**6.1 Total damage and flood risk estimates**

In this section, a few results will be examined more closely. First, the differences between the total damage and flood risk for extreme rainfall events and large-scale floods will be examined, for respectively the 'current situation' and the safety norms. Second, the differences between the 'current situation' and the safety norms will be discussed.

If we examine the results of the 'current situation' carefully, we see two important differences. One is the difference in exposure. While for extreme rainfall events, the area of exposure is almost the whole dike-ring area, for large-scale floods the inundation area is

Calculating the damages showed that the highest damages occurred in the urban – low density areas and to infrastructure. The total damages in the agricultural land uses are almost the same as the total damages seen in Table 7 with extreme rainfall events. In Figure 7, we see that even though the dike or dunes breach at all the possible locations, not all areas are inundated. For example, a few areas in the middle are not inundated. Furthermore, the damage map clearly shows the location of villages and infrastructure, because these are the

Fig. 7. Damage map for a large flood in Noord-Beveland (dike ring fills up completely)

annual damage of approximately 97,000 euro per year.

integrated flood risk model and our analysis.

**6.1 Total damage and flood risk estimates** 

**6. Discussion** 

For large-scale floods in this scenario, the total damage and flood risk are described in Table 8, where a total damage can be seen of approximately 388 million euro and an expected

**Return Period Total damage Flood risk in terms of EAD Total area (x €100,000) (x €100,000) (ha)**  1/4000 3880 0.97 5598

In this section we provide some critical discussion on the structure of our model and our results. First, we consider the results from the study area according to the 'existing situation' and the safety norms. Second, we identify possible methodological issues with the

In this section, a few results will be examined more closely. First, the differences between the total damage and flood risk for extreme rainfall events and large-scale floods will be examined, for respectively the 'current situation' and the safety norms. Second, the

If we examine the results of the 'current situation' carefully, we see two important differences. One is the difference in exposure. While for extreme rainfall events, the area of exposure is almost the whole dike-ring area, for large-scale floods the inundation area is

differences between the 'current situation' and the safety norms will be discussed.

Table 8. Total damage and flood risk for a large flood in Noord-Beveland

areas that incur the highest damages.

limited to a much smaller area. Reasons for this are that there are higher areas (roads or inner dikes) within Noord-Beveland that act as secondary defenses and that simply not much water flows into the dike ring after a breach near the dunes (where the difference in elevation between the water level and the land surface is limited). The other difference is in the damage distribution. While for extreme rainfall events the highest damages are found in agricultural land uses, the highest damages for large-scale floods mainly are found in urban areas and infrastructure.


Table 9. Total damage and flood risk for all the different scenarios

In Table 9, all the total flood risk values are listed for the 'current situation'. In the table can be seen that flood risk for extreme rainfall events is much higher than the flood risk of largescale floods, which is remarkable since the total damages of extreme rainfall events are in general much lower than that of large-scale floods. The main reason for this is that even though the total damages are much lower for extreme rainfall events, the probability of occurrence is much higher. This is an interesting result, since much more policy has been made to prevent or mitigate the chance of large-scale floods (Kok and Klopstra, 2009).

If we examine the results with respect to the safety norms closer, we see the same differences as in the 'current situation'. In Table 10, we see that even though the damages of large-scale floods are higher, the flood risk in terms of annual expected damage is much lower. This comparison is more interesting because of the dissimilarities between the two types of flood risk, described in section 3.3. The probabilities and safety norms for extreme rainfall events can be interpreted as 'accepted risk'. In other words, the area is allowed to inundate with these probability levels. Also important is to take into account what the effect is for both events on for example the insurances, indirect damage, human casualties and the social disturbance. These effects are not taken into account in the calculated annual expected damage but have, especially for large-scale floods, a very high effect on the total impact. Taking these unquantified effects into account would probably bring both types of risk

Comparing Extreme Rainfall and Large-Scale Flooding

**6.2 Methodological issues** 

damages will affect both estimates.

different types of flood risk.

**7. Conclusion** 

Induced Inundation Risk – Evidence from a Dutch Case-Study 23

The model used in this study seemed very useful when determining flood risk for both extreme rainfall events and large-scale flooding. But several methodological issues remain that should be taken into account. First, the aggregation of the built up areas in the land use maps could have been better. There are only three different urban land use classes, while more differentiation would be desirable. Second, there could have been more detailed investigation about the maximum damages for a number of land use classes. For a number of classes, determining the maximum damages was sometimes difficult, even though it was calculated with HIS-SSM damage maps and literature studies. Therefore, more research and investigation is required to provide consistent estimates of the maximum damages. Third, it was difficult to develop the model with different inundation maps as inputs. Several

adjustments must been made to fit the different inundation maps in the same model.

areas with different land uses are similar to those reported in this study.

compare the different types of flood risk in a plausible and consistent way.

Netherlands to test if this is the case for more dike rings.

Also important to take into account is that the model only has been used for determining the flood risk of a small, specific area. It is interesting to see whether results for larger areas or

Finally, it is always hard to validate the model in a consistent way when observation data is lacking. Since there have not been many large-scale floods or extreme rainfall events, it is hard to test if the model calculates realistic absolute damage estimates. As both types of risk are estimated using the same model, the influence of any bias in, for instance, maximum

The main objective of this study was to create a common methodology to assess flood risk of extreme rainfall and large-scale flooding in the Netherlands. Based on the literature we were able to incorporate both types of flood risk within an integrated model that allowed us to

We then applied the model to analyze flood risk in the 'Noord-Beveland' area. Results show that even though the highest total damages are found to result from inundations of largescale floods, the flood risk of extreme rainfall events are in general much higher when both are expressed in terms of annual expected damage. The reasons are that extreme rainfall events cause larger areas to inundate and occur with a higher probability, which combines to drive up flood risk. Further investigation should be done in other parts of the

Our model does not quantify some types of indirect damage, such as human casualties and social disturbances. These should be taken into account to provide an even more consistent comparison. We expect that they would have increase the damage associated with largescale floods. Nonetheless, we question whether the difference large difference (64 times) would be completely bridged by these additional effects. Aside from these unquantified factors, there are a number of data comparability issues, such as the differences in exposure and the distribution of the damage, which should also be kept in mind when comparing

Even though the model requires further refinements our initial results suggest it is possible to compare different forms of flood risk within an integrated model. Our finding that higher


closer together. It is, however, questionable whether the difference would completely be bridged by these additional effects given the large (64 times) difference.

Table 10. Total damage and flood risk for the safety norm maps

It is important to note that the inundation maps with respect to the safety norms for both types of events are hypothetical. For extreme rainfall events, it is not plausible that the whole area inundates with the same height, since the norm is a lower limit and many areas will probably be much safer than the norm. Similarly, for large-scale floods the compartmentalization within the dike-ring area will probably prevent the whole area from inundating unless there are many dike failures at all sides. Nevertheless, by contrasting these situations we could compare the types of risk as they are 'allowed' by current policy.

It is interesting that the total damage calculated with the integrated flood risk model is much lower than the total damage calculated with the Damage Scanner and the HIS-SSM for dike-ring 28. The total damage calculated with these latter models is respectively 583 and 653 million euro (Klijn et al., 2007), while the total damage calculated in the integrated flood risk model is only 388 million euro. That figure is more in line with total damage estimates of around 400 million euro determined by Klijn et al. (2004) and van der Klis et al. (2005). One explanation for the higher damages in the Damage Scanner and the HIS-SSM could be the difference in cell size. In the Damage Scanner, the grid cell size is 100x100 meter, instead of 25x25 meter in the integrated flood risk model, which can result in higher damages because of aggregation of multiple land uses in one grid cell, resulting from overestimation of residential land in the aggregation process. This can also be seen when one compares the land use map used in this study with the land use map used in the Damage Scanner; the amount of residential and commercial land-use is 7.5 per cent higher in the Damage Scanner than in the land use map used in this study. This overestimation results from the fact that residential land tends to dominate, but not completely fill cells at a coarser resolution, as has also been observed by Bouwer et al. (2009). Another reason could be that in the integrated flood risk model a greater variety of agricultural land uses are used with much lower maximum damages. For these classes, much lower maximum damages are chosen because it is not likely that inundations of more than 0.5 meter will cause any more damage to agricultural crops. Finally, the HIS-SSM model calculates the damages by using objects. In the integrated flood risk model, objects such as tractors and other agricultural machines are not taken into account, which results in lower damages.

Even though high flood risk values are found for extreme rainfall events in Noord-Beveland, this does not mean that this will also be the case for the rest of the Netherlands. Since Noord-Beveland has much agriculture, not many urban areas and many secondary defenses, it is not very representative for the rest of the Netherlands. For instance the 'Randstad' area (middle west of the Netherlands) is much more urban and will therefore probably have a different comparison of the examined types of flood risk.
