Methodology - Response of Flood Events to Land Use and Climate Change - page 70

3.9.2 Modeling Approaches
There are different types of flood modeling approaches:
1D model: In the 1D model, the principle is that the water level, velocity, and
discharge only change parallel to the stream direction, and flow characteristic
variation in any other direction is ignored. These models strongly depend on the
geometric data at selected locations and floodplain and assume that river geometry
for selected river sections only changes linearly along these sections.
2D model: In the 2D model approach, the principle is that the flow propagations
change along two flow directions (x and y directions). The main advantages for
using 2D model approach is that the flow in heterogeneous areas such as flood-
plains may be better represented in simulations, but flows across wide river beds
are also not only in one direction but they need a more detailed representation of
cross sections. Consequently, there are some problems with surface roughness
parameterization which causes the calibration of 2D models to be relatively
complex [ 57 ]. The 2D floodplain or overland flow module is based on a solution
for the continuity and momentum equations.
1D-2D model: A coupled 1D-2D model approach is when the channel flow is
simulated by a 1D representation and floodplain flow is simulated by a 2D rep-
resentation, which is expected to benefit from advantages of both 1D and 2D
models. The 1D-2D model is integrated by a 1D channel flow module with a 2D
overland flow module. In the 1D-2D model, the 2D floodplain layer is overlaid
with a 1D river layer, where layers are geometrically connected through map
coordinates at the center point of a 2D floodplain grid element and the center
points of 1D river section. The center points of the floodplain grid layer and the
center points of the river grid layer lead the calculation algorithm in the numeric
calculation procedures. In this case, the connectivity between channel and flood-
plain allows for overtopping of the river to floodplain inundations in case of a big
flood occurrence [ 57 ].
3.9.3 HEC-RAS: Calculation Procedures
The 1D modeling approach is based on the 1D solution of the De Saint-Venant
equation [ 58 ], such as the HEC-RAS model developed by the US Army Corps of
Engineers. When considering channel flow, it is assumed that the flow behavior
can be satisfactorily described as unsteady flow (flow characteristics may change
over time) in one spatial dimension by two state variables: velocity (u) and water
depth (h) as functions of time (t) and space (x)[ 59 ].
To solve u and h, two independent equations are required and usually the
continuity equation (based on the conservation of mass principle) and
the momentum equation (based on the conservation of momentum principle) are
applied. The equations derived by De Saint-Venant in 1871 are shown below in
their Eulerian form per unit width of channel with no lateral inflow [ 3 ]: