John Duder, David Bouma and Paul McCallum
The authors have been involved in the safety inspection and remediation of many older (pre-dating the 2004 Building Act) farm dams over the past decade coupled with considerable corporate knowledge from dams inspected by Tonkin & Taylor Ltd in its 50+ year history. This paper presents a summary of the varied benefits and risks of these older dams and the difficulties encountered in bringing them into alignment with current practice.
The many farm dams around New Zealand provide considerable benefit to the owners and often to the environment and wider community including the obvious stock water and irrigation, but also micro hydro, recreation, flood detention, release of environmental flows and flows for downstream users, and wetland habitat.
However, when applying current dam safety practice, and looking forward to the implementation of the Dam Safety Regulations, some of the older farm dams have significant dam safety issues that are often challenging to address. Although there is a high degree of variability, typical issues include:
Little or no documentation of geotechnical investigations, design or construction,
Design standards, particularly for spillway capacity have generally increased,
Little or no formal surveillance or maintenance carried out or recorded since commissioning,
Many farm dam owners have a poor understanding of their obligations under the Building Act and the Conditions of their Resource consents,
Consent conditions may not require dam safety related monitoring and maintenance, and/or the conditions may not have been historically enforced.
Many of these farm dams have been constructed by small contractors at the request of the farmers, often with only “standardised” engineering design and little specific geotechnical investigation. Typically there are no as-built records and the dam owners have been left with a general lack of understanding of owner’s responsibilities to monitor and maintain the dam.
Given that there are often very limited funds available for upgrade work, it has proved important to apply sound engineering judgement and a high degree of pragmatism to realise the greatest possible reduction in dam safety related risk for the available funds. Good cooperation between the Regional Authority, the Building Consent Authority for dams (often they are different organisations), the dam owner, and the dam engineer, together with a pragmatic approach is vital in moving toward current best practice for management of these dams.
Case studies are presented for the Northland Region, where the farm dams are typically homogenous earth fill dams in the order of 8 to 12 m high, fulfilling functions as irrigation, stock water supply, recreation and flood detention structures. The findings are considered relevant to earth fill farm dams across the country.
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Nanda Nandakumar and Stephen Farrelly
Fuseplug auxiliary spillways are used to increase the discharge capacity in dam upgrades for flood security. Hydrologic level-pool routings are used to determine the size and trigger levels for fuseplugs. In the level-pool routing, the water surface from the body of the storage to fuse bays is generally assumed to be horizontal and any drawdown effects on the water level are neglected. This paper assesses the validity of this assumption using the CFD model results for Keepit Dam. It is shown that equal spacing of trigger levels can result in premature activation, and the drawdown effects need to be taken into account in determining spacing of trigger levels. It was also shown that the design water levels for the intermediate AEPs are underestimated.
A comparison of inflow and outflow frequency curves showed that peak outflows can exceed the peak inflows due to fuseplug operations, but the downstream impact is expected to be negligible due the size of the flood in which the peak outflow will exceed the peak inflow.
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.
Richard Davidson, Jennifer Williams, Roger Raeburn and Jason Boomer
Ashton Dam is a 20-m high embankment dam located on the Henry’s Fork River in Eastern Idaho. It is a high hazard structure licensed with the FERC. The dam was completed in 1916 as a zoned earth and rockfill dam utilizing a low plasticity silt core. Ashton Dam is located approximately 13 km north of Teton Dam and is the sole remaining structure of four similarly designed dams. Over the years, the dam’s condition deteriorated, evidenced by periodic recurrence of sinkholes, sediment plumes and settlement.
PacifiCorp initiated a major 3-year rehabilitation project for the structure. Based on a risk-based design process, a new zoned embankment was reconstructed. Significant structural upgrades were also required for the powerhouse, training walls and gated spillway. To facilitate this construction, a new diversion tunnel and gated outlet structure were built to divert the river and manage flood flows. Cofferdams were required for both the upstream and downstream construction works.
Several challenges were encountered during construction, which were managed with a risk-based process. These included addressing the uncertainties that were known during design and the unknowns that were discovered during construction. Some of the construction challenges covered in the paper include utilization and processing of low plasticity silty material for embankment reconstruction, tunnel construction through fractured basalt with a major shear zone, a lake tap excavation in the wet, dewatering of the embankment excavation, left abutment treatment, real-time redesign of structural features, and fill placement in a constrained excavation.
This paper provides a synopsis of how these design and construction challenges were addressed and overcome on a “blue ribbon” trout stream with high public visibility and interest. Of particular concern was the need for cold weather concrete work, managing flood flows, lake tap and embankment excavation during the very limited construction seasons, and maintaining environmental river controls for the sensitive downstream ecosystem.
Keywords: Risk-based design, Embankment Reconstruction, Piping, Aged Concrete Repair
The dam surveillance industry relies on deformation survey data to assist in understanding and monitoring dam performance. My paper presents an overview of New Zealand dam deformation surveying. The fundamentals and best practice of deformation surveying are discussed, along with accuracies achieved and developments in automated measurements in real time. The key to achieving high accuracy in the results is using precise well calibrated survey instruments, many redundant measurements, quality survey marks and rigorous computational routines.
Simon Lang, David Stephens, Peter Hill, Mark Arnold and Tommie Conway
Considerable thought has been given in recent years to managing the risks associated with floods during the construction of new dams and dam upgrades. Both ANCOLD and the NSW DSC provide some limited advice on how this risk should be managed, with many dam owners aiming for societal risk during construction to be no higher than pre-construction. One approach to do this is to draw down the reservoir such that sufficient airspace is created to reduce the probability of overtopping the construction works to be equal to that of overtopping the dam crest pre-construction. However, this frequently leads to very large releases of valuable water resource being required. This approach also fails to consider that the conditional probabilities of failure may be quite different during construction than during normal operation. A risk-based approach was applied for the recent upgrade of Tarago Reservoir. Existing event trees from a failure modes analysis were adjusted to reflect the construction conditions. In some cases, the event probabilities increased (for example as a result of excavation of the dam embankment), however some also decreased (for example as a result of more rapid means of detecting and intervening in breach formation during construction). The conditional probabilities of failure during construction were then used to estimate the overall seasonal probability of failure, and it was found that a limited draw down of the reservoir would be sufficient to ensure that risks were no higher during construction than pre-construction. To reinforce this, the cost-to-save-a-statistical life was estimated for further drawdown of the reservoir and used to demonstrate that the risks were as low as reasonably practicable.