How urban flood warning systems work: Mumbai, Chennai show the way

Mumbai and Chennai use systems designed to predict where rain may cause flooding, but street-level warnings need much more than rainfall forecasts

Mumbai is witnessing torrential downpours, with the India Meteorological Department (IMD) forecasting heavy rainfall over the wider Konkan region this week. While IMD warnings indicate where and when heavy rain is likely, urban flood warning systems go a step further: they estimate which parts of a city may flood, how deep the water could be, and when civic agencies need to act.

 

Here is a look at what urban flood warning systems are, how they work, and why major Indian cities need them.

 

What is an urban flood warning system?

 

A heavy rain forecast does not automatically mean severe flooding everywhere. Flooding depends on local drainage, tide timing, blocked outfalls, nullah capacity, land slope and how intense the rain is over a short period.

 

 

This is where an urban flood warning system comes into the picture. While a weather forecast predicts rainfall, a flood warning system seeks to estimate the likely impact of that rainfall after it hits the city.

 

A city-scale flood warning model combines rainfall forecasts with information on the elevation and slope of the land, rivers and lakes, drainage capacity, soil conditions, buildings, roads and other surfaces that prevent water from draining quickly.

 

In coastal cities such as Mumbai and Chennai, tides are also part of the calculation because high tides can slow the discharge of stormwater into the sea. Reservoir releases can add water to rivers, while roads, buildings and other structures can affect the movement of water.

 

How does Mumbai’s urban flood warning system work?

 

Mumbai launched iFLOWS-Mumbai, an integrated flood warning system, in June 2020. Developed by the Ministry of Earth Sciences with the Brihanmumbai Municipal Corporation, it was designed to estimate flood risk across all 24 municipal wards.

 

According to the Ministry of Earth Sciences, the system uses rain-gauge readings and local data on land use, topography, drainage systems, water bodies, tide levels, infrastructure and population. It then models weather, rainfall, runoff, water movement, tides and storm surges to produce flood warnings.

 

The system is designed to account for flooding caused not only by heavy rain, but also by riverbank breaches, storm surges, high tides and obstructions to water flow created by roads, buildings and railway lines.

 

iFLOWS works through seven modules. The first brings together real-time information, including IMD weather forecasts and water levels in Mumbai’s rivers and lakes. An inundation module uses these inputs to estimate the extent of flooding up to three days in advance, while another module predicts how water will move through areas expected to be flooded.

 

The system then assesses the vulnerability of people, infrastructure and other exposed areas. These assessments feed into a decision-support system that helps authorities determine where action may be needed. A final dissemination module sends the information to field officials through different communication channels.

 

At its launch in 2020, the government said iFLOWS could estimate flood inundation three days in advance, provide immediate weather updates for the next three to six hours, and predict around 12 hours ahead whether a particular location could be flooded.

 

How does Chennai’s flood warning system work?

 

Chennai’s Real-Time Flood Forecasting and Spatial Decision Support System is designed to provide up to 72 hours of early warning. It combines forecasts from five weather models with real-time information on rainfall, rivers and lake levels to predict possible flooding.

 

The system covers 74 watersheds and 989 km of rivers and can produce localised inundation forecasts for major flood-prone areas. It also includes a reservoir-operation guidance mechanism to help authorities manage flood risks linked to lake levels and water releases.

 

Developed after the 2015 Chennai floods with World Bank funding, the system was implemented with assistance from IIT Madras.

 

Coastal cities such as Mumbai and Chennai face a more complex flooding problem because heavy rain, high tide, drainage congestion, river flows and dense construction can interact quickly. This is why rainfall warnings alone may not be enough for civic response.

 

Do other Indian cities have urban flood warning systems?

 

India does not yet have dedicated city-level flood forecasting systems across all its major urban centres. The IMD’s Vision 2047 document says urban flood warning systems are operational in Mumbai and Chennai and proposes extending them to other major cities in phases.

 

The need is much wider. A 2025 World Bank assessment estimated that around 318,000 km of urban roads in India, or 27 per cent of the country’s urban road network, are exposed to a one-in-100-year flood. As extreme rainfall and urban expansion increase flood risks, systems such as those in Mumbai and Chennai show why city-specific warnings may become essential for India’s major urban centres.