On predicting debris flows in arid mountain belts

The accurate prediction of debris flows occurrence that will allow the reduction or prevention of economic losses and human casualties is presently the most difficult aspect of debris flows studies but also the aspect that receives most attention. Most prediction methods are based on rainfall as the basic parameter, with the moment of occurrence as only result, and without a prediction of debris flow travel time and size. This paper takes Jiangjia Gully in Dongchuan of Yunnan Province as an example, and considers, on the basis of the fulfillment of the essential condition: the abundant availability of loose materials, the conditions for the formation of debris flows. Based on the mechanism of the initiation of debris flows in channels and the volume of rainfall in the basin, this paper also gives a systematic analysis on the travel time and size of the debris flow and suggests that the hydrological condition for forming debris flow is the unit discharge of the flood ≥0.35 m3/s.m. It uses the 10-min rainfall intensity to calculate both the run-off of the rainfall and the unit discharge caused by the run-off, thus predicting the occurrence of debris flows. The velocity and the travel time of a debris flow can also be determined using the unit discharge of the run-off. The total volume of debris flows can be calculated using the 10-min intensity of rainfall and the total volume of the run-off, together with the volume concentration of the sediment in a debris flow.

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.

Debris flow is one of the most common types of geological disasters in China. Owing to the influence of topography, geomorphology, geological conditions, human activity, and rainfall debris flow disasters frequently occur in the mountainous areas of Beijing. The research on debris flow in the Beijing area focuses on rainfall and risk evaluation, material sources, and early warning and prevention of debris flow. However, there are few studies on the development characteristics of single-gully debris flow and the prediction of hazardous areas in the Beijing area. Therefore, we chose the debris flow of Bangou Village in Yanqing District of Beijing as the research object. We analyzed the recharge conditions in the ditch domain and predicted the extent of the hazardous area around the gully, providing suggestions for control measures. The dynamic reserves of the loose deposits in the debris flow gully, currently in the development stage, were estimated as 15.48 × 104 m3, representing four supply sources: artificial deposits, alluvium and diluvium, residual slope deposits, and collapse. The peak flow is 24.49 m3/s for a 10-year rainfall event, 27.64 m3/s for a 20-year rainfall event, 31.79 m3/s for a 50-year rainfall event, and 34.93 m3/s for a 100-year rainfall event. The total amounts of solids washed out by a debris flow from the preceding events are 0.70 × 104 m3, 0.79 × 104 m3, 0.91 × 104 m3, and 1.00 × 104 m3, respectively. The size of the debris flow is small, with a maximum hazardous area of 0.2810 km2. We conclude that a small debris flow outbreak in the Bangou Village gully is possible. We expect that the results of this study will provide basic information and help improve debris flow research in Beijing.

Development and maintenance of a telescoping debris flow fan in response to human-induced fan surface channelization, Chalk Creek Valley Natural Debris Flow Laboratory, Colorado, USA

Micro accelerometers are crucial tools for monitoring and providing early warning of geological disasters. This study focuses on the potential debris flow in Wangjiayuan, Fang County, Hubei Province, aiming to explore the relationship between clay content and debris flow motion characteristics. Physical model experiments were designed with varying clay content to simulate debris flow motion, and micro accelerometers were employed to monitor and record vibration acceleration changes during debris flow initiation and motion. Frequency spectra were generated using Fourier transform to analyze the motion characteristics in the frequency domain. The results showed that the predominant period of debris flow initiation ranged from 15 Hz to 29 Hz, while the period during motion in the middle section of the flow area ranged from 36 Hz to 54 Hz. Higher clay particle content was associated with a decrease in both the frequency range and mean of the predominant period, as well as a reduction in the Fourier amplitude extremum. These findings provide a foundation for monitoring and predicting debris flows, and the vibration signal characteristics of debris flow initiation and motion can be used to infer the clay content, offering insights into early warning systems for such geological hazards.

Volumes of erosional and depositional landforms were estimated by reconnaissance methods in a steep, forested drainage basin in the Coast Mountains, British Columbia, to examine what insight might be gained into the sediment budget in the absence of direct measurements of sediment transfer processes. Sediment transfers in postglacial and contemporary time were inferred on hillslopes, from hillslopes to stream channels, and within stream channels. Data were collected by aerial photographic analysis and field observation. Postglacial sediment sources are major gullies created by debris slides and flows, failures in glaciolacustrine terraces, and stream channel degradation. Depositional landforms consist of talus slopes and colluvial and alluvial fans. Contemporary sediment sources include debris slides and flows in established gullies and minor processes on hillslopes. Debris slide and debris flow volumes were calculated, and other processes were estimated from regional values. Erosion rate averaged over postglacial time is 276 t·km–2·a–1(0.15 mm·a–1surface lowering), with gullies and stream channel degradation contributing 170 and 82 t·km–2·a–1, respectively. A terminal alluvial fan provides an independent check of the results. In contemporary time, erosion rates are calculated to be 350 t·km–2·a–1, with debris flows and slides contributing nearly all of this sediment. The contemporary rate is probably perturbed by recent land use history. Mass-movement processes appear to be the dominant mechanism of sediment transfer and, contributions from Pleistocene valley deposits have declined significantly during Holocene time.

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

Debris flow with intermittent surges is a major natural hazard. Accurate estimation of the total volume of debris flow is a challenge for academic researchers and engineering practitioners. This paper has proposed a new model for the total volume estimation based on 15 years of observations in Jiangjia Valley, China, from 1987 to 2004. The model uses two input variables: debris flow moving time and average surge peak discharge. The Weibull distribution formula is adopted to describe the relationship between the debris flow surge peak discharge and its relative frequency. By integrating the Weibull function and two-point curve fitting, the relationship between the maximum discharge and average surge peak discharge can be established. The total debris flow volume is linked with the debris flow moving time and the average peak discharge. With statistical regression, the debris flow moving time is derived from the debris flow total time. The proposed model has fitted very well with the validation data and outperformed the existing models. This study has provided a new and more accurate way for estimating the total volume of debris flows with intermittent surges in engineering practice.

Alluvial fan facies in the miocene-pliocene coastal plain sands, Niger Delta, Nigeria

Abstract. Traditionally, interactions between tributary alluvial fans and the main river have been studied in the field and in the laboratory, giving rise to different conceptual models that explain their role in the sediment cascade. On the other hand, numerical modeling of these complex interactions is still limited because the broad debris flow transport regimes are associated with different sediment transport models. Even though sophisticated models capable of simulating many transport mechanisms simultaneously exist, they are restricted to research purposes due to their high computational cost. In this article, we propose a workflow to model the response of the Crucecita Alta alluvial fan in the Huasco Valley, located in the Atacama Desert, Chile, during an extreme storm event. Five different deposits were identified and associated with four debris flow surges for this alluvial fan. Using a commercial software application, our workflow concatenates these surges into one model. This study depicts the significance of the mechanical classification of debris flows to reproduce how an alluvial fan controls the tributary–river junction connectivity. Once our model is calibrated, we use our workflow to test if a channel is large enough to mitigate the impacts of these flows and the effects on the tributary–river junction connectivity.

<p>The prediction of debris flows is relevant because this type of natural hazard can pose a threat to humans and infrastructure. Debris-flow (and landslide) early warning systems often rely on rainfall intensity–duration (ID) thresholds. Multiple competing methods exist for the determination of such ID thresholds but have not been objectively and thoroughly compared at multiple scales, and a validation and uncertainty assessment is often missing in their formulation. As a consequence, updating, interpreting, generalizing and comparing rainfall thresholds is challenging. Here, we present the findings of Hirschberg et al. (2021), which focused on (i) uncertainties related to ID thresholds, (ii) differences in local compared to regional ID thresholds, and (iii) how prediction can potentially be improved using statistical learning algorithms. The findings are of interest for debris-flow (and landslide) early-warning developers.</p><p>We use a 17-year record of rainfall and 67 debris flows in a Swiss Alpine catchment (Illgraben) to determine ID thresholds and associated uncertainties as a function of record du- ration. This included comparing two methods for rainfall threshold definition based on linear regression and/or true-skill-statistic maximization. The main difference between these approaches and the well-known frequentist method is that non-triggering rainfall events were additionally considered for obtaining ID-threshold parameters. Depending on the method applied, the ID-threshold parameters and their uncertainties differed significantly. We found that 25 debris flows are sufficient to constrain uncertainties in ID-threshold parameters to ±30% for our study site. We further demonstrated the change in predictive performance of the two methods if a regional landslide data set with a regional rainfall product was used instead of a local one with local rainfall measurements. Hence, an important finding is that the ideal method for ID- threshold determination depends on the available landslide and rainfall data sets. Furthermore, for the local data set we tested if the ID-threshold performance can be increased by considering other rainfall properties (e.g. antecedent rainfall, maximum intensity) in a multivariate statistical learning algorithm based on decision trees (random forest). The highest predictive power was reached when the peak 30 min rainfall intensity was added to the ID variables, while no improvement was achieved by considering antecedent rainfall for debris-flow predictions in Illgraben. Although the increase in predictive performance with the random forest model over the classical ID threshold was small, such a framework could be valuable for future studies if more predictors are available from measured or modelled data.</p>

Influence of bedrock geology on sedimentation in Pre-Late Wisconsinan alluvial fans in the Canadian Rocky Mountains

Initiation mechanisms and dynamics of a debris flow originated from debris-ice mixture slope failure in southeast Tibet, China

<p>Alluvial fans are sedimentary bodies typical for mountain environments of all latitudes and climatic zones. Sediment, building alluvial fans, is transported from the catchments area by fluvial and/or sedimentary gravity flow processes resulting in a variety of distinct sedimentary facies. Several different factors control the prevailing sedimentary process on alluvial fans such as climate, catchment and fan area, catchment relief, aspect, vegetation types and density, tectonic setting, and bedrock lithology. In this study we present a detailed study of fluvial and sedimentary gravity flow facies on two adjacent Holocene alluvial fans. The fans are located in the Planica Valley in the Julian Alps (NW Slovenia). Both alluvial fans have very similar dimension, same climatic conditions, relief, and vegetation cover, however, differ in the geology setting of the catchment areas. While the catchment area of the first fan consists entirely of Julian aged peritidal to shallow subtidal carbonates Conzen dolomite, the adjacent second catchment area is partly composed of Julian-Tuvalian shallow-water claystone, marlstone, dolomite, and marly limestone. We made twelve sedimentary logs in a scale of 1: 10 and performed a granulometric analysis of several sediment layers to determine sedimentary processes forming the two alluvial fans. The first alluvial fan is entirely built of either clast-supported open-framework gravel or sandy gravel layers. The layers, up to 20 centimetres thick, contain less than 1% of mud fraction and the clasts are Conzen dolomite. Gravel is predominantly pebble sized, with occasional cobbles and boulders in the upper and middle part of the fan. Cobble sized clasts are occasionally imbricated parallel with slope inclination. We attribute these layers to fluvial and/or sheetflood depositional processes. The second alluvial fan is also predominantly composed of the same fluvial to sheetflood deposited layers, which are intercalated with crudely stratified and up to 50 cm thick layers of muddy sandy gravel. These layers contain up to 16% of mud fraction with gravel clasts being matrix-to clast supported. Based on sediment texture and the amount of mud fraction contained we attribute these layers to cohesive and highly viscous debris flow deposits. The debris flow deposits however do not represent the predominant sedimentary process in the second alluvial fan. This suggests that debris flows occur only sporadically indicating to their stochastic depositional occurrence. The study show that the catchment bedrock is the most important parameter determining the sedimentary process in a typical alpine alluvial fan when all other controlling factors are equal.</p>

Earthquakes trigger large numbers of landslides that provide abundant material for rainfall-induced debris flows and hence increase the frequency and magnitude of post-quake debris flows. A physically-based model incorporating a quantitative function of how much landslide deposit transforms into debris flows is developed to calculate debris-flow peak discharge by dividing a catchment into non-erosion, hydraulic erosion, and gravitational erosion zones. The model was implemented with a distributed hydrological computation program, and applied to Qipan catchment that was strongly influenced by the 2008 Wenchuan earthquake and where the debris flow peak discharge in 2013 was more than 10 times that before the earthquake. Remote sensing images and digital elevation model (DEM) data were used to obtain the spatial distribution of the earthquake-induced landslides. In situ tests were conducted to measure the hydrological parameters of the landslides. Comparisons of field survey and simulation results were carried out to verify the model validity. At 14 cross-sections, the peak discharge errors range from 3 to 20%. Moreover, the error of the inundation area is 2.1%, which suggests that the model is highly applicable to hazard mapping.KeywordsDebris flowPeak dischargeEarthquake-induced landslidesDistributed hydrological computation