Overall Conclusion and Discussion - Response of Flood Events to Land Use and Climate Change - page 222

In order to have valuable and basic information available for any future
management, it is necessary to provide the authorities, stockholders, other scientists,
and the public with basic flood hazard maps which contain fundamental information
for any environmental, technical, economical, and political decisions regarding
flood event management. This shows the potential adverse consequences associated
with different flood scenarios for civil protection purposes. The results of different
scenarios are discussed as follows:
DEM scenario:
• The DEM must be a continuous surface that includes the bottom of the river
channel and the floodplain to be modeled. Because all cross-sectional data
should be extracted from the DEM, only high-resolution DEMs accurately
representing the ground surface should be considered for hydraulic modeling.
The calculated water velocity by SOBEK and HEC-RAS models shows that this
hydraulic parameter decreases with increasing DEM pixel size. This is related to
the fact that the model considers more roughness and friction as it uses the
higher resolution DEM. A major disadvantage of using the low resolution DEM
is the loss of some features such as dykes and levees. These features play an
important role on flood model simulation results. Visualization analysis shows
that, due to a low resolution of 10 m, compared to 5 m, the generated river
network for 10 m deviates more and does not agree well with the actual
drainage paths. But with the increase of horizontal and vertical resolutions, the
delineated river networks from 5 m DEM come in close agreement to the actual
flow path. Even in flat urban areas, the results of 5 m resolutions are more
satisfactory compared to 10 m resolutions. The results of the 10 m DEM sim-
ulation show that the flow distribution shifted from the actual flow paths in
locations of the generated river network. It was obvious that the flood mainly
occurred due to overflow in the rivers, which was perhaps due to the negligence
of the locally elevated lands such as dykes and roads in the model.
• The idea behind resampling the DEM was to estimate how much the resampled
data were close to the data source. As a general conclusion, in terms of RMSE
analysis, better results in the accuracy of the model were obtained when a DEM
was used after resampling. The mean error in resampled DEM was 4.292, much
less than the value of 25.928 for DEM before resampling. Therefore, as an
overall result, it could be concluded that a low resolution DEM could show
better and more accurate results when it is resampled to a higher resolution.
River Change Scenario
• Regeneration of past events was the major concern in this research. But with the
well-calibrated model based on observed data for the 2008 flood event, the
regeneration of other past events was possible and convincing enough.
The results were validated by available photos from the past event. The error
behind the results is due to a lack of high spatial and temporal data resolution,
especially in hourly discharge and in the correlation between observed hourly
water height and discharge.