IRRIGATION DEVELOPMENT AND SUN-KOSI PROJECT
By Dr. AB Thapa
The necessity of the irrigation development on a large scale in Nepal had been keenly felt since the middle of 1960s. Bearing in mind such necessity of large scale irrigation development, the experts of the Food and Agriculture Organization ( FAO) and the Government of Nepal had carried out the First Phase study of the Sun-Kosi Terai Project in 1965 and 1966 to determine the possibility of irrigation development in the Eastern Terai Plain by diverting the abundant water of the Sun-Kosi River. The Interim Report on Feasibility Study of the Sun-Kosi Terai Project was submitted in October, 1969. The plan to divert the Sun-Kosi River into the Kamla River has been described in that report. The final feasibility report was submitted in October, 1972.
The feasibility report makes it perfectly clear that the people in Nepal are mostly dependant for their food supply and livelihood on a narrow belt of cultivated land in Terai. The potential new land for cultivation is very limited in Nepal. Thus, there is a need to raise agricultural productivity by intensification; i.e. by increasing the average cropping intensity. The Sun-Kosi diversion could help to provide sufficient water round the year to increase the cropping intensity as well as the per hectare yield, as a result, the agricultural production could be substantially increased.
Sun-Kosi Kamla Diversion
Two possible diversion plans had been identified for further study at the end of the First Phase study. They were called “A” Plan and “B” Plan. Under Plan “A”, the Sun-Kosi River was to be diverted into the Trijuga River, a tributary of the Sapta Kosi River, through a tunnel about 7 kilometers long to irrigate lands on the right bank of the Sapta Kosi. Similarly under the Plan “B”. the Sun-Kosi River was to be diverted into the Kamla River through a headrace tunnel about 15 kilometers long to irrigate both sides of the downstream reaches of the Kamla.
The field survey activities were started in October 1967. The whole dry season of the year 1967/68 was spent in identifying the most feasible plan. Following the surveys and studies of both the alternative plans, it was found that the “B” Plan to divert the Sun-Kosi River into the Kamla was the most feasible irrigation project. The selection of the Plan “B” for further study was approved by the FAO Headquarter and also by the Government of Nepal. The dry season of the following year 1968/69 was devoted to conduct semi-detailed surveys and the studies of the Sun-Kosi Kamla Diversion Project. Meantime, the general studies of irrigation development was also undertaken that covered a vast region of cultivated lands in Terai approximately 500,000 ha in area from Kosi on the east to the Nepal-India border line ( about 20 km to the west from Birgunj ) on the west. According to one of the studies the diversion of the regulated flow of the Sun-Kosi River could provide more than sufficient water needed for year round irrigation of lands within the project area estimated at 300,000 ha in net coverage.
Agricultural Studies
Extensive agricultural studies of the whole project area were made in course of the feasibility study. General reconnaissance soil survey of the Eastern Terai plain is one of them. The survey report gives the results of the reconnaissance soil survey covering the entire project area of 540,000 hectares in gross. It was revealed that about 80 percent of the entire area has good soils for irrigation farming belonging to Grade I, II and III according to the USDA standard. No alkalinity or salinity problems exist.
A special pilot farm was set up at Hardinath in Dhanukha district to conduct agronomic studies. Such studies covered cultivation of crops like paddy, wheat, sugar cane, maize, mustard, fodders and other vegetables. Tests were conducted to determine the irrigation water requirement, optimum sowing time, optimum use of fertilizer, selection of best varieties etc.
Project Area
The Terai Plain consists of the alluvial fans and the recent river deposit which are gently sloped from north to south. The elevations at the foot of the Churia Hills range from 150 to slightly over 200 meters. The plain has gentle slopes ranging from 1/1,000 at the foothills to 1/5,000 at the border line with India. The countless small streams and rivers have their origin in Churia Hills. They feed the plain until they reach the Ganges or its main tributaries.
The Project area lies between the Kosi River and the Indo-Nepal border line at a distance of about 20 km to the west of the Birgung town. Similarly the Churia Foothill is the northern boundary while the Indo-Nepal border the southern. The project area is nearly rectangular in shape. The length is about 240 kilometers and the width varies between 20 and 40 kilometers. The total project area is about 8,500 sq. km. out of it in 1970s when the feasibility study was made the gross cultivated land, reclaimable land, non-reclaimable land (mainly river beds) and forests on flat land were 5,450 sq. km, 570 sq. km, 650 sq. km and 1,870 sq. km respectively.
The project area covers 8 districts of Terai. They are Parsa, Bara, Rautahat, Sarlahi, Mahottari, Dhanukha, Sirha and Saptari. The total population in the project area in early 1970s was about 1,770,000 whereas, according to the Statistical Book published in 1994 by the Central Bureau of Statistics it had increased to about 3,600,000.
The River
The Sun-Kosi River is the biggest tributary of the Kosi River. The total drainage area of this river at Tribeni (near Barahachetra ) is 17,960 sq. km. At this point all three main tributaries join and further downstream the river is called the Kosi. Hydrometric records covering the period between the years 1948 and 1967 showed that the yearly mean discharge of the Sun-Kosi River at Tribeni varied between 932 cu. m/sec. and 622 cu. m/sec. The estimated yearly mean discharge of the Sun-Kosi River ( based upon the data at Tribeni ) at the proposed dam site varies between 871 cu. m/sec. and 576 cu. m/sec.
The annual average sediment load of the Sun-Kosi River at Tribeni according to some studies is about 54 million cu. m The annual average sediment load of the Kosi at Tribeni is about 119 million cu. m.
The Sun-Kosi Dam
The FAO had provided Nepal technical and financial assistance to conduct the study of the Sun-Kosi dam project for the agricultural development in the Terai Plain that had three main functions. They were capital formation needed for social investment through the increased income of farmers, sufficient food supply for the whole country, and increased employment in the Terai Plains. The Government of India had also investigated the Sun-Kosi dam site in 1940s. At that time India was exploring the possibility of building the Sun-Kosi dam primarily for controlling the Kosi River floods. The Indian survey team had favoured Kurule dam site on the Sun-Kosi River because it appeared geologically sound and allowed to have a big storage reservoir that extended in Dudh-Kosi and Sun-Kosi valleys. In all subsequent studies that followed thereafter the location for the dam at Kurule has remained unchanged. The site for the Sun-Kosi dam proposed in the FAO supported study is also at Kurule.
The Master Plan study of the Kosi River water resources was made in 1985 by JICA ( Japan Government Agency) team. The Sun-Kosi River diversion for irrigating lands in the Eastern Terai had been identified as one of the top priority projects in the Master Plan study. That study too had adopted the Kurule as the appropriate location for the dam across the Sun-Kosi River to divert it into the Kamla River.
Water Right Issue
The FAO provided assistance to Nepal in revising the 1954 Kosi River Treaty. Dr. Dante A. Caponera, Chief, Legislation Branch of FAO, Rome was in Nepal in 1966. The Kosi Treaty was revised on 19th December, 1966. Nepal's full right over the Kosi River water has been very clearly explained in the revised treaty. The Article 4 ( i ) of the Treaty related with the water rights has been presented hereinafter:
"HMG shall have every right to withdraw for irrigation and for any other purpose in Nepal water from the Kosi river and from the Sun-Kosi river or within the Kosi basin from any other tributaries of the Kosi river as may be required from time to time. The Union ( it indicates India ) shall have the right to regulate all the balance of supplies in the Kosi river at the barrage site thus available from time to time and to generate power in the Eastern Canal."
Geology
A separate report on the Geology of “B” Plan Area as a part of the Sun-Kosi Project feasibility study was prepared by Dr. J. B. Auden in January 1967. Dr. Auden is well known for his great contribution in the field of Himalayan geology. An abstract of the geological investigations has been presented in the report “ Feasibility Study of Irrigation Development in the Terai Plain, 1972”.
Power Development Planning
In planning the Sun-Kosi diversion, three options were considered. They are the diversion plan exclusively for irrigation, non-peaking power development option, and peaking power development option.
At present the available minimum discharge of the Sun-Kosi River at the diversion site is about 140 cum. m/sec. It is proposed to build a 34 m high dam and a 15,000 m long diversion tunnel 6 m in diameter to divert 140 cu m/sec. Sun-Kosi River water if this project is to be planned exclusively to irrigate lands in Terai.
The next alternative is the Sun-Kosi diversion project along with a non-peaking hydropower station. The diversion project would be having a 73 m high dam and a 15,000 m long tunnel 8.4 m in diameter to divert 215 cum. m/sec flow. Installed capacity of the hydropower station would be 330 MW.
The third alternative is the Sun-Kosi diversion along with a peaking hydropower station. The diversion project would have a 115 m high dam and a 12,700 m long tunnel 8 m in diameter There would be another dam at Kakaru. The firm discharge 140 cu. m/sec diverted from the Sun-Kosi River would be stored in a pondage to be provided on the Kakaru Khola, at about 8 kilometers upstream from Amtai Bazar. The required storage capacity at that pondage was determined to be about 9.6 million cu. m to meet the peak load with load factor of 21 percent, which would correspond to daily peaking time of 5 hours. The installed capacity of the hydropower station would be 1,050 MW.
JICA Study
Japan International Cooperation Agency (JICA) had helped Nepal to carry out the Master Plan Study of the Kosi River water resources development. The final study report was submitted on March, 1985. Three reservoir type schemes have been planned for the Sun-Kosi River. They are the Sun-Kosi No. 1 Dam, Sun-Kosi No. 2 Dam and the Sun-Kosi No. 3 Dam schemes. The lowermost is the Sun- Kosi No. 1 Dam Scheme. The Sun-Kosi No. 1 Dam according to JICA study and the Sun-Kosi dam proposed in the FAO supported irrigation development study are virtually one and the same. The sole difference between them is over the dam height. The FAO study has proposed a dam about 120 meters in height, whereas the JICA study recommends dam height in between 147 and 195 meters.
(Dr. Thapa writes on water resources)
