Recent and past floods in the Alaknanda valley: causes and consequences
Recent and past floods in the Alaknanda valley: causes and consequences
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阿拉克南达山谷最近和过去的洪水：原因和后果
DOI：
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
N. Juyal
中科院分区：
文献类型：
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作者：
N. Rana;Sunil Kumar Singh;Y. Sundriyal;N. Juyal
北阿坎德喜马拉雅地区，特别是阿拉克南达和巴吉拉蒂山谷，经历了近年来最严重的灾害之一（图1）。山洪暴发在喜马拉雅山很常见，但这次目睹的破坏在近代历史上是前所未有的。房屋像纸牌一样倒塌，道路和桥梁被汹涌的沃茨冲走。也许世纪最糟糕的因果关系是Kedarnath山谷的破坏。根据各国日报公布的数据，近4000人死亡或失踪，2232所房屋受损，Garhwal不同地区的1520条道路严重受损，约170座桥梁被冲走。据经济学家称，北阿坎德的旅游业将遭受约12,000亿卢比的损失，约占该州GDP的30%。多年来，北阿坎德喜马拉雅地区山洪暴发的频率和规模不断增加，令当地居民感到担忧。这是因为直到20世纪80年代左右还普遍存在的商业森林砍伐，还是最近为了水电项目而大肆破坏地形？为了了解地形对暴雨等异常天气事件的敏感性，让我们看看两次重大山洪暴发的成因，即1894年8月26日和1970年7月20日。这些洪水有相当好的记录。1894年的洪水发生在商业森林砍伐延伸到阿拉克南达内集水区之前，而1970年的洪水发生在商业森林砍伐在阿拉克南达山谷达到顶峰时。然而，这两次洪水都是由于阿拉克南达河支流的山体滑坡造成的堤坝决堤造成的，这是季风主导的喜马拉雅山异常降雨事件期间的一种常见地貌表现。1893年9月6日，阿拉克南达河的一条支流Birahi Ganga（图1）被从900米高的山谷侧翼滚落的约50亿吨岩体堵塞。这些碎片阻塞了河流，形成了一个高270米，底部宽3公里，顶部宽600米的湖泊。据估计，这个湖至少需要一年才能填满。只有在水开始倾倒大坝后，大坝才会部分决口，这将导致下游的山洪暴发，直到哈里德瓦尔。当时的监理工程师普尔福德和他的团队，特别是加尔瓦尔地区测量员潘迪特·哈里·克里申·潘特（Pandit Hari Krishen Pant），不懈努力，帮助精确估计了下游洪水的规模。在Birahi Ganga和哈里德瓦尔之间安装了一个很好的电报系统，用于实时监测和及时预警洪水。大约在1894年5月，前往Kedarnath和Badrinath的朝圣者交通被转移到新的步行路线
Uttarakhand Himalaya in general and Alaknanda and Bhagirathi valleys in particular have experienced one of the worst forms of disaster in recent times (Figure 1). Flash floods are common in the Himalaya, but the kind of destruction witnessed this time was unparalleled in recent history. Houses collapsed like a pack of cards and the roads and bridges swept away in the turbulent flood waters. Probably the worst causality of the century was the destruction of Kedarnath valley. According to the data published in various national dailies, nearly 4000 people were either killed or lost, 2232 houses were damaged, 1520 roads in different parts of Garhwal were badly damaged and about 170 bridges have been washed away. According to economists, the tourism industry in Uttarakhand will suffer a loss of ~12,000 crore rupees, which is around 30% of the state’s GDP. Over the years increased frequency and magnitude of flash floods in Uttarakhand Himalaya is worrying the inhabitants. Was it due to the commercial forest felling that was prevalent until around 1980s, or the recent rampant terrain tampering for hydropower projects? In order to appreciate the sensitivity of the terrain towards unusual weather events like cloudburst, let us look into the genesis of two major flash floods, viz. 26 August 1894 and 20 July 1970. These floods are reasonably well documented. The 1894 flood occurred well before the commercial forest felling extended into the inner catchments of the Alaknanda, whereas the 1970 event occurred when the commercial forest felling was at its peak in the Alaknanda valley. However, both floods owe their genesis to the breaching of dams created by landslides on the tributaries of the Alaknanda river, a common geomorphic expression during unusual rainfall events in the monsoon-dominated Himalaya. On 6 September 1893, a tributary of the Alaknanda river called Birahi Ganga (Figure 1) was blocked by ~5000 million tonnes of rock mass that rolled from 900 m high valley flank. The debris blocked the river forming a lake 270 m high, 3 km wide at the base and 600 m wide at the summit. It was estimated that the lake would have taken at least a year to fill. The dam would partially breach only after the water began to topple it, which would cause flash floods in the downstream till Haridwar. The untiring efforts of Pulford, the then superintending engineer and his team, particularly Pandit Hari Krishen Pant (district surveyor of Garhwal), helped to meticulously estimate the magnitude of downstream inundation. An excellent telegraph system was installed between Birahi Ganga and Haridwar for real-time monitoring and timely warning of the flood. Around May 1894, pilgrim traffic on the way to Kedarnath and Badrinath was diverted to the new pedestrian route