Using nature-based solutions to manage flood risk in Thu Duc City

The challenge: A triple threat for a city surrounded by water

Situated between the Saigon and Dong Nai rivers, TDC is susceptible to three main flood drivers, including extreme rainfall, high-river water levels, and compound flooding.

These factors, in addition to land that lies only around 1.5 metres above sea level, have frequently led to flooded streets as drains overflow and rising tides cause water to overwhelm canals and embankments. This causes pollution problems due to effluent overflow as well as damage to buildings, assets, and infrastructure.

Our calculations show that if no action is taken, TDC will face a loss of 66.9 million USD every year in the current climate conditions. With climate change predictions factored in, this loss will grow to 157 million USD by 2100 – the equivalent of 8% of the city’s 2022 revenue.

But flooding isn’t the only problem. Without visibility into what future flood risk may look like, the city’s land use masterplan lacks vital information. This could lead to sub-optimal development decisions and hamper the city’s potential for growth.

The solution: A citywide flood risk management project

To help TDC address these challenges, Royal HaskoningDHV was brought in to conduct a rigorous technical assessment of potential risks and identify the best solutions to mitigate them.

Our Vietnam-based consultants began by using our flood risk and hazard modelling software to look at current and future risks, factoring in rising sea levels and the predicted effects of climate change.

In projects like this, people and the natural environment are always a primary consideration. So, alongside our risk modelling, we performed an urban fabric analysis dating back to the 1800s, which provided insight into the history of the city’s growth to help inform decisions about future plans.

In addition, we carried out a similar environmental assessment to ascertain how solutions may have a positive or negative impact on the city’s natural environment. Each of these elements would eventually feed into a multicriteria assessment of each of our proposed solutions.

Vitally, we also conducted community engagement to understand the lived experience of TDC’s citizens. To do so, our team members visited flood sites to ask community members about their experiences and take photos of affected areas.

Armed with this information, and after numerous workshops with city officials, we were able to conduct a detailed cost benefit assessment of an IFRM project divided into three investment phases, the first of which will begin in 2025.

A blue-green future for TDC

Our vision for TDC is to create a resilient blue-green network combining a green dike system integrated into the main river channels and a host of nature-based solutions.

Our work centred around four main pillars: flood delay, storm drains, discharge, and protection. The goal was to delay the water runoff from flood events, find natural areas suitable for storing and containing flood water, and update drainage and discharge infrastructure to protect urban environments.

Working with a landscape subconsultant, we came up with a series of nature-based solutions, including blue-green buildings, green corridors, urban forests, wetlands, canal parks, and bioswales. We then combined these solutions with some traditional hard infrastructure considerations, like sluice gates, pumps, and raised roads and walkways that provide leisure opportunities for residents.

Once we’d identified the best mitigation paths, these solutions were applied to a map of the city which was then passed through our flood model. This evaluated their performance against a range of future climate change, rainfall, and rising sea level scenarios between now and the year 2090. This modelling also helped identify the best locations for new wetlands and how they could be connected to those that already exist.

These projections were ultimately used to create a comprehensive project plan containing a series of investment opportunities for stakeholders to consider. The map below shows the difference they will make compared to a scenario where no action is taken at all.

The result: A complete flood mitigation roadmap – and more room for growth

By the end of our planning phase, every one of the suggested investment items in our project plan was approved by both the city government and the World Bank.

Once implemented, these solutions will contribute to a natural water detention and retention system, which will minimise the amount of water being discharged for each catchment and dramatically reduce the frequency and consequences of flood events.

As is often the case with nature-based solutions, there are many secondary benefits, too. The blue-green network will also reduce heat and greenhouse gasses across the city, and will add value to the community in the form of social and economic benefits.

In addition to this project plan, we offered a range of technical and advisory support services to the city. These include a behavioural change study to help shift public perceptions about green spaces, and support for nature-based solutions to educate contractors about the various species native to the area.

With plans approved, TDC now has a roadmap to help mitigate the impacts of flooding, and the information needed to make more informed decisions about future urban planning and expansion.