Urban Poverty & Climate Change in Dar es Salaam, Tanzania:

Flood Modeling
128
Annex 5
A flood modeling exercise was undertaken by Ardhi University to map potential changes
in rainfall regime and sea level rise on the flood extent and depth in the at-risk areas that
were covered in the socio-economic surveys. A combined 1D-2D hydrodynamic model
known as SOBEK was used (developed by Delft Hydraulics Software). The flood
propagation model required spatial data (including a digital elevation model (DEM) and
surface roughness estimates) and temporal data (such as initial water level, and
downstream and upstream boundary conditions).
Digital Elevation Model (DEM)
Aster Digital Elevation model (DEM) with 30m resolution was used in this study and the
30m grid size was resampled to 15m. The DEM was then converted to standard Arc Info
ASCII (.asc) format, required by SOBEK.
Surface Roughness
Surface roughness is another important spatial data parameter, indicating resistance to the
flow of water on a floodplain. The surface roughness map was derived from a land-use
map and was generated at the same resolution as the DEM to ensure that for each cell,
both elevation information and roughness values were available (Alkema et al., 2007).
Initial and boundary conditions
The initial condition describes the initial state of the system, in this case water levels and
fluxes at the start of the computation. In this study the restart file was used to define
initial condition of the model. The restart file was created by running the model starting
with dry conditions until the hydraulically stable starting point of simulation was reached
(Alkema et al., 2007).
Boundary conditions for flood model were applied to define the inflow and outflow
elements of the model domain. In this study, there were a number of upper boundary
conditions in which discharge data was used and one downstream boundary where a
constant water level was used.
Model schematization
Schematization involved adding the flood model components using a network editor
known as NETTER available in SOBEK. The model components added include: river
cross-section, flow calculation points, 1D boundary nodes (upstream and downstream),
1D-2D-internal boundary node, connection nodes, measurement stations, and hydraulic
structures (bridges). NETTER offers a way to assign attribute values for various added
nodes. The schematization starts by importing the 2D network, which represents the
terrain elevation in ASCII data format. For quick assessment of the model performance,
history boundaries (where water depth, and water velocity are recorded for a specific
pixel) were placed for selected areas. A 1D flow was represented by a series of crosssections
perpendicular to the direction of the river flow. The river is defined by reaches