Magnitude Of Debris Flows Research Articles

New rulers for estimating the magnitude of catastrophic debris flows

New rulers for estimating the magnitude of catastrophic debris flows

Constructing Rainfall Threshold for Debris Flows of a Defined Hazardous Magnitude

Debris flow can cause damage only when its discharge exceeds the drainage capacity of the prevention engineering. At present, most rainfall thresholds for debris flows mainly focus on the initiation of debris flow and do not adequately consider the magnitude and drainage measures of debris flows. These thresholds are likely to initiate numerous warnings that may not be related to hazardous processes. This study proposes a method for calculating the rainfall threshold that is related to a defined level of debris flow magnitude, over which certain damage may be caused. This method is constructed by using the transient rainfall infiltration analysis slope stability model (TRIGRS) and the fluid dynamics process simulation model (MassFlow). We first use the TRIGRS model to analyze slope stability in the study area and obtain the distribution of unstable slopes under different rainfall conditions. Afterward, the MassFlow model is employed to simulate the movement process of unstable slope units and to predict the depositional processes at the mouth of the catchment. Lastly a rainfall threshold is constructed by statistically analyzing the rainfall conditions that cause debris flows flushing out of the given drainage ditch. This method is useful to predict debris flow events of a hazardous magnitude, especially for areas with limited historical observational data.

Factors controlling volcanic debris flows two decades after the 1990–1995 eruption at Mount Unzen in Japan

Rainfall-triggered volcanic debris flows (VDFs), or lahars, frequently occur during and/or immediately after eruptions, often because of decreases in the infiltration capacity resulting from the deposition of fine volcanic materials. Although the infiltration capacity recovers and the frequency of VDFs decreases within a decade, VDFs have continued to occur over several decades after the eruption of some volcanoes. No study has yet identified the factors controlling sediment discharge of VDFs for a specific volcano after the recovery of infiltration capacity to the pre-eruptive level. We analyzed topographic changes and sediment volumes between 2003 and 2020, using 16 digital elevation models (DEMs) and orthophotographs from aerial lidar scanning for the Tansandani and Gokurakudani gullies of Mount Unzen where the infiltration capacity has recovered after the most recent eruption in 1990–1995. We also analyzed the frequency and magnitude of VDFs in 2003–2020. Topographic changes differed before and after three large-scale VDFs in 2015 and 2016. The total volume of the three VDFs was 140 × 103 m3. Between 2003 and 2014, the width and depth of the gullies continuously increased, with the changes in the intervals of successive DEMs averaging below 5 m. Between 2017 and 2020, gully cross-sections remained U-shaped and changed little, while the channel gradient gradually decreased. U-shaped gullies promote the dispersion of VDFs and their gentle slopes reduce the velocity of VDFs. As a result, the frequency and magnitude of VDFs decreased, and deposition and erosion in the gullies were almost balanced between 2017 and 2020. Thus, we conclude that topography is the main factor that controls the occurrence and magnitude of debris flows and therefore sediment discharge after the recovery of infiltration capacity to the pre-eruptive level.

Spatio-temporal mapping and long-term evolution of debris flow activity after a high magnitude earthquake

A high-magnitude earthquake can trigger substantial amounts of co-seismic debris and unstable hillslopes in mountainous regions, which leads to a significant increase in the frequency and magnitude of post-earthquake debris flows. The long-term impacts of earthquakes on the debris flow activity have not been fully understood, primarily because of a lack of multi-temporal mapping inventories of debris flow activity. Therefore, it is crucial to analyze the spatio-temporal evolution of debris flow activities and their driving factors after the 2008 Wenchuan earthquake for disaster risk management and mitigation from a dynamic perspective. We used the fusion of remote sensing data to develop the first systematic multi-temporal inventory of debris flow activity to quantify changes in spatial and temporal activity of post-earthquake debris flows from 2008 to 2021. Subsequently, we examined the control of material sources, topographic and hydrological factors on the evolution of post-earthquake debris flow activities. A close relationship between the decline in debris flow activity and the reduced active material sources was observed in both time and space, so the supply capacity of hillslope deposits after the earthquake was the first-order indicator controlling post-earthquake debris flow activity. With the progressive depletion and stabilization of co-seismic landslide materials over time, topographic and hydrological factors occupied a dominant role in the occurrence of debris flow. In general, an oscillatory decay pattern of debris flow activity over time was observed after the 2008 Wenchuan earthquake. We observed that the period of post-earthquake debris flow activity varied considerably in different regions attributed to the magnitude of the earthquake, and the geological and climatic environments. Based on the debris flow channel disturbances, the dynamic evolution of landslide materials and hydrodynamics, we divided the debris flow active period after the Wenchuan earthquake into three phases: high activity period (lasting 2 ∼ 5 years), medium activity period (lasting 3 ∼ 10 years), and low activity period (lasting 9 ∼ 18 years). Following the above three activity periods, the debris flow activity will evolve into a steady period. We also proposed a quantitative model for debris flow activeness in the main Wenchuan earthquake-affected area, which predicts that the activeness of debris flows will tend to pre-earthquake levels in 46 years following the earthquake.

Risk Assessment of Debris Flows Along the Karakoram Highway (Kashgar-Khunjerab Section) in the Context of Climate Change

The Karakoram highway (KKH) is renowned for its complex natural environment and geological conditions. The climate changes drastically and directly influences the frequency and magnitude of debris flows in this region, resulting in significant casualties and economic losses. However, the risk assessment of debris flows along the KKH in the context of climate change has been rarely explored. Therefore, in this study we used the debris flow data, historical meteorological data and future climate prediction data to assess the debris flow risk of the study region during the baseline period (2009–2018), 2025s (2021–2030), 2035s (2031–2040) and 2045s (2041–2050) under the Representative Concentration Pathway 8.5 scenario. The results show that the risk of debris flows increases with climate change, with the highest risk level in the 2025s. Among different parts of this highway, the upper reaches of the Ghez River and the second half of Tashkorgan-Khunjerab are the sections with the highest risk. These findings are helpful for debris flow prevention and can offer coping strategies for the existing line of the KKH. They also provide some reference for the renovation, improvement, operation, and maintenance of the KKH.

Modelling the evolution of debris flows after the 2008 Wenchuan earthquake

Strong earthquakes can trigger extensive landslides, generating abundant loose deposits that are prone to be remobilized as debris flows by subsequent rainfall events. The magnitude-frequency distribution of post-earthquake debris flows often changes significantly over time. However, the key parameters controlling the post-earthquake debris flow activity and magnitude were poorly understood. In this study, we selected a representative catchment in the Wenchuan earthquake-hit region to study the activity of debris flows from 2008 to 2020, as well as the changes in parameters due to material depletion, grain size coarsening and vegetation recovery. Samples were collected from the field in different years after the earthquake and were tested in the laboratory to obtain their geotechnical properties. A numerical model was improved by considering the changing properties to fit better to dynamic conditions after the earthquake. We found that the particle size of the loose material gradually coarsened, the shear strength showed an increasing trend, while the erodibility coefficient of loose material showed a decreasing trend. The evolution of these parameters controlled the magnitude-frequency of post-earthquake debris flows and also changed their behavior from slope failures in the first few years after the earthquake to concentrated surface runoff erosion (channel erosion) in later years. The results have an important implication for risk assessment and prediction of future debris flows in earthquake-prone regions.

Evolution of debris flow activities in the epicentral area, 10 years after the 2008 Wenchuan earthquake

The occurrence of debris flows after the earthquake not only damaged infrastructure but also caused large numbers of casualties. Analyzing the evolution of debris flows after the Wenchuan earthquake is significant for understanding the long-term impacts of the earthquake on the subsequent debris flow activities. This study made a continuous track of mass movements of loose deposits in all debris flow gullies from 2008 to 2018 along Provincial Road 303 (PR303) after the Wenchuan earthquake according to remote sensing images interpretation and field investigations. Four aspects, i.e. vegetation recovery, source materials, runout characteristics, and human activities were analysed to describe the evolution laws based on the 10-year inventory of debris flows. The loose deposits on the hillslope were found to be gradually transported into channels. A declining trend of mass transport rate from 2010 (24.5%) to 2018 (6.5%) was found. The occurrence frequency, magnitude, and mobility of debris flows were found to decrease. Recovery of vegetation made the loose deposits stable, whereas human activities of excavation of debris flows deposits promoted the occurrence of landslides. Three stages were divided for debris flows activities: the active period (2010–2014), the unstable period (2014–2017), and the recession period (after 2017). The decreasing trend of debris flows in the future is predicted.

Study of debris flow peak discharge at Kamikamihorizawa Creek

Prediction of the peak discharge of debris flow is one of the most important factors in mitigating debris flow disasters. Currently, empirical methods based on the relationship between the peak discharge and total debris flow volume (magnitude) for many debris flow events are used to estimate peak discharge for planning or designing debris flow countermeasures in Japan. In order to estimate the peak discharge of debris flow with high accuracy, the debris flow monitoring sensors with load cell and pressure sensor (hereafter referred to as a DFLP system), was installed to evaluate various characteristics during floods involving debris flows at Kamikamihorizawa Creek. During the debris flow that occurred on August 29, 2019 at Kamikamihorizawa Creek, seven surges corresponding to debris flow events were monitored by the DFLP system for a 20-minute period. Based on the relationship between the peak discharge and magnitude, the observation data of Kamikamihorizawa Creek, Illgraben and Schipfenbach has been added to previous data from the literature and the relationship has been updated. The peak discharge and magnitude of debris flow on August 29, 2019 at Kamikamihorizawa Creek exhibited distributions similar to past observation data, which shows that the relationship between peak discharge and magnitude follows a similar trend.

Future global debris flow susceptibility considering climate change, wildfire probability, and glacier retreat

The present-day impact of climate changes on debris flow magnitude, frequency, and susceptibility has been demonstrated in North and South America, Europe, Asia, and New Zealand. Such impacts are expected to increase under future emission scenarios. Future global debris flow susceptibility models provide an international perspective on areas worthy of further, more detailed analyses with regard to geographic changes in global debris flow susceptibility. In this study, future global debris flow susceptibility models are developed under RCP 2.6 and 8.5 IPCC Climate Change Scenarios. These models were further augmented with wildfire probability, and areas of potential glacier retreat, both of which can act as amplifiers to debris flow susceptibility. The results are projected against future urban centers, for a spatial view on potential human vulnerability. Key findings are (1) wildfire acts as a significant amplifier in area and magnitude of debris flow susceptibility in all modeling scenarios, (2) greater than 50% of the studied glaciers reside within higher susceptibility zones when wildfire is not considered, and greater than 75% when wildfire probability is considered, (3) 76 of the studied glaciers are within 5 km of eleven urban centers, (4) 11% of these “urban” glaciers are in higher susceptibility zones when wildfire probability is not considered, and 51% are in higher susceptibility zones when wildfire is considered, (5) about 12% of future urban centers will reside within higher susceptibility zones under both future climate change scenarios. Consideration of these factors, together with traditional environmental factors and triggers, and findings by local and regional glacier-related debris flow researchers, suggests a new paradigm in modeling debris flow susceptibility, at any scale.

Keynote lecture. Forecasting the inundation of postfire debris flows

In the semi-arid regions of the western United States, postfire debris flows are typically runoff generated. The U.S. Geological Survey has been studying the mechanisms of postfire debris-flow initiation for multiple decades to generate operational models for forecasting the timing, location, and magnitude of postfire debris flows. Here we discuss challenges and progress for extending operational capabilities to include modeling postfire debris-flow inundation extent. Analysis of volume and impacted area scaling relationships indicated that postfire debris flows do not conform to assumptions of geometric self-similarity. We documented sensitivity of impacted areas to rainfall intensity using a candidate methodology for generating inundation hazard assessments. Our results emphasize the importance of direct measurements of debris-flow volume, inundated area, and high temporal resolution rainfall intensity.

Initiation and runout characteristics of partially saturated debris flows in Ohya landslide scar, Japan

A partially saturated flow, which has an unsaturated layer in its upper part, has been monitored in the steep initiation zones of debris flows. Understanding the initiation and runout characteristics of partially saturated flows is essential for predicting the timing and magnitude of downstream debris flows. Monitoring performed using time-lapse cameras and water pressure sensors in the Ohya landslide scar in central Japan allowed us to obtain data on a series of partially saturated debris flow surges from initiation to termination on July 6, 2020, and July 13, 2021. Debris flow surges were mainly induced by repetitive mass movement of sediment deposit caused by the overland flow erosion of channel deposits, channel deposit slides, and water and sediment supply from channel banks and tributaries. The excess pore water pressure in a partially saturated flow on July 6, 2020, was clearly higher than that on July 13, 2021. Rainfall patterns, which control the water content in channel deposits, and the flow height likely affect the magnitude of the excess pore water pressure in partially saturated flows.

Hydrologic recovery after wildfire: A framework of approaches, metrics, criteria, trajectories, and timescales

Abstract Deviations in hydrologic processes due to wildfire can alter streamflows across the hydrograph, spanning peak flows to low flows. Fire-enhanced changes in hydrologic processes, including infiltration, interception, and evapotranspiration, and the resulting streamflow responses can affect water supplies, through effects on the quantity, quality, and timing of water availability. Post-fire shifts in hydrologic processes can also alter the timing and magnitude of floods and debris flows. The duration of hydrologic deviations from a pre-fire condition or function, sometimes termed hydrologic recovery, is a critical concern for land, water, and emergency managers. We reviewed and summarized terminology and approaches for defining and assessing hydrologic recovery after wildfire, focusing on statistical and functional definitions. We critically examined advantages and drawbacks of current recovery assessment methods, outline challenges to determining recovery, and call attention to selected opportunities for advancement of post-fire hydrologic recovery assessment. Selected challenges included hydroclimatic variability, post-fire land management, and spatial and temporal variability. The most promising opportunities for advancing assessment of hydrologic recovery include: (1) combining statistical and functional recovery approaches, (2) using a greater diversity of post-fire observations complemented with hydrologic modeling, and (3) defining optimal assemblages of recovery metrics and criteria for common hydrologic concerns and regions.

Estimating the debris-flow magnitude using landslide sediment connectivity, Qipan catchment, Wenchuan County, China

In an earthquake disturbed basin, the earthquake-induced landslides’ connectivity with debris-flow channels determines the landslide sediment volume that is transported into debris flows, and the debris-flow magnitude as well. In this study, the index of sediment connectivity (IC) was used to analyze the evolution of landslide sediment transport capacity. A normalized IC was developed to quantify the movable volume of the landslide sediment available for debris flows. Combined with a landslide empirical relationship, sediment connectivity calculation, particle model, and numerical algorithm, two debris-flow events after the 2008 Wenchuan earthquake were simulated for the Qipan catchment. The results show good agreement between the simulated and observed debris-flow magnitudes of peak discharges, inundated areas, and depths with approximately 9% average error, which indicates that the normalized landslide connectivity index is suitable for estimating the movable volume. Moreover, the IC and landslide characteristics (area and location) showed significant negative correlations, indicating that the spatial connectivity of landslide sediment generally decayed over time, with a corresponding reduction in debris-flow magnitude.

Sediment Recruitment and Redistribution in Mountain Channel Networks by Post‐Wildfire Debris Flows

AbstractWe used lidar differencing and field observations to map volumes, and interpret the origins of, sediment mobilized from mountain canyons by large post‐wildfire debris flows near Montecito, CA, USA in 2018. The debris flows progressively entrained and partially redeposited 550,000 m3 of previously stored channel sediments throughout the canyon networks. The observations that scour depths and volumes were highest where the largest volumes of bouldery colluvium and debris‐flow deposits had accumulated, and that scour persisted beyond the mountain front, indicates that debris‐flow volumes in this extreme event were ultimately controlled by the coarse sediment reservoir available for scour. Because the volumes of available stored sediment result from the stochastic interaction of colluvial mass wasting, the magnitude and frequency of previous debris flows, and the accommodation space provided by valley morphology, the study reinforces the importance of estimating stored sediment volumes when developing debris‐flow hazard assessments.

Quantitative investigation of the spatiotemporal evolution of coseismic deposits and its influence on the magnitude of postseismic debris flows in the Hongchun valley, Wenchuan County, China

Quantitative investigation of the spatiotemporal evolution of coseismic deposits and its influence on the magnitude of postseismic debris flows in the Hongchun valley, Wenchuan County, China

Quantitative analysis of artificial dam failure effects on debris flows – A case study of the Zhouqu ‘8.8’ debris flow in northwestern China

Artificial dams are one of the most common hydraulic structures for mitigating debris flow disasters in alpine valley regions. However, performance alteration and failure after successive debris flows can lead to dam failure, releasing large amounts of materials within a very short time; moreover, the contribution of artificial dam failures to debris flows is poorly understood. This study quantitatively analyzed the artificial dam failure effects based on the numerical simulations of the Zhouqu '8.8' debris flow, with three scenarios: all nine dams failed (S1); no dams were ever built (S2); all nine dams remained intact (S3). The results showed that artificial dam failures had a significant amplifying effect on the magnitude of a debris flow. The maximum velocity and flow depth decreased by 20% and 11.2% if all the dams did not collapse; comparison of S1 and S2 showed that discharge and velocity at the front of the debris flow increased by 54.6% and 89%, the bulk density and yield stress increased by 3.3% and 5.7%, due to artificial dam failures. This could increase the destructive capacity of a debris flow and the possibility of a river blockage. A single artificial dam failure could locally amplify the magnitude of debris flow. Overall, on the catchment scale, the magnitude of a debris flow was dominated by topography and channel geometry, which can reduce the amplification effect of dam failures at locations where the channel was curved. However, where the channel was straight and flat, the flow velocity and discharge increased cumulatively by 3 m/s and 637 m3/s due to cascading failure. In addition, a comprehensive scheme combining ecological and engineering measures to mitigate debris flow disasters is discussed. This quantitative study is important and urgent needed to understand the amplification effect of dam failures and to implement debris flow mitigation in alpine valley regions.

Forecasting the Frequency and Magnitude of Postfire Debris Flows Across Southern California

AbstractSouthern California has a long history of damaging debris flows after wildfire. Despite recurrent loss, forecasts of the frequency and magnitude of postfire debris flows are not available for the region like they are for earthquakes. Instead, debris flow hazards are typically assessed in a reactive manner after wildfires. Such assessments are crucial for evaluating debris flow risk by postfire emergency response teams; however, time between the fire and first rainstorm is often insufficient to fully develop and implement effective emergency response plans like those in place for earthquakes. Here, we use both historical distributions of fire and precipitation frequency and empirical models of postfire debris flow likelihood and volume to map the expected frequency and magnitude of postfire debris flows across southern California. We find that at least small debris flows can be expected almost every year, while major debris flows capable of damaging 40 or more structures have a recurrence interval between 10 and 13 years, a return interval that is comparable to a magnitude 6.7 earthquake. A sensitivity analysis to possible future changes in current fire and precipitation regimes indicates that debris flow activity in southern California is more sensitive to increases in precipitation intensity than increases in fire frequency and severity. Projected increases in rainfall intensity of 18% result in an overall 110% increase in the probability of major debris flows. Our results, in combination with an assessment of exposure, can be used to prioritize watersheds for further analysis and possible prefire mitigation.

Debris flow magnitude, frequency, and precipitation threshold in the eastern North Cascades, Washington, USA

We examine the magnitude, frequency, and precipitation threshold of the extreme flood hazard on 37 low-order streams in the lower Stehekin River Valley on the arid eastern slope of the North Cascades. Key morphometric variables identify the magnitude of the hazard by differentiating debris flood from debris flow systems. Thirty-two debris flow systems are fed by basins 10°. Five debris flood systems have larger drainage areas and debris fans with slopes 7–10°. The debris flood systems have Melton ruggedness ratios from 0.42–0.64 compared to 0.78–3.80 for debris flow basins. We record stratigraphy at seven sites where soil surfaces buried by successive debris flows limit the age of events spanning 6000 years. Eighteen radiocarbon ages from the soils are the basis for estimates of a 200 to1500-year range in recurrence interval for larger debris flows and a 450 ± 50-year average. Smaller events occur approximately every 100 years. Fifteen debris flows occurred in nine drainage systems in the last 15 years, including multiple flows on three streams. Summer storms in 2010 and 2013 with peak rainfall intensities of 7–9 mm/h sustained for 8–11 h triggered all but one flow; the fall 2015 event on Canyon Creek occurred after 170 mm of rain in 78 h. A direct link between fires and debris flows is unclear because several recent debris flows occurred in basins that did not burn or burned at low intensity, and basins that burned at high intensity did not carry debris flows. All but one of the recent flows and fires occurred on the valley’s southwest-facing wall. We conclude that fires and debris flows are linked by aspect at the landscape scale, where the sunny valley wall has flashy runoff due to sparse vegetation from frequent fires.

Debris flows in the Lushan earthquake area: formation characteristics, rainfall conditions, and evolutionary tendency

Debris flows often occur in the mountainous watersheds of earthquake-affected areas, and in the Lushan earthquake area of southwestern China, they have become a significant hazard. In this study, the influencing factors and spatial distribution of debris flows were analyzed through a review of their occurrence history. Debris flows are mainly distributed in the northwestern part of the study area, which hosts the greatest density of active faults. The debris flows are generally formed by the ‘progressive bulking’ effect in channels, and deep incision, lateral erosion, and blockage breaking are common processes that amplify the magnitude of such debris flows. Rainfall thresholds for different types of debris flow were proposed to explain the spatial differences between debris-flow regions, and the temporal variations of those thresholds highlighted how the rainfall conditions required for the occurrence of debris flows have changed. Natural vegetation recovery, reduction in the availability of solid material, and artificial debris-flow control projects play important roles in raising the threshold of the rainfall conditions required for triggering debris flows.

Time Since Burning and Rainfall Characteristics Impact Post-Fire Debris-Flow Initiation and Magnitude

ABSTRACTThe extreme heat from wildfire alters soil properties and incinerates vegetation, leading to changes in infiltration capacity, ground cover, soil erodibility, and rainfall interception. These changes promote elevated rates of runoff and sediment transport that increase the likelihood of runoff-generated debris flows. Debris flows are most common in the year immediately following wildfire, but temporal changes in the likelihood and magnitude of debris flows following wildfire are not well constrained. In this study, we combine measurements of soil-hydraulic properties with vegetation survey data and numerical modeling to understand how debris-flow threats are likely to change in steep, burned watersheds during the first 3 years of recovery. We focus on documenting recovery following the 2016 Fish Fire in the San Gabriel Mountains, California, and demonstrate how a numerical model can be used to predict temporal changes in debris-flow properties and initiation thresholds. Numerical modeling suggests that the 15-minute intensity-duration (ID) threshold for debris flows in post-fire year 1 can vary from 15 to 30 mm/hr, depending on how rainfall is temporally distributed within a storm. Simulations further demonstrate that expected debris-flow volumes would be reduced by more than a factor of three following 1 year of recovery and that the 15-minute rainfall ID threshold would increase from 15 to 30 mm/hr to greater than 60 mm/hr by post-fire year 3. These results provide constraints on debris-flow thresholds within the San Gabriel Mountains and highlight the importance of considering local rainfall characteristics when using numerical models to assess debris-flow and flood potential.