Preprints
https://doi.org/10.5194/egusphere-2024-513
https://doi.org/10.5194/egusphere-2024-513
28 Feb 2024
 | 28 Feb 2024

Local floods in Madeira Island between 2009 and 2021. Rainfall analysis and risk assessment in mountain streams

Sérgio Silva Lopes, Marcelo Fragoso, and Eusébio Reis

Abstract. The torrential floods that occurred in December 2020 and January 2021 in villages located mainly on the northern side of Madeira Island, are analysed in comparison with other local scale events that occurred in 2009, 2012 and 2013. The term torrential flood is adopted in this work to designate the hydrogeomorphologic events, characterized by the occurrence of interactions between slope movements (landslides, debris flows) and fluvial dynamics (flash floods), typical in mountainous regions, formed by deep and narrow-bottomed valleys, as is the case in Madeira Island.

The heavy rainfall episodes of 25 December 2020 and 7 January 2021 were particularly surprising for the localised incidence of numerous occurrences of landslides, debris flows, and flash floods triggered by them.

Intense precipitation episodes, with hourly maximums of more than 40 mm and accumulated amounts in 24 hours exceeding, in some rain-gauges, the critical threshold of 300 mm, triggered peak discharges in several catchments, the largest with an area of up to 50 km2 and the smallest with less than 20 km2. The flow hydrographs of some streams show several flood peaks, associated with secondary maximums of heavy rainfall.

Daily and sub-daily rainfall peaks over catchments are crucial conditions for triggering floods. It was found that there is a very strong correlation between maximum precipitation in 24 hours and 12 hours, which can be mathematically described by a linear regression law. On the other hand, antecedent daily rainfall may also influence the occurrence of torrential floods in Madeira Island: the analysis of the combinations of critical pairs, consisting of the maximum rainfall in 24 hours and the antecedent rainfall calibrated for different durations (in days), allowing the identification of polynomial regression rules, which can be used to detect, in time and space, critical scenarios of torrential floods. The constitution and maintenance of an inventory of critical precipitation values, represents a relevant source of information to ensure the quality of the results issued by any flood early warning system.

Finally, hydrogeomorphological susceptibility mapping, which was produced through the collection of field information in the aftermath of catastrophic floods, embodies a complementary approach that provides a distinct spatial perspective of the areas of sediment production, flooding and deposition.

Sérgio Silva Lopes, Marcelo Fragoso, and Eusébio Reis

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-513', Anonymous Referee #1, 03 Apr 2024
    • AC1: 'Reply on RC1', Sergio Lopes, 12 Apr 2024
  • RC2: 'Comment on egusphere-2024-513', Anonymous Referee #2, 05 Apr 2024
Sérgio Silva Lopes, Marcelo Fragoso, and Eusébio Reis
Sérgio Silva Lopes, Marcelo Fragoso, and Eusébio Reis

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Short summary
This article analyses the characteristics of intense rainfall that gives rise to torrential floods in Madeira Island. There is a very strong correlation between maximum precipitation in 24 hours and 12 hours. Antecedent daily rainfall may also influence the occurrence of torrential floods. It was possible to obtain the identification of polynomial regression rules, which can be used to detect, in time and space, critical scenarios of torrential floods.