Development of an inverse method for coastal risk management

Abstract

Abstract. Recent flooding events, like Katrina (USA, 2005) or Xynthia (France, 2010), illustrate the complexity of coastal systems and the limits of traditional flood risk analysis. Among other questions, these events raised issues such as: "how to choose flooding scenarios for risk management purposes?", "how to make a society more aware and prepared for such events?" and "which level of risk is acceptable to a population?". The present paper aims at developing an inverse approach that could seek to address these three issues. The main idea of the proposed method is the inversion of the usual risk assessment steps: starting from the maximum acceptable hazard level (defined by stakeholders as the one leading to the maximum tolerable consequences) to finally obtain the return period of this threshold. Such an "inverse" approach would allow for the identification of all the offshore forcing conditions (and their occurrence probability) inducing a threat for critical assets of the territory, such information being of great importance for coastal risk management. This paper presents the first stage in developing such a procedure. It focuses on estimation (through inversion of the flooding model) of the offshore conditions leading to the acceptable hazard level, estimation of the return period of the associated combinations, and thus of the maximum acceptable hazard level. A first application for a simplified case study (based on real data), located on the French Mediterranean coast, is presented, assuming a maximum acceptable hazard level. Even if only one part of the full inverse method has been developed, we demonstrate how the inverse method can be useful in (1) estimating the probability of exceeding the maximum inundation height for identified critical assets, (2) providing critical offshore conditions for flooding in early warning systems, and (3) raising awareness of stakeholders and eventually enhance preparedness for future flooding events by allowing them to assess risk to their territory. The next challenge is to develop a framework to properly identify the acceptable hazard level, as an input to the present inverse approach.

High-resolution marine flood modelling coupling overflow and overtopping processes: framing the hazard based on historical and statistical approaches

“…For Zone C, however, there are notable differences depending on the statistical method applied, reflecting a greater uncertainty in the estimation of the 100-year hazard for several neighbourhoods. To overcome this uncertainty arising from the choice of scenarios for propagation, one possible solution is to use a meta-model, which is, in essence, a mathematical approximation of a hydrodynamic model that predicts the modelled responses at a negligible cost in computing time (Idier et al, 2013). In this way, it becomes possible to estimate the response variables directly by "propagating" all the simulated combinations of forcing conditions obtained from the Monte Carlo simulation (see Sect.…”

“…As emphasized previously, this approach rests on the assumption that the return period of the response is equal to the return period of joint exceedance of the input variables. In reality, the joint exceedance probability of the input variables is an underestimation of the true exceedance probability of the response (Hawkes et al, 2002;Idier et al, 2013;Bulteau et al, 2018). The reason for this is simply that combinations which do not belong to the space (SWL > x H s > y) can still produce values for the response variable Z in excess of z T .…”

“…flooding in our case. However, the system response is often complex, and whenever a problem addressed has more than one dimension, the one-to-one relationship is rarely valid (Idier et al, 2013). For example, let us imagine we are interested in the 100-year return value of the inundated area.…”

Tracking the critical offshore conditions leading to inundation via active learning of full-process based models

“…the boundary of the set of offshore forcing conditions, which lead to an inundation at the coast. Adapted from [3] As discussed by Idier et al [2], it can be of high interest to identify the combination of all critical set of offshore conditions that lead to inundation on key assets for the studied territory (e.g., assembly points, evacuation routes, hospitals, etc. ), i.e.…”

“…Tracking such a critical frontier is the key element of an inverse methodology of coastal risk assessment. The main idea of such an inverse risk method is the inversion of the usual risk assessment steps [2]: starting from the maximum acceptable hazard level (defined by stakeholders as the one leading to the maximum tolerable consequences) to finally obtain the return period of this threshold. Such an "inverse" approach would allow the identification of all the offshore forcing conditions (and their occurrence probability) inducing a threat for critical assets of the territory.…”