Natural Flood Research Articles

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.

Intensified rice production negatively impacts plant biodiversity, diet, lifestyle and quality of life: transdisciplinary and gendered research in the Middle Senegal River Valley

A major programme of irrigated rice extension in the Middle Senegal River Valley has further limited the river’s natural flooding in the floodplain (Waalo), initially reduced by drought. We conducted a transdisciplinary (TD) and gendered study in the region to explore links between agricultural biodiversity and family diets using a social analysis of women’s practices. The results showed how rice expansion impacts local agrobiodiversity, diet quality and the cultural way of life. Disappearance of the singular agropastoral and fishing system of the Senegal River Valley is profoundly modifying the landscape, limiting wooded riverine settings, and is undermining the traditional diversified flood-recession cropping system in the Waalo. This is causing an overconsumption of rice by reducing alternative food sources, such as sorghum, vegetables and animal products (fish, milk and meat). In particular, flood-recession sorghums are in danger of disappearing, yet they are more nutritious than rice and now sell for twice as much, or more. The way of life is being disrupted, notably sociabilities previously based on territorial complementarities, and women are disadvantaged in terms of recognition and added workload. Women’s groups have launched collective irrigated gardens, organic or not, only supported by the local NGO, but any surplus is hardly ever sold on the weekly markets in the neighbourhood. Moreover, this diet imbalance increases nutritional risk factors for health, such as vitamin and iron deficiencies, especially for women, hypertension and diabetes. We argue that, firstly, gendered TD experiences are relevant for documenting women’s activities in order for them to gain political support and, secondly, that targeting women’s care tasks gives more value and impact to TD research results.

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.

Investigation of effect of logjam series for varying channel and barrier physical properties using a sparse input data 1D network model

Targeted design and placement of natural flood management leaky barriers, or engineered logjams, and accurate representation of logjams in hydraulic models across different scales is necessary to generate desired water storage and flood peak attenuation for effective river restoration and natural flood management projects. We systematically assess the effect of varying logjam spacing, extent of logjam-generated change in water surface profile, vertical height of lower gap, overflow to local floodplain, and varying channel slope, representing a series of logjams in a sparse input data 1D network model including the jam-generated backwater, which depends on loss of momentum within the jam. To guide catchment-scale representation, best-fit increased channel resistance was found to increase with number of jams, approaching an analytically determined maximum for close inter-jam spacing, useful for setting an upper envelope for efficacy.

From Sponge Cities to Sponge Watersheds: Enhancing Flood Resilience in the Sishui River Basin in Zhengzhou, China

Increasing extreme rainfall and increasingly severe flood disasters worldwide make it essential to enhance flood resilience research and practice. The Sponge City Approach (SCA) has been proposed as a nature-based solution for solving pressing water problems in cities, but there is an urgent need to apply the approach at the broader catchment scale to address the rising challenges driven by climate change. This study established a Sponge Watershed Approach (SWA) using the “Source-Flow-Sink” framework and applied it in the Sishui River Basin in Zhengzhou, China, to reconstruct a town after a heavy rainfall disaster and enhance its resilience to future floods. An integrated modeling approach utilizing hydrologic, hydraulic, and flood-loss curve models was used to analyze and evaluate the flood-reduction effects of different strategies under multiple scenarios. The results show that: (1) a strategy focusing solely on interventions at the source of runoff cannot adequately cope with extreme rainfall, and a whole-system approach to managing floods is necessary instead; (2) the effectiveness of the flow strategy depends on the intervention magnitude and the location where interventions are made; (3) the sink strategy offers the most effective way (approximately 30% decrease in flood loss) to adapt to extreme rainfall; (4) the risk and impact of flood disaster in the town was significantly reduced with the SWA (approximately 35–50% decrease in flood loss) compared to those without any interventions; (5) the effects of these strategies are interactive, and the optimal strategy depends on the rainfall condition. These results demonstrate the practicability of SWA by integrating the source, flow, and sink strategy at the whole-catchment scale, which will help promote natural flood management and enhance the climate resilience of catchments worldwide.

On (in)validating environmental models. 2. Implementation of a Turing‐like test to modelling hydrological processes

AbstractPart 1 of this study discussed the concept of using a form of Turing‐like test for model evaluation, together with eight principles for implementing such an approach. In this part, the framing of fitness‐for‐purpose as a Turing‐like test is discussed, together with an example application of trying to assess whether a rainfall‐runoff model might be an adequate representation of the discharge response in a catchment for predicting future natural flood management scenarios. It is shown that the variation between event runoff coefficients in the record can be used to create some limits of acceptability that implicitly take some account of the epistemic uncertainties arising from lack of knowledge about errors in rainfall and discharge observations. In the case study it is demonstrated that the model used cannot be validated in this way across all the range of observed discharges, but that behavioural models can be found for the peak flows that are the subject of interest in the application. Thinking in terms of the Turing‐like test focusses attention on the critical observations needed to test whether streamflow is being produced in the right way so that a model is considered as fit‐for‐purpose in predicting the impacts of future change scenarios. As is the case for uncertainty estimation in general, it is argued that the assumptions made in setting behavioural limits of acceptability should be stated explicitly to leave an audit trail in any application that can be reviewed by users of the model outputs.

Simulated effects of floodplain restoration on plant community types

AbstractAimsChannelization and artificial embankments have altered the natural flood regime of many rivers, impacting the hydrological characteristics of floodplain ecosystems and their biological communities. This study was undertaken on a floodplain meadow to assess spatial patterns of plant communities in relation to soil physical and chemical conditions, and the impacts of floodplain restoration that involved embankment‐removal.LocationRiver Glaven, Hunworth, Norfolk, UK.MethodsFine‐scale plant and soil chemistry sampling was conducted prior to embankment removal, and hydrological and climatological conditions were monitored prior to and after embankment removal. Hydrological/hydraulic modelling simulated groundwater levels for a 10‐year period to assess changes in soil aeration stresses and plant community composition following embankment‐removal.ResultsHydrology was identified as the primary driver of plant community composition. Soil fertility was also important. Unique continuous measurements of vadose dissolved oxygen concentrations using oxygen optodes indicated strong coupling between water table depth and root zone dissolved oxygen concentrations. Reinstatement of overbank flows did not substantially affect aeration stress across most of the meadow because of pre‐existing wet conditions. However, along the river‐floodplain ecotone, aeration stress increased substantially from conditions normally associated with dry grassland to those characteristic of fen communities (p < 0.05).ConclusionsThis restored water table regime may be suitable for more diverse plant assemblages. Benefits of flooding for increased species richness and transport of propagules may, however, be over‐ridden without accompanying water level management during the growing season, or hay removal to balance additional supply of nutrients from river floodwater and sediment. Our results show that hydrological/hydraulic modelling combined with quantitative measures of plant water‐requirements can provide practical and adaptive management tools to estimate the response of floodplain communities to changing water regimes.

Optimized flood control for the Three Gorges Reservoir considering evolving flood propagation trends in the Jingjiang Reach of the Yangtze River

The main objective for the Three Gorges Reservoir is to help control floods and to protect the Jingjiang and Chenglingji regions in the middle reach of the Yangtze River. Hydrological measurements have demonstrated that the character of flood propagation in the Jingjiang Reach has changed significantly since the Three Gorges Reservoir became operational, meaning that flood control management must be modified accordingly. To explore the effective flood control management, a hydraulic model was developed to simulate the potential causes and impacts. For a natural flood with gradually varying discharge propagated as a diffusive wave, the celerity downside the dam fasted due to the change of the dominant property of the flood wave. For a flood event controlled by a reservoir, the wave was propagated more quickly as a surge with rapidly varying discharge. The rating curve changed for flooding processes associated with a diffusive wave versus a surge event at a particular cross-section, which changed the flow capacity in the river channel. Observed flood events in 2016 and 2017 were also simulated in the hydraulic model using different management schemes, and the modified flood propagation trends along the lower river reach were analyzed. Lastly, from these simulations, we provide suggestions on how to optimize the operation of the Three Gorges Reservoir to control flood damage and protect the Jiangjiang Reach and Chenglingji areas more effectively.

Effects of sediment flushing operations versus natural floods on Chinook salmon survival

Flushing is a common measure to manage and reduce the amount of sediment stored in reservoirs. However, the sudden release of large volumes of sediment abruptly increases the suspended solids concentration and alters the riverbed composition. Similar effects can be produced also by natural flood events. Do flushing operations have more detrimental impacts than natural floods? To answer this question, we investigated the impact of flushing on the survival of the Chinook salmon (Oncorhynchus tshawytscha) in the Sandy River (OR, USA), assuming that sediment is flushed from hypothetical bottom gates of the, now decommissioned, Marmot Dam. The effects of several flushing scenarios are analyzed with a 2D morphodynamic model, together with habitat suitability curves and stress indicators. The results show that attention has to be paid to duration: the shorter the flushing operation, the lesser the stresses on fish survival and spawning habitats. Flushing causes high stress to salmon eggs and larvae, due to unbearable levels of suspended sediment concentrations. It also decreases the areas usable for spawning due to fine-sediment deposition, with up to 95% loss at peak flow. Without the dam, the corresponding natural flood event would produce similar effects, with up to 93% loss. The study shows that well-planned flushing operations could mimic a natural impact, but only partly. In the long-term, larger losses of spawning grounds can be expected, since the removal of fine sediment with the release of clear water from the reservoir is a lengthy process that may be undesirable due to water storage reduction.

Geomorphic effects of natural flood management woody dams in upland streams

AbstractOne popular Natural Flood Management (NFM) technique involves the construction of channel‐spanning woody dams in low‐order streams that maintain a clearance height above base flows. While extensive research has examined the geomorphic effects of natural wood accumulations, little has been documented of NFM woody dams, which are structurally distinct from natural accumulations and may produce different patterns of erosion and deposition. This consideration is crucial because changes in physical habitat characteristics have implications for flood management objectives as well as ecosystem structure and functioning. This study adopted a Before‐After Control‐Impact (BACI) design to assess the geomorphic effects of NFM woody dams in the upper River Cover catchment, United Kingdom. One baseline survey prior to and three monitoring surveys up to 2 years following dam construction were conducted. Structure‐from‐Motion (SfM) photogrammetry was employed to capture topographic change, supplemented by bathymetric surveys. Results highlight that where the dams remained secure in place, they promoted in‐stream habitat diversity by creating underflow pools. Sediment storage was observed only where the dams had clearance heights <0.3 m from the stream bed. Additionally, the dams commonly led to bank erosion, likely enhanced by inherent bank instability in the study catchment as observed along the control reaches. However, volumes of sediments eroded and deposited were not statistically different between the control and woody dam reaches. Longer monitoring is required to determine whether these effects on channel morphology and habitat diversity will persist, amplify, or diminish over time, and to better understand the longevity of NFM woody dams.

The rise of natural flood management

The rise of natural flood management

Comparison of Urbanization, Climate Change, and Drainage Design Impacts on Urban Flashfloods in an Arid Region: Case Study, New Cairo, Egypt

Urban flooding is considered one of the hazardous disasters in metropolitan areas, especially for those located in arid regions. Due to the associated risks of climate change in increasing the frequency of extreme rainfall events, climate-induced migration to urban areas leads to the intensification of urban settlements in arid regions as well as an increase in urban expansion towards arid land outskirts. This not only stresses the available infrastructure but also produces substantial social instability due to unplanned urban growth. Therefore, this study sheds light on the main factors that are increasing the flood risk, through examining the consequences of rapid urban growth and the performance of drainage networks on urban flood volumes and comparing it with the effects induced by climate change on the surface runoff. The effect of urbanization is assessed through land use maps showing the historical urbanization conditions for the past 30 years, while considering the role of urban planning and its effect on exacerbating surface runoff. Six climate projection scenarios adopted from three Global Climate Models under two Representative Concentration Pathways (4.5 and 8.5) during the period (2006–2020) were compared to ground observed rainfall data to identify which climate scenario we are likely following and then evaluate its effects on the current rainfall trends up to the year 2050. The significance of the drainage design in the mitigation or increase of surface runoff is evaluated through capacity-load balance during regular and extreme storms. It is found that using impervious surfaces coupled with poor planning causing the blockage of natural flood plains led to an increase in the total runoff of about 180%, which is three times more than the effect induced by climate change for the same analysis period. Climate change decreased the intensities of 2- and 5-year rainfall events by 6% while increasing the intensities of extreme events corresponds to 100-year by 17%. Finally, the urban drainage had a distinguished role in increasing surface runoff, as 70% of the network performed poorly during the smallest rainfall event of 2-year return period. The study emphasizes the urgency to re-evaluate the existing and future urban drainage design approach: although urban development and climate change have inevitable effects on the increase in urban flood volumes, it could be alleviated through improved infrastructures.

Strategic Design and Delivery of Integrated Catchment Restoration Monitoring: Emerging Lessons from a 12-Year Study in the UK

Despite growing interest in river and catchment restoration, including a focus on nature-based solutions, assessing effectiveness of restoration programmes continues to prove a challenge. The development of the Eddleston Water project, the Scottish Government’s empirical study of the impact of implementing natural flood management measures on flood risk and habitat restoration, provides the opportunity to review restoration monitoring at a strategic and operational level for this long-running catchment restoration programme. The project has implemented an extensive range of restoration measures along the river and across the 69 km2 catchment. This paper reviews the monitoring strategy and assesses both how the monitoring network developed meets its strategic aims and what subsequent changes were made in monitoring design and implementation. Covering hydrology, hydromorphology and ecology, we explore how all three are integrated to provide a comprehensive assessment of restoration success. Lessons to help inform other river rehabilitation monitoring programmes include the importance of a scoping study and capturing the full range of environmental variables pre-restoration; the limitations of BACI designs; and the need to focus integrated monitoring on a process-based framework and impact cascade, whilst also covering the full trajectory of recovery.

Seismic Performance Evaluation of Highway Bridges under Scour and Chloride Ion Corrosion

Cross-river bridges located in seismically active areas are exposed to two major natural hazards, namely earthquakes and flooding. As the scour depth increases, more parts of the bridge substructure will inevitably be exposed to unfavorable conditions such as chloride ion (Cl−) corrosion. To investigate the seismic performance of highway bridges under the action of scour and Cl− corrosion, a spatial finite element dynamic model of a continuous rigid bridge was established and a Cl−-accelerated electrochemical corrosion test and quasi-static test were carried out. The results showed that a reasonable scour depth and the combination sub-factors under the joint probability density of scour action and seismic action can be obtained to establish the combined expression of the action effect. Cl− corrosion can cause a reduction in displacement ductility, load-bearing, and energy dissipation capacity, and increase inequivalent viscous damping coefficient of the columns. Seismic damage of the columns grows linearly to twice the ultimate displacement under Cl− corrosion, which becomes more significant with the increase of the reinforcement ratio.

Exploring the nexus between urban growth trend and wetland loss in port harcourt municipality

Although Wetlands naturally serve phenomenal roles as natural flood control basins, habitats and breeding grounds for species, ecotourism sites, and biodiversity they have, over time, been depleted to meet man’s increasing demand for land space. This study is a longitudinal survey that looked at the nexus between urban growth and wetland loss in Port Harcourt municipality. Primary and secondary data were used. Primary data were obtained from field observations and ground-trotting. Satellite imageries for four epochs: 1990, 2000, 2010 and 2020 were acquired using Google Earth imageries. Data were analysed using the Geographic Information System (GIS) ArcGis10.4.1 software. The shape file of the study area boundary was generated using ArcGIS 10.4.1 and was used to clip the geo-referenced Google earth images for each year (1990, 2000, 2010, and 2020). The reclassified images were converted to polygons and edited. The area of each classification (land uses) was calculated in square meters using the Spatial Query Module in ArcGIS 10.4.1. Data presentation was done using maps, tables, and charts. Results of analysis revealed that there exists spatio-temporal variation in wetland loss in the study area between the years under review with wetland decreasing form 34633511.38m2 in 1990 to 30945235.9m2 in 2000, and to 23297313.11m2 in 2010 to 18688909.45m2 in year 2020. A very high positive correlation coefficient (Rs) of 0.99 obtained expresses the strong relationship between urban growth and wetland loss in the study area. The study recommends that government and its relevant Ministries, Departments and Agencies (MDAs) should embark on wetland creation, development, and conservation to ensure the continuity in the services, functions, and value of wetland ecosystems in the study area.

ASSESSMENT OF IMPACTS OF THE ROAD SYSTEM ON FLOOD REGIME IN THE COASTAL FLOODING AREA OF VU GIA - THU BAN RIVER BASIN, QUANG NAM PROVINCE

Flooding is one of the dangerous types of natural disasters, which has been causing great damage to human values and even human life, leading to negative impacts on socio-economic development, specifically in the lowland areas of the Vu Gia - Thu Bon river basin. The road system contributes to promoting intra-regional and inter-regional connectivity and is an important factor for socio-economic development. However, in certain cases, the road system continues to be expanded and enhanced, turning them into "dyke systems", interfering with the natural flood regime, increasing the area and flooding time. To assess the impact of the road system on the flood regime, the paper uses the Mike Flood modeling module to simulate the change of flood regime for flood events in 1999, 2007, 2020, and the 5% flood scenario. The comparison between the current road system and the old road system shows a certain increase in both depth and flooded area, especially in the case of 5% flood, the flood depth increases by 0.1 - 0.2 m in the upper part of the basin but only a small change in the lower part of the basin from 0.01 - 0.04 m.

Assessing the efficacy of offline water storage ponds for natural flood management

AbstractLand use changes, landscape modifications and changing climate have resulted in local to regional flood risk increases in recent decades. As an alternative to traditional engineering approaches, there has been a movement towards catchment‐based flood risk management, a subset of which is natural flood management (NFM). NFM aims to enhance flood resilience through the slowing and storing of runoff and flow, based on the restoration and augmentation of hydrological and morphological catchment features. However, despite research highlighting their potential benefits, there is a limited quantity of robust and science‐based empirical evidence on how these structures function in the landscape and their efficacy in reducing flood hazards. To address this knowledge gap and contribute to the growing NFM evidence base, this study evaluates the efficacy of offline water storage ponds for flow attenuation. Two contrasting pond sites in the Tone and Parrett catchment headwaters (SW‐England) were monitored from April 2018 to December 2020 for channel flow, pond volume, and rainfall. Field‐scale, high resolution (1.61 cm spatial support) digital elevation models from Structure‐from‐Motion and manual surveys were collected to quantify pond dynamics and connectivity. A comprehensive, storm definition methodology was developed for event separation, to enable quantification of the impact of pond behaviour on stream flow, across storm hydrographs. Results show that where offline ponds function as designed, with direct pond filling from the channel, peak flow attenuation can reach 7% (maximum event AEP of 83%). Smaller events, where pond filling occurs directly from rainfall or runoff display only a maximum of 3% peak flow reduction (event AEP from 83% –>99%). The ability of ponds to attenuate flow is heavily reliant on sufficient structural conditions for ponds to fill directly from the channel and to drain slowly following event peaks. This study provides an empirical database for future NFM applications, including key criteria for future design and for use as observational data in modelling applications, by upscaling the efficacy of ponds to reduce flood impacts at larger scales.

Physical representation of hillslope leaky barriers in 2D hydraulic models: A case study from the Calder Valley

AbstractThe resources of small‐scale community‐based flood risk action groups are often limited, hence studies to model and predict the effects of Natural Flood Management are often restrained by time and lack of empirical data to validate results. As a result, representations of hillslope leaky barriers are largely modelled as several equifinal approaches, often without survey data. The geometrical characteristics of hillslope leaky barriers were surveyed for the first time at Hardcastle Crags, Calder Valley. This data informed six 2D hydraulic model representation scenarios with varying combinations of topography modification and roughness increase, allowing the sensitivity of their results to be tested. Results from Scenario 3 (topography modification and roughness increase) estimated total hillslope runoff peak flow to reduce by 16.6% in a 1:1‐year design return period; however, this reduction diminished as rainfall intensity increased. Return periods of over 1:30 year estimated peak flow reductions of <5%. Only 14.3%–21.7% (98–148 m3) of the total additional storage provided by the barriers is mobilised during simulated events. A multi‐peaked rainfall event from December 2015 was also simulated. Although the initial peak flow was reduced by 22.7%, as storage became mobilised, effectiveness reduced significantly for subsequent peaks within the same event.