Delta Programme scenarios
What agreements have been made regarding the use of flood scenarios in the strategy for national vital and vulnerable functions?
A flood scenario mainly focuses on two aspects: the flood water depth and the probability of a flood occurring. An important consideration with respect to the evacuation of people in the threatened area is the warning time in the event of a potential flood (along with the number of residents and the available road capacity infrastructure). After a flood has occurred, an important factor – with a view to the recovery time – is the duration of the flood: recovery can commence only when the water is gone.
In actual practice in the Netherlands, the vulnerability of a function is determined on the basis of three types of scenarios. The upper limit is marked by the highest possible flood scenario, which distinguishes a (combined) threat from the rivers and the sea with an annual flood risk estimate of 1:100,000 to 1:1,000,000. A more realistic picture is painted by the Risk Map scenarios (as reported to Europe) and the scenarios used in the Delta Programme. The difference between the two sets of scenarios is mainly a matter for connoisseurs. The differences in water depth are slight and possibly even fall within the margin of uncertainty of the model calculations. The Vital and Vulnerable Functions Managers Consultative Body has confirmed that all the national vital and vulnerable functions use the Delta Programme scenarios to determine their vulnerability and the appropriate impact-reducing measures.
The Flood Story Maps in the Climate Impact Atlas feature map images and stories about the two defining elements in flood scenarios: 1. flood water depth, 2. the probability of a flood occurring. In the story maps, the first element is found under the heading of Flood Depth. It features maps of the Delta Programme scenarios and includes a map indicating the difference in water depth between the Delta Programme scenarios and the upper limit (the highest possible flood scenario). This comparative map enables local authorities to consider whether there is reason for deviating from the basic rule of taking the Delta Programme scenarios as the point of departure for all the national vital and vulnerable functions. The second defining element, the probability of a flood occurring, is included under the heading of Localised Flood Probability.
The warning time has a major impact on the evacuation options: along the rivers, residents have relatively ample time to safely reach non-threatened areas; along the coast and lakes, there is little time. For the time being, the decision has been made to include maps showing the evacuation options and flood durations in the Climate Impact Atlas. These two elements of the flood scenarios are presented on maps in the Climate Impact Atlas viewer.
An imminent flood offers a choice of two evacuation strategies: staying or leaving. Given sufficient warning time, leaving – horizontal evacuation – will be the best option. However, the “Horizontal Evacuation” map shows that large parts of the Netherlands only have limited evacuation opportunities. In many cases, vertical evacuation (staying) will hold more promise. A prerequisite in such cases is the presence of sufficient (elevated) dry locations in the threatened area. This information is reflected on the “Vertical Evacuation” map. In the years ahead, the Security Regions will be setting down evacuation strategies at postcode level, in the context of the WAVE 2020 programme.
The Climate Impact Atlas presents the most relevant information that can be used to set priorities and develop meaningful impact-reducing measures in the spatial domain, supplementary to the preventative measures that are being taken under the flood risk management policy. To this end, the Atlas includes two maps reflecting the likelihood of success of impact-reducing measures: a potential success map of impact-reducing measures indicating the flood probability and water depth in the event of failure of primary flood defence systems, and a potential success map of supplementary impact-reducing measures in the event of failure of secondary flood defence systems. The choices made in the map material are intended to simplify the discussions about which flood scenarios to use. However, the fact that choices have been made entails that more information is available. For those who wish to know more and delve deeper: all the information is contained in the Landelijk Informatiesysteem Water en Overstromingen (National Water and Flooding Information System, LIWO).