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# **5. Results of RDM Analysis for Ho Chi Minh City**

This section describes how we conducted the steps of the RDM analysis shown in Figure 2.1 using the simulation model and data presented in Section 4. This section focuses on population risk, which is a more stressing measure than economic risk in Ho Chi Minh City. The section first considers population risk for two alternative plausible futures. This initial discussion motivates the need for running the model many times over a wide range of conditions and introduces some of the visualizations we will subsequently use. We then examine how the Baseline Strategy performs over many plausible futures. A statistical scenario discovery confirms that the Baseline Strategy may not be sufficiently robust and confirms the need for an integrated flood risk management strategy that augments it with adaptation and retreat policies. We next examine the robustness of alternative combinations of such adaptation and retreat policies by analyzing how they how they each perform over many plausible futures. Finally we identify the tradeoffs between cost and robustness. The section concludes with a summary of a similar analysis for economic risk, and then offers some final observations.

This analysis aims to demonstrate how Ho Chi Minh City could use existing models and data to examine the robustness of integrated flood risk management strategies and potentially identify strategies more robust than those the city has heretofore considered. Such robust risk management strategies would reduce risk in a wide range of climate and socio-economic futures at reasonable cost.

## **Comparing Risk in Few Futures Is Insufficient for Decision Making**

We first assess the performance of the Baseline Strategy given our best estimate of future conditions, i.e., one that is consistent with official estimates and available information about future rainfall intensity in Ho Chi Minh City, Saigon River levels, the city’s population, population distribution, and poverty rates.<sup>14</sup> Figure 5.1 plots the risk to poor and non-poor populations in these conditions (blue mark) as calculated by the simulation model described in Section 4.<sup>15</sup> It shows that the Baseline Strategy could keep risk at acceptable levels (i.e., less than levels of risk experienced prior to the new infrastructure (the origin)) for both the poor and non-poor. This approach establishes a desired level of performance that can be used to compare alternative strategies over _________

> 14 This 20% increase in rainfall intensity for Ho Chi Minh City is consistent with the IPCC mean estimate for the increase in precipitation intensity for Southeast Asia in 2045-2065 (2012).  Consistent with MARD’s sea level rise estimate for 2040 and 2050 shown in Appendix A, we assume in this case a 30 cm rise in the level of the Saigon River.  Consistent with the Ministry of Planning and Investment’s estimates, we assume a 2045 Ho Chi Minh City population of 11.1 million. Lacking reliable predictions to the contrary, we assume the population distribution and poverty rate in 2045 remain at their recent levels.

> 15 To calculate each point on Figure 5.1, we run the model with the Baseline Strategy for a specific set of assumptions about the six hazard, vulnerability, and exposure uncertainties: the rainfall intensity, Saigon River level, Ho Chi Minh City population, population distribution, poverty rate, and population vulnerability.  We then run model again, without the JICA 1999 Infrastructure, to calculate the benchmark risk using the values for recent conditions for rainfall intensity, Saigon River level, Ho Chi Minh City population, population distribution, and poverty rate, but using the same population vulnerability value as in the corresponding Baseline Strategy run.  The difference between the risks in those two paired runs gives the location of a dot in Figure 5.1.