**3.3 Comparison of the different conditions**

When looking at the two sorts of flood risk described in the previous sections, a number of important differences can be determined, wherefore in Table 3 an overview is given. In this section, these dissimilarities will be further explained.


Table 3. Overview of the differences between extreme rainfall events and large-scale floods

A number of observations can be made based on the table. First, there is a large difference in probability of occurrence. While the flood risk related to extreme rainfall events has a relatively high probability of occurrence, flood risk resulting from large-scale flooding has a

around one billion euro (van Veen, 2005). With large-scale flooding, there is not only damage to crops and sewers, but also human casualties and damage to buildings and

Looking at exposure in the case of large-scale flooding, it is usually limited to one or maybe two dike rings or part of a dike ring, due to safety measurements before a flood will occur or during a flood. These safety measurements are for example strengthening of closely located weak parts of the dike, closing the breach or the closing of possible weirs that are in the area. In the Netherlands, the HIS-SSM ('Hoogwater Informatie Systeem - Schade- en Slachtoffermodule') is commonly used for the determination of the flood risk of rivers and sea. With the HIS-SSM model, expected damage and the expected amount of casualties because of large-scale floods can be calculated (Kok et al., 2005). Another model, the Damage Scanner, is a simplified model of the HIS-SSM that calculates the expected damage of a large-scale flood (de Bruijn, 2006; Klijn et al., 2007). Whilst the HIS-SSM model calculates the damage per object, the Damage Scanner calculates the damage per land-use class (van der Hoeven et al., 2009). Finally, in the case of large-scale floods, policy is mostly made by 'Rijkswaterstaat', which is a governmental institution that is responsible for national water management and the roads of national importance in the Netherlands.

When looking at the two sorts of flood risk described in the previous sections, a number of important differences can be determined, wherefore in Table 3 an overview is given. In this

**Factor Extreme rainfall event Large-scale flood**  Occurence Relatively frequent Relatively unfrequent

Exposure Relatively unlimited Relatively limited

Type of water Fresh water Fresh, salt or brackish water

Models Hoes (2007) HIS-SSM and Damage Scanner

Table 3. Overview of the differences between extreme rainfall events and large-scale floods

A number of observations can be made based on the table. First, there is a large difference in probability of occurrence. While the flood risk related to extreme rainfall events has a relatively high probability of occurrence, flood risk resulting from large-scale flooding has a

Safety norms Actual inundation Possibility of overflow

Amount of inundation Few decimeters Few meters

Human casualties None to few Few to many

Costs of prevention Relatively low Relatively high

Policy Regional Water Boards Rijkswaterstaat

infrastructure.

**3.3 Comparison of the different conditions** 

section, these dissimilarities will be further explained.

Impact Low High

Flow speed Low High

relatively low probability of occurrence. When looking at the differences in impact (damage), we see that extreme rainfall events have a relatively low impact in comparison with large-scale floods, which have a much higher impact. With these two conditions in mind, there is now one clear difference: high probability/low damage (extreme rainfall events) versus low probability/high damage (large-scale floods) (Merz et al., 2009).

Second, while exposure for extreme rainfall events concerns almost the whole of the Netherlands (extreme precipitation can happen anywhere), exposure to large-scale flooding is relatively limited since it is confined to those areas contained within the dike rings. Also important is the amount of inundation of both forms of flood risk. Whilst the inundation of extreme rainfall events is most of the time much lower than that of large-scale floods, usually a few decimeters, the inundation for large-scale floods is much higher (up to a few meters). Not only the amount of inundation determines the damage though, but also the speed of the water flow. A high speed will usually cause much more damage, especially in terms of human casualties. With extreme rainfall events, there is usually very little or almost no flow speed, while large-scale floods can have very high flow velocities, especially near the breach. The occurrence of human casualties is an important difference between the two forms of risk. For large-scale floods the chances of human casualties are much higher than for extreme rainfall events. There can also be a difference in the 'type' of water that inundates the area. While extreme rainfall events mainly involve fresh water, large scale floods are usually salt or brackish water. The latter is especially for agriculture much more harming than fresh water inundation (Nieuwenhuizen et al., 2003). Finally, flooding from extreme rainfall events mainly occurs due to minor bottlenecks in the regional water system, while flooding from large-scale floods mainly occur due to failure of primary water defenses. Due to this difference, for extreme rainfall events minor (relative cheap) measurements are expected to prevent flooding, while for large-scale floods much larger (and more expensive) measurements are expected to be implemented. Nevertheless, Kok and Klopstra (2009) found in a simple cost-benefit analysis that the cost-effectiveness of reducing the risk of large-scale floods is in general much higher than that of reducing the risk related to extreme rainfall events.

There are also clear differences in the probability criteria. As described before, the safety norms of extreme rainfall events are not only higher than those of large-scale floods, there is also a clear difference in the interpretation. The safety norms for extreme rainfall events mean the minimum probability that there will be an actual inundation, while the safety norms for large-scale floods are defined as the levels at which the dikes could possibly overflow.

Another important difference is the determination of flood risk, since both types of flood risk are determined in different models that use different input parameters to determine the risk. For extreme rainfall events, the damage model of Hoes (2007) has been developed, while for large-scale floods, the HIS-SSM of Kok et al. (2005) is most commonly used. While looking at these two models, there are already a few differences. Not only different inundation maps are used to determine the expected inundation (e.g. starting at different depths), but also different land-use maps with different land-use classes are used. While in the model of Hoes many more agriculture classes are used, the HIS-SSM provides more variety in urban classes. Other differences are observed in the definitions of maximum damages and damage curves.

Comparing Extreme Rainfall and Large-Scale Flooding

Annual Damage (EAD).

**4.2 Land-use and inundation data** 

the different flood probabilities.

included in airport and seaport.

high water levels the 'Delta Works' will close.

Induced Inundation Risk – Evidence from a Dutch Case-Study 11

damage, the standard deviation and the total area per land use. Once these damages are calculated, the final outcome can be determined. As described in the introduction, the flood risk is determined by multiplying the flood probability with the consequences, which can be described as the maximum amount of possible damage in a specific area that is calculated in the integrated flood risk model. The final outcome is the flood risk in terms of Expected

The key inputs to this model come from two different maps. One is the land use map and the other is the inundation map. For the land use map, a new land use map is made which is a combination of the land use map from Land Use Scanner (described in Riedijk et al., 2007 and used in the Damage Scanner) and the 'Landgebruikskaart Nederland' (LGN4, used in the model of Hoes, 2007). The former are derived from a land use model that is applied to simulate land use changes and that is mainly focused on urban areas (see, for example, Koomen et al., 2008 and Koomen and Borsboom-van Beurden, 2011). The latter dataset is more focused on agriculture and distinguishes more classes in these categories (de Wit and Clevers 2004; de Wit 2003; van Oort et al. 2004). Since extreme rainfall events mainly damage agriculture but large-scale floods also damage urban areas and infrastructure, we combine those two to cover enough land-uses for both types of flood risk. The other map we use is the inundation map, which shows us the maximum inundation in a specific area for

The combined land use map contains 25 different land-use classes which can be aggregated into four major land-uses: urban land-uses, agriculture, nature and infrastructure. The urban land-uses consist of five classes: Urban - high density, Urban - low-density, Urban - rural, Commerce and Building lot. Where 'Urban - high-density' are the main cities and towns (like Amsterdam or The Hague), 'Urban - low density' are suburbs and villages (like Egmond aan Zee) and 'Urban – rural' are farms and large houses between pastures and along rural roads. Commerce is all the commercial areas within the Netherlands. The agricultural land-uses consist of nine classes: Greenhouses, pastures, corn, potato, beet, wheat, orchard, bulbs and other agriculture. The nature land-uses consist of seven classes: fen meadow, forest, sand/dune, heath, peat/swamp, water and other nature. Finally, the infrastructure land-uses consist of three classes: Airport, seaport and infrastructure, where the 'infrastructure' class are all the roads, railways and other infrastructure that is not

The inundation maps depict the inundation of extreme rainfall events or large-scale floods. These maps show the inundation in a specific area for different return periods, varying from a probability of 1/10 to a probability of 1/40000. The inundation maps used in this study for large-scale floods, which are calculated for different scenarios, are obtained from the province of Zeeland. The inundation maps can be subdivided into four scenarios: 1/4000 with RTC, 1/4000 without RTC, 1/400 with RTC and 1/40000 with RTC. "RTC (Real Time Control) is a module in the SOBEK model which allows the system to react optimally to actual water levels and weirs, sluices and pumps'' (Deltares, 2010). Important to note is that for the 'North Sea-side' of Noord-Beveland all four scenarios are used, while for the 'Oosterschelde-side' only the first two scenarios are used. This is due to the fact that with

Of final importance are the differences in policy. Whilst for extreme events the Regional Water Boards are responsible for policy making, is 'Rijkswaterstaat' responsible for the policy making with large-scale floods. Due to this difference, other criteria or other processes are seen as important for flood policies.
