Countries globally are experiencing increasing rates of urbanisation, with the total population living in urban areas expected to reach 68% by 2050. Asia and Africa are expected to account for close to 90% of this growth.

As cities expand, growing populations will place greater demands on urban infrastructure. Urbanisation can also increase flood risk, as roads and buildings replace natural surfaces, reducing rainfall infiltration and increasing surface runoff.

Blue-Green Infrastructure (BGI) could help manage this runoff by incorporating natural features into urban environments. However, much of the research on BGI has focused on cities in the Global North, where established policies, technical expertise, and financial resources can support its implementation. This raises questions about whether these approaches can be adapted to densely populated cities elsewhere.

Mumbai, India, provides an important case study. The city experiences intense monsoon rainfall alongside extensive impermeable surfaces and limited space for new infrastructure. A recent study, published in the Open Access journal Water, modelled whether BGI could be integrated into two densely populated areas of Mumbai to help manage urban flooding.

Ms. Vedanti Kelkar, a PhD candidate at Dresden University of Technology, Germany, and lead author of the study, explains the study’s purpose:

Ultimately, we wanted to investigate the feasibility and performance of integrating BGI into an existing neighbourhood to manage urban flooding, while taking into account the formal–informal complexities of Mumbai.

This article explores further insights from Ms. Kelkar into Mumbai’s infrastructure, the challenges of implementing BGI in rapidly urbanising areas, and the potential for the developed model to inform flood management strategies worldwide.

Using Mumbai as a case study for flood management

Mumbai has experienced rapid urban expansion, resulting in substantial increases in built-up areas. In addition, more than half of the city’s population is thought to reside in informal settlements across the city.

The researchers focused on the F-North and G-North wards of Mumbai. These areas contain a mixture of planned residential neighbourhoods, commercial and industrial areas, and informal settlements, alongside an existing network of parks, gardens, open spaces, and footpaths.

Rather than replacing these existing spaces, Ms. Kelkar and a team of researchers investigated how BGI could be incorporated into the surrounding built environment:

We wanted to work with the existing character of the neighbourhood and investigate how additional BGI interventions could be integrated into the existing built environment alongside these spaces.

Several ways of incorporating BGI into existing urban infrastructure were proposed:

• Adapting footpaths to incorporate permeable pavements.
• Modifying road dividers to function as infiltration trenches.
• Converting commercial building rooftops into green roofs.

These interventions allowed the researchers to investigate how BGI could be distributed throughout an existing neighbourhood without relying on large areas of land for new infrastructure.

Mumbai’s unique stormwater challenge

Mumbai receives an average of around 2,300 mm of rainfall each year, much of which falls during the monsoon season between June and September. The city is also estimated to be home to nearly 22 million people.

Finding space for new infrastructure can be challenging in such a densely developed city, as Ms. Kelkar explains:

The availability of land for new conventional infrastructure is extremely limited. This means that Mumbai needs approaches to stormwater management that can work within the existing built environment while also contributing to the wider liveability and sustainability of the city. This is where BGI, and nature-based approaches more broadly, become particularly relevant. BGI has the potential to provide hydrologically and technically effective interventions that can be integrated into existing urban infrastructure and the city’s stormwater management network.

However, BGI cannot necessarily be transferred directly from cities with proven success, she continues:

The context matters because BGI cannot simply be transferred from one city to another without considering local constraints, including the intensity of monsoon rainfall, high population densities, existing drainage infrastructure, limited open space and the characteristics of the built environment.

BGI may also provide benefits beyond flood management by helping address some of the environmental pressures associated with urban development:

BGI offers an opportunity to address some of the wider consequences of rapid urbanisation such as increasing impermeability, loss of natural systems and environmental degradation while potentially contributing to more liveable neighbourhoods.

Identifying suitable locations for BGI in Mumbai

To determine where Blue-Green Infrastructure could be introduced, the researchers combined spatial analysis using QGIS with hydrological modelling using the EPA Storm Water Management Model (SWMM).

SWMM was used to simulate the existing drainage system under a 25-year rainfall event, an amount of rainfall that has a 4% chance of happening each year. Information such as the drainage network, catchment characteristics, rainfall data, and historical flooding records were used to identify locations where flooding was likely to occur.

QGIS was then used to analyse these results alongside information about roads, buildings, and land use in the study area, Ms. Kelkar expands on the approach used:

We combined EPA-SWMM and QGIS because they provided complementary capabilities for identifying flooding hotspots and assessing where BGI could realistically be implemented. EPA-SWMM (Storm Water Management Model) is a hydrological and hydraulic modelling tool used to simulate rainfall, runoff and urban drainage systems, while QGIS is a GIS platform used to manage, analyse and visualise spatial data.

By combining these two techniques, the researchers could target interventions at areas prone to flooding rather than applying BGI uniformly across the study area.

The effectiveness of simulating BGI in Mumbai

The researchers then used their model to assess how the proposed BGI interventions could affect flooding under the simulated 25-year rainfall event.

Overall, the introduction of BGI resulted in a 12.6% reduction in peak runoff and a 45% reduction in peak flooding flow. However, the results also revealed that the interventions did not reduce flooding at every location, as Ms. Kelkar explains:

One particularly interesting finding was that the green roofs on a hospital building did not necessarily reduce flooding at the expected locations. By slowing down and extending runoff, they altered the timing of flows, causing runoff from different areas to arrive at the same time and creating a larger combined flow downstream.

This highlights the importance of considering how individual BGI interventions interact with existing infrastructure:

BGI cannot simply be assumed to reduce flooding everywhere; its effectiveness depends on how the intervention interacts with the existing drainage network and the wider catchment.

The findings therefore demonstrate the value of testing BGI interventions through hydrological modelling before they are implemented. This could help identify where interventions are most likely to be effective while also highlighting potential unintended effects.

Developing a model to apply to other cities experiencing rapid urbanisation

Although this work focused on two wards in Mumbai, Ms Kelkar provided some valuable insights into the potential of developing such a model to assess the success of BGI implementation elsewhere:

The approach can be scaled and replicated across different catchments to test different BGI interventions under local conditions before they are implemented on the ground. This can help develop BGI plans, designs and policies that are based on robust scientific and technical evidence.

However, applying BGI elsewhere will require consideration of the local environment and how these factors can influence the performance of selected interventions:

This is particularly important in Indian cities because rainfall characteristics, soil and groundwater conditions, available space, drainage networks and the built environment vary considerably between locations. Therefore, BGI placement and design should be informed by technically sound simulations rather than a one-size-fits-all approach.

The availability of reliable, open source data may also influence how accurately BGI can be modelled:

However, the availability of reliable data remains a significant challenge. In this study, we relied on data from a scientific paper that documented the city’s soil classifications and groundwater conditions. However, we were unable to obtain robust municipal-level data to validate these findings.

Ms. Kelkar also emphasises the importance of collaboration when developing BGI interventions:

Collaboration among municipal authorities, urban planners, engineers, communities, researchers, and other relevant stakeholders can help ensure that BGI interventions respond effectively to local environmental and social conditions.

Together, these considerations could help support the development of flood management strategies that are adapted to the needs of rapidly urbanising cities.

MDPI’s commitment to impactful Open Access research

We would like to thank Ms. Vedanti Kelkar for sharing her valuable insights and providing detailed information on the team’s research for this article.

Research has the potential to influence scientific innovation and inform policies that address global challenges. By supporting researchers and making their work accessible, Open Access publishing can help broaden the reach and impact of important scientific findings.

Discover more research, advances in Open Access publishing, and insights into MDPI’s contribution to scientific research on the MDPI blog here.

Further research on developing flood infrastructure, the effects of urbanisation, and city planning can be found across MDPI’s Open Access journals Smart Cities, Sustainability, and Water.