A postwildfire debris flood in Gragnano, southern Italy, on September 11, 2024

Abstract

On September 11, 2024, a debris flood hit the urban area of Gragnano (in Naples) in the Lattari Mts. of southern Italy and resulted in the evacuation of nine families. This event was triggered by a storm that occurred 1 month after a wildfire affected two catchments located along the northern slopes of Mt. Pendolo. The increasing frequency of debris floods in peri-Vesuvian areas and effects of this event led to this analysis. In this study, we analyze the event and assess its magnitude and conditions of development in relation to the preceding wildfire. Field observations were supported by unmanned aerial vehicle (UAV)-aided photography, LiDAR data acquisition, digital elevation models (DEMs), and satellite imagery interpretation. Rainfall data and runoff modeling were also used. The results indicated that (i) the wildfire affected two catchments of 0.041 km 2 and 0.075 km 2 that contributed sediment to the debris flood, (ii) the wildfire severity ranged from moderate to moderate–high, (iii) the triggering rainfall produced a total of 24.7 mm of rain over a duration of 80 min, (iv) the sediment transported by the event contained mostly pumices, (v) the inundation area corresponded primarily to major roads and extended for a total of 0.05 km 2 , (vi) the total gross sediment volume entrained into the flow was estimated to be ~ 4100 m 3 , and (vii) the clear-water peak discharge was estimated to be 4.6 m 3 /s for the eastern catchment and 2.7 m 3 /s for the western catchment. These results provide a better understanding of events that are becoming increasingly frequent with ongoing climate change.

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