Geography Reference
In-Depth Information
4.3.2. Hydrodynamic model
The HEC-RAS model (Hydraulic Engineering Center - River Analysis System) was used to
define, for the full extension of the Séqua/Gilão River, the maximum flood levels to be
expected for 10- and 100-years recurrence periods. In the simulation it was assumed (i) a 1D
unsteady flow, (ii) inflow in the upstream boundary condition is defined by the previously
obtained design hydrographs, (iii) water level at the downstream boundary condition is
defined by expected spring tide levels at Faro bar and an additional climate change scenario
with a mean sea rise of 0.91 m was also used (mean sea rise value was imposed by national
authorities), (iv) flow resistance is approximated by the Manning-Strickler equation, (v)
densely urbanized areas are considered as non-effective flow areas,
i.e.
, where water
overflows and returns in the same river section and (iv) head loss in hydraulic structures
(
e.g.
, bridges) is due both to the head loss during flow along the hydraulic structure and the
localized head loss.
The main flow line was discretized by cross sections (STs) of the river, based in topographic
and bathymetric surveys. These STs were set where the river geometry showed important
changes and near hydraulic structures. Geometrical information of the latter was retrieved
from the structures final drawings and
in situ
measurements made for this purpose.
Riverbed material was classified by local observations (Chow, 1959). Maximum flood levels
in the Tavira urban area were obtained for the worst case scenario,
i.e.
, when the
downstream-moving river flood wave overlaps the upstream-moving tide wave. This case
scenario takes place when the river peak flow reaches the upstream boundary cross section
1 hour before the tidal high water occurs at the downstream boundary cross section.
4.3.3. Flood delimitation
The following cartographical data and the hydrodynamic model results were combined to
define the 10- and 100-years recurrence period flood-affected areas:
Portuguese Army topographical maps 1:25,000 and ortophotomaps;
Topographical map of Tavira with contour lines every 10 m;
Bathymetric survey of the Séqua/Gilão riverbed and the Tavira bar, Quatro Águas bar
and Gilão River.
The flood-affected areas were delimited for the 10- and 100-years return period and the 100-
years climate change scenario by retrieving, for each of the cross sections, the maximum
flood levels from the hydrodynamic model. After completion of this process, the areas
above the maximum flood which were within the flood delimitated area (island areas) were
trimmed out.
4.3.4. Comparison of the model results with observed flood levels
In the night of the 3
rd
of December 1989 a severe flood occurred in Tavira with 120 mm of
daily rainfall registered during that day at the São Brás de Alportel meteorological station.
