Risk involves exposure to a chance of injury or loss. The fact that risk
inherently involves chance leads directly to a need to describe and to
deal with uncertainty. Corps policy has long been (1) to acknowledge
risk and the uncertainty in predicting floods and flood impacts, and (2)
to plan accordingly. Historically, that planning relied on analysis of
the expected long-term performance of flood-damage-reduction measures,
on application of safety factors and freeboard, on designing for
worst-case scenarios, and on other indirect solutions to compensate for
uncertainty. These indirect approaches were necessary because of the
lack of technical knowledge of the complex interaction of uncertainties
in predicting hydrologic, hydraulic, and economic functions and because
of the complexities of the mathematics required to do otherwise.