This paper discusses the common environmental issues and requirements project lenders have when financing hydropower dam projects in developing countries. The environmental specialist’s role, as part of the Lender’s Technical Advisor team, is discussed throughout the main phases of project finance (credit approval, financial close, lending/construction and loan repayment/operation). Further, how environmental issues are reviewed and monitored, thereby minimising reputational risks to the lenders are outlined.
Lenders typically consider hydropower dam financing, especially reservoir schemes, as high reputational risk loans. Finance is usually syndicated and although most international lenders are Equator Principles signatories or use the International Financing Corporations (IFC) Performance Standards, some lenders have additional environmental guidelines and requirements to enable financing. These differences are discussed.
Common environmental concerns include loss of habitat of endangered and/or threatened species, changes to river flows, erosion and sediment control during construction, and the minimisation and disposal of project wastes.
These issues are discussed drawing on the author’s experience in monitoring environmental issues of hydropower projects in Asia Pacific and Africa, including both smaller run-of-river schemes and larger storage reservoir projects.
Keywords: Environment, impacts, project financing, concerns, lenders, lenders technical advisor.
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Richard R. Davidson, P.E., CPEng Kenneth B. Hansen, P.E.
Early in the twentieth century, placing concrete core walls within embankment dams was a popular construction technique for small to medium height dams. It became in vogue as a replacement for the popular British dam construction technology of puddle clay core dams which were used between the 1860’s and 1920’s. It avoided the many problems with semi-hydraulic / manned placement methods of the puddle clay cores within narrow trenches. However, after the mid 1930’s this concrete core wall construction fell out of favour because of the improvements made in embankment compaction methods and the difficulties in building reinforced concrete core walls to more significant heights.
Today concrete core wall embankment dams are now reaching an age where their continued performance is being questioned. This dam building technology has become extinct and is unknown to the last few generations of dam engineers. Therefore, it is relevant to re-examine this dam building technology in a modern context and work on answering the following questions. How have these dams performed after almost a century of service? Are there unanticipated performance features that have produced positive results when subjected to extreme flood and seismic events? Does the concrete provide enhanced performance over time? What role does steel reinforcement play in the performance of the core wall? Are there lessons here that can be applied to the more common concrete cutoff wall solutions being applied to embankment dams with seepage problems? This paper examines these questions with a number of illustrative case histories to provide a retrospective illumination of this forgotten dam building technology.
Keywords: Embankment dams, Concrete core walls, Dam construction history.
Kim Robinson, Andrew Pattle and Thomas Shurvell
Rowallan Dam is a 43m high clay core rock fill dam located in Northern Tasmania. The dam impounds 121GL used for hydro power generation and has a High A consequence category.
Over the summer of 2014/15 major reconstruction works were carried out on the dam to repair a piping incident from 1968. The work entailed reconstructing two sections of the dam down to foundation level and the upper 7m of the 568m dam crest. During the work, the dam was temporarily exposed to a significantly increased flood overtopping risk.
A range of measures were taken to manage the overtopping risk; such as increasing the dewatering capacity of the dam, lake draw down, installation of a sheetpile wall, development of emergency backfill procedures and a flood forecasting system.
The focus of this paper is on the flood forecasting system and how this was integrated into the overall management of overtopping risk during construction. The forecast models were run automatically on a 2 hour schedule using the latest BoM forecast, telemetered lake levels and rainfall from 7 gauges surrounding the catchment. The system provided a continuous 7 day lake level forecast which guided the site team on when to release water to manage the storage.
In the event that the lake level forecast reached a predetermined trigger level, the dam safety team would have been automatically notified and various emergency procedures would have been triggered in response to the flood warning.
This paper discusses the measures that were taken to manage the flood risk, how it worked in practice and conclusions which are applicable more generally to managing overtopping risk during dam works.
Keywords: dam construction flood risk, flood forecasting
Monique Eggenhuizen, Eric Lesleighter, Gamini Adikari
St Georges Dam is located on Creswick Creek approximately 2km southeast of the township of Creswick and 135km northwest of Melbourne. The reservoir, located within the Creswick Regional Park and originally constructed to supply water for the Creswick quartz crushing plant in the 1890s, has since been established as a popular recreational storage and is the responsibility of Parks Victoria. The dam is approximately 16m high and located across a relatively steep gully. The embankment consists of earthfill with an upstream face of rock beaching and a grass covered downstream face. The primary and secondary spillways are cut into the right and left abutments respectively.
At the completion of a detailed design review, St Georges Dam was assessed to be within the top three of Parks Victoria’s dams portfolio in regards to Public Safety Risks. The detailed design review assessed that the risk position for the dam plotted within the unacceptable region of the ANCOLD Guidelines for the static, earthquake and flood failure modes. As such, upgrade measures were considered to be required. In 2010 and 2011, a number of significant flood events emphasised the importance of upgrade works at this dam, particularly in regards to upgrading the spillway capacity, and consequently Parks Victoria assigned these works a high priority.
SMEC was engaged to design the upgrade works for the dam. A number of arrangements to increase the spillway capacity of the dam were considered, with the most cost effective option being assessed to be a secondary spillway over the dam embankment in the form of a rock chute.
This paper describes the decision making process associated with the option selection and the methodology for designing the overbank spillway which utilised the findings in ‘Riprap Design for Overtopping Flows (Abt & Johnson, 1991), and US Army Corps of Engineers, Waterways Experiment Station, publications of standard riprap gradations and computer program CHANLPRO.
Keywords: Embankment Dams, Spillway, Rock Chute, Erosion Protection
Maz Mahzari and Chi-Fai Wan
Upgrading of an existing dam often faces challenges in both static and seismic safety assessment. The use of new hydrological and seismological data and improved design methods often mean more severe loading which outdates the original design and demands expensive upgrade works. Establishing the design criteria for checking the structural adequacy of an existing dam for multiple unusual load events occurring within a relatively short time frame presents another challenge.
A probabilistic approach is presented to rigorously address the effects of multiple load events while maintaining a consistent risk of failure for the structure. This is based on a probabilistic conditional combination where probability of each event is defined and used to develop a joint probability distribution. For instance if an earthquake occurs following a severe flood, the seismic hazard curve of the site can be used to adjust the seismic loading with shorter average recurrence interval to be used in conjunction with the pre-earthquake flood when assessing the structural adequacy of the dam. With this method of adjustment, the design can benefit from the choice of a reduced seismic design loading and hence a more cost effective design solution.
The proposed method is straightforward and can be effectively used in most engineering practices, including the design of hydraulic structures such as dams.
Keywords: Dams, Seismic Hazard, Post-earthquake, Risk analysis
Maree Dalakis, Dr Saman de Silva, Siraj Perera and Dr Gamini Adikari
This paper describes the results of a statistical and qualitative analysis on historical dam safety incidents in Victoria, the first study of its kind conducted in the State. The study investigates trends arising from qualitative dam safety incident data collected by the Department of Environment, Land, Water and Planning since the year 1996. The reported incidents are categorised based on their severity and statistical trends are identified in relation to the types of incidents common to regulated and unregulated dams, as well as common responses to incidents, including their post-incident operation. The geographical distribution of incidents across the State is also analysed to determine the effects of seismicity on dam safety incident rates. Furthermore, the unique Victorian conditions of sustained drought and subsequent flooding and their impact on incident rates are investigated through the combined analysis of geographical incident distribution and streamflow data. The incident data is further assessed according to the frequency of visual inspection and reporting of the structures in order to gauge the relative influence of these practices, and dam regulation in general, on mitigating incident risk in dams. An understanding of dam safety incident trends and the impact of inspection and reporting practices is increasingly important given the increasing expectation for dam owners to properly operate and maintain their assets with minimal resources and finances.
Keywords: dam, safety, incident, historical, failure.