Slope Stability Report

6.4 Qualitative Risk Assessment

With reference to the Australian Geomechanics Society (AGS) ‘Landslide Risk Management Guideline
2007’, as per Section 8.0 ‘Risk Assessment’, the main objectives of risk evaluations is usually to decide
whether to accept or treat the risks and to set priorities. The ‘Tolerable Risk’ criteria are usually
imposed by the regulator (local Authority), unless agreed otherwise.

The AGS guidelines define ‘Risk’ as a measure of probability and severity of an adverse effect to health,
property or the environment. Risk is often estimated by the product of probability and consequences.
In these guidelines risk is further defined as;

    a) For Life Loss, the annual probability that the person most at risk will lose his or her life taking
       account of the landslide hazard and the temporal spatial probability and vulnerability of the
       person.
    b) For property loss – the annual probability of the consequence or the annualised loss taking
       account of the elements at risk, their temporal spatial probability and vulnerability.

A qualitative assessment involves an analysis which uses word form, descriptive or numeric rating
scales in order to describe the magnitude of potential consequences and the likelihood that those
consequences will occur. The Risk evaluation involves making judgements about the significance and
tolerability of the estimated risk. Evaluation may involve comparison of the assessed risks with other
risks or with risk acceptance criteria related to finance, loss of life or other values.

A qualitative landslide risk has been calculated taking into consideration the applied hazard rating and
the intended development as described by the client. It must be noted, the residual risk level
determined is conditional on the risk mitigation measures as outlined in Sections 7.0 of this report
being fully implemented and maintained for the expected life of the structure.

The elements considered within the risk assessment are as follows:

        The intended/ proposed buildings and associated outbuildings.
        Persons or Occupants on site.

As per the AGS guidelines, Qualitative Terminology used in assessing likelihood, consequence and risk
is presented in the following tables;

        Likelihood      Indicative Annual                              Description
                           Probability
  Barley Credible              10‐6          The event is inconceivable or fanciful over the design life.
  Rare                         10‐5          The event is conceivable but only under exceptional
                                             circumstances over the design life
  Unlikely                     10‐4          The event might occur under adverse conditions over the
                                             design life.
  Possible                     10‐3          The event could occur under adverse conditions over the
                                             design life.
  Likely                       10‐2          The event will probably occur under adverse conditions over
                                             the design life.
  Almost Certain               10‐1          The event is expected to occur.