- Scrivener Dam Dissipator Strengthening Project
National Capital Authority
2.1The National Capital Authority (NCA) seeks approval from the Committee to proceed with the proposed Scrivener Dam Dissipator Strengthening project.
2.2The NCA states that the dam ‘plays a critical role in controlling the flow of water in the Molonglo River and managing the risk of floods by controlling the water levels in the lake [Lake Burley Griffin]. That function protects the residents of Canberra from flooding and downstream areas from sudden changing water flows.’
2.3The strengthening of the Scrivener Dam dissipator will rectify known structural deficiencies in the dam and ensure the ongoing, safe operation of the dam for the next 100 years.
2.4The estimated cost of delivery of the project is $38.5 million (excluding GST).
2.5The project was referred to the Committee on 3 August 2023.
Conduct of the inquiry
2.6Following referral, the inquiry was publicised on the Committee’s website and via media release.
2.7The Committee received one submission and one confidential submission. A list of submissions is at Appendix A.
2.8On 15 September 2023, the Committee conducted a site inspection at Scrivener Dam, followed by public and in-camera hearings at Parliament House, Canberra. A transcript of the public hearing is available on the Committee’s website.
Need for the works
2.9The Scrivener Dam is located on the Molonglo River Corridor in the Canberra suburb of Yarralumla and creates Lake Burley Griffin, a recreational water body that forms the centrepiece of Canberra. The dam is bridged by Lady Denman Drive and is located adjacent to Government House and the National Zoo and Aquarium.
2.10The NCA reported to the Committee that although the dam has undergone regular maintenance, it has not undergone any significant works since its construction.
2.11The NCA explained the function of the dissipator, stating ‘the dissipater is located downstream of the dam and absorbs the energy of water flowing over the floodgates during heavy rains. The dissipater also mitigates against potentially damaging erosion in the river channels downstream of the dam.’
2.12A detailed design review of the Scrivener Dam in 2014, which was reviewed in 2016, identified several risks associated with the stability of the dissipator structure under flood conditions. The review noted that if the drainage system does not function as intended and significant uplift pressures develop, the dissipator slabs may lift – which could ultimately lead to the failure of the dam. Additionally, during routine inspections of the dam in 2015, gaseous bubbles were observed to be coming through the dissipator slab joints. This highlighted that none of the concrete slab joints were constructed with water stops, which could allow water to move through the joints in the slab, creating voids and uplift pressures on the underside of the dissipator.
2.13At the public hearing, the NCA reported to the Committee that it had undertaken extensive preparatory studies on the dam following the 2014 and 2016 reviews. The NCA also undertook investigative studies over the course of 2019 to 2022 with various parties, including contracting the University of New South Wales to build a scale model of the dam and studying how it would behave in a range of scenarios.
2.14The NCA advised the Committee that the water released through the sluice gates and flood gates possesses tremendous kinetic energy. If uncontrolled, the water has the potential to significantly erode unprotected areas downstream of the dam, which could undermine its foundations and cause failure of the dam. There is also the potential for the dissipator to be significantly damaged during a major flood event, which in turn, could lead to the failure and subsequent collapse of Scrivener Dam.
2.15The NCA has outlined three key objectives for the proposed works:
- anchor the dissipator slab to prevent damage and uplift of the structure during flood operations;
- strengthen the dissipator slab to prevent damage from transient pressures during flood operations; and
- install new erosion protection on the downstream abutments of the dam to minimise risk of erosion of these areas during flood operations.
Options considered
2.16The NCA engaged an engineering consultant who identified eleven options to strengthen the dam, shortlisting three for further development:
- Option 3 (Overlay slab with new anchors) was recommended as the preferred option as it scored the highest on the multi-criteria analysis undertaken in a preferred option workshop. Following Committee approval, the NCA will use one of the following three sub-options:
- Option 3a: construct a new thin reinforced concrete slab on the top of original slab with anchor bars
- Option 3b: construct a new thick reinforced concrete slab with less anchors
- Option 3c: construct a new reinforced concrete slab on the top of the original slab with anchor bars and dowel the top slab to the existing slab.
- Option 4 (Retrofit anchors with no new overlay slab) will retrofit new anchors to the existing slab. The number of the additional anchors will be determined as required so that the existing slab and additional anchors jointly are able to take the peak design net uplift loads. This option was discounted as although it is cheaper, it is not robust enough in the anchor head arrangement and does not achieve the overall intent of the project.
- Option 9 (Partial demolition of existing slab, new anchors and reconstruct slab) will partially demolish the top of the existing slab as required for the embedment of the new anchor heads and reconstruction of the slab to the original thickness. This option was discounted as it is considerably costlier, more difficult to carry out due to the amount of demolition required and would have a significant community impact due to waste and noise.
- The NCA undertook a multi-criteria analysis to assess the preferred options against the following criteria: technical merit and ability to meet project requirements, constructability, cost, operation and maintenance requirements and other aspects, including impact to operations of the dam during construction. Option 3 was determined to be the preferred method to address the requirements of the project.
- ‘Dewatering’ the dam (draining all the water from the lake) was not considered as an option by the NCA due to the associated potential health risks and the disruption of both commercial and recreational activities on and around the lake.
Scope of the works
2.19The scope of work for Option 3 is comprised of eight elements:
Element 1 – Project Approvals (not yet achieved)
- Parliamentary Works Committee (PWC) approval
- NCA Works approval
- Environmental approvals
- EPA Waterways works licence.
Element 2 – Procurement Activities
- Multiple procurement activities including; construction head contractor, legal support
- Designer involvement, project management support services and other ancillary roles.
Element 3 – Site Enabling Works
- Installation of site fencing, traffic management and safety measures
- Establishment of the site compound including access from existing roads, parking areas, site offices, equipment storage and laydown areas, assembly and handling areas and facilities
- Construction of vehicle access roadways into the dissipator worksite
- Construction of a coffer dam to separate work areas from water being released from the dam. The coffer dam configuration will be reconfigured during the project to enable works to occur in different locations
- Installation of dewatering equipment to drain the work area and enable safe and dry access to the worksites
- Installation of silt fencing and other environmental controls as required.
Element 4 – Dissipator Ground Anchors
Installation of 700 new ground anchors. The anchors are 57mm thick steel rods cast at a depth of between 8.5m and 11.5m into the foundation rock.
Element 5 – Dissipator Topping Slab
‘A new 500mm thick reinforced concrete topping slab to be constructed on top of the existing dissipator slab…The topping slab includes a mesh of 28mm thick steel reinforcement bars, installed at the top and bottom, that are encased in concrete’.
Element 6 – Reconstructed Baffle Blocks
Demolition of existing baffle blocks and installation of new blocks on top of new topping slab. The new baffle blocks will the same dimensions as the existing blocks.
Element 7 – Abutment Erosion Protection
- To protect against damaging erosion, the top layer of soil will be removed on the abutments and an erosion protection matting will be laid down. This matting will then be filled with soil and reseeded with grass. The result will be a grassed slope similar to current site conditions
- Backfill material behind the training (side) walls of the spillway will be improved with free draining material, and the walls will be strengthened to ensure the walls are structurally adequate to protect against high flows.
Element 8 – Site Remediation
Site remediation will include removal of all temporary works including roads, culverts and coffer dams, and remediation of all disturbed areas.
2.20In addition to the elements listed above, the project also includes the following general scope items:
- Security Measures: the Scrivener Dam operators have developed a security plan that will secure the physical and electronic elements at the dam, ensuring its ongoing safety. Prior to any works commencing, barriers and fencing will be installed to control unauthorised access to the construction site and the dam compound.
- Fire protection: all construction and fire protection requirements will, as a minimum, be in accordance with applicable codes and standards.
- Occupational Health and Safety: a Safety in Design approach and documented safety options will be adopted in construction. The Construction Contractor will be required to develop and adhere to a safety management plan for the construction phase, which incorporates Safety in Design mitigations identified by the Design Consultant as well as other relevant risk mitigations, prior to commencing any works.
- The NCA reports that there is no significant maintenance burden introduced by the project and no significant whole of life costs associated with the project.
Potential impacts
2.22At the public hearing, the NCA reported that they undertook a vibration assessment and noise assessment that included placing sensors along the zoo enclosures as part of their investigations. The NCA collaborated with zoo operators who also observed the animals’ behaviour, finding no impact.
2.23The NCA engaged a specialist acoustic consultant to undertake an acoustic and vibration investigation in 2022 to ‘understand the potential acoustic and vibration impacts of the project during construction.’ The NCA sought to ‘characterise the likely disruptive noise and vibration from the project, model propagation over surrounding areas, understand the impacts of the project and establish mitigation strategies to minimise impact if required.’ Government House and the National Zoo and Aquarium were identified as the entities most likely to be impacted by noise and vibration from the works. The report found:
- The noise levels modelled at adjacent areas in the Zoo and Government House were below ACT thresholds for construction activity in residential areas
- The construction noise could be a minor nuisance to guests at the Jamala Lodge at the National Zoo and Aquarium
- The construction noise was found to have little or no adverse impact on the Zoo animals
- Sound and vibration levels at Government house and surrounding suburbs were well below minimum thresholds.
- At the public hearing, the NCA reported to the Committee that although there would be overlap in the timing of major works with the Commonwealth Avenue Bridge and construction of the Light Rail System, there would not be any temporary or permanent road closures as part of the dam works. The NCA acknowledged that the works may require partial road closures and a slowing down of traffic for the delivery of equipment or items as needed.
Water levels
2.25Lake Burley Griffin was included on the Commonwealth Heritage list in 2022. The listing also extends to adjacent lands including Scrivener Dam. The NCA conducted a Heritage Impact Assessment as part of its submission, which found that there will be minor impacts to heritage values relating to historical processes and technical achievement. Works undertaken as part of the project will cover the existing dissipator slab and baffle blocks, and as such, the original construction of the dissipator will be hidden from view. However, the original form of the dissipator will be sympathetically reflected in the modifications.
2.26The Scrivener Dam is classified as a ‘large’ dam in line with standards published by the International Commission on Large Dams (ICOLD). The dam has been assessed as a 'High C' Hazard Category dam in accordance with the guidelines issued by the Australian National Committee on Large Dams (ANCOLD). This is primarily due to the population at risk and potential loss of life resulting from a failure of the dam. The Dam is designed to maintain Lake Burley Griffin at a constant level of 555.93 metres above sea level (+/- 150mm).
2.27In order to adequately manage construction risk while working in the dissipator, and to minimise the risk of inundation of the construction site, the NCA will lower the level of Lake Burley Griffin by a maximum of 500mm below normal lake level. The maximum drawdown level has been selected to minimise impact to lake users, to allow a buffer in the lake to accommodate rain events and increased inflows while minimising risk to construction and inundation of the site. This will mitigate potential risk of immersion and delay to construction and allow for the movement of staff and machinery out of the dissipator in the event of a weather event.
2.28Lake Burley Griffin was previously lowered by 500mm in 2011 after a safety audit uncovered corroded bolts in the flap gates of the dam. The lake remained at the lower level while the structural problems were addressed. The 500mm lowering level is also congruent to drought conditions in February 2003.
2.29The NCA advised the Committee that:
…as part of normal operations of the dam we continue to early monitor the weather and we undertake flood monitoring in the event that there is a significant weather event in the catchment. We will often have at least 12 hours notice of a potentially large weather event in the catchment. At that point in time, we can start to take steps to effect any urgent repairs that might be need to happen and move primary machinery out of the dissipater. Together with the drawdown of the reservoir, we expect that there will be plenty of time to be able to complete any necessary tasks.
Community consultation
2.30The NCA reported in its submission to the Committee that it has proactively engaged with relevant stakeholders, businesses and commercial operators, organisations and ACT Government departments regarding the project. The NCA has directly engaged with the following stakeholder groups:
- Sporting and recreational users: ANU Sailing Club, ANU Boar Club, YMCA Sailing Club, Capital Lakes Rowing Club, Canberra Yacht Club, Rowing ACT, Canberra Grammar School, Pedal Power ACT and the Molonglo River User Group (via EPA)
- Commercial operators: Canberra Cruises and Parties (commercial vessel), Go Boat Canberra (commercial operator), Capital Paddle (commercial operator), Southern Cross Club – MV Southern Cross (commercial vessel), Lake Burley Griffin Cruises (commercial vessel) and SupCbr (commercial operator)
- Other organisations and stakeholders: Friends of the Grasslands, The Canberra Ornithologists Group, Abstractors and the Yarralumla Nursery
- Adjacent organisations: Government House and the National Zoo and Aquarium
- Other organisations and government organisations: the Australian National Botanic Gardens, Environment, Planning and Sustainable Development Directorate - Parks and Conservation, Environment Protection Authority, Jerrabomberra Wetlands, City Renewal Authority, Transport Canberra and City Services, ACT Property Group, Major Projects Canberra, the National Arboretum and the Royal Canberra Golf Course.
- In its submission, the NCA reported that it had utilised the following community engagement methods:
- Attending Lake User Group (LUG) meetings: NCA staff have attended the quarterly meetings and provided briefings on the project. The NCA will continue to provide this service leading up to and throughout the proposed construction stage of the project
- Holding Dissipator Strengthening Project user group sessions: the NCA held two formal LUG consultation sessions on 30 November 2022 and 8 December 2022 to provide further details about the project to the community andallow the community to offer comment on the proposed works
- NCA website: the NCA has provided details about the project on its website
- Publishing media releases and social media updates
- Sending monthly email updates through a project newsletter
- Providing informal consultation, including on-site visits as needed with impacted stakeholders.
Cost of the works
2.32The estimated total capital delivery cost of the project is $38.5 million (excluding Goods and Services Tax). This includes all costs required for the delivery of the project including project management and design fees, construction costs, contingencies against known and unknown project risks and escalation provisions. Any additional cost will be met from within existing resources.
2.33The NCA provided further details on project costs in its confidential submission and during an in-camera hearing.
Revenue
2.34There will be no revenue generated by this project.
Public value
2.35The Scrivener Dam is a critical component of Canberra’s architecture and urban design. The dam creates Lake Burley Griffin which is the centrepiece of Canberra and an important resource for recreation, commercial activities, and tourism to the broader community. The lake is utilised by lake users, community groups, the wider ACT Community, and domestic and international visitors to Canberra.
2.36The dam allows for safe release of water in normal operation and during significant weather events. Strengthening the Scrivener Dam will ensure ongoing safe operation of the dam for the next 100 years and allow the NCA to maintain the lake as a public amenity.
2.37The project will also generate short-term employment within the construction sector and will provide employment opportunities in several areas.
Committee comment
2.38The Committee did not identify any issues or concerns with the proposal, and it is satisfied that the project has merit in terms of need, scope, and cost.
2.39The Committee recognises the importance of the Scrivener Dam and Lake Burley Griffin to the fabric of Canberra. It provides a public value as a resource for recreation, commercial activities and tourism to the local community, and domestic and international markets. Approval of these works will allow for the safe patronage of the lake for the future.
2.40The Committee understands that if the proposed works do not proceed, then the dam is at risk of failure, or even collapse, which could cause unacceptable risk to the city of Canberra and its inhabitants.
2.41The Committee notes that the NCA did not assess an option where the dam would be dewatered, as this could cause a potential health hazard and disrupt the recreational and commercial activities of the lake.
2.42The Committee was pleased to see that the NCA had undertaken an informative, comprehensive and consultative approach in their community and stakeholder engagement, especially in regard to the adjacent stakeholders Government House and the National Zoo and Aquarium.
2.43The Committee was satisfied that the proposed works would not have a detrimental impact on traffic flows in Canberra and that the NCA had a sufficient plan in place to manage traffic and noise incidents.
2.44Having regard to its role and responsibilities contained in the Public Works Committee Act 1969, the Committee is of the view that this project signifies value for money for the Commonwealth and constitutes a project which is fit-for-purpose, having regard to the established need.
2.45The Committee recommends that the House of Representatives resolve, pursuant to section 18(7) of the Public Works Committee Act 1969, that it is expedient to carry out the following proposed works: National Capital Authority—Scrivener Dam Dissipator Strengthening project.
2.46Proponent entities must notify the Committee of any changes to the project, scope, time, cost, function, or design. The Committee also requires that a post-implementation report be provided within three months of project completion. A report template can be found on the Committee’s website.