Over the past three weekends, the territory of Southern Switzerland has experienced three particularly intense weather events that have put not only the local communities, affected by significant damage, but also the entire national population in apprehension. The intensity of rainfall, river overflows, and, especially, debris flows underscored the vulnerability of the people and infrastructure that daily inhabit and experience the Alpine territories. 

These events have also provided valuable food for thought for the Hydrology Sector of the Institute of Earth Sciences (IST) of the Department of Environment Constructions and Design of SUPSI, which, on behalf of the Canton's Watercourses Office, is in charge of hydro-meteorological monitoring of the Ticino territory. The data complement MeteoSwiss's network of automatic weather stations, with the aim of more accurately representing atmospheric phenomena. 

The precipitation that occurred first in the Maggia Valley and the following week in the Mendrisiotto was very intense at all local meteorological stations. The cantonal rainfall station in Cavergno, which has been active since 2004, was probably closest to the epicenter of the most intense precipitation on June 29-30. Indeed, new extreme values were measured for durations of 1, 3, 6 and 12 hours. Notable new record values for the sum of precipitation in 6 and 12 hours, 156 and 169.8 mm, respectively, far exceeding the previous values of 100 and 137.2 mm. 

Even more intense rainfall was measured for the July 6-7 rainfall event that occurred in the Mendrisiotto: 198.8 mm respectively 200.9 mm were measured in just 12 hours at Cabbio and Coldrerio, while 228.6 mm and as much as 243.5 mm were recorded in 24 hours. Other stations on Monte Generoso, Stabio and Mendrisio also recorded values slightly above 200 mm in 24 hours. These amounts represent maximum values for the Mendrisio and Cabbio stations. For Mendrisio, in operation since 2004 with sub-daily data, new extreme values were measured for 3-hour (85.6 mm), 6-hour (144 mm) and 12-hour (177.2 mm) precipitation. In Cabbio, the new extreme values for the same durations were 104.6, 165.8 and 198.8 mm, with a total for the 24-hour period of 228.6 mm; however, it should be kept in mind that the station has only been operational since 2015. 

So that the mentioned data can be analyzed more quantitatively and to better understand the exceptional nature of the rainfall, in Coldrerio it rained twice the average amount of rain for the entire month of July in 24 hours, with an estimated return period exceeding 100 years (MeteoSwiss, 2022). In Stabio, on the other hand, for rainfall measured over 12 hours we get a return period of about 30 years (MeteoSwiss, 2022).

The intense rainfall generated heavy surface runoff of stormwater with a rapid and important rise in river levels. New extreme values of 87.65 and 80.77 cubic meters per second are measured for the Faloppia, monitored since 2015, and for the Laveggio at Riva San Vitale (since 1978), respectively. For the Breggia, a floodplain discharge of 138 cubic meters per second was measured, which is lower than the 182 cubic meters per second of 07/27/2021, however, a value with an estimated return period of about 30 years (observation period 1966-2020, FOEN).
 

Annual flood extent for the Laveggio Segoma (in green), the Breggia (in blue) and the Faloppia (in red) rivers (FOEN, UCA-SUPSI)

These intense events underscore the importance of continuous and meticulous monitoring of hydro-meteorological conditions, as well as the need to study climate change prediction, prevention, and adaptation strategies to protect vulnerable communities. Academic collaboration supported and reinforced by government agencies is crucial to meeting future challenges and ensuring an effective response to intense weather events.

Image captions:

• Faloppia stream at the August 1st Street Bridge, 08.07.2024 10:30 a.m. - flow rate of 86 m3/s (Andrea Mini)
• Ghitello Pond, Breggia River, 08.07.2024 11:00 am - flow rate of 135 m3/s (Andrea Mini)