Flood risk management
Sea-level rise, land subsidence (Storyline: Sinking deltas) and more frequent extreme rainfall and storm events enhance the risk of flooding in deltas. Flood risk management is required to maintain the population’s safety and to prevent and compensate for economic damage. The first parts of this story are on flood risk management in the Rhine-Meuse delta and mainly focus on river flooding. Subsequently, we zoom-out to other deltas and discuss the challenges of coastal flooding.
A long history of flood protection
In the low-lying and densely populated Rhine-Meuse delta in the Netherlands, flood protection has a long history. Since the Middle Ages, the Netherlands has been constructing dikes to protect the population against flooding. As flood intensity and frequency vary, flood risk management requires an assessment of costs and benefits of protection measures per region. After a major flood disaster in 1953, the national government started building the Delta Works which consist of an extensive system of dams, sluices and storm surge barriers. Moreover, they adopted stricter legal safety standards for primary flood barriers based on occurrence probabilities of certain peak water levels.
Risk-based safety standards
In 2017, these probabilistic standards have been revised into risk-based safety standards which combine a basic level of protection of the population living behind a dike, prevention of substantial economic damage and the limiting of flood protection costs. The new safety standards have been established for dike segments instead of entire dike rings. Also, the new approach considers multiple failure mechanisms of the primary embankments, such as piping or other processes that lead to dike instability even before it overtops. By 2050, all primary flood defences must meet these new safety standards.
Every 12 years, dikes must be inspected to assess whether they fulfil the safety standards. In case a dike needs to be reinforced, the responsible water authority can apply for subsidy for the reinforcement according to the national water protection programme. Every six years, the European Flood Directive additionally requires all member states to perform a risk assessment and, where necessary, update flood risk maps and management plans.
The traditional way of flood risk management needs to be revised
Current flood risk management in the Dutch part of the Rhine-Meuse delta has been very successful with minimal flood-related casualties and damage. "However", Willemijn van Doorn-Hoekveld, researcher in public, water and liability law, argues, “the traditional way of management needs to be revised as there is limited space in the landscape for standard dike reinforcement in order to adapt to sea-level rise and changing river discharge". Additionally, there is high risk in relying solely on flood protection measures considering the deep uncertainty about the rate of sea-level rise and the extent of changing river discharge. Moreover, there is currently little room for natural processes and related biodiversity in flood risk management. Dikes, for instance, hinder sediment deposition in embanked polders (Storyline: Sediments matter).
Sustainable, flexible and integral management
There is increased attention on integrated flood risk management to address these issues. This new perspective requires innovative measures that often work with nature to increase the resilience of both nature and society against floods. These innovative measures aim to enable multiple functions, such as offering possibilities for recreation, nature conservation and housing. A more integral flood risk management approach also includes a diversification of strategies e.g. multi-layer safety. It comprises:
- Water system management (flood defence and mitigation, e.g. protection infrastructure and retention basins)
- Spatial planning (flood prevention and mitigation, i.e. land and risk planning; Storyline: Spatial adaptation in deltas)
- Disaster management (flood preparation, e.g. forecasting and alert)
- Compensation for reconstruction (flood recovery, e.g. solidarity principle and insurance)
However, as Carel Dieperink, senior researcher in multilevel water governance, argues, “Several challenges emerge in cases in which an area wants to opt for more diversification and innovative measures. Addressing these challenges ask for well-functioning coordination mechanisms”.