modeling the flood level of the January 2000 storm, the results were shared with city
engineers, planners and other municipal officials for a qualitative validation. The maps
of flooding extent were in agreement with flood limits observed by officials during the
event. Three water levels were used to generate flood risk maps from the LIDAR DEM:
1) the peak water level for the 21-22 January 2000 storm event (4.23 m CD); 2) the
coming 100 years (4.93 m CD).
In order to map flood limits in this project, it was also necessary to relate the
LIDAR elevation data to hydrographic Chart Datum (CD), the vertical datum used in
the tide-gauge records from which storm-surge flood levels are obtained. This datum is
approximately equivalent to the lowest astronomical tide and varies from place to place
around the coast. The local Chart Datum at each secondary port has typically been
related to a local benchmark near the tide gauge, often a permanent marker on the wharf
or nearby structures. A separate component of the Prince Edward Island project focused
on determining the vertical differences between Chart Datum, the ellipsoid, and
CGVD28 throughout the study area (King et al., 2002). In the case of Charlottetown,
Chart Datum was determined to be 1.685 m below CGVD28, the vertical reference for
the LIDAR DEM. Thus, a simple translation of the 21-22 January 2000 water level and
future water levels to the DEM was made by subtracting 1.685 m from the water levels
referred to Chart Datum.
4.2 GIS FLOOD MODELING OF STORM-SURGE WATER LEVELS
Many sophisticated numerical models have been developed for simulating tidal
hydraulics and these can be particularly useful for flooding of tidal reaches in rivers. In
the present study of coastal flooding, it was decided to use existing GIS and image
processing capabilities combined with the LIDAR DEM to visualize the potential areas
of flooding. Galy and Sanders (2002) recently used a similar approach where they used
a DEM derived from a SAR data to map flood risk along the River Thames in the
United Kingdom.
In our study, the initial DEM was built using tools within Arc/Info and the data
were transferred to PCI Geomatics image processing software for visualization and the
generation of raster flood risk maps. The maps were then transferred back to Arc/Info
for final vector processing and overlay analysis. It was assumed that a given water level
from the storm-surge event would form a horizontal flood plane extending landward
from the open harbour. Thus hydraulic effects, associated time lags, or flood expansion
or dampening were not considered in the modeling effort.
With the water levels now referenced to CGVD28, a model was written to
threshold the DEM into two classes for a given water level, one wet and one dry. This
initial threshold procedure did not include any connectivity checks with the source
harbour area. The resultant raster image was converted to vector polygons. With the
flood extent data in this form, it was quite simple to select only those polygons that
were connected to the open harbour, thereby excluding low-lying areas landward of
barriers that would check the spread of the flood. Specific conditions such as bridges
and causeways with culverts had to be dealt with individually. Areas separated from the
harbour in the LIDAR DEM by an apparent barrier such as a bridge were included in
the flooded areas. Other less obvious situations, such as causeways with culverts, were
more problematic and local municipal officials were consulted in these cases. Many of
the culverts are equipped with one-way valves to allow water to flow toward the
same storm event superimposed on a moderate sea-level rise (4.73 m CD); and 3)
the same storm event superimposed on a realistic estimate of relative sea-level rise over the
175
Airborne Laser Altimetry
engineers, planners and other municipal officials for a qualitative validation. The maps
of flooding extent were in agreement with flood limits observed by officials during the
event. Three water levels were used to generate flood risk maps from the LIDAR DEM:
1) the peak water level for the 21-22 January 2000 storm event (4.23 m CD); 2) the
coming 100 years (4.93 m CD).
In order to map flood limits in this project, it was also necessary to relate the
LIDAR elevation data to hydrographic Chart Datum (CD), the vertical datum used in
the tide-gauge records from which storm-surge flood levels are obtained. This datum is
approximately equivalent to the lowest astronomical tide and varies from place to place
around the coast. The local Chart Datum at each secondary port has typically been
related to a local benchmark near the tide gauge, often a permanent marker on the wharf
or nearby structures. A separate component of the Prince Edward Island project focused
on determining the vertical differences between Chart Datum, the ellipsoid, and
CGVD28 throughout the study area (King et al., 2002). In the case of Charlottetown,
Chart Datum was determined to be 1.685 m below CGVD28, the vertical reference for
the LIDAR DEM. Thus, a simple translation of the 21-22 January 2000 water level and
future water levels to the DEM was made by subtracting 1.685 m from the water levels
referred to Chart Datum.
4.2 GIS FLOOD MODELING OF STORM-SURGE WATER LEVELS
Many sophisticated numerical models have been developed for simulating tidal
hydraulics and these can be particularly useful for flooding of tidal reaches in rivers. In
the present study of coastal flooding, it was decided to use existing GIS and image
processing capabilities combined with the LIDAR DEM to visualize the potential areas
of flooding. Galy and Sanders (2002) recently used a similar approach where they used
a DEM derived from a SAR data to map flood risk along the River Thames in the
United Kingdom.
In our study, the initial DEM was built using tools within Arc/Info and the data
were transferred to PCI Geomatics image processing software for visualization and the
generation of raster flood risk maps. The maps were then transferred back to Arc/Info
for final vector processing and overlay analysis. It was assumed that a given water level
from the storm-surge event would form a horizontal flood plane extending landward
from the open harbour. Thus hydraulic effects, associated time lags, or flood expansion
or dampening were not considered in the modeling effort.
With the water levels now referenced to CGVD28, a model was written to
threshold the DEM into two classes for a given water level, one wet and one dry. This
initial threshold procedure did not include any connectivity checks with the source
harbour area. The resultant raster image was converted to vector polygons. With the
flood extent data in this form, it was quite simple to select only those polygons that
were connected to the open harbour, thereby excluding low-lying areas landward of
barriers that would check the spread of the flood. Specific conditions such as bridges
and causeways with culverts had to be dealt with individually. Areas separated from the
harbour in the LIDAR DEM by an apparent barrier such as a bridge were included in
the flooded areas. Other less obvious situations, such as causeways with culverts, were
more problematic and local municipal officials were consulted in these cases. Many of
the culverts are equipped with one-way valves to allow water to flow toward the
same storm event superimposed on a moderate sea-level rise (4.73 m CD); and 3)
the same storm event superimposed on a realistic estimate of relative sea-level rise over the
175
Airborne Laser Altimetry