Full Flood Cost: Insights from a Risk Analysis Perspective

Traditionally, flood protection measures have focused on high investment alternatives for control infrastructures that will benefit a certain amount of people, based in a gross cost-benefit (GCB) for the defined design flood. This gross cost-benefit approach, however, does not fully incorporate risk assessment into the analysis, given that it assumes that the chosen flood protection measures will provide protection against a design flood by avoiding flood damage every year during the project’s lifespan. This paper presents a probabilistic view of the benefits of implementing flood control measures, incorporating the risk concept to a full flood cost analysis, for an example region in Brazil. By combining analysis on annual expected damage, which considered the likelihood of $n$ flood events along the year—and not only the project one—and two flood management measures (levee and land zoning flood hazard), this paper evaluates why, and by how much, the inclusion of the probability of effectiveness of a given flood protection measure differs from traditional methods based on gross benefit. Results demonstrated a large difference between verifying a measure’s benefits by the GCB and the expected damage: whereas the former indicates the levee structure resulting in a lower accumulated damage from the fifth year of measure implementation, if the expected damage is used, investing in land zoning will be always most cost effective. These findings are useful to highlight that full flood costs should be based on risk evaluation, which is consonant with the latest perception of the assessment and management of flood risks, such as the Floods Directive present in the EU Water Framework Directive.