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Flood risk reduction through natural retention and physical protection: The problem with residual riskand the ‘levee effect’

People generally have a poor understanding of residual risk. They tend to believe in the absolute safety of levees, dams, and other structures. Thus, it is common to see the value of assets ‘behind the wall’ increase. The phenomenon, sometimes called the ‘levee effect,’ leads to the problem that relative losses of assets protected by these structures tend to be higher than otherwise would be the case, if, contrary to expectations, the levee is breached or the dam topped. Generally, it is important to challenge the faith people have in what they believe is absolute safety. There will always be residual risk and people living and working in flood zones need to be aware of this. In particular, it is important to keep in mind that: • Levees are only built as safeguards to a certain flood level. If that level is topped, they will not provide protection. Levees might be said to shift the losses to the tail-end of the probability curve. But they will not reduce loss potentials, nor will they completely eliminate hazards. • Levees were designed at one point in time based on assumptions that led to the level the levee will protect to (the design level). Many conditions may change over time and alter how the levee can protect – including what is happening upstream or downstream of a levee. • Floods can occur due to events other than river flooding (precipitation, groundwater etc.). For such events, levees offer no protection.

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