Future rainfall extremes and pluvial flood risk assessment in Singapore

While Singapore has demonstrated concerted efforts and progress in pluvial flood management, the recent Third National Climate Change Study in Singapore (V3) projects an increase in future annual extreme rainfall, and flash floods are expected to intensify. However, occurrences of flash floods in Singapore generally do not result in fatalities and significant infrastructural damages, and the most salient impacts observed during flash flood events are disruptions to traffic networks and businesses, which translates to economic and productivity losses.

This study thus aims to 1) Characterise future flooding likelihood and its uncertainties in Singapore under various Shared Socioeconomic Pathways (SSPs) for mid and end-century scenarios, 2) Calculate potential economic loss associated with travelling time delay due to traffic disruptions during flood events.

A Bayesian probabilistic modelling approach was used to obtain the probability distribution of flooding as a function of various precipitation intensity values (i.e. maximum 30, 60, and 120 minutes rainfall) for present day and across different SSPs scenarios. Travel routing simulations of vehicle and public transit were conducted for dry and flood conditions to determine the total travel time delay during flood conditions.

Results showed that future flood likelihood is the highest and has the most widespread impacts beyond the 99^th percentile of maximum 30 minutes rainfall, compared to other intense rainfall durations. Interestingly, flooding likelihood is higher for present conditions than future scenarios due to projected longer drier days and more intense but infrequent extreme precipitation.

Intensification of precipitation extremes in the future poses hard limits to pluvial flood prevention in Singapore, which has significant implications to policy. This indicates that continual infrastructural flood prevention measures have limited efficacy under extreme rainfall events, and cross-sectoral adaptation measures for extreme rainfall and pluvial flooding should be prioritised to enhance flood resilience in Singapore.