Numerical modelling of flash flood event in steep river using Telemac 2D and Sisyphe

Extreme weather events, natural disasters as well as
failure of climate change mitigation and adaptation are the risks
with the highest likelihood of occurrence and largest global
impact. Historically, the attention has been on floods in the
larger, slow responding watercourses. Due to a changing climate,
it is both expected and experienced more frequent and more
extreme rainfalls, creating violent flash floods in small
catchments. In steep rivers, this induces rapid changing
discharges and large water forces resulting in erosion and rivers
taking new courses, destroying communities and threatening
livelihoods and live. Municipalities are responsible for mapping
the risks natural hazards induce. When it comes to the risks due
to floods in steep rivers, there is still a lack of approach and
methodologies to handle their analysis. For mapping of such an
event, the identification of critical points where the water can
find the way from the river channel due to erosion and
sedimentation is important. Therefore, the methodology has to
include sediment transport and bed evolution in the river
channel and the inundation area of the flood.
In this study, the flooding of Utvik (western Norway) in July
2017 is reconstructed using TELEMAC – MASCARET
numerical simulating software. The longitudinal slope of the
river in Utvik is ca. from 3 to 17 %. A 2D numerical model of
Utvik was built. The proposed solution consists of hydrodynamic
simulation carried out in Telemac 2D and morphodynamic
simulation (i.e. including bedload transport and bed evolution)
using coupling of Telemac 2D and Sisyphe. Instabilities in bed
evolution were observed in the river channel with steep
longitudinal slope and steep river banks. To avoid the
instabilities, non-erodible bed was set-up in the river channel in
the final morphodynamic simulation. The results of the
simulations were compared with the documentation of the flood
event.