- Research letter
- Open Access
Giant palaeo-landslide dammed the Yangtze river
© Higgitt et al.; licensee Springer. 2014
Received: 11 September 2013
Accepted: 11 February 2014
Published: 7 April 2014
Field evidence is presented to demonstrate that a very large landslide blocked the Jinsha River (the main stem of the Yangtze) near the present day town of Qiaojia, Yunnan Province. The discovery is significant because no persistent river-blocking landslide has been reported so far downstream in a major catchment. At the location of the landslide dam the upstream catchment area is 445 × 103 km^2. Sediments deposited behind the dam indicate that the minimum crest height was approximately 200 m with a lake volume of 11.4 +/− 1.3 km^3. The landslide occurred on the western (Sichuan) side of the river and displaced an estimated volume of at least 3.75 km^3, with material riding up to 550 m above the river on the eastern (Yunnan) side of the valley. The location is at the intersection of the Xiaojiang and Zemuhe fault zones which form part of the eastern boundary fault of the Sichuan-Yunnan Fault Block, an area where many earthquakes exceeding magnitude 7.0 have been documented in the historical record.
Evidence of a previously unrecognized ancient landslide which dammed the main stem of the Yangtze River, China is presented. The find is significant not only due to the huge dimensions of the landslide but because of the enormous catchment area impounded by the dam and the recognition that a river as large as the Yangtze has been – and can be - impacted by a persistent river-blocking landslide. The site of the dam is located 8 km upstream of the town of Qiaojia, Yunnan Province (26° 50’ 40” N; 102° 57’ 35” E). The Jinsha River (the name given to the main stem of the Yangtze upstream of Yibin) forms the provincial border between Sichuan and Yunnan. At the landslide dam location, the upstream catchment area is 445 × 103 km2. We believe this to be by far the largest catchment area ever reported for a significant landslide dam.
Landslides forming natural dams across rivers have been described from a number of mountain environments [1–3]. Such features pose considerable hazards and consequently the topic has received increasing attention in recent years, including the margins of the Tibetan Plateau . Upstream of the blockage, backwater inundates the valley floor as a lake is formed. This increases pore water pressure which may induce secondary slope failures with the risk of displacement waves. At the dam site there is risk of breaching by overtopping, piping or slope failure which generates an outburst flood which may be catastrophic. An elevated flood wave can travel long distances downstream at high velocity. In China, documentary records of large earthquakes and river blocking landslides have enabled inventories to be compiled [5, 6]. However, the remote and rugged terrain shelters evidence both of recent landslide dams which escaped the attention of official records and of ancient landslides which occurred before documentary records. Recognition and interpretation of palaeo-landslides and their lake impoundments is vital to extend understanding of the frequency-magnitude and spatial characteristics of landslide dams. Increasing awareness has also challenged geomorphologists to re-evaluate the role of landslide dams in the fluvial sediment cascade . It has been demonstrated that landslide dams arrest rates of bedrock incision as alluvial fills inhibit headward erosion, while catastrophic dam failures may produce sediment yields many orders of magnitude greater than ‘normal’ rates [4, 8–10].
The evidence for the landslide dam has been gathered from fieldwork reconnaissance together with analysis of geological maps, satellite imagery and digital elevation models. The initial impetus for the analysis was the recognition of substantial sand and gravel deposits in the Jinsha valley. These deposits are infrequently exposed at the land surface as most of the valley sides are draped by colluvial or landslide deposits derived from higher slopes. However, there are three active sand quarries in the vicinity and various exposures in incised gullies. The sediments are sub-horizontally laminated dark grey sands and gravels typically arranged in fining upward units capped with imbricated armoring. Typically couplets are at intervals of 8–10 cm apart indicating pulses of sedimentation in flowing water. The Geological Survey of China has indicated some areas of Quaternary deposition in this part of the Jinsha Valley, indicating that the presence of the sand and gravels has been recognized. However, it appears that Quaternary deposits have been assumed to be remnant fluvial terraces. Recent work  has demonstrated that many deposits along river corridors in high mountain environments may be related to landslide dams as either deposition of sediment within a lake impoundment or as the beveling of landslide and associated slope deposits along a temporary lake shore. In the present example, the distribution of sand and gravel deposits was mapped so that the horizontal and vertical extent could be determined. Direct field observations and sampling of sediments has been supplemented by field mapping of bench surfaces along the Jinsha valley and its tributaries.
Results and Discussion
The lake volume of the Jinsha landslide dam is similar to the largest known event in historic times – the 1911 Usoi Dam in Tajikistan . However, its significance is the recognition that landsliding is capable of disrupting one of the world’s largest rivers at a considerable distance downstream. This merits reappraisal of the potential hazard posed by earthquake triggered landslide dams. In comparison, the 2008 Wenchuan Earthquake generated more than 250 landslide dams, but the largest of these at Tangjiashan, which causes considerable hazard management challenges, impounds a lake volume three orders of magnitude smaller than the Jinsha landslide dam [17, 18]. An empirical relationship to predict peak discharge from catastrophic failure of natural dams , Qp = 0.3 [VD]0.49 where Qp = peak discharge (m3 s−1); V = lake volume (m3); D = lake depth (m), suggests a peak discharge > 300,000 m3 s−1 if the dam failed catastrophically with a resultant flood wave impacting the channel downstream for several hundred kilometers. Furthermore, as the river enters a steep gorge downstream of Qiaojia where the active channel width is only 120–200 m, the unit stream power of a dam failure flood would be extremely high.
The natural damming of rivers by landslides is a significant hazard in the seismically active, mountainous terrain of Southwest China. Geological and geomorphological methods to identify landslide hazard potential are especially important in a region experiencing rapid urban development, widespread land use change and the construction of water resources projects on major rivers. New techniques based on the interpretation of satellite imagery and digital elevation models are enabling more detail about past and present landslide distributions to be generated. There is increased awareness that river-blocking landslides have been more widespread than the documentary records suggest and that Quaternary sediments traditionally assumed to be fluvial terraces may need to be reinterpreted. The recognition of palaeo-landslides capable of creating very large lake volumes necessitates reappraisal of hazard assessment. In the case of eastern boundary fault of the Sichuan-Yunnan block it is apparent that high magnitude earthquakes have potential to generate massive slope failures. The range of hazards related to landslide dams in the Qiaojia area will soon be supplemented. In the gorge section downstream of Qiaojia, construction of the Beihetan Dam is now underway. Once completed it will be the third largest hydropower project in the world. Ironically, the design water level for the dam is 825 m so the reservoir will occupy the majority of the terrain once inundated by the landslide dam and, in doing so, will drown the majority of the field evidence for the palaeo-lake. The presence of the reservoir introduces the additional hazard of earthquake-generated landslides causing displacement waves. The need for vigilance in assessing seismic and slope stability hazards is apparent.
This study was supported by Chinese Academy of Sciences (Grant KZCX2-XB3-09) and by the Ministry of Education (Singapore) Academic Research Fund (Grant R-109-000-152-112).
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