Hydropeaking, rapid discharge fluctuations driven by hydropower operations, poses significant threats to aquatic ecosystems, particularly fish populations. This study assesses the effectiveness of various hydropeaking mitigation strategies on fish habitats in the Bavarian Lech River, Germany, with a focus on larvae of European grayling (Thymallus thymallus). The ecohydraulic model CASiMiR was employed to simulate four scenarios, ranging from a baseline, with no mitigation, to a combined operational and morphological interventions. Modelling results show that reducing downramping rates through modified turbine operation allows to decrease stranding risk for European grayling larvae by nearly sevenfold. Morphological measures result in further reduction of stranding risk while the scenario combining both operational and morphological strategies achieves the most substantial reduction of stranding risk. Similar trends are observed in the fish monitoring data with the last scenario resulting in the highest fish densities compared to baseline scenario. Decline in fish abundance due to natural flood events reveals the recruitment variability, underscoring the importance of considering both artificial and natural hydrological disturbances. Overall, the findings highlight that operational measures play a key role in reducing stranding risk, while morphological measures are essential in maintaining fish abundance and habitat persistence, both important for the good ecological status according to the European Framework Directive. The study results demonstrate the effectiveness of the combined mitigation approach in reconciling hydropower generation with riverine biodiversity conservation, which is supported by mid-term fish monitoring data.

The operational practice of "hydropeaking" allows hydropower plants to cover peaks and deficits in energy demand, but it also impacts river ecosystems. The assessment of hydropeaking impacts plays an important role in safeguarding ecosystem services, but is challenging due to the relative importance of impacts at different sites. To compare impacts in hydropeaking rivers, we elicit expert judgment on the relative impacts of hydropeaking on river ecosystem services. Using the best-worst scaling (BWS) method, we compare the impact on the three categories of river ecosystem services (provisioning, regulating and cultural). Our respondents include 98 hydropower experts. Our analysis accounted for individual heterogeneity to assess how perceptions vary across regions, attitudes and representative river characteristics. We find trade-offs between provisioning and regulating services at the regional and local levels, which represents a key issue in dealing with climate change and ecosystem degradation. The best-affected services were water for power generation, raw materials, water for industrial activities and water for irrigation. The worst-affected services were fisheries and aquaculture, maintenance of population and habitat, and wild animals. Our results have implications for the safeguarding of river ecosystem services and the design of regulatory and incentive schemes for mitigation.

The space-and time-averaged velocity profiles in gravel beds show some features that differentiates them from that established in smooth beds, but the obstruction posed by the gravel hampers the experimental measurements below the sediments. To overcome such difficulties, we coupled particle image velocimetry (PIV) with refractive index matching (RIM) by means of substituting the gravel with hydrogel spheres. We applied a superposition of models on the experimental velocity profile: the Darcy, the mixing, the logarithmic, and the wake law models. The resulting analytical profile is adherent to the experimental data in all the regions of the flow, from the bottom to the free surface. After including a second data set from literature, we carried out the dimensional analyses on the parameters in the profile, defining them as functions of the general characteristics of the flow and of the bed. In particular, the position of the inflection point, and thus the relative contribution of the mixing and the logarithmic layers on the overall profile, were found to be related to the permeability Reynolds number.

Restoration of streams to a good ecological status, as required by the European Water Framework Directive (WFD), has been slow, if not failing. Colmation, the clogging of hyporheic interstices by fine sediments, could be one cause for this failure by negatively influencing hyporheic invertebrates. Therefore, this study analysed the hyporheic communities in six German rivers representing all five ecological status classes using ecological, colmation- and grain size analysis. The study revealed that I) the amount of fine sediment and level of colmation had a significant impact on the taxa number and the abundance of hyporheic meiofauna, II) increased colmation led to a shift from coarse- to fine sediment dweller-dominated hyporheic communities, III) hyporheic communities were impacted most by land use in catchments, fine sediment levels, and colmation, and IV) faunal patterns associated with colmation corresponded to the ‘general degradation’ classification defined by the WFD. Presumably elevated fine sediment loads from open land use types, and thereby increasing colmation-levels, altered the porosity and the size of voids. This led to a loss of habitats within the sediments which influenced hyporheic invertebrates. While substrate conditions are important for benthic communities, they are especially critical for hyporheic fauna, which responds particularly strongly to changes in sediment structure. Oxygen and nutrient provision are influenced by sediment structure and further impact the hyporheic organisms. Consequently, the loss of functioning communities led to the deterioration of the ecological status of rivers. Colmation should therefore be considered when implementing the WFD.

Engineers, geomorphologists, and ecologists acknowledge the need for temporally and spatially resolved measurements of sediment clogging (also known as colmation) in permeable gravel-bed rivers due to its adverse impacts on water and habitat quality. In this paper, we present a novel method for non-destructive, real-time measurements of pore-scale sediment deposition and monitoring of clogging by using wire-mesh sensors (WMSs) embedded in spheres, forming a smart gravel bed (GravelSens). The measuring principle is based on one-by-one voltage excitation of transmitter electrodes, followed by simultaneous measurements of the resulting current by receiver electrodes at each crossing measuring pores. The currents are then linked to the conductive component of fluid impedance. The measurement performance of the developed sensor is validated by applying the Maxwell Garnett and parallel models to sensor data and comparing the results to data obtained by gamma ray computed tomography (CT). GravelSens is tested and validated under varying filling conditions of different particle sizes ranging from sand to fine gravel. The close agreement between GravelSens and CT measurements indicates the technology’s applicability in sediment–water research while also suggesting its potential for other solid–liquid two-phase flows. This pore-scale measurement and visualization system offers the capability to monitor clogging and de-clogging dynamics within pore spaces up to 10,000 Hz, making it the first laboratory equipment capable of performing such in situ measurements without radiation. Thus, GravelSens is a major improvement over existing methods and holds promise for advancing the understanding of flow–sediment–ecology interactions.

As the share of renewable energy grows worldwide, flexible energy production from peak-operating hydropower and the phenomenon of hydropeaking have received increasing attention. In this study, we collected open research questions from 220 experts in river science, practice, and policy across the globe using an online survey available in six languages related to hydropeaking. We used a systematic method of determining expert consensus (Delphi method) to identify 100 high-priority questions related to the following thematic fields: (a) hydrology, (b) physico-chemical properties of water, (c) river morphology and sediment dynamics, (d) ecology and biology, (e) socio-economic topics, (f) energy markets, (g) policy and regulation, and (h) management and mitigation measures. The consensus list of high-priority questions shall inform and guide researchers in focusing their efforts to foster a better science-policy interface, thereby improving the sustainability of peak-operating hydropower in a variety of settings. We find that there is already a strong understanding of the ecological impact of hydropeaking and efficient mitigation techniques to support sustainable hydropower. Yet, a disconnect remains in its policy and management implementation.

Riverbed clogging is key to assessing vertical connectivity in the hyporheic zone and is often quantified using single‐parameter or qualitative approaches. However, clogging is driven by multiple, interacting physical and bio‐geochemical parameters, which do not allow for a conclusive assessment of hyporheic connectivity with single‐parameter approaches. In addition, existing qualitative assessments lack transparency and repeatability. This study introduces a Multi‐Parameter Approach to quantify Clogging and vertical hyporheic connectivity (MultiPAC), which builds on standardized measurements of physical (grain size characteristics, porosity, hydraulic conductivity) and bio‐geochemical (interstitial dissolved oxygen) parameters. We apply MultiPAC at three gravel‐bed rivers and show how the set of parameters provides a representative appreciation of physical riverbed clogging, thus quantifying vertical hyporheic connectivity. However, more parameters are required to fully characterize biological clogging. In addition, MultiPAC locates clogged layers in the hyporheic zone through multi‐parameter vertical profiles over the riverbed depth. The discussion outlines the relevance of MultiPAC to guide field surveys.

Many water bodies are, as a result of anthropogenic influences, such as river straightening, river bank fixation, or damming in accordance with the EU Water Framework Directive (2000/60/EC) not in a good ecological state anymore. With the aim to return to a good ecological status for surface waters, restoration measures are implemented in many rivers. However, the success and sustainability of such measures are often site-dependent and require hence an objective assessment.In this study, the Multi-Parameter Approach to assess Clogging (MultiPAC) was used to assess the suitability and sustainability of different riverbed restoration strategies. MultiPAC is based on several measured physico-chemical parameters, which enable a detailed investigation of in-situ conditions of gravel-bed rivers. The approach includes measurements of the sediment composition for identifying surface and subsurface grain size distributions and fine sediment fractions. In addition, measurements of the porosity are obtained by using Structure-from-Motion and the Water Replacement Method of freeze-core samples. Finally, measurements of the interstitial oxygen concentration and so-called slurping rates, which are converted into hydraulic conductivity, were performed with a double-packer system called VertiCo.The residual river stretch between Jettenbach and Töging at the Inn River in Germany provided a means to evaluate riverbed restoration measures, implemented in February and March 2020. Investigations were performed for several gravel bars, where sediment was replenished and a mechanical break-up of the bed armour layer was conducted. The MultiPAC investigations were performed before measure implementation, shortly afterwards (March 2020) and in November 2020 to investigate the impact of a flood event with a 10-year return period, which occurred in August 2020, and thus may have influenced the sustainability of the restoration measures.From the measurements, it can be seen that sediment replenishment and the mechanical break-up of the armour layer significantly improved the ecological functioning of the riverbed. However, it became evident that the increase in the quality of the riverbed was only temporary. Hence, these measures will need to be repeated regularly with the aim of maintaining ecologically-valuable riverbed habitat conditions. The results of this study also showed that MultiPAC provides detailed insights into the riverbed sediments, their composition, and the permeability of the riverbed.

Hydropeaking has received increasing attention in the last years, but many knowledge gaps remain, potentially hampering effective policy and management efforts in rivers under such type of hydropower production. In this study, we collected open hydropeaking research questions from over 200 experts in river science, practice, and policy across the globe using an online survey available in five languages. We used a systematic method of determining expert consensus (Delphi method) to identify 100 core questions related to the following thematic fields: (i) hydrology, (ii) physico-chemical properties of water, (iii) river morphology and sedimentology, (iv) ecology and biology, (v) socio-economics and energy markets, (vi) policy and regulation, as well as (vii) management and mitigation measures. The consensus list of questions shall inform and guide researchers in focusing their efforts to foster a better science-policy interface, thereby improving the sustainability of peak-operating hydropower in a variety of settings.

The description of complex river environments requires interdisciplinary approaches to collect and manage manifold data types and sources. Deriving comprehensive knowledge from complex data sources is challenging and necessitates not only knowledge of environmental science but also statistics and Software engineering. This study introduces a relational database framed in an application called River Analyst for creating and managing river data with open-source standards (Python3 and Django). We conceptualize data models of river environments, which describe sediment characteristics and hydraulics related to hyporheic exchange. River Analyst enabled us to derive novel insights for restoring rivers affected by so-called riverbed clogging, notably, fine sediment infiltration in the hyporheic zone. The database analysis reveals that clogging is not a dominant control process when the fraction of fine sediment exceeds 50%–55%. In conclusion, the new Software holds promise for data-informed advancements in augmenting knowledge to restore ecologically functional hydro-environments.

An increase in the demand for renewable energy is driving hydropower development and its integration with variable renewable energy sources. When hydropower is produced flexibly from hydropower plants, it causes rapid and frequent artificial flow fluctuations in rivers, a phenomenon known as hydropeaking. Hydropeaking and associated hydrological alterations cause multiple impacts on riverine habitats with cascading effects on ecosystem functioning and structure. Given the significance of its ecological and socio‐economic implications, mitigation of hydropeaking requires an inter‐ and transdisciplinary approach. An interdisciplinary network called HyPeak has been conceived to enrich international research initiatives and support hydropower planning and policy. HyPeak has been founded based on exchange and networking activities linking scientists from several countries where hydropeaking has been widespread for decades and numerous studies dedicated to the topic have been carried out. HyPeak aims to integrate members from other countries and continents in which hydropower production plays a relevant role, and grow to be a reference group that provides expert advice on the topic to policy‐makers, as well as researchers, stakeholders, and practitioners in the field of hydropeaking.

The infiltration and accumulation of fine sediments in gravel-bed rivers leads to a reduction of the existing pore space and may lead in a worst case to a complete clogging of the river bed. To understand the highly dynamic process of sediment infiltration, measurements with high temporal and spatial resolution are required. Within this study, the development of sediment accumulations in an artificial river bed is investigated to gain further understanding on the process of colmation. The artificial river bed, implemented in a research flume, is made of spheres with two different diameters and in different packing arrangements. Three sediment mixtures with different grain size distributions are supplied to observe the dynamic infiltration process, and to get information on the distribution over depth. In addition, supply rates and supply masses are varied during the experiments. To observe the dynamic development of sediment accumulation, the gamma-ray attenuation method is used, which provides the opportunity of non-intrusive and undisturbed continuous measurements during the experiments at a certain position. Additionally, the accumulated sediment masses are obtained right after the supply of sediments and 28 minutes later, with a high vertical resolution to detect changes as result of consolidation within the pores. From the measured amount of infiltrated sediments can be seen that the accumulated sediment mass is strongly particle size-dependent. The measurements of the fine sediment mixture show that the filling started from the bottom until the accumulation reach the surface of the artificial river bed. The experiments with the coarse sediment mixture resulted in a clogging layer in the upper section of the river bed, and subsequently less sediments reached the flume bed. By varying the supply rate, it can be seen that a higher supply rate leads to an earlier start of the infiltration and a rapid filling, while the lower supply rate resulted in a later infiltration and slow filling process. The measurements 28 minutes after the end of the experiments show, in addition, that dynamic changes happen mainly in the upper layers due to the washing of surface sediments by the flow, and only to a smaller extent by further settlements due to solidification within the pores. The feeding mass itself has no considerable effect on the infiltration behavior of the current setup, as once the pores are filled, almost no additional particles penetrate the bed. The use of a high sophisticated measurement method made it possible to investigate the infiltration process of sediments in an artificial river bed with high temporal and spatial resolution. Due to the use of different sediment mixtures, and different supply conditions, further insight on the process of fine sediment infiltration could be gained within this study.

The hyporheic zone represents an important part of aquatic ecosystems and is known for its richness in biodiversity. Although it is well-known that infiltration and accumulation of fine sediments into gravel riverbeds (colmation) can lead to heavy impacts on the hyporheic fauna and on the reproduction cycle of gravel spawning fish, no standardized method to quantify or evaluate the degree of colmation is available. This article presents a Multi-Parameter Approach to assess Colmation (MultiPAC), by considering four parameters: particle size distribution, the porosity of the riverbed, hydraulic conductivity and dissolved oxygen content. Hence, existing (freeze coring) and newly developed measuring techniques are used that allow the measurement of vertical profiles of hydraulic conductivities and dissolved oxygen contents. After verification in the laboratory, MultiPAC is applied in an extensive field campaign that proved the capability to detect different sediment characteristics of riverbeds. The vertical profiles of hydraulic conductivity and dissolved oxygen show typically decreasing values with increasing sediment depths and can detect clogged layers by sharp reductions of the measured values within the profile. With additional information on porosity and particle size distribution, an objective database is available, towards a reliable and quantitative assessment and evaluation of the phenomenon of colmation in rivers.

The last decades witnessed a growing interest in the sediment faith in rivers, especially in relation to the operation of reservoirs. An efficient management requires a model for describing the interaction between the fine transported sediments and the armored gravel-bed. However, the definition of the streamwise velocity profile in gravel-bed rivers is still to be investigated, due to the inaccessibility of the deeper gravel substrate. We coupled the Refractive Index Matching (RIM) methodology with the Particle Image Velocimetry (PIV) technique to measure velocity inside the gravel voids. To that end, we filled a flume with gravel except for the measuring section, where we substituted the gravel with similar-sized hydrogel spheres. These spheres, originally similar to 2 mm sized, absorb water growing up to similar to 2.5 cm, keeping the same refractive index as water. Being invisible when submerged, they avoid optical disturbances. Such a set-up allowed us to measure velocity in low-submergence conditions in three longitudinal planes, from the flume bottom to the free surface. The resulting double-averaged velocity profiles show an inflection point above the crest level, usually associated with a mixing-layer type of turbulence. The mixing layer analogy is confirmed by the behavior of the Reynolds intensities, the skewness, and the ratio between the components of the Reynolds stress tensor. However, the mixing layer analogy alone does not describe the velocity profile from deep regions of the bed to the free surface, and our results indicate that a superposition of multiple laws better represents the entire velocity profile, coupling the surface and subsurface flows.

Riverbed clogging, also referred to as colmation, describes the infiltration of fine sediment in gravel bed rivers. The infiltrated fine sediment leads to a reduction of the pore space and, in the worst case, to a sealing of the riverbed. As a result of severe colmation, negative effects on the environment may occur, such as a limited oxygen supply for fish eggs or for macrozoobenthos. The quantification of the degree of colmation and its impact on the ecological status of a river is often based on an expert assessment or only on a single parameter, such as the amount of fine sediment. However, depending on the sediment matrix of the riverbed, the packing arrangement of particles, or the organic material in the riverbed, a single parameter may not be sufficient to evaluate the degree of colmation. In addition, most expert-based assessments, such as mapping of inner and outer colmation, are on the one hand biased due to subjectiveness and on the other hand, only investigate the surface layer of the riverbed. Knowledge on possible occurring colmation layers in deeper regions of the interstitial will not be gained by using these methods. In this study, a novel MultiParameter Approach to assess Colmation (MultiPAC), is presented, which measures several physical parameters, and provides insights into the status of colmation conditions in the interstitial. These are: * measurements of the sediment composition for identifying surface and subsurface grain size distributions and for assessing fine sediment fractions, * measurements of porosity by using Structure-from-Motion in combination with freeze-core sampling, and * measurements of oxygen concentration and hydraulic conductivity by using a newly developed double-packer system, called VertiCo. The VertiCo (Vertical profiles of hydraulic Conductivity and dissolved Oxygen) enables measurements with a high spatial resolution over the vertical axis of the riverbed to enable the quantification of possible colmation layers or changes of the conditions in the interstitial over depth. With the MultiPAC it is feasible for the first time to holistically assess the influence of oxygen and hydraulic conductivity in the interstitial. By taking also into account the properties of the sediment matrix and the porosity, the degree of colmation of a riverbed can be identified. In addition, these findings may provide important information to support the classification of the ecological state of river sections.

In many parts of the world, climate change has already caused a decline in groundwater recharge, whereas groundwater demand for drinking water production and irrigation continues to increase. In such regions, groundwater tables are steadily declining with major consequences for groundwater-surface water interactions. Predominantly gaining streams that rely on discharge of groundwater from the adjacent aquifer turn into predominantly losing streams whose water seeps into the underground. This reversal of groundwater-surface water interactions is associated with an increase of low river flows, drying of stream beds, and a switch of lotic ecosystems from perennial to intermittent, with consequences for fluvial and groundwater dependent ecosystems. Moreover, water infiltrating from rivers and streams can carry a complex mix of contaminants. Accordingly, the diversity and concentrations of compounds detected in groundwater has been increasing over the past decades. During low flow, stream and river discharge may consist mainly of treated wastewater. In losing stream systems, this contaminated water seeps into the adjoining aquifers. This threatens both ecosystems as well as drinking and irrigation water quality. Climate change is therefore severely altering landscape water balances, with groundwater-surface water-interactions having reached a tipping point in many cases. Current model projections harbor huge uncertainties and scientific evidence for these tipping points remains very limited. In particular, quantitative data on groundwater-surface water-interactions are scarce both on the local and the catchment scale. The result is poor public or political awareness, and appropriate management measures await implementation.

The infiltration and accumulation of fine sediments in gravel riverbeds (clogging, colmation) is a natural process, especially in rivers with heterogeneous particle size distributions. However, natural rivers are characterized by regularly occurring flood events that lead to bed alterations and hence, flushing of infiltrated fine sediments. In the case of high non-natural fine sediment inputs and/or regulated low flows, this fragile balance between clogging and de-clogging is disturbed and may finally lead to heavily clogged riverbeds with well-known ecological consequences, especially for macroinvertebrates and gravel-spawning fish. This study presents the application of a novel approach called MultiPAC (Multi-Parameter Approach to assess Colmation) that assesses the efficiency of an artificial flood event on de-clogging of the riverbed of a near-natural bypass channel. In contrast to existing methods for determining colmation, which typically use qualitative approaches (e.g., mapping) or single-parameters (e.g., fine sediment contents), MultiPAC is designed to measure four in-situ key parameters, notably the particle size distribution, the porosity, the hydraulic conductivity, and the dissolved oxygen content. In particular, the combined measurements of hydraulic conductivity and dissolved oxygen along vertical profiles of the riverbed (VertiCO – Vertical profiles of hydraulic Conductivity and dissolved Oxygen) with a spatial resolution of 3.0 cm enable insights into gravel riverbeds and provide an exact vertical localization of clogged layers. The sediment characteristics of the near natural bypass channel show a distinct difference before and after the artificial flood. The vertical profiles of the measured hydraulic conductivities show increasing values up to a sediment depth of approx. 10 - 15 cm, which proves the efficiency of the artificial flood regarding de-clogging. In addition, the particle size analyses of most freezecore samples show a reduction in fine sediment fractions along with increasing porosity, which confirms the effectiveness of the flood operation. However, the vertical profile measurements show a reduction in dissolved oxygen concentrations after the artificial flood, which cannot be explained by the changed sediment characteristics or differences in the water temperatures in the hyporheic zone. Most likely, an apparent and widely spread Algae layer on the riverbed significantly influenced the oxygen measurements and a before-and-after comparison is not feasible because the Algae layer was removed during the artificial flood. The conclusion of this study is twofold: On the one hand, it could be proven that the artificial flood was sufficient to trigger de-clogging effects. On the other hand, the application of MultiPAC showed its potential for evaluating clogging/de-clogging processes in gravel riverbeds. Especially the detection of the impact depth of de-clogging events represent highly valuable information for designing artificial floods.

The complex process of sediment infiltration in gravel beds has been studied widely. However, the temporal behavior of sediment infiltration and its clogging has not yet been investigated sufficiently. To understand the mechanisms involved in this phenomenon, measurements of the effects of various boundary conditions are required with high spatial and temporal resolution. The nonintrusive and undisturbed gamma-ray attenuation method was applied in this study to establish a measuring scheme to detect the dynamic development of river bed clogging and to investigate the effects of fine sediments' particle-size distributions, total supplied mass, and supply rate on the sediment infiltration process. In a series of experiments, vertical profiles of infiltrated sediment masses were measured at two instants during the experiments. Then the dynamic changes of the intiltrated sediments at a specific position of the bed were investigated using continuous measurements during the experiments. The results of the measured vertical profile showed that sediment infiltration and clogging in an artificial bed matched the sediment's existing bridging criterion of the porous gravel bed. Furthermore, temporal changes occurred mainly in the upper layers. The continuous one-point measurements indicated almost real-time sediment accumulation development and proved that higher supply rates lead to an earlier start of the infiltration and rapid filling, whereas lower supply rates result in later and slower infiltration of the sediments. (C) 2022 American Society of Civil Engineers.

----------------------------------------------------------------------------------------------------------------------------------------------------------------- Data used in the publication "Functional relationships between critical erosion thresholds of fine reservoir sediments and their sedimentological characteristics", which is accepted by the Journal of Hydraulic Engineering and will be published in a forthcoming issue. Beckers, F., K. Koca, S. Haun, M. Noack, S. U. Gerbersdorf, and S. Wieprecht. Forthcoming. „Functional relationships between critical erosion thresholds of fine reservoir sediments and their sedimentological characteristics.” J. Hydraul. Eng. https://doi.org/10.1061/(ASCE)HY.1943-7900.0001984 The data consists of two files: GBS.txt contains the sediment data of the reservoir Großer Brombachsee SBT.txt contains the sediment data of the reservoir Schwarzenbachtalsperre Both files contain information on the sediment depth and erosion stability separated into τc,0 and τc,S. Furthermore, both files contain a collection of physico-chemical sediment parameters, including bulk density, sediment composition (Clay, Silt, Sand), percentiles (d10, d50, d90),cation exchange capacity (CEC), and organic content (TOC). Additionally, the SBT data contains biological sediment parameters, including chlorophyll-a (CHL-a), and extracellular polymeric substances separated into proteins (EPS-p) and carbohydrates (EPS-c). ----------------------------------------------------------------------------------------------------------------------------------------------------------------- The data was collected within the transdisciplinary research project "CHARM - Challenges of Reservoir Management". -----------------------------------------------------------------------------------------------------------------------------------------------------------------

The complex phenomenon riverbed clogging constitutes one reason for degraded riverine ecosystems.However, measuring or monitoring of clogging is challenging because of its multifaceted character.This keynote addresses clogging from different perspectives including field and laboratory aspects and shows recent developments in measuring techniques to gather relevant involved parameters and processes.