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

6 Studies on Water Management Issues

Flooding from extreme rainfall events vary from upwelling groundwater levels, which occurs frequently but with little damage to very large inundations of land which occur less frequent but with lots of damage. Inundation due to extreme rainfall events occur when there is more rainfall than the water system in a specific area can handle (Hoes, 2007). There can be several reasons for this inundation: the rainfall is not able to infiltrate into the ground or not able to properly flow away, there is insufficient pumping capacity or there is a too

Besides the issue of storage capacity of a specific area, the duration of a rainfall event determines the amount of inundation. First, slow and fast reacting water systems can be distinguished in a specific area. Green houses and urban areas are for example fast reacting water systems, while pastures are an example of slow reacting systems and arable land responds usually between the two systems. For quick responsive water systems, a short period of several hours to several days with heavy rainfall is often necessary to have the land inundated, while the slow-reacting systems often need an event spread over several

Other important factors that determine the amount of inundation are the soil type, the water levels before a rainfall event and the time of occurrence in the year. The soil type determines how fast the rainfall can infiltrate in the ground. For example, if the soil is rich in sand, water can much more easily infiltrate then when the soil is rich in clay. The water levels before a rainfall event determine how much more water can be stored during the extreme rainfall event. High water levels before an extreme rainfall event means that less water can be stored, which results in faster inundation of the area. Furthermore, most of the extreme rainfall events, both short and large, occur at the end of the summer and in the autumn

The consequences of extreme rainfall can sometimes be relatively large. For example, the extreme rainfall event that occurred in the autumn of 1998 caused around half a billion euro in damage. Looking at the exposure, this can be relative large but also very local. In 1998, the south-western parts of the Netherlands had problems with this extreme rainfall event (Smits et al., 2004), while in 2006, the problems occurred at a much more local scale (i.e. minor flooding in Egmond aan Zee, which is a small town located in the northwestern part of the Netherlands). One of the models that is used to predict and determine the damage of

**Land-use type Probability criteria [1/yr]** 

Table 1. The probability that a certain land use type may become inundated. (Nationaal

Pastures 1/10 Agriculture 1/25 High quality agriculture and horticulture 1/50 Greenhouses 1/50 Urban area 1/100

**3.1 Flood risk of extreme rainfall events** 

days (Smits et al., 2004).

(Smits et al., 2004).

Bestuursakkoord Water, 2003)

little storage capacity in the area to store all the water.

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 of inundation up to several meters of inundation.

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 Bruijn, 2007).


Table 2. Flood probabilities for different areas in the Netherlands

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,

Comparing Extreme Rainfall and Large-Scale Flooding

risk related to extreme rainfall events.

damages and damage curves.

overflow.

Induced Inundation Risk – Evidence from a Dutch Case-Study 9

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

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

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

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

events) versus low probability/high damage (large-scale floods) (Merz et al., 2009).

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 infrastructure.

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.
