Monday, August 11, 2008

A document worth reading - "Taming the Teesta" by Dr Kalyan Rudra

An excerpt from the above document is placed below:-

"The Teesta is a rain and snow-fed river. The permanently snow-covered area of the basin is about 158.40 sq. km. The upper catchment receives a total annual rainfall of 1,328 mm. while the middle of the basin receives 2,619 mm. It has been recorded that about 77-84% of the annual rainfall is received between June and September. The heavy concentration of rainfall within a short period is common in the eastern Himalaya. Gangtok recorded 1,500 mm. rainfall between October 2 to 5, 1968. The highest one-day rainfall recorded at Darjeeling was 521 mm. The mountains have been extensively deforested with increasing population since the mid-19th century. This has altered the infiltration run-off ratio (the amount of rainwater absorbed by the soil relative to the amount which runs off) and slope failure has become a menace. Infiltration is generally high and run-off little in a forested tract. The Teesta basin is now one of the most landslide prone areas of the country, contributing a huge sediment load to the river. The mean annual discharge of the Teesta at Anderson bridge is about 580 cumecs and it declines to 90 cumecs in the lean months. The peak discharge may be as much as 4,000-5,000 cumecs. It was estimated that the peak discharge of the river at Jalpaiguri during the devastating flood of 1968 was 19,800 cumecs. The sediment load in the river increases with high monsoon discharge. It was observed that 72% of the suspended load is transported between July and August when the bulk of discharge flows through the river (Starkel et al, 1998). It seems certain that the dynamic equilibrium of the river will be impaired with the construction of a series of dams and the sediment load will be trapped within the reservoirs, reducing their capacity. This, in turn, could compel dam managers to release water during heavy rainfall, causing sudden flash floods downstream. On July 20, 1993, a severe cloudburst in and around Kathmandu generated 540 mm. of rainfall within 24 hours and brought down five million cubic metres of sediment into the reservoir of the Kulekhani dam (Dixit and Ahmed, 1998). This risk will exist for any dam constructed in the Himalayas.

The entire Himalaya is tectonically unstable. The Indian plate continues to subduct under the north Asian plate and rocks lying in between are severely compressed. The crust has broken up in a series of faults along the southern front of the Himalaya. These thrust faults are collectively termed the Main Boundary Thrust (Valdiya, 1998). The stretch between the two dam sites selected for stage III and stage IV is geologically fragile and already identified as seismic zone IV. There is a very real fear that the massive construction works and the reservoirs created would increase the risk of seismicity."

Those interested in reading the whole document can find it here:-

praful rao

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