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Natural dams are formed most often in narrow, steep valleys in high mountains. The outburst floods triggered by natural dam failures result in the topography and landforms successively being altered. Boulder bars are common natural structures that are selected here to quantitatively evaluate the impact of outburst floods on the topographical and landform variations in downstream channels. In this study, we selected the Sedongpu natural dam on the Yarlung Tsangpo River formed as a result of a landslide in 2018 as an example, and studied the geomorphological changes in a river reach located 173 km downstream of the Sedongpu natural dam. The sizes and shapes of the boulder bars in this area were statistically analyzed. The results show that there are three shape types of boulder bars in this area, i.e., sickle, bamboo leaf and oval. Furthermore, it found that the relationship between the lengths and widths of boulder bars is similar before and after outburst floods, as is the relationship between perimeters and lengths of boulder bars, which means these relationships are not affected by outburst floods. And the perimeters of boulder bars are almost twice their lengths. In addition, the relationship between the areas and lengths of boulder bars follows a power function. The most important finding is that the riverine morphological features conserved self-similarity due the influence of the outburst flood erosion triggered by a natural dam failure. This finding adds to the previous observations since dam failures introduce sudden and dominating impacts on river systems. [ABSTRACT FROM AUTHOR]

Outflow channels of the Tharsis province on Mars testify to the drainage of groundwater reservoirs through magmatic and tectonic processes. The Olympica – Jovis Fossae system extends over 500 km, but the significance of its deep and narrow channels for the Tharsis dome is poorly understood. We present a new geologic map of Olympica-Jovis Fossae where we mapped landforms and geological units, dated them relatively, and identified the chronological succession of outflow events and deposition of lava flows, airfall, and sedimentary units. Cross-cutting relationships indicate a complex succession of 28 channel incisions in the volcanic stratigraphy. The five identified morphological channel types indicate different flow dynamics at different times and places. The map highlights the role of intensive volcanic activity in releasing water from overpressurized aquifers and possible subglacial flows, and will assist in a better understanding of the general conditions for catastrophic groundwater flooding in a volcanic context.

Surface hazards can form hazard cascades which expand their reach. However, our understanding of their complex dynamics and ability to mitigate their impacts remain limited. In 2018, two landslides dammed the Jinsha River in the Tibetan plateau and formed landslide‐dammed lakes. Subsequent dam breaches prompted the evacuation of >120,000 people. An early warning system for floods using a regional seismic network has been proposed on the basis of catastrophic floods having been detected ∼100 km away, with seismic energy proportional to discharge. Surprisingly, we find that this catastrophic outburst flood was undetectable beyond a few kilometers, with peak seismic energy preceding peak discharge. We propose that river channel stability also controls seismic energy generation and should be considered for accurate monitoring of catastrophic floods. In contrast, we find that the various processes during dam breach can be well‐characterized seismically further away and provide warning ∼60 min before discharge exceeds monsoon flood levels. We also show that numerical modeling of dam breaches which typically lacks in situ measurements can benefit from incorporating seismic data as constraints. Finally, we show that this event drastically increased sediment fluxes ∼670 km downstream for years and may significantly reduce the capacity of hydropower plants. Our results reveal ways to improve early warning of catastrophic outburst floods and the need to consider surface hazards' long‐term impact when managing infrastructure in mountainous regions. Plain Language Summary: Mountainous regions are prone to surface hazards such as landslides and floods. However, our understanding of how these events evolve and interact is limited. In 2018, two landslides blocked a river in Tibet and created landslide‐dammed lakes. Subsequently, the dam broke and the resulting outburst floods severely damaged downstream infrastructure and led to the evacuation of >120,000 people. In contrast to what previous studies suggest, we find that this catastrophic outburst flood was undetectable beyond a few kilometers, with peak seismic energy preceding peak discharge. We propose that river channel stability also controls seismic energy generation and should be considered for accurate monitoring of catastrophic floods. Conversely, the dam breakage could be detected seismically further away and provide warning ∼60 min before discharge exceeded monsoon flood levels. Finally, we show that this event drastically increased sediment amounts downstream for years and may reduce the capacity of hydropower plants. Our results reveal ways to improve seismic monitoring and early warning of catastrophic floods and the need to consider the long‐term impact of surface hazards when managing downstream infrastructure. Key Points: Catastrophic outburst floods were surprisingly quiet with peak seismic energy preceding peak discharge due to changes in channel stabilityVarious dam breach processes can be well‐characterized seismically, which can constrain numerical modeling and provide earlier warningThese hazard cascades drastically increased sediment fluxes ≥670 km downstream for years and reduced hydropower plant capacity [ABSTRACT FROM AUTHOR]

Natural dams are formed most often in narrow, steep valleys in high mountains. The outburst floods triggered by natural dam failures result in the topography and landforms successively being altered. Boulder bars are common natural structures that are selected here to quantitatively evaluate the impact of outburst floods on the topographical and landform variations in downstream channels. In this study, we selected the Sedongpu natural dam on the Yarlung Tsangpo River formed as a result of a landslide in 2018 as an example, and studied the geomorphological changes in a river reach located 173 km downstream of the Sedongpu natural dam. The sizes and shapes of the boulder bars in this area were statistically analyzed. The results show that there are three shape types of boulder bars in this area, i.e., sickle, bamboo leaf and oval. Furthermore, it found that the relationship between the lengths and widths of boulder bars is similar before and after outburst floods, as is the relationship between perimeters and lengths of boulder bars, which means these relationships are not affected by outburst floods. And the perimeters of boulder bars are almost twice their lengths. In addition, the relationship between the areas and lengths of boulder bars follows a power function. The most important finding is that the riverine morphological features conserved self-similarity due the influence of the outburst flood erosion triggered by a natural dam failure. This finding adds to the previous observations since dam failures introduce sudden and dominating impacts on river systems.

The Jinsha River basin in the upper reaches of the Yangtze River in China is prone to strong geological activities, with numerous large-scale landslides along its banks that can potentially trigger a cascade of flood hazards. Recent seismic events such as the Wenchuan and Luding earthquakes have heightened the likelihood of landslide collapses along the slopes of the Jinsha River, thereby increasing the risk of a large-scale landslide-dam-break-flood hazard chain. Among these landslides, the ancient Woda landslide is currently in a state of slow deformation, and if reactivated, it can potentially obstruct the river and trigger catastrophic outburst floods. This study uses the integrated continuum method to simulate the dynamic processes associated with large-scale slope failures and the formation of landslide dams. Furthermore, the outburst flood resulting from the dam breach is modeled by combining the dammed lake flow model and the shallow water equation, allowing for the simulation of long-distance flood propagation. The findings indicate that the Woda landslide has the potential to create a dam of approximately 68.1 m in height, with a corresponding dammed-lake volume of about 7.10 × 108 m3. The peak flow rate of the resulting outburst flood can reach 4.4 × 104 m3/s, leading to an extensive impact zone reaching 140 km downstream. This flood inundates several downstream villages, towns, and even the Sichuan-Tibet Railway which is under construction. Moreover, the study reveals that the resistance coefficient of landslides significantly influences the entire hazard chain evolution process. Lowering the resistance coefficient of landslides leads to a considerable increase in the height of the landslide dam, amplification of the peak flow rate of the outburst flood, and an elevated risk for downstream elements situated at greater distances. [ABSTRACT FROM AUTHOR]

Outburst floods triggered by breaching of landslide dams may cause severe loss of life and property downstream. Accurate identification and assessment of such floods, especially when leading to secondary impacts, are critical. In 2018, the Baige landslide in the Tibetan Plateau twice blocked the Jinsha River, eventually resulting in a severe outburst flood. The Baige landslide remains active, and it is possible that a breach happens again. Based on numerical simulation using a hydrodynamic model, remote sensing, and field investigation, we reproduce the outburst flood process and assess the hazard associated with future floods. The results show that the hydrodynamic model could accurately simulate the outburst flood process, with overall accuracy and Kappa accuracy for the flood extent of 0.956 and 0.911. Three future dam break scenarios were considered with landslide dams of heights 30 m, 35 m, and 51 m. The potential storage capacity and length of upstream flow back up in the upstream valley for these heights were 142 × 106m3/32 km, 182 × 106m3/40 km, and 331 × 106m3/50 km. Failure of these three dams leads to maximum inundation extents of 0.18 km2, 0.34 km2, and 0.43 km2, which is significant out-of-bank flow and serious infrastructure impacts. These results demonstrate the seriousness of secondary hazards associated with this region. [ABSTRACT FROM AUTHOR]

Gya river, one of the main tributary of Indus, hosts several scattered palaeoflood deposits. The chronology, genesis and spatiotemporal relationships of these scattered deposits can throw light to the regional and global climatic fluctuations and their implications for the evolution of the valley. The valley structure of the Gya river consists of a broad middle reach alternating with narrow valleys and gorges in the upper and lower reaches which can be blocked by the slightest of sediment discharge damming the entire valley. The geomorphological, sedimentological and chronological study in this valley reveals multiple short-lived lake phases at 21–19.9 ka, 13 ka and 4.5 ka in the broader reach of the river during the transition periods when climate rapidly fluctuates between cold-dry and warm-wet. The damming in the valley is the result of the glacial lake outbursts in the head waters of the Gya catchment blocking the narrow lower reaches of the main channel by massive sediment. These lakes that are formed by valley damming contains ~108 m3 of water which subsequently breach out causing significant geomorphic changes on reach scale along the Gya river channel. [ABSTRACT FROM AUTHOR]

Tectonically active mountain regions such as Southwestern China are frequently hit by catastrophic floods caused by the breach of landslide dams. However, how these extreme events influence the morphodynamics of bedrock‐alluvial rivers is not well understood. More specifically, during a landslide dam outburst flood, bedrock can be eroded by intensive transport of sediment (i.e., tools effect), but can also be buried under thick alluvial cover (i.e., cover effect). Here, we implement the MRSAA‐c (Corrected Macro‐Roughness‐based Saltation‐Abrasion‐Alluviation) model to simulate the morphodynamics of bedrock‐alluvial rivers subject to landslide dam outburst flood(s). For a single dam breach event, the evolution of a bedrock‐alluvial river shows different characteristics at different time scales. At flood time scale (within one day), bedrock is eroded by about 10 mm. The landslide deposit then mainly provides a cover effect in the decadal‐century scale, which leads to a net uplift of the bedrock underneath, and thus the formation of a bedrock "bump." The bedrock profile returns to its initial equilibrium state at a millennial time scale, after the bedrock bump migrates upstream beyond the inlet. When subject to repeated dam breach events, the bedrock‐alluvial river can reach a dynamic equilibrium after sufficient time (tens of thousands of years). The equilibrium is characterized by sharply increased bedrock slope around the dam site, and gently decreased slope downstream. The impact of dam breach events is constrained within a scale of hundreds of kilometers, indicating the existence of a sedimentograph boundary layer. Plain Language Summary: Both seasonal floods and outburst floods (caused by e.g., breaching of landslide dams) contribute to bedrock channel downcutting in mountain rivers. Since outburst floods may carry much more water and sediment than seasonal floods, their impacts are important to understand how bedrock channels change in time. We implement a numerical model to simulate landslide dam outburst floods and associated channel evolution. Results show that a landslide dam outburst flood cuts into the bedrock channel during the flood event. In subsequent decades or centuries, landslide deposits can cover the channel and thus hinder bedrock downcutting, leading to a net rising of the bedrock surface. Eventually a bedrock bump moves upstream and beyond the river reach in tens of thousands of years. When outburst floods at a single site occur repeatedly, a dynamic balance is reached, with steeply increased bedrock slope near the dam site and gently decreased slope in the downstream. Our results show the great influence of landslide dam outburst floods on the behavior of bedrock channels. Key Points: A single dam breach event leads to bedrock incision over flood time scale, then forms a bedrock bump at decadal‐century scaleThe wave of alluvium from a dam breach shows diffusion when the bed is alluviated, and also advection when the bedrock is partially exposedUnder repeated dam breach events, the equilibrium profile of a bedrock‐alluvial river is characterized by a boundary layer and a knickpoint [ABSTRACT FROM AUTHOR]

The 2023 South-Lohnak Lake outburst flood event across the upstream Teesta catchment triggered short-lived, high-magnitude flooding and substantial socioeconomic disruption downstream. We performed landscape analysis across the Teesta catchment using topographic metrics to understand the geomorphic response to this extreme flood event. We estimated the various topographic metrics such as k sn anomaly (normalized channel steepness index) (∼500–2500 m0.9), SL-index (stream-length gradient index) (∼5000–7000 m), and K sn Q (channel steepness-weighted discharge) along the trunk channel draining through the South-Lohnak Lake, which shows high magnitude in flooded regions and numerous spikes in their longitudinal profiles across mass-wasting regions downstream. We used the K sn Q metric as an event characteristic to investigate the changes in the channel morphology due to high-magnitude flooding. There was a sudden increase in the magnitude of the K sn Q metric when the river traversed through the Chungthang dam over the high ranges of excess topography (∼0–1700 m). This shows that the downstream channel morphology changes rapidly during this flood event, and K sn Q is a significant indicator of geomorphic change. To understand the spatial relationship between the physical drivers that trigger this outburst flood, we quantified the causal relationship between glacial-hydrological drivers over the upstream Zemu-sub catchment. Our observations suggest that precipitation intensity and surface temperature had a significant direct causal influence on snowmelt and snow depth (in terms of water equivalent) over the event duration with a 5-day and 1-day lag composite. Through the inclusion of lagged composites and causal linkages, the drivers across the glaciated landscape serve as event anomalies, which trigger the glacier hillslope failure and subsequent high-magnitude outburst flooding. Therefore, we suggest that the interaction of topographic discontinuity with hydrological extremes and their causal interdependencies influences this outburst event in the upper reaches, followed by high-magnitude flooding in the downstream reaches of the Teesta River. However, the long-term geomorphic consequences of such events on the evolution of the regional landscape remain unclear. • The interaction of excess topography with hydrological extremes and their causal interconnections triggered short-lived high-magnitude flooding in the upper reaches of the Teesta catchment. • The inverse causal influence of surface temperature on snow-depth water equivalent has a significant impact at the base of a glacier hillslope, which over-steepens the channel gradient and causes it to collapse. [ABSTRACT FROM AUTHOR]

On August 7, 2021, a disastrous rainfall-induced landslide occurred at Mt. Silabaku in the Yusui Stream source area, Taiwan. The huge deposits from the Silabaku landslide severely damaged the Minbaklu Bridge, and minor collapses in the Yusui Stream disrupted the only major road, Expressway No. 20. More than 400 residents from the three adjacent villages were stranded for 19 days in the mountainous area. The analysis of rainfall data, seismic records, topographic and geologic data, and historical orthoimages of the slope conditions, landform evolution, and current status of the Silabaku landslide revealed that the landslide resulted from slate avalanche induced by heavy rains, with local failures during Typhoon Morakot in 2009. Based on the high-resolution topographic data of light detection and ranging and an unmanned aerial vehicle, the debris deposits accumulated along the Yusui Stream were estimated to be above 8 million m3. In addition, a fan volume of 2 million m3 changed the flow line of the Laonong River, and the thick deposits pose a considerable threat to the safety of the nearby villages and infrastructures. [ABSTRACT FROM AUTHOR]

We describe and model the evolution of a recent landslide, tsunami, outburst flood, and sediment plume in the southern Coast Mountains, British Columbia, Canada. On November 28, 2020, about 18 million m3 of rock descended 1,000 m from a steep valley wall and traveled across the toe of a glacier before entering a 0.6 km2 glacier lake and producing >100‐m high run‐up. Water overtopped the lake outlet and scoured a 10‐km long channel before depositing debris on a 2‐km2 fan below the lake outlet. Floodwater, organic debris, and fine sediment entered a fjord where it produced a 60+km long sediment plume and altered turbidity, water temperature, and water chemistry for weeks. The outburst flood destroyed forest and salmon spawning habitat. Physically based models of the landslide, tsunami, and flood provide real‐time simulations of the event and can improve understanding of similar hazard cascades and the risk they pose. Plain Language Summary: Glacier retreat exposes unstable slopes that can suddenly fail. We describe and model one such event with far‐reaching consequences. The landslide mass (50 million tonnes, equivalent to the combined mass of all Canadian automobiles) entered and suddenly drained a 0.6‐km2 alpine lake in the southern Coast Mountains of British Columbia. Displaced water destroyed salmon‐spawning habitat over a distance of 8.5 km and created a plume of sediment and organic matter more than 60 km from the head of the fjord into which the floodwaters discharged. Physically based models are able to simulate the hazard cascade, and such models could be used to improve hazard and risk assessments of these events under future climate change. Key Points: Rapid glacier retreat created conditions that allowed a geohazard cascade to occur in a steep mountain valleyWe describe and model a recent landslide, tsunami, and outburst flood that typifies a deglacial geohazard cascadeFuture work should predict new glacial lakes and use physically based models to simulate the slope, tsunami, and outburst flood hazards [ABSTRACT FROM AUTHOR]

[Display omitted] • A landslide occurred 13 ka ago in Wenchuan blocked the Minjiang River. • The more than 200 m high dam forced 53 km backwater and kept steady for about 8 ka. • The lake outburst at ∼ 4.7 ka, with peak discharge 7 times of the largest gauged flood. • Culture sites including Yingpanshan flourished in the shoreline of the lake. Dammed lakes are a common phenomenon in high relief river valleys and, depending on various circumstances, they can either remain stable for thousands of years or outburst as soon as they overtop. Here, we present geological evidence of a landslide dam (barrier lake) on the Minjiang River in Wenchuan, Sichuan province, on the eastern Qingzang Plateau, that formed a 27 km2 paleo-lake around 13 ka. This barrier lake was stable for about eight thousand years until the dam broke around 4.7 ka, creating an outburst flood with a peak discharge of 26,000 m3/s. After the barrier lake drained out into the plain, the 15 Neolithic sites situated along the banks of this paleo-lake that date from 5.3 to 4.7 ka and include the Yingpanshan site were rapidly abandoned. From this evidence, we argue that this barrier lake and the subsequent outburst flood highlight how local changes in the geomorphology of landforms during the Holocene can shape settlement patterns and migrations, exploring the cultural exchanges between the Majiayao Culture of North China and the Baodun Culture of the Chengdu Plain as a case study of this phenomenon. [ABSTRACT FROM AUTHOR]

• Dam breach and flood routing processes of landslide dams are studied by physical and numerical models. • Numerical simulations indicated that the evolution of breach morphology was influenced by hydrodynamics. • Channel terrain controls the spatial and temporal distribution of the flow velocity. • Channel topography and flood hydraulics are essential factors affecting deposition distribution. • Upstream inflow significantly affects landslide dam breach and flood propagation processes. Landslide dams are common hazards in mountainous regions, and outburst floods following dam breaches significantly impact people and infrastructure along rivers. The breaching processes associated with landslide dams and the interactions of outburst floods with channel terrain are very complex. The lack of event-based observations hampers the understanding of the mechanisms underlying these processes. Here, we combine a large-scale field experiment conducted on a natural river channel with a 3D numerical simulation. The numerical model was calibrated by a flume test and validated by experimental data from the large-scale physical model, demonstrating that it can accurately simulate the breaching process of landslide dams. The results show that the channel terrain controls the spatial and temporal distribution of the flow velocity and depth of the breach flood. The area of high flow velocity is mainly located in the block-confined narrow channel. In addition, outburst floods with high sediment concentrations led to extensive deposition in the downstream channels. The deposition thickness generally decreases in the flow direction and is affected by the channel topography. The measured changes in the particle size distribution of bed sediment before and after the dam breach support our findings. Moreover, the effect of upstream inflow on breach discharge was also investigated. It was found that the increasing inflow rate significantly accelerated the breaching process and increased the peak discharge. Our study enhances the understanding of the breach mechanism and reveals the connection between the hydrodynamic processes and geomorphic effects of breach floods, which can provide insight into the risk assessment of landslide dam hazards. [ABSTRACT FROM AUTHOR]

• The flood tended to change the planimetric form of the UR channel while to change the dimension of the DR channel. • There are three channel migration patterns: channel straightening, meander bend migration, and erosion at concave bank while deposition at convex bank. • The peaks of the bank displacement and the shear stress are staggered and the mean staggered distance is 846 m. • The maximum relative migration rate for both UR and DR occurs when (W/R)·U ∼ 1. High-energy outburst floods triggered by failure of landslide- and glacial dams have strongly impacted rivers in eastern Tibet, affecting river geomorphology, human settlements, and infrastructure. However, understanding the impact of these extreme outburst floods on river landscape evolution is hampered due to lack of event-based observations. The second-largest modern outburst flood recorded in the Yigong River in 2000 provides an opportunity to investigate changes in river morphology caused by such extreme floods. By combining remote sensing images from 1984 to 2020 and 2-D numerical flood simulations with the Hydrologic Engineering Center's River Analysis System (HEC-RAS) software, we show that the riverbank displacement during the Yigong outburst flood (YGOF) is an order of magnitude greater than that in other periods. Channel migration and variation in channel width in the upper reach (UR) straths are greater than those in the down reach (DR) gorges, and the curvature of the UR was greatly reduced by the YGOF while that of the DR increased. Furthermore, the channel morphology after the YGOF has become more dynamic. The relationship among the product of the dimensionless curvature (W/R), simulated flow velocity (U) and relative migration rate (M/W) in both the UR and DR follow similar distribution patterns, in which the maximum relative migration rates in the UR and DR occur where (W/R) · U ∼ 1. The results quantify the impact of the 2000 YGOF on the fluvial systems, which demonstrates that high-energy outburst floods play an extremely important role in mountainous river morphology and promotes our understanding of hydraulic process and geomorphological effects of paleofloods. [ABSTRACT FROM AUTHOR]