OPTIMISING MULTIPLE-USE OF WATER IN CASCADES OR SYSTEMS OF RESERVOIRS IN SMALL CATCHMENTS

Transcription

1 OPTIMISING MULTIPLE-USE OF WATER IN CASCADES OR SYSTEMS OF RESERVOIRS IN SMALL CATCHMENTS MK3 Optimising cascades of hydropower INSTITUTIONAL & CONCLUSIONS Conclusions and recommendations from the MK3 project December 2013 Peter- John Meynell

2 Author Produced by Suggested citation More information Image Project Team Copyright Peter-John Meynell Mekong Challenge Program for Water & Food Project 3 Optimising cascades of hydropower for multiple use Led by ICEM International Centre for Environmental Management P-J Meynell Project report: Challenge Program on Water & Food Mekong project MK3 Optimizing the management of a cascade of reservoirs at the catchment level. ICEM International Centre for Environmental Management, Hanoi Vietnam, Cover image: Is there enough water available for irrigation? Downstream of Theun Hinboun HPP (Photo Peter-John Meynell). Inside page: MK3 1st Cambodian Stakeholder Meeting, Stung Treng: H.E. Dueong Pecu, Deputy of Stung Treng Administration Office, and Team Leader Peter-John Meynell provide a rousing speech to formally open workshop proceedings (Photo Paradis Someth) Peter-John Meynell (Team Leader), Jeremy Carew-Reid, Peter Ward, Tarek Ketelsen, Matti Kummu, Timo Räsänen, Marko Keskinen, Eric Baran, Olivier Joffre, Simon Tilleard, Vikas Godara, Luke Taylor, Truong Hong, Tranh Thi Minh Hue, Paradis Someth, Chantha Sochiva, Khamfeuane Sioudom, Mai Ky Vinh, Tran Thanh Cong 2013 ICEM - International Centre for Environmental Management 6A Lane 49, Tô Ngoc Vân Tay Ho, HA NOI Socialist Republic of Viet Nam i

3 TABLE OF CONTENTS 1 INTRODUCTION OVERALL RECOMMENDATIONS AN INTEGRATED APPROACH Basin profiles Field visits GIS analysis Catchment hydrology and its modeling in Nam Theun-Nam Kading and in Sesan river catchments Landuse suitability for agriculture in the Sesan catchment: Potential for irrigation development Trade-offs between hydropower and irrigation development and their cumulative hydrological impacts A case study from the Sesan River Fish and fisheries in the Sesan river basin Potential for fish passage and implications for hydropower Case study of Lower Sesan Exploring environmental flow regimes in the Lower Sesan in Cambodia Enhancing ecological diversity of reservoirs with constructed wetlands Flood Control Challenges for large hydroelectric reservoirs, with examples from Nam Theun-Nam Kading Basin in Lao PDR Impacts of landuse change on sediment transport and implications for reservoirs Error! Bookmark not defined Institutional challenges for the management of multiple use of water in cascades of reservoirs.. Error! Bookmark not defined. 4 FINDINGS OF THE MK3 PROJECT Agriculture Sesan fisheries and fish passage Environmental flows on the Sesan Creating wetlands within reservoirs Flood control RECOMMENDATIONS Agriculture Sesan fisheries and fish passage Environmental flows on the Sesan Creating wetlands within reservoirs Flood control Soil erosion and sediment transport... Error! Bookmark not defined. 5.7 Managing multiple use for sustainability... Error! Bookmark not defined. ANNEX: PARAMETERS FOR ASSESSING MULTIPLE WATER USE OPTIONS IN RESERVOIRS

4 1 INTRODUCTION The MK3 project aimed to develop a systems approach to coordinated and equitable management of cascades of reservoirs through a suite of hydrological and hydraulic models designed to focus on multi-use, optimization and benefit sharing at the catchment level. The project assessed the opportunities and risks posed by the operation of many Water Storage Infrastructure (WSI) projects in a single catchment to show how such water uses and options may be modelled, planned and managed at a catchment level. The project researchers considered specific dams and the cascades of which they are a part in the Sesan and Nam Theun/Nam Kading river basins, to examine the pros and cons of assigning a fraction of the reservoir storage to irrigation, fisheries etc. They developed a series of related studies that focused on particular aspects of multiple use of water in hydropower reservoirs in the two river basins. All of these studies were underpinned by the use of a specifically designed set of hydrological and hydraulic models of the flows in the two basins and of the changes induced by operation of the reservoirs. GIS mapping was used for land use and suitability for different crops, and for soil erosion and sediment transport. Despite the usual gaps in hydrological, environmental and social data that commonly restrict studies and impact assessments, the various models provided considerable insight into the issue of multiple use of water in cascades of reservoirs. This final paper in the e-book pulls together all of these related aspects of multiple use of water in the cascades of dams found in these two catchments. These studies can either be read at an individual level, so that insight into the specific use of water can be gained, or they can be seen as a whole indicating what needs to be done to assess the potential for multiple-use in an existing system of dams, or for proposed dams that are being planned. Each of the multiple-uses depends upon the application of the models and illustrates the implications for the hydropower potential. The question being asked was if this multiple use was developed, how much water would be diverted from the hydropower dam and what would the reduction in electricity generated annually? This question highlights the trade-off that may be made between single- or multiple- use of hydropower dams. Finally, multiple-use of water in cascades of reservoirs is more difficult to manage, and the institutional challenges of doing this have been reviewed, especially when different private sector companies are operating the dams and trying to make a profit from the single use of electricity generation. This paper summarises all the key research undertaken by MK3, in terms of a) what was done as part of the integrated approach, b) the findings and c) the conclusions and recommendations. The overall recommendations are presented first, followed by the summaries of each area of research. 2

5 2 OVERALL RECOMMENDATIONS In considering all the papers that have been produced as part of the MK3 project, it becomes clear that the potential for multiple use of water in single reservoirs and in cascades of dams is generally much greater than at first assumed. The impacts of other uses upon the hydropower potential are often not as great as the proponents of single-use hydropower projects claim. In a time where there is an increasing emphasis upon the sustainability of hydropower projects and the opportunities for benefit sharing, it is essential that multiple use options are considered adequately at the feasibility stage and EIA stages of planning and design. Retrofitting of multiple-use is more difficult and may be more expensive, so incorporating the design of multiple-use at an early stage is essential. Nevertheless throughout the world, there are examples of sites where other uses for the water have been re-engineered, including fish passage, irrigation and recreational uses. The following overall recommendations are made: Regulatory agencies should insist that studies be carried out to assess the potential and feasibility of multiple use options for all proposed dams Developers should include technical, social and economic viability assessments of such multiple uses for proposed dams for example in feasibility studies and EIAs If multiple use options are considered to be viable and environmentally and socially sound they should be incorporated into the design of new projects River basin managers and planners should work with developers and local agencies to explore the possibilities for multiple-use of both existing and proposed hydropower projects, irrigation and water supply reservoirs. Operators of dams in cascade should work closely together in the coordinating water releases for flood management, irrigation water supply downstream, environmental flows and fish passage and other multiple uses of water. National and transnational level planning and management is essential to address the complexity and increased competition of water use as a foundation for water resource sustainability. 3

6 3 AN INTEGRATED APPROACH 3.1 BASIN PROFILES The starting point for our work was the development of the basin profiles for Upper Sesan in Vietnam, Lower Sesan in Cambodia and the Nam Theun-Nam Kading River basin Lao PDR. These were prepared from existing information, reports, e.g. EIA reports, MRC BDP sub-basin reports, and statistics about each of the basins to provide the team with baseline information and development trends. For most of the information about these basins, no detailed primary data was collected, although in the Nam Theun-Nam Kading district data about populations, land use and agriculture was collected. 3.2 FIELD VISITS Whilst this research was mainly desk based, drawing upon publically available information and modelling, two field visits were made to the Lower Sesan and Nam Theun Nam Kading river basins by a large group of the research team. Subsequent individual visits were also made by some of the team members. The two field visits were invaluable in that they provided opportunities to appreciate the conditions and issues in these two river basins. The visits provided opportunities to discuss the issues with dam operators, water resource and irrigation planners, and watershed managers. Stakeholder consultation meetings were also held during each field visit. The research focus for each of the studies developed out of the experiences of these field visits. 3.3 GIS ANALYSIS Each of the river basins were mapped using GIS techniques with data on land use and land cover, protected areas, existing and future hydropower and irrigation schemes and reservoir areas. GIS data was also analysed to provide information on each of the hydropower dams and reservoirs, and to develop parameters for scoping of multiple use. Initial assessments of the potential for multiple-use of water from reservoirs in cascade may use some of the parameters that we have identified for developing a typology of reservoirs indicating the suitability for different multiple uses. These are shown in the Annex. GIS techniques were also used to map land suitability for different crops and the potential for irrigation in areas adjacent to the rivers. 3.4 CATCHMENT HYDROLOGY AND ITS MODELING IN NAM THEUN- NAM KADING AND IN SESAN RIVER CATCHMENTS Catchment hydrology modelling underpins all of the studies undertaken in this project. A good understanding of the catchment hydrology is essential when assessing catchment development and its implications on water resources and flow regimes. However, often the observed data are scarce and inadequate to conduct such assessments. The key hydrological data are precipitation, temperature and river flow data, and these were obtained from the MRC. The gathered data were analysed and used to create a baseline understanding of the catchment hydrology. We used these to setup hydrological models for the case study catchments to simulate their hydrological processes and to supplement the scarce measurement data with simulated hydrology. These included the distributed physically-based model called VMod driven by measured hydrological data. The modelled catchments were based on grid presentation of the area in question, where in each grid cell hydrological computations take place. The Sesan catchment was modelled together with neighbouring rivers Sekong and Srepok with grid cell sizes of 3 x 3 km. The Nam Theun-Nam Kading catchment was modelled with grid cell sizes of 1 x 1 km, because this required a spatially more refined approach due to karst mountain topography. The Nam Theun-Nam Kading the model was able to reproduce 71% of the measured daily flow variability at Theun-Hinboun dam site during the period but the success of the model varied in different years. The model performance was higher in when it was able to replicate 84-89% of the daily flow variability. In the Sesan catchment the model was able to reproduce 87% of the measured daily flow variability at Ban Kamphun measurement station during the period Furthermore in Sesan, the 4

7 modelled annual flow in eleven existing and future dam sites was on average within 5% from the measured flow volume. 3.5 LANDUSE SUITABILITY FOR AGRICULTURE IN THE SESAN CATCHMENT: POTENTIAL FOR IRRIGATION DEVELOPMENT This study developed an agro-ecological zoning approach to understand the diversity of Sesan catchment in terms of climate, soil and land use. It identified the recent agriculture trends in the catchment and investigated the potential for agriculture development. The study went on to assess the potential for irrigation development from the cascades of existing and proposed dams. It highlighted suitable areas for irrigation development and estimated the water requirement for those potential cultivated areas. 3.6 TRADE - OFFS BETWEEN HYDROPOWER AND IRRIGATION DEVELOPMENT AND THEIR CUMULATIVE HYDROLOGICAL IMPACTS A CASE STUDY FROM THE SESAN RIVER Using the landuse suitability assessments from the earlier study this study focused on the land suitability and water requirements for dry and wet season rice cultivation. The catchment hydrology was then modeled including the requirements for hydropower operations and irrigation water abstraction over the annual cycle. The model allowed an assessment of the changes in hydropower generation and hydrological regime at catchment scale that would be required if full implementation of the irrigation potential was realised. 3.7 FISH AND FISHERIES IN THE SESAN RIVER BASIN The fish and fisheries of the Sesan were reviewed, including fish biodiversity in all three of the 3S Rivers (species richness, endemicity, endangered species); fish migrations and fisheries aspects (fish catch estimates, catches of migratory fish). The economic value of this resource and the fish dependency of riparian communities were discussed, with trends in fish resources and fisheries. The hydrology of the Sesan river, its main ecological zones and habitats were described, (this information was later used in the environmental flow paper) and the population density of the people and communities along the Sesan River and fisheries-based livelihoods were reviewed. 3.8 POTENTIAL FOR FISH PASSAGE AND IMPLICATIONS FOR HYDROPOWER CASE STUDY OF LOWER SESAN 2 The proposed Lower Sesan 2 + Lower Srepok dam will block the lower part of the Sesan and Srepok Rivers and will clearly have a negative impact on biodiversity, fish migrations, fish catch and food security in the 3S area and far beyond. There is an urgent need to reconsider the options for fish passage around the Lower Sesan 2 dam site, which have to date been discounted as impracticable by the developers. The paper considered the examples of three fish passage systems at high dams in South America including upstream migrations at the Lajeado Dam fish ladder, at the Peixe Angical Dam fish ladder, at the Canal da Piracema fish passage around Itaipu dam. The latter was found to be the most promising and we drew lessons from this design for a possible fish passage system at the Lower Sesan 2 site, without modifying the current design of the dam. Based upon flow requirements suggested for the Itaipu fish passage, we estimated the annual flow requirements and compared this to the loss in hydropower potential. 3.9 EXPLORING ENVIRONMENTAL FLOW REGIMES IN THE LOWER SESAN IN CAMBODIA The concept of allocating a certain proportion of water released from hydropower dams for maintaining the ecological functions and services in the downstream river has been developed over the last decade. The practice in South East Asia has been to identify minimum flow releases below dams based largely upon what can be negotiated by the hydropower company, who wish to use all the water they can for generating electricity. In this paper we examined the impacts of managing different flow regimes from a cascade of dams in the Upper Sesan upon the existing ecology and ecosystem services of the Lower Sesan. We postulated 5

8 that near natural flows could be achieved in the lower river if the higher cascade of dams was operated with minimum storage as a series of run-of-river dams. We recognized that this could have significant implications for the power generated, which would represent a trade-off between power generation and multiple use, in this case the use of water for environmental purposes. The characteristics of existing and proposed hydropower plants on the Sesan river were described. The previously studied river geomorphology and habitats in the lower part of the river up to the Yaly Falls HPP was used, including the important areas for fish spawning and feeding. This helped to identify specific reaches of the river that may be used to assess the geomorphological and habitat impacts of changes in flow regime. We compared the hydrology of the Sesan using the original flow regime before Yaly dam was built with the changes that have occurred during the past 15 years as a result of hydropower operation. We analysed a number of key flow characteristics important for determining environmental flows using the Flow Health modeling tool, with implications for the habitat, biodiversity and productivity of the lower part of the river. We recognized that changes in the quantities of sediment moved down the river due to trapping in the reservoirs will affect the geomorphology and habitats considerably over a long period of time. We then compared the losses in hydropower potential due to operating the dams in cascade with no regulation (i.e. as run-of-river dams) to the more usual operation as storage dams with full regulation, recognizing that trade-offs would be required in practice ENHANCING ECOLOGICAL DIVERSITY OF RESERVOIRS WITH CONSTRUCTED WETLANDS Increased fish production is usually promoted as an important secondary benefit of reservoirs created for hydropower. Dam proponents present increased fishery as a benefit for local people in compensation for disruption to livelihoods caused by dam construction. However, reservoir fisheries often fail to live up to expectations. The aquatic habitats that develop in reservoir are less diverse and less productive than the original river, especially for steep-sided, deep reservoirs with large annual drawdown. Fish diversity often decreases and there is a tendency for relatively large-bodied and valuable top predators to be replaced by smaller less valuable fish. Two linked papers are presented that explore the creation of wetlands within the drawdown of such reservoirs to improve the environment, increase fish diversity and support local livelihoods. The construction of permanent wetlands within the drawdown of reservoirs by building earth dykes and spillways in seasonal stream channels below the full supply level was proposed and a feasibility study for constructing wetland in Nam Gnouang reservoir was carried out. Five potential sites for such earth dykes and spillways were identified. Designs and estimates of costs were prepared, and the hydrological functioning and contact with the reservoir was examined. Such constructed wetlands could have conservation or fish production objectives acting as fish refuges and spawning grounds, serving the reservoir fishery as a whole or be used as dedicated aquaculture ponds FLOOD CONTROL CHALLENGES FOR LARGE HYDROELECTRIC RESERVOIRS, WITH EXAMPLES FROM NAM THEUN- NAM KADING BASIN IN LAO PDR The operation of large hydro-electric reservoirs for flood control can be a successful method of reducing inundation downstream. The challenge for the designers and operators of hydro-electric reservoirs is to foresee extreme events well enough to ensure a successful outcome in regard to flood control. Spillway design is sometimes compromised by inadequate information, e.g. the availability of river flow information for a short period prior (say of just a few years) prior to project initiation. Timeliness is the essence of flood control, and the difference of just one to two days in response to the onset of a major flood can make the difference between safe operation at the reservoir, and unsafe operation. The operation of the Nam Theun 2 dam in managing floods in the case of extreme events was modelled, and two scenarios were compared a) timely and effective spillway operation and b) with spillway malfunction and delay in flood releases. The requirements for preparedness and management of such extreme events are considered, together with coordination with all dam operators in the same cascade. 6

9 4 FINDINGS OF THE MK3 PROJECT 4.1 AGRICULTURE The land use suitability analysis of these two papers showed that the proposed hydropower development in the Lower Sesan would submerge some highly valuable land for agriculture, especially the proposed Lower Sesan 3. However, hydropower could provide a potentially interesting water source for irrigation development. If gravity-fed irrigation schemes are developed 1 km away from the river, i.e. not too costly in terms of infrastructure, the water demand required for irrigation development will require less than 10% of the active water storage. By analyzing the water demand for wet and dry season rice, we found that the irrigation of 3,894 ha, 6,490 ha and 9,086 ha reduced the annual hydropower generation of Sesan 3A by -1.8 GWh = -2.4%, - 3.1GWh = -4.0% and -4.3GWh = -5.6%, respectively. The irrigation from Sesan 3A required cascade operation with Yali and Sesan 3, as the Sesan 3A did not have sufficient storage to facilitate irrigation. When reservoir irrigation is up-scaled to catchment level, total irrigable area was estimated at 28,348 ha, which would lead to -1.6% (209 Gwh) reductions in total annual hydropower generation of nine projects with total capacity of 13,056 GWh. In terms of flow changes, the hydrological impacts of hydropower operations were much more significant than the impacts of irrigation. The hydropower operations increased dry season (November-April) flows on average by 167% and decreased the wet season (May-October) flows on average by 11% in the lower reaches of Sesan River. The impacts of irrigation resulted in annual water losses of 0.39 km 3 corresponding to 1.9% of the annual average flow of the Sesan River (20.5 km 3 ). It was further found that hydropower and irrigation development would turn the Sesan River into increasingly complex and managed system with increasing competition between different water users. The increasing complexity and competition will require improved national and transnational planning and management of water to guarantee equitable and sustainable use of water. The increased competition may for example result in decreased possibilities to consider environmental sustainability. 4.2 SESAN FISHERIES AND FISH PASSAGE The 3S system (Srepok, Sesan and Sekong Rivers) is characterized by a high level of fish biodiversity with 329 species. This corresponds to 42% of all Mekong fish species, for an area representing only 10% of the Mekong Basin. The Sesan River features 133 fish species, i.e. approximately 40 to 50% fewer species than the Sekong and Srepok Rivers. The 3S rivers are also home to 14 endangered fish species, including the critically endangered species Aaptosyax grypus (giant salmon carp), Catlocarpio siamensis (giant carp) and Pangasianodon gigas (giant catfish). Fifteen species are found exclusively in the Sekong River and two in the Srepok River; they are found in no other Mekong tributary and nowhere else in the world. No species is exclusively found in the Sesan River. The 3S system is characterized by at least 89 migratory fish species belonging to 15 families. Sekong, Sesan and Srepok Rivers are characterized by 64, 54 and 81 migratory fish species respectively. At least 41 migratory fish species are commonly caught by fishermen in the Sesan River, and these migratory species represent 60% of the fishermen s total catch. There has been a drastic decline in the abundance of most fish species, or more specifically in the catch per fisherman, along the 3S rivers. This decline applies to species in all families and to all three of the rivers. In the Sesan River for instance Henicorhynchus siamensis and H. lobatus had not been seen upstream for about ten years. The Srepok seems to be the river where the decline is smallest. The total catch in the Sesan River Basin is not precisely known and estimates range between 370 and 6,700 tonnes of fish per year. The total value of the fish production along the Sesan River ranges from USD 1 to 25 million, the central estimate being between 2-16 million USD, depending on estimates of total yield and of price per kilo. There is no substantial aquaculture sector in the Cambodian part of the 3S. In Vietnam no detailed and recent aquaculture statistics could be found; in 2003 about 5,000 tonnes (i.e. less than 1 percent of the national production) were produced in the 3S. 7

10 The population living along the Sesan River Basin in Cambodia and Vietnam reaches 840,000 people. The number of people dependent on riverine resources along the Sesan River is subject to multiple contradictory estimates; in Cambodia, this number ranges from 30,000 to 50,000 people. Dependency might be less in Vietnam. Fish is still regarded as the main source of protein in the diet of the Sesan people. Former contradictory estimates of fish consumption along this river varied between 15 and more than 100 kg/person/year, but a recent comprehensive study by the Fisheries Administration amounts average freshwater fish consumption to 32.6 kg of inland fish per person and per year in this region. Studies of fish passage systems at high dams in South America showed that in a species-rich tropical environment, up to 116 species were found in the fish passage systems, with a few species (mostly excellent swimmers) predominating. The efficiency of the fish passage seems closely linked to the water velocity in certain stretches and suggests that a velocity of 2.3 m/sec is acceptable (with 0.8 m minimum depth and low turbulence). Discharge in the pass system should be between 10 and 12 m 3 /sec year round. For downstream migration of adults, a fish passage may be more successful if it is directly connected to a part of a reservoir that is flowing and well oxygenated. More generally, the distribution of favorable habitats for reproduction upstream should be taken into account. Efficient fish passage systems for high dams must mimic the natural systems in order to facilitate the maintenance of natural fish populations. A variety of biotopes along an extended fish passage system creates beneficial conditions for more species to pass the obstacle. A review of local conditions at the Lower Sesan 2 site shows that the stream located 9.8 km downstream of the dam on the right bank would potentially be a good path to consider for the development of a fish passage system, similar to that of the Canal da Piracema at the Itaipu Dam in South America. That fish pass would consume at most 1.2% of the reservoir water and result in a loss of hydropower not superior to 1.1% of the planned production. 4.3 ENVIRONMENTAL FLOWS ON THE SESAN The key ecological zones and geomorphological habitats in Sesan river were defined, with potential e- flow study locations identified. Since construction of the Yali falls dam began there is a clear decrease in the seasonality flow shifts, a marked decrease in persistently higher flows sub-indicator, e.g. in dry season, the flows tend to be higher than previously. The low flow sub-indicator shows a slight decrease The reservoirs would be maintained at more or less constant level, and only inflow passed on down to the next dam below and so on down the cascade. Whilst this would mimic the natural flows in the catchment, such a regime would not pass on the sediment in the same way. Such a change in operation of the hydropower projects would have implications for electricity generation. These changes are compared to operating for maximum power production. Thus the two scenarios compared were: Full regulation scenario - the simulation allowed each project to use freely the full storage capacity of reservoirs in order to maximize energy production; No regulation scenario the reservoir levels of all projects were kept constant at full supply level so that the natural flow regime was passed through the dam unaltered. In simulations, each hydropower project was allowed to use only 3 Mm 3 of reservoir storage to improve the stability of the simulation/optimization process. The annual average hydropower generation of the eleven hydropower projects under these scenarios and their respective annual differences in hydropower generation was compared. The losses in hydropower generation due to the No regulation scenario varied from zero to 33% between the individual projects. The total annual average loss in hydropower generation across the eleven projects was 13.3%. The Full regulation scenario optimises the production of energy but will have significant impacts on downstream flow and ecosystem health. There is therefore a need to reach a compromise if flows for maintaining environmental conditions are to be considered. 8

11 For most indicators, if the annual flow is reduced to less than 70% of the reference mean annual flow then the sub-indicator score diminishes to zero. There is a sharp drop in most indicators when the mean annual flow is reduced between 90-70% of the reference mean annual flow. The overall Flow Health score does not change significantly with annual flow changes between 10-70% of the reference mean annual flow. If annual flow is to be less than 70% then it is likely that the benefits of improved energy production will outweigh the benefit for flow health and the annual flow may as well be reduced to 10% of the reference mean annual flow 4.4 CREATING WETLANDS WITHIN RESERVOIRS We identified five different locations with outline designs for building the dykes and spillways to create the permanent wetland. The hydrological functioning of these wetlands was considered with lengths of time when the wetland is connected to the main reservoir. These wetlands can be used for different functions for conservation as a refuge and breeding habitat for fish, which can then move back into the wider reservoir and increase the productivity, as a natural wetland from which fish and other aquatic animals can be harvested by local communities, or as dedicated fish ponds for more intensive aquaculture. Estimates are provided for the costs of construction, potential fish production, and the impact upon the water retained within the wetland upon hydropower potential. The five constructed wetlands in Nam Gnouang would have an area of 3.1 ha and cost around 20-30,000 USD to build using locally available materials and labour. The volume of water retained within wetland was estimated at 22,000 cu.m or 0.001% of the active storage of the reservoir. This water would have a value for power production of 1,320 USD per year. Assuming that the wetland would be managed for low intensity aquaculture, fish production could be about 3 tonnes per year with value of 6,000 USD per year. This more than outweighs the reduction in hydropower potential. 4.5 FLOOD CONTROL An example was studied from the Nam Theun 2 reservoir, using runoff values from the 2002 season, assuming that the dams that contain the reservoir had been built at that time. A combination of mechanical failure (one spillway gate at Nam Theun 2 dam not operating), and human/ management error (a one day late response) was shown to trigger water level conditions that constituted an extreme hazard, i.e. near the top levels of the saddle dams. In contrast, the timely opening of spillway gates, combined with capability to release water from all gates at full capacity, allowed for reservoir levels to stay well within the safe elevation range, and allowed a significant reduction in downstream flood discharge. 9

12 5 RECOMMENDATIONS 5.1 AGRICULTURE Based upon the assessment, the following suggestions are made: When planning any hydropower development, the losses of agricultural land should be assessed; The potential for irrigation of agricultural land from the reservoir at least within 1 km on each side of the river downstream should be assessed; Water requirements for irrigation may be compared with potential for electricity generation; More detailed and case specific investigations should be conducted for estimating the potentials and related trade-offs in multiple-use of hydropower reservoirs. We noted that this study was focussed on technical and hydrological aspects of irrigation from reservoirs, but there are many other aspects that should be included in such feasibility studies including environmental and social, livelihood and land ownership aspects. The full understanding of the impacts the considered development requires more holistic assessment approaches than those based on computational models. 5.2 SESAN FISHERIES AND FISH PASSAGE Based upon the studies, the following suggestions are made: The importance of the Sesan fishery resource should be fully recognised in any dam development on the Lower Sesan. There is an urgent need for a proper assessment of fish catches along the Sesan and Sre Pok Rivers. The contribution of the Sesan and Sre Pok Rivers to the fish production in the wider Mekong floodplains in Cambodia, in the Tonle Sap and in Vietnam should also be assessed. Identifying the location of fish breeding sites is essential to a better assessment of the impact of dams on migratory species The potential for developing suitable fish passage around high dams such as Lower Sesan + Lower Sre Pok 2 should not be immediately discounted. Because of the importance of the 3S rivers fishery to the wider Mekong, some provision for fish passage should be made. There is potential to use natural streams downstream of dams for fish passage directly to the reservoirs. The design of fish passage systems should take into account the requirements of different fish species (which requires studies of swimming requirements) and include resting pools. 5.3 ENVIRONMENTAL FLOWS ON THE SESAN Based upon the studies, the following suggestions are made: Techniques for assessing the flow requirements for maintaining environmental and ecosystem services are available, and it is relatively easy to identify locations and reaches in the river where such assessments may be carried out; Hydrological model systems for comparing different flow regimes are also available and may be used for considering the trade-offs that may be required when operating individual dams and those in cascades in order to determine the least damaging flow regimes to the river ecosystems downstream; When planning new dams, downstream flow release requirements should be assessed as part of feasibility studies and EIAs, rather than relying upon the concept of minimum flow requirements. This should be included as a requirement of MOUs with developers. 5.4 CREATING WETLANDS WITHIN RESERVOIRS Based upon the studies, the following suggestions are made: Design and effectiveness of constructed wetlands in reservoirs should be developed Trials of constructed wetlands in reservoirs require testing for conservation, wild fish capture and aquaculture; Such trials should cover the technical, social acceptability and management aspects; Ecosystem design and management of reservoirs for more than hydropower alone deserves the attention of developers and government. 10

13 5.5 FLOOD CONTROL Based on the flood control paper, we make suggestions for: The establishment of a strengthened capacity for the NT-NK River Basin Committee Secretariat; For emergency communications capability between dam operators and downstream stakeholders; For periodic, long-term requirements for engineering assessment concerning safe operations at the dams. We note the ongoing research work on climate periodicities in the Mekong region is encouraging, as it promises to offer ways of achieving advance notice of the likelihood of an extreme weather season. 11

14 ANNEX: PARAMETERS FOR ASSESSING MULTIPLE WATER USE OPTIONS IN RESERVOIRS The following parameters are suggested for analyzing the reservoirs in the Upper and Lower Sesan in Vietnam and Cambodia and in the Nam Theun/Nam Kadinh basin in Laos. The purpose of doing this is to provide a scoping method for assessing the different possible multiple uses. Design parameters of dams The following design parameters will be useful: Design head - m Design flow of turbines m3/sec Turbines type, number and capacity Installed capacity MW Mean Annual energy GWh Spillway design flow - m3/sec Riparian releases - m3/sec Dam dimensions, length, height m Type of scheme - Diversion, run-of river, storage The GIS-analysable parameters are as follows: Full supply Level metres above sea level Minimum Operating level metres above sea level Draw down m (difference between FSL and MOL) Area of reservoir at FSL sq km Area of reservoir at MOL sq km Perimeter of reservoir at FSL km Perimeter of reservoir at MOL km Total volume of reservoir at FSL million m3 Total volume of reservoir at MOL million m3 Active (Live) storage - difference between volumes at FSL and MOL - million m3 Average depth of reservoir m Maximum depth of reservoir m Area exposed at drawdown sq km Average slope of land in drawdown - Area exposed at drawdown which is between 0 5 % slope Catchment area above dam sq km Mean annual flow at the dam site m3/sec Distance from dam to confluence with the Mekong km Number of tributaries with annual flows flowing into each reservoir Number Tributaries with major centre of population (>10,000 people) upstream of reservoir Sub-catchment areas for each of these tributaries sq km Land use in each sub-catchment forest, agricultural land, swidden, protected area, urban etc. Number of roads (1o, 2o and 3o) leading to the reservoir shore - number Length of roads running along the reservoir shore line Population within 5km, 10 km and 50 km of shoreline numbers of people Number of villages within 5km of reservoir shoreline numbers of villages Distance to major centres of population - >10,000 persons - km Area of irrigable land downstream of the dam site ha Maximum distance of irrigable land downstream of dam site - km Population in irrigable area people per ha and number People resettled from reservoir area numbers of villages, people 12

15 Some ratios that may be useful for comparison MW installed/reservoir area Mean annual energy/reservoir area People resettled/reservoir area People resettled/mw installed and mean annual energy Active storage/mw installed and mean annual energy Active storage/mean annual flow at dam site Active storage/mean annual flow at confluence with the Mekong Active storage/length of river system % of catchment regulated at each dam site MW installed/% of catchment regulated Active storage/total storage Residence time in active storage of reservoir at mean annual flow and minimum mean monthly flows (April) Perimeter/Area of reservoir at FSL and MOL Shape of reservoir - This will be dependent upon the terrain and height of the dam, e.g. Long, narrow, steep-sided valley, simple perimeter, few tributaries Long, narrow, steep-sided valley, complex perimeter with many indentations and tributaries Broad, shallow reservoir with many tributaries Broad shallow reservoir with few tributaries 13