Probabilistic rainfall thresholds for debris flows occurred after the Wenchuan earthquake using a Bayesian technique

Preliminary study of the effect of earthquakes on the rainfall threshold of debris flows

The Ms 8.0 Wenchuan earthquake greatly altered the threshold for rainfall-triggered debris flows in the affected areas. It is of both scientific and practical significance to determine the rainfall thresholds. This study examines one of the regions most prone to debris flows to analyze the characteristics of rainfall that caused debris flows, and to explore local rainfall thresholds. We applied the relation between rainfall intensity and duration, peak intensity and event amount, and other single factor approaches. Comparison of effectiveness and accuracy indicates that the event rainfall is the most sensitive factor for forecasting. Analysis of the annual rainfall thresholds showed that the rainfall conditions required for debris flows have increased on the continent during the past 6 years. Besides the rainfall fluctuations over the past few years, material changes were the primary reason for threshold variability. Recovery of vegetation plays an important role in reducing potential loose material that supplies volume for debris flows. Natural solidification, decrease of the potential erosion depth, and surface coarsening make it more difficult to initiate a debris flow, and ultimately increased rainfall conditions required. The change in rainfall thresholds can be predicted and verified for the entire earthquake-affected region.

Critical rainfall thresholds can be the key to ensuring effective debris-flow forecasting. They are significant for the study of the trigger mechanisms of debris flows, for the forecast of the characteristics of future events, and for the development of engineering guidance for mitigation. Using a hydrological approach, we first calculated the flood peak discharge at different frequencies and then the corresponding rainfall thresholds for the initiation of different scales of debris flows in Zhouqu County, China. This was followed by the establishment of a functional relation between the intensity and duration of rainfall events that trigger debris flows of different warning levels in two initial soil conditions (dry and moist). This, in turn, yielded four warning levels and two preliminary warning levels. For the two early soil conditions (dry and moist) in the Sanyanyu Gully of Zhouqu County, the Level I (red) warning values for rainfall triggering debris flow are 56 and 51 mm h −1 , respectively; the Level II (orange) warning values are 41 and 38 mm h −1 , respectively; the Level III (yellow) warning values are 32 and 30 mm h −1 , respectively; and the Level IV (blue) warning values are 24 and 22 mm h −1 , respectively. The Level V preliminary warning values are 17 and 16 mm h −1 , respectively; and the Level VI preliminary warning values are 10 and 9.5 mm h −1 , respectively. The rainfall intensity and duration were found to exhibit a power function relation, I = αD β , where the values of α and β vary with the warning levels. Rainfall events capable of triggering debris flows in the new thresholds and intensity–duration relations presented here can be used for forecasting purposes and in operational geohazard warning systems. These research results also provide a scientific basis for regional hazard mitigation and reduction in Zhouqu County.

This work examines the period affected by extreme rainfall that triggers debris flow and the modification of the critical rainfall threshold for debris flows after extreme events in the Chenyulan watershed in central Taiwan. Three rainfall indices, Im (maximum hourly rainfall depth), Rd (maximum 24-h rainfall amount), and RI (= Im × Rd), were used to analyze the variations in rainfall thresholds for triggering debris flow. Five extreme rainfall events with RI > 365 cm2/h that triggered multiple debris flows are discussed. Critical rainfall (the rainfall threshold of Im, Rd, and RI for triggering debris flow) drops significantly at the initial stage after an extreme rainfall event; it then increases to revert to its original value after subsequent rainfall events. The lowered critical rainfall and the required recovery period were associated with the RI driven by an extreme rainfall event. Empirical relationships for modified critical rainfall and recovery period after an extreme rainfall event are also presented. The critical RI and Im (or Rd) could be reduced by 70% and 40%, respectively. The maximum recovery period was approximately three years after an extreme rainfall. The recovery period has an increasing trend with the increasing ratio of landslide area bought by an extreme rainfall.

Wildfire‐induced changes to soil and vegetation promote runoff‐generated debris flows in steep watersheds. Postfire debris flows are most commonly observed in steep watersheds during the first wet season following a wildfire, but it is unclear how long the elevated threat of debris flow persists and why debris‐flow potential changes in recovering burned areas. This work quantifies how rainfall intensity‐duration (ID) thresholds for debris‐flow initiation change with time since burning and provides a mechanistic explanation for these changes. We constrained a hydrologic model using field and remotely sensed measurements of soil‐infiltration capacity, vegetation cover, runoff, and debris‐flow activity. We applied this model to estimate rainfall ID thresholds for debris‐flow initiation within three burned areas in the southwestern United States over a postfire recovery period of three to four years. Modeling suggests ID thresholds are lowest immediately following the fire (below a one‐year recurrence interval [RI] storm) and increase with time, such that a 10‐ to 25‐year RI storm would be required to generate a debris flow after three years of recovery. Modeled changes in rainfall ID thresholds result from increases in soil infiltration capacity, canopy interception, hydraulic roughness, and median grain size of sediment entrained in an incipient debris flow. The relative importance of each of these factors varied among our three sites. Results improve our ability to assess temporal changes in postfire debris‐flow potential, highlight how site‐specific factors may alter the persistence of postfire debris‐flow hazards, and provide additional constraints on the timescale of recovery following wildfire.

Constructing rainfall thresholds for debris flow initiation based on critical discharge and S-hydrograph

Rainfall thresholds for the occurrence of debris flows are commonly defined by Intensity‐Duration curves (IDthresholds). Interestingly, many empiricalIDthresholds show up as straight lines in a log‐log plot and therefore can be expressed by power‐law functions of the formI = α ⋅ Dβ, whereαis the scaling coefficient andβis the exponent of the power function. The different values ofαandβreflect the variability of geological and hydrological conditions in the different areas. In most cases, however, field conditions are so complex that a quantitative interpretation of the empirical rainfall threshold is impossible and the physical meaning ofαandβremains obscure. In this work, we provide a physical interpretation of the rainfall thresholds that characterize an active debris flow catchment in the Eastern Italian Alps (the Dimai Basin, Belluno Province). The catchment is affected by frequent debris flows generated by surface‐water runoff and has been monitored since 2010 to investigate the initiation process. Monitoring data allowed for the detection of two rainfall thresholds: a lower one that identifies the arrival of water in the initiation area (Catchment Outflow Threshold) and an upper one that identifies the initiation of debris flows by channel runoff (Debris Flow Threshold). We demonstrate that these two thresholds can be satisfactorily reproduced by a simple physically based model in which the excess rainfall is routed over the catchment using a kinematic‐wave scheme. This simple analysis provides a sound explanation of the observed thresholds and can be used to develop physically based thresholds for runoff‐generated debris flows.

On 24 September 2008, 72 debris flows were triggered by heavy rainfall in the central part of the area affected by the Wenchuan Earthquake. This paper describes the process of debris-flow initiation and transport in the study areas and presents field observations on the roles of rainfall, lithology and the presence of faults. It is likely that following the ground shaking, the critical amount of accumulated precipitation and the hourly rainfall intensity necessary to initiate debris flow was reduced compared with values before the earthquake. A case study in the Xishanpo gully is presented; the debris flow from which caused a thick accumulation in the already devastated city of Beichuan. It is concluded that the whole of the area shaken by the Wenchuan Earthquake is now more susceptible to debris flows, which may be initiated by localized heavy rainfall. Care must be taken to properly assess this new type of geo-hazard.

Spatial and temporal evolution of long-term debris flow activity and the dynamic influence of condition factors in the Wenchuan earthquake-affected area, Sichuan, China

Controls on spatiotemporal heterogeneity and evolution of debris flows along a traffic corridor affected by the strong Wenchuan earthquake

<p>After the 2008 Wenchuan earthquake (Mw 7.9), increased occurrence of rainfall-induced debris flows was initially observed in the earthquake-hit region (Sichuan, China). In the following years, the frequency of debris flows gradually reduced, indicating a progressive recovery of stability of debris deposits accumulated along slopes and in gullies after the earthquake. To assess these dynamically changing conditions, empirical thresholds have been identified to predict post-seismic debris flow occurrence with two approaches: a meteorological approach based only on precipitation characteristics, and a hydrometeorological approach that also considers the hydrologic conditions before the onset of rainfall. Both used the available record of precipitations and debris flows that occurred between 2008 and 2015 in several gullies, tributary of the upper Minjiang river course, in Wenchuan county. Hydrometeorological thresholds for debris flows were identified at the gully catchment scale, by assessing the water balance with a simplified lumped hydrological model, based on the Budyko framework. The parameters of the model were estimated based on the scarce available information about the water balance of the entire watershed of the upper Minjiang. Simulated catchment water storage was used as a proxy of the moisture state of the slopes. The results indicate that both meteorological and hydrometeorological thresholds allow catching the progressive recovery of stability of the debris deposits. Specifically, the assessment of water balance at the catchment scale highlights the role played by the hydrological processes affecting the slopes, leading to the definition of reliable thresholds, that resulted robust despite the uncertainty of the estimated parameters of the hydrological model. Therefore, the hydrometeorological approach appears suitable to define thresholds for early warning of debris flows at the catchment scale.</p>

A strong earthquake could trigger a large number of co-seismic landslides and induce large amount of loose materials on steep slopes and in the gullies. Under strong rainfall conditions, these loose materials could induce devastating debris flows, which will endanger the resettled population and destroy the re-built infrastructures. From 19 to 20 August 2019, fourteen debris flows were triggered by extreme rainstorms near the epicentre of the Wenchuan Earthquake. Among the fourteen incidents, three of them produced debris flow dams, which changed the course of the Minjiang River and resulted in flooding at different parts of the reconstructed Miansi town. In addition, sixteen casualties, twenty two missing persons, and destruction of four main roads were reported. In this paper, one typical catchment, named as “Dengxi gully”, near the epicentre of the Wenchuan earthquake (Sichuan Province, China) was chosen as a case study for remote sensing analysis, field investigation of landslide evolution and debris flow development before and after the catastrophic events. The debris flow in the study area was initiated in four stages: (a) generation of a large amount of loose materials from the Wenchuan Earthquake; (b) run-off erosion from co-seismic landslide material on hilly slopes and repeated mobilizations in steep channels over the years; (c) development of high intensity localised rainfall events; (d) wash out of accumulated materials in gully by the flood. The study of “8.20 debris flows” can provide a benchmark for analysis of long-term evolution of debris flows in order to identify potential continuing hazards in the earthquake-affected areas and make proper engineering decisions.KeywordsDebris flowsExtreme rainstormsEpicentre of earthquakeWenchuan

Evaluating the post-earthquake landslides sediment supply capacity for debris flows

The Weigou Gully, a branch of Pingtong River, is located in Guixi Township, Beichuan County. The area was seriously affected by the Wenchuan earthquake and its aftermath. Debris flows occurred on September 24, 2008, July 14, 2009, and September 10, 2009, threatening the safety of the inhabitants and their property. Debris flows in Weigou Gully were mainly supplied by landslides triggered by the earthquake. The debris flows had characteristics of high frequency, high bulk density and coarse particle size. Following the earthquake, the frequency of debris flows increased. Based on the analysis of the disaster characteristics, the optimal cross-section design method for the “trapezoid-v” shaped debris flow drainage canal of the Weigou Gully is discussed. The depths (h1 and h2) of the cross-section of the “trapezoid- v” shaped drainage canal and the optimal crosssection parameters using these two measurements are defined. In addition, several formulas for calculating the cross-section measurements are deduced under optimal hydraulic conditions when discharge (Q), debris grain composition (D50 and D10) and longitudinal ratio (I) are known. Finally, the optimal dimensions for the cross-section of the Weigou Gully debris flow canal are determined.

The Wenchuan Earthquake triggered tens of thousands of landslides and many areas became susceptible to debris flow. From 12th to 14th, August 2010, heavy rain dropped in Wenchuan Earthquake disaster area, Sichuan province, China. The heavy rain triggered simultaneous occurrence of many debris flows in Qingping town in Mianzhu city, resulting in heavy casualty and property loss. Within the range of 4 km in Qingping Segment of Mianyuan River, 11 debris flows occurred simultaneously and the total runout deposits were about 600 × 104 m3 mainly from Wenjia and Zoumaling gully. Field investigation indicated that the sediment supply was mainly derived from landslide deposits. The debris flows could be classified into three types, including gully erosion starting, branch gully starting and source area starting. It is suggested that Wenchuan earthquake areas are still prone to debris flows. Therefore it is important to regulate and restrict use of hazard areas as well as to arrange relocation of people currently living in areas susceptible to debris flows.