Natural Flood Management Research Articles

Leaky dams augment afforestation to mitigate catchment scale flooding

AbstractDespite calls for large‐scale afforestation to alleviate flooding, the effectiveness of such action remains unclear. Simulations with the SHETRAN hydrological model are therefore carried out for the 335‐km2 Irthing catchment and its 1‐10‐km2 headwater catchments in northwest England to determine: (a) whether forests can reduce flood peak discharges in a large catchment; (b) the proportion of the catchment that requires afforestation to be effective; and (c) the extent to which a combination of afforestation and natural flood management features (leaky dams) improves upon afforestation on its own. Four‐year simulations were run with a range of forest covers and extents of leaky dam installation, the latter modelled as a channel hydraulic resistance. Hydrograph, flood frequency and peak discharge magnitude responses to forest cover simulated (and observed) in the headwater catchments are replicated in simulations at the full scale. Afforestation on its own can reduce the frequency of given flood magnitudes but has a variable impact on individual peak discharge magnitudes. For the Irthing, a 76% forest cover reduces the mean discharge of 20 peaks in a partial duration flood series by 10% relative to the current 21% forest cover but reduces the largest peak by only 2.5%. Accompanying adverse effects include 17.5% reduction in long‐term runoff and loss of agricultural land. By contrast, leaky dams mitigate flood frequencies and peak discharges effectively, over a range of discharge magnitudes, with no reduction of annual runoff. The dam installation required for a 10% mean peak discharge reduction reduces the largest peak by 22%. The flow resistance increases by which the dams are simulated have still to be translated into specific dam designs. Nevertheless, and considering the figures indicatively, dam installation in 20% of the Irthing streams with Strahler orders of 1–3 achieves a 10% peak discharge reduction with only 35% forest cover and 7.5% runoff reduction. The study illustrates the potential for a dense network of leaky dams to augment the impact of afforestation on flood mitigation, especially at the largest discharges, while minimizing adverse impacts on water resources and food security.

Field-based monitoring of instream leaky barrier backwater and storage during storm events

Engineered leaky barriers are increasingly used as natural flood management methods providing ecosystem and water quality benefits in addition to flood attenuation, complementing hard engineering flood defences. Field-based monitoring of a natural flood management site, Wilde Brook in the Corvedale catchment, England (UK) studied the rainfall-runoff relationship for a 5.36 km reach with 105 leaky barriers over two years. Paired pressure transducers were placed upstream and downstream of three channel spanning leaky barriers, allowing evaluation of upstream backwater rise relative to rainfall intensity, storm magnitude, and frequency. By increasing backwater rise, the leaky barriers caused overbank flows, resulting in a reduction in the cross-sectional area velocity after the event. The incidence of overbank flow depended on the local stream cross-sectional profile, barrier properties, location in the reach, and storm magnitude. Barrier operational flow conditions were classified into five modes according to relative bank inundation and barrier submergence extent. The backwater rise magnitude depended on barrier physical properties and evolution over time through cycles of accretion and build-up of brash and leafy material in the barrier, in addition to local bedload sediment transport dynamics, where instances of scour around the barriers were observed. Backwater rise and net volume hydrographs showed rapid filling up behind the barriers on the rising limb and slower water release on the falling limb. For a ∼ 4 yr return period storm event, results indicated that one leaky barrier increased storage volume by up to 102 m3, which translates to an overall net volume increase of ∼ 10,700 m3 for the full reach. These new findings provide quantitative evidence of leaky barrier backwater and storage performance, and leaky barrier design recommendations for storms up to a 4 yr return period. This evidence can be used to develop and validate flood modelling generalised approaches for smaller, more frequent storms, and work towards the development of an approach for modelling leaky barriers for larger storm magnitudes.

What approaches exist to evaluate the effectiveness of UK-relevant natural flood management measures? A systematic map

BackgroundThis systematic map principally sought to understand the different forms of effectiveness that existing studies evaluate in relation to Natural Flood Management (NFM) in the UK with a supplementary question of whether studies engaged with climate change and future flood risk. NFM measures seek to protect, enhance, emulate, or restore the natural function of rivers as part of approaches to flood risk management (FRM). While there is agreement in both academic and practice/policy literature that NFM should be part of a holistic FRM strategy to address current and future flood risk, the specifics of how to expand the application of and consistently implement NFM successfully in practice are less well known. A core focus of this study is on how the effectiveness of NFM measures is evaluated in different studies based on approaches drawn from the Environmental Impact Assessment (EIA) literature: procedural, substantive, transactive and normative. The systematic map also examines how studies account for climate change, which is a crucial issue given the connections between NFM and climate change adaptation and resilience.MethodsWe searched 13 bibliographic databases, Google scholar as a web-based search engine, and 21 organisational sites. Articles were screened by title, abstract, and full text based on defined eligibility criteria. Checks were performed for consistency amongst reviewers. Forms of effectiveness were coded on the basis of the included studies in the systematic map. The quantity and characteristics of the available evidence are summarised with the frequencies of effectiveness forms for each NFM measure are presented in heat maps.Review findingsA total of 216 articles reported eligible studies that were coded as part of the systematic map. Overall, the systematic map shows that the majority of studies considered at least one approach to effectiveness; however, very few studies considered multiple forms of effectiveness. The systematic map also demonstrates that climate change is considered systematically by around one-quarter of studies although many studies make claims about NFM’s effectiveness in the face of future climatic change.ConclusionsNFM can be effective in several different ways owing to their multiple benefits; however, there are evidence gaps around understanding these different forms of effectiveness. This is particularly marked for studies considering transactive and normative effectiveness. Interdisciplinary studies are more likely to consider multiple forms of effectiveness. This systematic map also found that whilst 75% of studies mention future climate change in their studies, only 24.1% contain a systematic consideration of the issue through, for example, using climate change projections. NFM is also at risk of climate change (e.g. through drought) and therefore it is imperative that study designs seek to incorporate consideration of effectiveness under future climate change. Policymakers should be made aware of the lack of understanding of how NFM measures perform under future climate change.

Land cover influence on catchment scale subsurface water storage investigated by multiple methods: Implications for UK Natural Flood Management

Study regionUnited Kingdom (UK). Study Focus‘Natural flood management’ (NFM) schemes manipulating land use and other catchment features to control runoff are increasingly promoted across the UK. Catchment water storage and mixing processes influence runoff, but our understanding of the effects of land cover change on these processes is still limited. This study combined hydrometric, isotopic and geochemical measurements to investigate land cover versus potential topographic, soil and geological controls. It compared storage-discharge dynamics in nine nested catchments within a 67 km2 managed upland catchment in southern Scotland. Storage and mixing dynamics were characterised from hydrometric data using recession analysis and from isotopic data using mean transit time and young water fraction estimates. To give information on water sources, groundwater fraction was estimated from end member mixing analysis based on acid neutralising capacity. New hydrological insightsThe analysis showed low but variable sub-catchment scale dynamic storage (16–200 mm), mean transit times (134–370 days) and groundwater fractions (0.20–0.52 of annual stream runoff). Soil hydraulic conductivity was most significantly positively correlated with storage and mixing measures, whilst percentage forest cover was inversely correlated. Any effects of forest cover on increasing catchment infiltration and storage are masked by soil hydraulic properties even in the most responsive catchments. This highlights the importance of understanding dominant controls on catchment storage when using tree planting as a flood management strategy.

Can hydrological models assess the impact of natural flood management in groundwater‐dominated catchments?

AbstractNatural flood management (NFM) is widely promoted for managing flood risks but the effectiveness of different types of NFM schemes at medium (100–1000 km2) and large scales (>1000 km2) remains widely unknown. This study demonstrates the importance of fully understanding the impact of model structure, calibration and uncertainty techniques on the results before the NFM assessment is undertaken. Land‐based NFM assessment is undertaken in two medium‐scale lowland catchments within the Thames River basin (UK) with a modelling approach that uses the Soil and Water Assessment Tool (SWAT) model within an uncertainty framework. The model performed poorly in groundwater‐dominated areas (P‐factor <0.5 and R‐factor >0.6). The model performed better in areas dominated by surface and interflow processes (P‐factor >0.5 and R‐factor <0.6) and here hypothetical experiments converting land to broadleaf woodland and cropland showed that the model offers good potential for the assessment of NFM effectiveness. However, the reduction of large flood flows greater than 4% in medium‐sized catchments would require afforestation of more than 75% of the area. Whilst hydrological models, and specifically SWAT, can be useful tools in assessing the effectiveness of NFM, these results demonstrate that they cannot be applied in all settings.

A systematic review of natural flood management modelling: Approaches, limitations, and potential solutions

AbstractThe Pitt Review of the 2007 summer floods in the UK, published in 2008, commended the potential of natural flood management (NFM) for reducing flood risk. NFM is a nature‐based approach that has since gained substantial interest from both practitioners and academics. The review further highlighted the need for catchment‐based flood management (CBFM) to enhance resilience to flooding and climate change by incorporating NFM and wider nature‐based solutions into hard flood protection systems. Such integrated approaches are considered to be more sustainable and adaptable than the traditional hard‐engineered measures. More recently, the European Commission's European Green Deal also highlighted the need for greater use of nature‐based solutions including NFM for managing flood risk. Whilst there have been many attempts to quantify the effects of NFM through hydraulic and hydrological modelling, there is still no systematic review conducted for these modelling works. This review aims to summarise the current NFM modelling approaches, as well as discussing their key limitations related to data, model methods, and real‐world applications. This paper then goes further to highlight potential solutions to some of these challenges and provides guidance to assist modellers to improve future modelling and data collection process.

Evaluating the effectiveness of land use management as a natural flood management intervention in reducing the impact of flooding for an upland catchment

AbstractNatural flood management (NFM) is a method for reducing flooding by using a catchment‐based approach to managing flood risk. Understanding and quantifying the impact of implementing NFM at the catchment scale remains ambiguous with a clear need for robust empirical evidence. A combination of fieldwork, laboratory analysis and modelling was applied to quantify the impacts of land use management changes on catchment flood hazard. Soil hydraulic conductivity was measured under varying land management regimes and used to parameterize a physically based spatially distributed hydrological model (SD‐TOPMODEL). A suite of stakeholder informed land management scenarios was modelled, permitting the quantification of the impact of NFM interventions on the timing and the intensity of the peak discharge at the catchment outlet. The findings support the implementation of NFM interventions as a means of reducing flood hazard within a rural upland catchment. Improved soil infiltration provided the greatest reduction in the intensity and delayed timing of the flood peak for a 10‐year occurrence storm event (7% reduction in peak runoff and 8% increase in lag time) with similar reductions observed for a 100‐year storm event. Catchment wide woodland planting reduced peak flow by 11% during the 100‐year event but was not effective during the 10‐year event. Riparian buffer strips provided consistent reductions in peak flow and in the timing of the peak across both storm events with no significant differences relating to vegetation age. Critically, we observed that the effect of implementing multiple NFM interventions was not additive and that efficiencies can be made in using this modelling approach to prioritize the most effective outcomes.

Perceptions and practice in Natural Flood Management: unpacking differences in community and practitioner perspectives

Divergent community and practitioner perceptions of Natural Flood Management (NFM) may impact wide-scale uptake, but are under-researched, especially in peri-urban environments. This mixed-methods study used picture- and scenario-based exercises, interviews and envisioning workshops in a post-industrial, peri-urban area of Greater Manchester, UK. Key differences were unpacked, with community members showing less confidence in NFM, and more confidence in grey infrastructure than professionals. Community confidence in installed NFM measures was, however, higher following a major storm event. Analysis suggests the value of demonstrating how NFM reduces flood risk, together with other co-benefits, through early engagement plus interpretation in the landscape. Uncertainty around effectiveness can be addressed using a learning-through-doing approach, enabled through field observation by community members. There is potential to engage more effectively around multifunctional benefits, framing NFM as green infrastructure that enhances biodiversity and recreation from the start. These findings hold significance for increasing uptake of NFM worldwide

Mainstreaming nature-based solutions: What role do Communities of Practice play in delivering a paradigm shift?

As the urgency to adapt to climate change intensifies, nature-based solutions (NBS) are receiving increasing attention. To mainstream NBS, a fundamental shift in environmental management is required. This study evaluates the role that Communities of Practice (CoP) can play as platforms to foster social learning to drive such a paradigm shift. A Natural Flood Management (NFM) CoP in Yorkshire, UK, was used as a case study. A unique research design combined opportunistic data collected prior to the inception of the CoP and purposive data collected during and after its formation. Opportunistic data captured information from stakeholders regarding NFM engagement and challenges around its instalment and delivery. Purposive data was used to examine the ability of a CoP to foster social learning, overcome the challenges identified prior to its establishment and evaluate the extent to which a CoP contributes to inducing a NBS paradigm shift, using a multi-loop social learning framework. Results demonstrate that the CoP was effective in delivering social learning and improving NFM instalment and delivery. While most evidence of social learning point to incremental rather than transformational changes, it did reveal abundant questioning of the current framing of flood management. Furthermore, the CoP seems to have encouraged some participants to re-think the current governance structures for NFM and the boundaries of current actor networks, raising promise that, if sustained in the longer term, the CoP could induce a paradigm shift. Further research should conduct longitudinal studies to examine the CoP’s development overtime and its potential for overcoming current constraints.

Modelling the effectiveness of land‐based natural flood management in a large, permeable catchment

AbstractIn the United Kingdom, woodland planting and soil and crop management are being promoted as approaches to tackling flooding. Although evidence is limited, it is thought tree planting and regenerative agriculture practices such as crop–herbal ley pasture rotations increase infiltration, soil water storage and evapotranspiration, potentially reducing flooding. A process‐based soil–water–vegetation model was coupled with a semi‐distributed groundwater model to explore the impact of these interventions on peak and low flows in a large, groundwater‐dominated catchment. Land use change and management were found to have limited potential to reduce flooding in this setting. Herbal ley–crop rotations produced a <1% reduction in flow for return periods >2 years, and levels of woodland planting judged to be the realistic maximum produced reductions of 0.2%–2.6%, depending on tree species. Broadscale spruce planting was the only scenario to produce significant reductions in peak flow (16.0%–24.7% at return periods 1–15 years); however, the level of spruce planting required to achieve these reductions was estimated to reduce Q95 flow by ~39%, which would likely have negative implications for water security and ecological river flows. The impact of land‐based natural flood management interventions for flood prevention in large, permeable catchments should not be overstated.

Methodological Framework for Assessing Hydromorphological Conditions of Heavily Modified and Artificial River Water Bodies in Croatia

Water Framework Directive (WFD) guidance documents from 2019 provide a comprehensive review of methods and guidelines to improve the comparability of heavily modified water bodies (HMWBs) in the European Union. However, there is currently no common, single methodology for monitoring HMWBs and artificial water bodies (AWBs) or for determining maximum (MEP) and good ecological potential (GEP). This study presents the first assessment of hydromorphological conditions of HMWBs and AWBs in Croatia based on type-specific indicators. The typology of HMWBs and AWBs was based on distinct hydromorphological characteristics and modifications in order to be easily related to the uses of the water bodies. The classes of hydromorphological potential were graded from the theoretical MEP, which was determined by the score scale as the tolerated deviation from natural reference conditions, considering potential mitigation measures. The use of the water body and/or the effects on the wider environment were considered while determining MEP and choosing indicators included in monitoring and assessment. In the case of AWBs, the parameters dependent on natural reference conditions, which are non-existent, were omitted from the assessment. Only 27% of HMWBs and 2 out of 51 AWBs achieved good potential for all three hydromorphological elements. The most significant hydromorphological modifications include channelization, straightening, deepening and removal of riparian vegetation due to flood-protection management practices. In order to achieve the environmental objectives set out by the WFD, the Croatian water management system has to start implementing mitigation measures, especially related to natural flood management.

Incorporating technical and farmer knowledge to improve land use and management for natural flood management in lowland catchments

Natural flood management (NFM) involves measures to restore and protect natural hydrology and geomorphology to minimise flood risk. NFM is promoted as part of the nation’s flood resilience response. Government agencies have created national maps of opportunities for specific NFM measure to target available grant aid and encourage landowners and managers to deliver effective NFM measures where most needed. Mapping opportunities within a regulatory framework ensures that measures are targeted towards areas with a presumption of suitability thus minimising the burden of the application process.The viability of NFM depends on the responsible authority and community at risk accepting the proposed measures as both effective and appropriate. Top-down initiatives can fail to meet their targets when landowners and managers feel left out of the decision making and their experiential local knowledge is at odds with the spatial prioritisation derived from national datasets.This interdisciplinary paper assesses the applicability of a technical-based national mapping approach with local stakeholders for identifying opportunities for woodland and runoff management measures to reduce flood risk within the lowland catchments of the West Thames River Basin, in southern England. Farmer surveys supported the technical map in terms of targeting areas of deep clay and silt soils over mudstone as high priority for improved land and soil management to reduce rapid runoff. However, while incentives correctly target the problem areas, farmers desired greater flexibility in choice of NFM measures to reflect local soil knowledge and the sustainability of management practices within farm systems. It is argued that a more integrated and co-designed mapping approach would enhance NFM uptake and thus achieve a greater reduction in downstream flood risk.

Nature-Based Solutions for Flood Mitigation and Soil Conservation in a Steep-Slope Olive-Orchard Catchment (Arquillos, SE Spain)

The frequency and magnitude of flash floods in the olive orchards of southern Spain have increased because of climate change and unsustainable olive-growing techniques. Affected surfaces occupy >85% of the rural regions of the Upper Guadalquivir Basin. Dangerous geomorphic processes record the increase of runoff, soil loss and streamflow through time. We report on ripple/dune growth over a plane bed on overland flows, deep incision of ephemeral gullies in olive groves and rock-bed erosion in streams, showing an extraordinary sediment transport capacity of sub-daily pluvial floods. We develop a novel method to design optimal solutions for natural flood management and erosion risk mitigation. We adopt physical-based equations and build a whole-system model that accurately reproduces the named processes. The approach yields the optimal targeted locations of nature-based solutions (NbSs) for active flow-control by choosing the physical-model parameters that minimise the peak discharge and the erosion-prone area, maximising the soil infiltration capacity. The sub-metric spatial resolution used to resolve microtopographic features of terrains/NbS yields a computational mesh with millions of cells, requiring a Graphics Processing Unit (GPU) to run massive numerical simulations. Our study could contribute to developing principles and standards for agricultural-management initiatives using NbSs in Mediterranean olive and vineyard orchards.

Natural flood management: Opportunities to implement nature‐based solutions on privately owned land

AbstractThe implementation of Natural Flood Management (NFM), as an example of a nature‐based solution (NbS), is promoted as a risk reduction strategy to support sustainable flood risk management and climate change adaptation more widely. Additionally, as an NbS, NFM aims to provide further multiple benefits, such as increased biodiversity and improved water quality as well as improved mental health. The implementation of NbS often needs private‐owned or managed land, yet can create conflicts between the different stakeholders which can undermine the social consensus required for successful implementation. Consequently, a main question is how the multiple benefits and requirements of NFM can be delivered to meet the different goals of the wide variety of stakeholders who must be involved. This article discusses the challenges and potential of implementing NFM as an alternative to the traditional technical mitigation measures in flood risk management. We outline four opportunities in the implementation of NFM: physical conditions of the catchment, social interaction, financial resources, and institutional setting. Their importance is then demonstrated and compared to different examples across the globe. Nevertheless, the core drivers reflect the social interaction and institutional setting and the role of stakeholders in the successful implementation of NFM.This article is categorized under:Engineering Water > Planning WaterHuman Water > Water GovernanceScience of Water > Water ExtremesWater and Life > Conservation, Management, and Awareness

Developing a “Sponge Catchment Management Plan (SCMP)” framework at the catchment scale: The case of Guiyang, SW China

AbstractCatchment floods are more challenging due to intensive urbanization and climate change. Enlightened by the Low Impact Development (LID), China initiated the Sponge City Program (SCP) to transform Urban Flood Management (UFM) to be more environmentally friendly in 2013. The China National Government (CNG) has subsidized municipal SCP facilities to enhance urban flood resilience while delivering multiple co‐benefits for urban ecosystems and social well‐being. Recent floods at Schleiden (Germany), Arizona State (USA), and Zhengzhou (China) in 2021 reflected the necessity of Catchment Flood Management (CFM) to cover the whole catchment scale. The SCP, designed to handle small‐scale urban pluvial floods, has brought concerns when facing larger‐scale fluvial floods after the Zhengzhou 2021 flood. Indeed, catchment‐scale Natural Flood Management (NFM) can manage fluvial floods while improving flood adaptations sustainably from upstream to downstream reaches. This research develops a new framework named the Sponge Catchment Management Plan (SCMP), including structural and Nonstructural elements. On the structural side, the SCMP framework integrates NFM with the SCP and Grey Engineering (GE) for reducing the fluvial flood discharge peaks in the whole‐catchment scale. On the nonstructural side, the SCMP encourages collaborative governance, revising technical standards, and improving “bottom‐up” participation. This research used Semi‐Structured Interviews (SSIs) and a Focus Group Approach (FGA) to explore 62 professional and Nonprofessional stakeholders’ perspectives on the SCMP framework. Some professional respondents did not know much about the NFM and were worried about the effectiveness of this practice. But most interviewees supported the SCMP pilot work and shared the co‐benefits from the NFM. This case study at the SW China, Guiyang, could be a lesson to encourage other Chinese cities further implement SCP to improve catchment‐scale flood resilience.

The influence of land management and seasonal changes in surface vegetation on flood mitigation in twoUKupland catchments

AbstractAs the frequency and magnitude of storm events increase with climate change, understanding how season and management influence flood peaks is essential. The influence of season and management of grasslands on flood peak timing and magnitude was modelled for Swindale and Calderdale, two catchments in northern England. Spatially‐Distributed TOPMODEL was used to investigate two scenarios across four storm events using empirically‐based soil and vegetation data. The first scenario applied seasonal changes in vegetative roughness, quantifying the effect on flood peaks at catchment scale. The second scenario modelled the influence of grassland management from historical high‐intensity grazing to a series of natural succession stages between grassland and woodland, and a conservation‐based management. Model outputs were analysed by flow type, measuring total, overland and base flow peaks at the catchment outlet. Seasonal changes to vegetation were found to increase overland flow peaks by up to +2.2% in winter and reduce them by −5.5% in summer compared to the annual average. Percentage changes in flood peak due to hillslope grassland management scenarios were more substantial; overland flow peaks were reduced by up to 41% in Calderdale where extensive woodland development was the most effective mitigation strategy, and up to 35% in Swindale, where a rank grassland dominated catchment was the most effective. Conservation‐based farming practices were also useful, reducing overland flow peak by up to 42% compared to the high intensity grazing scenario. Neither management nor seasonality changed the timing of runoff peaks by >45 min. Where overland flow dominates, especially in catchments with shallow soils, surface roughness was found to be more influential than soil permeability for flood mitigation. We recommend that seasonal changes to roughness are considered alongside the spatial distribution of Natural Flood Management in mosaiced upland catchments.

The Performance of Natural Flood Management at the Large Catchment-Scale: A Case Study in the Warwickshire Stour Valley

The limited understanding of Natural Flood Management (NFM) performance, especially at large hydrological scales, is considered a critical barrier for the further funding and implementation of these nature-based solutions to the increasing international problem of flooding. The publications of the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report and Environment Agency’s National Flood and Coastal Erosion Risk Management Strategy (NFCERMS) for England have shown that extreme weather, including increased likelihood of high magnitude flood events, will occur and will require more novel management methods. This study focused on the ability of co-designed NFM measures to ameliorate downstream fluvial flooding by attenuating catchment response through a highly spatially distributed network of attenuating and roughening measures. Performance was characterised by the ability of NFM to attenuate flood peaks at different spatial scales across a large (187 km2) dendritic catchment, including the lowering of flood peaks and delaying the time-to-peak. Using a coupled modelling methodology and applying it to the upper Stour Valley, Warwickshire-Avon, UK, a rural response to the application of a set of NFM interventions was developed using the hydrodynamic model Flood Modeller Pro and XPSWMM ©. The method demonstrated a means of incorporating local knowledge in a realistic set of NFM schemes, tested to multiple flood risk scenarios (including climate change). Under frequent, smaller design storm events (e.g., Index Flood (QMED) and 3.3% AEP), flood peaks were lowered across all hydrological scales tested (5.8 km2 to 187 km2). As the design flood event severity increases, impact from upstream NFM attenuation on downstream peak response diminished significantly, especially at the largest hydrological scales. However, even at the largest hydrological scale, delays in time-to-peak were noted, increasing the ability of downstream communities to respond and enact flood preparation activities, thus increasing resilience to potential flooding events. While the benefits were limited to large flood events, the modelling indicated that NFM has the potential to reduce downstream flood risk. However, greater integration of observed data to improve model confidence and reduce uncertainty in modelled events is needed, especially the uncertainty associated with using single peaked design storm events from the Flood Estimation Handbook (FEH). This paper proposes a future Before–After Control–Impact (BACI) monitoring programme that could be integrated with models and applied across non-tidally influenced catchments seeking to empirically test the hydrological performance of in-situ NFM.

The Role of Runoff Attenuation Features (RAFs) in Natural Flood Management

Natural Flood Management (NFM) and catchment-based solutions for flood risk management and environmental problems are wide-ranging and complex. Management of fluvial flood risk in the UK is undergoing a fundamental shift, with a change in emphasis from solely working with structural defences to considering catchment-based measures which attenuate flood runoff. At the heart of this change are NFM and nature-based solutions. One key type of intervention is the Runoff Attenuation Feature (RAF): a class of features that targets runoff flow pathways and creates new temporary flow storage (such as ponds and leaky barriers). However, there is currently a lack of evidence for the effectiveness of NFM and RAFs at larger catchment scales and for managing extreme flood events. Nevertheless, there is a strong evidence base to suggest that well-designed RAFs deliver a range of ecosystem services if installed in the correct location. This paper reviews and critiques the role of RAFs and NFM as an interventionist and holistic approach to lowering runoff rates. The link between RAF design types and their relationship to land use and scale is made. Recent novel innovations and attempts to scale up RAFs are discussed. The role of antecedent conditions, groundwater and the change in residence time of processes is highlighted. The uncertainty and complexity of proving NFM effectiveness underpin a view that new thinking in catchment flood management is needed. New research is required, and many questions are raised about RAFs and NFM. The direction of travel is that a positive and proactive NFM community can now embrace the problem. Proof that RAFs and NFM can address flood management is not likely to be resolved without a great deal of further research but confidence that RAFs do beneficial work is growing and an argument for greater amounts of runoff attenuation is made.

Deciding on fitness‐for‐purpose‐of models and of natural flood management

AbstractThis article has taken a distributed modelling approach to examine the potential for hillslope storage bunds as a natural flood management (NFM) strategy to mitigate the effects of downstream flooding if deployed extensively in the 209 km2 River Kent catchment in Cumbria (UK). To do so, a novel invalidation approach has been taken to calibrate an ensemble of Dynamic Topmodel parameter sets considered as fit‐for‐purpose for the prediction of flood peaks. Only 67 realizations out of 100 000 survived evaluation at the peaks of ranked flood hydrographs including the notable 2005, 2009 and 2015 events, together with a process constraint on the minimum area contributing saturation overland flow in the largest storm of record. These 67 events were used as the baseline (before intervention results) to simulate the effect of introducing 1549 storage bunds to temporarily retain saturation overland flows in optimal hillslope locations. The analysis is extended to simulating the reduction of inundation and damage estimates by running the ensemble of Dyanmic Topmodel simulations as inputs to the distributed hydraulic model JFlow for the 2015 Storm Desmond event. Even given such a large number of interventions the effects on the flood peaks and damages in this case were relatively small. The results of the study emphasize the large amounts of storage required in any NFM scheme, with individual features large enough that they are not too quickly overwhelmed, in order to have a significant impact on flood hydrographs and consequent flood damages.

Nature‐based solutions enhance sediment and nutrient storage in an agricultural lowland catchment

AbstractIn this paper, nature‐based solutions (NBS) include: (1) natural flood management (NFM) interventions with a primary function of flood risk reduction but with additional multiple benefits for water quality improvements through the mitigation of diffuse pollution; and (2) ponds with a primary function of water quality improvement. This study assesses the ability of these NBS to trap pollutants in run‐off within two small (3.4 km2) agricultural catchments (Upper Thames, UK). The masses of sediment, phosphorus, and organic carbon trapped by 14 features (since construction 2–3 years previously) were quantified through sediment surveying and sampling. Streamflow and suspended sediment monitoring downstream of features enabled catchment yields to be calculated. The features trapped a total of 83 t sediment, 122 kg phosphorus, and 4.3 t organic carbon. Although the footprint of the features was <1% of the catchment area, they drained 44% of the total land area and captured the equivalent of 15% of the total suspended sediment yield, 10% of the total phosphorus yield, and 8% of the particulate organic carbon yield as monitored at the catchment outlet over the monitoring period. Results reveal that accumulation rates were influenced by hydrological connectivity, with greater accumulation in features constructed directly on streams (online ponds), and those offline features that filled from overbank flows. The low to moderate accumulation rates observed in offline features suggests that their floodwater storage potential is only likely to significantly reduce in the medium term, necessitating maintenance after ~10 years. Compared with topsoil in each contributing area, trapped sediment was enriched in phosphorus and carbon in the majority of features, having on average 50% higher phosphorus and 17% higher organic carbon concentrations than surrounding arable soils, highlighting its potential value for redistribution on farmland. Monitoring results demonstrate the potential of NBS, including NFM, to mitigate diffuse pollution in lowland catchments.