Graduation of Joost Trommelen

How can the Dynamic Adaptive Policy Pathways (DAPP) approach be used to create and select effective flood risk strategies under highly uncertain conditions? 

• Professor of graduation: Prof. dr. ir. S.N. Jonkman

• Supervisors: Dr. ir. C. Mai Van, Dr. ir. M.M. Rutten, ir. M. Bos 

The rising sea level and increased storm surges because of climate change threaten coastal areas globally. 570 cities and some 800 million people are by 2050 estimated to be exposed to this when emissions do not decrease (UCCRN, 2018). It is, however, deeply uncertain if and to what extent emissions will decrease. Additionally, the effects of climate change are not fully understood due to its complexity resulting in a wide range of uncertainty. An effective flood risk strategy is necessary to prevent these cities and people to be heavily affected by the changing climate. Flood risk strategies are not only influenced by the changing climate but also by factors like: 

- Current water levels, 

- Discount rate, 

- Subsidence, 

- Economic growth rate, 

- Population growth, 

- Damages, 

- Costs of measures, 

- Inflation rate, and 

- Effectiveness of measures. 

These factors are of uncertain nature which requires this flood risk strategy to be flexible and/or robust. An approach to identify and implement such a strategy is the use of Dynamic Adaptation Policy Pathways (DAPP). DAPP is an approach made out of the concepts of Adaptation Pathways and Adaptive Policy-making. The approach aims to provide decision-makers with insight in what action to take when. DAPP is an approach that has been developed in research. The approach has for that purpose been applied to simplified illustrations, but not yet to more detailed master-planning resulting in practical issues. This results in the main research question of this thesis: 

How can the Dynamic Adaptive Policy Pathways (DAPP) approach be used to create and select effective flood risk strategies under highly uncertain conditions? 

DAPP requires the creation of Adaptation Pathways. Pathways can vary in terms of the time of implementation, safety level and (sequence of) measures leading to a near-infinite number of possible flood risk strategies. A framework that automatically creates these pathways has been created. The solution space of this framework is restricted by a selection of measures and sequences. The solution space is further reduced by applying an economic optimisation of safety levels. This optimisation be done by maximising the (annual) NPV, maximising the Benefit-Cost Ratio or minimising the Equivalent Annual Cost (EAC). Deterministic input of the factors influencing the desired flood risk strategy is required to perform this optimisation and create the pathways. The sea level rise, discount rate, socio-economic growth rate turn out to have a considerable impact on the obtained pathways. 

As not all possible futures are considered when using deterministic input, it can occur that the best performing pathway in design conditions does not perform comparably in a different future. Investments can be postponed to reduce the range of uncertainty in the input. However, it is not possible ii to postpone investments when a flood risk reduction strategy has to be chosen to prevent the actual safety level from dropping below the required safety level. A robust strategy is desirable in that case. The robustness can be assessed by conducting a probabilistic assessment. This probabilistic assessment shows the performance of a pathway in the full range of possible futures. Flexible and/or robust pathways will have a narrow ranges of outcomes in comparison to inflexible and/or frail pathways. Choosing such a pathway reduces the chance of underperformance. As no distribution can be assigned to climate scenarios, a sea level rise-scenario has to be chosen as design condition. Whether over-/under-adaptation is desired depends on the combination of factors that can influence the flood risk and is therefore project-specific. In general, it can be sad that under-adaptation is favorable for projects in which the minimum required safety level is above the economic optimum. 

Finally, trigger values must be set to give decision-makers insight into when to take what action. These values should prevent the safety level from dropping below the required one and exclusion of subsequent measures to ensure flexibility. This requires a timely analysis to obtain the desired subsequent measure. After this analysis, sufficient time should be available to implement all possible types of subsequent measures, even for the one with the longest implementation time. 

Overall, pathways can be created using the DAPP-approach. Optimisation has to be used to keep the number of pathways workable. Afterwards, the created pathways can be evaluated according to scenarios. An additional probabilistic assessment is required to assess pathways in the full range of possible futures. This assessment is applied to the three most promising pathways. This amount can be increased when this process is further automated. Afterwards, trigger values should be set and updated according to the latest known data. These trigger values should give enough time to determine the desired subsequent measure and implement it. This adaptive approach will ensure the flood safety requirements without overspending and can be a useful instrument to deal with the uncertain effects of climate change.