| Since the 2008 Wenchuan earthquake occurred so far, along the earthquake strongly influence zone of the region’s Valley mountain has more than the mass of the large-scale debris flow disaster event. As of September 24,2008, in Beichuan County, Pingwu county and other places in regional debris flow disaster, causing multiple blocked river and formed a barrier lake, and killed more than 40 people. In August 13, 2010, Mianzhu city in the Qing Pingxiang Wenjiagou bro ke out large debris flow, its out of the detrital material of about 4.5 million cubic meters, blocking the main river, killing 7 people,5 people missing and QingPing village is in a major disaster buried. These disasters not only to construct the risk range of buildings and infrastructure caused direct damage, causing heavy casualties, but also the ecological environment on the earthquake caused serious influence. On the one hand, these large-scale outbreak of debris flow disaster in mountainous rivers will produce rich source along in the earthquake zone, into the main river, causing river silting and increase, or even blocked lake. On the other hand, with the large amount of sediment discharge of the river in the lower reaches of rivers plain riverbed deposition, elevation, endanger the safety of flood control in plain area, the scope of geological disasters and broader extension in space. The research results show that the earthquake stricken area in Wenchuan earthquake area of geological disasters, there is a significant "after effect" phenolmenon, namely after the earthquake a long time (about 20～25), the frequency and scale of geological disasters will be significantly enhanced, along with the gradual recovery until ecological and geological disaster area, will significantly reduce and return to pre earthquake levels. The serious influence of this phenomenon is bound to the earthquake post disaster reconstruction and ecological environment evolution.This paper first sediment source of rivers in Wenchuan earthquake area in mountainous area were investigated, the types and distribution characteristics of sediment source are analyzed. On this basis, select the Hanwang Mianyuan river basin to Qingping segment (Han Qing section) is a typical river of study area, using the remote sensing data obtained as the main source of the sediment of landslide geological disaster in temporal and spatial scales of the distribution, and dynamic change characteristics of it are involved in the initiation of debris flow source volume and sediment source analysis. Then, based on the understanding of the study area 4 has occurred the typical gully debris flow disaster and sediment process, this study was carried out for 3 consecutive years of the river section monitoring work, the purpose lies in evolution, and further to obtain the study area river chain disaster effect and forecast the next period of time the channel transmission sediment variation characteristics. The main results and conclusions of this study as follows:(1) In the mountainous area of Wenchuan river sediment source types and distribution characteristics of the earthquake stricken area. With the general river sediment mainly comes from the river erosion is different, Wenchuan after the earthquake, the earthquake in the river sediment mainly comes from the earthquake induced secondary disasters caused by the loose deposits. Because of all kinds of geological disasters induced by earthquake area of the mountain and forest vegetation destroyed a large number of the formation of large area of bare surface, slope, broke the river basin water and soil in the cycle of the relative balance, storm, exacerbated by soil erosion. In Hanwang to Qingping segment Mianyuan River as an example, the Wenchuan earthquake, the geological disaster types of rock and soil evolution from bank slope into the river sediment source can be divided into bank collapse type, slope slip type, debris flow type and dammed lake type. According to the remote sensing image interpretation results, in which four kinds of sediment source, gully debris flow is the main controlling factors of the evolution of the study area of river short term. The statistical results suggest that, only the Wenjiagou, Zou Ma Ling Gou and Xiaogang sword this 3 debris flow gullies source accounted for the entire study section of the 79% sediment sources.(2)The temporal and spatial distribution characteristics of the study area of geological disasters and the resulting Provenance Variation characteristics. Based on the results of remote sensing interpretation, in 2008 after the Wenchuan earthquake and the 2010 storm, research is in District 80% landslide distribution between the range of elevation 1000-2000 meters,81% of the landslides distribution between 20 degrees -50 degrees, and the heavy disaster area of the landslide distribution similar. In addition, collapse and landslide in different slope inward are distributed, and the difference is not great. In time domain, the number of geological disasters in the study area show a downward trend in the wake of the three years to 2013 again after reaching a peak in the number of cycles will also fluctuate after achieving a stable level. It can be said that in the near future 20 years, loose solid substances produced by the geological disaster will still sediment to rivers produce supply, but overall showed a recharge less trend.(3)Based on the research section of riverbed natural conditions and using the remote sensing interpretation results of analysis, research section of scouring and silting is mainly characterized by the following rules:the river silting flat section, river bend section of the river bed scour; longitudinal slope section, small packing serious; and longitudinal gradient of large area, erosion, even on the other side the slope of highway subgrade is also damaged. The research section of river namely appears as Deposits-erosion accumulation rule. In addition, the main factors affecting the rainfall is also a research section scour and silting characteristics. In high rainfall years, generating heavy rain will lead to floods, erosion effect on increasing flood river, on the whole river, riverbed erosion than the accumulation of years of heavy rain. With the development of the next ten years, the geological disaster to the volume gradually reduced, the rainstorm years river channel scour was greater than the accumulation effect is more apparent. But on the river as a whole, the rainstorm years always in a few years, so the total amount of accumulation of river is increasing, but the rate of increase is decreasing year by year; similarly, scour is with the accumulation increased with the increase of amplitude is reduced year by year. But look on the whole, the amount of sediment accumulation of river bed showed > erosion(4)The spatiotemporal evolution rule research section of river. In order to analyze the Mianyuan river channel evolution characteristics, this study since 2010,3 consecutive years of Mianyuan river of hydrological monitoring. Monitoring range of Qingping township to the section of Mianzhu City, Mianzhu city. Mianyuan river is about 31km long, the river set up a total of 8 measuring points. The 1#-6# monitoring point is located in the valley on both sides of the middle and low mountain terrain area, while 7# and 8# monitoring point is located in the piedmont alluvial plain area of fan. Change character, monitoring content includes different frequency rainfall conditions mud parameters of gully erosion and siltation amplitude data. Monitoring data analysis result reveals the interannual variations of each monitoring point, sediment scouring and silting of the characteristics and law of sediment transport. With 1# monitoring point for example, between 2010 and 2012 the flood season, the main point to silting riverbed, making the overall elevation. While in the non flood season, to slow erosion mainly, river evolution trend for the erosion and incision. Because the geological disaster decrease with time, accumulated sediment volume throughout the year from 30.39 million square in 2010 to reduce to 22.68 million in 2012, while the cumulative erosion from 29.28 million party reduced to 22.71 million party. Different monitoring points for the number of the topography, river width, longitudinal slope, sediment source is different, which exhibits the sediment evolution characteristics will vary.(5)In the Wenchuan earthquake, evolution and sediment produced in the process of river source of the river shows a chain effect. On the one hand, occurrence of geological disaster mass of loose solid material blocking the river and formed a barrier lake, have an important impact on the bed:when the lake has not burst, the upstream riverbed sediment deposition and with persistent elevation; when the outburst, river downstream flood scouring, also will be a lot of weir body solid material to the downstream handling deposition, also caused the riverbed elevation. On the other hand, the sediment in the transport process of silting and scouring the acceleration channel evolution, evolution of river type and longitudinal and cross section.(6)In Wenchuan earthquake region River transported sediment monitoring of short time, less data characteristics, application of grey theory to construct the Mianyuan river basin soil erosion prediction model X(1)(k)= K/3 (80.481 e0.90925k -54.371) and siltation prediction model X(1)(k)=K/3(78.546 e0.76358k -56.516).According to the typical basin average annual sediment load value, the amount of sediment Mianyuan River Basin in 2015～2020 years was predicted. The results show that before 2015, the amount of sediment erosion of Mianyuan River (river sediment carrying capacity) than sedimentation, sediment content in rivers are far greater than the amount of sedimentation, sediment content of the river is high. From the beginning of 2016, the whole of Mianyuan river basin is dominated by sediment deposition, deposition amount higher than the maximum scouring volume is 4.453 million cubic meters, which also explains the Mianyuan River Basin will store large amounts of sediment provenance in a period in the future.The related study results in this paper are adapted to the range of high earthquake intensity area in the mountain valley. This paper reveals the evolution rules of geological hazard in a certain range of river sediment transport laws and mountain areas. But the geological environment in the region of high earthquake intensity recovery need a long period of time, so in the later work, on the one hand, to continue in-depth study; on the other hand, the proposed long-term continuous observation in the study area and collected with meteorological data, for the future lay the foundation for continued research. |
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