Analysis of the triggering conditions and erosion of a runoff-triggered debris flow in Miyun County,Beijing, China

The occurrence of debris flow from channel-bed failure is occasionally noted in small and steeply sloping watersheds where channelized water flow dominates debris flow initiation. On August 12, 2016, a debris flow from channel-bed failures occurred in the Caozhuangzi Watershed of the Longtan Basin, Miyun, Beijing. Rainfall records over 10-min intervals and field investigations including channel morphology measurements were used to study the triggering conditions and erosion process. The results indicated that the occurrence of this event lagged the peak 10-min rainfall interval and that the cumulative rainfall prior to the occurrence time played an important role in its formation. A mean 10-min rainfall intensity–duration expression in the form of I₁₀?=?5.0?×?D^?0.21, where I₁₀ denotes the mean 10-min rainfall intensity and D is the rainfall duration ranging from 10 to 60 h, was proposed. The debris flows have low proportions of grain size fractions <?0.1 mm and higher fractions of grains 0.1–2 mm in size, indicating that the flow had low viscosity and was coarse-grain dominated. Channel morphology analysis revealed that abrupt changes in topography in the study area, including a steep section, a concave stream bank area, and a partial concave stream section were eroded more extensively than other sites. The maximum sediment erosion volume and erosion depth were not proportional to the variation in stream gradient. Consideration of the degree of erosion in the channel at sites with abrupt morphology changes, the maximum sediment erosion volume, and the erosion depth and volume at the initial channel site and downstream region of forest area together showed that the prime factor controlling erosion was entrained sediment volume. This work, thus, provides a case study regarding the triggering conditions of runoff-triggered debris flows and the topographical changes by debris flow erosion.     

人工降雨条件下冲沟型泥石流起动试验研究

下垫面以位于贡嘎山东坡的熊家沟为模型,开展了不同降雨强度条件下冲沟型泥石流起动的模拟试验,初步研究了冲沟型泥石流的形成机理和演化特征.试验研究表明:(1)在强降雨条件下,水体入渗速度、不同深度土体含水量变化与降雨强度呈反比例关系,降雨强度越大,越不利于水体入渗,而有利于坡面汇流、冲沟径流和下切侵蚀; (2)在强降雨和径流条件下,土体破坏方式、破坏程度以及泥石流形成机理表现出差异性.相对较小雨强降雨条件下,土体破坏方式以滑坡为主,泥石流形成模式表现为滑坡液化与转化起动,雨强较大降雨条件下,土体破坏方式以侵蚀垮塌为主,泥石流形成模式为洪流席卷垮塌体和沟床揭底; (3)起动试验中泥石流阵性特征明显.在强降雨条件下,雨强与泥石流的规模、黏度之间没有正相关性,雨强越大,泥石流黏度越小,试验中多出现的是高含砂洪流,而相对较小雨强作用下由土体液化转化形成的泥石流黏度较大.试验现象和结果与熊家沟泥石流起动、发生过程具有较高的一致性.     

Rainfall-triggering response patterns of post-seismic debris flows in the Wenchuan earthquake area

Several giant debris flows occurred in southwestern China after the Wenchuan earthquake, causing serious casualties and economic losses. Debris flows were frequently triggered after the earthquake. A relatively accurate prediction of these post-seismic debris flows can help to reduce the consequent damages. Existing debris flow prediction is almost based on the study of the relationship between post-earthquake debris flows and rainfall. The relationship between the occurrence of post-seismic debris flows and characteristic rainfall patterns was studied in this paper. Fourteen rainfall events related to debris flows that occurred in four watersheds in the Wenchuan earthquake area were collected. By analyzing the rainfall data, characteristics of rainfall events that triggered debris flows after the earthquake were obtained. Both the critical maximum rainfall intensity and average rainfall intensity increased with the time. To describe the critical conditions for debris flow initiation, intensity–duration curves were constructed, which shows how the threshold for triggering debris flows increased each year. The time that the critical rainfall intensities of debris flow occurrences return to the value prior to the earthquake could not be estimated due to the absent rainfall data before the earthquake. Rainfall-triggering response patterns could be distinguished for rainfall-induced debris flows. The critical rainfall patterns related to debris flows could be divided on the basis of antecedent rainfall duration and intensity into three categories: (1) a rapid triggering response pattern, (2) an intermediate triggering response pattern, and (3) a slow triggering response pattern. The triggering response patterns are closely related to the initiation mechanisms of post-earthquake debris flows. The main difference in initiation mechanisms and difference in triggering patterns by rainfall is regulated by the infiltration process and determined by a number of parameters, such as hydro-mechanical soil characteristics, the thickness of the soil, and the slope gradient. In case of a rapid triggering response rainfall pattern, the hydraulic conductivity and initial moisture content are the main impact factors. Runoff erosion and rapid loading of solid material is the dominant process. In case of a rainfall pattern with a slow triggering response, the thickness and strength of the soil, high hydraulic conductivity, and rainfall intensity are the impact factors. Probably slope failure is the most dominant process initiating debris flows. In case of an intermediate triggering response pattern, both debris flow initiation mechanisms (runoff erosion and slope failure) can play a role.     

黏土颗粒含量对蒋家沟泥石流启动影响分析

黏土颗粒在泥石流中的含量并不大,但却显著地影响着泥石流的启动。在室内通过筛分配成9种不同黏粒含量级配的土体,在自行设计的模型槽内以1.64 g/cm3（松散干密度）,1.79、1.94 g/cm3（天然干密度）3种干密度堆成边坡模型,在雨强为85 mm/h下进行人工降雨试验,初步探讨了黏土颗粒含量对泥石流启动的影响,得到：黏粒含量在5%～18%时可以形成泥石流,其中黏粒含量10%时所需时间最短,低于5%或大于18%难以形成泥石流,黏粒含量具有临界性;填筑干密度越大,泥石流启动越困难,表现在启动时间长、深度浅、规模小、且填筑干密度不改变黏粒含量临界性;降雨条件下土体入渗率越高,泥石流越容易启动产流。通过试验的研究,可以深入揭示泥石流形成的内在机制,黏粒含量临界性为泥石流预测、预报提供了新思路。     

沟床起动型泥石流的10 min降雨预报模型

针对沟床起动型泥石流的诱发因素为高强度短历时的降雨,提出10 min降雨强度是这类泥石流暴发的关键。在1 h预报模型的基础上,基于云南蒋家沟的多年泥石流观测资料,修正了1 h预报模型的降雨参数,并得到了10 min降雨预报模型。10 min降雨预报模型在中国西部的其他流域,如云南浑水沟、贵州望谟县打易区域泥石流沟、四川三滩沟、四川雅安陆王沟和干溪沟、甘肃柳湾沟、甘肃马槽沟等流域的验证中,取得了较好的结果。10 min降雨预报模型是部分建立在泥石流的形成机理上的模型,并不是完全的统计模型,因此该模型也可以用于其他地区的沟床起动类型泥石流预报。     

暴雨型泥石流预警模型初步研究

2010年8月13日四川都江堰市龙池地区暴发了特大规模的群发性暴雨型泥石流过程,给龙池地区的震后恢复重建带来了巨大的灾难,因此,研究该区泥石流发生机理和预警十分必要。本文在总结前人关于泥石流暴发与降雨条件研究成果基础上,发现泥石流物源含水量及地表径流流深等是导致流域内松散物源启动的主要原因。根据其前期降雨量和有效降雨强度等特征,建立了泥石流流域物源土体颗粒个别启动、局部启动和大量启动的判别式,建立了适合该区域暴雨泥石流预警模型,将可能诱发泥石流暴发的不同降雨条件划分为蓝、黄和红色3个危险等级,为该区泥石流监测预警提供了科学依据。     

Analysis of rainfall preceding debris flows on the Smědavská hora Mt., Jizerské hory Mts., Czech Republic

In August 2010, extreme rainfall affected the north of the Czech Republic and caused regional floods and landslides. Three torrential debris flows originated in the Jizerské hory Mts., close to Bílý Potok on the north slope of the Smědavská hora Mt. The rainfall situation which triggered the debris flow was analyzed and compared with the rainfall situation in 1958 when a debris flow occurred in the same area. The rainfall data were obtained from rain gauges of the Czech Hydrometeorological Institute. Four rain gauges were chosen close to the Smědavská hora Mt. with data of daily amounts from 1983 to 2013 and 10-min intensity or hourly amounts from the specific period. The data from 1958 were available from three different rain gauges (only daily amounts). The data series were not complete so linear regression was applied to interpolate them. A number of analyses were carried out including daily rainfall, 2-day/3-day moving values, antecedent precipitation index (API) of 5/10/30 days, 10-min intensity, and hourly amounts, and the trigger factor of the debris flow in the study area was also investigated. It was determined that for the triggering of debris flows, both high API values as well as high-intensity short-duration rainfall is needed. It was documented that in cases of solely high API indices or high-intensity short-duration rainfalls, no debris flows were initiated.     

Catastrophic debris flows triggered by a 4 July 2013 rainfall in Shimian,SW China: formation mechanism,disaster characteristics and the lessons learned

On 4 July 2013, three catastrophic debris flows occurred in the Hougou, Majingzi, and Xiongjia gullies in Shimian county and produced debris dams and river blockages, resulting in serious casualties and huge economic loss. Though debris flows have been identified prior to the catastrophic events, their magnitudes and destructive power were far beyond early recognition and hazard assessment. Our primary objective for this study was to explore the formation mechanism and typical characteristics and to summarize the lessons learned from these disastrous events in order to avoid the repeat of such disasters in the future. Based on field investigation and imagery interpretation of remote sensing carried out following the catastrophic events, four conclusions were drawn: (1) The catastrophic debris flows were initiated from surface-water runoff, and the triggering factor was attributed to the local intensive rainfall with an hourly intensity of more than 46.7 mm. (2) Entrainment was the most important sediment-supplying method for the debris flow occurrence, and the source materials transported by debris flows from the three gullies were estimated to be about 97?×?10⁴ m³ in volume altogether. (3) As surface-water runoff eroded and entrained hillslope and channel materials persistently, debris flows were characterized by intensive incision at upper or middle reaches and significant magnification effect in flow discharge and volume downstream. Corresponding peak discharge surveyed at the outlets of the Hougou, Majingzi, and Xiongjia gullies was estimated up to 751.0 m³/s, 870.1 m³/s, and 758.7 m³/s, respectively. (4) Debris flows that occurred from the three gullies all belonged to viscous ones and the bulk densities were calculated more than 1.80 g/cm³, indicating a huge carrying capacity and destructive impacting power. In addition, the lessons learned from the catastrophic events were summarized, including recognition and assessment on debris flow hazard and utilization of deposition fan. In this paper, prevention suggestions on debris flow prone valleys with high-vegetation coverage and low occurrence frequency were also put forward. The results of this study contribute to a better understanding on the initiation mechanism, dynamic characteristics, and disaster mitigation of debris flows initiated from intense rainfall and surface-water runoff in mountainous areas.     

The formation of runoff-generated debris flow in Southwestern of China: take Gangou as an example

In the mountain area of Southwestern China, there are large quantities of runoff-generated debris flows that are threatening the local people and facilities seriously. Gangou is a typical runoff-generated debris flow; its source is old deposit from floods and the debris flows downstream of the channel. On June 30, 2005, Gangou occurred debris flow, the debris flow destroying the road, the communications facilities and the farmland at the gully mouth. Unlike the formation mechanisms of other debris flows, the formation of 2005 debris flow in Gangou has its distinctive characteristics as follows. (1) The supply of the loose sources is intensive and distribute near the mouth of the gully; it is rare to see any debris flow initiate at such a lower location. (2) The debris flow finishes its initiation, flow and deposition around the 700-m-long channel, accompanied with the blocking process in the gully when the debris flow ran out; however, 10 min later, it releases and amplifies the peak flow about three times. (3) The topographic condition of the basin does not contribute much to the formation of the 2005 debris flow; instead, its formation is the result of the co-effort of continuous rainfall and a short-time heavy rainfall. In other words, the previous cumulative precipitation enables the moisture content of the soil on the right bank of the gully to reach saturation; then the soil slides into the channel under the action of the heavy rainfall at a later time. Meanwhile, the heavy rainfall accelerates the formation of gully run-off and initiates the loose mass in the channel from slide, thus forming the debris flow.     

汶川强震区泥石流起动模型试验分析

强震使大量松散堆积物堆积于高山峡谷内,由于持续降雨的作用,导致泥石流在汶川灾区频繁发生。降雨强度和物源与泥石流的起动有着密切联系,分析雨强和物源与泥石流起动的相关关系,可得不同雨强下,不同物源区泥石流起动的特征。通过对典型泥石流沟（映秀镇烧火坪侧沟,牛莲花心沟）进行野外样品采集,将两条泥石流沟进行对比室内试验。控制起动坡度,降雨强度与颗粒级配得出不同坡度下泥石流起动的表面流形成流量,起动流量。同时,与非强震区（西昌市琅环乡）的泥石流简单对比,得到强震区泥石流起动的临界条件及雨强,为泥石流的预测和治理提供相应的理论依据。通过大量的试验结果,泥石流起动首先会形成表面流,然后形成冲沟,随着物源区雨强的增加,最终形成大面积松散堆积物失稳,整体起动。泥石流起动与汇水区面积成一定关系。     

温州群发性坡面泥石流临界雨量研究

温州地处浙江东南沿海,降雨充沛。台风等极端气候灾害引起的强降雨次数繁多,并可能引发地区周边泥石流等地质灾害。收集相关地区、时段的气象、水文资料,并对研究区降雨数据进行处理,在野外实地调查、遥感解译的基础上,根据研究区地质灾害调查结果,总结群发性坡面泥石流特征,分析其形成的地质环境条件。针对致灾因子降雨量与温州地区群发性泥石流灾害的因果关系,获取坡面泥石流发生周期内的各时段降雨量,并利用相关性分析确定最大1h、3h雨强为研究变量,在二维坐标平面上投影近百个案例点,拟合得出临界雨量方程,并根据泥石流发生与否划分区域计算超越概率和误报概率,得出温州地区泥石流爆发前期雨强满足"两倍关系"的经验结论,寻求解决温州地区实际泥石流地质灾害预警问题的可行性。     

A prediction model for debris flows triggered by a runoff-induced mechanism

Many debris flows were triggered within and also outside the Dayi area of the Guizhou Province, China, during a rainstorm in 2011. High-intensity short-duration rainfall was the main triggering factor for these gully-type debris flows which are probably triggered by a runoff-induced mechanism. A revised prediction model was introduced for this kind of gully-type debris flows with factors related to topography, geology, and hydrology (rainfall) and applied to the Wangmo River catchment. Regarding the geological factor, the “soft lithology” and “loose sediments” in the channel were added to the list of the average firmness coefficient for the lithology. Also, the chemical weathering was taken into account for the revised geological factor. Concerning the hydrological factor, a coefficient of variation of rainfall was introduced for the normalization of the rainfall factor. The prediction model for debris flows proposed in this paper delivered three classes of the probability of debris flow occurrence. The model was successfully validated in debris flow gullies with the same initiation mechanism in other areas of southwest China. The generic character of the model is explained by the fact that its factors are partly based on the initiation mechanisms and not only on the statistical analyses of a unique variety of local factors. The research provides a new way to predict the occurrence of debris flows initiated by a runoff-induced mechanism.     

汶川震区暴雨泥石流激发雨量特征

利用搜集的汶川震区典型泥石流暴发前后的降雨过程资料,分析了泥石流的激发雨量过程,获得了汶川震区的泥石流激发雨量特征,以期为泥石流的预测预报提供依据。结果表明,汶川地震区的泥石流激发雨型可分为快速激发型、中速激发型和慢速激发型3类,其差异主要体现在降雨的持续时间和强度方面。不同激发雨型下的泥石流形成过程的差别主要体现在松散土体饱和过程。雨型的差异(降雨的持续时间和强度)使得土体饱和产生超渗产流的时间出现差异,进而使得泥石流暴发的时间存在差异。激发雨强跟激发雨型存在一定的关系,激发雨强最大者为中速激发雨型,其次是慢速激发雨型,最小者为快速激发雨型。与地震之前相比,地震后的泥石流暴发时的累积雨量和临界雨量都有所降低。     

A size classification for debris flows

A 10-fold classification for debris flow size is proposed based on total volume, peak discharge and area inundated by debris. Size classes can be used for regional overview studies where detailed site investigations are either unnecessary, too costly or where the highest hazard and risk creeks need to be identified for further study. They are also useful to compare the regional impact between affected areas and the effects of rainstorms, and they allow lay-people to obtain an understanding of debris flow magnitude and consequences. Finally, different size classes allow the estimation of travel times to points of interest based on empirically derived equations. It is proposed that agencies concerned with debris flows should establish a documentation of debris flow size according to this classification, which serves as a data base for hazard and risk planning.     

Effects of the morphology of sediment-transporting channels on the erosion and deposition of debris flows

A series experiments are conducted to investigate the effects of streambed profile on the erosion and deposition of debris flows. It is found that straight channel can increase the run out of debris flows by 10–25%, compared to that of surfaces without channels, and that travel distance was positively correlated with the hydraulic radius of the channel. In addition, the presence of straight channels caused the volume of debris flow deposition to become normally distributed with respect to travel distance. In the case of curved channels, increases in the sinuosity index resulted in significant blockage and obstruction. In the deposition zone, the maximum deposition volume for a channel with a comparatively low sinuosity index (1.05) was <?50% of the minimum deposition volume for a straight channel. Furthermore, the channel curvature affected not only the positions of deposition peaks along the travel distance but also the debris flow magnitudes in each unit interval (0.5 m). This study demonstrates the effects of differences in channel morphology on the erosional and depositional processes of gully debris flows. These findings are of significant importance for guiding debris flow risk assessment and for the restoration and reconstruction of downstream regions.     

Objective definition of rainfall intensity–duration thresholds for the initiation of post-fire debris flows in southern California

Rainfall intensity–duration (ID) thresholds are commonly used to predict the temporal occurrence of debris flows and shallow landslides. Typically, thresholds are subjectively defined as the upper limit of peak rainstorm intensities that do not produce debris flows and landslides, or as the lower limit of peak rainstorm intensities that initiate debris flows and landslides. In addition, peak rainstorm intensities are often used to define thresholds, as data regarding the precise timing of debris flows and associated rainfall intensities are usually not available, and rainfall characteristics are often estimated from distant gauging locations. Here, we attempt to improve the performance of existing threshold-based predictions of post-fire debris-flow occurrence by utilizing data on the precise timing of debris flows relative to rainfall intensity, and develop an objective method to define the threshold intensities. We objectively defined the thresholds by maximizing the number of correct predictions of debris flow occurrence while minimizing the rate of both Type I (false positive) and Type II (false negative) errors. We identified that (1) there were statistically significant differences between peak storm and triggering intensities, (2) the objectively defined threshold model presents a better balance between predictive success, false alarms and failed alarms than previous subjectively defined thresholds, (3) thresholds based on measurements of rainfall intensity over shorter duration (≤60 min) are better predictors of post-fire debris-flow initiation than longer duration thresholds, and (4) the objectively defined thresholds were exceeded prior to the recorded time of debris flow at frequencies similar to or better than subjective thresholds. Our findings highlight the need to better constrain the timing and processes of initiation of landslides and debris flows for future threshold studies. In addition, the methods used to define rainfall thresholds in this study represent a computationally simple means of deriving critical values for other studies of nonlinear phenomena characterized by thresholds.     

Recent catastrophic debris flows in Luding county, SW China: geological hazards, rainfall analysis and dynamic characteristics

About 127 debris flow gullies have been identified, and debris flows have been an important type of geological hazards in Luding County, affecting cities, towns, rural areas, scenic spots and human’s engineering projects, such as mining and waterpower utilizing equipments. In this summary paper, recent two catastrophic debris flow events occurred on June 30, 2005, in Chuni town, in the central of the county, and on August 11, 2005, in Hailuogou scenic spot, in the southwest of the county, respectively, are reviewed. The debris flow events are introduced on the basis of field investigation and RS interpretation and the triggering factors for flow occurrence are identified. Furthermore, the rainfall related to flow occurrence including antecedent rainfall and intraday rainfall is analyzed, and a power-law function which can be used as a basic warning line is established based on both antecedent effective rainfall and intraday rainfall. Then dynamic parameters such as flow velocity and flow discharge are calculated, respectively. Through comparison and discussion, some conclusions are made including (1) The antecedent rainfall played an important role for debris flows which generated predominately based on the slope-instability due to the saturated loose sediments; (2) Despite slower flow velocity and smaller magnitude, the slope-type debris flows just like 2005-6-30 debris flows usually lead to serious damages for the difficulty to forecast and to prevent; (3) The mistaken recognition on debris flow hazards and lack of prevention consciousness strengthen the hazard and damage degree. This research is of certain significance for the prevention and mitigation of debris flow hazards and for the planning of the town building and tourism development in the future.     

"5·12" 地震后汶川县泥石流特征与演化分析

2008年“5·12”汶川地震极大地改变了震区泥石流的特征,不仅增强了泥石流的活动性,同时也使得震区在相当长的时间内都要面临泥石流的威胁。本文基于前人大量的研究成果,并利用遥感解译结合现场调查等手段,分析了汶川县泥石流沟道纵坡降、沟壑密度、两岸坡度等基本发育特征;进而分析了地震前后汶川县降雨分布及泥石流相关降雨参数变化特征。结果显示,流域内泥石流沟的沟壑密度在0.2~4之间,属于微度土壤侵蚀区域,泥石流的沟床纵坡降偏大,有利于泥石流的发生;泥石流流域内斜坡坡度多为30°~40°,有利于灾害的发生;震后汶川县年均降雨量增加了5.17%,降雨多集中在7~9月份,降雨量由南及北逐渐降低;震后泥石流的降雨阈值在2008~2013年呈现缓慢回升的趋势,但2019年又有所下降,预计恢复到震前水平尚需要一定时间;同时震后汶川县泥石流物源丰富,震后物源量呈现“震荡式衰减”的演化趋势,但体量仍然很大,对泥石流仍需坚持监测预警工作。     

An integrated model to assess critical rainfall thresholds for run-out distances of debris flows

A dramatic increase in debris flows occurred in the years after the 2008 Wenchuan earthquake in SW China due to the deposition of loose co-seismic landslide material. This paper proposes a preliminary integrated model, which describes the relationship between rain input and debris flow run-out in order to establish critical rain thresholds for mobilizing enough debris volume to reach the basin outlet. The model integrates in a simple way rainfall, surface runoff, and concentrated erosion of the loose material deposited in channels, propagation, and deposition of flow material. The model could be calibrated on total volumes of debris flow materials deposited at the outlet of the Shuida catchment during two successive rain events which occurred in August 2011. The calibrated model was used to construct critical rainfall intensity-duration graphs defining thresholds for a run-out distance until the outlet of the catchment. Model simulations show that threshold values increase after successive rain events due to a decrease in erodible material. The constructed rainfall intensity-duration threshold graphs for the Shuida catchment based on the current situation appeared to have basically the same exponential value as a threshold graph for debris flow occurrences, constructed for the Wenjia catchment on the basis of 5 observed triggering rain events. This may indicate that the triggering mechanism by intensive run-off erosion in channels in this catchment is the same. The model did not account for a supply of extra loose material by landslips transforming into debris flow or reaching the channels for transportation by run-off. In August 2012, two severe rain events were measured in the Shuida catchment, which did not produce debris flows. This could be confirmed by the threshold diagram constructed by the model.     

Formation and characteristics of post-earthquake debris flow: a case study from Wenjia gully in Mianzhu, Sichuan, SW China

During the three flood seasons following the Wenchuan earthquake in 2008, two catastrophic groups of debris flow events occurred in the earthquake-affected area: the 2008-9-24 debris flow events, which had a serious impact on rebuilding; and the 2010-8-13/14 debris flow events, which destroyed much of the progress made in rebuilding. The Wenjia gully is a typical post-earthquake debris flow gully and at least five debris flows have occurred there. As far as the 2010-8-13 debris flow is concerned, the deposits of the Wenjia gully debris flow reached a volume of 3.1 × 10⁶ m³ in volume and hundreds of newly built houses were buried. This study took the Wenjia gully debris flow as an example and discussed the formation and characteristics of post-earthquake debris flow on the basis of field investigations and a remote sensing interpretation. The conclusions drawn from the investigation and analysis were as follows: (1) Post-earthquake debris flows were a joint result of both the earthquake and heavy rainfall. (2) Gully incision and loose material provision are key processes in the initiation and occurrence of debris flows and a cycle can be presented as the following process: runoff—erosion—collapse—engulfment—debris flow—further erosion—further collapse—further engulfment—debris flow enlargement. (3) The amount of rainfall that triggered debris flows from the Wenjia gully was significantly less than the average daily rainfall, while the intraday rainfall threshold decreased by at least 23.3%. (4) The occurrence mechanism of Wenjia gully debris flow was an erosion type and there was a positive relationship between debris flow magnitude and rainfall, which fitted an exponential model. (5) There were five representative characteristics of Wenjia gully debris flow: the long duration of the occurring process; the long distance of deposition chain conversion during the process of damage; magnification in the scale of debris flow; and the high frequency of debris flow events.