Safety of Small/Rural Dams and Barrier Lake Management

Transcription

1 Safety of Small/Rural Dams and Barrier Lake Management A. Introduction 1. Lessons learnt and recommendations are provided for artificial dams and barrier lakes. In the case of artificial dams, emphasis has been placed on small, rural dams, rather than on larger, well-engineered dams because smaller dams represent a higher hazard in China 1. That higher risk from smaller dams is also confirmed by international experience. Barrier lakes and their management are highlighted as they are formed as a specific outcome of earthquakes. 2. Key issues and lessons learnt from international experience are presented with reference to three case studies of relevant World Bank supported projects: Gujarat s 2001 Earthquake- Dam safety aspects; Lake Sarez Risk Mitigation Project; Armenia Dam Safety Program. 3. Brief descriptions of these projects are provided in boxes at the end of the note. Further information is available at NDRC s request. B. Key Issues and Lessons Learnt Artificial Dams 4. After the impact of an earthquake, priority should be placed on emergency measures to counteract immediate risk to human life. At the same time, dam rehabilitation and modernization measures should be planned and later implemented. 5. In the case of Gujarat s 2001 earthquake, (see Annex 1) emergency measures were carried out in less than 5 months, before the beginning of monsoon rains. Rehabilitation and modernization measures took 8 years. 6. Cumulative probability of failure of many small rural dams is much higher than that of a few larger reservoirs (see Annex 1). This is valid under seismic and flood conditions. 7. Defensive measures to enhance earth/rock dam resilience to earthquakes (H.B. Seed, 1979) are listed in the following box. 1 Large dam: reservoir capacity > or = 100m cubic meters Medium dam: 100m cubic meters > reservoir capacity >or= 10m cubic meters Small dam: reservoir capacity < 10m cubic meters World Bank Good Practice Notes 1 Dams and Barrier Lakes

2 1) Ample freeboard to allow for settlement, slumping, or fault movements. 2) Use wide transition zones of material not vulnerable to cracking. 3) Use chimney drains near the central portion of the embankment. 4) Provide for ample drainage zones to allow for possible flow of water through cracks. 5) Use wide core zones of plastic materials not vulnerable to cracking. 6) Use well-graded filter zone upstream of the core to serve as crack-stopper. 7) Provide crest details that will prevent erosion in the event of overtopping. 8) Flare the embankment core at abutment contacts. 9) Locate the core to minimize the degree of saturation of materials. 10) Stabilize zones around reservoir rim to prevent slides into the reservoir. 11) Provide special details if there is danger of fault movement in the foundation. Barrier Lakes 8. Barrier lakes can be formed by diverse mechanisms such as glacier movements, moraine barriers, landslides, avalanches, volcanic calderas, etc. 9. Barrier lakes normally last days or weeks;, their life span depends mainly on: water and sediment flow size and shape of the barrier materials forming the barrier rate of seepage through the barrier. 10. Only a few barrier lakes survive for months or years, very few reach a long lasting equilibrium (see Annex 2). 11. After the occurrence of a barrier lake, the immediate priority is to facilitate river flow by removing, at least partly, the barrier. 12. In case of large barriers, depending on composition and on local climate, the hazard may last for longer, therefore long term solutions need to be studied and implemented. 13. Satellite imagery and remote sensing techniques allow detection of recently formed lakes by comparison of before and after pictures. In addition it also allows for the identification of landslide features such as escarpments, typical forms of landslide bodies and failure mechanisms (rockslide, earth slide, debris flows, etc.). 14. Basic measurements of the lake area and mean depth permit the use of empirical relationships to estimate lake volume peak discharge in case of barrier failure (e.g. Costa and Schuster, 1988). This information is of vital importance for a rapid assessment of the downstream areas that could be potentially flooded. World Bank Good Practice Notes 2 Dams and Barrier Lakes

3 15. When access becomes possible, ground checks by expert teams are absolutely necessary. Site visits allow verification of the landslide dam geometry/profile, material types and likely internal structure. Seepage points should also be mapped and flows documented. An assessment of landslide type and mode of emplacement is an essential part of the observations, including the type and nature of rock mass in the source zone, travel profile (distance & height), disaggregating with travel etc. 16. For long term monitoring of the highest hazards, essential elements are lake level (for both rise and fall in the event of failure) flow gauging stations downstream rainfall station 17. Time lapse camera(s) looking upstream at the dam (generally with a few minute intervals) may be required in some cases. C. Recommendations Artificial Dams 18. More than 2,300 dams were affected by the Wenchuan Earthquake, of these only 30 dams are large or medium size dams. The small dams affected by the earthquake were built 30 or 40, even 50 years ago without proper design. The conditions of these dams are not sound. Even worse, most of these dams have no proper operation, maintenance and surveillance (OMS). It is recommended that the owners of these dams work out an OMS manual for their dams to strengthen dam safety management. 19. Several international guidelines exist on post-earthquake safety inspections of dams, but they are by necessity quite generalized because each dam owner needs to set up safety inspection procedures which are case-specific. These procedures should be specified in the standing operation procedures for each dam, and in particular in the Emergency Preparedness Plans (EPP). Most of the small dams in China have no EPPs at present. It is recommended that the Chinese Government issue guidelines on EPP for small dams with reference to international experiences. 20. A uniform and consolidated risk assessment of all dams within the earthquake affected area needs to be completed/ compiled. 21. The risk assessments should guide the prioritization of rehabilitation and modernization measures, with the objective to reduce the risk profile of the entire group of dams (see Annexes 1 & 3). The Bank, together with Chinese Government, has been conducting Portfolio Risk Assessment (PRA) using quantitative risk assessment techniques (Risk-Based Profile System (RBPS)) for a number of dams to identify the risks and remedial measures prioritized based on risk indexes. World Bank Good Practice Notes 3 Dams and Barrier Lakes

4 22. Risk assessment need not be complex. The level of sophistication should be appropriate to the specific case; semi-quantitative, empirical, methods can be efficient tools for large sets of dams (see Annex 3). 23. In terms of preventive actions, good practice includes risk assessment which should be done for all high hazard dams. This should be updated on a regular basis. 24. Adequate technical assistance should be provided for design, construction, operation, and modernization of small rural dams. 25. A census of rural dams should be completed and made available in different regions/ counties; water level in the reservoir is the priority information to be monitored. Barrier Lakes 26. Barrier lake occurrences should be monitored in those areas where their likelihood is higher. Satellite imagery and remote sensing techniques are essential tools for managing these hazards. 27. After successful implementation of emergency measures, the barrier lakes that are expected to last for some time require monitoring and, as appropriate, early warning systems. In some cases structural measures to reduce/ remove risk of failure are required, these may represent opportunities for delivering social services such as water supply, recreation, hydropower, irrigation, etc. (see Annex 2). 28. Awareness and training of the population at risk is essential during the life of those barrier lakes which are expected to remain in place for long periods of time (see box 2). 29. In special cases, risk assessment can be a powerful tool for design and operation of monitoring and early warning systems (see Annex 2). 30. Long living barrier lakes should have in place emergency preparedness plans such as those used for man-made reservoirs (see Annex 2). 31. Given the safety conditions and risk level of Tangjiashan Barrier Lake, long-term monitoring and management are needed. It is recommended that Chinese Government establish gauging station(s) and rainfall stations downstream and upstream of the lake. In addition, a monitoring system and EPP for the lake management should be set up. World Bank Good Practice Notes 4 Dams and Barrier Lakes

5 Annex 1: Gujarat s 2001 Earthquake- Dam safety aspects 32. The Indian Subcontinent is characterized by a very large number of embankment dams for irrigation and rural water supply. Such dams are generally of small size (less than 30m high), and built by organizations with little or no experience in dam engineering. During the Bhuj earthquake of January 26, 2001 about 250 earth-fill dams were damaged. Soil liquefaction and slope failure caused extensive cracking, deformations and settlements. Courtesy Prof. S. K. Jain Courtesy Dr. R. Kishore 33. No major damage was reported in the larger, better engineered dams which were located in the earthquake affected region. Fortunately, the earthquake occurred before the monsoon period, when reservoirs were mostly empty. Had the earthquake happened during or after the monsoons, damage to saturated earth dams would have been much more severe and several reservoirs would have released water causing devastating floods. 34. Lack of design documents for most of the dams made the definition of emergency measures a difficult task. Such measures had to be implemented at 245 dams, from January to June 2001; repair works included opening and filling cracks, slope reinforcement, repair of intake works and spillways. Five dams, which could not be repaired, were breached and removed. Total expenditure in emergency repairs to dams was US$ 4 million. 35. Following the emergency, two teams composed of national and international experts identified the strengthening measures to be implemented at 225 dams. As there was no basic data available, about 12,000 soil samples were tested. Hydrological safety was updated at all dams. Stability analysis was carried out using updated seismic loading conditions. Design and construction was subject to independent review, seminars and workshops took place to strengthen capacity of field staff. 36. The Bhuj earthquake confirmed that small rural dams have a very high probability of failure and that such failures can occur over a very vast area, where several such dams are located storing water of similar volume as a large reservoir. The intuitive World Bank Good Practice Notes 5 Dams and Barrier Lakes

6 assumption that many smaller dams are safer than one or few large dams is incorrect. Besides, due to the fact that most rural dams were built by inexperienced organizations and no design records exist, it is almost impossible to assess their safety. One should therefore conclude that, especially in rural areas, the total earthquake risk of small dams may be significantly higher than that of large dams. World Bank Good Practice Notes 6 Dams and Barrier Lakes

7 Annex 2: Lake Sarez Risk Mitigation Project 37. Lake Sarez in Tajikistan was formed in 1911 when a massive earthquake-triggered landslide buried the village of Usoy under a 600m high obstruction of Murgab River. The resulting 60km long lake containing over 17 km3 of water is located behind the so-called Usoy landslide dam in the Pamir Range. The freeboard (i.e. the margin before overtopping occurs) is approximately 50m. Although the safety of the Lake has been studied over many years, there existed significant gaps and inconsistencies in the available data. At the same time, it was clear that the risk to the downstream population could be unacceptably high. The generated flood could affect up to 5 million people living along the Bartang, Pyandhz, and Amu Darya rivers. 38. In late 2000, the World Bank launched the Lake Sarez Risk Mitigation Project (LSRMP). The project s objective was to help alert and prepare vulnerable people in case of a disaster associated with an outburst flood from Lake Sarez, while seeking long-term solutions to this complex issue. LSRMP components were as follows: Design and installation of a monitoring system and of an early warning system. Social Component that identified safe havens and trained vulnerable communities within a disaster preparedness program. Studies on long term solutions. 39. A risk analysis guided the design and informed the operation of the monitoring system. A team of experts concluded that the highest probability of failure was associated with piping phenomena that could result in a sudden increase of seepage through the dam. The total probability of occurrence of this chain of events reached 6*10-5, which is comparable to that of man-made dams. The following figure shows the level of risk at Sarez together with generally accepted curves for man-made dams as well as for natural hazards. Sarez is located in between. World Bank Good Practice Notes 7 Dams and Barrier Lakes

8 40. Even without structural measures, the probability of uncontrolled release of water from Sarez is very low compared with other natural hazards. With long term structural measures, such as lowering the lake level, probability of failure could be reduced by one or two orders of magnitude and the lake could be used for hydropower generation. In any case, considering the number of people at risk, Usoy dam should have an emergency preparedness plan in the same way as other dams. Such a plan was put in place by the project. World Bank Good Practice Notes 8 Dams and Barrier Lakes

9 Annex 3: Armenia Dam Safety Program 41. The lack of resources allocated for maintenance of dams in Armenia since 1991 resulted in many deficiencies in the dams. In addition to the lack of maintenance, several dams had design deficiencies, such as inadequate spillway capacity and seismic vulnerability. The devastating earthquake that destroyed most of Spitak forced a revision of seismic criteria and stability requirements. This change in standards resulted in dams previously meeting safety standards, being classified as deficient. The need of dam safety improvements was also highlighted by the failures of Agarak Dam in 1974, Marmarik Dam in 1974 and Artik Dam in 1994; the latter resulted in two fatalities. 42. In addition to four dams that were never completed, there are 83 irrigation dams in Armenia. In the 1990s a long-term program started, with World Bank assistance, for the rehabilitation and restructuring of the irrigation sector. As part of that program, all 87 dams were investigated and, where necessary, improved to meet current dam safety standards. During the program, Armenian agencies had access to international experience and expertise in the field of dam safety through regular visits from World Bank and international specialists. 43. Risk assessment made use of a semi-quantitative method in which both the probability and the consequences of an event are ranked. This method, which has been used for assessing the safety of dams in Canada and the UK, is known as failure modes, effects and criticality analysis (FMECA). The following stages are required: i) identification of failure modes; ii) comparative assessment of probability of failure; iii) comparative assessment of consequence or impact of failure. 44. The impact score is the combination of the population at risk and the potential economic loss. The risk index is the product of the total impact score and the risk score. This score allowed ranking all dams and identifying priorities for remedial works. 45. The risk profile of the dams, as measured by the risk index, is presented in the figure below which also shows the reduction in risk that can be achieved by the implementation of Emergency Preparedness Plans (non-structural measure) and remedial works respectively. World Bank Good Practice Notes 9 Dams and Barrier Lakes

10 with EPP and remedial works reduction in risk due to remedial works reduction in risk due to EPP Risk index Landjaghbiur-1 V.Talin-1 Irind Tsilkar Shenik Maralik Agarak V Sasnashen Shgarshik Kaps V Talin - 2 V. Karmir Karaundsh Akhurian Metsavan Spandarian Davidashen Talish Kosh Kechout Tolors Getik Joghaz Aghakchi Brnakot Karagloukh Vosketas Ors Angehakot Ashnak-1 Aghavnadzor N. Talin Gndevaz Ashnak-2 Hatsashen World Bank Good Practice Notes 10 Dams and Barrier Lakes