Low-regret climate change adaptation in coastal megacities – evaluating large-scale flood protection and small-scale rainwater detention measures for Ho Chi Minh City, Vietnam

Abstract

Abstract. Urban flooding is a major challenge for many megacities in low-elevation coastal zones (LECZs), especially in Southeast Asia. In these regions, the effects of environmental stressors overlap with rapid urbanization, which significantly aggravates the hazard potential. Ho Chi Minh City (HCMC) in southern Vietnam is a prime example of this set of problems and therefore a suitable case study to apply the concept of low-regret disaster risk adaptation as defined by the Intergovernmental Panel on Climate Change (IPCC). In order to explore and evaluate potential options of hazard mitigation, a hydro-numerical model was employed to scrutinize the effectiveness of two adaptation strategies: (1) a classic flood protection scheme including a large-scale ring dike as currently constructed in HCMC and (2) the widespread installation of small-scale rainwater detention as envisioned in the framework of the Chinese Sponge City Program (SCP). A third adaptation scenario (3) assesses the combination of both approaches (1) and (2). From a hydrological point of view, the reduction in various flood intensity proxies that were computed within this study suggests that large-scale flood protection is comparable but slightly more effective than small-scale rainwater storage: for instance, the two adaptation options could reduce the normalized flood severity index (INFS), which is a measure combining flood depth and duration, by 17.9 % and 17.7 %, respectively. The number of flood-prone manufacturing firms that would be protected after adaptation, in turn, is nearly 2 times higher for the ring dike than for the Sponge City approach. However, the numerical results also reveal that both response options can be implemented in parallel, not only without reducing their individual effectiveness but also complementarily with considerable added value. Additionally, from a governance perspective, decentralized rainwater storage conforms ideally to the low-regret paradigm: while the existing large-scale ring dike depends on a binary commitment (to build or not to build), decentralized small- and micro-scale solutions can be implemented gradually (for example through targeted subsidies) and add technical redundancy to the overall system. In the end, both strategies are highly complementary in their spatial and temporal reduction in flood intensity. Local decision-makers may hence specifically seek combined strategies, adding to singular approaches, and design multi-faceted adaptation pathways in order to successfully prepare for a deeply uncertain future.

Macroscale impact assessment of flood damage reduction from climate change adaptation policies

“…Quantitative assessment of climate change is vital for justifying mitigation strategies to restrain global warming and minimize adverse consequences (Ciscar et al, 2011; Maranzoni et al, 2022; Ranger et al, 2011). Scheiber et al (2022) applied flood protection and mitigation measures in the Ho Chi Minh City, Vietnam, and quantified the extent of flood damage reduction using the flood severity index as metrics. Likewise, the quantitative assessment of the effect of adaptation policies can serve as the basis for selecting and implementing policy priorities by decision‐makers (Hammond et al, 2015).…”

Assessing Peri-Urbanisation and Urban Transitions between 2010 and 2020 in Ho Chi Minh City using an Urban Structure Type Approach

“…Previously centrally run cities such as HCMC only included the administrative units of urban districts (quận) subdivided into wards (phường), with rural districts subdivided into communes (xã) and townships (thị trấn). In response, to accommodate the growing population, the city has reshaped its form and rapidly expanded, suburbanising industries and populations as well as constructing infrastructure, resulting in reduced green space, encroachment of fertile cropland and low-lying marshland, and an increased extent of sealed surfaces [56][57][58][59]. Xu et al (2019) reported that the urban built-up area of Greater HCMC increased over 10-fold from a mere 5500 ha in 1990 to 63,500 ha by the year 2013 [30].…”