4 process. In particular, this paper presents a RDM analysis of flood risk management in the Nhieu Loc-Thi Nghe canal catchment area in Ho Chi Minh City, shown in Figure 1.1. This area faces high flood risk and has received significant investments in flood risk management.

The original infrastructure planning for Nhieu Loc-Thi Nghe catchment did not include a full uncertainty analysis. As mentioned, in the years since, actual climate and socioeconomic conditions have changed significantly from what was projected at the time. Building on Steering Center for Flood Control’s existing models and data, this study re-conducts the previous analysis, this time using an RDM framework to help manage uncertainty and help develop a robust, integrated plan. This choice of study design was motivated by two considerations. First, this study aims to demonstrate how the Steering Center for Flood Control and a wide range of other organizations can use RDM to augment their existing planning activities to improve their ability manage uncertainty. Thus, it was important to build this study on models and data that the Steering Center for Flood Control had previously used. Second, Steering Center for Flood Control is in the process of developing a more comprehensive integrated flood risk management strategy, using a new flood risk modeling system developed by the firms Royal Haskoning and Deltares. The plan resulting from this process will address flood risk over the entire city and is likely to be considerably more robust to future uncertainty than the infrastructure investments considered here. Nonetheless, the process used to develop this new plan does not yet take account the full range of plausible climatic and socio-economic futures facing Ho Chi Minh City and could benefit, as a next step, from the type of stress-testing described in this report.

The RDM analysis in this study found that the soon-to-be-completed infrastructure may reduce risk in best-estimate future conditions. However, the infrastructure may not be sufficiently robust over the full range of futures, that is, it may fail to reduce risk below current levels under many plausible future conditions. The analysis also suggests that adaptation and retreat measures, particularly when used adaptively, can play an important role in managing risk. In particular:

- The soon-to-be-completed infrastructure in Ho Chi Minh City’s Nhieu Loc-Thi Nghe catchment will reduce risk compared to current levels if three-hour rainfall event intensities increase by no more than approximately 6% and if the Saigon River rises less than 45 cm. However, scientific evidence suggests both these thresholds may be exceeded by mid-century, in which case risk may rise above current levels even with this infrastructure in place.

- Augmenting this infrastructure with a full range of adaptation and retreat measures would ensure risk reduction for rainfall intensity increases up to approximately 35% and Saigon River level increases up to 100 cm.  Little scientific evidence exists to suggest these levels would both be exceeded by mid-century. Additionally, uncertainties about trends in population and vulnerability appear less important that these climatic uncertainties in determining whether risk will rise above current levels.