Large wood debris that clogged bridges followed by a sudden release. The 2019 flash flood in Catalonia.

Francolí River (Catalonia, Iberian Peninsula). The aim is the reconstruction of October 2019 flash flood, that was documented through extensive field work: rainfall (300 mm in just a few hours), flood marks, times passage and witnesses' snapshots reports, channel changes, log drift (20,000 trees) woody debris at bridges, as well large damage six fatalities. methods are: hydrological model built for rainfall-runoff basin routing river, use hydraulic principles such flow waterfalls, against obstacles (trees), etc. finally 1D/2D free surface numerical models. uppermost 100 km2 produced discharges 700 m3/s (up to 50 m3/s/km2, locally). Three bridges failed, but their cascading failure (when one triggers next downstream) not proved. main widened more than 10 times, dragging away soil vegetation like bulldozer. resulting wood clogged two worsened inundation. An anomalous downstream, probably surge around 1090 m3/s, due jam narrow bridge, took lives. Water Authority now warning planners vegetated, torrential basins may cause catastrophic floods valley towns, if are sensitive debris.