How Nature-Based Solutions Can Help Reduce Flood Risks

Nature-based solutions can be used to enhance flood control systems. Photo credit: ADB.

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Published: 12 May 2022

In the Philippines, solutions that respect river dynamics and ecosystem functions were adopted to keep vulnerable communities safe from natural hazards.  

Introduction

In highly populated river and coastal areas in Asia and the Pacific, communities are vulnerable to the worsening and compound impacts of climate change, such as heavier rainfall, more frequent flooding, and severe cyclones. When these natural hazards occur, disaster strikes—damaging property, traumatizing communities, and making life harder for the most disadvantaged.

Governments can be better prepared with protective and adaptive measures to help decrease the crippling effect of these events. One way to strengthen the community’s disaster and climate resilience is by restoring and conserving nature through the concept of nature-based solutions. Three river basins in the Philippines used this approach to protect flood-prone communities from natural hazards.

What are nature-based solutions?

The United Nations Environment Assembly (UNEA) defines nature-based solutions as “actions to protect, conserve, restore, sustainably use, and manage natural or modified terrestrial, freshwater, coastal and marine ecosystems, which address social, economic, and environmental challenges effectively and adaptively.” UNEA also highlights that the goal of these solutions is to simultaneously provide human well-being, ecosystem services, and resilience, and biodiversity benefits.

Nature-based solutions can be used to enhance flood control systems, improve wastewater treatment systems, and render irrigation networks more environmentally sustainable. These are not standalone or quick-fix initiatives to solve problems. Instead, they are part of a comprehensive strategy that considers a community’s well-being, ecological resilience, and flexibility to achieve greener growth.

Mangrove planting is a nature-based solution to coastal erosion and flooding. Photo credit: ADB.

What are the advantages of this approach?

Nature-based solutions are attractive to policy makers and investors for several reasons. First, they yield economic, social, and environmental co-benefits for the community. Unlike traditional solutions that are optimized to attain one purpose, they can lead to advantages that go beyond a project’s main goal. Examples of co-benefits include the creation of green jobs, easier access to public green spaces, and the enrichment of coastal habitats and local flora and fauna, thereby increasing food security.

Second, nature-based solutions enable decision-makers to choose from a wider range of hybrid solutions instead of being limited to either gray or green options. They are adaptable and can complement traditional gray infrastructure. For example, seawalls, levees, and embankments can be built as earthen structures instead of concrete and with a clay top layer seeded with native grass species or covered with brush mattresses that promote the growth of native vegetation. Using natural and biodegradable materials can potentially result in locally sourced solutions and have minimal environmental footprint. 

Third, they can be built in harmony with existing natural resources by leaving space for present ecosystems and natural resources. For instance, including mangrove or beach forests in the flood risk mitigation scheme results in safer, more robust, and cheaper design. The natural system enhances the function of gray engineering structures by reducing the impacts of waves, wind, and current and by trapping sediment in front of it.

Finally, nature-based solutions can cope with climate uncertainties and reduce disaster risk. They can evolve with the changing environment and integrate indigenous and traditional knowledge with modern systems. These advantages show that people’s welfare is at the heart of this approach. As such, it is vital for stakeholders to actively take part in designing these solutions so that they reflect a holistic perspective, are socially inclusive, and add more value.

Application in flood risk management in the Philippines

In recent years, the Asian Development Bank (ADB) has been investing in nature-based solutions in several sectors, especially water-related investments where its strength and competitive advantage lie. Examples of technical assistance for integrating these solutions include piloting sponge cities to reduce urban flood risk, coastal protection with green and gray infrastructure, and managing drought and flood through integrated watershed management.  

To identify suitable solutions that can reduce flood risk, implementers can refer to the natural river management approach. This is a way to tap the natural functions of river systems to deliver climate resilience. At the same time, this approach reduces the unintended environmental and social impacts at lower costs.

In the Philippines, ADB has been supporting the government in preparing projects to lower and mitigate flood risks in six major river basins: Apayao–Abulog and Abra in Luzon, Jalaur in Visayas, and Ranao (Agus), Buayan–Malungon, and Tagum–Libuganon in Mindanao. The Department of Public Works and Highways prepared basin-wide flood risk management master plans to establish flood risk management strategies, assess combinations of various measures, and set out priority projects for the short, medium and long term. To support this initiative, the Protecting and Investing in Natural Capital in Asia and the Pacific project used the natural river management approach to quickly assess and propose nature-based solutions to be integrated into the flood master plans for three of the six river basins.

Mankayan in Benguet is one of the headwaters of the Abra River Basin. Photo credit: Gem B. Castillo.

Abra, Buayan–Malungon, and Tagum–Libuganon were prioritized due to their high-risk hotspot locations and their relatively large population in the downstream area close to the coast. Implementers chose green and inclusive solutions that respect river dynamics and ecosystem functions. Figure 1 shows the proposed types of nature-based solutions with traditional engineering and soft solutions.

Figure 1: Proposed Types of Nature-based Solutions for Selected River Basins

Room for the River* approach with gray infrastructure
River Basin Proposed nature-based solutions with traditional and non-structural interventions Goals Benefits
Buayan–Malungon
  • Planting and restoring mangroves
  • Demolition of the Old Buayan bridge and dike
  • Construction of jetties on each side of the Buayan river
  • Drainage improvement
  • Protecting flood vulnerable population
  • Improving flood conveyance
  • Reducing coastal flooding impacts
  • Health impact reduction
  • Recreation potential
  • Potential fish spawning ground
  • Increase in biodiversity
Tagum–Libuganon
  • Restoring the Ising wetland
  • Creating a green river connection between Tuganay and Ising rivers
  • Protecting urban centers by constructing local protection structures
  • Improvement of river and drainage works
  • Building bridges and culverts in the road to accommodate peak runoff during 100-year events
  • Comprehensive land use planning
  • Protecting flood-vulnerable populations
  • Increasing flood retention/drainage capacity
  • Improving water quality and biodiversity
  • Recreational bird watching and fishing
  • Increased agricultural production as a result of reduced waterlogging
  • Water purification and sediment trapping
Abra
  • Restoring and accommodating natural river meandering
  • Erosion protection with vegetation strips
  • Protecting urban centers by constructing local protection structures
  • Quarrying in strategic areas
  • Implementing land use management
  • Resettling vulnerable communities
  • Protecting flood vulnerable populations
  • Reducing riverbank erosion and flooding
  • Promoting livelihoods
  • Improved agricultural production, livelihoods and job opportunities
  • Low investment costs with less river ecosystem disturbances
  • Improved water transportation

*Room for the river is an approach in flood management that gives water more space by restoring the river’s natural landscape.

Three key lessons

Here are three insights on using nature-based solutions in the Philippine flood risk management project.

  • There is no one-size-fits-all solution. Each river basin presents unique challenges and opportunities. Proponents need to prepare a comprehensive strategy with a holistic perspective using approaches such as natural river management. The chosen strategy must consider social, environmental, engineering, policy, and institutional factors before it is carried out.
  • Aim to mix green and gray solutions for maximum impact. The range of possible solutions including green and gray solutions should be studied. Although hard infrastructure is still useful under specific conditions, mixing these with green ecosystem-based approaches provide opportunities to enhance sustainability and expected benefits. Hybrid approaches can unleash the potential of nature-based solutions to strengthen resilience, reduce disaster risk, and realize co-benefits.
  • Participation matters. Active engagement of stakeholders from the start is needed to gain a deeper understanding of river dynamics, the socioeconomic context, and the institutional system in the whole river system. Nature-based solutions work better when the community takes part in weighing the tradeoffs of different measures and discussing how solutions can be integrated with development processes.

Isao Endo
Environment Specialist, Environment Thematic Group, Sustainable Development and Climate Change Department, Asian Development Bank

Isao Endo is an Environment Specialist working on natural resource management at ADB. He manages technical assistance to promote natural capital investments with a focus on nature-based solutions and market-based instruments, supporting ADB’s operation to integrate these innovative approaches into project design. He holds a bachelor’s degree in Economics from Sophia University and a master's degree in Environmental Management from Yale University.

Junko Sagara
Water Resources Specialist, Southeast Asia Department, Asian Development Bank

Junko works on water resources management, irrigation, and flood risk management projects. Prior to joining ADB, she worked with a Japanese engineering consulting firm for more than 14 years on projects in water resources planning and management, flood control and forecasting, disaster risk management, and climate change adaptation. She holds a master’s degree in Civil and Environmental Engineering and a bachelor’s degree in Civil Engineering.

Bregje van Wesenbeeck
Senior Director, Deltares

Bregje is the scientific director of Deltares, a Dutch research institute for water management, and an expert in nature-based solutions. She is also an associate professor in nature-based solutions and ecosystem-based adaptation for climate change and disaster risk reduction at the Delft University of Technology. She has more than 15 years’ experience in working with development banks, UN, NGOs, and governments in Asia, Africa, US, and the Netherlands, providing advice on flood risk reduction, erosion mitigation, and climate change adaptation.

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