Flood Risk Assessment & Investment Framework

Abstract

The Hollandsche IJssel is a river between Rotterdam and Gouda and is protected from the main river system in the Netherlands by a storm surge barrier. The Hollandsche IJssel experiences several problems: the storm surge barrier dates from 1958 and has a relatively low closure reliability; three quarters of the dikes have been rejected according to the safety standards due to macrostabilty; subsidence of the dikes is approximately 90cm per century; there is limited space available to reinforce the dikes because of the dense housing next to the dikes. To determine the current flood probability and risk, the use of fragility curves is applied. They represent the state of the dikes and can be combined with a probability density function of water levels, which represents the state of the storm surge barrier. This will return the flood probability. By combining them with damage curves, the flood risk can be assessed. To determine cost-optimal flood risk reduction strategies, the use of equivalent annual cost is applied. The equivalent annual cost is defined as the net present value of a project devided by the present value of annuity. It can be seen as an interest weighted average annual cash flow. By keeping the equivalent annual cost as continuously as low as possible, flood risk reduction strategies can be determined which after manual post-processing proved to be cost-optimal. This proved to result in three strategies for the Hollandsche IJssel: the dike reinforcement strategy, the double barrier strategy and the canalization strategy. The dike reinforcement strategy gradually reinforces the dikes and keeps the current storm surge barrier until no longer possible after which it is replaced by a similar one (likely around 2060). The double barrier strategy suggests the construction of a second storm surge barrier to increase closure reliability. This leads to a postponement of dike reinforcements, which are still required due to subsidence. Eventually, because of the limited amount of space that is available due to the dense housing: both lead to inevitability of canalization somewhere after 2100 (with the used schematization around 2120). Because of this inevitability, the recommended strategy is the third strategy: implement canalization immediately.