51

# **Policy Levers (L)**

Section 4 describes four individual policies we use to construct integrated flood risk management strategies:

- Groundwater management

- Rainwater capture

- Relocation of vulnerable areas

- Elevating homes

Here, we describe these policies in greater detail.

Subsidence is a major contributor to increasing effective height of the Saigon River. High rates of groundwater extraction, in turn, contribute to subsidence, though other factors are also at play. We therefore consider groundwater management and recharge as one potential method of reducing hazard by reducing rates of subsidence.<sup>32</sup> The effects of groundwater extraction on subsidence, and the potential for groundwater management to mitigate subsidence, depends greatly on a variety of factors such as land use and soil composition, and is not well understood for Ho Chi Minh City. For the purposes of this study, we are agnostic towards the groundwater management policies used to reduce extraction and encourage recharge. Instead, we examine the risk effects of slowing the rate of subsidence, regardless of how it is brought about. We use a subsidence rate reduction of 0.33 cm/year, amounting to a maximum of 10 cm of avoided subsidence over the entire 30 years of our study.

Rainwater capture may offer a second method of reducing the flood hazard by reducing the amount of rain water that reaches the drainage system during the extreme rainfall event. Potential rainwater harvesting systems in Vietnam range from small plastic bins that can store 100 liters (0.1 cubic meters) of water to large cement tanks that can store 10,000 liters (10 cubic meters) (Patrick _et al._ 2009). While we were unable to find surveys of rainwater technology adoption in Vietnam, we drew on related studies from neighboring Thailand which found that ceramic jars with a 2000 liter (2 cubic meter) capacity were most popular and sufficient for storing water for a family of six (0.33 cubic meters per person).<sup>33</sup> For population risk experiments in which population is an exogenous uncertainty, we assume that a rainwater capture policy would seek to provide 0.33 cubic meters of storage capacity per person in our study area.<sup>34</sup> In our economic risk experiments, population is not an uncertainty and so we assume a fixed level of implementation that is the average of the rainwater capture capacities in the population experiments. This policy effectively reduces the intensity of the rainfall event by 10mm.

## _________

> 32 There are other mechanisms for counteracting subsidence, such as land fill.

> 33 http://www.unep.or.jp/ietc/Publications/Urban/UrbanEnv-2/9.asp

> 34 As an example of its effect, suppose that there are 2.23 million residents in the study area (consistent with the current population) and every resident has 0.33 cubic meters of rainwater storage capacity. Further suppose a 100mm 3-hour rainfall event, which produces a total of 8.7 million cubic meters of rainfall over the 87 square km of Ho Chi Minh City. The rainwater capture capacity in the study area would reduce this volume by 0.75 million cubic meters, resulting in 7.95 million cubic meters of rainfall reaching the drainage system. This is equivalent to a 90mm rainfall event without rainwater capture. Under these conditions, rainwater capture reduces the effective intensity of the rainfall event by 10%.