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WEST RAPTI HIGH DAM PROJECT AB Thapa West Rapti is a tiny river by comparison with the rivers like the Kosi, Gandak and Karnali. The average annual flows of the Kosi, Gandak and Karnali are in the range of about 1500 cumecs., whereas such flow of the West Rapti is only about 100 cumecs. The West Rapti river water is extensively used for irrigation in Nepalese territory, as a result, this river almost dries up throughout the dry seasons. However, during the monsoon season the damages due to the West Rapti floods are not at all insignificant despite the fact that this is relatively a very small river. In recent time almost every year widespread submersion and the loss of life and property in the Indian territory due to the West Rapti floods are reported. The rise of the West Rapti water level is exceptionally high at the time of big floods. In the last century until 1975 the recorded highest flood water level of the West Rapti river at Birdsghet near Gorakhpur area had exceeded the warning stage level by a huge margin of over 9 feet. By comparison with the West Rapti, the peak flood water level rises in other rivers are relatively small. In the same above period the recorded highest flood water level of the Ganges at Patna, Yamuna at Delhi, Gandak at Bhainsalotan and the Bagmati at Hayaghat around Samastipur had exceeded the warning stage level by far smaller margin of just 3.4 feet, 5.4 feet, 2 feet, and 4 feet respectively. No wonder the West Rapti high floods devastate vast area of lands despite the fact that it is relatively a small river. Unfortunately the poor people who bear the brunt of the West Rapti disaster in India are often wrongly informed by certain section of the Indian press and even by some politician who lay the entire blame on Nepal for the West Rapti flood disaster. They try to convince the Indian public that Nepal opens the flood gates of its vast West Rapti storage reservoirs whenever there is a heavy spell of rainfall across the border in Indian territory which is resulting in worsening of the West Rapti flood situation. Dr. K.L. Rao, a renowned expert in the field of water resources engineering and onetime Union Minister of Irrigation & Power, Government of India has very correctly explained the cause of the West Rapti flood problem. He has put forward his own ideas with a view to resolving the West Rapti flood problems in his book 'India's Water Wealth'. Dr. K. L. Rao's Observations Flood plains are generally heavily populated since they are very fertile and are easily accessible. The Ganga basin has nearly 40% of India's population. The flood plains in India are not being developed in a regulated manner. Consequently, the damage and the loss of life and cattle due to floods are increasing year after year. The increase in damage due to the floods can also be attributed to the urbanization of the flood plains and the indiscriminate development of industries in areas normally liable to floods. As a result, whenever high magnitude floods occur, the damage is considerably more than in the days when the flood plains were mostly used for cultivation purposes. In order to keep the discharge in the Ghaggar river within safe limits in the developed areas of Rajastan, a scheme had been implemented which provided for the diversion of 340 cumecs entering Rajastan areas into the depressions in the sand dunes where it is gradually absorbed or lost by evaporation. This diversion channel which is about 48 km long and benefiting an area of 54,000 ha has already been implemented. The scheme has given very good results. Damage due to inundation can be reduced by limiting the flood discharges at the damage centre. Reservoirs, detention basins, diversions and soil conservation measures help in reducing the magnitude of the flood flows. Reservoirs and detention basins store a part of the flood waters and moderate the flood peaks. Diversions and floodways take a part of the flood flows to another basin, and consequently reduce the flood discharge in the main river. The floods affect the most densely populated areas of the country. They can be checked ( in case of the West Rapti river ) only by constructing detention reservoirs particularly on the Rapti river. Laxmanpur Barrage Triggered Floodings Very soon a vast area of lands adjoining the West Rapti river inside the Nepal territory could be subjected to very severe and virtually perpetual flooding. The Laxmanpur barrage would be going to result in this type of situation. Very few of us might be knowing that the Laxmanpur barrage is an integral part of the Saryu canal now being expeditiously built hoping to draw the regulated water of the West Seti river from the Girjapur barrage. The total length of the Saryu canal running almost parallel to Indo-Nepal border is about 230 km. In the original design the barrage across the Saryu river was at a location about 328 meters to the south of the railway bridge at Nanpara. Later, the design was modified and the location of the barrage was shifted 10 km to the north from the original site. The longitudinal gradient of the Saryu canal has been reduced to a minimum. It is only one meter on a stretch of 9 km. The adoption of such extremely mild slope permitted the alignment of the canal to be pushed as far as possible to the north very close to Indo-Nepal border specially in its middle and lower reaches. Normally siphons are provided to deliver canal water across a river. A much higher average slope of the canal is required for the operation of this type of cross drainage structure, which has to operate under pressurized condition. A free flow hydraulic regime needs to be maintained at the river crossing if the average gradient of the canal is to be reduced. For reducing the average slope of the Saryu canal an altogether a different type of structures has been devised, which at first glance drives anybody crazy. Barrages have been built across the rivers to elevate the river water to such a level that would allow free passage of canal water across the river into the canal taking off from the other side of the river The contentious Laxmanpur barrage is one of such structures devised to deliver the water from the Girjapur barrage across the West Rapti river.. It would be necessary to maintain all the time pond water upstream of the barrage above certain fixed level to keep the Saryu canal running at desired capacity. Thus the pond would always be full, with the result that a sizeable area of lands in Nepalese territory could remain perpetually submerged. When the river is in flood, the water would be spreading out onto the lands that were never before regarded prone to flooding. Justification For Building High Dam Protection of life and property of innumerable people living in the Eastern UP of India against the West Rapti floods would strongly justify to build the West Rapti high dam. Thus Nepal should not fail in its duty as a good neighbour to try to build this dam on our own by mobilizing all available resources Such high dam could also help to a limited extent to minimize the negative impact of the Laxmanpur barrage on Nepalese territory if on one hand the construction of the Pancheshwor, Karnali and West Seti storage dams is delayed until the Saryu canal alignment problem is resolved, and on the other hand the West Rapti flood water is held back in the reservoir to prevent it from entering into its original course. Even the surplus unused regulated flow of the West Rapti water could as far as possible be diverted into the Gandak basin or perhaps into the Gandak western canal to prevent significant afflux from the pondage of the Laxmanpur barrage that would soon be resulting in the submersion of very large area of lands in Nepalese territory once the Saryu canal drawing water from the Girjaour barrage would be fully operational.. The regulated flow of the West Rapti river can be used to irrigate fairly large area of lands in the Deokhuri valley as well as the lands in the Kapilabastu and Rupandehi districts round the year. It would also be possible to generate electricity on a very large scale. The storage capacity of the West Rapti reservoir is expected to be sufficiently large to generate greater part of the electricity in dry seasin when the demand for the electricity as well as the water requirement for irrigation is the highest. The West Rapti High Dam The studies carried out in the past have identified that the Bhalubang location close to the point where the East- West Highway crosses the West Rapti river is the most suitable for building a high dam. There is another even better site further upstream at Naumure if this project is to be built only for flood control and power generation. Past studies have helped to find out that the proposed Bhalubang site is suitable to build a dam up to 120 meters in height. This dam height would correspond to the highest reservoir water level at an elevation of 420 meters a. s. l. At that dam height the total storage capacity of the reservoir would have been 3,000 million cubic meters. The geological conditions for the foundation of abutments in the valley floor are expected to be good. Needless to say that the geological condition needs to be thoroughly checked in course of the future investigation studies. Some studies carried out in the past have recommended that the most appropriate type of dam for the West Rapti river would be the embankment dam with a central impervious core. The toe of the proposed dam would be formed by the upstream cofferdam which would be an integral part of the dam. The top of the cofferdam at an elevation of 326 m a. s. l. would form the lower berm of the embankment. In one of the main variants selected for preliminary study in the past the dam height was 90 meters though the topography would have allowed to build a dam even up to 120 meters in height. Construction Materials Embankment dam design is very much site specific in its dependence on the availability of the local construction materials. There are three major components of an embankment dam with central impervious core that require local materials in large quantity. They are the embankment shells, impervious core materials and pervious (filter) materials needed to provide filter Embankment shells are usually randomly placed pervious material, which normally forms the bulk of the dam. Its function is to provide mass to the structure and support the impervious core. It is stated in the past studies that pervious materials are found in sufficient quantity upstream as well as the downstream of the proposed dam site at Bhalubang to construct the embankments of the main dam and the cofferdam. It is reported that suitable impervious materials to build the central core of the proposed embankment dam are also found in sufficient quantity at a distance of about 3 km downstream of the dam site on the right bank. However, clean sand or gravel deposits are not found in the project area close to the dam site, as a result, the filter materials would have be specially produced by putting through the process of washing and sieving of terrace materials and the recent river deposits. n |
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