Iain Lonie, Malcolm Barker and Colin Thompson
Consideration of flood mitigation benefits, water supply, irrigation and recreational usage played an instrumental role in developing the proposed upgrade for Maroon Dam to meet dam safety and flood capacity requirements. Maroon Dam is a 47.4 m high zoned earthfill dam completed in 1974. The dam is a multi-purpose reservoir which is now owned and operated by Seqwater and plays an important role in the local community. Key drivers for the proposed upgrade design included embankment stability, foundation concerns, piping, spillway capacity and erosion of the embankment toe.
Six options were reduced to three through a high level screening exercise. A more detailed assessment of the remaining options was undertaken using a Multi Criteria Analysis and a detailed risk assessment. Consideration of the competing uses of the reservoir was critical in the development and assessment of the preferred option. This paper will present the details of the analytical methods used as input for the Multi Criteria Analysis and the detailed risk assessment for the final proposed design option that will meet the requirements of dam safety and flood capacity without impacting on water supply, irrigation and recreational usage.
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.
Nicole Anderson and Nihal Vitharana
A large number of aging concrete dams in Australia may not meet the requirements of modern dam safety practices. In addition, there is an ever-increasing demand for the supply of water. Continuous concrete buttressing is a method of strengthening existing dams which allows the dam to be raised to augment the storage capacity at an incremental cost.
This paper explores the key design considerations involved in concrete-buttressing existing concrete gravity dams. Critical aspects considered include storage level during construction, interface drainage, interface shear transfer, the relative strength of existing and new concrete and the behaviour during the heating and cooling phases of the heat-of-hydration. The discussions will be of relevance to asset owners and water authorities faced with upgrading existing dams in a time where there is an increasing demand for security of supply of water resources.
Nigel Connell, Karina Dahl, Steve Agnew and Brent Walton
The Waimakariri Irrigation scheme was initially built from 1997 to 2001 and irrigates approximately 18,000 hectares in North Canterbury with canals between the Waimakariri and Ashely Rivers. This was an enlargement from an existing stockwater scheme originally constructed in 1890. The owner and operator of the scheme, Waimakariri Irrigation Ltd, propose to construct a storage pond to supplement irrigation supply when take is restricted due to low flow in the Waimakariri River.
The footprint of the proposed pond is approximately 1 km x 1 km, with maximum dam height of 12 m and an 8.2 Mm3 maximum storage capacity. Accommodation for hydro-power development has been incorporated into the design of the irrigation storage ponds to provide multiple use of the reservoir contents.
The embankments are to be constructed from on-site granular material that forms the Canterbury Plains and lined with geomembrane. Careful consideration has been given in the seismic design for this High Potential Impact Classification (PIC) structure, which takes into account lessons from recent major earthquakes in the Canterbury Region. In addition, an understanding of the rapidly growing community downstream of the proposed dam has been crucial to ensuring that the potential risk of the dam is managed appropriately.
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.
Jeong Yeul, Lim
For various historical reasons and some technical reasons, the safety of dams has been evaluated using an engineering standards-based approach, which was developed over many years. It was used initially for the design of new dams, but increasingly has been applied over the past few decades to assess the safety of existing dams. Some countries have carried out risk assessments of existing dams that included both the structural and hydraulic safety of the dam and social risk. These methods developed by other countries could be adapted to assist in decision-making for dam safety management. Unfortunately, methods for risk assessment of dams were not established in Korea. This study outlines a beginning risk analysis for structural safety management. The first stage consisted of research on the present domestic dam safety guidelines and reviewing operations for management systems of dam safety abroad. Also, dam risk analysis requires reliable data on dam failure, past construction history and management records of existing dams. A suitable risk analysis method of dams for structural safety management in Korea is use of event tree, fault tree and conditioning indexes methods. A pilot risk assessment was carried out for two dams. The dam risk assessment process was thus established, and we learned the importance of risk assessment. The future includes additional research and risk analysis to develop the system.