Monday, August 27, 2007


The Himalayas are one of the youngest mountain ranges on earth and represent a high energy environment very much prone to natural disasters. High relief, steep slopes, complex geological structures with active tectonic processes and continued seismic activities, and a climate characterised by great seasonality in rainfall, all combine to make natural disasters, especially water-induced hazards, common phenomena.
Flash floods are among the more devastating types of hazard as they occur rapidly with little lead time for warning, and transport tremendous amounts of water and debris at high velocity.
Flash floods affect thousands of people in the Himalayan region every year – their lives, homes, and livelihoods – along with expensive infrastructure.
According to a report by the International Centre for Integrated Mountain Development (ICIMOD), there are several different types of flash flood. The most common include intense rainfall floods (IRF); glacial lake outburst floods (GLOFs), landslide dam outburst floods (LDOF), and flash floods caused by rapid snowmelt (RSM) and ice. Failure of dams and other hydraulic structures can also lead to flash floods.
Intense rainfall is the most common cause of fl ash floods in the Himalayan region. These events may last from several minutes to several days. Such events may happen anywhere but are more common to mountain catchments. The main meteorological phenomenon causing intense rainfalls in the region are cloudbursts, stationarity of monsoon trough and monsoon depressions.
Landslide dam outburst
Debris from a landslide can temporarily block the flow of a river creating a reservoir in the upstream reach. The landslide dam can breach due to overtopping and cause huge floods known as landslide dam outburst floods (LDOF).
Glacial lake outburst
The glaciers in the Himalayas are mostly retreating, and as they retreat lakes can form from melt water held in by the now exposed terminal moraine acting as a dam. If the dam breaks the water can be released suddenly resulting in a glacial lake outburst flood.
Impact of climate change
Intense rainfall floods and landslide dam outburst floods are directly related to the hydrometeorological conditions and likely to be affected by climate change.
Climate models project an increase in monsoon precipitation in the region. Similarly the frequency and magnitude of extreme rainfall events are also anticipated to. GLOFs are related to glacial retreat which in turn is mainly due to climatic warming. It is therefore very likely that flash floods due to intense rainfall, landslide dam outbursts, and glacial lake outbursts will increase in the future.
Zemu Glacier Complex
Zemu Chhu originates as Poke Chhu from Zemu glacier, which is the largest glacier in Teesta basin and is about 25 km in length Zemu glacier lies on the eastern face of Khangchendzonga peak (8,598 m) and is joined by Simvo and Siniolchu glaciers on its right flank and Nepal Gap and Tent Peak glaciers on its left. It is joined by Hidden glacier on its left. Poke Chhu flows in W-E direction and receives drainange on its left bank from Thomphyak Chhu. Thereafter, it flows as Zemu Chhu to drain into Teesta River near Zema village. This glacier complex is comprised of 6 glaciers covering an area of 113 sq km.