Spatial impact and triggering conditions of the exceptional hydro-geomorphological event of December 1909 in Iberia.

According to the DISASTER database the 20-28 December 1909 was the hydrogeomorphologic event with the highest number of flood and landslide cases occurred in Portugal in the period 1865-2010 (Zêzere et al., 2014). This event also caused important social impacts over the Spanish territory, especially in the Douro basin, having triggered the highest floods in more than 100 years at the river's mouth in the city of Oporto. This work aims to characterize the spatial distribution and social impacts of the December 1909 hydro-geomorphologic event over Iberia. In addition, the meteorological conditions that triggered the event are analysed using the 20 Century Reanalysis dataset from NOAA and precipitation data from Iberian meteorological stations. The Iberian Peninsula was spatially affected during this event along the SW-NE direction spanning from Lisbon, Santarém, Oporto and Guarda (in Portugal), until Salamanca, Valladolid, Zamora, Orense, León and Palencia (in Spain). In Iberia, 134 DIS15 ASTER cases were recorded (130 flood cases; 4 landslides cases) having caused a total of 89 casualties (57 in floods and 32 in landslides) and a total of 3876 people were affected, including fatalities, injured, missing, evacuated and homeless people. This event was associated with some outstanding precipitation values at Guarda station (Portugal) in 22 December 1909 and unusual meteorological conditions characterized by the presence of a deep low pressure system located over NW Iberian Peninsula with a stationary frontal system striking the Western Iberian Peninsula. The presence of an upper-level jet (250 hPa) and low-level jet (900 hPa) located on SW-NE oriented towards the Iberia along with upper-level divergence and lower-level convergence favoured large-scale precipitation. Finally, associated with these features it is possible to state that this extreme event was clearly associated to the presence of an elongated Atmospheric River, crossing the entire northern Atlantic basin and providing a continuous supply of moisture that contributed to enhance precipitation. This work contributes to a comprehensive and systematic synoptic evaluation of the second most deadly hydro-geomorphologic Disaster event occurred in Portugal since 1865 and will help to better understand the meteorological system that was responsible for triggering the event. [ABSTRACT FROM AUTHOR]