Recent research has marked an interesting step forward in our knowledge of floods, the way they occur and hit communities, and our capability to predict them. New data availability and the advent of new methods, notably those based on artificial intelligence, convey exciting perspective to cope with floods in the future. At the same time, new solutions are emerging, spanning from nature based ones to structural and infrastructural interventions.

On the other hand, there is growing and data-based evidence that flood risk, in terms of expected damage is increasing. It is also increasingly clear that often floods take communities by surprise. The number of truly unexpected events that are continuously occurring is concerning. Therefore, we assist to a sort of paradox, where new knowledge and opportunities are associated to an increase of risk. What are the reasons for this paradox? For what reason we are not able to transfer new knowledge into operational practice to mitigate the risk of flood? These are interesting questions that are rooted into the science of floods and the way local and regional communities and civil protection manage the risk of flood and its implications.

From a scientific point of view, it is not yet clear the dynamic interaction between climate change, hydrological change (including land-use change) and societal changes. It is also not clear what is the reason of the above surprise. As a result, we are still not fully capable of identifying priorities for actions and solutions.

This talk aim to propose a closer look at the above paradox and questions, basing on the assumption that floods never occur for one reason only, but rather from an interaction of drivers. These need to be better explored by promoting an interdisciplinary approach, which hopefully will promote a transdisciplinary transformation. I will look at the key role of society and in particular economy to get to target and I will discuss the perspectives given by artificial intelligence, which is not a new opportunity but is becoming dramatically more accessible thus offering new options.

The case of the Po River, in Italy, will be used as an example case study, by emphasising that flood risk is not an isolated problem, but it is often accompanied by hydrological risk in general and in particular the risk of drought. Therefore, finding solutions need to be a synergetic effort.

How to cite: Montanari, A.: A closer look at flood risk and future perspectives after changes in climate, hydrology and society, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4731, https://doi.org/10.5194/egusphere-egu25-4731, 2025.

High Mountain Asia (HMA) faces significant hydrological and geomorphic challenges due to global warming and cryosphere loss, which are altering water supply patterns and the frequency of flood hazards such as glacial lake outburst floods (GLOFs) and landslide-dammed lake outburst floods (LLOFs). These floods have historically caused significant damage, destroying many hydropower projects (HPPs), including major events in 1981, 1985, 2016, and 2018. While reservoirs with large storage capacities can mitigate some impacts, many planned and existing HPPs remain vulnerable. Here we compile a new flood database between 1950 and 2023 in HMA. A total of 1,015 flood events are documented, including 261 pluvial floods (PFs), 220 snowmelt-induced floods (SFs), 427 GLOFs, and 107 LLOFs. The changing flood frequency is linked to warming temperatures, rising precipitation, and cascading interactions with glaciers, permafrost, and human exposure.

Floods threaten infrastructure, disrupt energy production, and mobilize sediments that degrade reservoirs and turbines, intensifying risks under ongoing climate change. However, strategic design, maintenance, and sediment management, supported by improved monitoring and early warning systems, can enhance the resilience of hydropower projects. Policymakers and stakeholders must urgently adopt sustainable strategies to address these flood hazards, ensuring the viability of hydropower and contributing to sustainable development in this critical region.

How to cite: Li, D., Ni, J., Lu, X., Walling, D., Lane, S., Steiner, J., Immerzeel, W., and Bolch, T.: Climate and geomorphic-driven river floods and related impacts on hydropower in High Mountain Asia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1860, https://doi.org/10.5194/egusphere-egu25-1860, 2025.

The Mekong River–Tonle Sap Lake interaction system plays a vital role in supporting livelihoods in the region through the reverse flow phenomenon. During the flood season, a substantial volume of water flows from the Mekong mainstream into the Tonle Sap Lake floodplain, which is then gradually drained during the dry season to provide additional water to the Vietnamese Delta. This interaction is critical for fisheries and agriculture, benefiting approximately 20 million residents across the Tonle Sap Lake and Mekong Delta regions.

However, since 2010, extensive dam construction in the upper Mekong River and local sand mining activities have significantly altered the flow regime, weakening the interaction in two key aspects: the duration of reverse flow and the volume of nutrient-sediment water entering the lake. Utilizing an integrated modeling framework comprising hydrodynamic and hydrological models, this study found that while the Tonle Sap Lake system demonstrated resilience to climate change between 2010 and 2024, the influence of human interventions has been profound.

Our results indicate that the average annual reverse flow volume, which was approximately 43 km³ during the historical period (1980–2000), has declined by about 25% to an average of 30 km³ in recent years. Additionally, the duration of the reverse flow has shortened by approximately 20 days. These changes underscore the dominant role of anthropogenic stressors in disrupting the Mekong River–Tonle Sap Lake system.

To sustain this critical interaction, urgent measures are needed to regulate local sand mining and foster transboundary collaboration with upstream states regarding dam operations and future reservoir construction. Such actions are essential to maintaining flow regimes that approximate natural conditions and securing the livelihoods of millions in the region.

How to cite: Morovati, K. and Tian, F.: Impacts of Climate Change, Sand Mining, and Hydropower Dams on the Mekong River–Tonle Sap Lake Interaction System, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2407, https://doi.org/10.5194/egusphere-egu25-2407, 2025.

Vegetation effects are particularly significant in dryland region channels, where flooding is mainly as flash floods, because large vegetation commonly grows within the channels. Measurements at long-term study sites in ephemeral channels of southeast Spain show that such vegetation can be highly resilient to a range of flows.  However, extreme events, such as occurred in September 2012, can effectively zero the vegetation in the channels and valley floor.  The effects of the vegetation in mature pre-flood and in sparse post-flood state on the hydraulics of flow and on flood levels have been measured in the field and calculated using a range of assumptions on roughness effects.  The vegetation state is shown to have large effects on flow velocities and on flood stage and inundation extent. Since that extreme event the recovery of vegetation has been measured annually.  Differential rates of recovery are evident between sites and within sites, varying with spatial position in relation to the main channel. In some locations vegetation has regrown to pre-2012 states but elsewhere occurrence of a series of moderate to large flows has restricted growth.  Drought conditions have also occurred over the past decade and in earlier periods, affecting growth. The vegetation dynamics have complex interactions with the sediment and morphological changes in these channels and together they contribute to variations in flood capacity and hydraulics of flow. Feedback effects through erosion and deposition processes are identified. Results of modelling the effects of the different vegetation coverage, assemblages and heights of plants are discussed. Effects of timing of flows and of hydrological balances, countered by extreme temperatures, are analysed. It is shown that the dynamics of vegetation, through succession of different size events and varying conditions, have a significant effect on flood levels and spatial patterns in ephemeral channels and these need to be incorporated in flood modelling and predictions.  Consideration of flood management strategies needs to recognise that presence and dynamics of vegetation can pose both challenges and benefits, especially in highly erodible catchments with very high sediment fluxes and under conditions of climate change.  

How to cite: Hooke, J.: Mediation of hydro-geomorphological  process effects on flooding by vegetation in dryland channels , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3099, https://doi.org/10.5194/egusphere-egu25-3099, 2025.

Compound flooding refers to the co-occurrence of multiple flooding mechanisms, leading to more severe and complex flood events. Low-lying coastal deltas and estuaries are especially prone to compound flooding as they face multiple drivers such as storm surges, high river discharge, intense rainfall, tides, and sea-level rise. These combined sources amplify the impacts, resulting in significant loss of life and property, as seen in Hurricane Katrina (New Orleans, 2005), Cyclone Nargis (Myanmar, 2008), and Storm Xynthia (French Atlantic coast, 2010). Globally, 2.15 billion people live in near-coastal areas, including 898 million in low-elevation coastal zones. The UK has a long history of estuarine flooding caused by compound events. Climate projections suggest hotter, drier summers and wetter winters, accompanied by more frequent and intense extreme events. Sea-level rise is expected to exacerbate vulnerabilities in the UK's coastal regions (UK Met Office, IPCC 2014). Coastal aquifers are frequently affected by flooding, making groundwater a critical factor in estuarine geomorphology. Recent studies have highlighted the significant volumetric and chemical importance of groundwater in river-dominated coastal systems, warranting further investigation under climate change scenarios.

In this study we have developed a coupled catchment and groundwater model using Caesar Lisflood to assess groundwater’s contribution to compound flood events. The model is calibrated using historical fluvial and tidal flow data to evaluate how river discharge, groundwater, and associated drivers shape flood magnitude, timing, and behavior. Additionally, the study analyzes the sensitivity of the estuary to changes in hydrogeological parameters by observing variations in modeled groundwater heads and simulated discharge in response to changes in aquifer properties. Our research focuses on the Conwy estuary in North Wales, a flashy catchment that experiences frequent flooding events. A notable compound flood occurred during Storm Ciara (February 2020), when record river levels, intense rainfall, and high storm tides combined to affect 172 properties. The Conwy River drains a 600 km² catchment with annual precipitation averaging 1,700 mm and a baseflow contribution of 27%. Baseflow, the component of streamflow discharged from groundwater storage, reacts slowly to rainfall and is notably influenced by topography, geology, vegetation, land use, and climatic factors. This research delves into the lesser-studied role of groundwater in estuarine hydrology, providing insights into its potential impact on compound flood dynamics under future climate scenarios.

How to cite: Bhattacharya, A.: Impact of Groundwater in Compound Flooding: A Case Study of the Conwy Estuary in Wales., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7245, https://doi.org/10.5194/egusphere-egu25-7245, 2025.

Extreme flood events can cause significant geomorphic changes in river systems, including bank erosion, sediment deposition, river braiding and the formation of new channels, resulting in extensive damage to settlements, infrastructure, and floodplain structures. At the same time, information on the extent, nature and distribution of the geomorphologic impacts of floods is often not systematically collected and is therefore not available to assess the consequences of flood events and associated risks, or to provide a basis for efficient water management and flood protection.

This study presents longitudinal research employing geomorphologic mapping and UAV photogrammetric reconstructions to track the intensity, type, and distribution of geomorphic changes to streams in the Opava River Basin, Jeseníky Mts., Central Europe, recurrently affected by devastating floods. The field survey was carried out repeatedly in the area after major floods in 1997, 2007, and 2024 using a consistent mapping methodology. This approach combined the mapping of geomorphologic flood effects with hydromorphological properties, including information on channel and floodplain modifications. The survey covered a contiguous stretch of river in the core flood zone over a length of 100 km, and the UAV campaigns following the 2024 flood focused on selected river segments representing hotspots of river dynamics.

The large-scale mapping results allowed for an assessment of the spatial distribution of flood effects, the identification of critical elements and structures, and the analysis of relationships between stream modifications and the nature of geomorphic impacts. High-resolution models from UAV monitoring allowed us to quantify detailed geomorphic analysis and determine bank erosion rates, sediment volumes, and channel migration patterns.

The analysis revealed substantial spatial variability in geomorphic responses, with particularly intense erosion and sediment deposition observed in narrow valley sections and areas of high flow velocity, as well as in relation to floodplain connectivity and channel modifications. The most significant geomorphological changes consistently reoccurred in the same locations, signaling the need for targeted river management and protection.

The study highlights the importance of geomorphic mapping of flood impacts for understanding the risks posed by high-intensity floods, improving risk assessment, and efficient post-flood recovery efforts. Methodologically, it emphasizes the efficiency of UAV photogrammetry for detailed and rapid post-flood assessments, providing comprehensive information to better understand flood dynamics and target river management strategies.

How to cite: Langhammer, J. and Lehký, M.: Geomorphological Response of Montane Streams to Extreme Floods, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9823, https://doi.org/10.5194/egusphere-egu25-9823, 2025.

Flood risk is increasing worldwide, however, studies on river geomorphological responses to floods often lack explicit consideration of anthropogenic impacts on rivers. A pervasive impact of human modification has been the conversion of anabranching riverscapes to incised, single thread, so called ‘fire-hose’ channels. However, the consequences of such modification for river functioning are poorly understood, restricting our ability to manage flood risk in modified systems or restore these riverscapes. Therefore, we aim to determine how incision of gravel bed rivers modifies their geomorphological response to flood events. We undertook an experiment in a large hydraulic flume, designed to simulate an alluvial gravel-bed river. The flume was filled with loose sand and seeded with alfalfa to represent riparian vegetation. Under our initial conditions of low flow and sediment input, a fully equilibrium anabranching channel developed. Subsequently, we simulated both small and large flood events. We then prompted incision by lowering the downstream base level, allowed the channel to reach a new equilibrium state, and conducted the same flood sequence. We compare the response of the anabranching and incised treatments to floods, in terms of geomorphic work done, morphological response and sediment output. First, we found that for the same input conditions, both anabranching and incised, single thread, equilibrium states existed, determined by the historical changes in base level modification. However, the two equilibrium states responded very differently to flood events. Riparian vegetation played a critical role in this process, reducing widening and channel migration associated with incision in non-vegetated experiments. Instead, channel morphological changes to high flows after incision were predominantly through adjustments to river depth. Second, incision reduced flooding because even the largest flows were fully contained within the channel. However, sediment export from the incised channel during floods was nearly double that of the anabranching treatment. Consequently, incision reduced flood extents locally, but may exacerbate flood risk overall by transporting water quickly downstream and exporting much greater amounts of sediment which could reduce channel capacity at other parts of the river network.

How to cite: Mason, R., Delorme, P., Murphy, B., McLelland, S., Baynes, E., Polvi, L., Rice, S., and Parsons, D.: Effect of incision on river response to floods: Insights from flume experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15572, https://doi.org/10.5194/egusphere-egu25-15572, 2025.