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COST EFFECTIVE APPROACHES FOR PRESCRIBED DAM ASSET MANAGEMENT

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COST EFFECTIVE APPROACHES FOR PRESCRIBED DAM ASSET MANAGEMENT Haris Pinidiya Sydney Water, NSW, Australia ABSTRACT Dams need to be dewatered in an emergency, typically by operating scour valves. Operating
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COST EFFECTIVE APPROACHES FOR PRESCRIBED DAM ASSET MANAGEMENT Haris Pinidiya Sydney Water, NSW, Australia ABSTRACT Dams need to be dewatered in an emergency, typically by operating scour valves. Operating the scour valve without knowing the conditions will lead to local flooding. Sydney Water manages over a dozen reservoirs within its water network which have been classified as dams under New South Wales Dam Safety Act 1978 Australia and they are located in densely populated areas. As a regulatory requirement, SW is obliged to assess and manage the critical assets associated with SW owned dams, including valves and pipes, to ensure their operability and reliability to dewater the Dams in an emergency. Sydney Water managed to identify and implement some engineering solutions to resolve the issue in a number of Dams. However the same solution could not be adopted at certain dams due to specific site conditions and other issues. Cost involved in most of the other identified engineering solutions, including construction of alternative pipelines and installation of pumps, were extremely high. In the absence of low cost alternatives, Sydney Water has developed cost effective, non-asset approaches with comprehensive emergency dewatering plans to promptly lower the water levels of these dams using systematic flow diversions without need to discharge water through scour mains. Sydney Water successfully trialled and adopted these methods to the satisfaction of the regulatory authority, the Dams Safety Committee, and other stakeholders. INTRODUCTION Australian National Committee on Large Dams (ANCOLD) guidelines and Sydney Water operational plans specify Operational Preparedness Tests (OPT) of the critical valves required to ensure the dam safety, including annual operation of valves under fully opened conditions. ANCOLD critical asset identification criteria rank all scours in the critical category. Therefore the operational preparedness testing of the scour valves becomes a regulatory requirement for dam asset management. Sydney Water performs operational tests in line with ANCOLD guidelines to ensure the operability and reliability of critical scour valves. However, 1 Sydney Water frequently experienced significant constraints in operating scour valves in a number of dams due to potential safety risks, leading to severe dam hazards including: Valve failures Scour valves were not exercised regularly because of high risks involved in operations and they are considered unreliable should they be called upon to dewater the reservoir in an emergency situation Uncontrollable heavy flow Downstream areas are now heavily populated and sudden discharges of water in a scour valve failure will have serious consequences to the downstream community and environment. These prescribed dams, ranging from a Significant to High A consequence rank according to ANCOLD and New South Wales Dam Safety Committee (DSC) criteria, are located in densely populated urban residential areas. Deeper manholes in a number of reservoirs prevent the installation of additional valves and laying large diameter scour pipes. Compared to large diameter outlets, scours installed at the lowest level to enable the Sydney Water dams to be emptied are smaller and not adequate to discharge a large volume of water in an emergency. These gave rise to the need of implementing integrated solutions rather than isolated asset based solutions to meet the regulatory requirements. PROCESS AND METHODOLOGY Sydney Water identified a number of dams where simple asset based isolated solutions were not practicable and feasible. These dams were ranked in order of their criticality in order to focus and optimise the resources. Risk assessment A desktop study identified potential risks of valve failure during full valve operation for all dams by considering the consequences of failure and likelihood. The qualitative assessment considered following significant consequences in the event of failure. Adverse impacts to the community and environment around reservoirs due to flooding. This included not only the financial cost of damaged private /public properties but also wider damages in the way of disruption to the other services including transport and telecommunications. Subsequent complex recovery process as many downstream areas are now urbanised and densely populated. Supply interruptions to the large customer base if the reservoir has to be out of service for prolonged periods. The magnitude depends on the water network configuration and the availability of alternative supply. It further assessed that the likelihood of failure is more or less similar for all the valves in the absence of data to suggest otherwise. The risk ranking was therefore more focused on the consequence. Table No 1 shows the identified potential risks categorised in to three levels of high, medium and low in a Matrix. Eight dams with high risk exposure were selected for the options development process. Table No 1 Outcome of Risk Assessment Dam No Valve failure based on condition Impacts to community Environmental Impacts Supply Interruptions High risks & need solution? 1 Low High High Low Yes 2 Med. Low High Med. Yes 3 High Med. High High Yes 4 Low Low Low Neg. No 5 Low High High Low Yes 6 Low High High Low Yes 7 High Med. Med. High Yes 8 Low High High Low Yes 9 Med. Med. Med. High Yes 10 Low Low Low Low No 11 Low Neg. Neg. Low No Stakeholder Engagement The high risk scour vales are located within several water delivery systems, where different operations managers were responsible for operating and 2 maintaining the systems. Additionally several external organisations and personnel were involved as service providers in dam safety management. This required active stakeholder engagement from the outset to develop well accepted, integrated solutions. Early and continuous stakeholder engagement also ensured the successful implantation of the solutions. Sydney Water conducted a workshop to develop and document a process for undertaking Operation Preparedness Tests, to verify the operational capability of dewatering Sydney Water s prescribed Dams. Sydney Catchment Authority, Dam Surveillance Inspectors as well as Operations & Maintenance personnel from Sydney Water were among the participants to deliberate in the issue resolution process to: Undertake options analysis (including asset solutions, non-asset solutions and workable operational solutions) and selection of preferred alternatives Make recommendations for trial and monitoring Option Analysis Lowering the reservoir level, by operating outlet valves to quickly discharge water into the system, is the preferred practice of operating scour valves to prevent potential adverse impacts in the event of a dam failure. Scour valves and pipes can be used as additional water discharge conduits if required. The inability to freely operate the existing scour valves at several Sydney Water reservoirs led to rethinking the approach previously adopted to meet the regulatory requirements in relation to dam safety. Deviating from simple asset based solutions such as constructing new assets, attention was paid to reviewing the requirements for flexibility, and improving policies and procedures. Key options identified are listed below. Asset solutions: These are focused on the asset itself and considered options of constructing new assets or modifying existing assets. These may sometimes be extremely capital intensive. Considered the following solutions as options Installation of a series of second scour valves along the existing scour pipe Plugging the scour pipe outlet to operate reservoirs using high capacity pumps Installation of additional valves inside reservoirs Extending and re-laying large diameter scour pipes & more valves to reservoir Non Asset solutions The focus was more on altering the operating and maintenance practices and changing the processes to optimise the use of existing assets taking the whole system view. Operational Solutions - Efficient and effective operational plans with an alternative dewatering system. This involves the development of a comprehensive emergency dewatering plan to promptly lower the water levels of these dams without the need to discharge water through scour mains. Plans include the use of systematic flow diversions through the large diameter outlet mains into the surrounding reservoirs and the water distribution zones by creating artificial demands Review of regulatory requirements to see if they reflect the current needs of stakeholders Review the maintenance policy Social studies and environmental impact assessments to verify that desktop risk assessment reasonably represents the actual risk and that the Operating Preparedness Test is mandatory Preferred Option All asset solutions considered required extremely high capital investments, and the least capital intensive option was the installation of a second scour valve in series along the scour pipe. This was selected to trial in four dams where geographic and technical conditions were suited. Figure No 1 shows the location of the secondary scour valve along the scour pipeline. Among the non-asset solutions, development of an operational plan with alternative dewatering arrangements was selected to trial in four other identified dams, due to the attractive results yielded through preliminary desktop analysis. Figure No 2 shows a schematic diagram of a dewatering process developed for a dam with sequential operation procedure. Other non-asset solutions, including the review of regulatory needs together with maintenance policies, environmental impact assessments and social studies evaluating the impacts, were considered not feasible during the issue resolution process due to time and extensive resource involvements. Trialling and monitoring Asset solution In the selected four dams, a secondary control valve at the scour outlet pipe, just downstream of the existing scour valve (Figure No 1) was installed and performed the safety valve operational checks including Operation Preparedness Tests (OPTs). This ensured safe scour valve operations in an emergency. Non Asset Solution The dewatering mechanisms were successfully trialled during recent periodic reservoir maintenance events, with no safety, service disruption or other asset failure issues. Sydney Water has gained sufficient confidence to adopt the newly identified process in the event of a dam failure to safely and effectively lower the reservoir levels. KEY FINIDINGS 1. Both trialled options (installation of a second valve along scour pipe and the alternative dewatering plan) provided a similar service level to the customer. However the alternative dewatering process proved to be the most cost effective option. 2. Asset based solutions take an isolated asset view while non-asset options take the whole system view. 3. The asset based option has the following disadvantages: Introducing a secondary valve to ensure the safety of the primary safety valve, could not be adopted at certain dams due to location specific construction difficulties and other issues. The cost involved in this method, including construction of alternative pipelines and installation of valves, were considerably high. Extended valve operation time because two valve operations may be a critical factor in an emergency 4. The alternative cost effective dewatering procedure eliminated the need for operation of the scour valves and provided a number of other major advantages: Quicker and safer dewatering compared to conventional use of scour pipes & valves. Discharging significant quantities of water through large diameter outlet mains into the system is more effective and faster than dewatering through the scour mains which usually consist of comparatively smaller diameter pipes (see Table No 3) that lead to limited capacity maintenance chambers and stormwater systems. Avoids adverse impacts to the community and environment caused if water were released through scours. 3 Eliminates the need to reach the scour valve in a dam failure, which places staff at risk. Provides a reliable alternative when other engineering solutions or installing a secondary control valve does not prove feasible, since secondary valves can be installed only if the appropriate conditions prevail. 5. The approximate time to dewater the selected dams following the described mechanism is estimated at 3.8 days (maximum) which is well within the evacuation time specified by the United States Bureau of Reclamation (USBR) guidelines See Table No 2 Table No 2 Estimated time taken to dewater the dams Dam No Emergency dewatering Capacity 5 The estimated time taken to dewater the reservoir is approximately 3.5 days with no scour valve operating. 6 Dewatering time is 3.8 Days with no scour valve operating. 9 Approximately 1 day per (half) side with no scour valve operating. Dam has two sides. 2 Approximately 3 days with no scour valve operating. Note: The estimated times taken to dewater the identified reservoirs are well within the evacuation time of 60 to 70 days required by United States Bureau of Reclamation (USBR) Guidelines Benefits The non-asset solutions proved to be the best, meeting technical feasibility, financial viability and social acceptability. Also this solution: satisfied all stipulated regulatory requirements and was endorsed by the NSW Dam Safety Committee is limited to less complex operational arrangements proved to be more effective than previously proposed capital intensive asset solutions and offers the lowest life cycle cost alternative minimises maintenance costs and safety risks involved in scour valve operation. dewaters the selected reservoirs quicker than passing water through scour pipes. 4 Maximum required dewatering time of the reservoirs remained well within international guidelines during test runs. Table No 3 Comparision of pipe diameters of the dams Dam No Inlet Pipe Diameter (mm) (pipes) CONCLUSION Outlet Pipe Diameter (mm) Scour Pipe Diameter (mm) Sydney Water has developed and trialled alternative safe dam dewatering protocols that do not require the conventional practice of operating scour valves but will achieve the same outcome in an emergency. This alternative process, which is limited to less complex operational arrangements, was more effective than previously proposed capital intensive asset solutions. It further minimises the maintenance costs and high safety risks involved in scour valve operations, which could generate adverse community and environmental impacts. Periodic scour valve tests (OPT) will no longer be undertaken at a number of Sydney Water owned prescribed dams where risk and operational constraints are high. Instead dams will be quickly dewatered following the alternative flow arrangements in an emergency. ACKNOWLEDGEMENTS The author would like to acknowledge the assistance of Sydney Water staff who contributed to the development of emergency dewatering plans for the prescribed dams. A word of appreciation is extended to the Water NSW (Sydney Catchment Authority) staff for their involvement in the process. Figure No 1 - Secondary control valve at scour outlet pipe Figure No 2 - Schematic diagram of the dewatering process for one dam. 5 REFERENCE ANCOLD, 2003, Guidelines on Dam Safety Management. Australian National Committee on Large Dams Inc., NSW, Australia AWT, 1996, Policy on Dam Safety Inspection and Exercising of Critical valves at Prescribed Dams, Australian Water Technologies, SW Australia, DSC 3A, 2012, Consequence Categories for Dams guidelines, Dam Safety Committee New South Wales, Australia DSC 3D, 2010, Reliability of spillway Flow Control Systems guidelines, Dam Safety Committee, New South Wales, Australia DSC 3D, 2010, Section B- Reliable Gate Systems Operation guidelines, Dam Safety Committee New South Wales, Australia DSC, 2013, Centennial Park No1 & 2, Maroubra and Thornleigh Reservoir Alternative Arrangements for Dewatering, Letter Ref. No , Dams Safety Committee, NSW, Australia Kanak. F., Pinidiya. H., 2014, Critical Valve Inspection Guidelines, Sydney Water NSW, Australia Pinidiya. H., Kanak. F, Murray. J., 2013, Sydney Water Dam Safety Scour Valve Operation, No 2013/ , Sydney Water, NSW, Australia Sydney Water, 2012, Operation Preparedness Tests (OPT) and Valve Assessments - Sydney Water Dams, Sydney Water, NSW Australia Sydney Water., 2013, Dam Safety- Scour Valve Operation Workshop , Sydney Water, NSW Australia 6
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