Flood defense systems can be defined as a series of flood defenses, and can be found in, for example, coastal and riverine regions. River flood defense systems can be defined as a series of multiple flood defenses alongside a river. An example of such a series of flood defenses can be found in the form of dike rings in the Dutch Rhine and Meuse river basin. Coastal flood defense systems can consist of a combination of defenses, sometimes even with multiple lines of defense. A typical combination, often found in estuaries, is that of a barrier separating a large water body (front defense) and levees surrounding the large water body (rear defense). Examples of this type of coastal flood defense system can be found in Lake IJssel and the Eastern Scheldt in the Netherlands, and in Neva Bay, close to Saint Petersburg in Russia.
Flood defenses in a flood defense system can influence each other’s flood risk. For example, flood defenses in a river flood defense system share a common denominator, a river, and any significant change to the river characteristics can influence the flood risk of each of the flood defenses in that same system. One such change to river characteristics can be due to breaching. The effect of failure, or breaching, of a flood defense can have a significant effect on the flood risk of other flood defenses in the same system downstream of the failed flood defense.
Because flood risk plays an important role in determining economically optimal safety targets, a change in flood risk will also influence the economically optimal safety targets. Therefore, incorporating the flood defense system behaviors into economically optimal safety targets for each flood defense in its system can lead to a more efficient allocation of funds regarding flood protection. In a broader perspective, the risk & optimization methods that include system behavior can also be used to contemplate economic growth being dependent on the spatial development of flood defenses.