2.5. Inland Inundation Modeling
A well-established 2D hydraulic model (FloodMap-Inertial, Yu and Lane
2011) is utilized to derive coastal flood inundation for each scenario.
The model, as well as its earlier diffusion-based version (FloodMap, Yu
and Lane 2006a,b) have been tested and verified in a number of different
environments (e.g., Tayefi et al., 2007; Lane et al., 2008; Casas et
al., 2010; Yin et al., 2013, 2016, 2019; Yu and Coulthard, 2015). It
adopts a simplified 2D solution to numerically solve the inertial form
of the 2D shallow water equations by neglecting the convective
acceleration term in a raster-based domain. Inland flood routing takes
the same structure as the inertial algorithm of Bates et al. (2010), but
with a slightly different method to calculate the time step. Rather than
using a global Courant–Freidrich–Levy Condition where the time step
for the next iteration is calculated based on the maximum water depth
and velocity found at the last time step (e.g. Bates and De Roo, 2000),
the Forward Courant–Freidrich–Levy Condition (FCFL) approach described
in Yu and Lane (2011) for the diffusion-based version of FloodMap is
used in the inertial model to maintain simulation stability and minimize
numerical diffusion.
To apply the model for inland flood simulation, two types of data are
required: floodplain topography and boundary conditions. In terms of the
floodplain topography, a city-wide Digital Elevation Model (DEM)
constructed from 0.5 m topographic contours is available (Yin et al.,
2013), with a grid cell resolution of 50 m and a vertical accuracy of
0.1-0.2 m. The locations and heights of sea dikes along the coast,
provided by Shanghai Water Authority (SWA), are then overlaid onto the
original ‘bare earth’ DEM. An empirically-based uniform manning’s n
roughness coefficient of 0.06 is used in the simulation to represent the
effect of urban features (e.g. buildings) on flood routing. For each
synthetic storm, water level time series (every 30 minutes) at locations
spaced at 1000 m intervals along the coast, derived from the ADCIRC
model run, were used as flow boundary conditions to drive the flood
modeling. The performance of the coupled ADCIRC-FloodMap method was
investigated by Yin et al., (2016), where the ADCIRC-FloodMap method
demonstrated higher efficiency and accuracy in inundation prediction at
the city scale than using the ADCIRC model alone, which is particularly
important for hazard modeling involving thousands of simulations.