A University of Madras study has linked Chennai’s expanding built-up area, shrinking vegetation and water bodies, drainage gaps and low-lying terrain to rising flood risk across several parts of the metropolitan region. Its findings point to a structural problem: the city is not only receiving intense rainfall, but is also replacing the land and natural systems that previously absorbed and carried water.
The study identified flood-risk hotspots in Velachery, Pallavaram, Chromepet and adjoining southern localities, along with parts of western Chennai, Koyambedu, Villivakkam and areas around Tondiarpet. It found that between 2000 and 2024, Chennai’s built-up area increased by 7.6%, while vegetation cover declined by 26.14% and water bodies shrank by 22.54%.
Those changes matter because urbanisation alters how rainfall moves through a city. Roads, buildings and other paved surfaces are less permeable than vegetation or open ground. When they replace surfaces that allow water to seep into the soil, more rain becomes surface runoff. The result is greater pressure on stormwater drains, channels, rivers and low-lying areas during intense rainfall.
The study’s first author, Rahul Debnath, said spatial analysis showed that flood-prone areas were concentrated in low-lying coastal zones, western and southern peri-urban areas and the Adyar River floodplain. These locations were identified through the overlap of urban expansion, high flow accumulation and historical flood occurrences. The overlap is significant because it connects three different dimensions of risk: how the city has grown, how terrain directs water and where flooding has already occurred.
The findings also place Chennai’s peri-urban expansion at the centre of the flood question. Development at the city’s edges has extended roads and buildings into areas where drainage infrastructure may not have expanded at the same pace. According to the study, drainage gaps in western and southern peripheral areas overlap with recent urban growth, suggesting that new development and supporting infrastructure have not always advanced together.
For the study, areas located more than one kilometre from mapped stormwater drains were classified as having drainage gaps. This measure does not by itself establish whether a location will flood during every major rainfall event. It does, however, provide a way to identify places where residents and built-up areas may be farther from the city’s formal drainage network.
The researchers used Landsat satellite imagery from five years, elevation data, India Meteorological Department rainfall records and historical flood locations. Geographic information systems and remote sensing were used to examine changes in land cover, terrain and flood exposure. This approach allows the researchers to compare how Chennai’s physical form changed over time with the locations where flood risk is concentrated.
The rainfall record cited in the study adds another layer to the city’s vulnerability. Rainfall in 2015 exceeded its estimated 100-year return-period intensity, while the 2023 event was associated with a nearly 25-year return period. These figures do not mean that future floods will follow a fixed cycle. They show instead that Chennai’s drainage and land-use systems have been tested by rainfall events of very different magnitudes, including events that place exceptional stress on urban infrastructure.
The central planning issue is therefore not simply whether Chennai has more drains than it did in the past. It is whether drainage capacity, land-use decisions and the city’s natural water-retention systems are being considered together. A drain network can expand while flood exposure also grows if construction continues to reduce permeable land, narrow water pathways or place more assets in low-lying areas.
The Greater Chennai Corporation has expanded its stormwater drain network in recent years. According to the corporation, 1,144.5 kilometres of stormwater drains were laid between 2022 and 2025. That figure indicates a substantial public infrastructure response, but the study’s identification of drainage gaps shows that network length alone does not settle the question of coverage or performance.
Drainage is a connected system rather than a collection of isolated roadside structures. Its effectiveness depends on where drains are located, how they connect to larger channels, whether their capacity matches runoff volumes and whether they can carry water away from low-lying areas. The information supplied with the study does not establish the capacity or operational condition of every drain in the identified hotspots. It does establish that gaps remain a relevant factor in the city’s flood-risk map.
The study’s recommendations reflect this wider planning challenge. Its authors called for land-use planning, terrain characteristics and historical flood records to be integrated into flood-risk management. They also recommended assessing drainage coverage and capacity to identify priority areas for intervention. This shifts the focus from responding to water after it accumulates to deciding where development should occur, what natural features must be protected and where infrastructure investment is most urgent.
Tamil Nadu’s disaster-management planning already uses location-specific assessments. Revenue administration and disaster management commissioner B Chandra Mohan said the state had identified around 1,030 locations with high or very high vulnerability and another 1,003 with moderate vulnerability. These classifications were based on experience, forecasts and water-stagnation depth.
According to Chandra Mohan, teams had assessed vulnerable locations and prepared mitigation and response plans for them. Each location has a plan that includes mitigation and response measures, as well as water-level triggers for action and evacuation. This operational system addresses what happens when flooding begins: when authorities act, how residents are moved and where people are taken as water levels rise.
The University of Madras study addresses an earlier stage of the same problem: why particular locations are repeatedly exposed and how urban growth may be increasing that exposure. The two approaches are related but different. Vulnerability plans help manage immediate risk, while land-use and drainage analysis can help identify the physical conditions that create or amplify that risk.
Chennai’s flood challenge is consequently distributed across several institutions and policy areas. The corporation is responsible for major urban drainage works, while state disaster-management authorities coordinate vulnerability assessments, mitigation and emergency response. Land-use decisions and development patterns determine how much open, vegetated or water-retaining space remains available. The study’s findings suggest that these functions need to be read as one urban system rather than as separate administrative tasks.
The data also complicates any simple narrative that attributes flooding only to extreme rainfall. Rainfall intensity is a major trigger, but the study connects flood exposure to the city’s changing surface, shrinking water bodies, terrain and drainage coverage. The 7.6% increase in built-up area, 26.14% decline in vegetation cover and 22.54% reduction in water bodies between 2000 and 2024 describe a city whose ability to absorb and route water has changed over the same period in which development has expanded.
What remains unclear from the supplied findings is the precise contribution of each factor at each hotspot. The study identifies relationships between urban expansion, land-cover loss, terrain, drainage gaps and historical flooding, but the available report does not provide location-by-location drainage capacities, flood depths or the performance of individual projects. Those details will matter when authorities decide which areas require new drains, upgraded capacity, protection of water bodies or changes in development control.
The immediate significance of the study is that it gives Chennai a spatial framework for understanding flood risk. It identifies where urban growth and drainage gaps overlap, highlights the role of the Adyar floodplain and low-lying coastal and peri-urban areas, and places the city’s changing land cover alongside rainfall history. The next test for the city’s flood strategy will be whether these findings are incorporated into drainage prioritisation, land-use planning and location-specific response plans.
