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The July 2021 flood in central Europe was one of the five costliest disasters in Europe in the last half century, with an estimated total damage of EUR 32 billion. The aim of this study is to analyze and assess the flood within an interdisciplinary approach along its entire process chain: the synoptic setting of the atmospheric pressure fields, the processes causing the high rainfall totals, the extraordinary streamflows and water levels in the affected catchments, the hydro-morphological effects, and the impacts on infrastructure and society. In addition, we address the question of what measures are possible to generate added value to early response management in the immediate aftermath of a disaster. The superposition of several factors resulted in widespread extreme precipitation totals and water levels well beyond a 100-year event: slow propagation of the low pressure system Bernd, convection embedded in a mesoscale precipitation field, unusually moist air masses associated with a significant positive anomaly in sea surface temperature over the Baltic Sea, wet soils, and steep terrain in the affected catchments. Various hydro-morphodynamic processes as well as changes in valley morphology observed during the event exacerbated the impact of the flood. Relevant effects included, among many others, the occurrence of extreme landscape erosion, rapidly evolving erosion and scour processes in the channel network and urban space, recruitment of debris from the natural and urban landscape, and deposition and clogging of bottlenecks in the channel network with eventual collapse. The estimation of inundation areas as well as the derived damage assessments were carried out during or directly after the flood and show the potential of near-real-time forensic disaster analyses for crisis management, emergency personnel on-site, and the provision of relief supplies. This study is part one of a two-paper series. The second part (Ludwig et al., 2022) puts the July 2021 flood into a historical context and into the context of climate change.

Heavy precipitation over western Germany and neighboring countries in July 2021 led to widespread floods, with the Ahr and Erft river catchments being particularly affected. Following the event characterization and process analysis in Part 1, here we put the 2021 event in the historical context regarding precipitation and discharge records and in terms of the temporal transformation of the valley morphology. Furthermore, we evaluated the role of ongoing and future climate change on the modification of rainfall totals and the associated flood hazard, as well as implications for flood management. The event was among the five heaviest precipitation events of the past 70 years in Germany. However, consideration of the large LAERTES-EU regional climate model (RCM) ensemble revealed a substantial underestimation of both return levels and periods based on extreme value statistics using only observations. An analysis of homogeneous hydrological data of the last 70 years demonstrated that the event discharges exceeded by far the statistical 100-year return levels. Nevertheless, the flood peaks at the Ahr river were comparable to the reconstructed major historical events of 1804 and 1910, which were not included in the flood risk assessment so far. A comparison between the 2021 and past events showed differences in terms of the observed hydro-morphodynamic processes which enhanced the flood risk due to changes in the landscape organization and occupation. The role of climate change and how the 2021 event would unfold under warmer or colder conditions (within a −2 to +3 K range) was considered based on both a pseudo global warming (PGW) model experiments and the analysis of an RCM ensemble. The PGW experiments showed that the spatial mean precipitation scales with the theoretical Clausius–Clapeyron (CC) relation, predicting a 7 % to 9 % increase per degree of warming. Using the PGW rainfall simulations as input to a hydrological model of the Ahr river basin revealed a strong and non-linear effect on flood peaks: for the +2 K scenario, the 18 % increase in areal rainfall led to a 39 % increase of the flood peak at gauge Altenahr. The analysis of the high-resolution convection-permitting KIT-KLIWA RCM ensemble confirmed the CC scaling for moderate spatial mean precipitation but showed a super CC scaling of up to 10 % for higher intensities. Moreover, the spatial extent of such precipitation events is also expected to increase.

In the last few decades, the energy demand is increasing globally which, given the present dependency of the energy generation on fossil fuels, results in a continuous increase in CO2 emissions. In an isolated electric grid system like in the SIDS, the majority of the electricity is produced by fossil fuels, therefore most of the SIDS nations are now focusing on Variable Renewable Energy sources (VREs). VREs such as wind and solar are hardly predictable and bring instabilities in the electric power system if not buffered by a storage system. Here we investigate the possibility of using Seawater Pump Storage Hydropower Systems (S-PSHS) as a renewable energy storage solution in an isolated electric grid. For this, the island of Curacao (one of the SIDS nations) is used as proof of the concept. For detecting potential locations for the S-PSHS sites on the island, GIS application was developed. The application of this conceptual proposed solution in similar systems is straightforward and it can be easily upscaled in other geographies. The concept of using seawater for the pumped hydro project is not common in practice and it is anticipated to have technical, environmental and financial challenges which are discussed in this paper.

In this study we introduce a new algorithm that allows the automatic segmentation of the contour of sediment particles collected directly in the field. The benefits of this method consist in better performance with respect to other available methods, especially in case of shadows around the sediment particle and in a more systematic and faster procedure with respect to manual segmentation. The method is compared with the one proposed by Roussillon et. al. 2009 and its performances are assessed using shapes having known morphometry descriptors. We moreover propose a way to assess the relative error one can expect in the estimates of particles travel distances using sediment morphometry as their proxy. The conclusions derived hold under the hypothesis of validity of a relationship between particles’ relative mass loss and circularity and on the empirical Sternberg’s law. The expected relative errors of travel distances result to depend on accuracy with which particles’ shape is identified and on the distance itself, therefore suggesting the existence of a window of “more accurate” estimation.

The European Water Framework Directive aims to achieve a good ecological status for all European rivers by 2027. Since the majority of rivers in Germany are in a highly altered state, large-scale restoration projects have been promoted by the federal and state governments. To plan and implement river restoration implies the integration of different interests and constraints such as flood protection, water supply, recreational use and ecology. In particular in urban environments, or otherwise spatially restricted conditions, there are serious problems to reach the ecological objectives which are set by authorities. Thus, the planning engineer is confronted with additional difficulties, especially from human-made contiguous infrastructures. Consequently, it is not possible to develop watercourses through their own dynamics. In these cases, purposefully selected instream structures can be used as alternative means to achieve morphodynamic development and improve the ecological conditions in the existing riverbed.Until now, many restoration measures by means of instream structures have been implemented empirically according to the experiences by river engineers and technical staff. As a consequence, the guidelines for instream structures provide suitable hydraulic conditions and focus on the technical implementation rather than indicating which type of river habitat can be restored by the selected instream structure. The used measures often showed morphodynamic changes. However, in many cases habitat quality shows only negligible improvement compared to the initial conditions. This demonstrates a lack of scientifically derived solutions that can specifically induce morphodynamic changes and thus create fish and macroinvertebrates habitats in a targeted manner.At KIT we investigate artificial measures to create functional habitats in pre-alpine to lowland rivers. The investigation is made in close collaboration with governmental bodies who locally specify the ecological objectives guided by the EU Water Framework Directive. An analysis of ecological needs determines the lack of several habitat types in the examined river systems. Together with the state authorities, several types of hydraulic structures such as groynes and other instream structures are then evaluated regarding their ability of habitat replacement.The selected designs are examined according to state-of-the-art methods of hybrid hydraulic modelling, including mobile bed experiments, complementary numerical simulations and monitoring field campaigns. Based on the hydraulic findings the habitat suitability for all relevant flow conditions is derived through aquatic habitat simulation. The promising variants are then optimized and evaluated in terms of their ecological impact as well as hydraulic requirements, e.g. flood and bank protection for all morphologically relevant discharges.The current research shows that nature-based solutions, inspired by practical empiricism and improved scientifically, can be used for developing instream structures that generate purposefully ecologically favourable conditions in rivers. In our presentation we will discuss that with optimization through scientific methods we expect to improve the planning reliability and ecological benefits of the use of instream structures for the enhancement of river habitats.

The increase in economic activities, population and rural electrification has significantly increased the energy demand in most of the developing nations. This demand has to be supplied from various sources, preferably renewable, among which hydropower is expected to be one of the major contributors. Though developed nations have already harnessed most of their hydropower potential, developing nations are still struggling in project identification and capacity assessment, mainly due to lack of data and difficulties of access. We present and test an assessment framework, developed for data scarce regions, to identify optimal location and installed capacity of multiple run-of-river hydropower projects within a river basin. The developed framework consists of two components: the first component is a hydrological model for flow duration curves, the second component is a so-called hydropower model. Flow duration curves are obtained using an existing probabilistic hydrological model which derives the probability distribution of streamflow as a function of few topographic and climatic parameters. A novel optimization procedure is developed, where viable hydropower projects are identified minimizing their specific cost, which depends mainly on discharge, head and length of conduit system. We tested the assessment framework in the West Rapti basin (Nepal). The application showed that the total potential of this basin maybe achieved with 79 different projects with capacity ranging from 1 to 17 MW. The framework was developed using open languages and software and can therefore be freely used after request to the corresponding author.

Producing hard carbon from lignocellulosic biomass has been the focus of recent studies as a promising source of anode material for Na-ion batteries. Woody biomass is a potential source, but it is already well valorized. Consequently, river driftwood can be an excellent alternative, especially since it is a disturbing waste for dam regulators. It can jeopardize dam safety, damage intake works, and sink in reservoirs, lowering water quality and decreasing reservoir volume. We examine the potential of river driftwood as a source of hard carbon for Na-ion batteries. Hydrothermal carbonization (HTC) was carried out at temperatures between 180 and 220 ◦C as the first step to produce hydrochar followed by an upgrading pyrolysis step at 1400 ◦C under an inert atmosphere to obtain hard carbon. We investigated the effect of HTC operational conditions and driftwood biomass (genera) on hydrochar and hard carbon properties, as well as the latter’s impact on Na-ion batteries. The produced carbon electrodes delivered a reversible capacity of 270–300 mAh⋅g-1 for the first cycle and showed high coulombic efficiencies of 77–83%. We also observed promising cyclability of a maximum 2% loss after 100 cycles. Moreover, results suggest that obtained hard carbon can compete with commercial materials and is capable to supply large battery factories with anode material.

Driftwood in river catchments might pose a hazard for the safety of infrastructures, such as dams and river dwellers, and thus is often removed. Genissiat dam in France presents a case study where annually approximately 1300 tons of driftwood are removed to prevent driftwood sinking and to protect the dam infrastructure. Collected river driftwood is rarely studied for utilization purposes and is commonly combusted or landfilled. However, driftwood can be valorized for biochar production through pyrolysis or hydrothermal carbonization (HTC). This study follows a novel approach in characterizing river driftwood by identifying the different common genera present at Genissiat dam on the upper Rhone, France. Moreover, the research provides for the first time a comprehensive analysis of river driftwood different physico-chemical properties, such as moisture content, major elemental composition (CHNSO), HHV, and macromolecular composition (cellulose, hemicellulose, lignin, and extractives). The study shows that the transportation of driftwood through rivers can enhance its properties by reducing the bark content resulting in lower ash content. Results indicate that driftwood can be mixed and further processed as a feedstock regardless of their genera and type for biochar production by pyrolysis or hydrothermal carbonization.

Assuming that the equilibrium geometry of river channels does not depend on their initial state but solely on boundary conditions, several formulas have been derived that relate the channel depth and width to the river bankfull discharge and bed material. However, due to the existence of a threshold for sediment motion and the strong non-linearity between sediment transport and flow rate, this assumption might not be generally valid for gravel-bed rivers. This research clarifies the role of the initial conditions, more specifically the initial channel width, on the geometry of gravel-bed rivers considering a variety of boundary conditions. The approach includes laboratory experiments and two-dimensional modeling, reproducing the evolution of alluvial channels with different starting widths, discharge regimes and sediment input rates. The experiments represent the Arc River (France). Thus, the characteristics of this river were used in the numerical model to obtain a realistic virtual case complementing the experiments. Different boundary and starting conditions resulted in either braided or single-thread channels. We found that the initial width strongly influences the evolution process and leaves a footprint on the river braid-belt extension. The active width of braided systems and the width of single-thread channels do not depend on the starting condition. They depend on sediment input rather than on discharge variability. Different initial widths result in different final bed levels. This indicates that the initial channel width may affect the degree of channel incision or aggradation. The results of this study justify the use of equilibrium formulas for single-thread rivers.

Significance Hydroelectric reservoirs emit substantial amounts of CO2, especially in the tropics. Since many such systems exist and many more will be built within decades, it is important to assess their role in the carbon cycle. A major source of emission that is rarely monitored and never at different timescales is the carbon released downstream of dams. We measured the seasonal and subdaily variability of CO2 emission downstream of one of the world’s largest artificial reservoirs and find that its contribution is relevant for unbiased quantification of reservoir carbon budgets. These findings highlight the importance of subdaily variability in hydropower operation for downstream emission rates and call for appropriate analysis schemes to reassess the greenhouse gas footprint of this energy source., Recent studies show that tropical hydroelectric reservoirs may be responsible for substantial greenhouse gas emissions to the atmosphere, yet emissions from the surface of released water downstream of the dam are poorly characterized if not neglected entirely from most assessments. We found that carbon dioxide (CO2) emission downstream of Kariba Dam (southern Africa) varied widely over different timescales and that accounting for downstream emissions and their fluctuations is critically important to the reservoir carbon budget. Seasonal variation was driven by reservoir stratification and the accumulation of CO2 in hypolimnetic waters, while subdaily variation was driven by hydropeaking events caused by dam operation in response to daily electricity demand. This “carbopeaking” resulted in hourly variations of CO2 emission up to 200% during stratification. Failing to account for seasonal or subdaily variations in downstream carbon emissions could lead to errors of up to 90% when estimating the reservoir’s annual emissions. These results demonstrate the critical need to include both limnological seasonality and dam operation at subdaily time steps in the assessment of carbon budgeting of reservoirs and carbon cycling along the aquatic continuum.

Rigorously derived shallow water equations (SWEs) are applied to results of large eddy simulation (LES) of a continuously fed gravity current in order to assess (1) sensitivity of current depth results to its definition; (2) coefficients in depth-averaged continuity and momentum equation due to the nonuniformity of density and velocity profiles; and (3) sensitivity of entrainment coefficient to definition of current depth. It is shown that using different definitions of the current depth may produce significantly different numerical results. The coefficients due to nonuniformity in the continuity equation are very close to unity, whereas the coefficients in the momentum flux and the pressure term in the momentum equation are different from unity by a margin that is very sensitive to the definition of current depth. The entrainment coefficient is more sensitive to the selected parameterization than to the definition of the current depth.

We investigate to what extent the initial conditions (in terms of buoyancy and geometry) of saline gravity currents flowing over a horizontal bottom influence their runout and entrainment capacity. In particular, to what extent the effect of the introduction of an inclined channel reach, just upstream from the lock gate, influences the hydrodynamics of gravity currents and consequently its potential erosion capacity is still an open question. The investigation presented herein focuses on the unknown effects of an inclined lock on the geometry of the current, on the streamwise velocity, on bed shear stress, and on the mechanisms of entrainment and mass exchange. Gravity currents were reproduced in the laboratory through the lock-exchange technique, and systematic tests were performed with different initial densities, combined with five initial volumes of release on horizontal and sloped locks. The inclination of the upstream reach of the channel (the lock) was varied from 0 % to 16 %, while the lock length was reduced by up to 1ĝ•4 of the initial reference case. We observed that the shape of the current is modified due to the enhanced entrainment of ambient water, which is the region of the current in which this happens most. A counterintuitive relation between slope and mean streamwise velocity was found, supporting previous findings that hypothesized that gravity currents flowing down small slopes experience an initial acceleration followed by a deceleration. For the steepest slope tested, two opposite mechanisms of mass exchange are identified and discussed, i.e., the current entrainment of water from the upper surface due to the enhanced friction at the interface and the head feeding by a rear-fed current. The bed shear stress and the corresponding potential erosion capacity are discussed, giving insights into the geomorphological implications of natural gravity currents caused in different topographic settings.

Around the world rivers transport large volumes of driftwood into lakes, seas and oceans. Recruited commonly during flooding events and transported by rivers, driftwood poses a hazard for the point of view of the safety of infrastructures and river dwellers. For that reason, it is many times extracted locally and stored; driftwood removal prevents sinking and protects the dam infrastructure. Collected driftwood is a neglected river resource that is generally combusted or landfilled. Génissiat dam on the Rhone River in France presents a case study where annually approximately 1300 tons of driftwood is intersected. Among the different processes that are capable of converting driftwood, HydroThermal Carbonization (HTC) is of high interest due its ability to process biomass with high moisture content, such as driftwood. HTC of biomass leads to the production of a solid product referred to as hydrochar, which is a high added-value material that can be used in different applications, such as fuel cooking, soil amendment, water treatment and energy storage. The goal of the study was to characterize the driftwood collected upstream of the Génissiat dam and to investigate its potential for hydrochar production as precursor of anode in sodium-ion batteries. Sodium-ion batteries have received more interest lately as an alternative for the resource intensive and expensive lithium-ion batteries. The study follows a novel approach in study driftwood by categorizing based on their genera. HTC of the different identified genera was conducted in a 2L batch reactor following a temperature of 200 °C for a residence time of 11.5 h. Results show that the impact of driftwood genera is not significant for processing of driftwood through HTC. Produced hydrochar had a high carbon content (from 55.4 to 57.0 %) and lower ash content (from 0.2 to 1.4 \% of dry biomass). Electrochemical results show that driftwood-based hydrochar is a promising precursor of hard carbon anodes in sodium-ion batteries due to its excellent electrochemical performance. Key words: Driftwood, Hydrothermal carbonization, Hydrochar, Rhone river, Sodium-ion batteries

Turbidity currents are in the range of highly sediment concentrated flows, challenging traditional (i.e. optical and acoustic) techniques that aim to measure concentration and velocity quantities. In typical laboratory conditions, difficulties increase in the presence of highly non-uniform and unsteady flows. However, the measurement of those quantities along with a longitudinal profile is necessary to quantify and depict key mechanisms of mass and momentum transport, related to the mean and turbulent flow fields. The possible solutions often require prohibitive costs or resources. In this work, visual, acoustic, electrical, and statistical tools are tested. The aim of these tests is to find appropriate techniques and strategies for measuring concentration and velocity quantities in the broader research scope involving turbidity currents triggered by a 2D water jet. The outcomes will be applied in the quantification of turbidity currents with various boundary and initial conditions in a flume 4 m long, 2 m deep, and 22 cm wide. Additionally, the findings can potentially be transferred to other laboratory applications involving turbidity currents or other types of sediment-laden flows.Acknowledgements: CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, a Foundation within the Ministry of Education in Brazil), grant number 88881.174820/2018-01.

Recent studies show that tropical hydroelectric reservoirs may be responsible for substantial greenhouse gas emissions to the atmosphere, yet emissions from the surface of released water downstream of the dam are poorly characterized if not neglected entirely from most assessments. We found that carbon dioxide (CO2) emission downstream of Kariba Dam (southern Africa) varied widely over different timescales and that accounting for downstream emissions and their fluctuations is critically important to the reservoir carbon budget. Seasonal variation was driven by reservoir stratification and the accumulation of CO2 in hypolimnetic waters, while subdaily variation was driven by hydropeaking events caused by dam operation in response to daily electricity demand. This “carbopeaking” resulted in hourly variations of CO2 emission up to 200% during stratification. Failing to account for seasonal or subdaily variations in downstream carbon emissions could lead to errors of up to 90% when estimating the reservoir’s annual emissions. These results demonstrate the critical need to include both limnological seasonality and dam operation at subdaily time steps in the assessment of carbon budgeting of reservoirs and carbon cycling along the aquatic continuum.

Hydro-meteorological risks are a growing issue for societies, economies and environments around the world. An effective, sustainable response to such risks and their future uncertainty requires a paradigm shift in our research and practical efforts. In this respect, Nature-Based Solutions (NBSs) offer the potential to achieve a more effective and flexible response to hydro-meteorological risks while also enhancing human well-being and biodiversity. The present paper describes a new methodology that incorporates stakeholders’ preferences into a multi-criteria analysis framework, as part of a tool for selecting risk mitigation measures. The methodology has been applied to Tamnava river basin in Serbia and Nangang river basin in Taiwan within the EC-funded RECONECT project. The results highlight the importance of involving stakeholders in the early stages of projects in order to achieve successful implementation of NBSs. The methodology can assist decision-makers in formulating desirable benefits and co-benefits and can enable a systematic and transparent NBSs planning process. Electronic supplementary material The online version of this article (10.1007/s13280-020-01419-4) contains supplementary material, which is available to authorized users.

The evidence to date shows that hydro-meteorological risks are likely to become more extreme in the foreseeable future. The continuously changing climate has also led to increasing pressure on the environment and human society. For these reasons, effective and sustainable methods for hydro-meteorological risk management are becoming more important. As an umbrella concept, Nature-Based Solutions (NBS) have been promoted due to their potential in reducing hydro-meteorological risk, adapting to climate change, and providing a wide range of co-benefits to nature and human well-being. The procedure of efficient planning and selection of NBS is a complex process that requires the involvement of multiple stakeholders. Measures need to be evaluated taking into account their primary function for hydro-meteorological risk reduction, potential co-benefits and specific local requirements. This paper presents a methodology to select NBS measures for reducing hydro-meteorological risk and increase co-benefits at the river basin scale. This is achieved by using stakeholder opinion to identify the importance of benefits and NBS in the area under consideration. A broad range of benefits has been included, such as risk reduction, water quality, habitat structure, biodiversity, socio-economic, and human well-being. This methodology has been applied to the case study of Tamnava River Basin in Serbia from RECONECT project. The results from this case study highlight the importance of involving local stakeholders in early stages of selection and implementation of NBS as part of the wider stakeholder co-creation process. The results also indicate the potential of the new methodology to assist decision-makers in the selection and implementation of NBS.

Large-Eddy Simulations (LES) are used to investigate the governing processes involved in mass and momentum transfer between the flow in the main channel and symmetrically-distributed lateral bank cavities. In-cavity free-surface velocities, based on laboratory measurements made in an open channel, are used to validate the numerical results. A main vortical structure dominates the in-cavity flow which, despite the shallow nature of the flow, features a remarked three dimensional dynamics. LES results outline the largest velocities through the mouth of the cavity are attained in two thin regions near the bottom-bed and free-surface. In the shear layers established between the main channel and cavities is where the main transfer of turbulent momentum is made between these two flow regions, and the numerical simulations capture well the instantaneous coherent flow structures, e.g. Kelvin-Helmholtz vortices. LES captures a low-frequency standing wave phenomenon even with a rigid-lid approximation adopted at the free-surface boundary. Momentum exchange between cavities and main channel is analysed using the Reynolds Averaged momentum equation in the transverse direction, revealing that the pressure gradient term is the unique contributor to flushing momentum out of the cavities whilst convection and Reynolds normal stress terms are responsible for its entraining into the cavity. Furthermore, sediment deposition areas documented in the laboratory experiments are linked with the simulated hydrodynamics, which correlate with regions of low turbulent kinetic energy and vertical velocities near the bottom of the channel. Overall, the results shed new light into the complex mechanisms involved in mass and momentum transfer; this will aid to design embayments more efficiently regarding sediment transport processes.

Field observations in a wide range of environments have shown that sediment availability is a major control on the suspended sediment observations in streams. Here we examine, via laboratory experiments, how the amount of proximal in-channel fine sediment storage relative to the upstream fine sediment distal supply influences the observations of suspended sediment concentrations in streams. Experiments under idealized conditions in a laboratory flume with different ratios of proximal and distal sediment supplies were conducted under a varying flow regime. In addition, the role of the sediment particle size of the supplied sediment on suspended sediment observations was explored. The combinations of proximal and distal sediment supply result in multiple responses of the channel bed and sediment quantity within the channel bed, and the responses adjust through aggradation and degradation. The signature of sediment concentration observed at the upstream section of the channel, given by the distal supply, differs from the downstream observations of the total conveyed sediment(distal and proximal), as shown by an in-phase analysis of sediment concentration-discharge plots.Furthermore, we show that nonuniform sediment mixtures may result in a change in the direction of the hysteresis observed between sediment concentration and discharge (i.e., from a clockwise hysteresis toa counterclockwise hysteresis). We also demonstrate that the ratio between sediment distal supply and proximal sediment availability modulates the magnitude of the aggradation/degradation processes in the channel reach and thus the joint observations of sediment concentration and discharge.

Lateral cavities are built in the banks of rivers for several purposes: to create harbors, to capture sediment, to keep a central navigable channel (i.e., Casiers de Girardon in the Rhone river) or to promote the formation of aquatic habitats if a limited amount of sediment is captured, providing hydraulic and morphologic diversity (i.e., the case of Japanese Wandos). This work is focused on this latter purpose: promotion of hydraulic and morphologic diversity. In these scenarios, an increase in the flow discharge in the main channel may, however, re-mobilize the deposit of sediment inside these lateral embayments and cause a sudden increase of the sediment concentration and turbidity in the main channel. It is thus of interest to characterize the resistance and resilience of these sedimentary deposits when the main channel is subjected to high flow or flushing events. Laboratory tests were carried out for five different normalized geometries of the cavities installed in the banks of an open channel and for five hydrographs with different levels of unsteadiness. Water depth, sediment deposit mass, sediment concentration and area covered by the settled sediments were recorded throughout each experiment. Although sediment deposits established at equilibrium before the flushing events are different depending on the geometry of the cavities, generally, they are recovered after being flushed by the high flow events. It is shown that the resistance and resilience of the sediment deposits are strongly dependent on the flow field and the mass exchange between the main channel and the cavities. This mass exchange is governed by the geometry of the cavities and the magnitude of the hydrographs applied.

Buoyancy driven flows such as gravity currents, present in nature and human made applications, are conveyors of particles or dissolved substances for long distances with clear implications for the environment. This transport depends on the triggering conditions of the current. Gravity currents are experimentally simulated under varying initial conditions by combining three different initial buoyancies and five volumes of dense fluid released. The horizontal and vertical structures of the gravity currents are analysed and it is shown that the variation on the initial configuration is conditioning for these. Vertical transport through the gravity current is influenced at the bottom by the solid wall over which the current flows, and at the upper interface by the contact with the ambient water. The relative contribution of shear stress at the bottom and at the upper interfaces are estimated and analysed in terms of the initial triggering conditions of the current; these two compete with the buoyancy, the driver of the current, determining mixing and entrainment. By using a proper parametrization, which accounts for both initial volume of release and location of the observation position relative to the lock, a relation between the resistance at the bottom and at the upper interfaces with the initial conditions of release (i.e. the lock-length) has been found; this is found to be independent of the initial density in the lock. The present study shows that the variation of the initial conditions have consequences on (1) the configuration of the currents and on (2) the hydrodynamics of the currents, including mass and momentum exchanges, which are in addition mutually dependent.

Sediment traps with partially open check dams are crucial elements for flood protection in alpine regions. The trapping of sediment is necessary when intense sediment transport occurs during floods that may endanger urban areas at downstream river reaches. In turn, the unwanted permanent trapping of sediment during small, non-hazardous floods can result in the ecological and morphological degradation of downstream reaches. This study experimentally analyses a novel concept for permeable sediment traps. For ensuring the sediment transfer up to small floods, a guiding channel implemented in the deposition area of a sediment trap was systematically studied. The bankfull discharge of the guiding channel corresponds to a dominant morphological discharge. At the downstream end of the guiding channel, a permeable barrier (check dam) triggers sediment retention and deposition. The permeable barrier consists of a bar screen for mechanical deposition control, superposed to a flow constriction for the hydraulic control. The barrier obstructs hazardous sediment transport for discharges that are higher than the bankfull discharge of the guiding channel without the risk of unwanted sediment flushing (massive self-cleaning).

River restoration works often include measures to promote morphological diversity and enhance habitat suitability. One of these measures is the creation of macro-roughness elements, such as lateral cavities and embayments, in the banks of channelized rivers. However, in flows that are heavily charged with fine sediments in suspension, such as glacier-fed streams and very low-gradient reaches of large catchment rivers, these lateral cavities may trap these sediments. Consequently, the morphological changes may be affected, and the functionality of the restoration interventions may be compromised. Herein, we analyse the influence of these macro-roughness elements on the transport of fine sediments in the main channel. Laboratory tests with uniform flow charged with sediments in a channel with banks equipped with large-scale rectangular roughness elements were carried out. The laboratory experiments covered a wide range of rectangular cavity geometrical configurations and shallowness ratios. The influence of key parameters such as flow shallowness, geometric ratios of the cavities and initial sediment concentration was tested. Surface particle image velocimetry, sediment samples and temporal turbidity records were collected during the experiments. The amount of sediments captured by the cavities, the temporal evolution of the concentration of sediments in suspension and the flow hydrodynamics are cross-analysed and discussed. It is shown that the trapping efficiency of the macro-roughness elements is a clear function of the channel geometry and the shallowness of the flow.

River training works performed in the last couple of centuries constrained the natural dynamics of channel networks in locations that include the confluences between tributaries and main channels. As a result, the dynamics of these confluences are currently characterized by homogeneous flow depths, flow velocities, and morphologic conditions, which are associated with impoverished ecosystems. The widening of river reaches is seen as a useful measure for river restoration, as it enhances the heterogeneity in flow depths, flow velocities, sediment transport, and bed substrates. The purpose of this study is to analyze the effects of local widening of the tributary mouth as well as the effects of the ratio between the width of the tributary and that of the main channel on the flow dynamics and bed morphology of river confluences. For that purpose, 12 experiments were conducted in a 70° laboratory confluence. In these experiments, three unit-discharge ratios were tested ( q r = 0.37, 0.50, and 0.77) with two width ratios and two tributary configurations. The unit-discharge ratio is defined as the unit discharge in the tributary divided by that of the main channel, measured upstream of the confluence. The width ratio, which is defined as the width of the tributary divided by that of the main channel, was modified by changing the width of the main channel from 0.50 to 1.00 m (corresponding to B r = 0.30 and 0.15 respectively). The tributary configurations consisted of (i) a straight reach with a constant width (the so-called reference configuration ) and (ii) a straight reach with a local widening at the downstream end (the so-called widened configuration ). During the experiments, a uniform sediment mixture was continuously supplied to both channels. This experimental setup is novel among existing experimental studies on confluence dynamics, as it addresses new confluence configurations and includes a continuous sediment supply to both channels. The experiments were run until the outgoing sediment rate was nearly the same as the incoming; i.e., equilibrium had been achieved. The bed topography and water surface were then recorded in both channels. The results reveal that the width ratio and the locally widened tributary reach influence the dynamics of the confluence. The different width ratios influenced the size of the bank-attached bar at equilibrium, which was wider and longer for B r = 0.15 than for B r = 0.30. Other morphological differences were observed at equilibrium for the different width ratios, such as deeper scour holes and increased penetration of the tributary into the main channel. These differences were attributed to the different values of the ratio between the unit momentum-flux of the tributary and that of the main channel that were noted at equilibrium for the different width ratios. The local widening of the downstream reach of the tributary significantly enhanced the heterogeneity in flow depth, flow velocity, and bed morphology within the widened reach. This heterogeneity contrasts with the homogeneity observed in the tributary without widening (reference configuration). Additionally, the effects of the local tributary widening were limited to the tributary, with minor or negligible effects on the main channel.

Gravity currents are often modelled by means of shallow water equations (SWEs). In these models, simplifications such as the consideration of a constant layer-averaged density are common. This note presents the complete and general derivation of a 2D depth-averaged momentum equation for gravity currents with density and velocity varying in the bed-normal direction. Special attention is given to the pressure term which is evaluated for constant, linear and exponential density profile. The shape of the density profile has implications for the momentum balance: the assumption of constant density leads to an overestimation of the driving force due to pressure gradient by a factor of 33% for linear density profile and up to 50% for an exponential profile. It also leads to an overestimation of celerity in numerical models based on traditional SWEs by factor of 22% and around 40% for linear end exponential density profiles respectively.

Mountain confluences are characterized by narrow and steep tributaries that provide important sediment load to the confluence, whereas themain channel supplies the dominant flow discharge. This results in a marked bed discordance between the tributary and main channel which induces key differences in the hydromorphodynamics of the confluence when compared to concordant bed and some common types of discordant bed confluences. The processes of initiation and maintenance of the morphology of confluences are still unknown, and research linking morphodynamics and hydrodynamics of river confluences is required to understand these phenomena. In this paper, eddy-resolving simulations based on laboratory experiments made in a live-bed model of a mountain-river confluence are used to provide a detailed description of flow hydrodynamics and implications for morphodynamics. The test case study corresponds to a confluence with an angle of 70°. Numerical simulations are performed for two extreme bathymetric conditions: those at the start of the experiment and when equilibriummorphological conditions are reached. Results of the simulations and experimental observations are used tomake inferences on erosion mechanisms during the initial and final states of the erosion/deposition process. The relationship between the coherent structures present in the near-bed region in the instantaneous and mean flow fields and sediment entrainment/transport is described. The present paper demonstrates the critical role played by large-scale turbulence generated in the shear layers forming on the side of the tributary flow as it enters the main channel, by the main vortex forming over the discordant bed region surrounding the downstream end of the tributary, as well as by several near-bed vortices induced by the deflection of the tributary flow by the incoming flow in the main channel. The predicted patterns of bed shear stress are linked to pathways of sediment movement

Gravity currents are geophysical flows having large impact on human life and on the environment. The need to prevent and mitigate their negative consequences justifies the multitude of studies carried out to understand their physics. However little has been done concerning gravity currents travelling over porous bed, which is a frequent scenario in real life. Existing studies of gravity currents over porous substrates deal with lock-exchange gravity currents. In this paper we will present preliminary results of 23 experiments reproducing continuouslyfed brine currents initially developing over a flat bed and then running over a porous region of varying porosities. In correspondence of this region, 2D density maps are acquired by means of an image-analysis technique. Simultaneously an Acoustic Doppler Velocimetry Profiler allows to record a 3D instantaneous velocity profile. Results describing the effect of the bottom porosity on density and velocity profiles are presented. Surprisingly the current does not slow down while travelling over the porous substrate: the relaxation of the no-slip boundary condition together with fresh water incorporation from the bottom cavities allows high velocities in the near bed region. This modifies significantly velocity and density gradients at the lower current interface. Conversely, varying the bed porosity has no remarkable effect on the upper current interface.

River reaches downstream dams where a constant residual flow discharge is imposed, often lack sediment supply and periodic inundation due to the absence of natural flood events. In this study, a two-year return flood was released from an upstream reservoir and combined with sediment replenishment to enhance instream habitat conditions downstream of Rossens hydropower dam on the Sarine River in western Switzerland. Sediment replenishment consisted of four sediment deposits distributed as alternate bars along the river banks, a solution which was previously tested in laboratory. The morphological evolution of the replenishment and of the downstream riverbed were surveyed including pre- and post-flood topography. A hydro-morphological index to evaluate the quality of riverine habitats, based on the variability of flow depth and flow velocity in the analyzed reach, was investigated. The combination of the artificial flood with sediment replenishment proved to be a robust measure to supply a river with sediment and to enhance hydraulic habitat suitability.

We present the results of laboratory investigations of continuously-fed density currents that propagate first over a smooth horizontal bed and then over a porous substrate of limited length. Inflow discharge, initial excess density, and substrate porosities are varied. Density measurements, acquired through an image analysis technique, are performed above the porous layer simultaneously with quasi-instantaneous vertical velocity profiles. After a first phase in which the current sinks into the substrate, freshwater entrainment from the bed begins and, gradually, a mixing layer forms at the interface between the surface flow and the porous bed. Shear-driven and Rayleigh-Taylor instabilities rule the dynamics of this mixing layer. The porous boundary effects are observed in the vertical distributions of both density and velocity, especially in the near-bed region. Here, larger flow velocities are recorded over porous substrates. We argue that these are due to the presence of a longitudinal pressure gradient, which in turn is a consequence of the current mass loss. Its presence over the porous substrate is proved by the current interface longitudinal slope. However, other effects of the presence of the porous substrate, such as the relaxation of the no-slip boundary condition and the bed-normal momentum exchange, also affect the velocity field. The turbulent structure changes significantly over the porous substrate: while streamwise turbulence decreases, shear and bed-normal Reynolds stresses increase in large part of the current depth. Buoyancy instabilities further enhance the bed-normal momentum flux and, in the near-bed region, contribute to turbulent kinetic energy generation together with shear. Published under license by AIP Publishing.

The interest in micro-hydropower in existing infrastructure is increasing since this is a technology with low environmental impacts and potential for energy recovery in different types of installation. The technologies available for these schemes are still restricted and it is a current subject for research. In this work an experimental characterization of an inline tubular propeller suitable for pressurized systems, such as water supply and distribution networks, is presented. In the framework of the European Project HYLOW, started in 2008, a first prototype has been developed using CFD analysis and tested. Nevertheless, optimization and validation were still necessary. An improved model has been now adequately tested in the laboratory and proves to be interesting in terms of potential application. The experimental investigation evidenced that the new turbine has efficiencies of around 60% for low-headed operations, below 50 m.