Environmental Engineering Reference
In-Depth Information
Fig. 12.4
Fully 2D model of the River Severn, UK. From Horritt and Bates (2002).
types. There are therefore benefits in making use
of these data andknowledge by continuing to build
1D river models or to use existing ones. In addi-
tion, the grid resolution needed to model a river in
2D is significantly finer than what is typically
applied on floodplains, resulting in significantly
increased computation times. These reasons ex-
plain the current enthusiasm for combined 1D/2D
modelling for river and floodplain systems (see
'Hybrid 1D/2D methods' below).
2007a), or the two-equation k-
e
model (Namin
et al., 2004). No appropriate methodology exists
to calibrate viscosity in flood inundation models,
because calibration data at an adequate level of
detail do not exist. The viscosity coefficient is
sometimes also used to introduce additional arti-
ficial viscosity to the flow in order to enhance
model stability.
The prediction of flow (velocity and flood wave
celerity) is crucially dependent on the friction
parameter values adopted in the model. Applica-
tions of 1Dmodels benefit from decades of hydro-
metric data collection, and user experience in
model calibration and validation; floodwave prop-
agation (at least in the case of in-bank floods) is
nowpredicted by 1Dmodels with an accuracy that
can be considered excellent for many engineering
applications. Nevertheless the issue as to whether
models should be parameterized using engineer-
ing judgment informed by experience, or simply
by calibration, or even by an ad hoc combination
of both, is still debated in the literature (Beven
2000; Cunge 2003). The parameterization of fric-
tion in 2D models benefits to some extent from
the knowledge and experience available in 1D
Model parameterization and terrain geometry
Parameterization
The friction coefficient (e.g. Manning's n) is the
main parameter for which values are required to
be set in 2D flood flow modelling. As already
mentioned, eddy viscosity is usually considered
a secondary parameter and is therefore often ig-
nored. When not ignored, viscosity may be dealt
with using a constant viscosity coefficient (see,
e.g., Sauvaget et al. 2000; Danish Hydraulic Insti-
tute 2007a), the Smagorinsky viscosity formula-
tion (Syme 1991; Danish Hydraulic Institute
