Land use planning based on overall risk measures

The population density in the EU is rather high, and buildings, industrial and agricultural activities practically spread over almost all the accessible areas. However, rather large portions of the land where people live, work or travel are submitted to not negligible risks, from natural and/or technological origin; moreover, in most areas different natural and/or technological risk sources are simultaneously present. Different land use planning criteria may be adopted depending on the specific risk source under exam: in most cases, for each type of development (industrial area, residential area, etc.) and/or activity (parking lot, school, etc.) proper safety distances from the risk source(s) are established, according to experience, or deriving from a “consequence based” or a “risk based” approach. When the area is also subject to some natural hazards (for example, earthquakes, or floods), more restrictive construction standards, or additional safety distances, may be required. In practice, any development in the “vicinity” of one or more Seveso establishments (i.e. those where a major accident may occur) should separately respect all the provisions enforced for any natural and/or technological hazard in the area. However, a land use planning policy based on risk maps relevant to single risk sources may result in an insufficient protection when both natural and technological risk sources are simultaneously present. In fact, the different risk sources may interact causing an increase in the overall risk level, due to various types of domino effects: for example, it is well known that natural disasters may trigger industrial accidents, originating na-tech (natural-technological) events. It is obvious that, in order to establish sound criteria for land use planning, all the risks in a given area should be accurately assessed: the arrangement of a comprehensive risk map would be extremely useful, but maps of this type cannot be easily obtained. In fact, generally, risk maps use specific risk measures depending on the involved phenomenon, making it very difficult to sum risks deriving from different sources and to get an effective picture of the overall risk in the area. As a matter of fact, for most natural hazards (earthquakes, floods, etc.), risk maps show some specific risk index, or indicate the expected intensity of the phenomenon, or locate the area interested by the phenomenon; in some cases, maps are referred to given time intervals between two subsequent occurrence of the same phenomenon (the so-called return time). Therefore, directly obtaining risk maps accounting for a number of different natural phenomena is practically impossible. As far as technological hazards are concerned, individual risk maps are often used, where the risk at any location is estimated based on the expected frequency of occurrence of some “top events” associated to the industrial activity under exam (process plant, transport of hazardous materials, etc.) and on the extension of the impact areas where the consequence of such top events may cause fatalities. In this case, due to the uniformity of the used risk measure, it is rather simple to obtain overall risk maps taking into account technological risks of different origin. Despite of the problems described above, a “risk based” approach appears largely preferable than a “consequence based” one in order to properly address land use planning in the presence of different types of hazards. In fact, the former takes into due account also the expected frequency of such catastrophes, which, in most case, assumes very low values. Nevertheless, the adoption of a “risk based” approach would require some homogeneous “risk measure”, which should be conveniently applied to assess the risk associated to any type of risk sources, to be defined. This paper discusses the possibility of defining a risk index, i.e. a quantitative measure of the risk, based on the information usually available for different natural and technological risk sources, and shows an application where this risk index is used to obtain individual and global risk maps of an area including both technological and natural risk sources