Mitigation planning based on the prediction of river blocking by a typical large-scale debris flow in the Wenchuan earthquake area
Due to scale amplification resulting from the blocking-bursting process associated with landslide dams in debris flow catchments, many of large-scale debris flows that occurred as a result of the Wenchuan earthquake blocked the main rivers and resulted in catastrophic dam-breaking floods. To decrease the damage caused by dam-breaking floods, mitigation works should be applied in debris flow gullies. Currently, few studies focus on how to determine the key parameters concerning the scale of debris flows (i.e., peak discharge and total volume) that need to be controlled for mitigation planning considering downstream objects affected by the flow. The Xiaojia debris flow is described and analyzed as a typical large-scale case from the Yingxiu area. A back-calculation method and numerical simulation were proposed for mitigation planning in Xiaojia gully based on predictions of river blocking. First, the maximum peak flood discharge that did not endanger the town of Yingxiu was calculated. Then, the permissible blockage from a debris flow was determined based on the back-calculation of a dam-breaking flood. Finally, the scale of the largest permissible debris flow was obtained for use in mitigation plans based on numerical simulations. The calculations showed that the peak discharge of a flood that would not endanger Yingxiu should be <1496.48 m3/s. Accordingly, based on the 1/3 and 1/2 breach modes, the permissible blocking height of a debris flow barrier dam should not exceed 43.09 and 31.64 m, respectively. The total volume and peak discharge of a single debris flow event should be controlled to not exceed 70.59 × 104 m3 and 784.59 m3/s for the 1/3 breach mode and 45.21 × 104 m3 and 551.77 m3/s for the 1/2 breach mode. Based on these determinations of the key debris flow parameters, the simulation results indicate that debris flow damage can be decreased to an acceptable level to ensure the safety of Yingxiu downstream by the implementation of two check dams in the downstream channel and a deposition works on the debris fan.
KeywordsDebris flow Disaster characteristics Wenchuan earthquake River-blocking prediction Mitigation planning
In the Wenchuan earthquake area, during the rainy seasons from 2008 to 2013, the frequency and scale of debris flows greatly increased due to the huge increase in loose materials from the earthquake and extreme weather conditions. On 7–9 July 2013, a heavy rainstorm with maximum daily rainfall of 751.4 mm (Xingfu Village, Dujiangyan, the highest recorded in the past 60 years) swept Wenchuan County and caused significant damage (Liu et al. 2014). Many large-scale debris flows occurred in Wenchuan County, destroying the Dujiangyan-Wenchuan highway (newly constructed and in service for only 7 months), the old National Road G213, and many other rural roads. Among them, the debris flow that occurred on 11 July 2013 in the Qipan gully was the largest in scale and resulted in the most catastrophic damage in this area. Eight people were confirmed dead, and six were listed as missing. In all, 285 buildings, 4 km of drainage channel, three transformer substations, and the manufacturing complexes of seven companies downstream from the gully were completely destroyed by the debris flow.
Due to the scale of amplification resulting from the blocking-bursting process of landslide dams in debris flow catchments (Cui et al. 2011; Huang et al. 2012; Cui et al. 2013a), many of the large-scale debris flows in the Wenchuan earthquake area that blocked main river channels resulted in severe catastrophes (Tang et al. 2009; Huang and Li 2009; Tang et al. 2011; Chen et al. 2012). For example, on 14 August 2010, debris flows in the Hongchun and Shaofang gullies in Wenchuan County blocked the Min River and produced barrier dams that changed the river course and resulted in flooding of the newly reconstructed town of Yingxiu (Tang et al. 2011; Cui et al. 2013b). When a debris flow barrier dam breaks up, the dam-breaking flood can cause devastating effects on downstream facilities, lives, and property (Chang et al. 2011). To decrease the damage of dam-breaking floods, mitigation works can be applied in debris flow gullies. To undertake such debris flow mitigation planning, the downstream objects affected by the dam-breaking flood should be considered in particular. However, few studies have focused on how to determine the key parameters controlling the scale of debris flows (i.e., peak discharge and total volume) that need to be regulated to undertake mitigation planning that considers downstream objects affected by such flows. In this paper, the Xiaojia debris flow is described and analyzed as a representative large-scale case from the Yingxiu area. A back-calculation method and numerical simulation are proposed to assist with mitigation planning for the Xiaojia gully based on a river-blocking prediction.
The study area is located on the eastern edge of the Qinghai–Tibetan Plateau. The terrain is complex and characterized by considerable elevation differences and steep slopes (Yin et al. 2009). High mountains and deep-cut rivers are the main topographic characteristics of the area (Zhang et al. 2006). The geological conditions of the study area are very complex. The gully is very near the deep fracture zone of the Yingxiu–Beichuan reverse fault (part of the Longmen mountain fault zone), running southwest to northeast (Fig. 1). The Ms 8.0 “5.12” Wenchuan earthquake that occurred in this area in 2008 was due to movement on the Yingxiu–Beichuan reverse fault (Wang et al. 2009). The Wenchuan earthquake caused widespread earth surface destruction across the whole catchment of Xiaojia gully and formed a large number of landslips and landslides, providing rich source of loose solid materials for debris flows (Fig. 1). Based on the interpretation of 0.5-m resolution aerial photography from 7 January 2011 (Fig. 1), the total volume of loose solid material available for the formation of debris flows in the catchment was 532.09 × 104 m3 distributed over 0.99 km2 of the mountain surface as the result of earthquake destruction (Liu et al. 2014).
The strata exposed in the gully include diorite (δ23) in the upper reaches, plagioclase granite (γo24) in the middle reaches, and granodiorite (γδ23) in the lower reaches (Fig. 1). The rock mass of the study area is part of a suite of magmatic rocks formed in the Jinning–Chengjing period with structural characteristics including high strength, jointed fissures, and strongly weathered layers at the surface. Consequently, it is prone to instability, collapse, and the formation of landslides.
The study area lies within the subtropical humid monsoon climatic zone of the Sichuan Basin border, and it is in the rainstorm center of the western Sichuan region. According to data from the Yingxiu weather station (Fig. 1), the average annual rainfall is 1253.1 mm, and the maximum and minimum annual recorded rainfall is 1688 and 836.7 mm, occurring in 1964 and 1974, respectively. The average number of annual precipitation days is up to 202.7 days, with a maximum daily rainfall of 269.8 mm. The precipitation over the year is distributed unevenly, concentrated mainly from June to September, which accounts for 60 to 70 % of the annual rainfall (Liu et al. 2010). This period also usually has a high occurrence of collapses, landslides, debris flows, and other geological disasters.
Debris flow events and damage
Data collection and preparation
River-blocking prediction method
The calculating results of related parameters for the possible debris flow peak discharge under different return periods
Numerical simulation method for mitigation planning
Input parameters for the Kanako simulator
Simulation continuance time
Time interval of calculation
Mass density of bed material
Mass density of fluid phase (water and mud, silt)
Concentration of movable bed
Coefficient of erosion rate
Coefficient of accumulation rate
Manning’s roughness coefficient
Number of calculation points in channel
Interval of calculation points in channel
Results and discussion
The calculating results of related parameters for the river-blocking prediction
The permissible flood at Yingxiu Town
The permissible blockage by debris flow
The permissible debris flow scale in Xiaojia gully
Further calculations show that the permissible total debris flow volume of a single debris flow event in Xiaojia gully should be smaller 70.59 × 10 and 45.21 × 104 m3 under the 1/3 and 1/2 breach modes, respectively (Table 3). As a result, the peak debris flow discharge should be controlled to be smaller than 784.59 and 551.77 m3/s under the 1/3 and 1/2 breach modes, respectively, to maintain the safety of downstream Yingxiu (Table 3).
As shown in Table 3, the possible peak debris flow discharges in Xiaojia gully were calculated as 790.50, 628.04, and 452.86 m3/s under 1, 2, and 5 % return periods, respectively. By comparing the permissible peak debris flow discharge (Qcp) and the possible peak debris flow discharge (Qc), the necessity of applying mitigation measures can be evaluated (Fig. 8). For the 1/3 break mode, the permissible peak debris flow discharge (784.59 m3/s) is similar to the possible peak debris flow discharge (790.50 m3/s) in the 1 % return period (Table 3). Therefore, for a single debris flow event in Xiaojia gully, it is necessary to take mitigation measures when the debris flow frequency is larger than the 100-year return period (P = 1 %). However, for the 1/3 break mode, the permissible peak debris flow discharge (551.77 m3/s) is between the 20-year (P = 5 %) and 50-year (P = 2 %) return periods for the possible peak debris flow discharge. So, mitigation measures should be applied to decrease the peak discharge when the debris flow frequency is greater than the 50-year return period (P = 2 %). Ultimately, the peak discharge of a single debris flow event with a 100-year return period (P = 1 %) was used for mitigation planning in Xiaojia gully.
Mitigation planning in Xiaojia gully
Based on the field investigation combined with the topographic conditions of the channel in the downstream part of the Xiaojia gully, two check dams (closed type, Liu et al. 2013) with effective heights of 18 m (check dam #1, Fig. 6) and 12 m (check dam #2, Fig. 6) were proposed. The effectiveness of controlling the debris flow peak discharge with the two check dams was simulated using the Kanako simulator. The simulation results are shown in Fig. 10. When the debris flow passes through check dam #1 (Figs. 6 and 9), the peak discharge upstream of the dam is 749.4 m3/s (Fig. 10a, obs. 3) and 619.3 m3/s downstream from the dam (Fig. 10a, obs. 4). The debris flow peak discharge reduction rate is 17.36 %. Similarly, for check dam #2, the debris flow peak discharge decreases from 619.3 m3/s upstream of the dam (Fig. 10a, obs. 4) to 504.3 m3/s downstream from the dam (Fig. 10a, obs. 5). The peak debris flow discharge reduction rate of check dam #2 is 18.57 %. Figure 10b shows the peak debris flow discharge comparison before and after the implementation of the check dams. The results indicate a fluctuation in the peak debris flow discharge from upstream to downstream, which decreases from obs. 1 to obs. 5 and increases from obs. 5 to obs. 6 (Fig. 10b). The reason for this is the change in the channel width from upstream to downstream. As shown in Fig. 10b, the debris flow peak discharge has an opposite change tendency with the channel width. In the section of obs. 4 and obs. 5 (Fig. 9), the broad channel, with an average width of 40 m forces the debris flow velocity to drop, and thus, deposition occurs to decrease the peak discharge. For the same reason, the debris flow peak discharge at obs. 6 (channel width of 40.8 m, Fig. 10b) is larger than the peak discharge at obs. 5 (channel width of 53.1 m, Fig. 10b). After comparing simulation results for the gully mouth (Fig. 10b), peak debris flow discharge decreases from 727.3 m3/s before mitigation to 531.2 m3/s after mitigation. The mitigation effect is remarkable, with a reduction rate of 26.96 % as a result of the two check dams in the downstream part of the Xiaojia gully.
After the implementation of the two check dams in the downstream channel of the Xiaojia gully, the peak discharge of a single debris flow event was decreased to 531.2 m3/s, which is smaller than the permissible peak discharge of the 1/3 and 1/2 breach mode (Table 3). Therefore, for the mitigation of a single debris flow with a frequency of P ≤ 1 %, the implementation of two check dams is an effective way to maintain the safety of the downstream town of Yingxiu. However, the debris flow occurrence frequency was very high in Xiaojia gully following the 5.12 Wenchuan earthquake. Debris flows of different scales occurred almost annually after the Wenchuan earthquake. The storage capacity of the two check dams could be quickly used up due to the high frequency of debris flows. Accordingly, the effectiveness of the full check dams is limited for decreasing the debris flow peak discharge to the permissible level for maintaining the safety of the Yingxiu downstream. Thus, additional mitigation works were needed to ensure the safety of human life and property downstream. Based on field investigations of the downstream channel condition in the Xiaojia gully, no appropriate sites could be used to implement an additional check dam. Therefore, a deposition work on the debris fan was recommended to trap debris flow sediments (Fig. 6). According to field topographic conditions and the debris flow peak discharge to be controlled (Table 3), the plain layout of the deposition work was designed (Fig. 6). The retaining wall of the deposition work was designed to be 12-m high, which can provide a 4.6 × 104 m2 depositional area for trapping 56.6 × 104 m3 of debris flow sediments.
Following the Wenchuan earthquake, Xiaojia gully exhibited increases in the frequency and scale of debris flow torrents due to the huge increase of loose solid materials in the catchment. The debris flows in Xiaojia gully have been very active in recent years. If debris flows occur again, the possibility of blocking the main river is very high. The resulting dam-breaking flood could cause damage to the newly constructed town of Yingxiu, which is just 5.4-km downstream of the Xiaojia gully. Therefore, mitigation measures should be applied to decrease the risk level.
A back-calculation method and numerical simulation to assist with mitigation planning in the Xiaojia gully were proposed based on the river-blocking prediction. The peak discharges of permissible floods that maintained the safety of Yingxiu were calculated first. Then, a permissible level of blockage by debris flows was determined based on the back-calculation of flooding resulting from dam breaking. Finally, the permissible scale of debris flows was obtained to facilitate mitigation planning for Xiaojia gully. The peak discharge of the permissible flood (QP) was calculated to be <1496.48 m3/s to ensure the safety of the Yingxiu. Accordingly, under 1/3 and 1/2 breach modes, the permissible peak discharge of a dam-breaking flood at the dam site (Qbp) should be smaller than 2018.73 and 2878.31 m3/s, the permissible blocking height (HP) of the debris flow barrier dam should be less than 43.09 and 31.64 m. Finally, the total volume (Vdp) and the peak discharge (Qcp) of a single debris flow event should be controlled to not exceed 70.59 × 104 m3 and 784.59 m3/s and 45.21 × 104 m3 and 551.77 m3/s, under the 1/3 and 1/2 breach modes, respectively.
After the comparison of the permissible (Qcp) and possible (Qc) peak discharges in Xiaojia gully, the peak discharge of a single debris flow event with a 100-year return period (Qc = 790.50 m3/s, P = 1 %) was used for mitigation planning in the gully. A numerical simulation using the Kanako simulator was carried out to facilitate the mitigation planning. By constructing the two check dams in the downstream channel, the simulation results indicate that the debris flow peak discharge decreases from 727.3 m3/s before mitigation to 531.2 m3/s after mitigation at the gully mouth. For the mitigation of a single debris flow event, the effect is remarkable with a peak discharge reduction rate of 26.96 %. However, the debris flow occurrence frequency was very high in the Xiaojia gully after the 5.12 Wenchuan earthquake. The storage capacity of the two check dams can be quickly used up because of the high frequency of debris flows. Therefore, a deposition work on the debris fan was recommended to trap debris flow sediments. For the mitigation of next possible debris flows, the removal of previous debris flow deposits in the deposition work is very essential for keep the trapping space. Above all, by implementing the two check dams in the downstream channel and the deposition works on the debris fan, debris flow damage can be decreased to an acceptable level for ensuring the safety of Yingxiu Town. Besides the mitigation works, early warning for the debris flow in the Xiaojia gully is also very important. As shown in Fig. 3, the distance between the Yingxiu Town and the gully is about 5.4 km. It takes about 12 min for the out-breaking flood (vP = 7.39, Table 2) reaching to the Yingxiu Town. Therefore, a scientific warning system is very essential for the safety of the Yingxiu Town.
For the above proposed method, a series of empirical equations were used for calculation. Some parameters of the empirical equations such as the roughness coefficient of the Yuzixi River and the Xiaojia gully, the blockage coefficient in the Xiaojia gully were determined empirically. Therefore, the uncertainties could be caused for the calculating results. For example, the average velocity (vP) of the permissible flood passing through the protection embankments at Yingxiu Town can increase about 14.3 % when the roughness coefficient of the Yuzixi River (nP) is determined as 0.035 instead of 0.04 (Eq. 1). Accordingly, the peak discharge of the permissible flood (QP) can increase about 1.14 times, causing the uncertainties for the calculating results of the peak discharge from the dam-breaking flood at the dam site (Qbp) and the permissible peak discharge (Qcp) of debris flow in Xiaojia gully. To make the calculating results more rational and accurate, the determination of some parameters in the used empirical equations needs to be improved in future studies. Due to the uncertainties in the application of the empirical equations, the calculating results need to be verified. However, the available observation data of the Xiaojia gully were not enough to verify the reliability of the calculating results. Subsequent observations and studies will be continued to carry out in the Xiaojia gully for collecting enough data for verifying the calculating results. In addition, the proposed method will also be applied to other real debris flow cases to ensure its accuracy.
This research was financially supported by the Key Projects of the Chinese Academy of Sciences (KZZD-EW-05-01-04), the National Science and Technology Support Program (2012BAC06B02), and the Applicative Fundamental Research Program of Sichuan Province (2012JY0104).
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