Gully-type debris flow induced by high-intensity and short-duration rainfall frequently causes great loss of properties and causalities in mountainous regions of southwest China. In order to reduce the risk by geohazards, early warning systems have been provided. A triggering index can be detected in an early stage by the monitoring of rainfall and the changes in physical properties of the deposited materials along debris flow channels. Based on the method of critical pore pressure for slope stability analysis, this study presents critical pore pressure threshold in combination with rainfall factors for gully-type debris flow early warning. The Wenjia gully, which contains an enormous amount of loose material, was selected as a case study to reveal the relationship between the rainfall and pore pressure by field monitoring data. A three-level early warning system (zero, attention, and warning) is adopted and the corresponding judgement conditions are defined in real time. Based on this threshold, there are several rainfall events in recent years have been validated in Wenjia gully, which prove that such a combined threshold may be a reliable approach for the early warning of gully-type debris flow to safeguard the population in the mountainous areas.
Gully-type debris flow, a kind of common geohazard in southwest China, is mainly triggered by high-intensity short-duration rainfall causing a runoff-induced effect. Most of them are initiated by shallow landslides distributed along the gullies in mountainous regions (Kean et al., 2013). The fast growth of the population and economic development in these areas increase the frequency of catastrophic accidents and consequent socio-economic losses. The Wenchuan earthquake on 12 May 2008, Yushu earthquake on 14 April 2010, Lushan earthquake on 20 April 2013, Ludian earthquake on 3 August 2014, and the Nepal earthquake on 25 April 2015 triggered thousands of landslides and cracked mountains, which made these areas prone to debris flow development under rainstorm conditions (Shieh et al., 2009). In the Chi-Chi earthquake area in Taiwan, numerous co-seismic landslides were triggered as well, causing the continuous triggering of debris flows for 10 years after the earthquake (Yu et al., 2013b). These catastrophic events have greatly shocked the local people and government because of the human vulnerability to natural hazards as well as the lack of knowledge on natural disaster prevention and mitigation. There is an urgent demand for an effective method to reduce the hazard and risk. Therefore, researchers have been working on the forecast of debris flow occurrence and setting up of early warning systems for several decades. At the regional scale, the methods for shallow-landslide early warning are mostly based on statistical models and empirical conclusion and have already proven their importance in landslide prevention and mitigation (Keefer et al., 1987; Guzzetti et al., 2007a; Baum and Godt, 2009; Segoni et al., 2014; Shuin et al., 2012; Tropeano and Turconi, 2004). In the beginning, one or two parameters were selected for the assessment of rainfall thresholds to forecast landslide occurrence, e.g. rainfall intensity and duration (Keefer et al., 1987; Guzzetti et al., 2007a, b; Cannon et al., 2008), antecedent precipitation (Glade et al., 2000), and cumulative rainfall (Guo et al., 2013). Hereafter, Baum and Godt (2009) presented a combination threshold, including cumulative rainfall threshold, rainfall intensity–duration threshold, and antecedent water index or soil wetness for the shallow landslide forecasting. At the local scale, physical methods (e.g. numerical simulation) were used to find relationships among rainfall, soil properties, and pore pressure and their contributions to slope stability (Iverson, 1997; Peng et al., 2014; van Asch et al., 2013; Thiebes, 2012; Chae and Kim, 2011; Michel and Kobiyama, 2016; Beven and Kirkby, 1979; Deb and El-Kadi, 2009). However, detailed information related to landslide triggering is required to establish the site-specific thresholds, which are very difficult to extrapolate to other places due to the large variation in soil properties between different regions. Yu et al. (2013a) selected several identified factors related to topography, geology, and hydrology to develop a normalised critical rainfall factor combined with an effective cumulative precipitation and maximum hourly rainfall intensity index for the forecast of gully-type debris flows. The model, which is partly based on a runoff-induced mechanism, has been successfully applied to the Wangmo River catchment, Guizhou Province, China (Yu et al., 2014).
Unfortunately, strong earthquake shocks in southwest China caused a significant rise in the frequency of debris flow during recent years. The long-term effect of earthquakes causes the region to become a high-risk area, and particularly the gullies in mountains with no debris flows before become the debris flow gullies at present. The mechanism, movement characteristics, and thresholds of debris flow in these shocked areas, therefore, have been paid great attention by researchers, e.g. Guo et al. (2013), Huang et al. (2015a), Yin et al. (2010), Yu et al. (2014), Zhou and Tang (2013), and so on. But these models still mainly focused on rainfall threshold, with no consideration about the rise of loose deposited material and unstable slope distributed along the catchments. Therefore, during this study, pore pressure in slope stability analysis has been considered for establishing a combined threshold. The goal of the presented study is to propose a comprehensive method for gully-type debris flow early warning by real-time monitoring of rainfall and changes in pore pressure in the deposited material along channels in southwest China. The infinite slope stability analysis was applied to identify the critical stability conditions of the deposited material. Then, a comprehensive warning threshold for rainfall and critical pore pressure will be presented, which includes both rainfall conditions and soil properties. Finally, verification and revision will be discussed to search for a practical and useful method for reducing the risks of gully-type debris flow in southwest China.
Location of Wenjia gully modified from Huang et al. (2013). The inset photograph of Wenjia gully at the left bottom was taken from the other side of Mianyuan River on 10 August 2008.
The Wenjia gully is located to the north of Qingping town, Mianzhu city,
Sichuan Province, southwest China, and has a catchment area of 7.8 km
DEM map of Wenjia gully and photo of the debris flow event (13 August 2010).
Aerial image of Wenjia gully
Before the Wenchuan earthquake on 12 May 2008, the Wenjia catchment was
covered by rich vegetation, and the channel was smooth and stable, as shown
in Fig. 3a. At that time, few geological disasters had occurred in this region.
Therefore, many farmers settled down at the foothills along the Mianyuan
River at Qingping town downstream of the Wenjia channel's outlet (Fig. 3a, b).
During the earthquake, a giant landslide occurred upstream in the catchment
at the top of the watershed, which generated abundant co-seismic rockfall
material and finer landslide deposits on a platform with an elevation of
1300 m a.s.l. (Fig. 1, the photograph at the left bottom of the main
map). These loose solid erodible materials could easily transform into debris
flows during a rainstorm. Shortly after the earthquake on 24 September 2008,
one rainfall event caused the first debris flow in this gully. The
catastrophic debris flow triggered by a heavy rainfall on 13 August 2010,
with a peak discharge of 1530 m
Geological profile of the main channel of Wenjia gully.
Figure 4 is the geological profile of cross section A
According to Terzaghi theory in soil mechanics, the shear strength of
material at a point within a slope can be expressed as Eq. (1).
Rainfall infiltrates into a hillslope, always accumulating in a saturated
zone above a permeability barrier, and increases the pore pressures within
the slope material. Based on the Terzaghi's work, the increase in
Since the deposited material along the channel usually is loose and has a grain shape, it can be regarded as an infinite slope composed of cohesionless materials. Therefore, the critical pore pressure (Eq. 2) can be used to calculate the stability of the source area. Then pore pressure and rainfall monitoring sensors were installed in the Wenjia gully to capture the real-time data and put forward a comprehensive warning threshold for forecasting debris flow occurrence. The history events of rainfall with debris flow occurrences and non-occurrences have been collected for this study from 2008 to 2018. Fortunately, three debris flow events with detailed rainfall and pore pressure monitoring data have been recorded, which could be important evidence to prove the presented methodology.
Data were collected from the literature about the occurrence of debris flows in the Wenjia gully and from technical reports and documents presented by government agencies. Since there is a large difference in debris flow frequency before and after the Wenchuan earthquake, only the data after the quake were used for the analyses and set-up of an early warning system (Table 1). There were no debris flow events after 2014, so the rainfall data are omitted in the table.
The frequency of debris flow events in Wenjia gully from 2008 to 2017.
Primary rainfall events in the catchment of Wenjia gully (2008–2018), added from Xu (2010) and Yu et al. (2013b).
Table 1 and Fig. 5 show that the number of debris flows decreases with time. Several years after the earthquake, however, giant debris flows still caused catastrophic losses, which alarmed the public and government because of their huge destructive power and long-term impact. Particularly on 13 August 2010, a great rainstorm lasting for 2 h during the night, triggered a giant debris flow, which buried Qingping town in the Mianyuan River floodplain. According to the inventory report, the maximum deposition height was up to 6 m. Most of the check dams located in the downstream part of the Wenjia gully collapsed and lost their effectiveness after passing of the debris flow. Meanwhile, it eroded the channel bottom over a depth of about 13 m (Yu et al., 2013b).
Pore pressure and rainfall monitoring sensors have been installed for understanding their relationship and the link with debris flow occurrence. The real-time monitoring system in the Wenjia gully includes seven automatic rain gauges and five pore pressure monitoring instruments. The installation was finished by 1 April 2012 (see Table 2, Figs. 4 and 6). It can be seen that all rain gauges are arranged in the upstream part of the Wenjia gully catchment, while pore pressure monitoring sensors are distributed along the main stream of the Wenjia gully, with a depth of 1 m below the ground surface.
Layout map of the monitoring devices installed in the Wenjia gully (the base map is from Google Earth, the date of the background image is 18 December 2010).
The 2012 heavy rainfall event on 14 August, which triggered a debris flow,
was completely recorded by the real-time monitoring system. During the
rainstorm, monitoring sensors YL05, YL06 and SY02, SY05 lost connection
with the monitoring centre. The other monitoring sensors worked well, as
shown in Figs. 7 and 8. The figures show that the rainfall was almost
concentrated in 2 h from 17:00 until 19:00 UTC
List of monitoring devices in the Wenjia gully.
The rainfall in Wenjia gully on 14 August 2012 (the column graphs are hourly rainfall and the single line curves are cumulative rainfall).
The maximum hourly rainfall and cumulative rainfall are not found in the highest part of the catchment. The variability in cumulative maximum rainfall is larger than the variability in maximum hourly rainfall intensity. Figure 8 shows the relation between hourly rainfall and pore pressure: the small amount of rain from 02:00 to 05:00 with a maximum hourly rainfall of 12.5 mm did not trigger any change in pore pressure. However, during the concentrated rain period between 15:00 and 18:00 there was a sudden rise in the pore pressure. The debris flow was triggered adjacently when it reached the maximum rise in the pore pressure. The highest value of the pore pressure is 9.1 kPa (SY01) at 18:00, 5.7 kPa (SY03) at 20:00, and 7.8 kPa (SY04) at 17:00. The sudden rise in pore pressure may therefore be a good indicator for contributing to the gully-type debris flow occurrence.
The rainfall and pore pressure in Wenjia gully on 14 August 2012 (the column graphs are hourly rainfall and the single line curves are pore pressure).
In order to improve the warning thresholds for forecasting the debris flow
occurrence, which do not just represent a simple relationship between
rainfall and debris flow occurrence, the pore pressure of landslide deposits
was incorporated into the assessment of a threshold. Critical pore pressure
for bed-failure-generating debris flows can be estimated with Eq. (2). The total
unit weight of deposit material at the 1300 platform in
the Wenjia gully is around
The critical pore pressure with probable depth of slip surface in the Wenjia gully.
Considering the acquired available data, the maximum hourly rainfall
(
Warning threshold combined with rainfall and pore pressure.
Above the red line, the probability of debris flow occurrence is higher. But among these points, there are eight rectangular points with debris flow. The possibility of debris flow occurrence can be predicted correctly up to 62 % by rainfall threshold, which seems to be fine as a preliminary assessment. If the pore pressure monitoring data have been considered, in Fig. 10, there are three blue circular points without debris flow, but two magenta points show debris flow happened. The difference between them is that maximum pore pressure has exceeded the critical pore pressure line (8.22 kPa). Therefore, the rainfall threshold and pore pressure threshold need to be combined while forecasting debris flow occurrence; then there must be a much higher possibility of successful prediction. For a given rainfall event, the starting point and its trend can be calculated and plotted in Fig. 10 in order to verify in real time whether the trend line exceeds the warning threshold. More detailed information will be discussed with an example as follows.
In order to make better use of the presented method, early warning criteria have to be simplified to make a clear understanding for the study area. Therefore, a three-level early warning system has been proposed for the Wenjia gully, as shown in Table 3. At level one there is a low possibility of debris flow occurrence. At level two there is a chance of debris flow occurrence in the near future, and warning messages need to be sent to local authorities and countermeasures need to be discussed. At level three debris flow is very likely to occur immediately; therefore, local residents need to be alerted and prevented from entering threatened areas.
Recommended warning levels for Wenjia gully.
In order to explain how the presented method can be used in a real-time
debris flow early warning, the rainfall on 8 July 2013 has been
selected as an application (Fig. 11). The small, circular, solid magenta points
connected by a magenta line shows the course of the real-time monitored data
during this rainstorm, with the cumulative rainfall on the
Case application of the presented method in Wenjia gully (8 July 2013).
The case study shows how to use this presented combined warning threshold in a real-time way during a rainstorm. In 2014, two heavy rainstorms (Table 1) both exceeded the rainfall threshold, but pore pressure did not cross the critical pore pressure during the whole course of the rainfall. Therefore, a warning message has been sent to the local government with a median possibility of debris flow occurrence. At last, fortunately no debris flow occurred during these rainstorms. Therefore, the presented comprehensive warning threshold can be used as a helpful tool for debris flow prediction in mountainous areas, especially in this earthquake area of southwest China where a lot of loose material is available.
Gully-type debris flow, usually triggered by high-intensity and short-duration rainstorms, causes serious harm to human lives and property every year in the mountainous region of southwest China. Therefore, in order to prevent such natural disasters, there is an urgent requirement for an effective method to predict debris flow occurrence. The combined warning threshold proposed and discussed in this paper not only uses the common rainfall threshold, but also includes the critical pore pressure determined by a hydromechanical stability model.
Two rainfall triggering factors, maximum hourly rainfall and cumulative rainfall, have been selected to establish a simple rainfall threshold as a baseline for debris flow early warning. Critical pore pressure can be used as a combined threshold to make the warning threshold better in practical usage. The Wenjia gully was selected as a case study for a detailed explanation of the presented method, for the great volume of deposited materials triggered by Wenchuan earthquake along the channel. The results show that the combined threshold can play a great role in debris flow prediction, at least reducing the mistaken alerts for debris flow occurrence compared to use of only a rainfall threshold. However, such a combined warning threshold still has some restrictions. First, the critical pore pressure is a linear function with the depth of a potential slip surface, which is difficult to determine in an actual gully by the real-time monitoring system. In this study, 1 m depth of the slip surface was selected as a possible condition for this preliminary study. Second, the study area still focused on the Wenjia gully; therefore the presented method cannot be used in other gullies directly. But in the near future, different gully-type debris flows will be researched, and more subsequent work needs to be carried out for a better understanding of debris flow prediction. Finally, the most complex problem is the final determination of whether to alert the local population, and whether action needs to be taken immediately or later. Debris flow early warning is not an imminent hazard but is just regarded as a potential danger. In spite of these limitations, the methodology presented in this paper has reached the goal of establishing a preliminary combined warning threshold for gully-type debris flow prediction. In future studies, the critical pore pressure threshold, which is dependent on topography, geology, and soil properties, can be determined by long-term field monitoring and more important by debris flow tests in a laboratory with different slope angles and depths of slip surface to reduce the loss of property and lives.
All data included in this study are available upon request by contacting the corresponding author.
JH conducted the analysis and wrote the paper, TWJvA helped with the language, and CW & QL helped with the data analysis.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Flood risk assessment and management”. It is a result of the EGU General Assembly 2018, Vienna, Austria, 8–13 April 2018.
This study was financially supported by the Funds for Creative Research Groups of China (grant no. 41521002), International Cooperation (NSFC-RCUK_NERC), Resilience to Earthquake-induced landslide risk in China (grant no. 41661134010), and State Key Laboratory of Geo-hazard Prevention and Geo-environment Protection (Chengdu University of Technology) (grant no. SKLGP2017Z006). The authors also give sincere thanks to Niek Rengers for his review of an earlier version of this paper and for his suggestions to polish the language, which greatly improved the quality of the paper. Edited by: Dhruvesh Patel Reviewed by: Qiang Dai