Multi-hazard early-warning systems (MHEWS) are critical for mitigating extreme weather impacts and enhancing disaster resilience. However, quantitative empirical evidence on how different types of early warnings individually and collectively trigger preventive actions and influence resilience remains limited. Here, using location-based human mobility data aggregated from over 1.1 billion mobile devices across Chinese cities, we quantified daily intracity human mobility responses to 21,126 early warning signals during 19 tropical cyclones (TCs) from 2021 to 2023. To represent disaster resilience under MHEWS protection, we developed a protected resilience index that integrates both the magnitude of mobility changes and recovery durations. We found that, compared with city-level TC warnings alone, combined multi-level, multi-hazard warnings resulted in a 52.4 % reduction in mobility during TC exposure days, thereby increasing avoided direct population exposure by around 57.1 %. Each additional warning type further shortened recovery times, collectively reducing recovery durations by at least 55.6 %, with larger effects observed for stronger TCs. Under MHEWS protection, protected resilience remained statistically similar between moderate-intensity TCs (34 kt and 50 kt) but declined significantly under severe (>= 64 kt) conditions. Although absolute reductions in exposure were greater in high-frequency, coastal, and wealthier cities, relative improvements from MHEWS were more pronounced in less frequently affected, inland, and socioeconomically disadvantaged areas. Consequently, MHEWS significantly narrowed resilience disparities among cities facing equivalent hazard exposures. This study introduces a scalable, behaviour-based framework for quantifying early-warning effectiveness, highlighting the essential role of integrated multi-level and multi-hazard warnings in disaster preparedness across cities amid escalating climate risks.

Over a billion people globally are already exposed to the risk of flooding, but by 2050 this number is expected to double due to human-induced climate change, population growth, and encroachment into at-risk areas. Global Flood Models (GFMs) are vital tools for producing flood hazard maps supporting impact estimates and policy interventions. These GFMs represent river channels by typically assuming that the bankfull flow-carrying capacity equates to a river flow with a specified return period (RP) that is spatially and temporally invariant. However, bankfull capacity is determined by channel size, shape and roughness and so varies in response to erosion and sedimentation. To quantify the extent to which channel variability biases GFM predictions here we employ a typical GFM, the Fathom model, to a 135,000 km2 region of the Mississippi floodplain in a sensitivity analysis that evaluates how inundated areas and associated population exposures respond when forced with empirically-derived bankfull capacities. Our results show that since the typical RPs (< 1 year) of these present-day bankfull flows differ from the 2-year value normally assumed in GFMs, substantial underestimates of flood extent (9 to 59%, depending on flood magnitude) and populations exposed (15 to 118%) result. We also show that, over multi-decadal timescales, changes in past channel morphology are, depending on emissions scenario, of equal or greater importance in driving changes in simulated flood hazard and risk than changes in future climate. The evolution of bankfull capacity through space and time is therefore a first order control on flood hazard and risk, meaning it is vital that river channel variability and change is represented accurately in GFMs.

Despite the importance of floodplain vegetation (including dead wood) in fluvial geomorphology and its influence on the aquatic-terrestrial transition zone, current research is dominated by an ecological perspective which misses the bi-directional feedbacks between ecological and geomorphological processes in the river corridor. Prior studies have focused attention on the important roles of climate (light availability and temperature) and fluvial disturbance in controlling river corridor vegetation dynamics, but most of these previous studies have focused not only on trees, but also specific attributes such as canopy height and diameter at breast height. In contrast, much less attention has been paid to the role of understorey vegetation within fluvial systems, despite its potential role in modulating overbank flow (roughness), stabilising banks, and sequestering carbon. Here we define ‘understorey’ vegetation to mean all biomass up to a metre above the ground, irrespective of it being under a canopy or not and we also include large wood and leaf litter. Within this context, this research aims to quantify how hydraulic roughness and understorey vegetation co-vary seasonally along river corridors representing different disturbance regimes and river types, by quantifying structural aspects of understorey vegetation and its interactions with flow. Here we present work that is focused on Highland Water, a small flashy stream located in the New Forest, UK, which has riparian vegetation comprising predominantly a heavily-grazed deciduous canopy. The stream and its floodplain are also affected by the presence of developed log jams promoting overbank flow with multiple side channels. The study site is being surveyed monthly as well as during high flows to monitor flood extent. The structural complexity of riparian and floodplain understorey vegetation (

Over 70% of flood events recorded in the past two decades in the Global Flood Database and WorldFloods dataset have occurred in locations where complex channel systems occur. Here we define complex channel systems as parts of the river network that diverge, such as bifurcations, multi-threaded channels, canals and deltas. Yet, large scale flood models have, until now, used only single-threaded networks due to the lack of a river network that reflects complex channel systems . Therefore, these large-scale models fundamentally misrepresent the physical processes in these often highly populated areas, leading to sub-optimal estimates of flood risk.Using the new Global River Topology (GRIT) dataset, a global bifurcation and multi-directional river network (Wortmann et al. 2023), we extend the river channel bathymetry estimation routine of Neal et al. (2021) to model multi-channels with LISFLOOD-FP. We compare the multi-thread model results to observations and to previous versions of LISFLOOD-FP using a single-threaded river network in the Indus, Mekong and Niger rivers at 1 arc second (~30m). By using GRIT, we find marked improvements in model results, observing better connectivity to areas of the floodplain that are far from the main channel and more channel floodplain interactions in wetlands. This work paves the way to further our understanding of global flood risk and to finally consider the diverse, evolving nature of geomorphologically active river networks. As this work progresses, we will continue to model a typology of bifurcations and multi-directional rivers to help further our understanding of the significance of complex river systems.Neal, J., Hawker, L., Savage, J., Durand, M., Bates, P., & Sampson, C. (2021). Estimating river channel bathymetry in large scale flood inundation models. Water Resources Research, 57(5), e2020WR028301.Wortmann, M., Slater, L., Hawker, L., Liu, Y., & Neal, J. (2023). Global River Topology (GRIT) (0.4) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.7629908

Existing global river networks underpin a wide range of hydrological applications but do not represent channels with divergent river flows (bifurcations, multi‐threaded channels, canals), as these features defy the convergent flow assumption that elevation‐derived networks (e.g., HydroSHEDS, MERIT Hydro) are based on. Yet, bifurcations are important features of the global river drainage system, especially on large floodplains and river deltas, and are also often found in densely populated regions. Here we developed the first raster and vector‐based Global RIver Topology that not only represents the tributaries of the global drainage network but also the distributaries, including multi‐threaded rivers, canals and deltas. We achieve this by merging a 30 m Landsat‐based river mask with elevation‐generated streams to ensure a homogeneous drainage density outside of the river mask for rivers narrower than approximately 30 m. Crucially, we employ the new 30 m digital terrain model, FABDEM, based on TanDEM‐X, which shows greater accuracy over the traditionally used SRTM derivatives. After vectorization and pruning, directionality is assigned by a series of elevation, flow angle and continuity approaches. The new global network and its attributes are validated using gauging stations, comparison with existing networks, and randomized manual checks. The new network represents 19.6 million km of streams and rivers with drainage areas greater than 50 km 2 and includes 67,495 bifurcations. With the advent of hyper‐resolution modeling and artificial intelligence, GRIT is expected to greatly improve the accuracy of many river‐based applications such as flood forecasting, water availability and quality simulations, or riverine habitat mapping.

Knowledge of the Quaternary history of the lower Mekong, the major river within Cambodia, is basic. Herein we advance understanding by investigations of river terrace topographic expression and stratigraphy. Satellite images, digital elevation models and fieldwork have been used to define the terrace elevations and extent. Three terrace levels can be recognized, separated in the vertical, lateral and temporal dimensions by distinctive sedimentary signatures. Strath surfaces and alluvial cover have been dated using terrestrial cosmogenic and optical luminescence protocols. The highest level (T1: notionally +100 m above present sea level) is a discontinuous, degraded, bedrock strath with a patchy veneer of well-weathered fluvial cobble gravel. T1 is younger than a regionally significant meteorite impact similar to 800 ka (Marine Isotope Stage(2) 20), and older than basalt flows on its surface (600 ka?). The T1 level was abandoned before 99.42 +/- 7.52 ka (the end of the glacial MIS 5d), as the river incised in response to a rapidly falling sea level, to form a broad continuous strath terrace (level T2) exhibiting a thin alluvial cover, between 70 m and 40 m above sea level. The T2 terrace is composed of partially lateritic, interlayered, sand and gravel beds lying above weathered bedrock (blue/red clay). The basal deposits on the T2 level date to 70.65 +/- 5.13 ka, following a sea level rise to a short-lived elevation of around +30 m around 80 ka (MIS 5a). The T2 level was progressively down cut between 57.73 +/- 5.31 ka and 38.66 +/- 2.40 ka (MIS 3). Steadily falling sea level sustained MIS 3 incision which reached c., 10 m above the modern river level c., 33.03 +/- 3.09 ka, before the offshore minimum in sea level, c., 23 ka, i.e., towards the end of the Last Glacial Maximum. A loam-rich sandy terrace (T3; c., 0.45 ka (MIS 1)) is developed locally at c., +20 m above sea level. The timing of abrupt incisions, leading to the abandonment of the T1 and T2 levels, coincide with the onset of cool glacial stadials and falls in global sea level, whilst initial aggradation on the T2 level broadly can be associated with MIS 4. Despite a reduction in the contribution of glacial runoff from the Himalaya and Tibet towards the end of the Pleistocene, channel narrowing from T1 onwards has sustained the erosive power of the river, such that the rate of incision has only slowed within the Holocene.

Climate variability is a significant driver of flood events. However, geomorphological changes in river channels, including variations in local and upstream sediment supply, play a crucial role in determining flood conveyance capacity and flood stage variations. The interplay between hydrology and geomorphology, and their relative impact on flood conveyance, can vary in different river systems depending on both the degree of internal channel dynamics and the nature and magnitude of external forcings. For example, rates of bank erosion, vegetation establishment on bar surfaces, and overbank sedimentation control the time required for floodplain reworking, the adjustment of channel morphology and the associated evolution of river flow conveyance capacity and stage-discharge relations.To investigate the relative significance of hydrological and geomorphological controls on flood-stage variability, we employ a new computationally-efficient model of river and floodplain morphodynamics. This model simulates the evolution of river morphology and flow conveyance capacity by representing the interaction between processes of bank erosion, floodplain construction and river bed-level change over multiple centuries. The simple nature of the model enables its application at large spatial scales – e.g., to explore global variations in the controls on flood conveyance and its sensitivity to future environmental change. Simulated changes in conveyance capacity for a range of environmental settings were evaluated against trends in observed river gauging datasets. Convergent cross-mapping analysis was then applied to investigate the cause-and-effect relationships between controlling factors, including: (i) hydrologic regime; (ii) river sediment load; (iii) floodplain composition (e.g., fine versus coarse sediment); and (iv) lateral river dynamics (e.g., rates of erosion and accretion). Our analysis quantifies the causality between these factors and the resulting variability in river morphology (width and bed level), flood stage and channel conveyance capacity. Results indicate that in dynamic river systems, while the importance of climate-driven hydrological changes in driving conveyance capacity changes are acknowledged, geomorphological changes – specifically, variations in sediment supply and lateral sediment sources – may dominate over climate-driven trends.

The accurate estimation of bankfull discharge (QBF) plays a central role in multiple disciplines including geomorphology, hydrology, and ecology. For example, bankfull discharge is an essential input in many large-scale flood models which are widely used in understanding flood risk across large scales. However, in the context of extremely limited bankfull discharge observations, these Global Flood Models (GFMs) typically assume that bankfull discharge has a spatially uniform recurrence interval, with a value of 1-2 years widely adopted. In reality, many studies have found that the recurrence of bankfull discharge is highly variable. Therefore, more reliable estimates of bankfull discharge that account for river variability across different regions and climate zones are vital. Here, we train a random forest model to estimate bankfull discharge from global datasets encompassing river catchment characteristics, river geometry, topography, reservoir capacity, hydrological and climate indicators, alongside a newly compiled bankfull discharge database with over two thousand observations. The trained machine learning model is then used to develop the first estimate of bankfull discharge for 22 million km of rivers globally, using a newly developed, high-resolution, multi-threaded river network, Global River Topology (GRIT, Wortmann et al., 2023). Independent testing against observed values of QBF shows that the random forest model has good performance (R2=0.79), and the estimated QBF has better accuracy compared to the use of uniform recurrence-interval flows. This is the first study to estimate bankfull discharge for rivers at the global scale. Our dataset aims to improve bankfull representation in large-scale flood modelling, and to support river and water resources research more generally.Wortmann, M., Slater, L., Hawker, L., Liu, Y., & Neal, J. (2023). Global River Topology (GRIT) (0.4) [Data set]. Zenodo. 10.5281/zenodo.7629907

Sediment is an intrinsic component of the fluvial network, supplying material for floodplains and coastal landforms which provide resilience during flooding and storms. As a result, an understanding of the fluvial processes that control how much sediment moves through our river systems, and how this varies across the globe, is of fundamental importance. For the purpose of estimating sediment delivery through the fluvial network, it is often assumed that rivers are well mixed through their vertical extent. However, empirical data reveals that there is frequently large variability in the concentration of sediment through the water column. Better understanding this variability is of interest to the geomorphological community to help explain variations in sediment transport and improve estimates of sediment flux. In this research, we utilise a collection of Acoustic Doppler Current Profiler (ADCP) data from large rivers across the globe to investigate variations in the vertical distribution of suspended sediment. Calibrations of ADCP backscatter to Suspended Sediment Concentration (SSC) from the wider literature are used, alongside median grainsize and acoustic frequency, to create a Machine Learning (ML) model from which SSC from uncalibrated ADCPs can be estimated. This new ML model is subsequently implemented to explore the variations in the vertical mixing of suspended sediment both temporally and spatially. This variability is explored to identify the importance of catchment characteristics in determining variations in suspended sediment concentration within the water column. Comparison of multiple river systems and their catchment characteristics, both between sites and through time, enables the identification of key attributes which exert a greater control on this variation through the water column. Subsequently, this leads to an improved understanding of sediment flux through the river system, whereby knowing the variation in sediment concentration within the water column can help to better calibrate current methods of estimating flux.

Despite its energy benefits, hydropower dam development often causes ecological damages and social disruption, including downstream livelihood impacts, and biodiversity loss. Current methods for analyzing changes in downstream inundation extent due to dam operation typically rely on historical ground or satellite observations, or on coupled hydrological‐hydrodynamic modeling. However, while the former fails to isolate hydropower impacts from climate variations, the latter suffers from extensive input data requirements and high computational burden. This study proposes a novel hybrid framework integrating satellite data‐driven Forecasting Inundation Extents using REOF (Rotated Empirical Orthogonal Function) analysis (FIER), and the process‐based Hydrological Predictions for the Environment (HYPE) model incorporating the Integrated Reservoir Operation Scheme (IROS). The framework enables the isolated assessment of long‐term hydropower impacts on downstream inundation dynamics with computational efficiency and reduced ground data requirements, making it suitable for poorly gauged regions. Applying FIER‐HYPE‐IROS to the Lower Mekong River basin (LMB), a region significantly affected by dam proliferation impacting fisheries and agriculture, we found that dam operations decreased decadal‐average wet season water levels by up to 5% and increased dry season levels by up to 11%. Wet season inundation occurrence decreased by 11 days and the inundated area by 6%, while dry season inundation occurrence extended by 6 days and the surface water area increased by 40%. Although the current framework does not explicitly assess the downstream hydrological modifications, it offers a cost‐effective alternative for evaluating upstream alterations on inundation dynamics, such as dam operations, particularly in poorly gauged regions.

Aspects of the Quaternary sedimentary geology of South-East Asia have proven problematic in terms of interpretation as to the origins and relationships of the surface sediment layers. The MIS 20 large meteorite impact (c., 788 to 785 ka) occurred within mainland South-East Asia, evident from the well-researched ‘Australasian Tektite Strewn Field’ which extends over at least one tenth of the surface of the Earth. Key questions include: 1) whether the sedimentary impact signature is preserved in the Quaternary sediment cover of the region and 2) whether stratigraphic indicators and dating methods can discriminate meteorite impact-related associations of sedimentary strata, despite subsequent reworking and diagenesis. The importance of the questions raised relate to the search for the impact site, which has not been located conclusively. Moreover, the sedimentary signatures of meteorite impacts are not well known and the descriptions in this study should aid the recognition of impact signatures elsewhere in the world. An hypothesis was developed: Surface Quaternary sediments across a wide area of mainland South-East Asia represent the effects of a regionally significant meteorite impact. Over one hundred sedimentary sections were logged across five countries in mainland South-East Asia. Methods used, defining the stratigraphy and sedimentology, include computed tomography and X-ray scanning, geochemistry, magnetic susceptibility, and environmental luminescence as well as conventional grain size analyses. Luminescence analyses were applied to samples from key strata to provide age constraints and indications of reworking through dose distributional analysis of quartz fractions. The results of the investigation explain the nature of the stratigraphy and relate it specifically to the meteorite impact. In this manner, the strata and sedimentary signatures of the ejecta from a large cosmic impact are defined across a broad region, rather than being described at singular and isolated sections. The novelty is the spatial scale of the investigation which nevertheless remains detailed. A summary model of impact stratigraphy is presented that applies to the regional ejecta blanket covering at least 300,000 km2. Tektites were co-deposited with the ejecta and not introduced by surface processes reworking the deposits. Similar models may be applicable outside of mainland South-East Asia, wherever other large impacts are suspected to have occurred.

Storm surges are the most important driver of flooding in many coastal areas. Understanding the spatial extent of storm surge events has important financial and practical implications for flood risk management, reinsurance, infrastructure reliability and emergency response. In this paper, we apply a new tracking algorithm to a high-resolution surge hindcast (CODEC, 1980–2017) to characterize the spatial dependence and temporal evolution of extreme surge events along the coastline of the UK and Ireland. We quantify the severity of each spatial event based on its footprint extremity to select and rank the collection of events. Several surge footprint types are obtained based on the most impacted coastal stretch from each particular event, and these are linked to the driving storm tracks. Using the collection of the extreme surge events, we assess the spatial distribution and interannual variability of the duration, size, severity, and type. We find that the northeast coastline is most impacted by the longest and largest storm surge events, while the English Channel experiences the shortest and smallest storm surge events. The interannual variability indicates that the winter seasons of 1989-90 and 2013–14 were the most serious in terms of the number of events and their severity, based on the return period along the affected coastlines. The most extreme surge event and the highest number of events occurred in the winter season 1989–90, while the proportion of events with larger severities was higher during the winter season 2013–14. This new spatial analysis approach of surge extremes allows us to distinguish several categories of spatial footprints of events around the UK/Ireland coast and link these to distinct storm tracks. The spatial dependence structures detected can improve multivariate statistical methods which are crucial inputs to coastal flooding assessments.

The Tonle Sap Lake (TSL), a vital component of the Mekong River, is renowned as one of the world’s most productive lake-wetland systems. The lake’s high productivity is intimately related to an annual flood pulse that is driven by Mekong River flood waters forcing a unique flow reversal along the Tonle Sap River into the lake. During the dry season the floodwaters are returned to the Mekong River, sustaining vital freshwater fluxes to the downstream delta, inhabited by 23 million people. Recent observations have revealed notable changes in the timing and duration of the reverse flow into the TSL, resulting in associated reductions in lake inundation extents. Previous work has identified changes in flow regimes as a possible cause of the observed decline of the reverse flow. In contrast, here we show how riverbed lowering along the mainstem of the Mekong River – driven by accelerating channel bed sand mining and trapping of sediments through upstream hydropower damming – of 3.06 m (σ= 2.03 m), has resulted in a reduction of the water flux into the TSL by up to 47% from 1998 to 2018. We additionally show that projected future (to the year 2038) riverbed lowering, resulting from ongoing sandmining, of up to 5.92 m (σ) = 2.84 m), would result in a further decline of water flux into the TSL of ~ 69% relative to the bathymetry condition in 1998. These ongoing reductions are reducing the maximum extent of seasonally flooded areas by ~ 40% around the lake, presenting a critical threat to its biological productivity and the entire functioning of the TSL flood pulse system. Additionally, these changes in the reverse flow would increase, by around 26 billion m³, the flow that would be transmitted downstream into the Mekong delta during the monsoon season, potentially contributing to increased flood risk downstream as well as reducing dry season ‘return’ water fluxes to the delta by 59%, presenting risks of accelerated saltwater intrusion and reduced agricultural productivity within the delta. Taken together our modelling results show the importance of sediment and river bed levels to the sustainability of the TSL flood pulse and that its future function will be significantly diminished if current levels of sediment extraction from the Mekong system continue.

Tropical river deltas such as the Amazon, the Ganges-Brahmaputra-Meghna, and the Mekong are facing increasing pressures from climate change, upstream infrastructure building, and rapid economic development. Many deltas are shrinking and sinking, risking national and global food security. To promote the sustainability of the Vietnamese Mekong Delta, the Vietnamese government has developed a strategic spatial plan that introduces legal and institutional innovations designed to recalibrate central-local relationships and to increase policy effectiveness. We analyse these strategic changes by drawing on three sets of literature: strategic spatial planning, environmental states, and sustainability transitions in multi-level governance systems. We conducted interviews with provincial government officials in the delta to provide fresh insights into the real-time changes that local officials are facing. Our findings suggest that the state remains crucially important to facilitate transitions towards sustainability. By analysing the ongoing restructuring of state-wide governance regimes and resource relationships across a multi-level playing field, we view the state as a dynamic multi-level system where the dispersal of power is constantly in motion. Such a view allows us to observe in real time how states cope with sustainability crises. Applying this to the Vietnamese Mekong Delta enables us to locate the delta within a changing state-wide multi-level governance structure for strategic spatial planning in which the Vietnamese government enacts policy innovations to build local capacity while binding provinces closer to the centre. This recalibration of central-local relationships needs to be underpinned by investment in staff, the mobilisation of financial resources, and detailed guidance for implementation in order to stabilise the vertical and horizontal governance structures that are developing to transition vulnerable landscapes into a sustainable future.

In this study, we present a synthetic model summarising the main sedimentary and morphologic factors that drive spatial-temporal variations in bank stability through an exemplar macrotidal estuary. In contrast to previous studies that tend to only consider localised variations in the stability of small-scale banks, here the focus is on understanding the bank stability patterns at the scale of the whole Severn Estuary (UK). The results show that during falling tides, the bank sediments persist in a near-saturated state giving elevated bank pore pressures that coincide in time with declines in the hydrostatic confining pressure, leading to destabilisation of the bank. In contrast, bank stabilisation predominantly occurs during rising tides when the hydrostatic confining pressure is able to dominate over the destabilisation processes. Cohesive macrotidal estuaries similar to the Severn Estuary, tend to present a generalised decrease in the instability moving from the outer estuary where the tidal oscillations are more significant, to the inner part of the system where such oscillations are reduced and coupled with less high banks.

Recognition of the important physical and ecological roles played by large wood in channels and on floodplains has grown substantially during recent decades. Although large wood continues to be routinely removed from many river corridors worldwide, the practice of wood reintroduction has spread across the United States, the United Kingdom and western Europe, Australia, and New Zealand. The state-of-science regarding working with wood in rivers was discussed during a workshop held in Colorado, USA, in September 2022 with 40 participants who are scientists and practitioners from across the USA, UK, Europe, and Japan. The objectives of this paper are to present the findings from the workshop; summarize two case studies of wood in river restoration in the western United States; and provide suggestions for advancing the practice of wood in river management. We summarize the workshop results based on participant judgements and recommendations with respect to: (i) limitations and key barriers to using wood, which reflect perceptions and practicalities; (ii) gaps in the use of large wood in river management; (iii) scenarios in which wood is generally used effectively; and (iv) scenarios in which wood is generally not used effectively. The case studies illustrate the importance of the local geomorphic context, the configuration complexity of the wood, and the potential for modification of river corridor morphology to enhance desired benefits. Moving forward, we stress the importance of collaboration across disciplines and across communities of research scientists, practitioners, regulators, and potential stakeholders; accounting for stakeholder perceptions of the use of large wood; and increasing non-scientist access to the latest state-of-science knowledge.

Floods are consistently identified as the most serious global natural hazard, causing devastating loss of life and economic damage that runs into multiple billions of US dollars each year. At the coastline, many flood disasters are in fact compound flood events, with two or more flood drivers occurring concurrently or in quick succession. In coastal regions the combined effect of fluvial (river) and coastal (storm tides – storm surges and high astronomical tides) floods has a greater impact than if each occurred separately. Deltas in south-east Asia are particularly exposed to coastal compound floods as they are low-lying, densely populated regions subject to the intense rainfall storm surges frequently associated with tropical cyclone (TC) activity. For our study we used a sophisticated 1D river model, combined with 2D storm tide levels, to analyse past–present and future compound flood hazard and exposure for the Mekong River delta, one of the most flood-vulnerable deltas in the world. We found that with compound flooding, a greater area of the delta will be inundated, and some parts will flood to greater flood depth. Central areas around An Giang and the Dong Thap provinces are particularly impacted in our plausible scenario, where a TC makes landfall near the mouth of one Mekong River distributary. In the future delta, the impact of compound flooding is potentially more significant, as the same compound flood scenario inundates a greater area relative to the present case and to greater depth in many locations, and floods last longer. Compound flooding therefore has clear implications for flood managers of the future delta, who will need to ensure that existing and future flood defences are to the right standard and in the right locations to offer effective protection against this future risk.

This study proposes a sustainability assessment framework for managing the water resources of the Vietnamese portion of the Srepok River Basin (VSRB). A sustainability hierarchy was developed using five, ten, seven and five sustainability indicators to characterize economic, environmental, social and management dimensions of the basin's water resources. Reliable weights for each of these sustainability components were assigned using fuzzy analytical hierarchy process evaluations of judgements obtained from five highly experienced experts. The sustainability levels were ranked in increasing order of Management > Economic > Environment > Social, allowing a composite sustainability index to be assessed. By taking into account economic, environmental, social and management perspectives, the assessment provides a comprehensive understanding of the sustainability of water resources in the VSRB.