Single Flood Research Articles

Cost-benefit analysis of working with recovery in river management using Marxan

With the impact and cost of continued river degradation and climate change-induced extreme rainfall and floods, there is growing urgency to achieve and maintain good river health to meet global sustainable development goals. Up-scaling and working with nature-based solutions at regional, catchment and fluvial corridor scales is critical. One way to achieve this is to work within a recovery-enhanced approach to river rehabilitation, incorporating processes of geomorphic and vegetative recovery to build fluvial corridors in areas where they have become fragmented. Geomorphologically-informed cost-benefit analysis has not previously been applied in a riverine or riparian context, at the catchment or regional scale needed. We applied Marxan in a novel manner to undertake the first cost-benefit analysis for geomorphologically-informed rehabilitation of river systems. We estimated that to rehabilitate 75,500 km of streams in coastal catchments of NSW will cost $8.2 billion, comparing favourably to recent single flood event insurance losses, and projected future losses. We developed Marxan scenarios based on three broad approaches to river management: ad hoc and reactive, working with recovery and corridors, across current and future time periods. We found there are considerable current and flow-on future financial and non-financial benefits, and lower initial total and per-hectare rehabilitation costs, by fully adopting working with recovery or corridors approaches and moving away from the ad hoc and reactive approaches which dominate current practice. Implementing targeted rehabilitation based on a rolling sequence over time of corridors scenarios provides optimal holistic solutions to improve geomorphic condition and enhance recovery potential at landscape-scale. Our study demonstrates the use of Marxan as an accessible tool to address prioritisation complexity, and to run and cost landscape-scale rehabilitation scenarios over time. Our study also demonstrates the positive offsite feedbacks that occur through multiplier effects, as recovery occurs, and corridors are built. Geomorphologically-informed decision making becomes more robust, transparent, cost-effective, consistent across catchments, and adaptive to local situations and evolving river management priorities.

Climate change and urban sprawl: Unveiling the escalating flood risks in river deltas with a deep dive into the GBM river delta

River deltas, such as the Ganges-Brahmaputra-Meghna (GBM) delta, are highly vulnerable to flooding, exacerbated by intense human activities and rapid urban growth. This study explores the evolution of urban flood risks in the GBM delta under the combined impacts of climate change and urban expansion. Unlike traditional assessments that focus on a single flood source, we consider multiple sources—coastal, fluvial, and pluvial. Our findings indicate that future urban expansion will significantly increase flood exposure, with a substantial rise in flood risk from all sources by the end of this century. Climate change is the main driver of increased coastal flood risks, while urban growth primarily amplifies fluvial, and pluvial flood risks. This highlights the urgent need for adaptive urban planning strategies to mitigate future flooding and support sustainable urban development. The extreme high emissions future scenario (SSP5–8.5) shows the largest urban growth and consequent flood risk, emphasizing the necessity for preemptive measures to mitigate future urban flooding. Our study provides crucial insights into flood risk dynamics in delta environments, aiding policymakers and planners in developing resilience strategies against escalating flood threats.

Compound and consecutive drought-flood events at a global scale

Flooding during or after droughts poses significant challenges to disaster risk management. However, interactions between droughts and floods are often overlooked as studies typically analyse these events in isolation. Here we explore historical occurrences of compound and consecutive drought-flood events and drought effects on flood severity and timing by analysing global datasets of hydrometeorological and biophysical variables for 8255 catchments worldwide. These data show that 24% of floods globally (anomalies above the 85th percentile) are preceded by, or happen during, drought conditions. Flood events occurring during drought conditions are typically of lower magnitude, especially in arid regions, while floods following drought events have a severity distribution comparable to single flood events. For most drought-flood events, flood timing appears relatively unaffected by drought conditions, but almost a quarter of the drought-flood events had flood timings occurring two to three months later than expected. These shifts in flood timing suggest droughts potentially affect flood-generating processes. As both drought and flood occurrences are projected to increase in a warming climate, interactions between them may become more common and need to be accounted for in flood risk assessment and management.

The freshwater discharge into the Adriatic Sea revisited

The present study reconstructs the river discharge climatology and its respective historical series for all rivers of the Adriatic Sea with averaged climatological daily river discharge above 1 m3s−1, to reach a better representation of the Adriatic rivers in hydrodynamic models and, consequently, to develop a more realistic freshwater balance in the different regions of the hydrographic basin. Based on the European Flood Awareness System (EFAS) data set, a careful method of identification and selection of the Adriatic rivers, followed by a rigorous assessment against observational data, was developed to evaluate the current state of the Adriatic river discharges and their respective trends throughout several climate indicators from 1991 to 2022. Observational data are limited to 85% of the identified rivers, totaling 98% of the overall freshwater input into the Adriatic Sea. The results confirm that the Shallow Northern Adriatic receives the largest freshwater inputs with a daily average exceeding 2,400 m3s−1, which amounts to 61% of the overall Adriatic discharges. Consequently, this region guides the freshwater seasonal cycle of the Adriatic Sea, which presents a well-defined pattern of two flood peaks in late autumn and late spring, separated by a minimum discharge period at mid-summer. From the Central to the Southern Adriatic subregions, the absence of snow-melting effects prevents the secondary flood peak during the spring, shaping the seasonal cycle of river discharges from a single flood peak in late autumn to a drought period in August. The 32 years of continuous river discharge data reveal a negligible trend in the overall Adriatic Sea but a negative trend for the last decade (2013–2022). This decadal decrease is driven by the extreme drought that drastically pounded the northern Adriatic in 2022.

Does a braided river aggrade or degrade in response to changes in water and sediment fluxes?

Braided rivers undergo continuous morphological evolution in response to variations in water and sediment fluxes associated with a plethora of natural and anthropogenic disturbances, which has to date remained poorly understood. Here, a one-dimensional hydro-sediment-morphodynamic model is proposed for braided rivers. It is built upon an extended synthetic channel geometric model which encodes the essential, three-dimensional morphological information into a one-dimensional synthetic channel. It is first applied to the Rees River (New Zealand) to investigate overall morphological evolution (aggradation/degradation) during a single flood event. It is shown that the channel tends to degrade with increasing peak discharge and decreasing sediment supply, while the effect of baseflow discharge on aggradation/degradation is negligible unless the baseflow discharge exceeds a critical value. Then the proposed model is applied to resolve long-term morphological evolution in the Tuotuo River (China) reach (up to the year 2050), the source region of the Yangtze River, in response to projected water and sediment fluxes. Sustained degradation in the river reach is observed. The present study yields a viable physics-based method to predict overall aggradation or degradation of braided rivers under unsteady water and sediment fluxes over both short and long time scales.

Impact of large wood on sediment (dis)connectivity in a meandering river

Large wood (LW) can exert strong influence on sediment storage patterns in fluvial systems. However its specific impact on sediment routing is largely dependent on the stability of individual LW pieces or LW jams. We used sediment (dis)connectivity principles to quantify the impact of LW on sediment erosion and storage along a meandering river reach. Decadal spatiotemporal changes of LW and fluvial landforms are appraised along a 3.65 km long reach of the Odra River (Czechia). Detailed orthophotos at two-year intervals from 2012 to 2022 are analysed alongside field inventories. During this study period, there was a single flood event with a 5–10-year recurrence interval, and three smaller flow events with a recurrence interval of up to 2 years. We assessed the sediment deposition, bank protection, and erosion effects of LW, finding that the impact of LW on decelerating sediment fluxes was more pronounced. Specifically, LW-related deposition varied between 13.6 m2 and 36.7 m2 per 100 m of channel length during the individual two-year periods. In contrast, we detected only 0.5 m2 to 7.8 m2 per 100 m of channel length of newly eroded material during the same periods. Additionally, 6.3 % to 7.8 % of the lengths of the outer meander bends were protected by persistent LW elements against erosion during these two-year periods. Finally, we developed a three-stage conceptual model depicting the impact of LW on sediment (dis)connectivity in a meandering river at different relative timescales. Our model accounts for the dependence of interactions between LW and fluvial sediments based on the LW stability and frequency-magnitude of the hydrogeomorphic processes. In general, our observations suggest that the presence of stable LW in meandering rivers primarily leads to the deceleration of the lateral and longitudinal dimensions of sediment (dis)connectivity.

Theory and practice of basin-wide floodwater utilization: typical implementing measures in China

Floodwater utilization (FU) helps alleviate water shortages in regions with uneven rainfall. This study focuses on basin-wide FU, considering diverse water infrastructure and the delicate balance between flood risks and water scarcity. FU measures including optimizing reservoir flood limited water level (FLWL), extending the use of flood detention areas, and compensatory use of channel capacities, were reviewed and exemplified by real-world applications in China. The proposed procedure for implementing FU involves assessing FU potential, selecting appropriate utilization methods, and conducting a comprehensive evaluation of the associated risks and benefits. Using the Nansi Lake basin in China, as a synthesis example, the current-level FU potential and FU rate are estimated to be 1.47 km3/yr and 65%. This FU value exceeds the theoretical potential, indicating the severe crowding out of ecological water uses. Multi-stage FLWL through raising the post-flood FLWL from 34.20 m to 34.35 m could increase the reservoir (the upper Nansi Lake) ability to refill by 19.1%. Appropriate control of the sluices of interconnected rivers could conserve up to 0.135 km3 of floodwater from a single flood event without increasing flood risk.

Exploring an intelligent adaptation method of hydrological model parameters for flood simulations based on the light gradient-boosting machine

Traditional hydrological modeling methods use a set of parameters to simulate flood processes with complex causes and variable intensity, which can easily lead to parameter instability. To address the problem of parameter instability, this study proposes an approach integrating the hydrological model with Intelligent Adaptation Parameters (IAP), whose intelligent adaptation relationship is established by the light gradient-boosting machine (LightGBM) based on individual calibration parameters by each flood event and flood characteristics including flood-caused rainstorm information and initial soil moisture. A widely used hydrological model, Xin 'anjiang (XAJ) model, is chosen to be integrated with IAP (XAJ-IAP) in this study, which has a relatively complex structure and a total of 15 model parameters. The obtained findings demonstrate that: (1) recalibrating the sensitive runoff concentration and separation parameters with a single flood leads to a notable enhancement in simulation accuracy, while simultaneously considering the model's physical significance; (2) the XAJ overestimates large floods and underestimates small floods. Compared with the XAJ, the XAJ-IAP has a better rain-flood response relationship and simulation accuracy for floods of different magnitudes, solving the problem of parameter instability that exists in XAJ; and (3) evaluated in terms of information gain, sensitive parameters contribute the most to the establishment of the intelligent adaptation relationship in the LightGBM compared to flood-caused rainstorm information and initial soil moisture, indicating that sensitive parameters are the most important input features of the LightGBM. It can be concluded that the intelligent adaptation system can not only solve the problem of parameter instability that exists when traditional hydrological models simulate complex and changeable floods, but also further reveal the relationship between the model and floods.

Hazard assessment of a pair of glacial lakes in Nepal Himalaya: unfolding combined outbursts of Upper and Lower Barun

Due to climate change, the future Glacial Lake Outburst Flood (GLOF) risk is more likely in the Himalayas. Despite several GLOF studies, little attention has been given to the combined surge that can result from a pair of upstream-downstream glacial lakes. We chose Upper and Lower Barun glacial lakes as a pilot study due to the region’s increasing population and hydropower investments and the possible outbursts of upper-lower glacial lakes. This study mainly considered four scenarios (100 and 50% breach combination of upper-lower lakes), using a two-dimensional dam break model and inundation propagation. Based on our observations, the glaciers in the study area shrank by 33 km2, and the two glacial lakes expanded by 2.06 km2 between 1976 and 2020. The modeling result suggests that the single flood triggered by Upper Barun cannot travel beyond 50 km; however, the combined flow from the lakes could reach over 85 km. Our results illustrate that at least 60 buildings, motorable bridges, and infrastructures of Arun hydropower along the river course have high damage potential. The study insights can be helpful for effectively planning and formulating various disaster risk reduction initiatives to mitigate the likely effects of glacial lake(s) outbursts.

Determination of the maximum run‐off sediment transport capacity during a rainstorm flood in a small watershed on the Loess Plateau

AbstractThe channel sediment transport capacity is an important parameter in soil erosion models, and it determines the proportion of soil erosion that is sent outside the watershed. However, the calculation and judgment methods for the sediment transport capacity of channel are still incomplete, and further research is needed. Therefore, this paper takes Chabagou, a typical watershed in the hilly and gully region of the Loess Plateau, as an example and, based on the measured hydrological and sediment data collected during rainstorm floods in 1961–1969 at three hydrological stations, Caoping, Shejiagou, and Tuanshangou, studies the sediment transport capacity of channels in a small watershed from the perspective of energy dissipation. Clearly, the sediment transport capacity is better determined by using a single flood and sediment concentration peak process. The discriminant conditions are determined for the sediment concentration to reach sediment transport capacity during the flood rising and falling sections of the sediment process of the small watershed storm flood. When determining sediment transport capacity based on the characteristics of the flood and sediment concentration peak, we find that sediment transport capacity is reached in the flood discharge peak when the flood discharge peak occurs earlier than the sediment concentration peak. When determining sediment transport capacity based on the flow discharge and sediment concentration characteristics of the sediment fall section, we find that when the flood event exhibits a counterclockwise hysteresis curve and a time‐lagged effect occurs, the sediment concentration at that time reaches sediment transport capacity. In this study, the sediment concentration as a function of run‐off amount is logarithmic at different subbasin scales when run‐off reaches its maximum capacity to transport sediment. The R2 is high, ranging from 0.78 to 0.92. The results of this research provide a reference for the study of sediment transport capacity.

The heterogeneous geomorphological impact of an exceptional flood event and the role of floodplain vegetation

AbstractApplication of an integrated spatial approach combining several sources of remote sensing data to include both channel and floodplain morphological and sedimentological impacts has identified the geomorphological effects of three extreme flood events during a single flood season (2019–2020) along a 16‐km reach of the River Teme, UK. This combined approach allowed the assessment of in‐channel pattern development, incision and aggradation; lateral bank migration; and overbank sedimentation and scour by out‐of‐channel flows. Rates of change during the event period were compared with those in the previous 10 years. The approach also allowed the role of vegetation and cultivation, both bankside and out on floodplains, to be assessed with variations in the extent of riparian wood and channel slope driving contrasts in the extent of the response. The spatial impacts from such extreme events are highly localised, varied in kind, and can be considered for both rivers and floodplains together. Erosional effects were distinctively distributed and not simply contributions to ongoing meander development; channel aggradation was localised, and overbank sedimentation explicably patchy. In reaches without woody vegetation, differences in channel and floodplain slope, local floodplain relief as created by prior events, and the impact of man‐made structures were factors that drove variations in flood response. This study strongly underlines the role of continuous riparian vegetation in maintaining bank stability and constraining lateral channel migration, but also to the potential influence of floodplain vegetation and planting for ‘natural engineering’ in the context of floodplains as well as channels in comparable environments. Maintenance of riparian vegetation in the context of landowner and Natural England conflict and management of future flood risk is important, but also highlighted is the need to consider the role of hedgerows and wider planting for constraining soil and riverbank erosion during flood events.

MAPPING NEWLY INUNDATED AREAS IN POST-FLOOD LANDSAT IMAGES USING THRESHOLDING TECHNIQUES

Abstract. Identifying newly inundated areas following flood events is essential for planning rescue missions. These maps must be generated quickly as the spatial extent of the inundated areas might change during a single flood event. Several methods exist for generating such maps and several rely on one or more geospatial data to exclude existing waterbodies in an affected area. In this study, we tested a rapid flood mapping method that uses a pair of pre- and post-flood satellite images on seven sites throughout the US. We derived Normalized Difference Water Index (NDWI) and Modified NDWI (MNDWI) images from pre- and post-flood Landsat images and identified the optimal threshold values that highlighted newly inundated areas at these sites. The accuracy of the inundation maps was determined using manually interpreted verification data from the pairs of satellite images. Image analysts have identified the optimal threshold values between 25 and 40 minutes. Maps of newly inundated areas derived from differencing MNDWI and NDWI images had higher overall accuracy > 93%. Results obtained in this study confirms the utility of this rapid flood mapping technique to identify inundated areas using pre- and post-flood satellite images.

River widening in mountain and foothill areas during floods: Insights from a meta-analysis of 51 European Rivers

River widening, defined as a lateral expansion of the channel, is a critical process that maintains fluvial ecosystems and is part of the regular functioning of rivers. However, in areas with high population density, channel widening can cause damage during floods. Therefore, for effective flood risk management it is essential to identify river reaches where abrupt channel widening may occur. Despite numerous efforts to predict channel widening, most studies have been limited to single rivers and single flood events, which may not be representative of other conditions. Moreover, a multi-catchment scale approach that covers various settings and flood magnitudes has been lacking. In this study, we fill this gap by compiling a large database comprising 1564 river reaches in several mountain regions in Europe affected by floods of varying magnitudes in the last six decades. By applying a meta-analysis, we aimed to identify the types of floods responsible for more extensive widening, the river reach types where intense widening is more likely to occur, and the hydraulic and morphological variables that explain widening and can aid in predicting widening. Our analysis revealed seven groups of reaches with significantly different responses to floods regarding width ratios (i.e., the ratio between channel width after and before a flood). Among these groups, the river reaches located in the Mediterranean region and affected by extreme floods triggered by short and intense precipitation events showed significantly larger widening than other river reaches in other regions. Additionally, the meta-analysis confirmed valley confinement as a critical morphological variable that controls channel widening but showed that it is not the only controlling factor. We proposed new statistical models to identify river reaches prone to widening, estimate potential channel width after a flood, and compute upper bound width ratios. These findings can inform flood hazard evaluations and the design of mitigation measures.

Characterizing the 2020 summer floods in South China and effects on croplands

Floods occur more frequently in the context of climate change; however, flood monitoring capacity has not been well established. Here, we used a synergic mapping framework to characterize summer floods in the middle and lower reaches of the Yangtze River Plain and the effects on croplands in 2020, from both floodextent and intensity perspectives. We found that the total flood extent was 4936km2 from July to August, and for flood intensity, 1658, 1382, and 1896km2 of areas experienced triple, double, and single floods. A total of 2282km2 croplands (46% of the flooded area) were inundated mainly from Poyang and Dongting Lake Basins, containing a high ratio of moderate damage croplands (47%). The newly increased flooding extent in 2020 was 29% larger than the maximum ever-flooded extent in 2015-2019. This study is expected to provide a reference for rapid regional flood disaster assessment and serving mitigation.

Probabilistic compound flood hazard analysis for coastal risk assessment: A case study in Charleston, South Carolina

Coastal communities are susceptible to flooding due to flood drivers such as high tides, surge, waves, rainfall, and river discharges. Recent hurricanes such as Harvey, Florence, and Ian brought devastating impacts from combinations of high rainfall and storm surge, highlighting the need for resilience and adaptation planning to consider compound flood events when evaluating options to reduce future flood risk. Flood risk assessments often focus on a single flood driver (e.g. storm surge) due to the complexity of accounting for compound flood drivers. However, neglecting these compound flood effects can grossly underestimate the total flood risk. A probabilistic compound flood hazard analysis considers all compound events that lead to flooding, estimates their joint probabilities, simulates the flood response, and applies a probabilistic computation technique to translate flood responses and probabilities into probabilistic flood maps (such as the 100-year flood map). Probabilistic flood maps based on compound events can be used to assess risk more accurately for current and future conditions, with and without additional adaptation measures. In this paper we present an example of a probabilistic compound flood hazard analysis for the city of Charleston, South Carolina, considering tide, surge, and rainfall, for both hurricane and non-hurricane events. Charleston is regularly confronted with compound flood events, which are expected to worsen with sea level rise and more frequent tropical storms. Starting with an initial set of over 1,000 synthetic compound events, selection techniques described in the paper led to a final set of 207 compound events. The fast compound flood model SFINCS simulated the flood response for each event and, using numerical integration, compound flood return-period maps were created for Charleston, under current and future sea level rise conditions.

Assessing 40 Years of Flood Risk Evolution at the Micro-Scale Using an Innovative Modeling Approach: The Effects of Urbanization and Land Planning

The present work is aimed at assessing the change in time of flood risk as a consequence of landscape modifications. The town of San Donà di Piave (Italy) is taken as a representative case study because, as most parts of the North Italy floodplains, it was strongly urbanized and anthropized in the last several decades. As a proxy for flood risk, we use flood damage to residential buildings. The analysis is carried out at the local scale, accounting for changes to single buildings; GIS data such as high-resolution topography, technical maps, and aerial images taken over time are used to track how the landscape evolves over time, both in terms of urbanized areas and of hydraulically relevant structures (e.g., embankments). Flood hazard is determined using a physics-based, finite element hydrodynamic code that models in a coupled way the flood routing within the Piave River, the formation of levee failures, and the flooding of adjacent areas. The expected flood damage to residential buildings is estimated using an innovative method, recently proposed in the literature, which allows estimating how the damage evolves during a single flood event. The decade-scale change in the expected flood damage reveals the detrimental effect of urbanization, with flood risk growing at the pace of a fraction of urbanized areas. The within-event time evolution of the flood damage, i.e., how it progresses in the course of past or recent flood events, reflects changes in the hydrodynamic process of flooding. The general methodology used in the present work can be viewed as a promising technique to analyze the effects on the flood risk of past landscape evolution and, more importantly, a valuable tool toward an improved, well-informed, and sustainable land planning.

Flood Policy and Governance: A Pathway for Policy Coherence in Nigeria

In recent years, Nigeria is witnessing increasing frequency of flood occurrence with devastating impact translating into significant loss of lives (in Nigeria, over 300 people died in September 2022) and properties. Addressing flood disaster requires holistic approach from policy and governance perspectives, integration of policies and programs and synergies between institutions. Using synergies and eliminating trade-offs, flood governance and policy coherence integrate all relevant policy fields and institutions to achieve common policy outcomes. The objective of this study is to examine and understand how flood governance and policy coherence are approached, as well as institutional design and implementation for coherence in Nigeria. The findings revealed that there is no single flood policy in Nigeria. Due to this, there is no focus and no defined objectives for flood governance, prevention, control, and management, and no imperative for the government to seek both short-term and long-term flood solutions. There is no synergy and coordination among institutions for flood governance in the country. Since the country established the federal ministry of environment in 1999, the environment, floods, and climate-related hazards were given less priority. State and local governments handle most flood disasters and emergencies. Federal assistance is provided, however, when flood disasters exceed the capabilities of local and state governments. This study recommends that across the country, flood policy needs to be designed, formulated, and implemented while assigning governance responsibility and decentralizing policy to state and local governments.

Transcriptome analysis reveals the molecular mechanisms of Phragmites australis tolerance to CuO-nanoparticles and/or flood stress induced by arbuscular mycorrhizal fungi.

The molecular mechanism of arbuscular mycorrhizal fungi (AMF) in vertical flow constructed wetlands (VFCWs) for the purification of copper oxide nanoparticles (CuO-NPs) contaminated wastewater remains unclear. In this study, transcriptome analysis was used to explore the effect of AMF inoculation on the gene expression profile of Phragmites australis roots under different concentrations of CuO-NPs and/or flood stress. 551, 429 and 2281 differentially expressed genes (DEGs) were specially regulated by AMF under combined stresses of CuO-NPs and flood, single CuO-NPs stress and single flood stress, respectively. Based on the results of DEG function annotation and enrichment analyses, AMF inoculation under CuO-NPs and/or flood stress up-regulated the expression of a number of genes involved in antioxidant defense systems, cell wall biosynthesis and transporter protein, which may contribute to plant tolerance. The expression of 30 transcription factors (TFs) was up-regulated by AMF inoculation under combined stresses of CuO-NPs and flood, and 44 and 44 TFs were up-regulated under single CuO-NPs or flood condition, respectively, which may contribute to the alleviating effect of symbiosis on CuO-NPs and/or flood stress. These results provided a theoretical basis for enhancing the ecological restoration function of wetland plants for metallic nanoparticles (MNPs) by mycorrhizal technology in the future.

Multi-Hazard property buyouts: Making a case for the acquisition of flood and contaminant-prone residential properties in Galena Park, TX

The aftermath of extreme flood events can be particularly devastating for residential communities in proximity to flood-prone petrochemical facilities, as they are likely to experience multiple impacts from a single flood event. Hazard impacts could be from flood inundation to their properties, and floodwaters carrying contaminants from nearby facilities into their homes. While property acquisition or buyouts have been effectively used as a flood mitigation strategy, current buyout selection processes only factor in flood exposure, thereby ignoring other hazards such as exposure of properties to potential chemical substance transfer. In this paper, we identify properties that are eligible for flood buyouts but are also at a high risk of contaminant transferal during extreme flood events. We apply a benefit–cost analysis methodology to assess the economic viability of buyouts and proximity metrics to prioritize buyouts of contaminant-prone properties in Galena Park, Texas. Results indicate that, in selected flood-only property acquisition scenarios, cumulative avoided flood losses exceed the cost of property acquisition. However, although with lower cost-benefit values, a selection criterion that factors a combination of flood and contaminant-prone properties as buyouts results in multiple properties being removed from harm’s way. Our findings emphasize the potential economic benefits of applying a multi-hazard selection criterion in dealing with flood property buyouts, especially in socially vulnerable communities with high exposure to both flooding and contaminant transfer.

Resilience of ecosystem service delivery in grasslands in response to single and compound extreme weather events

Extreme weather events are increasing in frequency and magnitude with profound effects on ecosystem functioning. Further, there is now a greater likelihood that multiple extreme events are occurring within a single year. Here we investigated the effect of a single drought, flood or compound (flood + drought) extreme event on temperate grassland ecosystem processes in a field experiment. To assess system resistance and resilience, we studied changes in a wide range of above- and below-ground indicators (plant diversity and productivity, greenhouse gas emissions, soil chemical, physical and biological metrics) during the 8 week stress events and then for 2 years post-stress. We hypothesized that agricultural grasslands would have different degrees of resistance and resilience to flood and drought stress. We also investigated two alternative hypotheses that the combined flood + drought treatment would either, (A) promote ecosystem resilience through more rapid recovery of soil moisture conditions or (B) exacerbate the impact of the single flood or drought event. Our results showed that flooding had a much greater effect than drought on ecosystem processes and that the grassland was more resistant and resilient to drought than to flood. The immediate impact of flooding on all indicators was negative, especially for those related to production, and climate and water regulation. Flooding stress caused pronounced and persistent shifts in soil microbial and plant communities with large implications for nutrient cycling and long-term ecosystem function. The compound flood + drought treatment failed to show a more severe impact than the single extreme events. Rather, there was an indication of quicker recovery of soil and microbial parameters suggesting greater resilience in line with hypothesis (A). This study clearly reveals that contrasting extreme weather events differentially affect grassland ecosystem function but that concurrent events of a contrasting nature may promote ecosystem resilience to future stress.