Have you noticed Singapore’s erratic weather in recent years? Do you realise that flash floods are becoming of a norm in Singapore?
Yes, you are right!
Figure 1: Singapore’s Latest Flashflood Incident on 24 Jun 2020 (Soon Ho, 2020)
Numbers are great testament to statements. From the rainfall statistics published by Meteorological Service Singapore, there is an increasing trend in the frequency of heavy rainfall events over Singapore. Flash floods have occurred on 14 days in 2017, compared to 10 in 2016 and six in 2015 (Tang, 2017).
Besides the inconvenience caused and the damage to infrastructure as well as personal equipment such as cars , you will be surprised how these flash floods is a potential source of water pollution as well.
Perhaps, a little nugget of wisdom if you are wondering why flash floods are becoming a norm in Singapore. Not surprisingly, it is due to global warming . When the air over ocean warms, its water-holding capacity increases by 7% for every 1 degree Celsius of temperature rise (Hausefather, 2018). This increased moisture translates to a heavier downpour in the traditionally wet areas and as a tropical country heavily influenced by the monsoon season, Singapore is not spared from this heavy downpour onslaught.
Figure 2: Why Urbanised Landscape Increases Rate of Stormwater Runoff (US EPA, 2020)
Yet, our heavily urbanised landscape with a reduction in forests and soils would mean that these stormwaters continue to run along the streets of our cities, picking up pollutants and nutrients in the process which eventually end up into our rivers and streams. At an ecological level, the deposition of nutrients into waterbodies can cause eutrophication, in which the waterbody becomes overly enriched with minerals and nutrients, causing algal bloom (F. Chislock, 2013). Algal bloom limit light penetrations, reducing growth and causing die-offs of plants in littoral zones (Lehtiniemi, Engström-Öst and Viitasalo, 2005). The death of aquatic plants disrupts the food chain and deprives other marine organisms of oxygen, eventually killing most aquatic life. Eventually, hypoxia occurs when algae and other organisms die from the lack of oxygen and available nutrients, creating a dead zone in which most aquatic species cannot survive (Society, 2020).
Figure 3: Sanitary Sewage Overflow (US EPA, 2020)
Sanitary sewage overflow (SSO) can also occur, which can pose a substantial health risk to human upon exposure and contamination. SSO refers to a condition that occurs mainly in developed countries, in which untreated sewage is discharged from a sanitary sewer before reaching sewage treatment facilities (US EPA, 2020). During episodes of heavy rainfall, the combined flow of the stormwater runoff and sewage can exceed the capacity of the sanitary sewer system, causing the sewage to flow out of the sewage system. When this happens, people can be exposed to the sewage through the consumption of contaminated drinking water sources and direct contact in areas of high public access or recreational water activities. Exposure to raw sewage poses great risk because they carry bacteria and viruses that can cause disease ranging from mild gastroenteritis to life threatening conditions such as cholera. In fact, historical data shows the number of gastroenteritis illness cases have been shown to increase in the community following rainfall (Drayna et al., 2010).
In Singapore, we are very fortunate that our stormwater drainage system and sewer system are completely separated and hence, the latter effects of stormwater runoff is unlikely to happen. However, in other developed countries such as U.S and France, SSO is a common occurrence due to their outdated sanitation system and is indeed a cause of concern. Having said so, the issue of nutrient pollution and contamination of our drinking water in water catchment areas remain a possible risk in the event of flash floods as we can see how these stormwater runoffs can act as a non source pollution.
References:
1. Soon Ho, W., 2020. Flash Floods, Fallen Tree As Heavy Rain Pelts Down. [online] The Straits Times. Available at: <https://www.straitstimes.com/singapore/flash-floods-fallen-tree-as-heavy-rain-pelts-down> [Accessed 23 July 2020].
2. Tang, L., 2017. S$500 Million To Be Pumped Into Upgrading Drains Over The Next 3 Years: PUB. [online] TODAYonline. Available at: <https://www.todayonline.com/singapore/s500-million-be-pumped-upgrading-drains-over-next-3-years-pub> [Accessed 23 July 2020].
3. Hausefather, Z., 2018. Explainer: What Climate Models Tell Us About Future Rainfall | Carbon Brief. [online] Carbon Brief. Available at: <https://www.carbonbrief.org/explainer-what-climate-models-tell-us-about-future-rainfall> [Accessed 17 July 2020].
4. US EPA. 2020. Polluted Runoff: Nonpoint Source (NPS) Pollution | US EPA. [online] Available at: <https://www.epa.gov/nps> [Accessed 17 July 2020].
5. Chislock, M., 2013. Eutrophication: Causes, Consequences, And Controls In Aquatic Ecosystems | Learn Science At Scitable. [online] Nature.com. Available at: <https://www.nature.com/scitable/knowledge/library/eutrophication-causes-consequences-and-controls-in-aquatic-102364466/> [Accessed 17 July 2020].
6. Lehtiniemi, M., Engström-Öst, J. and Viitasalo, M., 2005. Turbidity decreases anti-predator behaviour in pike larvae, Esox lucius. Environmental Biology of Fishes, 73(1), pp.1-8.
7. Society, N., 2020. Dead Zone. [online] National Geographic Society. Available at: <https://www.nationalgeographic.org/encyclopedia/dead-zone/> [Accessed 17 July 2020].
8. US EPA. 2020. Sanitary Sewer Overflows (Ssos) | US EPA. [online] Available at: <https://www.epa.gov/npdes/sanitary-sewer-overflows-ssos> [Accessed 17 July 2020].
9. Drayna, P., McLellan, S., Simpson, P., Li, S. and Gorelick, M., 2010. Association between Rainfall and Pediatric Emergency Department Visits for Acute Gastrointestinal Illness. Environmental Health Perspectives, 118(10), pp.1439-1443.
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