Variability in rainfall threshold for debris flow after the Chi-Chi earthquake in central Taiwan, China

The purpose of this study is to analyze variability in rainfall threshold for debris flow （critical rainfall for debris flow triggering） after the ML 7.3 Chi-Chi earthquake in central Taiwan in 1999. Two study sites with different geological conditions were surveyed in the earthquake area. Streambed surveys were conducted to continuously monitor debris flows between 1999 and 2006. During the 7-year study period, every debris flow event was identified, and the streambed characterized. Results show that the rainfall threshold for debris flow was remarkably lower just after the Chi-Chi Earthquake, but gradually recovered. To date, this rainfall threshold is still lower than the original level prior to the earthquake. This variability in rainfall threshold is closely related to the mount of sediment material in the initiation area of debris flow, which increased rapidly due to landslides resulting from the earthquake. With the increase in sediment material, the rainfall threshold was lowered severely during the first year following the Chi-Chi earthquake. However, heavy rainfalls mobilized the sediment material, causing debris flows and transporting sediment downstream. With the decrease in sediment material, the rainfall threshold recovered gradually over time. Furthermore, debris flows occurred only in the subbasins that had sufficient sediment material to cause significant movement. Hence, these results confirm that the sediment material in the initiation area of debris flow is a crucial component of the rainfall threshold for debris flow.     

Estimating the size and travel distance of Klapperhorn Mountain debris flows for risk analysis along railway, Canada

Debris flows occurring on Klapperhorn Mountain in the Yellowhead Pass in the Canadian Rocky Mountains pose a significant hazard to railway operations at the base of the mountain. The size （volume） and travel distance of these debris flows play an important role in assessing the risk to the railway. GIS analysis, airphoto interpretation together with field work were undertaken on two debris flows located at track mileage 54.0 and 54.3. Characteristics of these two debris flow events were analyzed, including debris flow path morphology and event behavior. Their sizes and travel distances were estimated using an empirical-statistical model （UBCDFLOW） under different initiation conditions. Their potential impact on the railway bridge was evaluated using a bridge blockage ratio.     

Experimental study of debris flow caused by domino failures of landslide dams

The formation of landslide dams is often induced by earthquakes in mountainous areas.The failure of a landslide dam typically results in catastrophic flash floods or debris flows downstream.Significant attention has been given to the processes and mechanisms involved in the failure of individual landslide dams.However,the processes leading to domino failures of multiple landslide dams remain unclear.In this study,experimental tests were carried out to investigate the domino failure of landslide dams and the consequent enlargement of downstream debris flows.Different blockage conditions were considered,including complete blockage,partial blockage and erodible bed（no blockage）.The mean velocity of the flow front was estimated by videos.Total stress transducers（TSTs）and Laser range finders（LRFs） were employed to measure the total stress and the depth of the flow front,respectively.Under a complete blockage pattern,a portion of the debris flow was trapped in front of each retained landslide dam before the latter collapsed completely.This was accompanied by a dramatic decrease in the mean velocity of the flow front.Conversely,under both partial blockage and erodible bed conditions,the mean velocity of the flow front increased gradually downward along the sloping channel.Domino failures of the landslide dams were triggered when a series of dams（complete blockage and partial blockage） were distributed along the flume.However,not all of these domino failures led to enlarged debris flows.The modes of dam failures have significant impacts on the enlargement of debris flows.Therefore,further research is necessary to understand the mechanisms of domino failures of landslide dams and their effects on the enlargement of debris flows.     

Possible effect of ENSO on annual sediment discharge of debris flows in the Jiangjia Ravine based on Morlet wavelet transforms

The multi-time-scale structures of an annual sediment discharge series of debris flow in the Jiangjia Ravine and the Southern Oscillation index are analysed using the method of Morlet wavelet transformations. The possible effects of E1 Nirio episodes on the annual sediment discharge are discussed by comparing the period variations of ENSO and the discharge. The results show that the annual sediment discharge series of debris flow is related to E1 Nifio episodes. Generally, the annual sediment discharge of debris flow is less than usual during an E1 Nifio episode and debris flow is less active. On the contrary, the annual sediment discharge of debris flows is greater than usual during a La Nifia episode and debris flows are more frequent. There is a relationship between the annual sediment discharges of debris flow in the Jiangjia Ravine and the summer Southern Oscillation index, with both having quasi-periodic variations of 2 and 5-6 years.     

Geomorphic evidence of ancient catastrophic flow type landslides in the mid-mountain ridges of the Western Flysch Carpathian Mountains （Czech Republic）

Ancient flow type landslides are relatively frequent fossil forms of the relief in mid-mountain conditions of the Czech Carpathian Mountains. Sixty rather distinctive displays of debris flows, rock avalanches, and debris avalanches have been mapped in the uppermost part of the territory. Unlike contemporary sporadic and low volume debris flows, ancient （Pleistocene and Lower Holocene） accumulations are a few orders of magnitude more extensive and were of considerable geomorphologic significance in forming the steep sections of mountain valleys and slopes. This geomorphic pattern does not hold for flow type slides, the source of which is material released as a consequence of numerous deep-seated landslides. Due to deep disruption of slopes, a few high-magnitude flow type landslides （e.g., rock avalanches）, quite rare in flysch mid-mountain conditions, also occurred in the Late Holocene.     

Predicting global landslide spatiotemporal distribution： Integrating landslide susceptibility zoning techniques and real-time satellite rainfall estimates

Landslides triggered by rainfall can possibly be foreseen in real time by jointly using rainfall intensity-duration thresholds and information related to land surface susceptibility. However, no system exists at either a national or a global scale to monitor or detect rainfall conditions that may trigger landslides due to the lack of sufficient ground-based observing network in many parts of the world. Recent advances in satellite remote sensing technology and increasing availability of high-resolution geospatial products around the globe have provided an unprecedented opportunity for such a study. In this paper, a framework for developing a preliminary real-time prediction system to identify where rainfall-triggered landslides will occur is proposed by combining two necessary components： surface landslide susceptibility and a real-time space-based rainfall analysis system （http：//trmm.gsfc.nasa.gov）. First, a global landslide susceptibility map is derived from a combination of semi-static global surface characteristics （digital elevation topography, slope, soil types, soil texture, land cover classification, etc.） using a GIS weighted linear combination approach. Second, an adjusted empirical relationship between rainfall intensity-duration and landslide occurrence is used to assess landslide hazards at areas with high susceptibility. A major outcome of this work is the availability for the first time of a global assessment of landslide hazards, which is only possible because of the utilization of global satellite remote sensing products. This preliminary system can be updated continuously using the new satellite remote sensing products. This proposed system, if pursued through wide interdisciplinary efforts as recommended herein, bears the promise to grow many local landslide hazard analyses into a global decision-making support system for landslide disaster preparedness and mitigation activities across the world.     

LANDSLIDE POTENTIALITY OF THE TSENGWEN RESERVOIR WATERSHED, TAIWAN, CHINA

To recognize the geographical characteristics of the landslide areas will be helpful for the watershed management in the reservoir watershed. According to the quantitative analysis, we‘ll take different scores and weighting for the potential parameters of the landslide areas in the Tsengwen reservoir watershed, and in the meanwhile, we‘ll extract the different factors, including the slope, aspect, altitude, soil and geological textures etc., and the results shown as maximum one-day rainfall, ratio of forests and average relief is the most affecting parameters on the potential risk map of landslide areas.     

四川及邻区地壳流与动力特征研究

Using the broadband seismic data of the regional stations in the Sichuan Digital Seismic Network and the mobile seismic stations in this region,the receiver function inversion method was adopted to study the characteristics of crustal flow and dynamic effects in Sichuan and adjacent areas. The results show that: Velocity in the crust and upper mantle of the Sichuan basin is significantly higher than that beneath the eastern margin of the Qinghai-Tibetan plateau. The velocity v_S is from 3. 6 to 3. 8km / s in the crust and4. 5- 4. 8km / s in the upper mantle beneath the basin,and there is no low-velocity layer in the crust. The lithology shows a hard block. The v_S velocity in the eastern margin of the Qinghai-Tibetan plateau is lower,with average v_Sof 3. 0- 3. 4km / s in the mid crust and4. 0- 4. 5km / s in the upper mantle. Low-velocity layers are distributed widely in the crust,most of which are in the mid crust at a depth of 20km- 40 km,and there are also a few low-velocity layers appearing in the upper crust at depths of 10km- 20 km and the lower crust at depths of 40km- 60 km. Affected by the northward pushing of the Indian plate,the eastward movement of the eastern margin of the Qinghai-Tibetan plateau is blocked by the hard Sichuan basin,producing a southward and southeastward component.Such movement process is produced by the complicated forces acting in this area. Just under the action of these forces, the eastern margin of the Qinghai-Tibetan plateau becomes a region with complicated geology and intensive earthquake activity. Obstructed by the hard Sichuan basin,the low-velocity crustal flow is delaminated and split into two or three upward and downward tributaries. The upward flow intruded into the upper crust,causing uplift of the earths urface,forming mountain crests; the downward flow intruded into the lower crust and upper mantle,resulting in thickening of the crust and depression of the Moho. The crustal flow in the eastern margin of the Qinghai-Tibetan plateau is mainly distributed along the active faults. The crustal flow flows out from the Qiangtang block in the middle part of the eastern margin of the Qinghai-Tibetan plateau,the mainstream flows along the NW-SE trending Xianshuihe fault zone,then turns NS and flows to the south along the Anninghe and Xiaojiang faults. There is another crustal flow in the north of the study area,flowing in the NE and E-W directions to the Longmenshan faults.     

Determination of the suspension competence of debris flows based on particle size analysis

The determination of the critical particle size between solid and fluid phases, i.e., the suspension competence, is fundamental for debris flow. A method for determining suspension competence based on particle size analysis is presented in this paper. Suspension competence of static experimental water-debris mixtures prepared with the sediment of Jiangjia Gully is -0.025 mm if the bulk density is less than 1,800 kg m-3 and it increases with bulk density of more concentrated mixtures. Suspension competence of natural debris flows in Jiangjia Gully increases exponentially with the bulk density. These two data sets are compared in order to understand the suspension mechanism. It is concluded that turbulence may play a leading role in particle suspension in non-viscous and sub-viscous debris flows, while in viscous debris flows both matrix strength and excess pore water pressure play important roles.     

Sensitivity analysis of influencing parameters on slit-type barrier performance against debris flow using 3D-based numerical approach

Extreme rainfall-induced debris flow can be catastrophic to an urban area,and installation of slit-type barriers can prevent such damage while minimizing negative impact on environments.However,the performance of slit-type barriers against debris flows remains poorly identified partly due to the innate complexity in interactions between debris flow and solid structure.This paper investigated the flow behaviors of debris affected by slit-type barriers using the computational fluid dynamics(CFD)method,in which the numerical model based on the volume of fluid method was verified using the physical modeling results.The sensitivity analysis was performed by building metamodels to determine the primary parameters influencing the barrier performance against debris flows among various variables,in which the effect of input properties and design parameters,particularly the soil concentration in fluidized debris,initial velocity and volume of debris,the barrier height,and the opening ratio,was evaluated from the perspectives of the flow energy reduction and debris trapping.The initial velocity and volume of debris were found to play a significant role in determining the debris flow characteristics.A decrease in the opening ratio in the channel primarily facilitated the energy reduction and trapping due to the reduced opening size.However,the barrier height exhibited a limited effect when the height was sufficiently high to block the debris flow volume.In addition,it was observed that the double barrier system effectively increased the energy reduction while keeping the benefit of open-type barrier.The developed simulation method and obtained results provide an effective tool and an insight that can contribute to an optimum design of the debris-flow barrier.     

Thresholds for post-wildfire debris flows: Insights from the Pinal Fire,Arizona, USA

Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff-generated debris flows. The initiation of debris flows in recently burned areas is well characterized by rainfall intensity-duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post-wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris-flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western United States where ID thresholds for post-wildfire debris flows are well established, namely the Transverse Ranges of southern California. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post-wildfire debris-flow studies in Arizona only constrain the peak rainfall intensity within debris-flow-producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within five burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm h⁻¹ lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15-min rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris-flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern California may be attributed, in large part, to differences in the hydraulic properties of burned soils. © 2019 John Wiley & Sons, Ltd.     

Estimation of discharges of water flows and debris floods in a small watershed

Floods in small mountainous watersheds cover a wide spectrum of flow. They can range from clear water flows and hyperconcentrated flows to debris floods and debris flows, and calculation of the peak discharge is crucial for predicting and mitigating such hazards. To determine the optimal approach for discharge estimation, this study compared water flow monitoring hydrographs to investigate the performance of five hydrological models that incorporate different runoff yields and influx calculation methods. Two of the models performed well in simulating the peak discharge, peak time, and total flow volume of the water flood. The ratio (γ) of the monitored debris flood discharge (Q_d) to the simulated water flow discharge (Q_w) was investigated. Qualitatively, γ initially increased with Q_w but then decreased when Q_w exceeded a certain threshold, which corresponded to rainfall of 95 and 120 mm in a 6- and 24-h event with a normal distribution of precipitation, respectively. The decrease might be attributable to a threshold of sediment availability being reached, beyond which increased flow rate is not matched by increased sediment input in the large watershed. Uncertainty of hydrological calculation was evaluated by dividing the catchment into sub-basins and adopting different rainfall time steps as input. The efficiency of using a distributed simulation exhibited marginal improvement potential compared with a lumped simulation. Conversely, the rainfall time step input significantly affected the simulation results by delaying the peak time and decreasing the peak discharge. This research demonstrates the applicability of a discharge estimation method that combines a hydrological water flow simulation and an estimation of γ. The results were verified on the basis of monitored flow densities and videos obtained in two watersheds with areas of 2.34 and 32.4 km².     

Predicting post-fire debris flow grain sizes and depositional volumes in the Intermountain West,United States

Post-fire debris flows represent one of the most erosive consequences associated with increasing wildfire severity and investigations into their downstream impacts have been limited. Recent advances have linked existing hydrogeomorphic models to predict potential impacts of post-fire erosion at watershed scales on downstream water resources. Here we address two key limitations in current models: (1) accurate predictions of post-fire debris flow volumes in the absence of triggering storm rainfall intensities and (2) understanding controls on grain sizes produced by post-fire debris flows. We compiled and analysed a novel dataset of depositional volumes and grain size distributions (GSDs) for 59 post-fire debris flows across the Intermountain West (IMW) collected via fieldwork and from the literature. We first evaluated the utility of existing models for post-fire debris flow volume prediction, which were largely developed for Southern California. We then constructed a new post-fire debris flow volume prediction model for the IMW using a combination of Random Forest modelling and regression analysis. We found topography and burn severity to be important variables, and that the percentage of pre-fire soil organic matter was an essential predictor variable. Our model was also capable of predicting debris flow volumes without data for the triggering storm, suggesting that rainfall may be more important as a presence/absence predictor, rather than a scaling variable. We also constructed the first models that predict the median, 16th percentile, and 84th percentile grain sizes, as well as boulder size, produced by post-fire debris flows. These models demonstrate consistent landscape controls on debris flow GSDs that are related to land cover, physical and chemical weathering, and hillslope sediment transport processes. This work advances our ability to predict how post-fire sediment pulses are transported through watersheds. Our models allow for improved pre- and post-fire risk assessments across diverse ranges of watersheds in the IMW.     

Uncertainty in floodplain delineation: expression of flood hazard and risk in a Gulf Coast watershed

This paper investigates the development of flood hazard and flood risk delineations that account for uncertainty as improvements to standard floodplain maps for coastal watersheds. Current regulatory floodplain maps for the Gulf Coastal United States present 1% flood hazards as polygon features developed using deterministic, steady‐state models that do not consider data uncertainty or natural variability of input parameters. Using the techniques presented here, a standard binary deterministic floodplain delineation is replaced with a flood inundation map showing the underlying flood hazard structure. Additionally, the hazard uncertainty is further transformed to show flood risk as a spatially distributed probable flood depth using concepts familiar to practicing engineers and software tools accepted and understood by regulators. A case study of the proposed hazard and risk assessment methodology is presented for a Gulf Coast watershed, which suggests that storm duration and stage boundary conditions are important variable parameters, whereas rainfall distribution, storm movement, and roughness coefficients contribute less variability. The floodplain with uncertainty for this coastal watershed showed the highest variability in the tidally influenced reaches and showed little variability in the inland riverine reaches. Additionally, comparison of flood hazard maps to flood risk maps shows that they are not directly correlated, as areas of high hazard do not always represent high risk. Copyright © 2012 John Wiley & Sons, Ltd.     

Post-wildfire sediment cascades: A modeling framework linking debris flow generation and network-scale sediment routing

Wildfires represent one of the largest disturbances in watersheds of the Intermountain West. Yet, we lack models capable of predicting post-wildfire impacts on downstream ecosystems and infrastructure. Here we present a novel modeling framework that links new and existing models to simulate the post-wildfire sediment cascade, including spatially explicit predictions of debris flows, storage of debris flow sediment within valleys, delivery of debris flow sediment to active channels, and the downstream routing of sediment through river networks. We apply the model to sediment dynamics in Clear Creek watershed following the 2010 Twitchell Canyon Fire in the Tushar Mountains of southern Utah. The debris flow generation model performed well, correctly predicting 19 out of 20 debris flows from the largest catchments, with only four false positives and two false negatives at observed rainfall intensities. In total, the model predicts the occurrence of 160 post-wildfire debris flows across the Clear Creek watershed, generating more than 650 000 m³ of sediment. Our new storage and delivery model predicts the vast majority of this sediment is stored within valleys, and only 13% is delivered to the river network. The sediment routing model identifies numerous sediment bottlenecks within the network, which alter transport dynamics and may be hotspots for aggradation and aquatic habitat alteration. The volume of sediment exported from the watershed after seven years of simulation totals 17% of that delivered, or 2% of the total generated debris flow sediment. In the case of the Twitchell Canyon Fire, this highlights that significant post-wildfire sediment volumes can be stored in valleys (87%) and within the stream network (11%). Finally, we discuss useful insights that can be gleaned from the model framework, as well as the limitations and need for more monitoring and theory development in order to better constrain essential inputs, process rates, and morphodynamics. © 2019 John Wiley & Sons, Ltd.     

Spatial and temporal diversity for debris‐flow meteorological control in subarctic oceanic periglacial environments in Iceland

Slopes in fjord environments of Iceland are prone to debris‐flow initiation, responding to a wide variety of meteorological triggering factors, such as rain on snow, rapid snowmelt, long‐lasting rainfall or intense rainfall. If all fjord regions have similar debris flows with regards to their magnitude, their meteorological control is diverse both in space and in time. Debris flows in Northwest Iceland are triggered mostly by rain‐on‐snow and long‐lasting rainfall, while snowmelt is more characteristic in North Iceland, and rainfall has a clear impact in East Iceland. Most debris‐flow events occur on a single slope, and only a few are recorded at the same time in different regions. Observations of the threshold values underline the diversity of debris‐flow initiation, occurring with huge amounts of sudden water supply as well as with very moderate ones. Copyright © 2007 John Wiley & Sons, Ltd.     

Investigating the impact of the Chi‐Chi earthquake on the occurrence of debris flows using artificial neural networks

Debris flows have caused enormous losses of property and human life in Taiwan during the last two decades. An efficient and reliable method for predicting the occurrence of debris flows is required. The major goal of this study is to explore the impact of the Chi‐Chi earthquake on the occurrence of debris flows by applying the artificial neural network (ANN) that takes both hydrological and geomorphologic influences into account. The Chen‐Yu‐Lan River watershed, which is located in central Taiwan, is chosen for evaluating the critical rainfall triggering debris flows. A total of 1151 data sets were collected for calibrating model parameters with two training strategies. Significant differences before and after the earthquake have been found: (1) The size of landslide area is proportioned to the occurrence of debris flows; (2) the amount of critical rainfall required for triggering debris flows has reduced significantly, about half of the original critical rainfall in the study case; and (3) the frequency of the occurrence of debris flows is largely increased. The overall accuracy of model prediction in testing phase has reached 96·5%; moreover, the accuracy of occurrence prediction is largely increased from 24 to 80% as the network trained with data from before the Chi‐Chi earthquake sets and with data from the lumped before and after the earthquake sets. The results demonstrated that the ANN is capable of learning the complex mechanism of debris flows and producing satisfactory predictions. Copyright © 2009 John Wiley & Sons, Ltd.     

A clear and present danger: Ladakh's increasing vulnerability to flash floods and debris flows

This preliminary investigation of the recent spate of deadly flash floods and debris flows in Ladakh (India) over the last decade identifies uncontrolled development in hazardous locations as an important factor contributing to loss of life and property damage in this high mountain desert. The sediments exposed in the channel banks and on the alluvial fans of several mountain streams in the area indicate a long history of flash floods and debris flows resulting from intense storms, which appear to have increased in frequency within the last decade. The signposts of these recurrent hazards are being ignored as a growing economy, which is boosted by a well‐established tourism industry, is now driving development onto lands that are susceptible to floods and debris flow hazards. In this science briefing we argue that the increasing vulnerability in Ladakh should be addressed with sound disaster governance strategies that are proactive, rather than reactionary. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatio-temporal modelling of hydro-meteorological derived risk using a Bayesian approach: a case study in Venezuela

Extreme environmental events have considerable impacts on society. Preparation to mitigate or forecast accurately these events is a growing concern for governments. In this regard, policy and decision makers require accurate tools for risk estimation in order to take informed decisions. This work proposes a Bayesian framework for a unified treatment and statistical modeling of the main components of risk: hazard, vulnerability and exposure. Risk is defined as the expected economic loss or population affected as a consequence of a hazard event. The vulnerability is interpreted as the loss experienced by an exposed population due to hazard events. The framework combines data of different spatial and temporal supports. It produces a sequence of temporal risk maps for the domain of interest including a measure of uncertainty for the hazard and vulnerability. In particular, the considered hazard (rainfall) is interpolated from point-based measured rainfall data using a hierarchical spatio-temporal Kriging model, whose parameters are estimated using the Bayesian paradigm. Vulnerability is modeled using zero-inflated distributions with parameters dependent on climatic variables at local and large scales. Exposure is defined as the total population settled in the spatial domain and is interpolated using census data. The proposed methodology was applied to the Vargas state of Venezuela to map the spatio-temporal risk for the period 1970–2006. The framework highlights both high and low risk areas given extreme rainfall events.