The mountain torrent disasters and its effect on sediment transport after the Wenchuan earthquake

By analyzing the formation causes of the mountain torrent disasters as well as the distribution characteristics of rainstorm in Longmen Mountain area, and citing the typical post-seismic the mountain torrent disasters within 2008~2013 after the Wenchuan earthquake in the area as example, the character of post-seismic floods, the spatial distribution of the devasting post-seismic debris flows, the critical rainfall to trigger the mountain torrent disasters and so on are explored. Besides, the sediment before and after the Wenchuan earthquake of the Minjiang watershed are compared. The conclusions are as following: (1) The time of the post-seismic flash floods and debris flows after the Wenchuan earthquake are centered from July to September, which is consistent with the rainfall of the area. (2) Rather than the debris flows are triggered when the rainfall intensity reached the critical rainfall intensity, those are closely linked with the pre-accumulated rainfall. That is to say, the more pre-accumulated rainfall, the more probability of the debris flows. (3) After the Wenchuan earthquake, the total sediment increases 13.8%~70%. But the increasing range is varied from year to year. In the year when the debris flow occurred, the sediment shall increase drastically.


Introduction
Debris flow is one kind of special flows which is caused by flash flood, snowstorm, or other natural disaster in a mountainous area with deep gullies and steep terrain. In the paper, the post-seismic debris flow means the debris flow caused by flash flood after the Wenchuan earthquake [1]. It is the superposition effect of the earthquake and the results of the storm, mainly occurred in the process of a sudden rainstorm after the earthquake or the outburst of the earthquake barrier lake. It has the characters of sudden, high flow rate, large flow, large material capacity and destructive power.
On May 12, 2008 14:28 Wenchuan 8.0 strong earthquake triggered a large number of collapse, landslide and other geological disasters, which are rare in the world due to their large number, wide distribution, complex type and huge damage [2]. After the Wenchuan earthquake, the flash flood and debris flow disasters occurred frequently in Longmen Mountain area, especially during the flood season from July to September, 2008-2013. Through a large number of field research work, experts and scholars have done a lot of research work in terms of the formation conditions, the dynamic mechanism, the characteristics and causes of flash flood and debris flow disaster, and obtained a large number of research results. However, the previous studies on the relationship among heavy rainfall, flash flood and sediment transport are not in-depth enough. In fact, co-seismic landslides, the erosion, sediment transport and diffusion, and sedimentation caused by the post-seismic flash flood are important factors to affect the surface erosion and geomorphic growth of mountains [3,4]. The co-seismic landslides, post-earthquake flash floods and debris flows can bring a large amount of sediment input to the basin, which results in a huge increase in sediment transport. The increase range of sediment transport is 2-10 times, and the erosion rate can reach 0.2-7 mm/a [5]. Predecessors have long been put forward in the study that the debris flow caused by post-seismic flood should be studied, because of the pernicious characters of the sudden outbreak, large inertia, high velocity, a strong momentum, and carrying heavy loads of sediment into the river. If serious, it can jam the river and form the barrier lake, whose burst can cause the secondary disasters for upstream and downstream. Even the part jams can change river regime, sediment conditions and water flow. After the Wenchuan earthquake, large amount of the debris and sediment caused by landslide and sediments are still preserved in the riverbed, whose transmission process in the river system will have an important impact on the water conservancy projects and agricultural irrigation in the basin and downstream areas. Therefore, it is of great theoretical and practical significance to study the effect of flash flood, debris flows and their effects on sediment transport after Wenchuan earthquake for the post-disaster reconstruction in Wenchuan earthquake area and the water conservancy construction, water and soil conservation, and secondary disaster prevention and control in the Minjiang, Tuojiang and Fujiang river basins and their downstream areas in Longmenshan area. Longmen mountain area is located in the abrupt change zone of geology, landform and climate in  3 western China, which is the birthplace of Jialing River, Minjiang River, Fuljiang River, Tuo River, Dadu River and Qingyi River (figure 1). The main type of river system in the area is transverse river, whose flow direction is nearly vertical to Longmen Mountain. The river in the area can be divided into two kinds [5]. The first kind is through type rivers (the middle and southern of the upper reaches of Minjiang river, fu river and Jialing river, etc.), which originated in the west of Longmen Mountain. The Minshan, and Longmen Mountain act as watershed. Those rivers are composed of east-west sections and north-south sections, cross Longmen Mountain, and enter Sichuan basin. The second type is the piedmont rivers of Longmen Mountain, which originated in the east range of Longmen Mountain and consist of only east-west transverse river sections. Those cut through the piedmont area of Longmen Mountain (such as the north upper reaches of the Minjiang river) and enter the Sichuan basin ( figure 1).

The main factors to form the mountain torrent disasters in the area
Longmen Mountain is located in the eastern edge of Qinghai-tibet plateau. Rainfall is mainly controlled by the east Asian monsoon. From July to September, east Asian monsoon airflow passes through the Longmen Mountains from southeast to northwest. On the windward side of the eastern side of the Longmen Mountains, the enhanced precipitation under the influence of the terrain reaches 1600~2000 m/a, which is significantly. Heavy rainfall will inevitably lead to a large number of landslides and rapid mass loss of Longmen Mountain. While on the leeward side of the western Longmen Mountains, the precipitation is greatly reduced, which result in arid grasslands and deserts, and the erosion is significantly reduced. Those are beneficial to the formation of the Qinghai-tibet plateau on the west (behind) of the Longmen Mountains.

Distribution character of rain storm in the area
The distribution of rainfall in the Longmenshan area, which is on the eastern edge of the Qinghai-tibet plateau is mainly affected by the warm and humid air flow from the southeast. The ease slope of the mountain is windward slope with abundant rainfall. It is one of the rainstorm centers in Sichuan province and known as Lutoushan rainstorm area. The warm and humid airflow of the southeast monsoon is blocked by the Longmen Mountain, and the precipitation is formed by the uplift of the airflow. The rainstorm center is formed in the mountain front area, while the Minjiang river valley area, located on the west lee slope, has a dry climate and a rainless area.

Rainfall character in the area after the Wenchuan earthquake
It can be seen from table 1 that: (1) the annual average rainfall of the region where the flash flood and debris flow disaster occurred is large. (2) Comparing the critical rainfall intensity and maximum 1 h precipitation, the higher the previous accumulative rainfall, the easier to trig debris flow. (3) Because the cumulative rainfall increase rapidly near the maximum rainfall intensity, flash floods and debris flows outburst suddenly within short period. And most of the flash floods and debris flows occur at the maximum or near the maximum rainfall intensity.

Sediment transport comparison before and after the Wenchuan earthquake in the upper reaches of Minjiang River
After the Wenchuan earthquake, the author has repeatedly field trips to the earthquake zone. Along the way, it was found that a large amount of loose debris caused by the earthquake accumulated in the valleys. When heavy rain, the debris shall be carried into the river in the form of debris flow, which leads to the river water become muddy and high sediment, far higher than that before the Wenchuan earthquake. Taking the upper minjiang river basin as an example, the effects of rainstorm and flood on sediment transport before and after the Wenchuan earthquake shall be compared. According to the analysis of relationship between monthly precipitation and monthly sediment  Table 2 shows that, according to the correlation equations between precipitation and sediment discharge established by Dujiangyan hydrometric station, the calculated annual sediment discharge is greater than the measured values. The main reason is that Dujiangyan is located in the heavy rain zone of Longmen Mountain areas, and the annual precipitation of the zone is greater than the average annual precipitation values of the whole area. Because the precipitation and sediment discharge are high correlated, the calculated sediment discharge value is greater than the measured values. While the calculated annual sediment discharge values are with little difference from the measured values according to the correlation equations between precipitation and sediment discharge established by Zipingpu hydrometric station and Minjiang hydrometric station. And the calculated sediment discharge value according to the correlation equations established by Zipingpu hydrometric station is slightly less than that calculated according to the correlation equations established by Minjiang hydrometric station. Based on comprehensive consideration, it is considered that the calculated value according to correlation equations established by Zipingpu hydrometric station is close to the measured value.
As can be clearly seen from figure 2, the annual sediment discharge volume of Zipingpu in 2007 was 6.3028 million t. And that in 2008 was 9.0571 million t, which was 27.56% higher than the average annual sediment discharge volume (7.1 million t) before the earthquake. The annual sediment discharge volume in 2009 was 8.0805 million t, 13.8% higher than that before the earthquake. In 2008 and 2009, there were no floods or debris flows in the upper Minjiang river basin, but the sediment discharge volume increased by about 30% compared with the annual average. In 2010, the annual sediment discharge at Zipingpu was 12.3919 million tons, which was 1.75 times of the annual average before the earthquake. The significant increase in sediment discharge is mainly due to the devastating floods and debris flows on August 13 and August 18 in the basin in 2010, and large amounts of loose solid debris were transported to the river in the form of debris flows. In 2010, the annual precipitation reached 2545.8 mm, much higher than the average multi-years rainfall before the earthquake. Only in August 2010, the sediment discharge volume in Zipingpu accounted for 58.55% of the total sediment discharge in the year. It can be seen that after the Wenchuan earthquake, the