Historia del Coaching

Flood risk management is a continuous process of analysis, assessment and action to keep flood risk at an acceptable level against acceptable societal costs. Flood risk management strategies are combinations of measures (both structural and non- structural) to improve flood risk management as part of the sustainable development of a region. Therefore, to assess flood risk management strategies, decision criteria should not only include investment costs and flood risk, but also sustainability criteria such as equity, biodiversity and economic side-effects, and criteria to deal with uncertainty such as robustness and flexibility (De Bruijn et al. 2008). The method developed in this thesis allows quantifying system robustness to use it as additional criterion in flood risk management decision making.

When robustness is valued in addition to acceptable flood risk, the preference for a flood risk management strategy may change. Many strategies are possible and the most feasible one depends on the current system and the socio-economic and cultural context. Some countries may prefer an embanked river valley such as in the Netherlands (high resistance threshold), whereas other countries prefer a natural, unembanked floodplain such as in the Mekong delta (see De Bruijn 2005). De Bruijn showed that it is often expensive to enhance resilience in a system with already a high resistance threshold, because there is often less space due to the ‘levee effect’ (Tobin 1995, White 1945). The ‘levee effect’ means that well-protected areas attract socio- economic development, which increases the area’s vulnerability to flooding. This may require a higher protection level by raising the embankments, attracting even more socio-economic development, etc. Significantly reducing vulnerability in these systems requires large investments. This thesis has explored other possibilities to increase the resilience range, even if the resistance threshold is high. Promising measures to increase robustness in a highly resistant system include differentiating protection levels and strengthening existing embankments so they become nearly unbreachable. Although costs were not taken into account, it is believed that these measures are feasible in the context of the Netherlands.

Current flood risk management in the Netherlands is based on design standards for flood protection infrastructure. The flood-prone area is considered safe when this

protection can withstand water levels with a predefined return period. This is a risk- based approach in the sense that areas with potentially large impacts receive higher protection. An advanced risk-based approach, however, would also include measures to limit the potential flood impact. This is subject to ongoing debate in the Netherlands. In line with the European flood risk directive (European-Parliament 2007), the Netherlands National Government introduced the ‘multiple-tiered flood risk management approach’ (NationalGovernment 2009), which states that flood risk management should build on three tiers: 1) protection against flooding, 2) spatial planning to limit flood impact, and 3) emergency management to limit casualties. This approach acknowledges that flood risk can also be reduced by spatial planning measures. In theory, implementing spatial planning measures that reduce flood impact may avoid further raising and strengthening of embankments when flood risk otherwise increases due to climate change and socio-economic developments. In practice, however, raising embankments often seems more cost-effective than spatial planning measures, and therefore potential flood impact is still expected to increase in the future.

Although the multiple-tiered approach stimulates thinking about flood impact reduction, it may give the wrong impression that the first layer can only be used to avoid flooding, and that impacts can only be reduced by spatial planning (second layer). This thesis has shown that impacts can also be reduced by adapting the first layer; differentiating protection standards and unbreachable embankments are typical examples of measures that enhance protection against flooding and at the same time reduce the potential flood impacts. Furthermore, robust systems have shown to be better able to cope with extreme events, which may be an additional argument to choose measures that reduce the flood impact.

Recommendations for flood risk management

It is recommended to analyse a system’s robustness to a large range of plausible floods as part of a flood risk analysis. This thesis showed that robustness analysis adds to a narrow risk approach in flood risk management. Assessing an area’s flood risk solely on a single-value risk metric implies that as long as the probability is small enough, the

corresponding flood impact is acceptable. Additionally using the robustness criteria may aid to avoid societally unacceptable flood impacts.

It is recommended to define a wide range of flood magnitudes for which to provide system robustness. It is common practice to provide protection (resistance) against a limited range of flood magnitudes. Additionally, strategies to reduce the impact and/or increase the recovery threshold should be considered in order to deal with beyond- design flood magnitudes. The total range of flood magnitudes considered should be chosen large enough to include all plausible ones.

It is recommended to always consider measures aimed at flood impact reduction. From a robustness perspective, measures aimed at impact reduction are often desirable, even if the risk reduction is small. If measures reduce the impact for all extreme flood events, they automatically increase the robustness range.

It is recommended to manage flood risks for entire river valleys instead of individual dike ring areas. In this way, potential floods affecting the entire river valley are also taken into account and more vulnerable areas could be protected by allowing controlled flooding of less-vulnerable areas if this reduces the total impact in the entire river valley.

7.5 Implications and recommendations for drought risk