Ensemble strategy for decision-support tool : a case study of the Alex storm in 2020 in la Roya and la Vésubie valleys

Best allocation of ressources from stakeholders that face fast-flooding implies a dynamic representation of the risk, exposure, and danger, in a situation where parameters (roughness, infiltration, drainage network etc.), and input (bathymetry and rainfall) can be both uncertain and volatile. Ensemble strategy simulation appears as a good approach to deal with these issues.
Fast-flood event are also typically events where meteorological predictions can underestimate the actual rainfalls until very late. Urban microcharacteristics can also make models sensitive to spatial resolution, and « events » such as log jam can even modify DEM. 
At Strane Innovation, we develop a decision-support tool called BlueMapping. To be operational, that is, fast to deploy and reliable, we use this ensemble strategy together with the fastests simulation models deployed on powerful computers. It also requires quick and robust routines for the setup of the model, with proxies when data is not available at the moment, and inputs that are easy to modify if necessary.
We will ilustrate this discussion through the test case of the Alex storm that hit la Vésubie and la Roya valleys. After a quick benchmark with a standard model, we will compare the outcome between the flooding predicted by different models and the actual outcome. 
Since BlueMapping can be integrated in an alarm system, it is important to assess the values of the confusion matrix, in particular the false alarm ratio, to make sure our tool keeps its value over time.