Distribution of the extreme peak floods of the Tiber River from the XV century

The peak stages of the extreme floods inundating the town of Rome are known since the XV century. This record covering more than 500 years can be considered as a censored sample of the population of the peak flood stages at the site. To evaluate the peak flow of the observed events the following information is available: (a) a long record of daily stage measurements up to the XVIII century; (b) several records of daily rainfall depth measurement at two rain gauges in the Tiber catchment, extending at least up to the middle of the XIX century; (c) detailed surveys executed immediately after the great flood of 1870, including the elevation plan of the town, a survey of the riverbed and the flood hydrograph of the 1870 flood; (d) a less detailed survey of the riverbed executed in 1744; (e) a rich iconography, showing the conditions of the Tiber banks from the XVI century onward; (f) contemporary descriptions of several extreme floodings; and (g) a rich series of flow measurements and bed surveys after the great flood of 1870 to present days. A two-dimensional hydrodynamic model was used to simulate the flooding of the old town in order to determine the flow–discharge relationship, thus allowing reconstruction of a record of the past extreme floods. A statistical analysis was developed for evaluating the risk hazard, using the censored sample of all the extreme floods from the XV century and the sample of all the maximum annual floods from the end of the XVIII century.