Methodology - Response of Flood Events to Land Use and Climate Change - page 77

Post process:
HEC GeoRAS &
Arc GIS & ILWIS

Pre process:HEC
GeoRAS & Arc GIS

Modeling

Cross section
RAS Geometry
Boundary
conditions
Roughness

Inundation
Velocity
Depth
Duration

1D (HEC RAS)
1D 2D
(SOBEK)

Fig. 3.13 Input and output information in HEC-RAS and SOBEK modeling

Roughness value: An estimation of channel and floodplain roughness

coefficients.
Geometric data: Modeling input data are included in spatial data and temporal
data. The primary geometrical inputs required for 1D modeling were flow path,
center line, main channels, banks, and cross section cut lines. The optional geo-
metrical input was the application of left and right dykes in urban areas. In order to
model flood propagation, a series of input data created with the HEC-GeoRAS
model were applied in HEC-RAS, such as cross sections of a Y-Z profile. The
location and extent of these cross sections were represented by the cross sectional
cut lines layer in the HEC-GeoRAS model, an ArcGIS extension. Figure 3.13
shows a schematic representation of the necessary input data required for both
modeling and output information as a result of the simulation.
Boundary conditions: They were defined in the form of upstream hydrograph
data and downstream water level. Boundary conditions were applied to define the
inflows and outflows at the model boundary.
Alemseged and Rientjes [ 57 ] defined the boundary conditions as in commonly
specified terms as inflow and outflow elements of the model domain. In both
modeling approaches, the same discharge hydrograph was used for the upstream
boundary condition. Due to lack of data and no available recorded downstream
data, an imaginary lake condition was assumed in the SOBEK model, which is set
to have a water holding capacity of up to 2 m from its immediate value of ele-
vation. To preserve the lake conditions, a constant water level was specified and a
free flow was allowed throughout the simulation [ 11 ].
It is assumed that it will constrain the flow in the model and could eventually
even create a backwater effect, as it seems the water returned back again to the
watershed. The ideal downstream boundary condition would be a rating curve that
reproduces the relational discharge and water level for any situation. In particular,
with detailed data, the HEC-RAS allows to include a rating curve, stage, and flow
hydrograph and the slope of river course as a downstream boundary condition.
Figure 3.13 represents the schematic figure of modeling processes and some input
and output information in HEC-RAS and SOBEK modeling.