The application of nature-based solutions for flood protection in low-lying coastal areas has recently received much attention. The solution is attractive because it may lower the costs for coastal defense, increase the effectiveness and resilience of defense measures, curb negative trends in biodiversity of globally important animal populations and contribute to carbon capture and storage.

Nature-based flood defense measures are based on relatively well-established biogeomorphological principles of self-organisation in coastal landscapes. Vegetated foreshores in particular contribute to wave damping, accumulation and stabilization of sediments maintaining a shallow bathymetry, capacity to adjust bed level to sea level rise and, in some conditions, provision of accommodation space for storm surges. Uncertainties in the contribution of ecosystems to flood safety arise from incomplete modeling capacity for sediment dynamics, inclusion of sub-grid phenomena, complex spatial structure, limited understanding of edge dynamics and sparse knowledge of ecosystem behavior under extreme loading. For application at regional scale, the most difficult problem is that nature-based solutions are needed most where natural processes are weakest, e.g. due to extreme encroachment of the coast.

In the broader context of use of nature-based solutions as measure to adapt to increased relative sea level rise, a crucial question is how these solutions will behave at longer term. Geological evidence shows that increased rates of sea level rise forces coastal wetlands to become narrower and to move faster inland. An important factor determining this behavior is the sediment mass balance, inhibiting the vertical rise of an ever-larger surface of marshes with increasing sea level. A second factor is the stability of the seaward edge under conditions of increased wave attack in deeper coastal profiles. These phenomena are observed in contemporary systems with high rates of subsidence.

Future-proof nature-based solutions for flood defense will have to resolve the issues related to the regional sediment mass balance and long-term stability of the coastal ecosystems. Whether the strategy is to hold the line of the coast or to withdraw, future coastlines will need sufficient sediment sources, sufficient sediment transport capacity and sufficient ecosystem resilience to provide extended services combining flood defense and biodiversity conservation. Major scientific challenges in the design and use of nature-based solutions are identified at this system scale.

How to cite: Herman, P.: Nature-based solutions for flood protection in a systems perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17143, https://doi.org/10.5194/egusphere-egu23-17143, 2023.

The fundamental water resources problem to be solved in adaptation to climate change is increasing rainfall variability: generally, flood peaks are increasing, and dry-season water availability is decreasing. Large, centralized adaptation strategies such as reservoirs impounded behind tall dams typically perform well at both attenuation of flood peaks, and augmentation of dry season flows. They come with substantial costs, however, in terms of capital cost outlays and damages to local ecological and human environments. The Intergovernmental Panel on Climate Change, the United States (US) Government, the European Union, the United Nations, and many other institutions and agencies are currently advocating for the adoption of “nature-based solutions” (NbS), which are believed able to reduce adverse climate change impacts at community scale, while supporting biodiversity and securing ecosystem services. However, the potential of NbS to provide the intended benefits has not been rigorously assessed. This talk presents a preliminary assessment of climate change vulnerabilities for the Chimanimani biosphere reserve in Zimbabwe, and an evaluation of the tradeoffs between conventional “hard” civil infrastructure and decentralized NbS.

How to cite: Ray, P. and Tracy, J.: Quantitative assessment of the tradeoffs between conventional “hard” civil infrastructure and nature-based solutions for climate change adaptation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16603, https://doi.org/10.5194/egusphere-egu23-16603, 2023.

Nature-based features (NNBFs) have emerged over the past two decades as tools to leverage natural processes that provide a range of functions from flood reduction to pollutant removal. Despite their growing popularity, a notable gap remains between our understanding of internal NNBF processes and our ability to design NNBFs for specific objectives (e.g., x% reduction of peak storm flow, x% nitrogen reduction) by leveraging such processes. NNBF benefits are, as a result, difficult to quantify, making them a riskier investment compared to tried-and-true grey infrastructure alternatives. This knowledge gap must be filled if we want to design effective and sustainable NNBFs that are viewed on equal footing with grey infrastructure. This presentation will discuss the development of a non-tidal constructed wetland model for the purpose of evaluating design suitability across a range of performance metrics. This model, written in MATLAB, couples hydrologic, hydraulic, and water quality modules. It also allows the user to adjust the constructed wetland configuration (i.e., shape, area, grid cell water depths, vegetation placement, etc.) to maximize specific performance objectives including flood control, wildlife habitat, and water quality. Current efforts are also underway to build upon this design model concept to (1) expand to tidal wetland environments like salt marshes, (2) incorporate vegetation growth/death and morphologic processes, and (3) to incorporate future uncertainty into the design process to support the development of robust and sustainable NNBFs across a range of potential futures and landscape contexts. The overall aim of this work is to develop a framework that allows engineers and policy-makers to evaluate NNBF performance on level footing with grey infrastructure alternatives.

How to cite: Olszewski, J., Kucharski, J., Smith, M., Abellera, M., and Steissberg, T.: Nature-Based Features: developing a framework to shift them from risky investments to reliable and robust solutions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3548, https://doi.org/10.5194/egusphere-egu23-3548, 2023.

Intertidal vegetation is renowned for its capacity to reduce wave energy and thereby enhance coastal safety. At present, this concept is well established on local scales (meters to marsh-mudflat system) and relatively short terms (hours to years). However, the development of intertidal vegetation on larger scales (i.e. the spatial scale of an estuary and the temporal scale of decades) is unknown. Consequently, it is impossible at the moment to quantify the wave damping capacity at these scales. In this paper, the decadal spatiotemporal characteristics and dynamics of intertidal vegetation are studied by combining available data (e.g. from field surveys) and remote sensing techniques (Combining SAR and optical remote sensing). For this analysis, the Scheldt estuary, with a tidal reach of more than 160km from the mouth near Vlissingen, the Netherlands, till Gent, Belgium, is used as study area. Intertidal vegetation located in salt, brackish and freshwater environments is considered to cover the changes in salinity regimes in estuaries. More specifically, the vegetation species considered in this study are: Common glasswort (Salicornia europaea), Common cordgrass (Spartina anglica), Saltmarsh bulrush (Scirpus maritimus), Common reed (Phragmites australis) and Common willow (Salix alba). The variability in vegetation distribution in this study has three components: (1) the areal extent i.e. total marsh dimensions and associated variability in this (expansion/retreat); (2) the variability in vegetation characteristics e.g. density, biomass, height and seasonality; and (3) shifts in vegetation species i.e. going from one species to another at the same location over time. Since the wave damping capacity of a vegetated intertidal area is a combination of the wave attenuation from the bottom friction and the attenuation due to the presence of vegetation, the bed topography corresponding to the vegetation pattern is established from laser altimetry (LiDAR) data. Ultimately, the wave damping capacity of intertidal vegetated areas under various storm conditions is determined using a Simulating WAves Nearshore (SWAN) model, based on the spatial vegetation patterns and characteristics, as well as the variation in bathymetry. Quantification of the decadal dynamics of intertidal vegetation in estuaries and its impact on the wave damping capacity will help us to give recommendations for the effectiveness and the design of nature-based coastal protection measures.

How to cite: Bootsma, J., Borsje, B., van der Wal, D., and Hulscher, S.: Decadal intertidal vegetation development in an estuary and its effect on the wave damping capacity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11683, https://doi.org/10.5194/egusphere-egu23-11683, 2023.

We investigated the ecohydraulic effects of a recently implemented hydropeaking mitigation measure in the Upper Noce Stream (NE Italy, Italian Alps), which also allows for additional hydropower production. The Upper Noce, a 3^rd order gravel-bed stream, was affected since the mid-1920s by storage hydropower production and associated hydropeaking. The mitigation measure consisted in the diversion of most of the released hydropeaks into a sequence of three newly-installed, cascading run-of-the-river power plants, fed by a penstock running almost parallel to the former hydropeaking reach. The hydropeaking-diversion mitigation measure markedly reduced hydropeaking on a 10-km stream reach, and hydropeaking is now released immediately upstream the confluence with a major free-flowing tributary, which increases the hydropeaking baseflow. The flow regime in the mitigated reach shifted from hydropeaking-dominated to baseflow-dominated regime in winter, with flow variability represented only by snowmelt and rainfall in late spring and summer. We applied two sets of indicators (the Hydropeaking Indicators HP1, HP2 and the COSH method) and conducted a simplified hydraulic analysis of the hydropeaking wave propagation. We assessed the ecological effects of the mitigation measure using three complementary data sources: the analysis of (a) the benthic and (b) hyporheic invertebrate communities, based on datasets collected before and after the implementation of the diversion measure, and (c) ancillary data monitored by the diversion plant manager for required environmental monitoring, which included the suspended sediment regime and the Extended Biotic Index, measured yearly from the year before to the four subsequent years after the implementation of the mitigation measure.

Three main changes in eco-hydraulic processes associated with hydropeaking mitigation were detected. i) The flow regime in the mitigated reach changed to a residual flow type, with much less frequent residual hydropeaks, with an average two-fold increase in downramping rates that were recorded downstream the junction with a major tributary. ii) The functional composition of the macrobenthic communities shifted slightly in response to flow mitigation, but the taxonomic composition did not recover to conditions typical of more natural flow regimes. This was likely due to the reduced dilution of pollutants and resulting slight worsening in water quality. iii) The hyporheic communities conversely showed an increase in diversity and abundance of interstitial taxa, especially in the sites most affected by hydropeaking, and this effect was likely due to changes in the interstitial space availability, brought by an alteration of the previous time-space pattern of fine sediment transport, which eventually resulted in reduction of fine sediments clogging of the gravel bed interstices.

Besides illustrating a feasible hydropeaking mitigation option for Alpine streams, this work suggests the importance of monitoring both benthic and hyporheic communities, together with the flow and sediment supply regimes, and physico-chemical water quality parameters, for carefully detecting changes in eco-hydraulic processes associated with hydropeaking mitigation that may not be fully expected in the design phase.

How to cite: Zolezzi, G., Vallefuoco, F., Casari, A., Larsen, S., Dallafior, V., and Bruno, M. C.: Assessing the eco-hydraulic effects of a hydropeaking mitigation measure with increased energy production in the Noce River (Italian Alps), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14621, https://doi.org/10.5194/egusphere-egu23-14621, 2023.

Zeeshan Tahir Virk^a, Faisal bin Ashraf^a,b, Ali Torabi Haghighi^a, Bjorn Klove^a, Seppo Hellsten^c, Hannu Marttila^a

^a University of Oulu, Faculty of Technology, Water, Energy, Environmental engineering Research Unit

^b Stockholm Environment Institute (SEI)

^c Finnish Environment Institute (SYKE)

Fluctuating energy prices in the Nordic region call for short-term river flow regulation at hydropower plants (HPPs). This short-term regulation leads to Hydropeaking – the pulsating water flow downstream of an HPP. Hydropeaking is detrimental to the overall health of the river, impacting all riverine and riparian ecosystem services. One of the major ecosystem services affected by hydropeaking in Nordic rivers is the socio–recreational ecosystem service (SRES), which holds significant value for Nordic culture and human wellbeing. We examine how SRES are affected by hourly hydropeaking events in a large Nordic River reach. Employing two indicators based on normalized daily maximum flow difference and sub-daily flow ramping we estimated annual and seasonal trends of hydropeaking in the studied reach of the Kemijoki River system. The study reach was found to be under “High Pressure” peaking class in all seasons. For SRES impact assessment, we applied a novel methodological approach to multiple layers of high-resolution spatio-temporal data. An overlay analysis of inundation maps derived from 2D-hydrodynamic modeling and a customized land classification map based on a machine learning algorithm resulted in identification of SRES areas under influence of sub-daily hydropeaking within the study reach. The degree of impact on SRES corresponded to seasonal variations of sub-daily hydropeaking where the highest impact was observed in the summer season The most affected recreational land uses were the shore and litorine areas. Furthermore, as intraday flow ramping results in peaking waves downstream of the HPP, our results show that most part of the river channel becomes hydraulically unsafe during hydropeaking events, especially in summer, which is popular in the context of Nordic culture and tourism. Consequently, hydropeaking can seriously impact the social, and recreational services offered by Nordic rivers; therefore, regulation practices at the HPPs should also consider SPES aspects.  We recommend further research to evaluate these services so that tradeoffs between energy production at HPPs and ecosystem services of rivers can be balanced.

How to cite: Virk, Z.: Nordic socio-recreational ecosystem services in a hydropeaked river system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4122, https://doi.org/10.5194/egusphere-egu23-4122, 2023.

The artificial pulsed flows occurring downstream of hydropower plants due to electricity demand, i.e. hydropeaking, affect habitat selection by fish. This effect is particularly unknown for cyprinids, which are the most representative freshwater fish family in European rivers. This study aimed to evaluate the utility of two types of flow-refuge by Iberian barbel (Luciobarbus bocagei) at an indoor flume (6.5m x 0.7m x 0.8m) as a potential solution to mitigate the effects of pulsed flows associated to hydropower production. Based on previous comprehensive research conducted on cyprinids and with the results of this study, the best type of flow-refuge was selected, up-scaled and implemented downstream of small hydropower plants. Two different approach angles with the flume wall - 45⁰ and 70⁰ - were tested to assess the effectiveness of the created hydraulic conditions on attracting fish to the flow-refuge. For each type we tested a base flow event (7 l.s^-1), simulating river natural conditions, and a peak flow event (60 l.s^-1), simulating pulsed flows. For each setting, two flow-refuges (downstream and upstream) were installed in the flume and tested with a school of five Iberian barbels. The utility of the flow-refuges was assessed by the frequency and time of use by fish at two distinct flow-refuge locations i.e., downstream (area between the flume and the adjacent flow-refuge walls), and inside (the effective covered area of the flow-refuge). Blood glucose and lactate levels were quantified to identify potential physiological adjustments associated with the pulsed flows and the flow-refuge type. Preliminary results indicate that fish behavior differs according to flow event and the type of flow-refuge. The frequency of a single fish using the flow-refuge was higher in the 45⁰ refuge during pulsed flows than in the 70⁰. Overall, the average time spent inside the flow-refuges was higher during pulsed flows for both types and higher in the 45⁰ refuge.  After the 60 l.s^-1 events, the blood glucose and lactate levels were higher than in the 7 l.s^-1 events. In addition, lactate levels for the 45⁰ flow-refuge during the 60 l.s^-1 events, were the highest when compared to 7 l.s^-1 events. These results may be explained by the higher velocities created in the presence of the 45⁰ flow-refuge, shown by ADV results, that favoured individual use and rheotactic behaviour, setting off physiological adjustments, increasing residency time and the efficiency to use the flow-refuge.

How to cite: Boavida, I., Leite, R., Costa, M. J., Merianne, A., Mameri, D., Afonso, F., Santos, J. M., and Pinheiro, A.: Mitigating the hydropeaking using flow refuge: an experimental case-study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16568, https://doi.org/10.5194/egusphere-egu23-16568, 2023.

Artificial sub-daily flow fluctuations caused by peak-operating hydropower plants are considered one of the most significant impacts on riverine ecosystems downstream of dams. These rivers have, therefore, been subject to numerous studies in recent decades. However, cyprinid fish, in contrast to salmonids, have hardly been addressed in hydropeaking studies yet, and extensive knowledge gaps remain. Therefore, our experimental study aims to assess the effects of rapid flow reductions on the early life stages of two European cyprinid indicator species, the common barbel (Barbus barbus L.) and the common nase (Chondrostoma nasus L.).

We conducted mesocosm experiments (2.25×2 m) under semi-natural conditions at an outdoor experimental facility (http://hydropeaking.boku.ac.at), simulating different hydropeaking scenarios with varying down-ramping rates during day and night. At each trial, 100 fish from one species (body length <20 mm) were stocked at peak flow (80 L.s^-1). After an acclimation time (15 min.), the flow rate was reduced with variable ramping rates (0.3–1.8 cm.min^-1) to constant low flow conditions (10 L.s^-1). As a response parameter, larval stranding on a gently sloped shoreline mimicking typical nursery habitats was quantified during day and night.

The results reveal distinct diurnal patterns for both species, with increased stranding rates at night for all experimental scenarios. In addition, the data indicate differences between the tested down-ramping rates and show interaction effects between both parameters. The difference between species may result from water temperature and ecological factors. The study outcomes will benefit the ongoing discussion on species-specific hydropeaking mitigation by providing first insights on the direct effects of artificial flow down-ramping on early life stages of cyprinid fish.

How to cite: Führer, S., Hayes, D. S., Hasler, T., Graf, D. R. M., Stoisser, F., Fauchery, E., Coudrais, A., Olejarz, A., Mameri, D., Schmutz, S., and Auer, S.: Stranding of early cyprinid life stages: effects of artificial flow down-ramping on Barbus barbus L. and Chondrostoma nasus L. under experimental conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5678, https://doi.org/10.5194/egusphere-egu23-5678, 2023.

Rivers are increasingly recognised as active players in the global carbon cycle. They are able to transport, transform, and exchange organic matter, and can emit considerable fluxes of greenhouse gases (e.g., CO₂) into the atmosphere, with a magnitude comparable to the global carbon input to the oceans. However, the quantification of these processes is still affected by considerable uncertainties, driven by an incomplete understanding of the interplay between physical, geochemical, and biological parameters, and by a lack of spatially and temporally resolved high-quality data. For instance, and despite a potentially strong impact on kilometres of rivers worldwide, the effects of hydropeaking on riverine CO₂ emissions have been almost completely neglected until recently (Calamita et al., Unaccounted CO₂ leaks downstream of a large tropical hydroelectric reservoir, PNAS 2020). As a contribution to filling this knowledge gap, we present the results of a field-measurement campaign performed in a single-thread Alpine river (River Noce, Italy) during multiple hydropeaking events. Data of water-dissolved CO₂, water temperature, and flow discharge, were collected sub-hourly both downstream and upstream of the outlets of a hydropower plant, revealing a complex pattern of variation in time at both locations. Water released from the hydropower plant during hydropeaking had oversaturated CO₂ concentrations relative to the atmosphere, in close agreement with water samples collected in the hypolimnion of the upstream reservoir. Higher flow rates during hydropeaking events were associated with higher rates of gas exchange through the water-air interface. Higher exchange rates and higher CO₂ concentrations in water during hydropeaking events enhanced CO₂ fluxes, as confirmed by measurements with a floating CO₂ flux chamber. Meanwhile, the CO₂ concentration upstream of the outlets displayed strong diel fluctuations around the atmospheric equilibrium concentration, which were likely driven by primary production within the residual flow during the day. It is shown that the residual flow can have a previously unacknowledged added value as a CO₂ sink during the day, fueled by its biological activity. Hydropower releases bypassed the residual flow and discharged hypolimnetic water oversaturated with CO₂ at high flow rates during hydropeaking, offsetting CO₂ concentration and fluxes downstream of the outlets and increasing emissions on average. These results highlight the ubiquity of hydropeaking impacts also with respect to greenhouse gas emissions. They illustrate the complexity of the riverine carbon cycle and demonstrate the importance of temporally and spatially-resolved data for the accurate assessment of the riverine carbon balance.

How to cite: Dolcetti, G., Piccolroaz, S., Bruno, M. C., Calamita, E., Larsen, S., Zolezzi, G., and Siviglia, A.: Measured temporal variations of CO2 concentration and atmospheric emissions in a hydropeaking-impacted river, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-560, https://doi.org/10.5194/egusphere-egu23-560, 2023.