Grasping the Heterogeneity of the Subsurface: Using Buildup Scenarios for Assessing Flood Risk

The subsurface plays an important role when assessing flood risk and its build up has to be known in considerable detail to properly determine the chance of flood-protection failure. Currently, the subsurface below flood-protections is often characterized by geotechnical cross sections that are constructed using e.g. cone penetration tests (CPTs). In most cases, however, the available density of information is insufficient to account for the relevant heterogeneity of the subsurface. By ignoring this it is possible that buildups that have a negative impact on flood risk are missed and the strength of the flood protection is overestimated. In addition, due to the uncertainty about the buildup of the subsurface, the assessment criteria have to be conservative meaning that parts of flood protections will be assessed ‘unsafe’ unnecessarily. The existing uncertainty can be reduced significantly by incorporating additional, geological knowledge that exists about the general principles behind the buildup of the subsurface and to use both this knowledge and the measurements to make stochastic schematizations using subsurface buildup scenarios. The latter consist of a set of alternative buildup scenarios that characterize the subsurface in a certain segment. Each scenario has its own probability of encountering and is made up of stacked units that are described in terms of their geotechnical properties. The scenarios can be used in probabilistic assessments. The approach is flexible, meaning that scenarios can be easily expanded and adapted to incorporate additional data or information on the subsurface.