PROBABILITY OF FLOOD SCHEME FAILURE

There is significant uncertainty in the degree of flood protection offered by flood control schemes due to failures which can occur below the scheme design flood level. This can be from mechanisms such as piping, debris jams, out-flanking, bank scouring, bank slumping, landslides and capacity loss through in-channel deposition. By considering the likelihood of flood control schemes being compromised at levels other than the design flood an overall, better estimate of probability of scheme failure can be determined. Full technical investigation of all aspects of all flood control schemes in New Zealand would be prohibitively expensive, however, flood practitioners can heuristically estimate the probability that the schemes will fail under different flood loadings for schemes with which they are familiar. From these data the overall annual exceedance probability of failure of a scheme can be calculated. This study presents such an analysis for a sample comprising about half of New Zealand's flood control schemes and provides estimates of the effectiveness of flood hazard mitigation measures in NZ. Unless scheme upgrades are undertaken (several are already underway), the study indicates that the probability of serious failures on some NZ flood schemes is high.