Probability of flood scheme failure

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Graeme Smart

Wednesday 1 july 2015

12:00 - 12:15h at South America (level 0)

Themes: (T) Flood risk management and adaptation, (ST) Flood risk assessment

Parallel session: 9J. Floodrisk - Assessment

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Flood protection works are designed for a specified level of protection, expressed as a return interval or annual exceedance probability. While different design flows (for say a 1/100 AEP flood) can be determined depending on climatic variations over the period for which data is analysed, this is not the only major source of uncertainty in the level of flood protection. On one hand, flood control schemes have a “freeboard” allowance to give a conservative design. On the other, failures of flood defences do occur below their supposed design flood level due to mechanisms such as piping, debris jams, out-flanking, bank scouring, landslides or capacity loss through in-channel deposition, as well as overtopping. While uncertainty regarding flood return interval is recognised, the subject of quantifying pre-design-capacity scheme failure has received less attention and is the main subject of this talk. For the study, river practitioners familiar with flood control schemes, were surveyed to evaluate the probability of failure of individual protection schemes during floods different from the design protection level. For example a scheme designed to withstand a 1/100 AEP flood may be judged to have a 5% probability of failing in a 1/50 AEP flood, 25% failure probability for the 1/100 AEP flood and 100% failure probability in a 1/500 AEP flood. By considering the likelihood of flood control schemes being compromised at levels other than the design flood an overall, more accurate probability of scheme failure can be estimated. Formally, the basis for the analysis is that the annual exceedance probability of failure of a scheme is the integral of the product of the annual exceedance probabilities for different floods and the probability density of failure in the floods. The talk presents the results of such an analysis for New Zealand and gives a technique for more realistic estimation of the risk of flood disasters.