Atlas of Flood Maps

EU Flood Risk Maps Q4310 September, 2007
Examples from EU countries
WL | Delft Hydraulics
events. Maps normally cover only a certain village or community.
More detailed information can be found on the internet 1 .
Comments on the maps
Four maps are shown as examples of flood maps in Austria for the same region (fictitious
region ‘Muster’, Figure 4.2 - Figure 4.5). As a general comment the maps are very clear and
especially the cadastre background of the urbanization and infrastructure make them easy to
read. It may be useful for anyone not familiar to the region to have a small map added that
shows the location within Austria. Another comment is that the North indication on the map
is rather small and may obtain a more prominent position, especially in this case where
North is not at the top of the map.
In Figure 4.2 both flood extension and expected water depth are given. In fact both water
depth and water level (absolute value) are given. The latter might give an indication of the
flow direction, although a flood extension map is normally a representation of a static
situation, not of the inundation process. For the flood extension, three standard return
periods of 1/30, 1/100 and 1/300 yr are used that are the norm in Austria, but the water
depth is given for the 1/100 yr flood event. The differences between the extensions
corresponding to the three return periods are shown by symbols on lines instead of a system
of coloured surfaces. As an alternative use can be made of an indication of altitude similar
to topographic maps, either with colours and/or putting the actual value (30 – 100 – 300)
within the lines.
The water depths are given in an interval of 0.25 m, which is probably in line with the
accuracy of the information, but the combination of colours is less common. The smallest
depth (0 - 0.25 m) is shown in light blue and darker hues of blue are used for larger depths
up to 1.25 m, but then a shift is made to green with increasing depth indicated by lighter
hues (up to 3.00 m). Green is normally used as an indication of safety and as was explained
in Chapter 3.1. Water depth is preferably indicated by hues of blue. In case there are many
intervals the differences between the various shades of blue may become obliterate and it
might be an option to change to a larger interval (e.g. 0.5 m) instead of using a combination
of colours.
Although it can be deduced from the legend that the flood extension and water depth in
numbers refer to the event with a return period of 1/100 yr, this is not indicated in the
subtitle of the map.
In Figure 4.3 flood hazard zones are indicated using four levels of hazard (blue, orange,
yellow and red). Although red is normally the highest hazard, it is not immediately clear
from the map what the order of the hazards is. The colour blue is already being used for
inundation depth and may be left out of the colour palette here. It is very useful in this map
that the hazard zones are combined with land-use information, although the overall use of
the same colour for this purpose (green) does not allow for an easy distinction between
various types of landuse. There is however also another map available showing these hazard
zones on top of an aerial photograph.
1 http://www.wassernet.at/
4—2