Reconstruction and simulation of an extreme flood event in the Lago Maggiore catchment in 1868
2018
Abstract. Heavy precipitation on the south side of the central Alps
produced a catastrophic flood in October 1868. We assess the damage and
societal impacts, as well as the atmospheric and hydrological drivers using
documentary evidence, observations and novel numerical weather and runoff
simulations. The greatest damage was concentrated close to the Alpine divide and Lago
Maggiore. An atmospheric reanalysis emphasizes the repeated occurrence of
streamers of high potential vorticity as precursors of heavy precipitation.
Dynamical downscaling indicates high freezing levels (4000 m a.s.l.),
extreme precipitation rates (max. 270 mm 24 h −1 ) and weather dynamics that agree well with observed
precipitation and damage, and with existing concepts of forced low-level
convergence, mid-level uplift and iterative northeastward propagation of
convective cells. Simulated and observed peak levels of Lago Maggiore differ
by 2 m, possibly because the exact cross section of the lake outflow is
unknown. The extreme response of Lago Maggiore cannot be attributed to low
forest cover. Nevertheless, such a paradigm was adopted by policy makers
following the 1868 flood, and used to implement nationwide afforestation
policies and hydraulic structures. These findings illustrate the potential of high-resolution,
hydrometeorological models – strongly supported by historical methods –
to shed new light on weather events and their socio-economic implications in
the 19th century.
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