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.
