**5.1.2 Large-scale floods**

For large-scale floods, two scenarios are used to determine to the total damage in the area. One scenario is a flood that results from a dune breach at the 'North sea-side' of 'Noord-Beveland', the other scenario is a flood that results from a dike breach at the 'Oosterscheldeside' of 'Noord-Beveland'. For the 'North sea-side' the flood scenario is sub-divided into four more sub scenarios, which are 1/4000 with RTC, 1/4000 without RTC, 1/400 with RTC and 1/40000 with RTC. For the 'Oosterschelde-side', the flood scenario is sub-divided into two more sub scenarios, which are 1/4000 with RTC and 1/4000 without RTC (see section 4.2). The breach at the 'North Sea-side' is chosen because there is simply only one place where the dune could breach. The breach at the 'Oosterschelde-side' is chosen since this section in the dike has not been reinforced yet and has therefore at the moment a higher possibility to breach compared to other dike sections at the 'Oosterschelde-side'.

After determining the damages for the dune breach at the North Sea, results for this scenario show that the highest damages occur in the agricultural areas. In Figure 4, which shows the damage in the area with respect to the four sub scenarios, it can be seen that the flood from the North Sea mainly inundates the western part of Noord-Beveland. This area mainly consists of agricultural areas (see Figure 1). Only the inundation with the probability of 1/40000 inundates a much larger area, including a village. This can be seen in the damage map as a much darker spot in the middle of the area that inundates.

For the other breach location at the 'Oosterschelde-side', it is interesting that the results show major differences between the two sub scenarios. In the sub scenario with the RTCmodule, there is much more damage. Especially the damage in the infrastructure changes

in the northwestern part of the area. This is mainly due to the fact that there are higher inundation levels in these areas and agricultural land uses that undergo damage at even low inundation levels. If we compare this with the land use map of the region (Figure 1), we see

that these are all agricultural crops (wheat, beet, and grass).

Fig. 3. Damage maps for the four return periods for extreme rainfall events

possibility to breach compared to other dike sections at the 'Oosterschelde-side'.

map as a much darker spot in the middle of the area that inundates.

For large-scale floods, two scenarios are used to determine to the total damage in the area. One scenario is a flood that results from a dune breach at the 'North sea-side' of 'Noord-Beveland', the other scenario is a flood that results from a dike breach at the 'Oosterscheldeside' of 'Noord-Beveland'. For the 'North sea-side' the flood scenario is sub-divided into four more sub scenarios, which are 1/4000 with RTC, 1/4000 without RTC, 1/400 with RTC and 1/40000 with RTC. For the 'Oosterschelde-side', the flood scenario is sub-divided into two more sub scenarios, which are 1/4000 with RTC and 1/4000 without RTC (see section 4.2). The breach at the 'North Sea-side' is chosen because there is simply only one place where the dune could breach. The breach at the 'Oosterschelde-side' is chosen since this section in the dike has not been reinforced yet and has therefore at the moment a higher

After determining the damages for the dune breach at the North Sea, results for this scenario show that the highest damages occur in the agricultural areas. In Figure 4, which shows the damage in the area with respect to the four sub scenarios, it can be seen that the flood from the North Sea mainly inundates the western part of Noord-Beveland. This area mainly consists of agricultural areas (see Figure 1). Only the inundation with the probability of 1/40000 inundates a much larger area, including a village. This can be seen in the damage

For the other breach location at the 'Oosterschelde-side', it is interesting that the results show major differences between the two sub scenarios. In the sub scenario with the RTCmodule, there is much more damage. Especially the damage in the infrastructure changes

**5.1.2 Large-scale floods** 

from 22.3 million euro to 1.7 million euro in the sub scenario without the RTC-module. When looking at Figure 5, we see that the main reason for the higher damages is that there is a much larger area that inundates in the sub scenario with the RTC-module even though the inundation depth is lower.

Fig. 4. Damage maps for the four different sub scenarios with the 'North Sea breach'

Fig. 5. Damage maps for the two different sub scenarios with the 'Oosterschelde breach'

Finally, the flood risk per sub scenario was calculated (Table 6). At first, we see the highest total damages in the 'Oosterschelde sub scenario 1/4000 with RTC' and the 'North Sea sub scenario 1/40000 with RTC'. This is mainly due to the fact that, as described above, a much larger area inundates with a lot more urban area in both these sub scenarios and a lot more infrastructural areas in the first sub scenario. If we closer examine the flood risk values, we see the highest flood risk in the 'North Sea' sub scenario 1/400 with RTC and the 'Oosterschelde' sub scenario 1/4000 with RTC. The reason why the first sub scenario has a much higher flood risk is because it has a much higher probability of occurrence. The reason why the latter has a high flood risk is simply because there are very high total damages.

Comparing Extreme Rainfall and Large-Scale Flooding

the damage map are associated with pastures.

0.165 meters

**5.2.2 Large-scale floods** 

Induced Inundation Risk – Evidence from a Dutch Case-Study 19

Figure 6 shows the spatial distribution of the damage. In this figure, there can be seen that in urban areas the highest damages occur, which is consistent with the data for this scenario. The figure also highlights the difference in agricultural land uses. Much of lighter areas in

**Land use Maximum damage Flood risk in terms of EAD Area** 

1 - Urban - high density 2.9 0.03 0.56 2 - Urban - low density 340 3.4 118 3 - Rural area 62 0.6 105 4 – Commerce 6.7 0.07 3.7 7 – Infrastructure 210 2.1 330 8 - Building lot 1.3 0.01 2.1 9 - Holiday accomodation 7.4 0.07 25.6 11 – Pastures 140 14 1305 12 – Corn 28 1.1 157.4 13 – Potato 250 10 1363.4 14 – Beet 170 6.8 938 15 – Wheat 280 11.2 1538 16 - Other agriculture 220 8.8 1203 17 – Orchard 140 5.6 138.7 20 – Forest 0 0 59.7 21 - Sand/dune 0 0 1.7 23 - Peat/swamp 0 0 1.8 24 - Other Nature 0 0 57 25 – Water 0 0 4

*Total 1860 64 5598*  Table 7. Total damages and flood risk for an extreme rainfall event with an inundation of

To determine what the maximum damage will be in 'Noord-Beveland' when looking at the safety norms for large-scale floods, the Risk Map for the Netherlands. For the creation of this map it was assumed that the complete dike ring inundates in case of flooding up to a level

where flood water would spill out of the dike ring (RWS-DWW, 2005).

**(x €100,000) (x €100,000) (ha)** 


Table 6. Flood risk for all the sub scenarios with large-scale floods

#### **5.2 Safety norms**

#### **5.2.1 Extreme rainfall events**

After looking at the current situation, it is also interesting to see what the maximum damage could be if we assume that the probability of flooding equals exactly the safety standards for every cell, regardless of breach scenarios or the local water system. The safety norms for extreme rainfall events, described in Table 7, imply that different areas are allowed to inundate with different probabilities. In Table 7, we see the maximum damages and flood risk per land use if all the land is inundated with 0.165 meters of water. This inundation level is chosen because this is the average inundation above ground level for all four inundation maps.

Fig. 6. Damage maps for an extreme event in Noord-Beveland (inundation of 0.165 meters)

In Table 7, the annual expected damage is calculated per land use, according to the different safety norms described in Table 1. If we look at the maximum damages, we now see that highest amount of damages are in the urban areas and infrastructure, which is in contrast with the highest damages in the 'current situation' where we saw that the highest damages were found in the agricultural land uses. Important to note is that the damage in agricultural land-uses are still much higher than in the 'current situation'. When examining the flood risk more closely, we see that the highest flood risk occurs in agricultural areas. This is mainly due to the fact that these areas have low safety norms.

**(x €100,000)** 

**Flood risk in terms of EAD** 

**(x €100,000)** 

**Return period Total damage** 

1/4000 without RTC 162 0.04 1/400 with RTC 137 0.3 1/40000 with RTC 575 0.014

1/4000 without RTC 224 0.06

After looking at the current situation, it is also interesting to see what the maximum damage could be if we assume that the probability of flooding equals exactly the safety standards for every cell, regardless of breach scenarios or the local water system. The safety norms for extreme rainfall events, described in Table 7, imply that different areas are allowed to inundate with different probabilities. In Table 7, we see the maximum damages and flood risk per land use if all the land is inundated with 0.165 meters of water. This inundation level is chosen because this is the average inundation above ground level for all four

Fig. 6. Damage maps for an extreme event in Noord-Beveland (inundation of 0.165 meters)

In Table 7, the annual expected damage is calculated per land use, according to the different safety norms described in Table 1. If we look at the maximum damages, we now see that highest amount of damages are in the urban areas and infrastructure, which is in contrast with the highest damages in the 'current situation' where we saw that the highest damages were found in the agricultural land uses. Important to note is that the damage in agricultural land-uses are still much higher than in the 'current situation'. When examining the flood risk more closely, we see that the highest flood risk occurs in agricultural areas.

This is mainly due to the fact that these areas have low safety norms.

North Sea 1/4000 with RTC 162 0.04

Oosterschelde 1/4000 with RTC 943 0.2

Table 6. Flood risk for all the sub scenarios with large-scale floods

**5.2 Safety norms** 

inundation maps.

**5.2.1 Extreme rainfall events** 

Figure 6 shows the spatial distribution of the damage. In this figure, there can be seen that in urban areas the highest damages occur, which is consistent with the data for this scenario. The figure also highlights the difference in agricultural land uses. Much of lighter areas in the damage map are associated with pastures.


Table 7. Total damages and flood risk for an extreme rainfall event with an inundation of 0.165 meters

### **5.2.2 Large-scale floods**

To determine what the maximum damage will be in 'Noord-Beveland' when looking at the safety norms for large-scale floods, the Risk Map for the Netherlands. For the creation of this map it was assumed that the complete dike ring inundates in case of flooding up to a level where flood water would spill out of the dike ring (RWS-DWW, 2005).

Comparing Extreme Rainfall and Large-Scale Flooding

 **Return period Total damage** 

 1/10 9.5 95000 1/25 30 133000 1/50 60 125000 1/100 100 99000

North Sea 1/4000 with RTC 162 4000

 1/400 with RTC 137 34000 1/40000 without RTC 575 1400 Oosterschelde 1/4000 with RTC 943 24000 1/4000 without RTC 22.4 6000

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

areas and infrastructure.

Extreme rainfall events

Large-scale floods

Induced Inundation Risk – Evidence from a Dutch Case-Study 21

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

**(x €100,000)** 

1/4000 without RTC 162 4000

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

**Flood risk** 

**(in terms of EAD)** 

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 areas that incur the highest damages.

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

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 annual damage of approximately 97,000 euro per year.


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