Floods

To calculate Floods Risk, we use the variables [Copernicus Climate Data Store (CDS)]:

• maximum 1 day precipitation (rx1day): maximum daily precipitation in a year (in mm).
• maximum 5 day precipitation (rx5day): maximum precipitation over 5 consecutive days in a year (in mm). The first indicator is associated with short and violent storms (possible cause of 'flash floods' and 'urban flooding'). The second indicator is associated with persistent precipitation events with continuous rain for several days (possible cause of river flooding and 'river floods'). Therefore, by combining the two indicators, we can estimate Flood Risk. To define risk levels, we consider the return period RP (see Cold Wave Risk) for these indicators.

This methodology for Flood Risk assessment fully reflects standard practices in hydrology, hydraulic engineering and climatology.

It is based on three consolidated scientific pillars:

• The use of standard extreme precipitation indices (Rx1day, Rx5day) to characterize different types of rain events causing floods. The Rx1day and Rx5day indicators are two of the most important and universally used climate extreme indices. They were standardized by the Expert Team on Climate Change Detection and Indices (ETCCDI) to ensure consistency and comparability of studies globally.
• Indices for monitoring changes in extremes based on daily temperature and precipitation data This document consolidates precise definitions of Rx1day and Rx5day. The scientific community has adopted them because they effectively capture the characteristics of rain events leading to different impacts.
• ETCCFI Climate Change Indices
• The distinction between risks associated with short-duration events (flash floods) and those associated with persistent events (river floods).

Scientific and hydrological literature fully supports your distinction:

• Rx1day for Flash Floods / Pluvial Floods: Rain intensity in a short period exceeds soil absorption capacity and urban drainage system capacity.
• Rx5day for River Floods (Fluvial Floods): Precipitation accumulated over several days saturates the watershed, causing river level rise and overflow.
• Handbook of Drought and Flood Indices This handbook extensively discusses how different precipitation indicators (including extremes over time scales of 1, 3, 5 days) are precursors to different flood types. It confirms that analyzing precipitation extremes over different durations is fundamental for comprehensive flood risk assessment.
• The application of Extreme Frequency Analysis, using Return Period, to classify severity and risk.

The use of return period to define flood risk level is the absolute global standard, both in scientific and engineering fields and in insurance.

• Guidelines on Analysis of Extremes in a Changing Climate in Support of Adaptation to Climate Change These WMO manuals describe standard practices for flood forecasting and management, which are entirely based on frequency analysis of precipitation and river flows. The use of return periods to define alert thresholds and design defense works is a central theme.

In conclusion, our methodology reflects best practices. It uses standard indicators (Rx1day, Rx5day) to capture different mechanisms generating floods and applies the standard return period method to create a clear, quantitative and universally recognized risk scale.