Multivariate statistical modelling of compound flooding events for
southern African coasts
Abstract
In March 2019 cyclone Idai led to a compound flooding event in
Mozambique combining high river runoff and storm induced water level
extremes and causing damages up to 2 billion USD and more than 1200
fatalities. The co-occurrence of storm surges, wind waves, and flooding
through heavy precipitation and runoff increases the risk of flooding
and exacerbates the impacts along the vulnerable Southern African
coasts. To mitigate the associated high-impacts, it is essential to know
the probability of theses compound events (multivariate extreme event
analysis) and understand the processes driving them (Wahl et al. 2015).
In the project CASISAC*, we propose a regionalized multivariate
assessment of extreme events to model extremes as combination of storm
surge, waves, river discharge and high precipitation at southern African
coasts (Namibia, South Africa, Mozambique). We develop a multivariate
statistical model based on copulas to represent and analyze the physical
mechanism underlying compound events and their return periods under
present day climate conditions. The African regions are particularly
poor sampled by tide gauges and available records include large gaps. To
overcome the data scarcity, ancillary data from high-resolution ocean
model hindcasts (Schwarzkopf et al. 2019) based on the NEMO model will
complement the analysis. An integrated analysis of observational records
from tide gauges in combination with ocean model hindcasts allows us to
regionalize compound extreme events corresponding to certain return
periods along the entire coastline. In this presentation, we will show
preliminary results of the multivariate extreme value model analysis
chain. *CASISAC (Changes in the Agulhas System and its Impact on
Southern African Coasts: Sea level and coastal extremes) is funded by
the German Federal Ministry of Education and Research (BMBF) under the
grant number 03F0796C. Schwarzkopf et al. (2019): The INALT family – a
set of high-resolution nests for the Agulhas Current system within
global NEMO ocean/sea-ice configurations. In: Geoscientific Model
Development, Vol. 12, 7, 3329-3355, doi: 10.5194/gmd-12-3329-2019. Wahl
et al. (2015): Increasing risk of compound flooding from storm surge and
rainfall for major US cities. In: Nature Climate Change, Vol. 5, 12,
1093-1097, doi: 10.1038/nclimate2736.