Since engineering projects for protecting the city entail lead times for planning and construction that are measured in decades, the acceleration of sea-level rise presents a difficult policy challenge. The protection plan addresses this challenge through an iterative approach which builds incrementally upon the existing system, selectively raising defences and taking other measures to elevate the protection standards of the current system (see figure IV.3). The black arrows point to alternative measures which may be applied once a measure is no longer effective.