Due Diligence – Land Use Planning
Land Use Planning
Possibly the most unfortunate outcome of the use hazard based analysis using target risk criteria is in land use safety planning for hazardous chemical facilities.[1]
Firstly, if the criteria are not satisfied it tends to sterilise planning areas from development. From an engineering perspective at least, this is just silly. Any site has issues, including windstorm hazards, geotechnical and earthquake potentials, storm surge, flooding and inundation, lightning strike potentials etc. For the design to be successful, all these must be addressed. The fact that there is a chemical exposure is just another hazard to be managed. If in order to be safe, people wind up in an unaffordable, unattractive, air-conditioned bunker, then it may be that the project will not proceed. But this will be for commercial reasons, not safety ones.
Precautionary vs. target risk level approach to land use planning
Secondly, it ignores rare, catastrophic hazards. For example, if a plot of the over-pressures at Buncefield (an unconfined vapour cloud that detonated in the UK in 2005) were to be mapped to any major hazard fuel farm in Australia, the area that can cause fatalities is huge. But although monstrous, this is historically a very rare event. If the event is discounted by the unlikelihood of its occurrence (at say 1 x 10-9 pa) in accordance with the risk target approach (typically 1 x 10-6 pa or 3 orders of magnitude larger) it is a much smaller area. Under most current planning regimes, structures developed beyond such 10-6 pa individual risk contours need only be building code compliant. No building is permitted closer to the major hazard facility. The diagram above describes the concept.
Adopting the precautionary approach to land use planning in these circumstances means that, the closer to the hazard a structure is, the greater the precautions need to be. In principle, provided the level of protection is high enough, there are no limits to where a structure could be built in relation to the major hazard facility presented above. For example, immediately adjacent to the explosion, the protection required may be a concrete bunker as death may result directly from the overpressure. The direct overpressure danger may be reduced at some distance but a house with laminated windows may be required to prevent glass shards shrapnelling occupants. Beyond the credible worst-case contour no protection from this hazard is required.
The QRA (quantified risk assessment) calculation of the risk contour is beneficial only in terms of determining the level of protection that is required at a given location, enabling the common law test of the balance of the significance of the risk vs. the effort required to reduce it to be applied. For example, between the 10-6 pa risk contour and the credible worst case consequence contour the cost of the provision of sheet metal roofs and laminated glass windows rather than tiles and ordinary glass, especially for new structures is very, very small indeed.
If buildings are permitted between the designated 10-6 pa individual risk contour and accelerative glass over-pressure limits without such precautions, and an (admittedly rare) explosion resulting in deaths or injuries occurs, then the responsible officers of PCBUs responsible for approving and building such structures (town planners, developers, architects, engineers, builders etc.) may be found negligent under common law and criminally reckless under the provisions of the new WHS legislation (knew or made or let it happen).
[1] Robinson Richard M, Gaye E Francis, Peter Hurley et al (2013). Risk and Reliability: Engineering Due Diligence (9th Edition). R2A Pty Ltd. Page 169