**3. Differences between flood risk resulting from extreme rainfall events and large-scale floods**

In this section, a number of conditions will be described that need to be taken into account to make a consistent comparison between the flood risk induced by extreme events and that resulting from large-scale floods since it is important to assess both types of risk in a comparable way. This should be done to avoid methodological biases as much as possible. Besides the issue of probability (large differences in probability for both forms of risk), there are many more fundamental differences between inundation due to heavy rainfall and large scale flooding with respect to their processes, consequences, exposure and the way they are dealt with. These differences should first be identified properly in order to allow for proper comparisons from which appropriate conclusions can be drawn.

Comparing Extreme Rainfall and Large-Scale Flooding

mostly the responsibility of Regional Water Boards.

of inundation up to several meters of inundation.

**3.2 Flood risk of large-scale flooding** 

annual damage per pixel.

Bruijn, 2007).

Induced Inundation Risk – Evidence from a Dutch Case-Study 7

extreme rainfall events, is the model made by Hoes (2007), which determines the expected

For extreme rainfall events, a number of probabilities have been authorized in the 'Nationaal Bestuursakkoord Water' (2003), which are necessary to take into account in the comparison between the two different types of risk. Important to note that in the Netherlands the probabilities of extreme rainfall events define the safety norms that describe how often different land uses are allowed to inundate. In Table 1 an overview of the different probabilities is given. Finally, policy on water management in the Netherlands is

Large-scale flooding can be defined as a temporary covering of land by water outside its normal confines due to flooding or breaching of primary flood defenses, which can result in large inundation depths, high damages and even human casualties (Kok and Klopstra, 2009). Important to notice for floods in the Netherlands, is that the Dutch area is divided into so-called dike rings. Dike rings are areas that are surrounded by levees, dunes or other higher areas that protect the inner area of the dike ring from flooding. Large-scale flooding happens when a dike or dune cannot stop the water from flowing into the inner part of a dike-ring. This happens when water levels exceed the height of the defense or when the water barriers breach. The results of large-scale floods can vary from only a few decimeters

Different safety norms apply to different dike-ring areas, which are described in the 'Water Protection Act'. These safety norms can be defined as the probability of occurrence of a certain water level and wave conditions that are higher than the dike or dune, as described in Table 2. Important to note is that these water level exceedence probabilities are different than the flood probabilities. It can happen that a dike or dune already breaches before the water level is higher than the dike or dune due to various failure mechanism (Vrijling, 2001; RWS-DWW, 2005). This means that the probabilities which are described in the 'Water Protection Act' are not always the exact flood probabilities. Also other factors, such as the probability of breaching and thus the strength of the dike at a certain place play a role (de

Furthermore, as discussed in section 3.1, a number of other factors should be taken into account when describing flood risk. The consequences of large-scale floods are in general quite large. For example, the economic damage of the flood of 1953 was, in present value,

**Area Probability criteria [1/yr]**  Coastal areas Between 1/2000 to 1/10000 Areas along large lakes Between 1/2000 to 1/4000

Areas along tidal rivers 1/2000 Other areas among main rivers 1/1250

Table 2. Flood probabilities for different areas in the Netherlands
