A new joint probability appraisal of flood risk

Prior to the construction of the Cardiff Bay Barrage the risk of flooding of the Cardiff Bay waterfront was from inundation by surge tides. The barrage has substantially eliminated the probability of tidal flooding, and the residual flood risk now depends upon the ability of the inland bay to provide sufficient storage for river floodwater during periods of high tide. In the course of the feasibility and design studies the actual reduction in flood hazard was not quantified, but the design was carried out against specific combinations of tidal levels and fluvial flows. Recent research for MAFF at HR Wallingford has now made viable the assessment of the actual degree of flood risk, where flooding depends on the combination of two or more events. This paper presents a new joint probability appraisal of the standard of flood defence for the land around the inland bay, and demonstrates the conservatism incorporated in the original design methodology for flood risks. An examination of individual floods and sur...