Runout Distances Of Debris Flows Research Articles

Determining the debris flow yield strength of weathered soils: a case study of the Miryang debris flow in the Republic of Korea

Debris flow hazards are often interpreted through back-calculated simulation analysis or empirical methods. The mobility of a debris flow is greatly influenced by mechanical and hydrological parameters. The strength parameters play important roles in the debris flow initiation and flow stages. In particular, the rheological parameters of yield strength and plastic viscosity directly affect the debris flow runout distance and velocity. One of the most important parameters to consider when evaluating debris flow hazards is the shear strength. This strength is called the residual shear strength in the failure stage and the yield strength in the post-failure stage. The residual shear strength obtained from ring shear tests can be related to the initiation of mass movements; the yield strength obtained from rheological tests can be related to the mobilization of debris flows. The residual shear stresses obtained from ring shear tests of weathered soils typically range between 10 and 100 kPa and strongly depend on the normal stress and shear velocity. When progressive slope failure (i.e., strain-softening behavior) occurs at a relatively shallow slope depth (e.g., < 1 m), the soil strength ranges from approximately 5–10 kPa. If the liquid limit state (i.e., solid‒liquid transition) is reached, the shear strength of the soil is approximately 2 kPa. Once the soil fails and mixes with ambient water along the slip surface, the yield strength decreases dramatically, resulting in high mobilization. A suggestion on how strength parameters can be applied to estimate debris flow mobility is presented by considering the 2011 Miryang debris flow, which occurred in weathered soil deposits in Miryang city, Republic of Korea. The best approach for debris flow yield strength estimation would be to consider the residual shear strength in the initiation stage, the yield strength in the flow stage, and the reduction in yield strength with the entrainment effect of the flow in the rapid fluidization stage.

An AI-Based Method for Estimating the Potential Runout Distance of Post-Seismic Debris Flows

The widely distributed sediments following an earthquake presents a continuous threat to local residential areas and infrastructure. These materials become more easily mobilized due to reduced rainfall thresholds. Before establishing an effective management plan for debris flow hazards, it is crucial to determine the potential reach of these sediments. In this study, a deep learning-based method—Dual Attention Network (DAN)—was developed to predict the runout distance of potential debris flows after the 2022 Luding Earthquake, taking into account the topography and precipitation conditions. Given that the availability of reliable precipitation data remains a challenge, attributable to the scarcity of rain gauge stations and the relatively coarse resolution of satellite-based observations, our approach involved three key steps. First, we employed the DAN model to refine the Global Precipitation Measurement (GPM) data, enhancing its spatial and temporal resolution. This refinement was achieved by leveraging the correlation between precipitation and regional environment factors (REVs) at a seasonal scale. Second, the downscaled GPM underwent calibration using observations from rain gauge stations. Third, mean absolute error (MAE), mean square error (MSE), and root mean square error (RMSE) were employed to evaluate the performance of both the downscaling and calibration processes. Then the calibrated precipitation, catchment area, channel length, average channel gradient, and sediment volume were selected to develop a prediction model based on debris flows following the Wenchuan Earthquake. This model was applied to estimate the runout distance of potential debris flows after the Luding Earthquake. The results show that: (1) The calibrated GPM achieves an average MAE of 1.56 mm, surpassing the MAEs of original GPM (4.25 mm) and downscaled GPM (3.83 mm); (2) The developed prediction model reduces the prediction error by 40 m in comparison to an empirical equation; (3) The potential runout distance of debris flows after the Luding Earthquake reaches 0.77 km when intraday rainfall is 100 mm, while the minimum distance value is only 0.06 km. Overall, the developed model offers a scientific support for decision makers in taking reasonable measurements for loss reduction caused by post-seismic debris flows.

Modeling Shallow Landslide Runout Distance in Eocene Flysch Facies Using Empirical–Statistical Models (Western Black Sea Region of Türkiye)

Uncertainties related to runout distances in shallow landslide analyses may not only affect lives but may also result in economic losses. Owing to the increase in shallow landslides, which are especially triggered by heavy rainfall, runout distances have been investigated to decipher whether applications of a functional runout distance are feasible. This paper aims to give insights into the modeling of the shallow landslide runout probability in Eocene flysch facies in the Western Black Sea region of Türkiye. There are two main stages in this study—which are dominated by empirical models, the detection of initiation points, and propagation—which help us to understand and visualize the possible runout distances in the study area. Shallow landslide initiation point determination using machine learning has a critical role in the ordered tasks in this study. Modified Holmgren and simplified friction-limited model (SFLM) parameters were applied to provide a good approximation of runout distances during the propagation stage using Flow-R software. The empirical model parameters suggested for debris flows and shallow landslides were investigated comparatively. The runout distance models had approximately the same performance depending on the debris flow and shallow landslide parameters. While the impacted total runout areas for the debris flow parameters were predicted to amount to approximately 146 km2, the impacted total runout areas for the shallow landslide parameters were estimated to be about 101 km2. Considering the inclusion of the RCP 4.5 and RCP 8.5 precipitation scenarios in the analyses, this also shows that the shallow landslide and debris flow runout distance impact areas will decrease. The investigation of runout distance analyses and the inclusion of the RCP scenarios in the runout analyses are highly intriguing for landslide researchers.

A Study on the Comparative Analysis of the FLO-2D Model According to Debris Flow Sediment Amount

It is very important to predict the extent of the damage in order to reduce or prevent damage by the debris flow. In the Republic of Korea, various methods are used to understand the characteristics and to estimate the occurrence of the debris flow in an undamaged area, such as simulating disasters using the estimation of debris flow sediment amount based on field survey data. In this study, the runout distance of debris flow was analyzed by using different methods for estimating the debris flow sediment amount, at Wondeok-eup, Samcheok-si, Gangwon-do, where debris flow occurred due to Typhoon Mitak in 2019. The simulation results of the damage area were compared with the actual damage area. The result showed that the simulations generally corresponded to the actual area of damage caused by the sedimentation of debris flow. However, the estimation of damage area varied according to the used method of calculating the debris flow sediment amount.

Copula-Based Probabilistic Approaches for Predicting Debris-Flow Runout Distances in the Wenchuan Earthquake Zone

After the Wenchuan earthquake, frequent debris-flow disasters caused catastrophic damage to the infrastructures and inhabitants in the downstream. It is very important to properly determine the runout distances of debris flows to delineate potential hazard areas for the postearthquake mitigation work design and effective risk management. However, this is a difficult task due to the high variability and uncertainty of debris flows. This paper developed copula-based probabilistic approaches for predicting the runout distances of debris flows on depositional fans. The proposed approaches integrate the debris flow’s maximum runout distance (L), debris-flow volume (VD), and catchment internal relief (H) into a three-dimensional copula-based probabilistic model. A probabilistic model of debris flows in the Wenchuan earthquake zone was developed based on the field investigation data of 133 channelized debris-flow events. The developed model is able to predict the possible maximum runout distances for a specific hazard level. The proposed approaches were validated using an independent field investigation dataset of debris flows. Results show that the proposed approaches properly estimate the maximum runout distances of debris flows in the Wenchuan earthquake zone for a specific probability level of 0.88.

Simulation of Debris-Flow Runout Near a Construction Site in Korea

This study analyzed landslide susceptibility and numerically simulated the runout distance of debris flows near a construction site in Korea. Landslide susceptibility was based on a landslide prediction map of the study area. In the prediction map, 3.5% of the area had a 70–90% landslide probability, while 0.79% had over 90% probability. Based on the landslide susceptibility analysis, debris flows in four watersheds were simulated to assess possible damage to the construction site. According to the simulations, debris flow in Watershed C approaches to within 9.6 m of the site. Therefore, the construction site could be impacted by debris flow in Watershed C. Although the simulated flows in Watersheds A and D do not directly influence the construction site, they could damage the nearby road and other facilities. The simulations also show that debris runout distance is strongly influenced by the volume of debris in the on-slope source area and by the slope angles along the debris-flow path.

Early identification of river blocking induced by tributary debris flow based on dimensionless volume index

River-blocking events induced by tributary debris flows occur frequently in mountainous areas. If a dam formed by a river-blocking debris flow were damaged, a massive outburst flood could induce downstream hazards. Therefore, early identification of areas potentially at risk of river blocking is essential for hazard mitigation, particularly in extensive mountainous areas. This study proposed a dimensionless volume index (DVI) to evaluate river-blocking formation by considering the relationship between the deposition volume of a tributary debris flow in a river and the minimum river-blocking volume. The values of both parameters can be established based on the theory of debris flow run-out distance, deposition width, and deposition thickness with consideration of the effects of drag force, hydrostatic force, and buoyancy. The efficacy of the proposed method was demonstrated through two case studies of mainstream blocking/nonblocking by tributary debris flow events in Taiwan during Typhoon Morakot. The results indicated that early identification of river blocking could be achieved using the DVI based on a pre-established database of potential debris flow hazard areas, including local site information, rainfall intensities at potential debris flow hazard areas under varying recurrence periods, and mainstream discharge at varying recurrence periods.

Empirical relationships for the estimation of debris flow runout distances on depositional fans in the Wenchuan earthquake zone

The Wenchuan area has become highly susceptible to landslides and debris flows since the earthquake occurred on 2008. A detailed debris flow hazard assessment is necessary to provide information for future risk management. Debris flow runout on a depositional fan is an important factor for assessing debris flow hazard, and its estimate becomes essential for the planning of the mitigation works that must be built. We therefore developed two simple empirical relationships based both on univariate and multivariate approaches, which provide the runout distance using the data for 134 channelized debris flow events in 134 catchments in the Wenchuan earthquake zone. We generated a correlation matrix of the debris flow runout distances and the relevant variables. Because of the high correlations, the independent variable debris flow volume (VD) was selected for the univariate approach. Using a multicollinearity analysis and a stepwise regression technique, the catchment (basin) internal relief (H), and VD were used to develop a multivariate runout relationship. The coefficients of the variables were obtained using 80% of the dataset (training dataset); the remaining 20% was used for test of estimated performance by comparing the computed runout distances with observed values. The validation demonstrated that the proposed relationships are suitable for estimating the runout distances of debris flows on depositional fans in the Wenchuan earthquake zone. The univariate runout relationship has the advantage of being simple, whereas the multivariate runout relationship provides higher accuracy. Additionally we rearranged and reformulated existing relationships using the same training dataset and compared the results with those from the relationships here proposed. The estimations provided by our relationships were the closest to the observed values. The presented approaches may be applied to estimate debris flow runout distances in other areas after they are retrained using local datasets.

Dimensionless Assessment Method of Landslide Dam Formation Caused by Tributary Debris Flow Events

In this study, we develop a dimensionless assessment method to evaluate landslide dam formation by considering the relationship between the run-out distance of a tributary debris flow and the width of the main stream, deposition thickness of the tributary debris flow, and the water depth of the main stream. Based on the theory of debris flow run-out distance and fan formation, landslide dam formation may result from a tributary debris flow as a result of two concurrent formation processes: (1) the run-out distance of the tributary debris flow must be greater than the width of the main stream, and (2) the minimum deposition thickness of the tributary debris flow must be higher than the in situ water depth of the main stream. At the confluence, one of four types of depositional scenarios may result: (1) the tributary debris flow enters into the main stream and forms a landslide dam; (2) the tributary debris flow enters into the main stream but overflow occurs, thus preventing complete blockage of the main stream; (3) the tributary debris flow enters into the main stream, does not reach the far bank, and sediment remains partially above the water elevation of the main stream; or (4) the tributary debris flow enters into the main stream, does not reach the far bank, and sediment is fully submerged in the main stream. This method was applied to the analysis of 11 tributary debris flow events during Typhoon Morakot, and the results indicate that the dimensionless assessment method can be used to estimate potential areas of landslide dam formation caused by tributary debris flows. Based on this method, government authorities can determine potential areas of landslide dam formation caused by debris flows and mitigate possible disasters accordingly through a properly prepared response plan, especially for early identification.

Modelling the role of material depletion, grain coarsening and revegetation in debris flow occurrences after the 2008 Wenchuan earthquake

A large amount of debris was generated by the co-seismic mass wasting associated with the 2008 Mw 7.9 Wenchuan earthquake. The abundance of this loose material along the slopes caused more frequent debris flows, triggered by less intense and/or shorter rainfalls. However, both the triggering rainfall and the debris flow frequency seem to have normalised progressively during the past decade. Although changes of rainfall thresholds for post-seismic debris flows were recorded after several major earthquakes, the factors controlling these changes remain poorly constrained. With the aid of a virtual experiment, we investigate the roles of material depletion, grain coarsening and revegetation of the co-seismic debris on the propagation and deposition of debris flows initiated by runoff, as well as their influence on the triggering rainfall thresholds. We employ a Geographic Information System (GIS)-based simulation of debris flow initiation by runoff erosion, which we first calibrate on the 14th August 2010 Hongchun gully event that occurred near the Wenchuan earthquake epicentre. We obtain, by investigating each of the aforementioned processes, changing critical rainfall intensity-duration thresholds for given debris flow runout distances. Grain coarsening appears to play a major role, which is consistent with published laboratory experiments, while material depletion and revegetation do not seem able to account alone for the actual quick decay of debris flow frequency. While the virtual experiment has proven useful in identifying the first-order controls on this decay, model improvements and verification over multiple catchments are needed to make the results useful in hazard assessments.

GIS tools for preliminary debris-flow assessment at regional scale

The assessment of the areas endangered by debris flows is a major issue in the context of mountain watershed management. Depending on the scale of analysis, different methods are required for the assessment of the areas exposed to debris flows. While 2-D numerical models are advised for detailed mapping of inundation areas on individual alluvial fans, preliminary recognition of hazard areas at the regional scale can be adequately performed by less data-demanding methods, which enable priority ranking of channels and alluvial fans at risk by debris flows. This contribution focuses on a simple and fast procedure that has been implemented for regional-scale identification of debris-flow prone channels and prioritization of the related alluvial fans. The methodology is based on the analysis of morphometric parameters derived from Digital Elevation Models (DEMs). Potential initiation sites of debris flows are identified as the DEM cells that exceed a threshold of slope-dependent contributing area. Channel reaches corresponding to debris flows propagation, deceleration and stopping conditions are derived from thresholds of local slope. An analysis of longitudinal profiles is used for the computation of the runout distance of debris flows. Information on erosion-resistant bedrock channels and sediment availability surveyed in the field are taken into account in the applications. A set of software tools was developed and made available (https://github.com/HydrogeomorphologyTools) to facilitate the application of the procedure. This approach, which has been extensively validated by means of field checks, has been extensively applied in the eastern Italian Alps. This contribution discusses potential and limitations of the method in the frame of the management of small mountain watersheds.

Application of paleostress analysis for the identification of potential instability precursors within the Benue Trough Nigeria

Structures such as faults, joints and fractures of diverse patterns have acted as precursors of several slope instability cases within the Benue Trough Nigeria. In some cases, the structures by their nature weakened and also created avenues that streams took advantage to further destabilize the rock slopes. In other cases, structure orientation played significant roles in the mobility and eventual runout distance of debris flow and avalanches in the region. Detailed field-based structural, fracture and paleostress analyses were therefore carried out to determine the fractural patterns that correlate to reported instability and landslide cases in the region; and to produce models that reveal areas with heightened risk. Three fracture sets were isolated from analysis of fracture orientations and field relationships: Pre-folding (JT), Syn-Folding (JS) and Post Folding (JC) fracture systems. Paleostress analysis carried out on these fracture systems using the TENSOR™ software tool yielded three paleostress tensors corresponding to transtensional stress tensor with ENE-WSW direction of maximum extension (SHMIN), oblique compressive (transpressional) tensor with NW-SE direction of maximum shortening (SHMAX), and transtensional tensor with WNW-ESE direction of maximum extension (SHMIN). These tensors are related to the prevailing plate tectonic stress regimes affecting the entire Benue trough and the West and Central African Rift System (WCARS). Our pre- and post-tectonic models have revealed the reasons for instability and the likely places where future failures may be located. This is the first such analyses in the region and it is hoped that the results can broaden the use and applicability of paleostresses in failure-prone terrains for future risk and disaster reduction/assessment within the Trough and in other areas prone to structure-controlled landslides disaster.

An energy-based approach to predict debris flow mobility and analyze empirical relationships

Several empirical relationships allowing a preliminary estimate of debris flow runout distances have been proposed to correlate the runout length to the volume of the sliding granular mass, delimit potentially hazardous areas, and design safeguarding measures. To overcome their large variability and define their fields of applicability, an energy-based model, predicting debris flow mobility, is developed. The power balance of a granular mass sliding along two planar surfaces is written by taking into account the volume of the debris mass, the slopes of the sliding surfaces, an assigned interstitial pressure, the possible mass variation along the motion, the energy dissipation due to the grain inelastic collisions (“granular temperature” within a basal “shear layer”), and friction. A system of ordinary differential equations is obtained; its numerical solution allows, through parametrical analyses: (i) highlighting of the role of physical and mechanical parameters on the runout distance, such as grain size material, interstitial pressures, grain collisions, and erodibility of the crossed channel; and (ii) defining of the favourable conditions for debris flows mechanism generation. Finally, through the back-analysis of some cases, an original relationship to estimate the runout length, as well as to interpret the results of the empirical formulas, is proposed.

Linking rainfall-induced landslides with predictions of debris flow runout distances

Rapid debris flows are among the most destructive natural hazards in steep mountainous terrains. Prediction of their path and impact hinges on knowledge of initiation location and the size and constitution of the released mass. To better link mass release initiation with debris flow paths and runout lengths, we propose to capitalize on a newly developed model for rainfall-induced landslide initiation (“Catchment-scale Hydro-mechanical Landslide Triggering” CHLT model, von Ruette et al. 2013) and couple it with simple estimates of debris flow runout distances and pathways. Landslide locations and volumes provided by the CHLT model are used as inputs to simulate debris flow runout distances with two empirical- and two physically-based models. The debris flow runout models were calibrated using two landslide inventories in the Swiss Alps obtained following a large rainfall event in 2005. We first fitted and tested the models for the “Prattigau” inventory, where detailed information on runout path was available, and then applied the models to landslides inventoried from a different catchment (“Napf”). The predicted debris flow runout distances (emanating from CHLT simulated landslide positions) were well in the range of observed values for the physically-based approaches. The empirical approaches tend to overestimate runout distances relative to observations. These preliminary results demonstrate the added value of linking shallow landslide triggering models with predictions of debris flow runout pathways for a range of soil states and triggering events, thus providing a more complete hazard assessment picture for debris flow exposure at the catchment scale.

Damming of large river by debris flow: Dynamic process and particle composition

The frequency and extent of debris flows have increased tremendously due to the extreme weather and the Wenchuan earthquake on May 12, 2008. Previous studies focused on the debris flow from gullies damming the mountain streams. In this paper, an equation for the run-out distance of debris flow in the main river is proposed based on the dynamic equation of debris flow at different slopes given by Takahashi. By undertaking field investigations and flume experiments, a new calculation method of the volume of debris flow damming large river is obtained. Using the percolation theory and the renormalization group theory it was deduced that the large particles should comprise more than 50% for forming a stable debris flow dam. Hence, the criteria of damming large river by debris flow is presented in terms of run-out distance and grain composition which was then validated through the event of damming river by debris flow at Gaojia gully, the upper reaches of the Minjiang River, Sichuan, China, on July 3, 2011.

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.

High-frequency monitoring of debris-flow propagation along the Réal Torrent, Southern French Prealps

The Réal Torrent (2.3km2 drainage area) is a very active debris-flow torrent in the French Prealps. It was equipped with three high-frequency monitoring stations in late 2010 to investigate the triggering conditions and propagation of debris flows along the 2-km long torrent channel. The monitoring system combines different techniques including rain gauges and measurements of ground vibrations and flow elevations.In spring and summer of 2011, five rainfall-triggered debris-flow events occurred along the torrent. Most debris fronts deposited in the upstream part of the channel but one large front (1.9–2.0m high) propagated to the catchment outlet with a mean velocity of about 3ms−1. The volume of this main debris surge was 3900m3 near the hillslope sources and increased downstream: 5500m3 in the middle part and 8600m3 near the catchment outlet. The debris flow developed a more mature flow pattern as it progressed downstream. Multiple debris and hyperconcentrated surges were observed upstream, while a single debris surge occurred near the catchment outlet. The front-height remained relatively constant during its propagation, whereas its viscosity and velocity tended to decrease downstream as the channel slope decreases. The growth of this main debris surge could be explained by the entrainment of in-channel sediments and the coalescence of the multiple preliminary surges.For the 2011 events, rainfall burst duration and intensity were the main factors controlling the occurrence, volume and runout distance of debris flows. Antecedent channel conditions (sediment recharge, bed wetness and antecedent rainfall) are also important factors, but further investigations are necessary to better understand the role of these factors regarding the triggering and propagation of debris flows in the Réal Torrent.

유동학적 인자에 따른 토석류의 이동 및 퇴적 특성

Abstract Most of the landslides induced by rainfall in summer rainy season appear in the type of debris flow. Debris flow gives a lot of economic losses and human casualties due to high moving velocity and volume of debris flow. In order to analyze movement and deposition characteristics of debris flow, numerical analysis using FLO-2D program was conducted with various viscosities and yield stresses. As a result of numerical analysis, velocity and runout distance of debris flow decreased as its viscosity increased due to resisting force between particles of debris flow. Consequently, flow depth of debris flow increased and impact force decreased. Yield stress of debris flow affected its initiation and deposition characteristics. As yield stress increased, runout distance of debris flow decreased and its impact force increased. Based on the results of numerical analysis, it was found that velocity of debris flow mainly depended on viscosity, while deposition characteristics (runout distance, deposition width, deposition area) of debris flow depended on both viscosity and yield stress.

Changes in runout distances of debris flows over time in the Wenchuan earthquake zone

A large number of debris flows occurred in the Wenchuan earthquake zone after the 12 May 2008 earthquake. The risks posed by these debris flows were rather high. An appropriate model is required to predict the possible runout distance and impacted area. This paper describes a study on the runout characteristics of the debris flows that occurred in the Wenchuan earthquake zone over the past four years. A total of 120 debris flows are analyzed. Separate multivariate regression models are established for the runout distances of hil l-slope debris flows and channelized debris flows. The control variables include type of debris flow, debris flow volume, and elevation difference . Comparison of the debris flows occurring before and after the earthquake shows that the runout distance increased after the earthquake due to sufficient material supply and increased mobility of the source materials. In addition, the runout distances of annual debris flow events in 2008, 2010 and 2011 are analyzed and compared. There is a tendency that the runout distance decreases over time due to the decreasing source material volume and possible changes of debris flow type. Comparison between the debris flows in the earthquake zone and the debris flows in Swiss Alps, Canada, Austria, and Japan shows that the former have a smaller mobility.

Prediction of the run-out distance of the debris flow based on the velocity attenuation coefficient

Our aim is to determine the run-out distance of the debris flow that is crucial in the assessment, prevention and control of the debris flow hazard. Based on the variation characteristic of debris flow velocity in the alluvial fan, this paper proposes the calculation method of the velocity attenuation coefficient of the debris flow. By defining the velocity attenuation coefficient and deducing its calculating formula, this paper puts forward a new method to determine the run-out distance of the debris flow based on the velocity attenuation coefficient, and Gangou debris flow in Luding County, Sichuan Province is selected as a case for calculation and verification. Having 10 m as its measuring spacing, this paper measured 19 sections at the alluvial fan of the Gangou debris flow (among them, 11 sets of data are valid). And based on the measurement, this paper analyzes the characteristic of the velocity attenuation and calculates its velocity attenuation coefficient after the 2005 debris flow. The study indicates that when the velocity of Gangou debris flow at the alluvial fan is greater than 12 % of the initial velocity (at the mouth of gully), the attenuation is quite remarkable. But when the velocity at the alluvial fan is less than 12 % of the initial velocity, the attenuation is quite slow. Besides, when Gangou debris flow rushes out of the gully mouth (the initial velocity is 10 m/s) and when it attenuates to the 32 time, its velocity is less than 0.1 m/s, the debris flow is considered to stop flowing, and the run-out distance of Gangou debris flow is calculated to be 320 m. But the present alluvial fan of Gangou debris flow is measured to be 285 m in length, and the calculated run-out distance is 320 m, which is 35 m longer than its present length. This means when the debris flow runs out in 2005, it blocked up the main river (Dadu River) in some extent. And this finding is generally in accordance with that from the field survey. The findings can be of theoretical and practical significance in the debris flow hazard assessment, as well as its prevention and mitigation.