A critical assessment of the performance of standard 2D flood models
based on results of 3D URANS simulations
D.V. Horna Munoz
IIHR-Hydroscience and Engineering, The University of Iowa, Iowa City, Iowa, United States of America
ABSTRACT: Evaluating the accuracy of 2D depth-averaged solvers to predict flood propagation in natural
environments is one of the most important challenges to mitigate floods. This paper discusses the performance
of SRH-2D, a standard 2D flood propagation solver in terms of predicted flood extent and depth-averaged
velocity profiles in a complex bathymetry river reach for high flow conditions. The accuracy of the 2D solver
is mainly evaluated based on comparison with results obtained using a 3D URANS two-phase flow model
developed using the commercial software STAR-CCM+. The 2D model performance is evaluated for a steadystate test case. The domain contains a 7-km reach of the Iowa River near Iowa City and 2 river dams. Even
though the SRH-2D depth-averaged velocities show the same pattern as the 3D depth-averaged results, the 2D
model tends to underpredict the location and magnitude of the peak unit discharge inside the channel, especially
in the regions where 3D effects are significant.
1 INTRODUCTION
The standard practice for simulating realistic flood
events is the application of numerical models based
on solving the 1-D, 2-D or hybrid 1-D/2-D shallow
water (Saint-Venant) equations. The use of 1-D codes
is highly popular since they are able to compute flood
events in natural environments in large domains in a
relatively short amount of time and using readily
available computing resources. 1-D codes assume the
flow is essentially unidirectional making them incapable of simulating lateral diffusion of flood waves
into the floodplain (Hunter, et al 2007). 2-D codes
overcome these limitations by solving the 2-D Saint
Venant equations, which are obtained by depth-averaging the Navier-Stokes equations. The main limitations of the 2-D Saint Venant equations arise from the
derivation of the equations, in which hydrostatic pressure distribution is assumed over the vertical direction. It is because of these limitations that 2-D codes
are unable to accurately predict mean flow and turbulence in regions where the flow is highly 3D and nonisotropic (flows with separation, flow around hydraulic structures, river confluences, etc.). It is well documented that the flow field increases its level of three
dimensionality during unsteady events such as floods,
especially in regions of high stream curvature, around
hydraulic structures and in the transition region be-
tween main channel and floodplain. These flow complexities make a strong case for the use of a 3-D nonhydrostatic Navier Stokes model with deformable
free-surface capabilities to simulate unsteady flood
wave propagation in the domain. Such models should
be able to offer a better representation of the mean
flow field across the domain. By simulating exactly
the same test case using the 3-D and the 2-D model,
one can get a better idea about the accuracy of the latter model. This is important, given the lack of extended validation data for flood predictions.
In the present paper, we report a case study in
which the flood extent and depth-averaged velocity
profiles were obtained in a 6-km reach of the Iowa
River near Iowa City (Iowa, United States of America) for steady state under high flow conditions. The
reach contains two dams (Fig. 1). The bathymetry and
topography information were provided by the Iowa
Flood Center. Results obtained using SRH-2D, a
standard 2-D depth-averaged shallow flow solver, are
compared to those obtained with STAR-CCM+, a 3D non-hydrostatic viscous solver using the k-ε turbulence model and a deformable free-surface module
based on the Volume-of-Fluid (VOF) method. A
mesh with 6 million cells was used in 3D simulation.
SRH-2D solves the full 2-D Shallow Water equations
with a parabolic turbulence model. Roughness parametrization was accounted by specifying regions with
different values of Manning’s coefficient across the
domain.
3 CONCLUSIONS AND FUTURE WORK
The results reported in this paper suggests that the
predictive capabilities of 2D solvers used to model
flood propagation can decay significantly in regions
where 3D effects are important. Future research plans
include performing an unsteady simulation in the
same river reach to assess the performance of the 2D
model during a fast extreme flood event.
Figure 1. Close-up view showing the start of computational domain (1’-1’), the end of the computational domain (2’-2’), the
location of the first river dam (2-2) and the location of the second
river dam (3-3).
2 RESULTS
Based on comparison with USGS gage at the only location where stage was measured, the results obtained
with the calibrated SRH-2D model overestimate the
free-surface elevation by approximately 1 ft (30 cm)
while STAR-CCM+ underestimate the measured
data. It is important to mention that while SRH-2D
had to be calibrated for low and high flow conditions,
STAR-CCM+ results were obtained with no calibration. At steady state, the difference in flood extent
area is approximately 21% with respect to the results
obtained with STAR-CCM+. Flood extents for
STAR-CCM+ (3D) and SRH-2D (2D) are shown in
Figure 2 and 3 respectively.
Qualitatively, there is little difference between the
flood extents. The most noticeable difference is observed near the area between cross-sections 1 and 2,
in which SRH-2D inundates a significant amount of
area more compared to STAR-CCM+. Areas close to
cross-sections 7 and 8 also show a slight difference
between the 2 solutions.
The full paper contains a detailed comparison between the unit-discharge profiles in representative
cross-sections between the 2D and 3D models. The
comparison will show that several regions are present
where fairly significant differences occur between the
two models. They generally happen in regions where
the degree of flow three-dimensionality is high (e.g.,
regions of high channel curvature, regions containing
large-scale deformations at the bed).
Figure 2. Aerial view of flood extent for results obtained with
STAR-CCM+
Figure 3. Aerial view of flood extent for results obtained with
SRH-2D
REFERENCES
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Blain, 99-108. Southhampton, UK: WIT Press, 2001
Hunter, N. M., Bates, P., Horrit, M., Wilson, M. (2007) “Simple
spatially-distributed models for predicting flood inundation:
A review” Geomorphology: 208-225