A comparison of approaches to risk-based flood modelling in the US and Europe

The approach to floodplain risk management in the US has evolved around the National Flood
Insurance Program (NFIP) and as such is largely focused on establishing Flood Insurance Rate Maps
(FIRM) that specify insurance rates in zones that are anticipated to fall within the zone of inundation
that is determined to be associated with the 0.01 percent chance of occurrence during any given year
(i.e. 100-yr Flood). While this approach offers a wide range of information for floodplain managers
and communities to make decisions about how to best manage at risk portions of their communities,
it is not a truly risk-based approach to floodplain management. Furthermore, in order to sustainably
fund the US National Flood Insurance Program (NFIP), a way has to be found to determine the
actuarially based premiums which should be paid by home owners. This imperative has driven the
work that will be reported in this paper.

Risk, defined as the probability of consequence (e.g. in dollars), enables a broader understanding of:
a) the variability of flooding in time and space; b) the pathways that exist between the flooding source
and the flood prone area, including the effect of levees (flood defences); and c) the variable
probability of occurrence across the floodplain. The paper reviews various full risk based mapping
approaches that have either been proposed for the US by various organisations or already adopted in
a number of European countries and within the insurance industry. It identifies key challenges for
changing the approach in the US in terms of data requirements, methodological tool development,
upskilling and communication and concludes by setting out a tentative road map for the US to move
from the current system through to the full systems-based flood risk analysis method.