Structure-based framework for the design and risk assessment of hydraulic structures, with application to offline flood detention basins

The design of hydraulic structures for water control is based on the principle of accepting that structures might fail to fulfill their intended purpose as consequence of extreme, yet rare events producing damages. Traditionally, structures are sized to resist to a "design event", i.e. a hyetograph or hydrograph associated to a fixed probability of occurrence. This implicitly assumes that the probability of structure failure equals that of the hydrological load, although unjustified under general conditions. Indeed, structure failure also depends on the failure mechanism - which is a characteristic of the specific structure and problem at hand - ruling the interaction between the hydrological load and the structure itself. To overcome the limitations of the hydrological design event approach, we provide here a general, consistent, "structure-based" risk framework, for design or risk assessment. The framework combines the multivariate statistical distribution of the hydrological process and the hydraulic response of the structure. Remarkably, it reduces the potential complexity of the problem to the statistical analysis of the univariate probability distribution of the damage, i.e. the consequence of structure failure. The approach is exemplified for an offline reservoir for flood mitigation, by adopting both a simplified routing model and a Monte Carlo procedure based on a detailed hydrodynamic simulation. Results demonstrate the potential of the framework, regardless of the accuracy of the modeling approach adopted for its implementation. Specifically, the framework allows disentangling the roles of the different design parameters involved, by providing clear indications for practitioners.