The Present & Future Flood Risks In Bangkok

By Dr. Chanita Duangyiwa 20 October, 2020

The nature of flood risks in Bangkok are complex. Chulalongkorn University's Dr Duangyiwa breaks them down & looks ahead

Managing coastal threats is more than just tackling sea level rise (SLR). Some cities, such as Bangkok, are also facing combined threats from land subsidence (LS) and storm surge (SS), which would lead to more complicated and abrupt disasters if unabated. We sat down with Dr. Chanita Duangyiwa, a geography lecturer at Chulalongkorn University in Bangkok, to learn about the complex nature of flood risks in this low-lying city, which was the focus of her PhD dissertation published in 2017.

CWR: To start off, could you tell us what inspired you to conduct this research for your PhD study?

Dr. Chanita Duangyiwa (CD): The great flood of Thailand in 2011 inspired me to conduct this research. The flood was the same year I started my career as a lecturer in Geography after my master study in Earth Science. After the floods occurred, I realised that I didn’t actually know much about Bangkok’s flood risks like it is located on a floodplain and is at great risk of coastal flooding. I also found that only a few studies have investigated the combined impacts of main influencing factors. These motivated me to continue my PhD on the topic about the present and future flood risks in the Bangkok Metropolitan Region (BMR).

CWR: Can you briefly walk us through some of the key findings from your PhD?

CD: For coastal flooding, scenarios designed were based on three contributing factors: sea level rise (SLR), land subsidence (LS) and storm surge (SS). The highest water level (2.11m from mean sea level) during the great flood in 2011 was used as a baseline scenario, and subsequent scenarios were built for 2050, 2080 and 2100.

The projections of SLR for low and high scenario are based on RCP2.6 (0.26m-0.55m) and RCP8.5 (0.45m-0.82m) respectively in the IPCC fifth Assessment Report (AR5). The extreme SLR projected by NOAA was used as the worst-case scenario, with mean sea level increasing 20.60mm per year.

BMR could sink as much as 25mm per year & will reach 2.2m by 2100

According to the World Bank, BMR is suffering an uniform rate of LS of 1.2mm-7.1mm per year. However, studies have also shown that it could sink as much as 25mm per year. If this sinking rate is unabated, it will reach 2.2m by 2100.

SS is rare in the region as typhoons have rarely entered the Upper Gulf of Thailand. Therefore, the highest level of SS at the mouth of the Chao Phraya river ever recorded during Typhoon Linda in 1997 (~0.61m) was used to build storm surge scenarios.

My projected sea level change ranges from from 0.11m-3.9m by 2100

My study showed that in 2050, the projection of sea level change based on the components of SLR, LS and SS is ranged from 0.05m-1.94 m, while in 2100, the projection of sea level change is ranged from 0.11m-3.9 m.

CWR: Based on your study, which areas or key infrastructure in Bangkok face most risks from coastal flooding? Does it worry you?

CD: Due to the lack of topographic data at the river mouth, the coastal flood modelling was only undertaken at two sites, extending ~28 km to the eastern side and 36 km to the western side from the river mouth of the Chao Phraya River. The eastern study site But includes Samut Prakarn province while the western study site includes part of Bangkok, Samut Prakarn and Samut Sakhon provinces.

Critical infrastructure points such as hospitals, daycare and nursing homes, power substations, gas stations, government properties, schools, temples, etc. will already be flooded as early as 2050. For example, under the low-end scenario of SLR+LS+SS by 2050, as much as 16.2% of schools and 18% of temples are at risks of coastal flooding of 0.67m. When sea levels continue to rise, these impacts will be worsened.

Temples & schools, which act as temporary evacuation centers, will be flooded as early as 2050; new centres are needed

In Thailand, during the flood events, temples and schools could be changed to a temporary evacuation center. This is the reason why temples and schools were also included as infrastructure in this study. But worryingly, these sites will be impacted by the floods in 2050 thus new evacuation zones need to be identified.

The distinctive landuse types of the two sites also provide an interesting comparison regarding the potential risks and impacts. The number of buildings flooded in the western site is comparably fewer due to its rural nature.

No. of buildings flooded in the Western site is fewer than in the Eastern site

On the contrary, the eastern site, which is in the south of Bangkok, has experienced the most rapid industrialisation in Thailand since the late 1980s. There are located in the east study site: Bang Pu Industrial Estate and Bang Phli Industrial Estate. Results show that in 2050, part of Bang Pu Industrial Estate may be flooded due to the combined impact of SLR, LS and SS while Bang Phli Industrial Estate will not be flooded as it is more distant from the coastal area.

In the past decades, industrialisation and urbanisation have increased rapidly in the BMR. The inland mangrove forests were transformed to shrimp farms and salt fields in both study sites. It is reported that floodplain and agricultural areas in BMR have decreased while the residential, industrial, and commercial zones have increased during the past decades. As a result of these transformation, BMR is becoming more vulnerable to coastal flooding.

Yet, studies about local climate change projection models are still insufficient to inform policy. Public awareness and concern about the impact of climate change and flooding also need to be strengthened.

CWR: What were the main challenges you encountered in this research?

CD: One of the main challenges is that historical flood data with detailed information and high-resolution Digital Elevation Model (DEM) are not available. Although there are a few studies that have looked at historical SLR in the region, there are no authoritative national and local projections of how SLR will evolve for Bangkok.

Gaps in data were a big challenge…

…also, that there is no authoritative national & local projections of how SLR will evolve for Bangkok

The obtained DEM data in my study does not cover the whole area of BMR. Data of large areas surrounding the main river channel where sensitive areas are located are missing. A coarser resolution data was used to fill the gap of these missing data. However, the results are not satisfactory due to the poor quality of the data.

Similar issues exist with LS records that are typically short and incomplete. Recent studies have shed light on the spatial heterogeneity of LS in Bangkok using radar images, providing a detailed picture of LS covering the city area. However, future projections of LS are not available.

This led to the challenges in the treatment of future projections, therefore various approaches were applied to the different drivers to meet the data requirements for the individual types of flooding investigated.

CWR: Has the overall situation in Thailand improved since the major flood in 2011? What measures has the government taken to prevent it from happening again?

CD: Adaptation strategies to prevent future flooding in Thailand involved both structural and non-structural measures. The action plans and budgets were planned after the great flood in 2011.

The flood management master plan focuses on preventing fluvial/river & surface flooding…

…not much on coastal flooding

Therefore, the flood management master plan mostly focused on preventing fluvial/river and surface flooding, but not for long-term coastal flooding. This includes the plans for a) building the reservoirs to prevent and mitigate impacts of floods in the Chao Phraya River basin, b) developing a forecasting and warning system, and c) improving physical structures. The roads have been raised and flood dikes have been built along the rivers, canals, and coastal areas to prevent the overflow from entering the areas.

However, these might not be true holistic planning as the floodwalls built in some areas may cause flooding in other areas. This may increase the exposure to future flood risks for the people living outside the floodwalls.

The nature of flood risks in BMR are complex. The potential co-occurrence of various types of flooding and multiple key contributing factors need to be considered for flood management. For example, surface flooding during high tides may intensify the impacts as the flood water may not be able to drain to natural channels due to the higher water in the river, which influenced by the tidal level. Similarly, as happened in 2011, high river flow from the catchment might coincide with high tidal level, preventing water flowing to the ocean and delayed flood recession.

CWR: After the 2011 flood and with the growing focus on climate change and sea level rise research, are corporates more aware of flood risks now and adapting for it?

CD: Yes, they are more aware of the impacts of fluvial flood risks, but the awareness for coastal flooding is still low.

In the 2011 flood, seven industrial estates located in Chao Phraya River basin were forced to shut down and led to tremendous economic losses in the manufacturing sector. As a result, the Industrial Estate Authority of Thailand prepared the prevention plans for fluvial flood, such as building the permanent dikes around the seven estates. However, detailed implementation plans may be different due to the management policy and financing of each industrial estates. The Bank of Thailand also granted soft loans for flood victims and lowering the taxes to help people restore their homes and business.

CWR: Recent research and news coverage indicate that Bangkok is expected to be one of the cities most impacted by SLR. For example, Climate Central predicts that Bangkok could be under water as early as 2050. Have you seen these predictions? How do these findings compare to your own?

CD: I found that the Climate Centra’s map may overstate the impact of SLR by 2050 and 2100. In my study, the projected SLR (excluding LS and SS) for 2050 (0.18-0.80m) and 2100 (0.39-1.83m) don’t look as bad as those of Climate Central’s worst-case scenarios. Even the highest estimated SLR in my study (combining impacts of SLR, LS and SS) in 2050 (1.94m) and 2100 (3.9m) also look less severe compared to Climate Central’s worst-case scenarios that only consider SLR projection.

[Note: CWR has also written its own analysis on whether Climate Central has used the right map – read here.]

CWR: What is the research gap you would like to close?

CD: The challenge for future research is to simulate flood scenarios for the combination of drivers on the whole site when more datasets become available.

The challenge for future research is to simulate the combination of drivers

Moreover, I wish to develop more informative risk analysis methods to account for cascading impacts of flooding in the wide social, economic, and environmental domain. Further research may investigate more closely on the affected key infrastructure, for example, the number of beds in hospitals, the number of houses affected by the shutdown of a flooded power substation.

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Further Reading

• Thirsty And Underwater: Rising Risks In Greater Bay Area – How will water & climate risks, including rising sea levels & droughts, threaten the already water-stressed Greater Bay Area (GBA)? CWR’s Tan & Mirando explain in their latest CLSA report and highlight companies’ failure in climate risk disclosures
• No-Sense Climate Strategies: From DSD To HSBC – Hong Kong’s shortsighted & unrealistic climate plans will leave key assets & infrastructure exposed that mean the government, companies, investors and the public are even more exposed. China Water Risk’s Dharisha Mirando & Debra Tan expand
• It Happened – Central Banks And Water Risks – Half a dozen new reports by the NGFS means that CWR has achieved a key milestone in embedding water risks in finance. Debra Tan and Dharisha Mirando expand on these game-changing moves by the central banks. The credit evolution has started
• Capital Threats Remain Post COVID – There is no vaccine for climate & water risks, yet some in the financial sector are still burying their heads. CWR’s Dharisho Mirando reminds us how our capital is at risk & steps we can take to reduce them while going green
• HK Submerged? Is This Map For Real? – Rising sea level is a catastrophe waiting to happen but we have to avoid alarmism & choose the right map to visualise the risks. Getting the right scenarios also matter. Find out more in our review