series isotopes, demonstrate that throughout the geologic record large rivers show a similarity in sediment dynamics expressed by large variability in sediment residence time ranging up to several hundreds of thousands of years. [ABSTRACT FROM AUTHOR]

Hyperconcentrated peak discharge increase often occurs in the lower Yellow River (China) aggregating the flood risk along the course. Though a few work has been done to explain this phenomenon individually, no consensus has been achieved and the similarities and differences of the underlying physics among different increasing discharge events remain unknown. Based on the concept that flood wave is a kind of disturbance whose propagation can be described by the characteristic theory, we therefore derive a compatibility equation for flow discharge along the characteristics based on the cross sectional-averaged equations of sediment-laden flow. The critical parameters for the local discharge calculation consist of the up-and down-stream discharge and sediment concentration, the cross-sectional flow area and the time scale of flood wave propagation. By applying this equation to the 18 increasing peak discharge events in the lower Yellow River over the last five decades, we aim to uncover the mechanisms of such events in a uniform way and find out their similarities and differences. The results illustrate that the most and second dominant factors for the increasing peak discharge are the temporal accumulative effect of external forces and the spatial variation of pressure energy respectively whereas the discharge increase is suppressed by the imbalanced convection. The combination of increasing upstream discharge and local flow area or that of increasing discharge and decreasing sediment concentration at the upstream may be efficient for the reduction of local discharge increase. Despite the increasing upstream discharge may raise the flood risk, there remains an opportunity to balance the prevention of discharge increase and the occurrence of floodplain inundation. All the findings could provide scientific bases for a better river regulation in the lower Yellow River. [ABSTRACT FROM AUTHOR]

Tropical cyclones can generate huge precipitation leading to flash floods in coastal and island areas. The consequences for infrastructures and populations can be dramatic as large amounts of sediments are transported downstream. The ReNovRisk project aims studying the erosion and sediment transport in flooding rivers of La Réunion Island in the Indian Ocean during tropical cyclones. In this frame, we analysed recordings made at three seismic stations installed along a river in the Salazie Basin on the northern side of La Réunion Island, together with hydrological and meteorological data to characterize microseismic noise during the tropical cyclone Dumazile (March, 2018). We evidenced a very good correlation (r2 ~ 0.9) between the seismic noise amplitude and the water level, particularly during the rising limb of the river flood at high frequencies (>1 Hz). We also observe a clear influence of the station distance to the river on the noise amplitude, with strong noise attenuation with distance, particularly at high frequency, suggesting that the sediment transport monitoring has to be performed by stations located very close to the riverbed. We evidence a clockwise hysteresis between the power spectral density and the water level in both low and high frequency ranges dominated by water turbulence and sediment transport, respectively. Polarization measurements performed in multiple frequency ranges show an overall weak polarization, suggesting that the noise sources generated by the flooding river are not focused in the single point but instead, that the whole riverbed is emitting noise that reach the station from numerous backazimuths. [ABSTRACT FROM AUTHOR]

The landscape development in the Kosi River basin in the central Himalaya is strongly influenced by large tectonic and climatic gradients. Dynamic behaviour of the Kosi River in the alluvial part has been well-documented and is strongly linked to high sediment supply from the upstream hinterland area. Hence, reach scale erosion mapping in the hinterland area is a major issue, which can be tackled by highlighting sediment generating areas within the Kosi River basin. Sediments generated through hinterland erosion are routed downstream and spatially distributed in lower reaches of the Kosi River. The spatial distribution of energy available for sediment transport is governed by stream power (function of discharge and slope). This sediment transport competency approach has been applied in this paper to unravel erosion at basin/reach scale using spatial distribution pattern of specific stream power (SSP). A physically-based semi-distributed hydrological model, Soil and Water Assessment Tool (SWAT) has been used to simulate the reach scale stream power variability through estimation of channel slope and discharge. Stream power distribution pattern for different months was extracted at reach scale, through SWAT model by utilizing basic inputs such as precipitation, temperature, elevation, land use land cover and soil data. We observed that stream power is higher in channel reaches of the Sun Kosi and Arun. Downstream variation in SSP plot shows that mid-channel reaches in the Higher Himalaya have higher stream power in the Kosi River basin. The results also highlight a significant role of channel slope on stream power distribution pattern. Spatial distribution of slope data shows higher channel slope in western and main channel of the Kosi River that also results in higher stream power. This increased fluvial incision event is manifested in the spatial distribution of sediment yield. Further, since the estimation of SSP incorporates spatial variation in channel width, the peaks in specific stream power shift downstream in comparison to stream power distribution due to spatial variability in channel width. The reaches of high specific stream power and stream power therefore represent the major erosional zones, and therefore this approach can be used as a proxy for sediment erosion to understand sediment dynamics in the Kosi River basin. [ABSTRACT FROM AUTHOR]

The Chicken Creek catchment was constructed as a research platform to study initial ecosystem development at the landscape scale (Hüttl et al. 2014). The underlying conceptual framework was based on the idea that 1. patterns and processes evolving during the very first phase of development will have a long-lasting impact on later ecosystem stages, 2. defined boundary conditions and knowledge about subsurface structures of a constructed catchment would allow to better upscale and integrate processes compared to natural catchments, and3. lower complexity of interacting components in young ecosystems allow to better understand the role of abiotic and biotic feedback mechanisms compared to mature systems.After completion of the construction in 2005 (Gerwin et al. 2009), a major challenge was to develop and install a cross-disciplinary long-term monitoring program for the 6 ha area to record major environmental parameters adapted to the development of the site (Schaaf et al. 2013).During its first 15 years, Chicken Creek showed a very dynamic development (Elmer et al 2013). Whereas the abiotic geosystem of the first 2-3 years was characterized by heavy erosion and sediment transport, primary succession by invading vegetation and the unexpected formation of soil crusts within few years resulted in more biotic-abiotic feedbacks that controlled catchment hydrology. Our observations over a period of 15 years indicate that even minor variations in initial substrate characteristics (e.g. texture) can have lasting impacts on geomorphical, hydrological and biological development like erosion intensity, groundwater levels or establishment of vegetation patterns.The time-series of monitoring data combined with a structure model of the catchment (Gerke et al. 2013) allowed the closure of the water balance by relatively simple calculations of water storage volumes and the estimation of evapotranspiration (Schaaf et al. 2017). From these data three stages of ecosystem development were derived which are characterized by increasing influence of biota and increasing system complexity.Elmer, M., Gerwin, W., Schaaf, W., Zaplata, M.K., Hohberg, K., Nenov, R., Bens, O. & Hüttl, R.F. 2013: Dynamics of initial ecosystem development at the artificial catchment Chicken Creek, Lusatia, Germany. Environ. Earth Sci., 69, 491-505.Gerke, H.H., Maurer, T. & Schneider, A., 2013: A Three-Dimensional Structure and Process Model for Integrated Hydro-Geo-Pedologic Analysis of a Constructed Hydrological Catchment. Vadose Zone Journal, 12.Gerwin, W., Schaaf, W., Biemelt, D., Fischer, A., Winter, S. & Hüttl, R.F., 2009: The artificial catchment "Chicken Creek" (Lusatia, Germany) - A landscape laboratory for interdisciplinary studies of initial ecosystem development. Ecological Engineering, 35, 1786-1796.Hüttl, R.F., Gerwin, W., Kögel-Knabner, I., Schulin, R., Hinz, C. & Subke, J.-A., 2014: Ecosystems in transition: interactions and feedbacks with an emphasis on the initial development. Biogeosciences, 11, 195-200.Schaaf, W., Elmer, M., Fischer, A., Gerwin, W., Nenov, R., Pretzsch, H., Seifert, S., Winter, S., Zaplata, M.K., 2013: Monitoring the formation of structures and patterns during initial development of an artificial catchment. Environ. Monit. Assess., 185, 5965-5986.Schaaf, W., Pohle, I., Maurer, T., Gerwin, W., Hinz, C. & Badorreck, A., 2017: Water Balance Dynamics during Ten Years of Ecological Development at Chicken Creek Catchment. Vadose Zone Journal, 16. [ABSTRACT FROM AUTHOR]