Tim McMorran and Alan Hull
Accurate assessment of potential fault rupture hazard in dam sites is a critical factor in managing dam safety. Assessment of the location and activity of a surface fault within or near an existing or proposed dam can be technically challenging, expensive and affect design and construction schedules.
Three examples from regions of relatively high, moderate and low tectonic activity are used to illustrate that fault rupture hazard assessment is generally feasible in regions with high rates of tectonic activity, historic earthquake occurrence and the presence of Quaternary and Holocene-age landforms and sediments. In regions with relatively low rates of tectonic activity and landscape development, the fault rupture hazard assessment is more challenging.
The examples illustrate that robust geologic and geomorphic analysis provides critical information on the fault rupture hazard at existing and proposed dams. These analyses assist dam owners to obtain a more complete understanding of the fault rupture hazard at their facility, and support their longer term risk assessments.
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Monique de Moel and Gamini Adikari
Parks Victoria manages over 4 million hectares of parkland and a portfolio of over $1.9 billion worth of infrastructure assets. Within this portfolio, Parks Victoria is responsible for a large number of dams and their associated structures. Consequence category of these dams varies from Extreme to Very Low. Parks Victoria recognised that these assets required a dam safety management and monitoring program. The development of a program commenced with a portfolio risk assessment in 1998 which progressed to detailed design reviews of a selected number of dams and the initiation of an ongoing dam safety and surveillance program. This initial work identified the need for dam safety upgrade works within this asset portfolio which Parks Victoria has been progressively addressing. In 2012 Parks Victoria identified that a review of the risk profile of the dams was warranted. The review included consideration of alternative options such as staging of works, reducing storage volume and decommissioning, as well as non-technical considerations such as increasing the recreational use and the environmental value of these assets. This paper outlines the approach adopted by Parks Victoria in developing and improving its dam safety program and how it has assisted in minimising dam safety risks. Specifically, Parks Victoria’s approach of adopting measures that recognize the purpose and benefits of the individual storages, whilst being sympathetic to the requirements of the other infrastructure within its diverse portfolio of assets is highlighted. Since this work commenced in 1998, Parks Victoria have been successful in the development of an effective dam safety and management program which has resulted in the reduction of risks associated with this portfolio of assets.
Matthew Sentry and Darren Loidl
To triple Yass’ water storage capacity, Yass Valley Council was required to increase the height of their existing concrete weir by 3.0 m. The 100 m wide weir was originally constructed back in the 1920’s. Upgrade works to the weir included raising the height of the existing concrete weir by 3.0 m with reinforced concrete; install 33 number 27 strand post-tensioned ground anchors vertically into the crest; construct a new outlet structure; upgrade existing mechanical pipe works; and replace the existing pedestrian bridge with a concrete bridge capable of vehicle traffic.
The key project constraints during construction were to maintain constant water to the town’s water treatment plant and maintain minimum 70% reservoir storage.
The original weir had no auxiliary means of flow diversion and the construction constraints meant that the water storage could only be reduced by 1.0 m from the existing crest during construction, resulting in the construction work being carried out in an active water course with minimal means of flow diversion. These key project constraints meant that there was a high risk of flooding during construction work.
Geotechnical Engineering was engaged by Yass Valley Council to carry out the required upgrade work at Yass Dam. Prior to construction work commencing, risk workshops with client and designers clarified the flood risks during construction. To minimise the impact of flood events during construction, Geotech implemented several flood mitigation measures which were controlled by a detailed construction flood management plan. These control measures included construction of two temporary diversion slots cut into the existing concrete weir capable of supporting a 1 in 2 year rain event whilst allowing construction work to continue; re-design of concrete works to minimise the volume of concrete which was to be cut from the existing wall’s downstream face; detailed construction sequencing to minimise impact to existing and new wall during construction work; and the early installation and stressing of anchors.
Although a detailed construction flood management plan was developed and implemented, the Yass Dam site was impacted by 13 floods during the 20 month construction period. Several floods recorded water levels between 1.5 m and 1.9 m above the existing crest, resulting in work ceasing for weeks if not months at a time. As a result of the consistent flooding, Geotech was able to develop stronger and more resilient methods to be able to effectively work within an active watercourse on dam structures where minimal flow diversions are available. This paper presents the unique techniques implemented through the Yass Dam Upgrade project and discusses the effectiveness of these techniques and lessons learnt through the 13 flood events experienced.
Peter Mulvihill and Ian Walsh
The Falls Dam was constructed in the 1930’s to provide storage for several irrigation schemes in the Manuherikia Valley situated in New Zealand’s South Island region of Central Otago.
The opportunity to retrofit a small hydropower plant to the concrete faced rock fill dam was taken in 2003, utilising existing tunnels complemented by an innovative syphonic penstock system. The key design and construction features of this integrated scheme are described, along with experience from the first 10 years of the generation performance.
Looking ahead, there may be further integration challenges as current investigation of irrigation storage requirements leads to major redevelopment at this dam site and substantial changes to generation parameters.
Alan Collins and Michelle Archer
The Waikato River is the longest river in New Zealand. Mighty River Power operates nine dams on the river with a combined net head of 335 m. The reservoirs have limited storage capacity so that the Waikato Hydro System is effectively a continuous run of the river scheme, providing constant generation for the New Zealand electricity grid. The river is also the habitat of the New Zealand Longfin and Shortfin Eel. Before the dams were constructed, eels naturally migrated as small elvers and lived as far upstream as the Arapuni gorge, where a waterfall prevented them from travelling further upstream. The commissioning of the Karapiro dam in 1947 reduced the natural habitat of the eels. In recent years, the eel population has been declining through a variety of anthropogenic factors and protective status is being called for. An elver catch and release program commenced at Karapiro Dam in 1992. This transferred elvers as far upstream as Lake Ohakuri and significantly increased the available habitat for the elvers to grow into adult eels. Spawning adults migrate downstream and back out to sea and as a result most of these eels are killed by turbines at the hydro stations. While consent conditions don’t stipulate it, Mighty River Power is committed to being an environmentally responsible custodian of the Waikato River and is dedicated in efforts to preserve the eel fishery. Mighty River Power recognises the importance of eel to local iwi; particularly highlighted by the emphasis on eel in the Waikato River Independent Scoping Study. The Karapiro eel bypass project, started in 2010, sought to investigate and research means to assist downstream eel migration. Research was gathered into eel searching patterns, timing of eel migration, durability in high velocities and other survival factors. This information was used to design, construct, and test a prototype downstream eel bypass at the Karapiro dam, something that had not been built on a dam this size before. In the 2013 migration season, three eels safely used the bypass. Plans are in place to improve the performance of the bypass in the coming seasons. Mighty River Power wishes to share the lessons learnt from this project with other dam operators for the conservation of this important species.
Earthquake ground motions were developed for the Tekapo Canal Remediation Project, including both Canal and Bridge sites. This work involved the specifications of the parameters of active faults and seismic source zones, the development of an aftershock sequence, and the review and selection of suitable ground motion prediction equations. The seismic hazard at the project sites is dominated by earthquakes occurring on the Irishman Creek fault. The characteristics of an inferred active (unnamed) fault shown crossing the Tekapo Canal near Forks Stream and the hazard it poses to the canal were also assessed, and it was concluded that there was no need to further investigate it as part of the canal upgrade project. A probabilistic seismic hazard analysis was used to develop response spectra for mainshock events for the various return periods relevant to components of the canal system having different PIC categories. A deterministic seismic hazard analysis methodology was used to estimate the aftershock spectra. Depending on the PIC category, time histories were developed to represent the response spectrum for both mainshock and aftershock events at some canal sites.