A detailed systemic analysis of the hydrometeorological event of Oct. 14-15, 2018, made it possible to identify, for the first time, all the predisposing, triggering, and aggravating/mitigating factors that explain the genesis, impacts, and management of the event in the Aude watershed. Attention then focused on one of the parts of the watershed that received the highest cumulative rainfall, namely the Rieu Sec watershed, a right-bank tributary of the Orbiel River in the Minervois. It has been shown that the Oct. 2018 event was extraordinary in several respects (meteorology; hydrology; geomorphology; sediment budget; specific degradation rate; impact on riparian vegetation; recurrence interval estimated at 204-378 yrs). A model of small, mountainous watersheds (< 40 km2) functioning is proposed, in which high-magnitude/low-frequency flood events play a key role. The significant variability in precipitation observed from one sub-watershed to another complicates the management of rainfall and floods. The challenge for the future lies in improving flood forecasting, and the effective dissemination of these forecasts.

A severe and complex, polygenetic flood event occurred in Muktinath area of Mustang, Nepal on the evening of August 13, 2023 causing significant damage to property and infra-structures worth approximately of USD 7.4 million at Kagbeni Village, which is nestled along both banks of Kagkhola, a major left bank tributary of the Kali Gandaki River. About 29 houses, 1 motorable bridge, 1 steel truss bridge and 3 temporary bridges were destroyed, while more than 25 cows and other livestock were killed. Fortunately, human lives were spared because the community was warned to move to safety before the mud and sludge hit the village. A study was conducted in order to know what had caused this unusual flash-flood in Mustang. Kagbeni (2810 m) lies in the north Himalayan, rain-shadow area and normally receives few rainfall (

The encroachment of floodplains by anthropogenic activities is an increasing trend in developing Nepal. However, unplanned infrastructures like bridges constructed in hazard prone areas in the valley floor will never be sustainable. From a few study cases, we illustrate the concept of “river functional space” (lateral space, erosion, deposition) as an approach respective of nature and river-flows dynamics, and economically positive. We discuss the possible options between bridges and fords.

Located in the northern, rain-shadow zone (< 300 mm/yr) of the Nepal Himalaya, Kagbeni village (2810 m, 420 inhabitants, at the junction between Jhong Khola and Kali Gandaki) was affected by a disastrous, unusual hyper-concentrated flood on August 13, 2023. It caused significant damage to property and infrastructure (cost estimated at USD 7.4 millions), fortunately without fatalities. For several years, increased rainfall has been recorded at Jomsom station (2720 m), a trend confirmed by residents. However, no single extreme rainfall event that could have triggered the flood was recorded at Jomsom station or in CHIRPS rainfall data for the Jhong Khola valley. Considering the geomorphological context, we hypothesized that the flood event was most likely triggered by a landslide-lake outburst. The landslide existence and source area (upstream from Chhiongur gorges) were confirmed by analysis of Sentinel-1 InSAR coherence time series. Downstream, the flood spread over the entire valley floor, as evidenced by new deposits, bank cuttings and reactivation of landslides providing additional debris and making downstream flooding even more destructive (bridges, buildings, livestock, orchards...). The volume of debris transported was estimated at 647,000 m(3), followed by rapid post-flood re-incision (215,000 m(3)). The inhabitants of Kagbeni also contributed to disasters by settling on very low terraces and encroaching on the Jhong Khola riverbed. Upstream of the village, the Upper Mustang Road bridge also amplified the damage through a bottleneck effect: its concrete deck collapsed, and its transport was very destructive downstream. Given the general trend towards climate change, the probability of future, similar flash floods remains high in Kagbeni, yet some residents have rebuilt their homes and continue to live on hazardous floodplains. We suggest some measures for preventing future risks, such as (i) applying the Freedom Space for Rivers concept to avoid encroachment of floodplains by anthropogenic activities, and (ii) managing a concrete ford-type structure instead of poorly calibrated bridge deck along the Upper Mustang Road. More generally, a nationwide natural risk-reduction policy should be implemented.

The article synthesizes 25 years of hydromorphological restoration in the high-energy, gravel-bed rivers of southern France. In the Minervois region, the authors highlight the challenges and advancements made in riverbed restoration following the catastrophic flood of November 1999. This event catalyzed research in fluvial geomorphology and led to significant restoration initiatives implemented by the SMMAR. The article pays particular attention to the role of floods and ordinary flow events in the hydromorphological functioning of the channels, specifically focusing on hydraulic geometry, channel pattern, bedload transport, and sediment balance. The methods developed rely on field observations and measurements, as well as image processing. Between 2012 and 2024, the transport distances of cobbles were monitored in the watersheds using RFID technology (passive and active microchips). From 2016 to 2021, sediment balances of the restored river sections were quantified through LiDAR surveys to better understand how rivers respond to the new flow conditions and channel morphology resulting from restoration efforts. The results indicate that the restored sections function as preferential sediment storage areas. Although the restoration has improved sediment transport conditions during floods, several tributaries do not contribute sufficiently to the sediment recharge of the main channels, leading to chronic sediment deficits. The data also emphasize the importance of ordinary flows in sediment transport, which can often result in riverbed incision. To achieve a more balanced sediment supply, the authors advocate for the reactivation of sediment sources at the watershed scale.

Pokhara (ca. 850 m a.s.l.), Nepal's second-largest city, lies at the foot of the Higher Himalayas and has more than tripled its population in the past 3 decades. Construction materials are in high demand in rapidly expanding built-up areas, and several informal settlements cater to unregulated sand and gravel mining in the Pokhara Valley's main river, the Seti Khola. This river is fed by the Sabche glacier below Annapurna III (7555 m a.s.l.), some 35 km upstream of the city, and traverses one of the steepest topographic gradients in the Himalayas. In May 2012 a sudden flood caused >70 fatalities and intense damage along this river and rekindled concerns about flood risk management. We estimate the flow dynamics and inundation depths of flood scenarios using the hydrodynamic model HEC-RAS (Hydrologic Engineering Center’s River Analysis System). We simulate the potential impacts of peak discharges from 1000 to 10 000 m3 s−1 on land cover based on high-resolution Maxar satellite imagery and OpenStreetMap data (buildings and road network). We also trace the dynamics of two informal settlements near Kaseri and Yamdi with high potential flood impact from RapidEye, PlanetScope, and Google Earth imagery of the past 2 decades. Our hydrodynamic simulations highlight several sites of potential hydraulic ponding that would largely affect these informal settlements and sites of sand and gravel mining. These built-up areas grew between 3- and 20-fold, thus likely raising local flood exposure well beyond changes in flood hazard. Besides these drastic local changes, about 1 % of Pokhara's built-up urban area and essential rural road network is in the highest-hazard zones highlighted by our flood simulations. Our results stress the need to adapt early-warning strategies for locally differing hydrological and geomorphic conditions in this rapidly growing urban watershed.

During my career, my research fields in the Himalayas (Nepal and NW India) addressed human/environment relations. From the high glacial valleys where the story of their Quaternary past was the subject of my doctoral thesis, my focus has gradually shifted towards the warmer and more inhabited valleys, with slopes covered with terraced crops. The mapping of active processes (landslides, floods and other natural hazards) and their triggering processes (monsoon rains, earthquakes), and the rapid evolution of landscapes, observed from one mission to the next, gradually enabled me to analyze the real "risks" for the populations of this rapidly developing country. The opening of roads and a denser urbanization have also changed my perspective, broadened my approaches, and led me to discuss with academics, Nepalese or foreign, and local officials on new, more sustainable practices to reduce risks.

Highest geomorphic activity in central Nepal is mostly driven by monsoon rainfall, yet the recent development of infrastructure has increased this activity and the risks for the locals and travelers. Our aim is to illustrate recent cascading hazards and their interactions with, and impacts on, socio-economic development along an important road corridor. We focus on the middle Kali Gandaki valley reach, within the High Himalayan Crystalline Series HHC where the river deeply incised and the topography is characterized by steep hillslopes and high relief. This 20-25 km long reach experiences strong monsoon rainfall enhanced by orographic effects, with rainfall rates >2000 mm/a. In the last years between 2018 to 2021 the monsoon season was very strong and experienced several strong and long lasting rainstorm events with amplified catastrophic events such as debris flows, landsliding and river activities. On the basis of repeated field surveys, satellite images (Pléiades, Sentinel and Planet) analysis, Global Precipitation Measurement (GPM) data, UAV, river flow seismic noise records, we observed that once destabilized, hillslopes and steep, small tributary catchments evolved very rapidly during the years, all the more since road constructions for the upgrading to a 2-lane road contributed to destabilization of the hillslopes. This rapid disequilibrium has several consequences. (1) First it reworked old colluvium deposits, including old landslide material, old glacial and/or fluvial alluvium and related lacustrine deposits, hence revealing a former, complex paleo-topography of this deep valley (as observed north of Ghasa, along the Kahiku khola and Kali Gandaki). (2) Second, in providing looser material, it has accelerated the cascading system and transfer of sediments into the main Kali Gandaki River, as shown in the Rupse site, famous for its waterfall, and that was destroyed by a debris flood (July 20, 2020) generated by intense rainfall that triggered landslides in the upper catchment, with impacts at the junction with Kali Gandaki (destruction of road, bridge, settlements). Similarly, the Thaplyang site, active since 2014, was repeatedly affected by strong rainfall since 2018, with progressive erosion of an old landslide material – the active area increased from 9100 m² (March 2018) to 9600 m² (Oct. 2018) and 32300 m² (Nov 2021) – hence threatening small settlements upstream. (3) Third, the repeated disasters (river bank collapses and settlements destruction; traffic obstruction) affect the tourism economy and development along this major link between south China and north India. Further work, including SAR analyses, is ongoing to better quantify the overall sediment exported volumes and the impacts of this changing geomorphology on future infrastructure development and settlements.

Multiple hazards (e.g. floods, landslides, earthquakes, glacial and landslide lake outburst floods) are threatening people, their goods and infrastructures in the high mountains of Nepal Himalaya. Floods and landslides are mainly driven by monsoonal precipitation. However, human impact often increases natural risks, like in the Kali Gandaki (KG) valley, the deepest valley (>5500 m) on earth, where the new two-lane road construction (since 2017) has caused many undercut and instable slopes. In the light of previous events, we intend to assess the cascading multi-hazard events of 2020 in three tributary catchments of KG. We adopted a pluri-disciplinary approach: interpretation of Sentinel-2 satellite images (March and November 2020), analysis of precipitation (stations of Lete and Tatopani, GPM satellite precipitation measurements), hydrologic and seismic data (Beni), geomorphological mapping, hydrological modelling in HEC-RAS, and field visits in July and November 2020, including interviews with locals. On 20 July 2020 major hyper-concentrated flood events and landslides occurred in the Rupse, Thaplyang and Kahiku catchments (between Tatopani and Lete) destroying parts of the KG road, road bridges and a hotel (Rupse site). We focus on the Rupse River entering the KG valley at Rupse waterfall (height 108 m; kyanitic gneisses) then flowing down to the KG road and to KG River 200 m below. The major flood event lasted two hours and reached a max. flood level of 35 m at the edge of the waterfall. Upstream of the waterfall, four landslides (each about 250m wide, 200 m high) were triggered. Due to cloud coverage satellite scenes are missing to unravel whether the landslides caused the damming of the river and a landslide lake outburst flood or if the landslides were mainly triggered by the flood and increased sediment input to it. Floods from these tributary catchments caused a major KG flood especially south of the Rupse catchment, which led to severe erosion and sedimentation in the channel; i.e. destruction of a pole of the national electricity grid, reactivation of the Kham Bhitta deep-seated landslide, destruction of the KG road (the construction of which probably contributed to this reactivation). Seismic data from Beni, approximately 27 km downstream of the affected catchments, provide constraints on the timing and relative magnitude of the flood in the KG. The data show that a short duration high magnitude flood with a very rapid rise and recession passed through Beni on the afternoon of 20 July. In addition, station data of Lete and Tatopani shows that yearly rainfall totals of 1839.5 and 2140.2 mm, respectively, were the highest since 1970. March and April were already very wet, followed by extremely monthly rainfall totals of 499.7 mm and 551.5 mm at Lete and Tatopani, respectively. Assessing the 2020 events demonstrates how important localized events in relatively small areas are to understand cascading multi-hazard processes in Himalayan mountain regions. In addition, such hydro-geomorphic functioning and related hazards should be carefully considered when planning road design and bridge sites together with landslide and water level monitoring, for a better traffic maintenance and safety.

Monique Fort (Pr. Emerite, Universite de Paris, UMR 8586 PRODIG CNRS) Yvette Dewolf nous a quittes le 5 avril 2021. Nee en 1928, elle debuta sa carriere a l’Universite de Caen dans l’equipe d’Andre Journaux, et elle s’investit tres vite au Centre de Geomorphologie ou elle participa, entre autres avec Jean-Pierre Lautridou et Jean-Pierre Coutard, aux experiences sur les milieux periglaciaires et la cryoclastie. Nommee a Paris en 1965, elle co-dirigea, avec F. Durand-Dastes et sous la Direction...

The Seti River originates from the Annapurna Massif in the Higher Himalaya of Nepal and flows through the Pokhara valley in the Lesser Himalaya. The Seti River witnessed a disastrous flash flood on May 5th, 2012 causing the death of 72 people, obliterating dozens of homes and damaging infrastructures worth millions of dollars. Despite the 2012 flood event and several warnings by scientists for more yet bigger scale future floods in the Seti valley, fluvial risk is being aggravated by anthropogenic activities such as unplanned human settlement, encroachment of riverbanks, haphazard construction of road, drinking water, and hydropower projects in potential flood hazard areas in addition to the increased impacts of climate change on geological and hydro-metrological hazards as in other parts of Hindu Kush Himalayan Range. Covering some 40-km distance from the Seti headwater (Sabche Cirque) down to Pokhara city, the study is carried out based on hydro-geomorphological mapping, analysis of land-use and land-cover change, hydrological analysis including HEC-RAS modelling, historical archives, and interviews with local people. The study shows a significant change on the land use and land cover of the Seti catchment, mainly the urban/built-up area, which is increased by 405% in 24 years period (1996 to 2020) and by 47% in 7 years period (2013 to 2020). Further the study reveals that anthropogenic activities along the Seti valley have increased fluvial risk and are likely to invite more disasters. From the HEC-RAS analysis, two motor bridges built over Seti River were found to have insufficient freeboard to safely pass the highest flood discharge for 100 years return-period. Instead of relocating people to safer places, the government and local authorities rather seem to have encouraged people to live in the floodplain by providing basic amenities such as drinking water, electricity and access road. Given the context of climate change and Pokhara valley and the Seti catchment being in a high-seismic gap zone, there is a strong possibility of similar flood to the scale of 2012 or even greater in Seti River. Though the fluvial risk can be managed in a sustainable way through the application of functional space concept, i.e., by allowing more space (freedom) for rivers, this economic and environment friendly approach of the fluvial risk management has not been implemented yet in the Seti valley nor in Nepal. Rather the encroachment of floodplains by anthropogenic activities along the Seti valley is on an increasing trend. Many settlements and infrastructures along the valley have been identified vulnerable to hydro-torrential hazards, therefore it is utmost necessity to implement functional space river concept, land use and land plan policy, early warning system and public awareness education in order to mitigate and manage the future impact of fluvial hazards along the Seti valley.

The Nepalese Himalaya is affected by a major rift valley, the Thakkhola half graben (THG). Along this fault-bounded basin, the Kali Gandaki (KG) flows from the Tibetan plateau southwards to the Dhaulagiri and Annapurna massifs, where it forms the deepest gorge on earth. The THG has been filled with up to 1 km thick Plio- and Pleistocene sediments, underlain by clay shales of the Jurassic Spiti Formation that are strongly water swellable and prone to landslides. These pre-conditions led to a series of large and complex landslides, particularly along the eastern flank of the THG, with strong effects on infrastructure and the local population. One of these landslide systems (c. 15 km²) is located in the semi-arid Muktinath Valley, a tributary basin of the KG (c. 92.5 km²). Water as most important driver of the system is provided by precipitation mainly during the summer monsoon (annual rainfall: ~ 350 mm), snowmelt and irrigation.Against this background, we aim i) to better understand regional-scale landslide systems (spatial pattern, drivers/controls), ii) to establish a long-term monitoring of local-scale landsliding in the Muktinath Valley, and iii) to share our findings with local communities to support the development of mitigation strategies.Reconstruction of landslide dynamics over the past 30 years is based on local information (interviews), field observations (damaged buildings and walls), geomorphological mapping and multi-temporal (ortho-) photo analyses (WorldView, Pleiades). Since 2018, annual UAV surveying is applied.Results include a geomorphological map of the area focusing on landslide related processes and landforms, indicators of recent landslide activity, hydrologic characteristics and irrigation infrastructure, as well as the distribution of Spiti shale outcrops. Surrounding the presently most active landslide, we observed an average displacement of c. 20 cm/a since 1988 with an increasing trend towards present (30 - 50 cm/a since 2011). In the center of the most active landslide significantly higher displacements of up to 15 m have been detected since 2011, which corresponds to an average of about 2 m/a. The landslide monitoring based on UAV surveying, structure-from-motion processing and different approaches of high-resolution topographic change and error modelling (DEM resolution: 2.6 - 4.3 cm) shows massive change between April 2018 and March 2019 (gain: 33395 ± 5489 m³; loss: 50276 ± 10781 m³), accompanied by a total sediment export of 16881 ± 12098 m³ to the Jhong River. Detailed orthophotos (resolution: 1.29 - 2.15 cm) provide valuable supplementary information not only on recent landslide propagation and dynamics but also with regard to future threatened areas (opening cracks). Boosted landslide activity in 2018 is associated to the strong monsoon that heavily impacted in the larger region as well (debris flows, flash floods, multiple bank collapses): In August 2018 Muktinath recorded the highest monthly rainfall since 1978 (172 mm, DHM Nepal).The research is located at the interface between humans and the environment. The "symbiosis" of the local population and the landslide system is unique - and enables to deconstruct various interacting landslide processes driven and modified by climate (change) and human impact.

Nepal experienced disastrous earthquake events in 2015. The first one (magnitude of 7.8) with epicenter in Barpak, Gorkha district, occurred on 25th April 2015, followed by another event (7.3 magnitude) on 12thMay 2015, with epicenter in 19 km south east of Kodari, Sindhupalchok district. Those earthquake events induced different types of geo-hazard and they are widely distributed and caused serious damages and losses. This paper discusses the types of geo-hazards induced by these Gorkha and Sindhupalchok earthquake events and the losses and damages from those events; and the future risk from those geo-hazards in Pharak area covering 305 km2 in Solukhumbu district, Nepal. Satellite images of before and after the events were used to map landslide, debris flow, landslide dam and other geomorphic changes after earthquake. Information on the losses, damages and future risk were collected through focus group discussion, key informants’ interview, observation, and measurement. A total of 79 landslides, 13 rock falls, 5 debris flow and one site of river damming were identified and mapped. The losses and damages included private and public buildings, cultivated land, crops and other infrastructure such as trails, canals for hydropower plant. The losses and damages associated with landslide and debris flow induced by earthquake is comparatively higher than the losses and damages from other geomorphic hazards such as rock fall and landslide dam and landslide dam outburst flood. The risk from those geo-hazards induced by earthquake is also high. Community mobilization with activities of regular monitoring of those hazards, skill development for rescue operation, design and implementation of mitigation measures are some of the efforts necessary for better management of disaster risk.

In the past 20 years, motor road and bridge construction works have increased significantly in Nepal, in most cases with poor consideration of geo-hazards (rockfalls, landslides, debris flows) and river-flow dynamics. The National Road Construction (NRC) is further fostered by China's Belt and Road Initiative (BRI). Currently, the Kali Gandaki (KG) road corridor is being upgraded to a 2-way black-top road as a project under BRI. The river dynamics of the KG sub-catchments is rarely considered by engineers despite it represents a common threat to the KG road, road bridges, and human lives. Here we study the Ghatte Khola (GK) sub-catchment that is located along the KG corridor in Dana, Myagdi district, West Nepal. From hydro-geomorphological mapping, hydrological and HEC - RAS analysis, historical archives and interviews, several questions have arisen regarding to the size of the under construction GK motor bridge. The water way andfreeboard of this bridge were found to be insufficient to safely pass the highest flood discharge for 100 years return-period, hence the bridge was found to be vulnerable to extreme floods. Actually, the nearly completed GK bridge was destroyed by a severe hyper-concentrated flood on May 25, 2019. Active geo-hazards (rock falls, landslides) in the upper GK catchment appear as persistent drivers susceptible to aggravate sporadically the GK flows, and may pose serious threat to the future GK road bridge and to nearby infrastructure and settlements. We discuss these issues and eventually propose new technical, more sustainable solutions (eg., Fords).