The risk assessment

The risk assessment study objectives were to:

• identify the significant engineering, environmental and community risk events associated with the approvals, construction, operation and after-care phases for each of the schemes;

Table 18.7—Mine waste management options

Scheme Summary

A Closure B Tailings

C No dredge D Dredge

Immediate closure of the mine

Continuation of the current dredging scheme to the end of 2001 to build an embankment in which to store tailings, and the piping and storing of tailings until the end of mine life in 2010

Demobilization of the dredge at the end of the trial period in 1999 Continuation of the current trial dredging scheme (nominally 20 Mt/a) until the end of mine life in 2010

• quantify the risk events associated with each phase of each scheme, i.e. quantify the frequency of occurrence and the financial consequences of the identified engineering, environmental and community issues that may occur during the life of the project;

• combine the engineering risks with the environmental and community risks to provide a total scheme risk for each phase of each scheme;

• quantify OTML's highest realistic financial exposure associated with each scheme; and

• present the base costs and the total risk costs in a way that enabled comparison of the schemes.

The risk assessment proceeded at several levels, outlined in Figure 18.4 and shown in more detail in Figure 18.5. At the highest level, the process followed a structure similar to the risk management processes described in this book. This was supported at lower levels by many detailed studies concerned with health and ecological risk assessment (HERA), engineering, and social and economic aspects of the mine and its impacts (Figure 18.6). OTML has made many of the individual studies available on its web site,

Each of the detailed studies conformed to good practice in its own discipline (for example, EPA, 1998). In particular, the scientific aspects of the work were subject to intensive review by an independent international peer review group (PRG) of eminent scientists appointed by OTML for this purpose. The HERA was itself conducted at two levels, beginning with a screening level risk assessment and followed by a detailed level risk assessment, according to accepted principles, and subject to detailed PRG scrutiny.

A key part of the risk management process was the construction and validation of a detailed quantitative risk model, which is outside the scope of this chapter. The model consolidated the risk information from all the individual studies to assist the OTML board in its evaluation of the four main mine waste management options.

The overall risk assessment process, managed by OTML, and the quantitative risk analysis model, were also subject to an independent audit, conducted by the first author.

One important feature of the process was OTML's instruction that the risk assessment be directed to a comparison of mine waste management options and information relevant

High-level process:

High-level process:

Figure 18.4— Case: multi-level risk management process


Primary processes

(overall analyses and models)

• process management

• option definition

• quantitative analyses

• risk management

• process audit

Objectives Stakeholders Criteria Structure

Supporting processes

(detailed analyses and models)

• engineering

• sediment transport

• acid generation

• aquatic environment

• terrestrial environment

• human health and nutrition

Establish the context








the risks

the risks

the risks

Treat the risks

Identify options

Select the best responses

Develop risk treatment plans Implement

Monitor and review

Review processes • PRG

• internal review

—Case: detailed structure of the assessment

Acid rock Bioavailability

Toxicity Habitat Fish biomass Isotope tests Food webs


Sediment transport




Imaging Human health

Ground truth Wildlife

Phototoxicity Rehabilitation

Figure 18.6—Case: supporting models

Dredge trial Surveys

Hydrology to decision making in this context. This affected the way in which some of the studies and activities were conducted, and they generated comparative rather than absolute assessments of risks in many cases. While this may seem an unnecessary constraint to some interested parties with other objectives, a key question is whether the risk assessment was adequate in its approach, processes and use of available data for the purpose for which OTML commissioned it; that is, for generating information to assist decision-makers to compare and select options.

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