Hydrology in Mountainous Regions. I - Hydrological Measurements; the Water Cycle (Proceedings of two Lausanne Symposia, August 1990). IAHS Publ. no. 193, 1990.
Hydrology and development of the Arun River, Nepal
RICHARD KATTELMANN Center for Remote Sensing and Environmental Optics Computer Systems Lab, 1140 Girvetz Hall University of California, Santa Barbara, CA 93106.USA
ABSTRACT The Arun River drains a large area of the Tibetan Plateau before crossing the Himalaya into Nepal where its discharge increases dramatically. The steep gradient and relatively high dry-season flow of the Arun have led to plans for major hydroelectric development. Little information about the hydrology of the Arun basin was available to guide the planning efforts. This review of the limited knowledge concerning this river basin illustrates the unique features of Himalayan hydrology that must be considered when assessing the potential for water resources development in this region.
INTRODUCTION Water resources of the Himalaya are often viewed as the key to successful development of much of the Indian subcontinent. Year-round flow in the mountain rivers provides water for agriculture during the long dry-season and represents enormous hydroelectric potential. Hydroelectric generation in the Himalaya began in 1897 in Darjeeling. Large projects began to be built after Partition of India and Pakistan. In the past decade, much of the hydropower attention has been directed at Nepal, which is seeking rapid economic development, a product for export, and an alternative energy source to fuelwood. The combination of high precipitation and steep terrain provide Nepal with a theoretical potential for hydroelectric production of 83,000 MW, although only a fraction of this amount can be feasibly developed (Shrestha, 1983). Although many large-scale projects are under consideration in Nepal, hydroelectric development in the Arun Valley in the eastern part of the country appears to be the best prospect for the 1990's. Initially, a cascade of power projects was envisaged for the Arun in a broad plan for the development of the Kosi River basin (Japan International Co-operation Agency [JICA], 1984). Subsequent planning has concentrated on two sites where the river channel has dramatic bends (Figure 1), which minimize the tunnel length for a substantial head loss. These two proposed projects are called Arun-3 and Upper Arun. Pledges of US$550 million in international financing for Arun-3 have already been obtained (Bhattarai, 1989). Planning for these projects has necessarily proceeded with little hydrologie and climatic information. The Arun River example provides a case study of water resource assessment in a mountain region where data is lacking and the hydrology is poorly understood.
GENERAL GEOGRAPHY The Arun is the largest trans-Himalayan river passing through Nepal and also has the greatest snow- and ice-covered area of any Nepalese river basin. The Arun drains more than half of the area contributing to the Sapt Kosi river system but provides only about a quarter of the total discharge. This apparent contradiction is caused by the location of more than 80 percent of the Arun's drainage area of about 30,000 km2 in the rain shadow of the Himalaya. Average annual precipitation in Tibet is about 300 mm (Liu, 1989).
777 Richard Kattelmann 778
' *»_/
-border (China-Nepal) Upper Arun site
Leguvaghat ®É^ j ^ of rQad (1M())
FIG. 1 More than 80 percent of the Aran's drainage area is in Tibet.
In Tibet, the river is known as the Men Qu (Moinqu) in its upper reaches north of Xixabangma and then as the Peng Qu (Pumqu) for most of its course north of the Himalayan crest. After progressing eastward through arid grasslands, the Peng Qu turns south at the 4050 m elevation confluence with the Yarn Qu (Yeyuzangbu). The Peng Qu then flows through the narrow Yo Ri gorge and a broad valley before entering the Longdui gorge at 3500 m at a point below Kharta. The climate changes abruptly in this area from rain-shadow to monsoon-soaked (Howard-Bury, 1922). This portion of the Peng Qu basin may generate much of the streamflow that crosses the border. The Peng Qu crosses the Himalayan crest at an elevation of about 2175 m and becomes known as the Arun in Nepal. South of the Himalayan crest, the flow of the Arun increases rapidly downstream in the seasonally-humid environment of east Nepal. The 5000 km2 of land contributing water to the Arun inside Nepal is only 17 percent of the total basin area, but it provides more than 70 percent of the Aran's total flowa t its confluence with the Sapt Kosi (JICA, 1984). The landscape south of the border tends to be steep with less than 15 percent of the area having a sustained slope of less than 15° and is strongly dissected by stream channels. Many of the hillslopes are structurally unstable, and the region is seismically active (Kansaker, 1988). An earthquake in August 1988 with an epicenter more than 50 km south of the Aran basin had a magnitude of 6.7 on the Richter scale and resulted in more than 100 deaths in the Aran basin alone (Dunsmore, 1988). Soils tend to be shallow (generally less than 20 cm deep) and stony (Goldsmith, 1981). The alpine zone above 4000 m covers about 5-10 percent of the lower Aran basin (Shrestha, 1988). Several large glaciers are found in the Baran River tributary near the 8000 m peak, Makalu. 779 Hydrology and development of the Aran River, Nepal
The northern third of the Nepalese portion of the Arun basin supports a rich, though human-modified, forest of mixed hardwoods, Chir pine, fir, and rhododendron at elevations of 1000 to 4000 m (Cronin, 1979; Shrestha, 1988). The vegetation in the southern two-thirds of the area has been extensively modified for subsistence agriculture. Most of the half-million people in the Arun basin live in this southern area between 300 and 2200 m in widely scattered villages near the slopes they farm (Dunsmore, 1988). None of the four towns in the basin had more than 14,000 people in 1988. Less than 80 km of motorable road has been built in the low-elevation southeast comer of the basin. The potential for the basin to support either the existing or a growing population under subsistence agriculture is problematic and depends on active conservation of soil and forests (Dunsmore, 1988).
HYDROELECTRIC DEVELOPMENT PLANS The immense hydropower potential of the Arun River has long been recognized from considerations of the large discharge and steep gradient. The first detailed assessment of the basin estimated that more than 1100 MW of capacity could be developed in a cascade of six generating stations (JICA, 1984). One proposed site, known as Arun-3, was particularly attractive because of a great S-shaped curve in the channel around a ridge of resistant rock. Because a tunnel could cut off the bend and drop more than 200 m in 11 km, this project was judged to be the most efficient development (Nepal Electricity Authority, 1986). Arun-3 would involve diverting up to 150 m3 s_1 from a dam across the Arun upstream of Num to a powerhouse of 400 MW capacity at Pikuwa. As detailed design work progressed, the estimated construction cost more than doubled from US$240 million (JICA, 1984) to US$550 million (Bhattarai, 1988). An access road of at least 170 km length is necessary to begin construction of the dam, tunnel, powerhouse and transmission line. A minimal road could cost about US$35 million (US$200,000 per km) and take 2 to 3 years to complete (Dunsmore, 1988). However, the planned route of the road has been extended to 193 km to serve as many towns as possible. This alternative alignment has delayed funding and extended construction time to at least 4 years. Consequently, the Arun-3 project is unlikely to be completed before the year 2000. Another site with characteristics similar to Arun-3 was identified in 1985 near the border with China. This so-called Upper Arun site also takes advantage of a dramatic curve in the channel to minimize the length of the headrace tunnel. This project as currently proposed would divert up to 120 m3 s-1 from a dam no more than 12 m high through a 7 km tunnel to an underground powerhouse (Nepal Electricity Authority, 1987). The Upper Arun project would have an installed capacity of 350 MW and could generate more than 3000 GWH annually. The total project cost including access road and transmission line is about US$400 million (Nepal Electricity Authority, 1987). All planning and design work for these potential hydropower developments on the Arun has been conducted with far less information about basic hydrology than is available for projects in North America or Europe. For example, the minimal information that was available at the time of the Kosi Basin Master Plan gave the appearance that streamflow was greater than precipitation in some basins. The critical lack of data and knowledge about the Himalayan hydrologie system has been a persistent difficulty in water resources development throughout the region (Kattelmann, 1987; Gyawali, 1989). The two principal questions that must be asked early in any planning efforts are: (a) How much water is reliably available for hydroelectric generation? (b) Is the environment conducive to construction and operation of generating facilities? Two recent hydroelectric projects elsewhere in Nepal illustrate the risks of constructing projects in the Himalaya with inadequate hydrologie information: (a) in August 1985, a nearly-completed small facility on the upper Dudh Kosi in the Mt. Richard Kattelmann 780
Everest region was destroyed by flood waters from a glacial lake outburst only 12 km upstream; (b) the reservoir that supplies electricity to the capital city of Kathmandu proved to have insufficient storage to operate continuously following below-average precipitation during the 1988 monsoon. The failure of this project to provide year-round electricity only five years after completion tarnished the promise of hydroelectricity in the minds of many Nepalis. These experiences demonstrate the need for cautious planning and conservative design in Himalayan projects where our understanding of the hydrologie system is limited. The remainder of this paper attempts to synthesize our knowledge of the hydrology of the Arun River.
PRECIPITATION The climate of the Arun basin obviously reflects the dominating influence of the Indian monsoon. Precipitation is concentrated in the months of June through September, but relatively high precipitation also occurs during the months of April, May, and October. The eastern location of the basin and the topographic trough of the Arun Valley tend to increase the total precipitation and extend the wet season beyond those of most areas of Nepal (Shrestha, 1988). The regional atmospheric circulation during the summer is partially controlled by the creation of a thermal low over much of Tibet, where the ground receives intense solar radiation (Ye, 1981; Liu, 1989). The Arun Valley should enhance the movement of air and water vapor movement from the Bay of Bengal toward the Tibetan Plateau by providing a route through rather than over the Himalaya. Rainfall in the basin has been measured at six stations for more than 20 years. Lower elevation stations in the southern part of the basin have average annual rainfalls between 1000 and 2000 mm. Average rainfall at a station called Num near the Arun-3 project site is more than 3700 mm per year. Records from Num show a maximum of 350 mm of rainfall in one day and eight days in 26 years with rainfall exceeding 200 mm. The extent of high precipitation up the Arun Valley past the Nepal-China border is unknown. Although no long-term records are available, vegetation distribution documented in photographs and personal accounts of the weather suggest that rainfall is substantial (perhaps 1000-2000 mm) as far north as Kharta (Howard-Bury, 1922; Wager, 1937; Mukhopadhyay, 1982; Webster, 1989). One early explorer noted, "Within a mile [of Kharta] you may pass from the dry climate of Tibet to the moist, steamy air of a Nepalese character, with its luxuriant vegetation" (Wollaston, 1922:298). The area downstream of Kharta and north of the border is about 2600 km2 (Academia Sinica, 1981).
STREAMFLOW REGIME Knowledge of the streamflow regime of the Arun is largely based on ten years of record from a single gage at Tumlingtar, about 50 km downstream of the Arun-3 site. Thus, the streamflow data is strongly biased toward the zone of intense rainfall, the area of which more than doubles between the project site and gaging station. The feasibility studies have attempted to compensate for the influence of this downstream region and have generated several estimates of flow at the project sites. Estimates of dependable low flow and probable peak flow at the project site have been developed that provide some basis for engineering design (Nepal Electricity Authority, 1986 and 1987). In addition to the record of streamflow at Tumlingtar since 1975, a gage was operated farther downstream at Leguwaghat from 1979 to 1983, and new gages were installed at Uwa Gaon (between Arun-3 and Upper Arun) in 1986 and just above the Upper Arun dam site in 1989 (Fahlbusch, F., Morrison-Knudsen Engineers, personal communication). Staff gages have also been monitored near the Arun-3 site and on the Barun tributary (Thapa, 1987). 781 Hydrology and development oftheArun River, Nepal
Mean annual streamflow at Tumlingtar from 1975 to 1984 was 420 nr s . At this gage, streamflow is about 1000 m3 s_1 during the peak month of August, on the average, and about 120 m3 s_1 during the minimum flow period in February. Crude annual hydrographs based on monthly flows at Tumlingtar and extrapolations to the two project sites (Nepal Electricity Authority, 1986 and 1987) illustrate the seasonal distribution of streamflow (Figure 2).
000- — Tumlingtar • •• Upper Aran 500-
o >- > 1 F 1 M 1 A 1 M J J A S O N D
FIG. 2 The difference between streamflow (m3s-1) measured at Tumlingtar and that estimated at the Upper Aran site (NEA, 1987) increases dramatically during the monsoon period of June to September.
Flow characteristics at the China-Nepal border are less well-known because of the absence of any continuous records. However, measurements were made at the border and at several stations upstream and of tributaries over the course of a year during a multidisciplinary expedition in 1976 (Academia Sinica, 1981). These studies estimated mean annual flow of the Peng Qu at the border to be 156-170 m3 s-1. However, a large fraction of this flow is generated in the two southernmost tributaries, Ganmazangbo and Natang Qu, which together drain only 1,650 km2 (7 percent of the Peng Qu area), but contribute more than 45 percent of the total flow of the Peng Qu. A recent expedition to the southern part of the Peng Qu estimated the mean annual flowa t the border to be about 100 m3 s-1 based on one month of measurements and apparent application of the seasonal distribution of flow in another Tibetan river, Yarlung Zangbo at Nugesha (Guan and Chen, 1981). Concurrent measurements of the tributaries mentioned above suggested that their flowwa s again about 45 percent of that of the Peng Qu at the border. The rate of increase of discharge with distance downstream of the border is a critical unknown relating to the basic question of how much runoff is generated in different portions of the basin. We made a series of crude float-velocity—area measurements as a first approximation in November, 1986. This field survey suggested that streamflow in the post-monsoon season increased at least four-fold over a 30 km reach upstream of the Arun-3 site whereas drainage area increased only a few percent. Our estimates of bankfull discharge from channel dimensions also supported this large increase in streamflow with only a small change in drainage area. Given their inherent uncertainty, these estimates of bankfull discharge correspond surprisingly well with independent estimates of mean annual flood at the Upper Aran and Tumlingtar sites.
Dry-Season Flow Dependable flow during the dry portion of the year is, of course, crucial to the success of run-of-river projects. The Aran is particularly favorable in this regard because its dry- season flow is greater in absolute terms than any other river of east Nepal with comparable elevation or channel gradient near the measurement site. Based on tabulations of average flow in dry and wet seasons (Nepal Electricity Authority, 1987), Richard Kattelmann 782
the ratio of average streamflow in the dry season to that in the wet season is much higher in the Aran (0.23) than in other tributaries of the Sapt Kosi (average about 0.15). Low- season discharge in the Aran also tends to be consistent between years with a coefficient of variation of 0.12 in February over the ten-year record at Tumlingtar. Most of the flow in the Aran at Tumlingtar during the dry season has been assumed to come from Tibet (Nepal Electricity Authority, 1986 and 1987; Liu and Sharma, 1989). However, dry- season contributions from the Nepalese tributaries to the Aran may have been underestimated. For example, the Baran River, which has a small low-elevation drainage area (less than 200 km2), has a minimum flow of about 7 m3 s_1 (Thapa, 1987). Discharge measured below the glacierized portion of the Barun, about 200 km2 of alpine terrain, provided about one percent of the flowa t the Arun-3 dam site in November, 1986. A simple set of discharge measurements at several sites (every bridge between the stream gages at the Upper Aran site and Tumlingtar and all major tributaries) over a week in January or February in a couple of years would help resolve the question of where low- season flow is generated.
Floods The Himalayan foothills in the Aran basin are subject to intense and prolonged rainfall during summer that produces locally-high river levels and contributes to downstream flooding. Flood-generating overland flow has even been observed in the cloud forests of the upper Aran (Cronin, 1979). Several regional flood-frequency analyses have been performed in the Kosi basin (i.e., Karmacharya, 1982; JICA, 1984; Nepal Electricity Authority, 1986 and 1987). The mean annual flood of the Aran has been estimated to be about 2250 m3 s_1 at Tumlingtar and about 650 m3 s_1 at the Upper Aran diversion site (Nepal Electricity Authority, 1987). Earlier estimates from the Arun-3 feasibility study were substantially lower (Nepal Electricity Authority, 1986). Such a comparison is not intended as a criticism of the analyses, but it clearly illustrates the problems of working with limited data sets. The greatest peak flows in the Himalaya tend to result from sudden releases of water following failure of some natural impoundment (Ives, 1986). Such dams include glaciers, glacial moraines, and mass movement deposits. Several million m3 of water may enter the river in just a few hours. Such floods, where they occur, far exceed peak flows resulting from rainfall or snow- and ice-melt in upstream areas. Even though these flash floods are attenuated downstream, the potential for destruction from these dense debris flows is likely to be far above that caused by rainfall floods. A recently completed study identified more than 200 glacial lakes in the entire Aran basin (Liu and Sharma, 1988). Most of these lakes were thought to be stable and present little flood hazard. Our field survey of the Barun basin led to similar conclusions. Outburst floodsi n the Barun valley are inevitable but should lose much of their power and sediment passing through two broad flat areas. A substantial glacial-lake outburst flood is known to have swept through the Aran valley in recent decades. On the afternoon of 21 September, 1964, large amounts of ice from a glacier slid into Gelhaipuco, a 25 million nr lake in the Natang Qu basin just north of the China-Nepal border (Academia Sinica, 1979). The wave generated by the falling ice burst the moraine dam impounding the lake. Discharge of the ensuing debris flow was about 6000 m3s-1 at the confluence with Natang Qu and about 3200 nr s-1 30 km downstream. The'flood and debris flow damaged a road in the Natang Qu basin and carried 12 logging tracks downstream. This event was recalled by Nepalis who reported seeing concrete, logs, and truck bodies in the debris (Nepal Electricity Authority, 1987). The Arun-3 and Upper Aran feasibility studies have recommended designs resistant to glacial lake outburst floods of 7,700 and 10,000 m3s-1, respectively. Planning for such risks is a necessary part of water resources development in the Himalaya. 783 Hydrology and development oftheArun River, Nepal
Sediment The Aran is generally recognized to transport the least sediment of the major tributaries to the Sapt Kosi (JICA, 1984). Most of the area in Tibet is both of low slope and low drainage density and contributes relatively little sediment to the rivers. However, the estimates of 10 million m3 per year at the upper Aran site (Nepal Electricity Authority, 1987) and 35 million m3 per year at the confluence with the Kosi (JICA, 1984) are still enormous by comparison to most other areas of the world. Simply stated, the Himalayan zone produces vast quantities of weathering products for eventual transport by streams. The relative importance of suspended load, bed load, and transport during and after infrequent debris-flows in the Aran is unknown. The occasional destabilization of river channels and deposition of huge quantities of sediment from outburst floods can affect the load of the river for several years (Ives, 1986). Measurements of suspended load at the new Uwa Gaon stream gage and geomorphic studies during the design of the proposed projects should improve understanding of the sediment dynamics of the basin.
PROSPECTS Although the rerouting of the access road will undoubtedly increase construction costs and delay completion of the hydropower project(s), it also provides the opportunity to improve basic data collection and begin process-oriented studies of the Aran's hydrology. Improved knowledge will reduce levels of uncertainty and assumptions, and it will also provide a better basis for project design and operation. The experience in the Aran demonstrates the need to install a basic streamflow monitoring program as soon as a project with any chance of implementation is proposed. If a gage had been installed near the China-Nepal border in 1985 when flow from Tibet was recognized as critical unknown, we would now have several years of data and an adequate basis for assessing the geographic distribution of runoff-production. Another factor in the development of the Aran River basin is the planned eastward extension of Sagarmatha National Park to include the Makalu-Baran area (Taylor-Ide and Shrestha, 1985). An international peace park in the Peng Qu / Aran border area was proposed and endorsed at the Mohonk Mountain Conference in April, 1986 (Ives and Ives, 1987). Most of the western part of the Peng Qu basin has now been designated as a large park called the Qomolungma Nature Preserve (Himal, 1989). If the plans for the Makalu-Baran Conservation Area are carried out, then a large fraction of the Peng Qu / Aran catchment will be be under active management conducive to protection and enhancement of the water resource.
ACKNOWLEDGEMENTS Nima Wangchu and Mike Zika provided invaluable assistance with the field measurements. Fred Fahlbusch, Krishana Malla, Nick Mandeville, Ramiro Mayor-Mora, Shi Jiancheng, Bhaskar Thapa, and Xu Daoming were most helpful in providing data and information about the Aran basin.
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