CUMULATIVE IMPACT ANALYSIS AND NAM THEUN 2 CONTRIBUTIONS

Transcription

1 GOVERNMENT OF LAO PDR ASIAN DEVELOPMENT BANK CUMULATIVE IMPACT ANALYSIS AND NAM THEUN 2 CONTRIBUTIONS Annex 3: Hydropower Development Operation of reservoirs, assumptions and input data. Prepared by: Anders Korvald November 2004

2 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page i TABLE OF CONTENTS 1 INTRODUCTION ASSUMPTIONS Model description Technical assumptions RESERVOIR AND HYDROPOWER DATA Reservoirs in Lancang, Yunnan Reservoirs in Lao PDR Reservoirs in Thailand Reservoirs in Vietnam Reservoirs in Cambodia Summary of reservoirs in Mekong FLOODING LIST OF FIGURES Figure 3-1. Location of large existing and planned reservoirs for hydropower and irrigation...6 Figure 3-2. Monthly river flow at border to Lao PDR before (natural) and after construction (in percentiles) of planned hydropower projects in Yunnan....7 Figure 4-1. Daily discharges for Mekong just upstream Tonle Sap and the discharge in Nam Theun, during a high flood in Nam Theun LIST OF TABLES Table 3-1 Hydropower projects in Lancang, Yunnan (projects in bold are included in the 2010-scenario)...5 Table 3-2. Storage hydropower projects in Nam Ngum, Lao PDR (projects in bold are included in the 2010-scenario)....8 Table 3-3 Storage hydropower projects in Nam Theun, Lao PDR (projects in bold are included in the 2010-scenario)....9 Table 3-4 Storage hydropower projects in Se Kong, Lao PDR (projects in bold are included in the 2010-scenario)....9 Table 3-5. Existing reservoirs in Thailand Table 3-6 Storage Hydropower projects in Se San and Sre Pok, Vietnam (projects in bold are included in the 2010-scenario). PS! The sum includes the other smaller projects (active storage less than 100 mill.m 3 )...10 Table 3-7 Storage Hydropower projects in Se San and Sre Pok, Cambodia. (no projects are included in the 2010-scenario) Table 3-8. Active storage in the Mekong basin...11

3 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 1 1 INTRODUCTION One main task of the Cumulative Impact Analysis (CIA) has been to estimate the cumulative impact on hydrology. All reservoirs have an impact on the hydrological balance their purpose being flood control, water supply, irrigation or hydropower generation. This section presents the hydrological changes caused by future large projects, mainly hydropower, within Mekong catchment, whereas the resulting flow and water levels in Mekong are presented in the main report and the methodology in Annex 2. The future variation in water flow in Mekong depends on the future number and size of active storage of seasonal reservoirs. The cumulative impact of Nam Theun 2 depends on other reservoir development and the regulation of the other reservoirs. The largest potential projects are mainly located in Lancang, and in the Nam Ngum, Nam Theun, Se Kong and Se San basins. In the CIA one scenario is the situation in 2010 and the other in The selection of projects within scenario 2010 is based on projects under construction or close to financial closure. For the 2025 lists from MRC and National Power Development Plans has been used. Because of limited time available, the work on the hydrological impact of future reservoirs have been simplified. Small HPP and Run-of-river projects are not included. For multipurpose projects the irrigation demand will have an impact on the reservoir operation and thereby the potential energy production and downstream water balance. An optimisation of the total output from the basin, both energy production and irrigation utilisation, has not been carried out. Optimal reservoir operations are particularly difficult to forecast for multipurpose projects and model assumptions must be made regarding abstraction for hydropower, the volume of storage dedicated to flood protection and/or irrigation. 2 ASSUMPTIONS 2.1 Model description The calculation is based on the simplification of having an equal tariff throughout the year. A simulation of and reservoir operation by maximising the profit should have been carried out, but since the future market situation in the different countries is unknown, the criteria of maximising the energy production has been used instead. Another simplification is the disregard of storage allocated for flood control. It is expressed in certain references that the regulation of the reservoirs will take into account flood storage. To what extent this is planned is not known and the practical operation of this obligations are even more uncertain. Another simplification is ignoring the impact of existing power plants on the historical record used for Mekong ( ). In this period some large reservoirs have been established in Lao PDR (Nam Ngum) and Thailand. Inflow and outflow records from these projects should preferably have been used to generate a natural flow series in Mekong. Due to lack of data and since only the second half of the record is influenced, the impact has been ignored. A

4 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 2 simulation could have been carried out to generate the impact, but actual filling procedures, minimum discharges, irrigation demand (Thailand), operation pattern etc. are not known. Development stages (e.g. Nam Ngum: 1972: 30 MW, 1979: 110 MW and 1985: 150 MW) make it even more difficult. Two different scenarios are considered, one presenting the status in 2010 and one presenting the status in The existing stations (with storage) are included in both scenarios. The operation of the reservoirs may be represented by either a rule curve or by a system approach where water values are used. Because the future transmission system and value of energy (or irrigation or flood control) is unpredictable, a rule curve approach is sufficient at this stage. By using rule curves the reservoir operation tend to be similar for all years. In a larger system a higher flexibility in the reservoir operation is necessary, which is best achieved by using a system approach. In the present model there is only one rule curve: if the reservoir level is below this curve the power plant will produce less and when the reservoir level is above the curve the plant output is increased. The determination of the rule curve is carried out by iteration trying to find the highest income or energy production of the power plant(s). If there are several reservoirs or hydropower plants in the basin the total energy production in the basin is maximised according to the same rule curve. Operation of the reservoir in the model is not strictly following the rule curve. If there is a high inflow and the turbines are running at full capacity and the water level in the reservoir is above the rule curve, the excess water will be stored. Spilling will not take place until full supply level is reached. The reservoir operation has not been optimised by trying to find the potential highest firm power and/or firm energy. A firm power demand will determine whether the inflow is used for storage or power production. A high firm power demand will result in high reservoir levels in order to secure the power obligations, but the risk of spillage will also be high and hence the total power production may drop. Firm power and firm energy is used to characterise the quality of the supplied power and energy. In a system dominated by thermal power, the main problem in planning further development is to ensure that total plant capacity is sufficient to cover the maximum peak load that will occur. The term firm power is used to characterise this capacity. There are many uncertainties in the simulations. Although there is a gap in the energy balance at present, the grid or market might be a limitation in the future production since the future regional demand and location of transmission lines are unknown. To meet this situation or to meet financial requirements, a project might be developed in stages (as Nam Ngum) and thereby the reservoir is not utilised. 2.2 Technical assumptions The technical and energy production data referred to in Chapter 3 are taken from [4]. The following general assumptions have been made: Inflow records has been generated by using some few gauge records of long measurements period; Chiang Saen, Nam Ngum, Ban Signo and Se

5 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 3 Kong- Se San. These records have been scaled by the average inflow for each project. Reservoir capacity curves provide the relationship between elevation, storage and surface area. This data is required to calculate power production, to determine reservoir operation and evaporation. Where these data have not been available, simplified reservoir curves have been estimated based on active storage, total storage capacities and dam heights. Optimisation of reservoirs by finding the rule curves giving the highest energy production. Usually the operation is optimised by finding the highest income, but the future tariff structure is uncertain, the market is uncertain (domestic/export) and the project features might change due to upstream development. Determining the real daily reservoir operation would have to be based on expected energy prices, expected inflow and environmental requirements. Large reservoirs will have water loss due to evaporation. However, it can be argued that difference between the present evaporation and evapotranspiration pre-construction, is small and neglectable. Evaporation from reservoirs is usually included in reservoir simulations. For the projects in the actual area the evaporation could have been included by mm/year. In e.g. in Yunnan the impact on total flow would be less than 0.1 %. The reason is the topography of the reservoir area consisting of steep hillsides implying small reservoir areas in relation to total storage (large regulation zone). In any case the following assumptions have been made: If the pre-inundated area originally consisted of evergreen forest or slash and burn areas, the evaporation would have been almost equal to the evaporation after the construction of the dam. In addition the reservoir surface shrinks in the dry season and bare sand banks are exposed giving almost no evaporation. In many projects detailed data have not been available and the following technical assumptions have been made to carry out the energy calculation: Losses in waterways: At full capacity the total head losses in the waterways are calculated based on rated head, maximum discharge and rated capacity. At full capacity a total efficiency factor of has been used, containing an efficiency of for the turbines (old-new and Pelton- Francis), 0.97 for the generators and for the transformers. Because every project of interest has a reservoir the turbines are operated at optimum discharge or at maximum discharge. The power plant discharge depend on the head as follow: Q = Q max (h/h rated ) 0.5 The energy losses in waterways and efficiency of turbine, generator and transformer depend on the operation mode. Running the turbines at full capacity during peak hours and partly in the off-peak period gives a higher loss than running the power plant at same load throughout the day. In the simulation no peak power operation is assumed, i.e. same load throughout the day. A minimum flow is only included in the Nam Theun projects (only data available).

6 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 4 Gravity constant (g) of 9.81 m/sec 2 is used (actually dependent on latitude and elevation). The total of forced and planned outages are assumed to be 1% in general. The outages are caused by: - Maintenance of electrical and mechanical equipment. - Maintenance of tunnels and shafts. Cleaning of sand-trap, inspections of tunnels, painting of steel lined shaft and penstocks. - Flood or high sediment concentrations, intake area flushing. - Unpredictable outages: E.g. strikes, transmission line break-down, earthquakes, landslides giving very high sediment load in the water, etc. The transmission line losses between the power plant switchyard and the load centre are not included. A 8-10% loss may be used for rough estimates for transmission and distribution, but shorter lines, refurbishment and new transmission lines will change this figure. The reference point for the energy production calculation is therefore the power station switchyard. The energy production has been calculated based on the following equation: (100%-outages) x Efficiency x g x Power plant discharge x Net head x 24 hours x No. of days per month [kwh/month] The data on simulated energy differs from the project specific data because upstream development is not foreseen in the specific projects studies.

7 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 5 3 RESERVOIR AND HYDROPOWER DATA 3.1 Reservoirs in Lancang, Yunnan Based on data from Sustainable development of Lancang Mekong by He Damming [1], the energy production and resulting outflow of 8 of the largest hydropower plants in Lancang (Upper Mekong), Yunnan, have been simulated. The location of the projects are presented in a map in Figure 3-1. Because the average flow is known but not the flow variation, the inflow has been estimated by using the monthly inflow data from the gauge station just downstream at Chiang Saen and Luang Prabang in the period and , respectively. The total energy production based on He Damming is GWh/year, whereas the calculated energy production based on the record from Chiang Saen and Luang Prabang is and GWh/year, respectively. The calculation is based on the data presented in Table 3-1. Table 3-1 Hydropower projects in Lancang, Yunnan (projects in bold are included in the 2010-scenario). Reference No. In Map Power plant Manwan DachaoshaXiaowan Gonguoqia Jinghong Nuozhadu Mengsong Ganlanba SUM Completed year before Average inflow m 3 /sec Total storage Mm Active storage Mm Full Supply Level m a.s.l Min. Operation Level m a.s.l Surface at FSL km Net Head 1) m Dam height m Plant capacity 1) MW No. of turbines Energy GWh Sim. energy 2), Chiang Sae GWh Sim. energy 3), Luang Prab GWh Resettlement persons Reservoir-% 0.7 % 0.9 % 25.7 % 0.4 % 0.4 % 22.3 % 0.0 % 0.0 % Reservoir-% ref. Mengsong 0.4 % 0.6 % 15.5 % 0.2 % 0.4 % 19.3 % 0.0 % 0.0 % 36.4 % 1) Efficiency curve is estimated based on expected normal values. Based on Net head and general layout the head losses and tailwater level is generated thereafter. 2) Inflow record is based on monthly flow at Chiang Saen, All u/s hydropower are developped. 3) Inflow record is based on monthly flow at Luang Prabang gauge station, All u/s hydropower are developped. Ref. Water Resources and Hydropower - Policies and Strategies for the Sustainable Development of the Lancang River Basin, by Prof He Daming and Dr David Plinston, Sep-99. Since two of the projects already exist (Manwan and Dachaoshan) and construction of third project has started, three projects are included in the scenario whereas the other five projects are scheduled between 2010 and The present impact of existing regulations in Yunnan is small. The storage of the existing projects is only 1% of the total annual inflow (ref. Mengsong), but the construction of Xiaowan HPP would increase the regulation to 17% in Two of the future projects, the Nuozhadu HPP and Xiaowan HPP, would be able to store (active) almost 35 % of the total inflow (ref. Mengsong).

8 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 6 Figure 3-1. Location of large existing and planned reservoirs for hydropower and irrigation.

9 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 7 Dr Adamson has analysed the impact of hydropower development in Yunnan on the Lower Mekong [3]. He refer to the impact of 10% and 20% regulation. However, the scenario would provide a regulation of 17%, whereas the scenario would imply a regulation of 36 %. In addition, the paper do only present the monthly mean flow and do not present the variation in monthly flows. Figure 3-2 shows the water flow downstream Mengsong before (Natural Inflow mean average) and after the scheduled hydropower development in the year 2025 (50% percentile approx. mean average) Discharge (m 3 /sec) Natural Inflow 100 % 90 % 75 % 50 % 25 % 10 % 0 % Month Figure 3-2. Monthly river flow at border to Lao PDR before (natural) and after construction (in percentiles) of planned hydropower projects in Yunnan. In the 2025-scenario there will be insignificant difference between dry season and wet season flow. In fact, in some years the low flow may occur in July to October if the value of energy is higher during winter. The trend in the 2010-scenario will be the same, although the period with highest flow in Lancang will still be during the normal wet season. Irrigation According to [2] the agriculture cultivation in Yunnan is mainly carried out in small valley areas along the middle and lower stretch of Lancang. The total of the major irrigation area is about ha (1500 km 2 ) in the lower mainstream. Based on [1] there is ha farmland in Yunnan within the Lancang basin among which paddy field account for 39%. From the reservoir at Manwan HPP the amount of water has capacity for irrigating ha. By assuming an average consumption of l/s per ha, the total consumption may be estimated. However, much water is returning either as surface run-off/drainage or fed to the groundwater. By assuming an annual evaporation of 1000 mm/year from paddy fields the water loss is 0.3 l/s per ha. However, there was evapotranspiration from

10 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 8 the area before the paddies were established. This might have been the same rate (depending on type of original vegetation cover). Assuming dry season irrigation of paddy the water loss in existing paddies would represent 0.39 x x 0.3 /1000 m 3 /sec = 64 m 3 /sec. Planned irrigation of ha by diverting water from Manwan would in the same way imply a water loss of about 48 m 3 /sec. This number is, however, not included in the simulation because the natural/original evapotranspiration in unknown. 3.2 Reservoirs in Lao PDR In Lao PDR there are a large number potential hydropower sites on Mekong tributaries. Based on EdL power development plan 20 projects have been selected [4]. The location of the projects are presented in Figure 3.1. Because the average flow is known but not the variation in inflow the variation has been estimated by using the monthly inflow data from the Inflow Nam Ngum -record and the flow at Ban Signo gauge station (Nam Theun 2). The Nam Ngum record has been extrapolated to be used in the period In Se Kong a hydrological record has been generated by using the monthly inflow data from the Ban Signo gauge station and correlating this record with the monthly distribution in Se Kong given in [6]. Based on trend analysis, see Annex 2, all inflow records in Lao PDR have been reduced by 8%. In spite of this, the simulated basin-wise sum of energy becomes slightly higher that the energy production figures given in the project documents. (Nam Ngum and Nam Theun). The main reason is probably that reservoirs have secondary benefits for all downstream projects. The calculations of hydrological inflow to Mekong is based on the data in Table 3-2, Table 3-3 and Table 3-4 referring to Nam Ngum basin, Nam Theun basin and Se Kong basin, respectively. Table 3-2. Storage hydropower projects in Nam Ngum, Lao PDR (projects in bold are included in the 2010-scenario). Reference No. In Map Power plant N Ngum N Leuk N Song N Lik N Ngum 3EN Ngum 2BN Ngum 5 N Ngum 4AN Bak 2B SUM Completed year Average inflow m 3 /sec Total storage Mm Active storage Mm Full Supply Level m a.s.l Min. Operation Level m a.s.l Surface at FSL km Net Head 1) m Dam height m Plant capacity MW No. of turbines Energy GWh Sim. energy 2), GWh Resettlement persons 2264 > Reservoir-% 48.9 % 34.8 % 29 % 29.4 % 2.5 % 34.7 % 39.6 % 25.2 % 79.4 % Ref. Power Sector Strategy Study, by Electrowatt-Ekono and PA Consulting Group, Sep.-02. Ref. Nam Ngum 1 Hydropower Station Extension, by Lahmeyer and Worley, Aug ) Efficiency curve is estimated based on expected normal values. Based on Net head and general layout the head losses and tailwater level is generated thereafter. 2) Given inflow. Inflow record is based on monthly flow at Nam Ngum 1, , correlated by Ban Signo and later reduced by 8%. 3) EdL is reffering the alternative Nam Ngum 3B (690 MW). In the reference list data [5] NN3 and NN3E is available, but the active storage is the same. NN3E (588 MW) is used.

11 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 9 Table 3-3 Storage hydropower projects in Nam Theun, Lao PDR (projects in bold are included in the 2010-scenario). Reference No. In Map Power plant N Theun 2 TH. Ext SUM Completed year Average inflow 2) m 3 /sec Total storage Mm Active storage Mm Full Supply Level m a.s.l Min. Operation Level m a.s.l Surface at FSL km Net Head 1) m 341 Dam height m Plant capacity MW No. of turbines 6 1 Energy GWh Sim. energy 2) GWh Resettlement persons Reservoir-% 47 % 83 % Ref. Power Sector Strategy Study, by Electrowatt-Ekono and PA Consulting Group, Sep.-02. Ref. NT2. Environmental Assessment & Managment Plan, October ) Efficiency curve is estimated based on expected normal values etc. 2) Given inflow. The inflow record is based on monthly flow at Ban Signo, ,1and later re Table 3-4 Storage hydropower projects in Se Kong, Lao PDR (projects in bold are included in the 2010-scenario). Reference No. In Map Power plant Huoay Ho Xepon X Kaman 3 X Kaman 1 X Kong 5 N Kong 3 Xe Xou SUM Completed year Average inflow m 3 /sec Total storage Mm Active storage Mm Full Supply Level m a.s.l Min. Operation Level m a.s.l Surface at FSL km Net Head 1) m Dam height m Plant capacity MW No. of turbines Energy GWh Sim. energy 2) GWh Resettlement persons 1500 > Reservoir-% 67 % 10 % 66 % 81 % 28 % 70 % Ref. Power Sector Strategy Study, by Electrowatt-Ekono and PA Consulting Group, Sep.-02. X Kaman 3: Vietnamese study in ) Efficiency curve is estimated based on expected normal values. Based on Net head and general layout the head losses and tailwater level is generated thereafter. 2) Given inflow. Inflow record is based on monthly flow at Se Kong outlet and later reduced by 8%. 3.3 Reservoirs in Thailand Nine projects with reservoirs of significant storage exist in Thailand, see Table 3-5. Six of these projects are combined multipurpose projects, used for irrigation, hydropower and water supply. No plans exist for new large reservoirs projects) for the next 20 years. Hence, the present situation is expected to be the same as in 2010 and 2025.

12 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 10 Table 3-5. Existing reservoirs in Thailand. Reference No. In Map Power plant Nam Pung Ubol Ratana Sirindhorn Huai Kum Chulabhor Lam Phra Nam Pao Huai Luang Nam Oon Lam Takhon SUM Completed year Average inflow m 3/ sec Total storage Mm / 10 Active storage Mm Full Supply Level m a.s.l. Min. Operation Level m a.s.l. Surface at FSL km Net Head 1) m Dam height m Plant capacity 1) MW No. of turbines Energy GWh (?) 302 Sim. energy 2 ) GWh Irrigation Hectare Ref. EGAT webside and JICA/MRC- Paper IV [9] Pump storage 3.4 Reservoirs in Vietnam In Vietnam there are two existing and many planned power stations on Se San, see Table 3-6. The hydropower projects have been selected based on the preliminary National Hydropower Plan [10]. There exist only two hydropower power station in the basin within Vietnam, the large Yali HPP and Dray Linh. In addition there are two projects under construction; Se San 3 and Se San 3A, However, only Yali has an active storage above 100 mill.m 3. The other project is not included in the analyses. In addition, there exist several small reservoirs for irrigation purpose which in total has a capacity of approx. 100 mill.m 3. Because the average flow is known but not the variation in inflow the variation has been estimated by using the monthly inflow data from the Ban Signo gauge station, but each month is correlated by monthly distribution in Se San given in [6]. Table 3-6 Storage Hydropower projects in Se San and Sre Pok, Vietnam (projects in bold are included in the 2010-scenario). PS! The sum includes the other smaller projects (active storage less than 100 mill.m 3 ) Se San: Sre Pok: Reference No. In Map Power plant Yali U. KontumPleikrong Se San 4 D. Xuyen B. Tou Srah SUM Completed year 1999 Average inflow m 3/ sec Total storage Mm Active storage Mm Full Supply Level m a.s.l Min. Operation Level m a.s.l Surface at FSL km Net Head 1) m Dam height m /72 83 Plant capacity 1) MW No. of turbines Energy GWh Sim. energy 2) GWh Resettlement persons Reservoir-% 26 % 26 % 5 % 39 % 15 % Ref. Power Sector Strategy Study, by Electrowatt-Ekono and PA Consulting Group, Sep.-02. Ref. National Master Plan, 2004 and Se Kong - Se San and Nam Theun River Basins Hydropower Study, by Halcrow, Jan ) Efficiency curve is estimated based on expected normal values. Based on Net head and general layout the head losses and tailwater level is generated 2) Given inflow. Inflow record is based on monthly flow at Se Kong outlet and later reduced by 8%.

13 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page Reservoirs in Cambodia In Cambodia there are at present no existing large reservoirs. The construction of Prek Thnot multipurpose dam has been interrupted since 1973 due to the political unrest in the country as well as the financial situation. Kirirom I (12 MW) is the only hydropower plant in operation at present, but the station is a run-of river project. On Se San and Sre Pok there are several plans, see Table 3-7. Because the average flow is known but not the variation in inflow the variation has been estimated by using the monthly inflow data from the Ban Signo gauge station, but each month is correlated by monthy distribution in Se San given in [6]. Table 3-7 Storage Hydropower projects in Se San and Sre Pok, Cambodia. (no projects are included in the 2010-scenario). Reference No. In Map Power plant L. Se San L L. Se San U L. Sre Pok SUM Completed year Average inflow m 3/ sec Total storage Mm 3 Active storage Mm 3 Full Supply Level m a.s.l Min. Operation Level m a.s.l. Surface at FSL km Net Head 1) m Dam height m Plant capacity 1) MW No. of turbines Energy GWh Sim. energy 2) GWh Resettlement persons Reservoir-% Ref. Report analysis of Sub-Area 7V, BDP, July 2003 "L"=Lower, "U"=Upper 3.6 Summary of reservoirs in Mekong The sum of active storage in each country and in total for all countries are presented in Table 3-8. Table 3-8. Active storage in the Mekong basin. China Laos Thailand Cambodia Vietnam SUM NT2-portion Present N/A N/A % N/A % JICA [9] has summarised the present total (active?) capacity of large scale reservoirs in the entire Mekong Basin to 12,147 mill.m 3. Average annual flow volume of the entire Mekong basin is 475,000 mill.m 3. The active storage created by other water users than hydropower is small. An exception is the reservoirs in Thailand of which 2000 mill.m 3 is storage made with the only purpose of irrigating paddies. The remaining 3500 mill.m 3 are used both for hydropower and irrigation. The extractions of surface water by pumping stations are comparable small. In sum, only 18 m 3 /sec and 1.5 m 3 /sec are withdrawn from Mekong in Thailand and Lao PDR, respectively. [9].

14 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 12 4 FLOODING The floods in upper Mekong and tributaries to Mekong are normally created by rain storms lasting for some hours or a few days at most. They are difficult to predict, but only a small reduction of water level in the reservoirs could halt the flood. Since most available inflow records are monthly data a flood event of only a few days could not be simulated for the whole basin. Normally the floods are reduced by dam structures. There are two reasons for this: 1. The reservoir is not full and flood events during in particular the first period of the wet season will be trapped by the reservoir. 2. The reservoir is dampening the flood because of surcharging between full supply level and maximum flood level. The dams might however increase the flood in rivers further downstream if the flood from the area normally passes before the peak flood occurs in the main river. This might be the case for Nam Theun and Mekong. Although the size of the flood from the tributary will be reduced due to dam construction, the flood might be prolonged, the peak could be delayed and come on top of an existing flood in the main river. By comparing the daily record for Nam Theun (1998-2) with the record from Mekong just upstream Tonle Sap ( ) the annual maximum discharge in Nam Theun might as well occur before the maximum level in Mekong as after the maximum level. The floods in Nam Theun may be characterises as typical flush floods compared to the slowly changes in Mekong, see flood occurring in Sept. 2 (approx. 2.0 m 3 /s km 2 ) plotted in Figure 4-1. Extreme floods incidents recently in other tributaries to Mekong was Se San 1.2 m 3 /s km 2 on 3 Nov and Sre Pok (Krong Kno) 1.3 m 3 /s km 2 in 2000 [11]. The extreme flood in Nam Theun had an impact on Mekong, but since the base flow in Mekong was on the way down a delay in the flood (caused by NT2) would also reduce the flood in Mekong. The peak flood in Nam Theun may occur both before and after the peak flood in Mekong. Hence, if the flow in Mekong is increasing, a delayed flood from NT2 might increase the flood in Mekong. If the flow in Mekong is decreasing the NT2 will reduce the flow (compared to the natural situation)

15 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 13 Figure 4-1. Daily discharges for Mekong just upstream Tonle Sap and the discharge in Nam Theun, during a high flood in Nam Theun Discharge (m 3 /sec) Mekong at Kompong Cham Flood in Nam Theun

16 Cumulative Impact Analysis Annex 3: Future Hydropower Development Page 14 REFERENCES: [1] Sustainable development of Lancang Mekong river basin and integrated multiobjective utilization research of water resources by He Damming, Yunnan Institute of Geography and Zhang Jiazhan, Beijing Institute of Geography. [2] Water Resources Simulation Model of the Lancang River (in Yunnan Portion of China) by Liu Hong, Nanjing Institute of Hydrology and Water Resources. [3] The potential impacts of hydropower developments in Yunnan on the hydrology of the Lower Mekong by Dr P.T. Adamson, Halcrow Water. [4] Power System Development Plan for Lao PDR by Meritec Ltd and Lahmeyer GmbH, March [5] Power Sector Strategy Study, by Electrowatt-Ekono and PA Consulting Group, September 2. [7] Power system Planning in the Ministry of Hydropower and Handicraft by Knight Piesold, Draft Final Report, September [8] Se Kong Se San and Nam Theun River Basins Hydropower Study, Interim Report, Volume 1, by Halcrow, January [9] The Study on Hydro-meteorological monitoring for water quantity rules in Mekong river basin, Paper IV Preliminary check of changes in low flow regime on Mekong mainstream and Major Tributaries, by JICA and MRC, February 2. [10] National Hydropower Plan Study, Stage 2, phase 1, by SWECO, NORPLAN et al, April [11] Analysis of the Sub-area 7V. Basin Development Plan. By Institute of Water Resources Planning and Vietnam National Mekong Committee, Hanoi, July 2003.