Rainfall Thresholds For Debris Flows Research Articles

Constructing Rainfall Threshold for Debris Flows of a Defined Hazardous Magnitude

Debris flow can cause damage only when its discharge exceeds the drainage capacity of the prevention engineering. At present, most rainfall thresholds for debris flows mainly focus on the initiation of debris flow and do not adequately consider the magnitude and drainage measures of debris flows. These thresholds are likely to initiate numerous warnings that may not be related to hazardous processes. This study proposes a method for calculating the rainfall threshold that is related to a defined level of debris flow magnitude, over which certain damage may be caused. This method is constructed by using the transient rainfall infiltration analysis slope stability model (TRIGRS) and the fluid dynamics process simulation model (MassFlow). We first use the TRIGRS model to analyze slope stability in the study area and obtain the distribution of unstable slopes under different rainfall conditions. Afterward, the MassFlow model is employed to simulate the movement process of unstable slope units and to predict the depositional processes at the mouth of the catchment. Lastly a rainfall threshold is constructed by statistically analyzing the rainfall conditions that cause debris flows flushing out of the given drainage ditch. This method is useful to predict debris flow events of a hazardous magnitude, especially for areas with limited historical observational data.

Hydrogeomorphic Recovery and Temporal Changes in Rainfall Thresholds for Debris Flows Following Wildfire

AbstractWildfire‐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.

Spatial uncertainty of rainfall and its impact on hydrological hazard forecasting in a small semiarid mountainous watershed

Rainfall in mountainous watershed presents high spatial variability due to elevation effects, and this introduces uncertainty in forecasting hydrological hazards such as water floods and debris flows. This study investigated the spatial variation of rainfall in a small watershed with a network of 10 rain gauges. A rainfall-elevation relationship was established based on data from 52 rainstorm events, which provides a method for rainfall estimation within the watershed. Result indicated that lower errors of interpolation occur when the rainfall amount is high, and that it is more difficult to estimate rainfall in high-elevation regions. Rain gauges become less representative when the distance between gauges is >3.0 km. The spatial variation of rainfall suggests that the gauge at the lowest elevation, or a single gauge within the source region, shows non-negligible errors with regard to calculating water flood discharge and identifying rainfall thresholds for debris flows. This study contributes to the understanding of event rainfall distribution and its impact on hydrological hazard forecasting in a small mountainous watershed.

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

Empirically derived rainfall thresholds of debris flows are used for regional-scale early warning. However, triggering rainfall intensities of post-seismic debris flows evolve with time, causing high false alarms since thresholds estimated by conventional methods ignore the uncertainty of non-triggering rainfall events. Based on 172 triggering rainfalls and 2396 non-triggering rainfalls from 2008 to 2013 after the Wenchuan earthquake, we analyzed the evolution of probabilistic rainfall thresholds for post-seismic debris flows using a Bayesian technique. We found, rainfall thresholds significantly decrease compared with pre-earthquake events initially and later tend to increase annually. Meanwhile, the triggering rainfall characteristics tend to gradually change from a short-duration high-intensity pattern to a long-duration and low-intensity pattern. We also checked the effect of antecedent precipitation on debris flows by defining an IET (inter-event time). Our results suggest the antecedent precipitation plays an important role in low-intensity long-duration rainfall-induced debris flows and has little effect on the short-duration high-intensity rainfall-induced debris flows. The characteristics of triggering rainfall for debris flows after the Wenchuan earthquake can be best reflected by IET = 7 h. By employing a Naïve Bayes algorithm, uncertainties of three rainfall threshold models, I-D (mean intensity-duration), IT-D (triggering intensity-duration), and IT-E (triggering intensity-cumulative rainfall) were investigated, and the IT-D model was found to perform best. The characteristics and evolution of debris flow rainfall thresholds over the years after the Wenchuan earthquake was best presented by the Bayesian probabilistic rainfall thresholds. We believe the results can be used to improve the precision of an early warning system for post-seismic debris flows.

Variability in the characteristics of extreme rainfall events triggering debris flows: a case study in the Chenyulan watershed, Taiwan

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.

Characteristics of rainfall responsible for debris flows in Wenchuan Earthquake area

The ability to forecast debris flows is important because of their frequent occurrence and potential for large-scale damage in the Wenchuan Earthquake-hit area. Accurate understanding of the characteristics of rainfall responsible for triggering debris flows is necessary for an effective early warning procedure. This study examined the general characteristics and spatiotemporal variation of rainfall events that responsible for debris flows. Analysis was performed on rainfall data extracted during 2008–2013. The results revealed that the rainfall thresholds for debris flows varied annually and showed tendency toward higher conditions required. The coefficient of variation of relevant rainfall events indicated that the characteristics of the rainstorms changed. In addition, the coefficient of deviation revealed the spatial variation of rainfall in subregions, which was attributed to differences of the local climatic conditions that control debris flow occurrence. The primary controlling rainfall indices for triggering debris flows were filtered, and the most suitable models for establishing rainfall thresholds were selected. Ultimately, the rainfall thresholds were obtained for each subregion within the study area.

Spatial-Temporal Assessment of Debris Flow Risk in the Ms8.0 Wenchuan Earthquake-Disturbed Area

For 5 years (2009–2013) after the 2008 Ms8.0 Wenchuan earthquake, rainfall led to the transformation of unconsolidated co-seismic deposits into extensive and severe debris flows, causing significant loss of life and property. For debris flows in the earthquake-disturbed area, a few common concerns exist. What is their spatial-temporal distribution? What are the controlling factors? How much is the rainfall threshold for debris flows? What areas are more susceptible? Where suffered the most severe losses of life and property? Using debris flow characteristics, this study analyzes the relationships between seismic geological factors, geomorphologic factors, extreme rainfall, and debris flows in the 5 years following the earthquake, and draws the following conclusions. (1) There are regional differences in the rainfall threshold for generation of debris flows, and the annual maximum 72-hour accumulated rainfall for triggering a debris flow decreases from pre-seismic periods (135–325 mm) to post-seismic periods (75–160 mm) by 44.4–50.8% in study area. (2) Areas with high debris flow susceptibility and hazard are primarily controlled by seismic geological conditions. (3) The long-term risk of debris flows will fall to moderate, and the affected area will shrink to that around the seismogenic fault. The results of this study will help with meteorological early warning systems, deployment of disaster prevention and control projects, and reconstruction site selection in the post-seismic Longmen Mountain area.

Variability in rainfall threshold for debris flow after Wenchuan earthquake in Gaochuan River watershed, Southwest China

Wenchuan earthquake occurred 7 years ago, and more than 100 landslides and rock avalanches were triggered by the earthquake in Gaochuan river watershed, in Southwest China, including the Daguangbao landslide, which is the largest one triggered by Wenchuan earthquake, having a volume around 1.15 billion m3. The massive landslide deposits provided uncountable material source for debris flow in the study area, resulting in 25 debris flow events occurred in the past 6 years (2009–2014). The purpose of this paper is to analyze the variability in the rainfall threshold for debris flow after the earthquake in the study area. Results suggest that the critical rainfall threshold is closely related to the rainfall intensity (I) and the antecedent accumulated precipitation (P). There was a sudden drop of the rainfall threshold when the earthquake happened. During the 6 years, the rainfall threshold value kept increasing gradually in a linear trend. However, it is now still much lower than the original rainfall threshold prior to the earthquake. The threshold is expected to reach the original level around 2022 based on the analysis of existing data. Our study indicates that the increase of rainfall threshold is related to the decrease of sediments. And there are still lots of loose deposits in the gullies. Thus, the debris flow can still happen in a high possibility in the next few years.

Temporal differentiation of rainfall thresholds for debris flows in Wenchuan earthquake-affected areas

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.

Intensity–duration threshold of rainfall-triggered debris flows in the Wenchuan Earthquake affected area, China

The Ms 8.0 Wenchuan Earthquake has greatly altered the rainfall threshold for debris flows in the affected areas. This study explores the local intensity–duration (I–D) relationship based on 252 post-earthquake debris flows. It was found that I=5.25 D-0.76 accounts for more than 98% of the debris flow occurrences with rainfall duration between 1 and 135h; therefore the curve defines the threshold for debris flows in the study area. This gives much lower thresholds than those proposed by the previous studies, suggesting that the earthquake has greatly decreased the thresholds in the past years. Moreover, the rainfall thresholds appear to increase annually in the period of 2008–2013, and present a logarithmic increasing tendency, indicating that the thresholds will recover in the future decades.

Critical rainfall thresholds for debris flows in Sanyanyu, Zhouqu County, Gansu Province, China

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.

The formation and development of debris flows in large watersheds after the 2008 Wenchuan Earthquake

The Wenchuan earthquake has caused abundance of loose materials supplies for debris flows. Many debris flows have occurred in watersheds in area beyond 20 km(2), presenting characteristics differing from those in small watersheds. The debris flows yearly frequency decreases exponentially, and the average debris flow magnitude increases linearly with watershed size. The rainfall thresholds for debris flows in large watersheds were expressed as I = 14.7 D (-0.79) (2 h < D < 56 h), which is considerably higher than those in small watersheds as I = 4.4 D (-0.70) (2 h < D < 37 h). A case study is conducted in Ergou, 39.4 km(2) in area, to illustrate the formation and development processes of debris flows in large watersheds. A debris flow develops in a large watershed only when the rainfall was high enough to trigger the wide-spread failures and erosions on slope and realize the confluence in the watershed. The debris flow was supplied by the widely distributed failures dominated by rill erosions (14 in 22 sources in this case). The intermittent supplying increased the size and duration of debris flow. While the landslide dam failures provided most amounts for debris flows (57 % of the total amount), and amplified the discharge suddenly. During these processes, the debris flow velocity and density increased as well. The similar processes were observed in other large watersheds, indicating this case is representative.

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

Numerous landslides were triggered in earthquake-affected areas and the largest one was located close to the rupture fault. Abundant loose solid erodible materials from co-seismic rock falls and landslides were deposited in gullies, which formed the source material for debris flows. Numerous debris flows were triggered in the years after the earthquake during the rainy season. The Wenchuan and Chi-Chi earthquakes are examples where after these events the triggering thresholds for debris flows significantly decreased. Some studies emphasized that the increased debris flow activity in the earthquake-affected areas will last 20 to 40years. However it is still an open question because no detailed research was conducted on the temporal occurrence of debris flows over a long time span after a strong earthquake. In this paper the ratio of rainfall thresholds after and before earthquakes is studied in relation to the peak ground acceleration (PGA) of earthquakes and the number of rainy seasons after the earthquake. The debris flows in the Chi-Chi and Wenchuan earthquake areas were selected for this study. The preliminary results are: (1) the earthquakes caused an instantaneous decrease of the rainfall threshold for debris flows. The ratio (r) of threshold after the earthquake over the threshold before the earthquake decreases with a power function with the increasing dimensionless peak ground acceleration (PGA) of the earthquake; (2) in the first 7 rainy seasons after the earthquake the ratio (r) linearly increases with the increasing number of elapsed rainy seasons; and (3) the earthquake not only has an increasing effect on the debris flow activity but also has an influence on the duration of this activity. The duration of the enhanced activity increases with a larger PGA.

Characteristic rainfall for warning of debris flows

A characteristic rainfall is introduced to overcome the difficulties encountered in determining a critical rainfall value for triggering debris flow. The characteristic value is defined as the rainfall at which debris-flow occurrence probability shows a rapid increase, and can be used as a warning rainfall threshold for debris flows. Investigation of recorded debris flows and 24-hour rainfall data at Jiangjia basin, Yunnan Province, in southwestern China, demonstrates the existence of such a characteristic rainfall. It was found that the characteristic rainfall corresponds to the daily rainfall of 90% cumulative probability by analyzing the basin’s daily rainfall histogram. The result provides a simple and useful method for estimating a debris-flow warning rainfall threshold from the daily rainfall distribution. It was applied to estimate the debris-flow warning rainfall threshold for the Subaohe basin, a watershed in the 2008 Wenchuan earthquake zone with many physical characteristics similar to those of the Jiangjia basin.

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

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