Otago Regional Council (ORC) own and operate the Lower Taieri, Lower Clutha, and Alexandra Flood Protection Schemes. Collectively the schemes comprise over 220 km of earthfill levees, together with numerous appurtenant structures, such as major spillways, flood gates and pumping stations. The schemes provide flood protection to significant and varied communities and infrastructure adjacent to the Clutha and Taieri rivers, for example Dunedin Airport, and towns such as Balclutha and Outram. The works were constructed at various times since the 19th century to a range of standards, and assets are at various lifecycle stages.
Regular and systematic condition and structural integrity assessment is a key aspect of operating flood protection schemes for resilient communities. This can be challenging due to the large spatial extent of multiple schemes. Efficient and effective on-the-ground visual inspection of the entire network is key. A field assessment methodology was developed which combined on-the-ground visual assessment with innovative use of GIS technology, for field data capture, recording, analysis and presentation.
The structural assessment methodology used LiDAR-derived digital elevation models (DEMs) integrated with the field data to screen the levee networks based on geometry condition, to identify critical locations for analysis. Levee susceptibility to hazards such as overtopping scour, piping, seismic performance and slope instability was assessed utilising a semi-quantitative multi criteria analysis. Subsequent efforts were focused on critical locations enabling analysis which would not be efficient on a scheme-wide scale. An outcome included a GIS database to enable rapid future review of asset information and condition.
The assessment coincided with the July 2017 Taieri River flood – the largest event in almost forty years, and a timely reminder of the importance of flood protection infrastructure for community resilience. This event also highlighted the importance of making use of such events to field-truth assessment results and test assumptions about scheme performance and vulnerable locations.
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Following the failure of Paloona Dam’s intake trashrack during the 2016 floods in northern Tasmania, a replacement trashrack and support structure was designed, manufactured and installed (by diver) within five months. This was a remarkable feat and hailed as a success at the time.
The euphoria, however, was short lived. A routine dive inspection in January 2018 revealed cracked
trashrack bars on one of the panels and this was after less than twelve months’ operation. This prompted a rigorous investigation where it was determined that the bars suffered fatigue due to flow induced vibration. Indeed it is possible that the bars cracked within a few weeks of returning to service.
The science of flow induced vibration is relatively mature, having been extensively researched over several decades. Its application to trashracks is well documented. However, this experience has shown that the common design approach overly simplifies the fluid-structure interaction. For Paloona, the result was a trashrack design which has proven to be inadequate, not having the resilience required for a dam outlet works component.
This paper revisits flow induced vibration theory as it pertains to trashracks, outlines the findings of vibration testing at Paloona, and suggests a design approach which will avoid similar issues. It is hoped that similar failures can be prevented and the design life expected of trashracks achieved.
This paper discusses the current regulatory requirements and guidelines, which address to varying degrees the need for recovery controls and the engagement of Owners with Impacted Communities (ICs) within a Dam Safety Emergency Response Plan. The planning and application of appropriate recovery controls, which are applicable from the moment of failure, help to build resilience and reduce the ultimate consequence of TSF failure. The application of such controls, developed with close engagement with impacted communities has a strong precedent, being recommended as a result of the International Council on Mining and Metals (ICMM) review of good practice for emergency preparedness (Emery, 2005).
This paper presents a simple method to assess various recovery controls, with risk minimisation as its basis, and the use of existing risk assessment techniques such as bow-tie diagrams or the inclusion of recovery controls to other qualitative assessment methods. This will be illustrated through application to some relevant historical TSF failures.
Satellite remote sensing data can be used to monitor environmental processes and inform disaster risk reduction and hazard early warning. This paper describes the analysis of satellite remote sensing images to investigate the partial wall collapse of a tailings dam at the Cadia gold-copper mine in Australia that occurred on 9th March 2018. Our case study uses freely available remote sensing imagery acquired by the Copernicus Sentinel-1 (radar) and Sentinel-2 (multispectral) satellite constellations to monitor land surface changes in the Cadia mine area before and after the collapse. In this paper we discuss the benefits of utilising both radar and multispectral remote sensing imagery in a holistic approach to remote sensing, which could be used for continuous, near-real time monitoring of risk-related infrastructure such as dams without the need for in-situ measurement equipment.
We applied the Interferometric Synthetic Aperture Radar (InSAR) technique to measure surface displacements and interferometric coherence maps from a stack of Sentinel-1 radar images acquired between 2nd December 2015 and 25th June 2018 at regular 12 day intervals. The time series of surface displacements show a significant increase in the rate of movement of the dam wall in the area that eventually breached in the two months prior to the collapse. This change in movement behaviour was not observed at parts of the dam wall that remained intact. This analysis demonstrates the potential for InSAR monitoring to identify issues in advance of infrastructure failure, which could allow risk mitigation strategies to be implemented by the mine operator. We used interferometric coherence data to observe changes in the dam wall and surrounding areas before and after the collapse. A drop in coherence occurred in the breached section of dam wall, indicating the surface change caused by the collapse. Coherence for unaffected parts of the dam wall remained stable. Sentinel-2 multispectral imagery acquired between 2nd July 2017 and 24th June 2018 show the timing, extent and effects of the collapse as well as the rate of tailings movement.
A new operating arrangement at Hume Dam is being developed to improve the transition from flood operations to the release of water set aside for delivering environmental flow demands. The arrangement also aims to help manage inherent downstream flood risks associated with this transition and with the requirement to fill the storage.
This paper describes particular flood risks and environmental impacts resulting from the current approach required to meet asset and water resource security priorities during airspace management operations at Hume Dam. It then considers how the new environmental demands have interacted with long-standing operating objectives and airspace management during high inflow periods in ways that have altered the dam operations required to meet operating priorities and manage flood risks.
Critically, requests by environmental managers to start releases can arise sooner and with greater uncertainty compared with releases for meeting irrigation demand following a period of flood operations or airspace management. This difference has led to a more rapid storage filling curve to maximise water resource during periods when inflow rates remain relatively high and catchments are still responsive to rainfall.
The paper details how the new operating arrangement provides greater volumes and more flexible flood mitigation airspace using a discretionary volume of ‘held’ environmental water without otherwise impacting on the flood operations decision-making process. A number of challenges in defining the potential level of benefit and risk, and in understanding trade-offs were faced in negotiating the arrangement. However, the successful development of the approach and agreement to trial it were ultimately achieved by framing the issue as an opportunity to adjust dam operations in a way that seeks mutual benefits for dam operators and environmental managers.
Full adoption of the arrangement would result in greater airspace flexibility during flood operations to better manage risks without affecting water resource. Simultaneously, it provides environmental benefit due to changes in the pattern of releases during the transition period from flood operations to the commencement of environmental water releases as well as during the pre-spill release period.
Failure modes of seepage and internal erosion have been identified as one of the key issues for the
ongoing safety of dams and canals in New Zealand. Accordingly, many dams and canals have had
improvement works carried out to mitigate this issue. This paper examines the long-term performance of these measures including three case studies. It is concluded that the performance of these measures has been variable, but ongoing monitoring and periodic review has identified deterioration in performance. There are a number of technical areas where uncertainties on long-term performance may still remain, such as geotextiles in important filter functions and waterstops of various types.