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.
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As the Panama Canal is upgraded to accommodate larger vessels, hydrological and ecological elements of the project are being closely monitored, along with the effects of the increased usage that is projected to accompany the upgrade when it opens to traffic in 2015. Each of the 14,000 ships that annually pass through the Panama Canal requires 200 ML of fresh water – drawn from Gatun Lake and other Chagres River reservoirs – to navigate through the locks. The reliability of a sustainable water supply is thus vital to the canal’s operation and, by extension, to the world’s economy.
Hydrologic, hydraulic, and sedimentation studies are providing baseline data for comparison with projected operational scenarios. Several projects are currently being undertaken to restore and protect the widely recognised and highly valued biodiversity within Gatun Lake’s catchment area. Efforts to promote biodiversity conservation during the construction and operation of the expansion project are being coordinated with the concurrent efforts of a variety of academic, scientific, and private institutions, including the Smithsonian Tropical Research Institute, which is located on the largest island in Gatun Lake.
This paper examines the implications of growth and expansion on Gatun Dam and Lake. Current studies are assessing the impacts of deforestation on sedimentation and temporal flow distribution into Gatun Lake. Methodologies and results are presented for the USAID-funded Panama Canal Watershed Biodiversity Conservation Project, an undertaking that engages public and private sector partners in an effort to improve the management and conservation of critical areas through the implementation of sustainable practices and engagement of local stakeholders.
Tim Gillon and Grant Murray
Chelsea Estate is located on the edge of the Waitemata Harbour, and is only ten minutes drive from Auckland central business district. Within Chelsea Estate are four ‘low’ potential impact classification (PIC) dams, which cascade along Duck Creek. Three of the dams are over 100 years old and all dams were built from 1884 to 1917. The dams and the reservoirs have served, and continue to serve, several purposes including stormwater retention, recreational use and water supply for the adjacent sugar factory. In 2008 Auckland Council (AC) purchased the Chelsea Estate from the New Zealand Sugar Company (NZSC) and in 2009 the Estate was registered in the New Zealand Historic Places Trust (NZHPT). This paper discusses the history and functionality of the multi-function Chelsea Estate dams, the development of the site and how it impacts our understanding of the dams today.
Keywords: Chelsea Estate, multi-function dams, heritage dams.
At the time that Contact Energy Ltd renewed the consents to operate its hydro generation dams in the Clutha catchment of Central Otago, it entered into an agreement with the kayaking community to construct a whitewater park on the Hawea River. The construction of the whitewater park was part of a package designed to mitigate the adverse effect on the natural whitewater features in the catchment caused by the construction of hydro generation dams
This paper outlines the process involved in identifying the preferred option, obtaining the necessary consent, design and construction and commissioning of the Hawea Whitewater Park.
Upstream construction methodology has been used to raise tailings dams in Western Australia (WA) for more than three decades, and the tailings storage facilities (TSFs) built in this manner have performed satisfactorily so far. The maximum design earthquake (MDE) for most of the existing, upstream-raised TSFs in WA was that corresponding to a 1-in-1,000 year annual exceedance probability (1:1,000 AEP). However, the recommended MDE loading for the High/Extreme Failure Consequence Category in the 2012 ANCOLD Guidelines on Tailings Dams is that of a 1:10,000 AEP. This more stringent seismic design criterion may restrict the use of upstream TSF construction in some areas of WA and Australia in general.
To evaluate the viability of upstream construction for a new or existing TSF, the effects of the earthquake design ground motion (EDGM) on the liquefaction and deformation response of the structure must be understood. The results of such analyses are an essential component in determining whether upstream raising will be feasible, or whether more robust but much more costly centreline or downstream construction methods are required.
A parametric study was completed to investigate the liquefaction and deformation behaviour of a typical, upstream-raised tailings dam under different earthquake design ground motions with different response spectra. The study utilized two-dimensional finite difference code FLAC2D effective stress dynamic analysis, in which the UBCSAND constitutive soil model was incorporated. Twenty-eight earthquake ground motions (matched and unmatched to the target response spectrum) were used in the study and the liquefaction response of the tailings dam model under those ground motions was analysed.
The results of the study demonstrate the importance of appropriate ground motion and response spectrum selection in assessing the seismic performance of an upstream-raised TSF. Liquefaction response was shown to vary with different response spectra, even though the corresponding EDGMs had similar peak ground acceleration (PGA) values. The importance of earthquake frequency content and duration, which in turn are affected by earthquake magnitude, distance and ground motion response, is emphasized. Scaling and matching the earthquake input motion to the uniform hazard response spectrum (UHRS) may result in overly-conservative design. Thus, selection of the most representative EDGM is essential to evaluating expected seismic performance for an upstream-raised TSF, and scaling or matching the earthquake input motions must be done cautiously.
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.