**1. Introduction**

Flood risk is an important force in shaping land use patterns. Attention for flood risk is even more important in view of climatic changes that will impact sea-level rise, river discharge and precipitation patterns. Flooding typically results from two types of events: extreme rainfall events and large-scale floods. The former can be defined as inundation due to more rainfall than the water system in a specific area can handle and the latter as a temporary covering of land by water outside its normal confines due to flooding or breaching of the primary or regional defense structures such as dikes.

In recent history, the Netherlands has seen a number of events with both extreme rainfall and large-scale flooding. For example, an extreme rainfall event occurred in 1998 that caused substantial damage in the southwestern part of the Netherlands (Smits et al., 2004) and a large-scale flood almost occurred in 1993 and 1995 (Wind et al., 1999). Extreme rainfall events generally have a high probability of occurrence but a low impact, while large-scale floods have a low probability but a high impact (Merz et al., 2009). It is, therefore, interesting to compare the flood risk for both types of events. Flood risk is interpreted here as the product of the probability of a certain flood event and the (economic) impact that the event would cause if it occurred (Sayer et al., 2002).

Recent studies have already made initial progress with comparing flood risk from largescale floods and extreme rainfall events (van Veen, 2005; Merz et al., 2009). Other studies examined several aspects differing between extreme rainfall events and large-scale floods, including the regional differentiation in precipitation, communication, types of measures, possibilities to reduce flood risk and the possibility of insurance (Kok and Klopstra, 2009). It is still not known, however, how the flood risk (in terms of expected annual damage) of extreme rainfall exactly compares to the risk of large-scale floods. Since both inundation due to extreme rainfall and large floods from the sea or river can cause economic damage, it is interesting and valuable to calculate them in a consistent way in order to compare them.

Comparing Extreme Rainfall and Large-Scale Flooding

(see www.lgn.nl and Riedijk et al., 2007)

**large-scale floods** 

Induced Inundation Risk – Evidence from a Dutch Case-Study 5

Fig. 1. Land use map of 'Noord-Beveland'. Based on the LGN4 and Land Use Scanner maps

**3. Differences between flood risk resulting from extreme rainfall events and** 

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.

At the moment, more policy measures are taken to mitigate and prevent large-scale floods in the Netherlands, than to mitigate extreme rainfall events (Kok and Klopstra, 2009). Due to climate change, there is the expectation that not only the occurrence of extreme weather events will increase in the future (IPCC, 2007), but also a possible increase in large-scale floods (Milly et al., 2002; Te Linde et al., 2010). This can result, in combination with various socio-economic changes, in an increase in economic damage from floods (Bouwer et al., 2010; De Moel et al., 2011). Therefore, the risk of both extreme rainfall events and large-scale floods are both likely to increase. In order to prioritize it is interesting to have both types of flood risk calculated in the same way.

The objective of this research is to assess the flood risk, in terms of 'annual expected damage' (AED) of inundation due to extreme rainfall and large floods in a consistent way in order to compare the respective types of risk. Therefore, it is interesting to know to what extent flood risk from extreme rainfall events and large-scale flooding can be compared and how they relate to each other. To do this, the main objective of this research is to make an integrated model to compare the different types of flood risk in a plausible and consistent way.

In section 2, the study area, 'Noord-Beveland', which will be used to test the model, will be briefly discussed and described. In section 3, we then discuss the different conditions that must be taken into account to be able to put the comparison of the different forms of flood risk in a proper perspective. In section 4, the methodology will be described that was used to make to integrated flood risk model. In section 5, the results will be described after using the data from the study area as input for the integrated flood risk model. In section 6, the results will be discussed to see whether or not the model matches our expectations. Finally, in section 7, conclusions will be drawn.
