Structural Residual Risk Due to Levee Failures in Flood Mapping

A conceptual framework considering, in a statistical sense, the residual risk related to possible levee failures in flood hazard mapping is presented. The residual risk is separated into a “design hydraulic residual risk”, RRD, associated to levee failures caused by overtopping for low probability events with return period higher than the design value, and into the “structural residual risk”, RRS, in case of failure of the protection because of mechanisms other than overtopping, as piping, erosion, structural instability for medium and high probability events. Statistics of levee failure type and breach size on the Po, Piave, Tagliamento and Adige rivers, in Northern Italy, are estimated, as a function of river morphology and, for the Adige river, of flood intensity. A stochastic framework for probabilistic flood mapping is then discussed, taking into account in a Monte Carlo approach the effect of position, size, density of levees failures on the statistics of depth and velocity of inundated areas. An example is shown of the resulting water velocity uncertainty map which contributes to the residual hazard for the 100-year return period flooding map for a 23 km-reach in the middle-lower portion of the Po River. Implications for flood hazard mapping as requested, for instance, by the European Flood Directive 2007/60/EC are briefly discussed.