Flood Modelling with Climate Change in Singapore

Written by Ismail Weiliang and Ryan Liang, Inland Modeler, RHDHV Singapore.

20 MAY 24

3 MINS READ

ISMAIL WEILIANG

The Climatebender

RYAN LIANG

Inland Modeller

Views are entirely ours

and not connected to any company

Effects of Climate Change for Singapore

Singapore is not insulated from the impact of climate change. Singapore is a low-lying island city-state with limited land. Around 30 percent of the island lies under 5 meters above sea level, rendering it highly susceptible to rising sea levels. Considering the third national climate change study by the Centre of Climate Research Singapore (CCRS), projections indicate that:

  1. Singapore's sea levels might rise by up to 1m by 2100.
  2. Singapore's sea levels may rise even up to 4-5m considering factors like storm surges, higher tides, increased rainfall and land subsidence by 2100.
  3. Rainfall intensity will increase with 58% rainfall increase during rainy season for low emissions scenario by 2100

Sea Level Rise projection for South of Singapore (CCRS, 2024)

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Quick Take:

  • Climate projections indicate that Singapore's mean sea levels might rise by up to 1m by 2100 or even up to 4-5m considering factors like storm surges, higher tides, increased rainfall and land subsidence.

  • With Climate change, rainfall intensity will increase in Singapore.

Modelling for Flood Risk

Flood modeling is essential to understand the flood hazard and hence flood risk. It forms the basis of proposed coastal adaptation pathways and proposed coastal protection and drainage improvement solutions to cater for climate change. A 1D-2D dynamically coupled flood modeling system provides the most accurate representation of complex urban flood phenomena. Modeling methodology in Singapore general focuses on the ability of the model to represent three types of flooding mechanisms common in Singapore:

  1. Coastal flooding, through overflow from the coast, as well as penetration of seawater to upstream areas through the drainage network;
  2. Inland flooding, through overflow of the drainage network; and
  3. Backwater effect when flood as a result of the interaction of high-water levels downstream and high flow from upstream rainfall runoff.

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Quick Take:

  • Flood modeling is essential to understand the flood hazard and hence flood risk. 

  • There are 3 types of flooding mechanisms common in Singapore: Coastal Flooding, Inland Flooding and Backwater effect.

What goes into the flood model?

Data Input: For model set-up, large amounts of data is used. This includes inputs such as Topography, Bathymetry, Catchment boundaries, Land-use, drain and hydraulic structures, Meteorological data, and reservoir operations.

Scenarios: For model scenarios, scenarios are modeled for a) Rainfall events using rainfall return period and climate change factors such as % increase of rainfall and b) Extreme Sea levels from the combined effect of current Mean Sea Level, Projected Sea level Rise, Storm Surges and Land Subsidence.

Testing: Stability tests will be conducted for models before actual simulation, as such tests help to ensure that the numerical solution of the model does not diverge. It also verifies the model performance, which makes sure that all the hydraulic structures inside the model are working well and consistently under various conditions.

Calibration: For calibration and validation, we use historical events to adjust hydraulic parameters of the model. Calibration aims to obtain representative values for both hydrologic and hydraulic parameters: existing catchment characteristics (SCS-CN, Flow Path Length, and Slope) and hydraulic conditions of the drains (manning’s n-friction factor). Validation aims to determine whether the model is “good” in terms of its ability to reproduce the past with the required accuracy and is characterized by comparison of model output with an independent set of measured data.

Coupling: For 1D-2D Coupling, the validated 1D flood model is coupled with the modified DEM. For the 2D mesh preparation, drains with a width greater than the grid size will be excluded from the DEM to avoid double conveyance. The 2D Mesh will then be simplified with lesser elements and vertices, reducing the calculations per time step and hence computation load. For coupling, any open drains will be coupled to the 2D mesh. Closed drains with short lengths (eg 1m) will be coupled to the 2D mesh as well, as water can flow out from the openings and fill up the nearby areas easily in reality.

Simulation: After coupling, the flood model can then simulate design events to give results on flood extent, depth and timing.

Flood Modelling Workflow (Ismail Weiliang, 2024)

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Quick Take:

  • Flood models require huge amounts of data, various scenarios, calibration and validation with past events to simulate design events to produce results for flood depth, extent and timing.

Insights from flood modeling

A state-of-the-art modelling system that is capable of simulating and evaluating both inland and coastal flood risks in tandem is in the works for Singapore. This Coastal-Inland Flood Model will enable Singapore to better assess the impact of climate change on Singapore's coastal areas. 

This flood model will be developed based on Singapore's densely built-up and urbanised environment, and will enable holistic flood risk assessment by estimating the combined effects of extreme sea levels and inland floods caused by intense rainfall. 

The Flood model will help support site-specific climate change studies that Singapore is currently doing. Flood modelling can give insights on:

  1. Effectiveness of proposed inland and coastal flood adaptation measures
  2. Development of adaptation pathways
  3. Climate adaptation policies
  4. Operations management.

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Quick Take:

  • Flood modeling can help give insights for inland and coastal adaptation measures, adaptation pathways, policies and operations management.

Authors:

Ryan Liang is an inland flood modeller at RHDHV Singapore. He graduated from NTU in Environmental Engineering and has 6 years of experience in both Singapore and China. His experience includes stormwater managemen, Water Sensitive Urban Design (WSUD) and flood modelling.

Ismail Weiliang is a consultant that provides technical advice on climate resilience with half a decade of experience in flood risk advisory for Asia. He also founded "The Climatebender” a non-profit organisation that provides humanitarian relief to communities vulnerable to the climate crisis.

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