Wednesday, August 22, 2012

Urban centres and flood risk

A recent BBC article (Shanghai ‘most vulnerable to flood risk’) reports on a paper published in Natural Hazards by a team of researchers. The paper ‘A flood vulnerability index for coastal cities and its use in assessing climate change impacts’ in the journal Natural Hazards by Balcia, Wright and van der Meulen, follows in a tradition of trying to quantify risk using a set of key variables. (I think the paper is on open access so you should be able to read it via the link). The authors develop what they call the Coastal City Flood Vulnerability Index (CCFVI) that is composed of three parts: the hydro-geologic, the socio-economic and the politico-administrative. These parts represent the three key interacting subsystems that affect coastal flooding, the natural subsystem, the social-economic subsystem and the administrative and institutional subsystem. Within each of these the authors identify variables that indicate the degree of exposure to hazard, the susceptibility to the hazard and the resilience to the hazard. The hydro-geologic part only has indicators of exposure whilst the ‘human’ parts have indicators of all three.
Exposure is defined as the predisposing of a system to be disrupted by a flood event due to its location. Susceptibility is defined as the elements exposed within the system that influence the probability of being damaged during the flood event. Resilience is defined as the ability of a system, community or society to adapt to a hazard. This term is assessed through political, administrative, environmental and social organisational evaluation. Variables selected include sea-level rise, storm surge, number of cyclones in last five years, river discharge, foreshore slope, soil subsidence for the hydrogeologic subsystem. For the socio-economic subsystem the population close to the shoreline, the growing coastal population as well as cultural heritage are included as exposure factors whilst uncontrolled planning zones are an exposure variable for the political and administrative subsystem.  Susceptibility variables include the percentage of the population disabled or young or old and flood hazard maps. Resilience variables include shelters, level of awareness, institutional organisations and flood protection.
The paper carries out a detailed analysis of each subsystem and then combines the indicators into a single equation to determine overall vulnerability.  The selection of variables is well argued and the complexity and issues of using such indexes is discussed well, so the authors do not have a simplistic interpretation of hazards and vulnerability. Any paper that tries to squeeze and freeze the complex and dynamic concept of risk into a single index will always have the problem of simplification. Simplification, not only of the subsystems but also of the interpretation by others of the index itself.
The variables selected may reflect the data readily available plus a particular view of how the flood hazard should be alleviated. The focus on institutional organisations as resilience does imply a rather hierarchical view of hazard management and prevention (maybe a valid argument with a set of large urban areas with low social cohesion). Interpretation of the index, as in the BBC report, tends to focus on the final product rather than on the variables used in its construction and the ratings of the subsystems. Discussions could be made as to the appropriateness of the same variables for cities across the globe or for the selection of those variables anyway. Looking in detail at the breakdown of the index, it is clear that Shanghai is the most ‘vulnerable’ city on variables used to determine the hydro-geologic subsystem because of its high length of coastline and high river discharge (plus high soil subsidence). Manila, however, is ranked second because of its exposure to tropical cyclones and flooding – can both the same index combine both types of exposure? Does this mean that Manila is a more vulnerable location, as tropical cyclones are more frequent than high discharges? Can degrees of difference in vulnerability or rather exposure be assessed using a combined index? Shanghai is not, however, the top ranked city for all subsystems. For the economic variables, Shanghai is ranked fourth meaning that it is likely to recover quickly, economically at least, from the affects of a flood event.
An index like the one presented in this paper are very, very useful. They can be used, as the authors have done, to try to predict how changes in climate could impact on hazards and as such can be of great use in planning and management. A single index should, however, be used with caution, particularly if the choice of variables reflects a particular view of hazard management. Similarly, understanding how the index is constructed and how different parts of the index contribute to the whole is vital in understanding where vulnerability (and resilience) lie and how these might be improved.

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