Should Levee Goes Up or Down? A Risk Model for Flood Protection Standards

Many cities, towns, and areas in the world rely on levee systems protecting them from the threat of flood. Levee systems are designed to provide a specific level of flood protection rather than completely avoid the risk. Due to more and more extreme hydrological condition in recent years, people expect raising the height of levees with higher protection standards as an exclusive means of enhancing protection. The levees system actually is not panacea for flood protection. A flood protection infrastructure may increase flood losses, known as "levee effect". From this perspective, if levees are strategically removed or lowered, the result could be reduced flood risk and increased goods and services. Facing current climate condition, should we consider higher or lower levee systems need to be further examined. To answer this ambiguous question, this study develops a conceptual risk model to evaluate the overall performance of levee system. A general decision rule of flood protection standard is proposed for better understanding of this issue. We indicate that the key factor is the ratio of the rate of change of benefit loss to the rate of change of risk increase. According to the conceptual, numerical examples are simulated. The numerical examples apply appropriate flood damage functions and use different probability distributions of flood to demonstrate levee effect and evaluate the result of changing protection standards. The decision is based on the tradeoff of the reduction of current loss and the possible increase of future threat. For most cases, it is found when the flood level increase, the risk decreases much faster than the increase rate of flood loss. As the result, we prove the increase of flood protection standard is beneficial expect some expect some special cases.