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10.2 Rylstone Dam Safety Review REPORT BY THE MANAGER, WATER & SEWER TO 19 JUNE 2019 ORDINARY MEETING GOV400067, WAT500056

RECOMMENDATION That Council: 1. receive the report by the Manager, Water & Sewer on the Rylstone Dam Safety

Review;

2. receive the report by AECOM for the Rylstone Dam Safety Review.

3. note that the dam generally satisfies the NSW Dam Safety Committee (DSC) and the Australian National Committee on Large Dams (ANCOLD) criteria, however some major upgrade works are required; and

4. pursue grant funding from Safe & Secure, for two separable portions being design and construction phases.

Executive summary The Dam Safety Review of Rylstone Dam to assess the structural stability and condition of Rylstone Dam has been completed. The safety review was performed in accordance with NSW DSC and ANCOLD guidelines. The dam generally satisfies criteria of the guidelines, however some major upgrade works are required. The report was reviewed by an independent expert consultant and Council’s Water and Sewer Management. Works are required to both the left and right abutments, there is a possibility of over topping in a major flood event that could allow erosion of the abutments. The dam wall structure could then be compromised by side scouring impacting on the structural integrity of the dam. It is recommended that Council note the findings of the report and review recommendations for ongoing monitoring and upgrades. It is recommended that Council apply for safe and secure funding for two separable portions. These being Stage 1 Design and Review by an independent consultant and Stage 2 Construction by a suitably qualified structural dam building company. An Executive Summary of the AECOM Report attached for information. The full report is available on requested. Disclosure of Interest Nil Detailed report Council identified the need to perform a Dam Safety Review of Rylstone Dam in accordance with ANCOLD and NSW DSC guidelines. The dam safety review was completed as part of the process of addressing the concerns and recommendations resulting from the 2002 Portfolio Risk

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Assessment for Rylstone Dam. The previous structural review of the dam performed in 1999 by the NSW Department of Public Works and Services recommended the installation of anchors. The general scope of works from the Dam Safety Review included:

• Review the available information pertaining to the dam • Perform an engineering geological assessment of the site including a site investigation

drilling programme • Perform a Probabilistic Seismic Hazard Assessment (PSHA) for Rylstone Dam. • Undertake three-dimensional structural analyses of the dam for static and pseudo-static

analysis • Prepare a Dam Safety Review Report presenting the information collected, the results of

the analysis and the key conclusions and outcomes. The key conclusions from the Dam Safety Review are that the dam is in a satisfactory condition. The risk assessment and overtopping of the abutments in a flood event could erode the remaining embankment to the crest level of the dam. Flood Handling Capacity The current flood capacity (i.e. dam crest flood) was assessed to be a 1 in 100 AEP event based on the low point of the left and right abutments. This flood capacity does not meet acceptable flood capacity criteria for ANCOLD and DSC requirements for a high C Consequence Category Dam. Foundation Geology Based of the available information and the assessment of the geology and foundations of the dam, the following conclusions were made:

• The dam is constructed on a hard, strong bedrock foundation consisting mainly of the Rylstone Volcanics, a pale rhyolitic tuff. Foundation investigations did not identify any sheared or crushed zones that could represent low angle weak seams in the rock mass;

• There are no known faults nearby to the Dam. The nearest potential active fault sources is the Lapstone and Nepean faults located approximately 89km from the dam site;

• The foundation was assessed to be kinematically stable against sliding failure based on the joints and intersections defining wedges and confirmation from the site investigations that it is unlikely there are unfavourable low angle joints.

Seismic Hazard A seismic hazard assessment was performed as part of the safety review. The assessment developed site specific response spectra which were used to confirm seismic loading is not critical to the dam. Stability Evaluation The dam was evaluated for static (normal operations and flood loads) and pseudo-static seismic loads in accordance with ANCOLD Guidelines and additional arch dam analysis criterion. Stresses, base forces and displacements were extracted from a three-dimensional finite element analysis which has been developed using the commercial software program ANSYS. The results from the model were processed to evaluate the dam against the acceptance criteria. The flood loading at Rylstone Dam is such that the tailwater levels increase more rapidly than the reservoir levels and the critical loading of the dam is the 1 in 500 AEP flood event. The dam was

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also assessed for the 1 in 2,000 AEP and the 1 in 10,000 AEP flood events in accordance with ANCOLD flood criteria. The outcomes from the evaluation indicates that Rylstone Dam satisfies strength and sliding stability criteria for the static and pseudo-static load combinations assessed. The 1 in 500 AEP unusual flood load combination is considered to be critical loading of dam. The key recommendations from the Dam Safety Review and capital works are:

• Construct concrete cutoff walls on the right and left abutments to limit erosion in an overtopping event; and

• Undertake an updated risk assessment of the dam to account for the revised consequence assessment and the removal of the left and right abutments.

Additional recommendations were related to ongoing operation and maintenance and are outstanding from previous inspections. Beyond the initial capital costs, the following items should have minimal ongoing costs:

• Develop formal Operations and Maintenance manual; • Update Dam Safety Emergency Plan; • Continued surveillance of the dam through routine and annual inspections and perform

comprehensive dam safety inspections on a 5 yearly basis; • Construct permanent survey monuments and perform annual surveys of the dam.

Community Plan implications Theme Protecting Our Natural Environment Goal Provide total water cycle management Strategy Protect and improve catchments across the Region by supporting relevant agencies

Strategic implications Council Strategies This project will contribute to the improvement and upgrade of Council assets. A direct Council Strategy wasn’t identified from the Community Plan on the Council website, however these works are compliant with Council Strategy for securing water supply. The safety improvements will benefit the community and the downstream residents, insuring the dam integrity will not be compromised. Council Policies Not Applicable Legislation The Dam Safety Review and the O&M recommendations are requirements of NSW DSC. Having these in place and up to date may be required to secure funding for capital works.

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Financial implications There are costs associated with the Capital Works recommendation. AECOM has made no attempt to quantify these costs. When the design is complete, we will be in a position to estimate the construction cost and seek further grant funding. The ongoing O&M costs are anticipated to be minimal and/or not increase the current costs of operating and maintaining the dam. It is expected that the maintenance costs will be comparable with future maintenance estimates and therefore no change to the Operating Performance Ratio. Council Strategies This project will contribute to the improvement and upgrade of Council assets. The undertaking of this recommendation will improve the safety and operational procedures for Rylstone Dam. Associated Risks It is suggested that the associated risks will decrease based on an improved understanding of the structure and improving O&M procedures and documentation.

TOM BALDWIN MANAGER, WATER & SEWER

GARRY HEMSWORTH DIRECTOR OPERATIONS

29 May 2019 Attachments: 1. Rylstone Dam Safety Review - Executive Summary. APPROVED FOR SUBMISSION: BRAD CAM GENERAL MANAGER

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