Navin Singh Khadka's informative piece on the sedimentation problem of the Kosi River ('The mother of all floods', #157) is marred by an ill-informed quote from an expert in the Department of Electricity Development. If the Kosi Barrage cannot clear accumulated sediment, how on earth will sediment in a reservoir be flushed? I am intrigued by the so-called 'new' technology available that allows deposited sediment from reservoirs to be flushed. Accumulation of sediment in reservoirs is an unavoidable consequence of the hydrological cycle. Excessive sedimentation poses both physical and economic risks. The latter is particularly important to Nepal because projects such as the proposed Kosi Dam is likely to cost billions of dollars. Researchers all over the world are making attempts to minimise the rate of reservoir sedimentation. One of the suggested approaches is watershed management. Flushing by using outlets built for the purpose is another suggested method. Flushing works only under certain favourable conditions, depending upon a river's hydrology, shape and size of the reservoir, sediment hydraulics within the reservoir and size range of deposits. The water column in the reservoir consolidates the deposited sediment mass and introduces limitations. Heavy sedimentation triggered by the 1993 cloudburst drastically reduced Kulekhani\'s economic life. Flushing was not considered an option when the project was rehabilitated.

Anil Pokhrel,
email