An empirical rainfall threshold approach for the regional civil protection flood warning system: the case study of the Milan urban area

In recent years the interest towards flood forecasting in urban areas has increased significantly: urban areas and the people living in there are generally very exposed to floods; also, this problem may worsen in the future due to climate and land-use change.

Empirical rainfall thresholds could represent a useful tool especially for those river basins where applying hydrological models is difficult. Such an approach can potentially be used for every river cross section whenever enough rainfall-discharge data are collected.

In this study, we developed some empirical rainfall threshold with the aim of validating and enhancing the Regional Civil Protection warning system operating in the Lombardy Region.

Rainfall data were collected from a total of 92 stations taken from the official regional Environmental Protection Agency of the Lombardy Region (ARPA-Lombardia) and from the network of a citizens science association “Meteonetwork”. Data on hydrometric levels and discharge were obtained entirely through the regional Environmental Protection Agency of the Lombardy Region.

The study focused on the small and well urbanized river basins which drain their water towards the city of Milan (mainly Seveso, Olona and Lambro River basins). Simple rainfall-runoff methods and decision theory, as indicated on the local civil protection directory, were used to validate the results obtained.

Preliminary results given by this methodology have shown that it could be a helpful tool for local civil protection authorities to take preventive actions for the population, as well as to validate the existing warning systems based on rainfall thresholds.