Sea-level rise, like the change of many other climate variables, will be experienced mainly as an increase in the frequency or likelihood (probability) of extreme events, rather than simply as a steady increase in an otherwise constant state. One of the most obvious adaptations see more to sea-level rise is to raise an asset (or its protection) by an amount that is sufficient to achieve a required level of precaution. The selection of such an allowance has often, unfortunately, been quite subjective and qualitative, involving

concepts such as ‘plausible’ or ‘high-end’ projections. Hunter (2012) described a simple technique for estimating an allowance for sea-level rise using extreme-value theory. This allowance ensures that the expected, or average, number of extreme (flooding) events in a given period is preserved. In other words, any asset raised by this allowance would experience the same frequency of flooding events under sea-level rise as it would without the allowance and without

sea-level rise. It is important to note that this allowance only relates to the effect of sea-level rise on inundation and not on the recession of soft (e.g. sandy) shorelines or on other impacts. Under conditions of uncertain sea-level rise, the ‘expected number of flooding events in a given period’ is here defined in the following way. It is supposed that there are n Dasatinib supplier   possible futures, each with a probability, P  i, of being realised. For each of these futures, the expected number Staurosporine concentration of flooding events in a given period is given by N  i. The effective, or overall, expected number of flooding events (considering all possible futures) is then considered to be ∑i=1nPiNi, where ∑i=1nPi=1. In the terminology of risk assessment (e.g. ISO, 2009), the expected number of flooding events in a given period is known as the likelihood. If a specific cost may be attributed to one flooding event, then this cost is termed the consequence, and the combined effect (generally the product) of the likelihood and the consequence is the risk (i.e. the total effective cost of damage from flooding over the given period). The allowance is the height

that an asset needs to be raised under sea-level rise in order to keep the flooding likelihood the same. If the cost, or consequence, of a single flooding event is constant than this also preserves the flooding risk. An important property of the allowance is that it is independent of the required level of precaution (when measured in terms of likelihood of flooding). In the case of coastal infrastructure, an appropriate height should first be selected, based on present conditions and an acceptable degree of precaution (e.g. an average of one flooding event in 100 years). If this height is then raised by the allowance calculated for a specific period, the required level of precaution will be sustained until the end of this period.