**Surface Water and Groundwater Management**

**1** 

**Comparing Extreme Rainfall** 

**– Evidence from a Dutch Case-Study** 

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

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

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

**1. Introduction** 

them.

primary or regional defense structures such as dikes.

would cause if it occurred (Sayer et al., 2002).

**and Large-Scale Flooding** 

**Induced Inundation Risk** 

E.E. Koks, H. de Moel and E. Koomen

*VU University Amsterdam* 

*The Nederlands* 
