Jamie Campbell, Gregg Barker, Paul Southcott and Michael Wallis
The assessment of consequences of dambreak is used as input to the design parameters of dams, dam safety requirements and dam risk assessments. For many low consequence category dams, the consequences of failure can be dominated by itinerants, in particular vehicles on roads within the dambreak inundation area. Estimating the population at risk (PAR) and potential loss of life (PLL) rigorously is mathematically complex, requires significant user judgment and can be very sensitive to input assumptions. This paper presents a simple, practical tool that has been developed to assist engineers and analysts in assessing the PLL of itinerant road users within a dambreak inundation zone. The tool allows for a logical and defensible analysis based on an event tree approach and provides guidance on appropriate factors to be used in calculating the overall fatality rate of people exposed to the dambreak hazard. This paper details the tool and how to apply it to typical dambreak problems, providing the reader with the information required to estimate the consequences on itinerant road users; the paper also details how the concepts discussed can be applied to other itinerants.
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.
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.
Sean Ladiges, James Willey, Matthew Norbert and Andrew Barclay
The Enlarged Cotter Dam (ECD) Project, located in ACT, consisted of the construction of a new 87 m high roller-compacted concrete (RCC) dam and two central core zoned earth and rockfill saddle dams up to 23 m high on the low points of a ridge to the south-west of the Main Dam. The Main Dam is the highest RCC dam constructed in Australia.
A continuous single-line grout curtain with a total length of 1.2 km was constructed across the full extent of the Main Dam and the two saddle dams with the aim of reducing future seepage losses through the dam foundations. The grouting processes were similar for the main and saddle dams respectively, however the grouting of the Main Dam and saddle dams was carried out as two separate contract packages by different subcontractors. As such, the project provides a unique opportunity to undertake a comparison of the foundation conditions and control equipment and outcomes from the two packages of work. The foundation conditions were different at each of the three dams, with varying geology across the site.
The saddle dam grouting was completed in 2010, with the grouting carried out at the base of the core trench prior to construction of the embankments, while the grouting of the Main Dam was conducted in 2012-13, with most of the grouting works executed from the drainage gallery within the constructed dam.
Consistent throughout construction of the ECD grout curtain was a similar philosophy of real-time computer control, the use of Grout Intensity Number (GIN) parameters, water pressure testing, desirable grout mix properties and the avoidance of damage to the foundations. There were a number of key differences in the grouting process for the saddle dams and Main Dam; these include the ground conditions, the pumping control systems used, the GIN parameters adopted and the grout materials and mixes selected.
This paper provides a critical evaluation of the two grouting programmes, an assessment of the effectiveness of the grouting, comments on tools and methods used, and proposes a set of recommendations for curtain grouting over a range of ground conditions based on lessons learnt during the project.
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.
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.