Abstract
Rivers typically flow through multiple flood-protected areas which are clearly interconnected, as risk reduction measures taken at one area, e.g., heightening dikes or building flood storage areas, affect risk elsewhere. We call these interconnections ‘hydraulic interactions’. The current approach to flood risk management, however, neglects hydraulic interactions for two reasons: They are uncertain and, furthermore, considering them would require the design of policies not only striving for risk reduction, but also accounting for risk transfers across flood-protected areas. In the present paper, we compare the performance of policies identified according to the current approach with those of two alternative formulations: One acknowledging hydraulic interactions and the other also including an additional decision criterion to account for equity in risk distribution across flood-protected areas. Optimal policies are first identified under deterministic hydraulic interactions, and, next, they are stress-tested under uncertainty. We found that the current approach leads to a false sense of equal risk distribution. It does, however, perform efficiently when a risk-averse approach towards uncertain hydraulic interactions is taken. Accounting for hydraulic interactions in the design of policies, instead, increases efficiency and both efficiency and equity when hydraulic interactions are considered deterministically and as uncertain, respectively.
Highlights
It is well known that structural flood risk reduction measures alter a river’s hydraulic regime, as demonstrated by, for example, increased downstream flood peaks due to upstream dike heightening [1]or downstream flood load reduction due to upstream flooding [2,3]
Of the two alternative formulations, one formulation acknowledges only hydraulic interactions, while the other considers both hydraulic interactions and uses an additional decision criterion to account for equity in risk distribution across flood-protected areas
The performance performance of of the the policy policy resulting resulting from from the the first first problem problem formulation, formulation, i.e., i.e. neglecting neglecting hydraulic interactions, is shown based on its original values as well as by considering hydraulic interactions, is shown based on its original values as well as by considering hydraulic hydraulic interactions in order to make it comparable with those of policies identified by the other interactions in order to make it comparable with those of policies identified by the other problem problem formulations
Summary
It is well known that structural flood risk reduction measures alter a river’s hydraulic regime, as demonstrated by, for example, increased downstream flood peaks due to upstream dike heightening [1]. Downstream flood load reduction due to upstream flooding [2,3]. These phenomena are hereafter referred to as ‘hydraulic interactions’. Structural measures, such as dams and dikes, are designed according to a certain load (i.e., the design event), such that they should ideally withstand every load of lower magnitude. Besides the uncertainty on when and where dikes will fail, the way failure occurs, i.e., the breach growth rate and the final breach width, is uncertain. Predictions of hydraulic interactions can be flawed and mistakes in such predictions can lead to undesired outcomes
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