Simulation of flooding due to embankment breaches by Delft3D Flexible Mesh: Case study of coastal area in Shanghai

Abstract

As a low-lying
city, Shanghai faces threats from typhoon and spring tide under the condition
of climate change and land subsidence. With high water level at the toe, the
sea embankment is likely to be overtopped and breached, finally resulting in
inundation inland.  The objective of this
research is to study climate change and land subsidence effects on Shanghai
inland inundation due to dike overtopping and breaching under extreme weather
condition.  A hydrodynamic model and a
wave model have been established by Delft3D-FM and Delft3D respectively.
Through validations on historical events, the hydrodynamic model and wave model
are proved to be valid. The water level and wave condition along the coast,
which are concerned as the results of these two models, are also essential
inputs for overtopping and breach discharge calculation. In overtopping and
breach discharge calculation, the threshold of breaching is estimated as an
overtopping rate of 0.1 m3/m/s. The resulting overtopping and breach discharge
gives the boundary condition of the overland simulation. The inundation map
over Shanghai area can then be achieved by the overland simulation. A
sensitivity analysis of the breach widths is also done.   Ten hypothetical typhoon events are provided
by the Met Office Hadley Center under past and future climate conditions. These
cases are applied to the whole process to study the effects of climate change
on coastal flooding in Shanghai. The relative sea level rise is also considered
for both past and future climate conditions.   The results show that places with high water
level and low sea dike elevation are more likely to get high overtopping that
can finally result in breaching. For Shanghai city, such vulnerable places can
be found along Hangzhou Bay, especially in Jinshan District and the south-east
corner of Shanghai. Besides, the entrance of Shanghai Yangtze River Tunnel is
also vulnerable due to land subsidence. For some extreme cases, the whole
Shanghai coast is in danger.  For the
past climate and land elevation around the year 2000 with the wind speed return
period of 1.3 yr and the breach width assumed to be 300 m, it is simulated that
the maximum inundation area in Shanghai can be 1,805 km2 (33.3% of the
simulated area in Shanghai). In the future, given the challenge of climate
change and land subsidence, the sea level is relatively rising. The intensity
of typhoon will generally strengthen. For the future climate and land elevation
around the year 2100 with the wind speed return period of 4.5 yr, it is
simulated that the inundation area in Shanghai can be 3,388 km2 (62.4% of the
simulated area in Shanghai), which is almost twice of the inundation area
around the year 2000.  The breach width
also affects the inundation situation. If the breach width becomes larger, the
inundation situation will be worse. However, as the breach width grows, the
increase of the inundation area decreases.