Evaluating Nature-based Solutions for small catchments and streams: Incorporating floods, droughts and biodiversity

IN DEPTH NATURE–BASED SOLUTIONS Introduction Climate change is increasing extreme fluctuations in both floods and droughts worldwide, resulting also in an increased vulnerability of ecosystems already under pressure from human activities. This creates a need for increasing the climate resilience to better withstand both of these types of hydro-meteorological events using methods that also consider their impact on ecosystems at landscape scale. Nature-based Solutions (NbS) for flooding, water management and droughts are therefore gaining traction as a logical option of choice as they inherently focus on creating benefits for both human use and biodiversity 1 . Upstream catchments used to be dominated by a hydrology in which forests, wetlands, streams, and groundwater play the key roles as natural ecosystem types. However, these catchments have undergone significant change due to human use in the last century; intensification of agriculture has created a loss in natural ecosystems, urbanisation has increased drainage velocities by paving soils, limiting infiltration towards the groundwater. As a result, infiltration is limited, groundwater is not easily recharged, and rainfall events result in high runoff that not only increase flood risk but also increase soil erosion. This loss of the ‘sponge’ capacity of the landscape where it can absorb, retain, and slowly drain water is one of the reasons landscapes are vulnerable to increasing frequencies of hydrometeorological events. Figure 1 depicts the increase of water retention in the catchment through the increased use of the surface water, soil and groundwater systems, and how using this ‘sponge capacity’ leads to reduced flooding and droughts. Figure 1 | Schematic showing how the increase in water retained in the surface, soil and ground systems reduces flooding and droughts. Source: www.spongescapes.eu policybrief 1. Incorporating floods, droughts and biodiversity. for small catchments and streams: By Ellis Penning, Catherine Wilson, Christopher Wittmann, Guglielmo Sonnino Sorisio and Daniel Jones Evaluating Nature-based Solutions #HydrolinkMagazine | 05 IAHR.org 06 | #HydrolinkMagazine IAHR.org IN DEPTH > NATURE–BASED SOLUTIONS 2025 | 01 Nature-based Solutions that can help restore this ‘sponge function’ are numerous, including: methods that improve soil conditions in agriculture (e.g. through different cropping, tillage, and irrigation approaches); changes in local land-use to be more in line with the landscape features; and restoration of wetlands, floodplains and streams themselves to give more space to water where it falls. Yet the evidence base related to the performance of these NbS is not yet fully developed, which limits the uptake of these measures. Scientific evidence collected from implemented NbS often focuses only on one aspect of their functioning (e.g. the performance under floods) and lack data on other aspects, such as the response during dry weather spells, the gained biodiversity values, or related response of the hydrological system during droughts. Despite these challenges, the evidence is slowly growing, and many initiatives are under way to increase this evidence base, both in depth and breadth of application. In this article we show two examples of NbS for small catchments and streams as examples of this progress. EXAMPLE 1 Assessing catchments for floods and droughts Drought mitigation measures (DMM) have gained major traction amongst practitioners and water authorities to combat extreme dry conditions due to the significant increase in droughts in central and western Europe over the last few decades 2 . However, the historically wet year of 2023, where Europe experienced around 7% more precipitation than the annual average 3 , has highlighted the other (extreme wet) side of the coin of climate change impacts. These two opposite extremes highlight the variability between dry and wet extremes, particularly between summer and winter. Using an integrated ground and surface water modelling approach, we assessed the potential impact of ambitious nature-based DMM such as raising stream beds and blocking drainage channels during winter floods in the Chaamse beken catchment (Netherlands). This catchment lies in the sandy soil region in the south of the Netherlands and the drainage area is 50 km 2 . The catchment is dominated by forested areas (37%) and agricultural land (42%), mainly consisting of pastures, with a considerable urban area (9%) covering the mid to lower catchment, For a future climate scenario (2°C global mean temperature rise by 2050 4 ), the assessment included the long-term simulation of groundwater levels as well as the short-term response to different heavy rainfall events. This was achieved by integrating agroundwater/ unsaturated zone model (MODFLOW-MetaSWAP) to a hydrodynamic surface water model (Delft3D FM 1D2D) 5 . The implementation of ambitious drought mitigation measures was shown to be capable of compensating for climate change induced summer groundwater deficits by significantly reducing the drainage of groundwater throughout the catchment, by raising groundwater levels. The reduced drainage is also present during winter, indicated by the reduced baseflow at the downstream outlet, as shown in Figure 2. This also causes significantly raised groundwater levels during the wetter winter months, leading to a reduced floodwater retention capacity in the soils and groundwater during this time. As a result, the peak discharge at the stream outlet increased by 10% during a 1-in10-year rainfall event, and the flood inundated area doubled in size. Thus, had these ambitious DMM been implemented, it would undoubtedly have led to an even worsened outcome during the 2023 winter floods. The results show that while implementing DMM are beneficial for their primary function (drought mitigation), they can create trade-offs during storm events Figure 2 | Hydrograph of the streamflow response at the outlet of the Chaamse beken following a 1-in-10-year rainfall event during winter under the reference scenario with implemented drought mitigation measures (DMM, green), and without implemented measures (red). #HydrolinkMagazine | 07 IAHR.org IN DEPTH > NATURE–BASED SOLUTIONS 2025 | 01 EXAMPLE 2 Assessing catchments for flooding considering antecedent conditions The increase in storm frequency due to climate change will cause flooding to become more likely at times when it would not traditionally occur. Pre-existing wet soil and higher river flowrate conditions are a major driver of this increased likelihood. Using rainfall, hydrometric and photogrammetry data from the Nature based Solutions Wilde Brook test site, in Shropshire, UK, we assessed the effect of wet antecedent soil conditions and higher pre-existing river flowrates on the rainfall-runoff response and water storage provided by the instream wood structures, known as leaky barriers, during low and moderate magnitude storm events (see Figure 3). The monitored NbS test catchment is 5.3 km 2 with 105 leaky barriers constructed using wood sourced locally along a reach length of 5.36 km, with barrier located every 15-50 m. Two named storms, Dennis (15th Feb 2020) and Babet (18th October 2023) caused wide-spread national flooding throughout England and Wales (UK), including at the Wilde Brook NbS test catchment. In both storms the water volume was held back by the leaky barriers and released over a period of one to two weeks, Figure 3 | Quantifying water storage of a leaky barrier (left) at Wilde Brook test catchment (UK) using mobile phone photogrammetry using software Agisoft Metashape (Agisoft LLC). Wireframe mesh in Fusion360 CAD software (right) is generated to enable a relationship between water storage and water height interval to be established. Flow direction is from top to bottom. which led to a reduction in the flow velocity as well as attenuation of the storm hydrograph. Dennis was preceded by another storm event, seven days prior, leading to high soil moisture content and elevated pre-existing baseflow conditions. Although the total rainfall occurring during Dennis was 40% lower than during Babet, the wet soil conditions from the storm prior to Dennis led to a significant reduction in water being soaked up by the soil and retained on the surface and an identical amount of effective rainfall entering the river. During storm Dennis this resulted in higher backwater rise upstream of each monitored leaky barrier due to the reduced water storage capacity of the soil and initial higher river flowrate, and greater total water storage utilisation by the barriers along the reach corresponding to around 10,700 m3 which is equivalent to four Olympic size swimming pools, as shown in Figure 4. Consideration of the impact of pre-existing conditions on the flood performance of a protection scheme is crucial and this example illustrates the capability of leaky barriers in providing significant instream surface water storage during a storm where adverse antecedent ground and river level conditions are present. Moving forward Working towards climate resilience at the landscape scale will benefit from an approach in which we use the motto ‘green where we can and grey where we must’. A wide variety of measures are already available to use as inspiration at the landscape scale 6 , but attention needs to be paid to evaluate their function more fully. For example, Nature based Solutions have not yet been sufficiently tested for moderate and extreme events nor on their interactions between opposite extremes (floods and droughts), so until we have evidence on how they perform during these conditions, we need to be cautious in their design to avoid trade-offs or bad practices, and consider combinations of both flood and drought measures to mitigate potential disbenefits of a measure for a single purpose. This requires an evaluation for a range of hydro-meteoro-logical events, from high frequency low flow magnitude events to rarer extreme events, as well as considerations of the catch-ment response to the cumulative effect of successive weather events including dry weather spells. in shallow groundwater environments. This means that additional flood mitigation measures to improve surface water storage alongside DMM placement need to be considered in the planning process. In this context, it has become increasingly important to begin assessing the implementation of climate adaptation measures more integrally to avoid generating trade-offs. Furthermore, exploring alternative land use schemes that are guided by prevalent environmental conditions need to be developed to build more resilience into catchments to combat drier, wetter, and/or more variable conditions. However, the reduced baseflow that results from the reduced drainage depth of open water bodies, is caused by the removal of drainage ditches and raising of stream beds. Apart from leading to higher flood risk in wet conditions, this increases the risk for aquatic ecology during dry periods. Furthermore, for a scheme to be truly integrative, its evaluation needs to include the effects beyond hydrology such as crop yield, terrestrial biodiversity, and water quality. 08 | #HydrolinkMagazine IAHR.org IN DEPTH > NATURE–BASED SOLUTIONS 2025 | 01 Figure 4 | Total net water storage provided by the 105 leaky barriers at the Wilde Brook test reach (UK) for nine storms where the flood return period (t) is t > 2 years. Total and effective rainfall volumes are presented which show the effect of dry (orange arrows) and wet (blue arrows) antecedent conditions on the proportion of rainfall that enters the river. Storms Babet and Dennis are comparative storms with identical effective rainfall and wet soil conditions. Effective rainfall is calculated from the river discharge over the storm duration. Furthermore, investment is needed in monitoring and evaluation of NbS schemes which measure pertinent indicators at appropriate temporal scales (e.g. minutes for flow peaks in small catchments, days/weeks for water storage and soil moisture content, weeks/months for groundwater level). Performance indicators should not be limited to one metric (e.g. changes in peak flow) and consideration should instead be given to several metrics, such as river level, groundwater level, water storage volume, soil moisture, stream velocity, and peak flow lag time, and be reported in the context of the catchment scale and event magnitude. In addition, there is also a need to evaluate the impact of measures in terms of their role in providing other ecosystem services and biodiversity benefits. The selection of which measure is appropriate in a given situation is determined not only by the local biophysical situation, but also by the social context and the creation of overarching strategies together with the stake-holders of the region. This calls for a close link with social and socioeconomic experts to help make the transition to climate resilient landscapes. The IAHR Science community can be part of this transition by bringing valuable insights in the system functioning and building a multi-faceted evidence base to help stakeholders choose and decide based on objective information. IN DEPTH > NATURE–BASED SOLUTIONS #HydrolinkMagazine | 09 IAHR.org 2025 | 01 References 1 | UNEA, 2022. Resolution 5 of the United Nations Environment Assembly at its fifth session: Strengthening actions for nature to achieve the Sustainable Development Goals, UNEP/EA.5/Res.5: Nature-based Solutions for supporting sustainable development. https://wedocs.unep.org/bitstream/handle/20.500.11822/39864/NATURE-BASED%20SOLUTIONS%20FOR%20SUPPORTING%20SUSTAINABLE%20 DEVELOPMENT.%20English.pdf?sequence=1&isAllowed=y (accessed 10.2.2025) 2 | Aalbers EE, van Meijgaard E., Lenderink G, de Vries H, van den Hurk B, 2023. The 2018 west-central European drought projected in a warmer climate: how much drier can it get? NHESS 23, 1921–1946. https://doi.org/10.5194/nhess-23-1921-2023 3 | C3S, WMO, 2024. Europe experiences widespread flooding and severe heatwaves in 2023 [WWW Document]. Press Release - Copernicus Climate Change Service and World Meteorological Organization: European State of the Climate 2023. URL https://climate.copernicus.eu/europe-experienceswidespread-flooding-and-severe-heatwaves-2023 (accessed 6.18.24). 4 | van den Hurk B, Siegmund P, Tank et al., 2014. KNMI’14: Climate Change scenarios for the 21st Century – A Netherlands perspective (No. WR201401). KNMI, De Bilt. 5 | Wittmann CD, de Louw P, Schoonderwoerd E, Kingma V, Dahm R, Peerdeman K, Penning E (in review). Evaluating the Effects of Drought Mitigation Measures During Flood Events. SSRN: http://dx.doi.org/10.2139/ssrn.5031130 6 | Environment Agency 2018, ‘Working with Natural Processes – Evidence Directory’ SC150005, https://assets.publishing.service.gov.uk/media/6036c5468fa8f5480a5386e9/Working_with_natural_processes_evidence_directory.pdf Christopher Wittmann Christopher Wittmann is a PhD Candidate at Wageningen University and Deltares, The Netherlands focussing on the Upscaling the Sponge Functioning of European Landscapes to Enhance Climate Resilience. He has a MSc in Hydro Science and Engineering from the Technical University Dresden. His research interests include the integrated assessment of NbS to quantify their hydrological functioning for floods and droughts using numerical models. Guglielmo Sonnino Sorisio Guglielmo Sonnino Sorisio obtained his PhD in 2024 from Cardiff University where he is currently as a post-doctoral researcher in Nature-based Solutions in the Hydro-environmental Research Centre. His research focuses on Natural Flood Management solutions for UK catchments and on fish passage at anthropogenic migratory barriers. His work is conducted through laboratory and field based studies. He is a member of the IAHR Working Group on Nature-based Solutions. Daniel Jones Daniel Jones has a MEng in Civil Engineering from Cardiff University. His research interests include Nature-based Solutions and he has expertise in the evaluation of their performance with time through his work on the Wilde Brook Natural Flood Management test site. He currently is working in the Catchments and Modelling team for Arup in Cardiff (UK). Ellis Penning Ellis Penning is an expert on Nature-based Solutions and Aquatic Ecology at Deltares, The Netherlands. She leads the IAHR Working Group on Nature-based Solutions and works in a wide variety of projects related to the topic. She coordinates the EU project SpongeScapes and co-coordinates the EU project SpongeWorks and contributes to the EU projects on large-scale freshwater ecosystem restoration MERLIN and Danube4all. She was the PI for the development of the Dutch vegetation monitor. Catherine Wilson Catherine Wilson is a Professor in Environmental Hydraulics at Cardiff University. Her research uses experimental and modelling approaches to understand the transport of sediment and plastics, the hydro-environmental impact of hydrokinetic turbines, and efficacy of Nature-based Solutions. She is the deputy Director of the Hydro-environmental Research Centre and leads the hydraulics laboratories at Cardiff. She is PI on a number of Nature-based Solutions field monitoring projects for the UK Government member of the NERC Flood and Droughts Research Infrastructure group for the UK.