A Short Review of Nature-Based Solutions for Flood Mitigation

Failed Infrastructure and Unimproving Flood Events

2025 has been the year that Filipinos were reminded of our yearly sufferings from floods, rains and typhoons. More so, we have been reminded that it is not merely influenced by the strength of a cyclone or a rainfall level. It is an equation of the natural hazards brought by rain and wind, the physical infrastructures we have, and the preparedness of people to brace such storm events.

Prior to floods by Typhoon Tino (international name Kalmaegi), Uwan (international name Fung-wong) and the June-July monsoon rains, the country was appalled by corruption scandals that plague flood control projects. These criminal and rather fatal acts ultimately jeopardize the intent of flood control projects through poor construction, underfunding (and thus siphoning the funds supposedly spent for the construction), lack of actual experts and credentials or downright non-execution of these projects: more popularly known as ‘ghost projects’ — projects only done or in-progress on paper, but nothing built at all. 

Failed flood control gray infrastructure worsen flood impacts by causing unfounded confidence to residents that they would be spared from immediate flood impacts, which is compromised by poor construction. Furthermore, these gray infrastructure typically are part of a flood control master plan, all of which need each others to work well to be effective in a landscape or regional perspective. If one or multiple are compromised, the whole system would be less effective.

In other perspectives, we have also seen that infrastructure (and its absence) is not the only one to blame. Cebu province suffered badly in the floods caused by Typhoon Tino, tallying a death toll of 150 casualties. Beyond gray flood control infrastructure, deforestation, land use neglect and mountaintop luxury developments came under scrutiny as they were perceived to have worsened the flooding downstream. Such is also the perennial issue for Metro Manila, given the worsening deforestation of Sierra Madre in Rizal province. Marikina river not only reaches greater water levels, Typhoons such as Ondoy (international name Ketsana, 2009) and Ulysses (international name Vamco, 2020) leave layers upon layers of mud after inundating low-lying communities in Marikina City and Rizal municipalities.

Man clears up flood washed debris, in the aftermath of Typhoon Ulysses (Vamco), 2020.

Photo from Reuters / Eloisa Lopez

Deforestation and residential or settlement development in the mountains result in higher amounts of water runoff flowing downstream, thus increasing stormwater reaching lowland barangays. Furthermore, especially for barren lands and sites under construction, stormwaters are more likely to be brown or mocha in color as more soil and sediments flow downstream as well. To make matters worse, people living beside waterways are likely to be poor as these were remaining areas for settlement. Hence, the poor suffer further. If the uplands were instead kept intact as forests or natural areas, less water and less sediments would reach downstream. 

Gray Infrastructure for Flood Mitigation

The Philippines has long leaned towards building gray infrastructure in mitigating floods. Gray infrastructure means traditional, rigid engineering solutions such as culverts, seawalls, and flood barriers, designed to withstand water pressures and prevent inundation but often have limited ecological benefits, if not outright ecological disadvantages. These structural interventions can be expensive, and offer results that can easily be seen and quantified directly. 

In Metro Manila, the colossal example would be Manggahan Floodway, which redirects storm water flowing from the upper reaches of Marikina River, directly into Laguna de Bay. This prevents the vicinities along the Marikina River, the Pasig River and downstream regions in the City of Manila from being inundated with such amounts of storm water. This, however, transfers such flooding risks to the lakeside communities instead, in Taguig, Taytay and other Rizal and Laguna municipalities.

In the 1970s, the original master plan included another spillway that would add an outlet from Laguna de Bay to Manila Bay across Paranaque, also to lessen the volume passing through the Pasig River, and a direct parallel to the water to be added by Manggahan Floodway. This, however, did not materialize due to high financial costs and impacts on residents. Today, a modern version of the project, which is now a tunnel, would push through, as well as a Marikina Dam. 

Watershed basin extent of Pasig and Marikina Rivers extend to the uplands of Rodriguez and Antipolo, Rizal.

Another component of this plan is the Napindan Hydraulic Control Structure, which controls the flows of Marikina River and Napindan Channel. During heavy rains, the structure can divert excess water from the Marikina River into Laguna de Bay to reduce downstream flooding in Pasig, Manila, and other low-lying areas. Conversely, it can prevent backflow from the lake when water levels in Laguna de Bay are high. This, however, disrupts the flow of nutrients, particularly salinity coming from Manila Bay through the Pasig River, to the lake, prompting fisherfolks to oppose it and demand the structure to be deactivated and keep the flows open.

Aside from this, other gray infrastructure is also visible everywhere in Metro Manila. Major rivers such as Pasig River, Marikina River and San Juan River have retaining walls, losing the natural banks of the rivers; pumping stations facilitate the discharge of urban stormwater that prevents drainage systems from getting overwhelmed; seawalls such as the CAVITEX and in Roxas Boulevard. 

In the rural regions, dikes and spillways have been built, such as in Pampanga, which has long been a region vulnerable to flooding events, being at the outlets of the Pampanga River basin. In other existing rivers, such as the Pansipit river in Batangas province, river protection structures were built. In Tacloban, as a response to the deadly storm surge that hit the city, seawalls were built, which cost the residents and local fisherfolk their access to the shores and the sea.

Looking at a Watershed-level Perspective: Nature-based Solutions

Gray Infrastructure, especially those poorly planned, poorly built and poorly run, may redistribute risk. Flood control measures around Metro Manila, including embankments, levees, and diversion channels, often shift flood risk from the city to peri-urban and lakeshore communities of Laguna Lake, notably in the case of Manggahan Floodway. This means thInfrastructure designed to protect urban centers can inadvertently flood fishing villages, agricultural lands, and informal settlements along the lake, creating a “hazardscape” where vulnerability is redistributed rather than eliminated. This was further ascertained when the Paranaque Spillway was never materialized. 

The Paranaque Spillway was intended to channel water out from Laguna de Bay to the Manila Bay, especially with the excess runoff caused by the Manggahan Floodway. Effective flood management in this context requires recognizing these socio-ecological interconnections, integrating the lake and its surrounding communities into planning, and combining engineered structures with community-based and ecosystem-oriented strategies to reduce both direct and displaced risks. Hence, when planning to mitigate flood, it cannot be engaged in smaller, narrow-sighted scales, or by piecemeal initiatives. 

In contrast to monolithic gray infrastructure, nature-based solutions (NbS) reference natural and ecological processes in introducing interventions, incorporating the ecosystem in the solution. This effectively protects and healthily manages our natural environments, towards their continued benefit to people, while still addressing people’s needs. NbS can be seen as shield from the worsening climate change, and a foundation for resilient livelihoods.

In the context of flood management this manifests by understanding the type of flooding that occurs and looking at the proper scale. The most prominent case in the Philippines is that: flooding events happen as an effect of strong rain events, moved through hydrological processes within the scale of a watershed basin. Thus, we establish: floods from storms and monsoon events are not always localized. 

Flood waters that reach the cities beside rivers and near the seas can come dozens of kilometers away from the ridges and mountaintops. Hence, solutions would be needed to be placed within the watershed and different goals would be set depending on its position in the watershed. Watersheds are areas shaped by water and rain; as it moves down slope by gravity from the ridges of mountains, it forms streams and tributaries, converging with other small streams to form a larger river channel. Water moves further downstream through the channel until it reaches the sea. It facilitates movement of sediments and reshapes the terrain, nourishing habitats and ecosystems along the way.

Concepts of Nature-based Solutions for Flood Management

Nature-based solutions are typically lower in cost, have a longer lifespan (as an infrastructure) and have multiple benefits besides its intended purpose. In managing floods in river basins there are four types of solutions: flood conveyance, water retention and detention, erosion control and flood impact reduction. These four concepts are planned, mixed and matched in a given planning area, ensuring interventions are part of a larger network of solutions.

1. Flood Conveyance increases the capacities of streams and river channels to transport water, especially in high flooding events. It helps water to reach the sea or  the outlets in a shorter period of time. This is typically needed in the lower and middle regions of the basin, where faster water flows would reach the sea or the outlet. The lower reaches’ flatter characteristics would benefit from this as water is more likely to move slower in flatter areas. Flood conveyance interventions includes:

• River Widening and Deepening increases the width and depth of channels, enhancing the river’s capacity to carry larger volumes of water during heavy rainfall or flooding events, reducing the risk of inundation in surrounding areas. This also helps accommodate seasonal variations in flow and minimizes the impact of extreme weather, particularly near tidal zones or outlets.

• Removing embankments in the middle or upper reaches slows water flows, allowing excess water to spread safely across floodplains. This reduces pressure on downstream channels and prevents sudden surges that could overwhelm urban or agricultural areas. Often, this is done alongside controlled spillways or designated floodways to guide water safely.

• Reviving old channels and removing obstructions involves reopening historical or secondary river pathways blocked by sediment, vegetation, or infrastructure. This helps water reconnect with natural courses, spreading flows more evenly, reducing localized flooding, and improving the health of river ecosystems.

2. Water retention and detention capture water in the upstream regions, to prevent them from immediately reaching the lower reaches of the basin, thereby lessening the water that could flood them at a given time. These are temporary stormwater storages, such as detention basins and detention tanks. 

Pussian SWIP (Pussian Forest Lake), Barangay Pussian, Alcala, Cagayan. The lake also functions as a local park and tourist attraction. Photo from Mindtrip.

• Retention ponds and basins are excavated depressions or natural pools designed to temporarily store excess stormwater from rivers or runoff, reducing the risk of river swelling and lowering peak flows downstream. Water is gradually released once rainfall and river levels subside. These measures are typically implemented in the upstream and middle reaches of a river basin, where they can intercept runoff before it reaches more populated or vulnerable areas. In regional contexts, this includes retention basins, stormwater detention tanks, small water impounding ponds (SWIPs) and small farm reservoirs (SFRs).

• Stormwater Detention Tanks are engineered storage structures that capture runoff in urban and densely built areas, where impermeable surfaces reduce infiltration and drainage systems can become overwhelmed during heavy rain events. These tanks temporarily hold excess water and release it slowly into waterways, protecting urban infrastructure, streets, and settlements from flooding. While not an NbS per se, this is noteworthy given the increasingly limited opportunities for Philippine cities with less and less space, such as in Metro Manila where there are far less sites for larger NbS interventions

• Wetland restoration enhances the natural capacity of wetland ecosystems to store and regulate water. This includes both coastal wetlands, such as mangroves, and inland or freshwater wetlands, such as peatlands, flooded grasslands, and swamps. Restored wetlands not only reduce flood risks by retaining excess water but also serve as biodiversity hotspots, providing habitat for numerous species.

3. Erosion Control lessens the sediments that waterways carry especially during storm events. Sediments, such as muds, rocks, debris worsen the impact of flood. Erosion control is done by reducing flows and capturing sediments along the way.

• Wetland and riparian forest restoration or conservation stabilizes the banks of waterways by reintroducing native vegetation along river margins. The roots of these trees and plants bind the soil, significantly reducing bank erosion during high flows, while also filtering sediments and slowing water runoff. 

• Bioengineering techniques combine vegetation, natural materials, and gentle engineering to stabilize riverbanks, stream edges, and coastal shorelines. Instead of using hard structures like concrete walls or riprap, these methods use plants (e.g., willows, reeds, grasses), coir logs, fascines, or other biodegradable materials to reinforce soil and absorb the energy of flowing water or waves.

4. Flood impact reduction can be done through structural and non-structural means. Among the non-structural ones, effective no-build zoning and early warning systems. Gray infrastructure such as dikes and flood walls lessen exposure of settlements to flood inundation.

• Early warning systems help coordinate response, particularly on the public side; encouraging evacuation and related pre-emptive measures.  These also improve lead time for decision-makers, allowing agencies to mobilize resources before hazards escalate.

• Strict zoning avoids settlements being built in flood-prone areas and includes preservation of critical land covers, particularly forests. This reduces exposure and maintains natural buffers that slow runoff and absorb excess water.

• Embankments and levees prevent floodwaters from reaching settlements, especially during stronger rainfall events. They also help guide river flows in a controlled manner, reducing the risk of sudden overbank flooding.

Fragmented Governance as Constraint

The highly decentralized governance structure of the Philippines has been one of the key issues in addressing cross-border problems, including flooding. National government agencies have overlapping functions, and thus lead to more difficult coordinations, as well as redundant measures. 

Watershed boundaries are not yet widely mainstream into local government planning; moreso, most municipalities are part of larger watershed systems. Hence, they cannot, alone, pursue watershed-level planning for flood management. Say, the flooding in the City of Manila (downstream, coastal level) will be influenced by the flood waters coming all the way from Marikina (mid to upstream); or the floods in Pampanga’s coastal towns are well-established to come from the upper stream regions in the province and in Nueva Ecija, thus flooding is likely to prolong. The floods coming to Naga City (Camarines Sur) are generated from the uplands of Mount Isarog to its east, but also from the rest of the larger Bicol River Basin coming from as far as the Mayon Volcano at more than 90km southeast.

How Local Governments and Private Citizens can Contribute to Local Blue-Green Infrastructure

Moving within the realities of fragmented decentralized institutions, flood control master plans have been written for various large planning areas such as Metro Manila (Metro Manila Flood Management Project (MMFMP)) and in the Cagayan Valley (Cagayan Valley Flood Mitigation Master Plan 2026 to 2030), that provide a comprehensive study of flooding, towards detailing a list of projects for government agencies and local government units (i. e. towns and municipalities) to follow. These are typically a combination of gray and nature-based infrastructures.

For Metro Manila, the master plan lists proposed projects befitting for a highly congested metropolis, particularly: a network of pumping stations, increase of water retention capacities, resettlement of informal households and waterway waste management. For Cagayan Valley and its watershed basin, which stretches from Cagayan, Isabela and Nueva Vizcaya, a network of combined gray and NbS infrastructure was introduced, such as river protection (mostly concrete infrastructure and slope protection), small water impounding ponds (SWIPs) and small farm reservoirs (SFR). SWIPs and SFRs help retain water from the upper reaches, thus lessening the volume of water flowing down at a given time. For this master plan to succeed, municipalities must align with existing regional and national flood control plans for guidance and success of these regional plans. 

Local governments and private citizens play a crucial role in expanding blue-green infrastructure by integrating nature-based solutions into everyday spaces. Blue-green infrastructure refers to networks of natural and semi-natural systems—such as waterways, parks, rain gardens, and permeable surfaces—that manage water and mitigate stormwater flooding as well as reduce heat, and improve urban ecosystems. While these have crosscutting benefits for flood management and environment, these are also significantly lower in cost and do not need highly technical expertise to implement, making scalability easier. 

Local governments (e. g. municipalities and barangays) may be able to provide these blue-green infrastructure solutions at the urban or settlement level:

1. Improving Water Retention by introducing permeable surfaces through:

Curbside/streetside rain garden design. WTA Labs. 2025.

• Rain gardens: small gardens that absorb rainwater. Can be within existing office properties and be made by converting concreted surfaces into rain gardens. 

2. Bioretention swales: gardens or vegetated stretches that absorb rainwater down until it reaches a permeable drainage pipe layer

Bioswale diagram. From Rainsmart Solutions.

3. Rainwater Harvesting: attached in building gutter systems, this stores water for other uses.

Malabon Rainwater Harvesting System. From Malabon City Government

4. Planting Trees: in the community and street levels lessens water running off through its leaves and its roots. This also reduces pollution in water runoff.

5. Reducing concrete and non-permeable surfaces in parks: keep parks green to improve their permeability as well as improve the overall amenity of the park.

6. Maintaining waterways to improve flood flow and conveyance.

   1. Clean waterways: waterways must be clean and free of debris and built-structures.

   2. Creek Linear Parks and Green Buffer Zones: maintain waterways as non-built, vegetated zones. They may also be transformed into linear parks.

7. Lessen landslides and runoffs through vegetation strips in steep and hilly areas.

8. Maintain and/or restore inland and coastal wetlands, which can also serve as tourist spots or public space amenities.

9. Faithfully preserve forest lands, especially in the uplands and steep slopes. The loss of forest lands would lead to worsen floods through larger sediments and frequent flash floods which cause more damages.

We must rethink how we deal with floods. Restoring and promoting our ecosystems would be the foundation for strong and resilient communities.

Nature-based Solutions, including blue-green infrastructures offer a holistic way of improving our adaptability and resilience from natural hazards, while ensuring the health of the environment, and in effect, improving the daily life of residents. These are actionable from all scales: from the national or regional levels, down to the municipal, barangay and household levels and across contexts be it urban or rural; cost-effective from its intended impacts to its multiplier effects. While gray infrastructure would still hold a significant role in flood mitigation, combining and promoting nature-based solutions would amplify such solutions, while adding additional safeguards and mitigation.