SEA LEVEL RISE

What makes sea level rise different

Sea level rise is a hazard of trend rather than of event. The water arrives slowly and stays, and it changes the baseline against which every other hazard is measured: the same storm surge produces a different flood depending on the decade it occurs in, and drainage that works today ceases to work without any change in rainfall.

The consequence for assessment is that the counterfactual is not what exists now but what will exist. Comparing an intervention with the present condition understates its value, because the alternative to intervening is not the status quo but a deteriorating one. The appraisal has to be run against a projection, with the uncertainty in that projection carried through rather than collapsed to a single central value.

How it is assessed

The modelling follows the flood chain but with the downstream boundary condition changing over time, so that the same event is assessed at several points in the planning horizon. Where the intervention is a natural system — a dune, a salt marsh, a mangrove belt, a managed realignment — the assessment has to represent its capacity to grow vertically and to migrate inland, because that capacity is what determines whether it keeps pace with the rise. An intervention that is assessed as a static structure will appear to fail at a date at which a growing one would not.

The damage side follows the flood methodology, with the additional requirement that the exposed asset base itself changes over the period: what is behind the defence in 2050 is not what is there today. Co-benefits matter particularly here, because coastal natural systems deliver several services simultaneously and their value is often argued on that combination rather than on hazard reduction alone.

Because the timescale exceeds most planning and insurance horizons, the assessment is a natural candidate for the adaptive approach: decisions that are reversible are sequenced, and the trigger conditions under which the plan changes are identified in advance.

Where the methods are described

The adaptive planning page sets out the approach for decisions with long horizons and deep uncertainty. The hazards and risk modelling approaches page covers the flood chain that the coastal case inherits. The co-benefits valuation material covers the multiple services that coastal natural systems deliver.