The peculiarity of urban floods
What are urban floods and why are we witnessing them?
Urbanisation affects disasters just as profoundly as disasters can affect urbanisation,” writes Mark Pelling, geographer and climate change expert at King’s College, London, in his book, The Vulnerability of Cities.
In recent times, urban flooding has emerged as a major concern.As the weather gets more erratic and one-time rainfall, like the one recently witnessed in Srinagar, increases, large concentrated populations in urban areas face increased risk of flood.
There are some peculiarities of flooding in urban areas. Its primary reason is surface water runoff. Surface runoff is the excess water from rain or melting snow that flows over the earth’s surface without getting absorbed. In the urban landscape, it is controlled and managed artificially through drains which flush out the runoff from the city, unlike in rural settings where the runoff is absorbed naturally by farmlands and ponds.
Urban areas are characterised by impervious surfaces like roads, pavements and buildings. High rate of development and construction in these areas has resulted in the loss of soft landscape. This decreases a city’s capacity to absorb water, making it dependent solely on the outflow of surface water runoff. Under such circumstances, even moderate rainfall can lead to flash floods in low-lying areas. Cities located along a river might face an added problem if the river flows at a higher level within its embankment. Guwahati, a low-lying city on the bank of the Brahmaputra river, faced unprecedented flooding this year.
In the past few years, flooding in Delhi
due to overflow of the city’s 18 major drains has become a common phenomenon. Heavy rain in the Yamuna’s upstream increases its water level in Delhi, due to which the drains in the city experience reverse flow.
As more and more farmlands and green areas are being urbanised, the amount of surface area for water percolation is getting reduced. As a result, all the runoff flows on the land, without being absorbed. This increases the chance of floods.
Another reason for urban flooding is the lack of drainage system in an urban area. As there is little open soil to absorb water, nearly all the excess rainwater needs to be transported to the drainage system. Highintensity rainfall can cause floods when a city’s drainage network does not have the capacity to drain away excess water in adequate time.
A 2003 study by C P Konard, published in the US Geological Survey, shows that the streams in the urban areas of the US rise more quickly than those in rural areas during storms and have higher discharge. Thus, urban spaces flood more rapidly.It also shows that debris from broken bridges and other construction that the streams collect further restrict the water’s flow from the city, increasing its level and causing floods.
URBANISATION THE ROOT CAUSE
The risk of infrastructural damage increases with increasing urbanisation. A 2013 research paper by U S De, former additional director-general of meteorology (research), Pune, analysed flooding in four megacities of the country—Delhi, Chennai, Kolkata and Mumbai.The paper demonstrated how rapid and uncontrolled urbanisation is at the root of floods and flood-related damages in these cities. It noted that the mechanism for urban flooding is complex and locationspecific. Hence, each city needs its own flood management practices.
Encroachment is another fallout of urbanisation. The paper mentions that the number of water bodies in Delhi has been reduced to 600 from the original 800 due to encroachment. It notes that the floodplains of the Yamuna—home to thousands of illegal colonies—are the most populated parts of Delhi. High population density demands more infrastructure, leading to environmental degradation. By 2025, the population of tropical Asia is estimated to rise to 2.4 billion. Many of the most populated cities of the world—Tokyo, Mumbai, Shanghai, Kolkata, Jakarta, Delhi, Seoul, Manila and Dhaka—are located in Asia, three of which are in India.
Kolkata has been built on wetlands. Chennai, too, has seen massive construction in recent decades, reducing soil cover and vegetation. While other cities can expand in adjoining areas, Mumbai cannot due to its long coastline.The city was built by merging seven islands and hilly areas. Nearly 60 per cent of Mumbai’s population lives in poorly built temporary settlements. Only three
Highintensity rainfall can cause floods when a city's drainage network loses capacity to drain precipitation
outfalls (discharge point of a waste stream into a body of water) to the sea have floodgates.The remaining 102 outfalls in the city open directly into the sea. During high tide, the sea water enters the drainage system through these outfalls, causing floods.
WEATHER: THE MISSING LINK
The year 2005 was recorded as the hottest year of the century. Incidentally, in the same year, the worst urban flooding was reported in Mumbai on July 26-27. During those two days, the city witnessed an unprecedented 944 mm of rainfall in 24 hours. In the same year, 10 severe urban floods were reported from across the country. Three-fourths of Chennai was inundated. It affected more than 500,000 people.
In 2006,22 cities in India reported floods. The increasing trend of urban flooding was carried into 2007, where the number of affected cities rose to 35.Extreme weather events have increased in recent times. “Floods and droughts will become more frequent. One projection shows that the intensity and the number of tropical cyclones will increase in the next 40-100 years,” De says.
Anil K Gupta, director of the Wadia Institute of Himalayan Geology in Dehradun, Uttarakhand, adds that the entire Himalayan range is vulnerable because of rising temperatures. “Each and every valley—be it Kashmir, Kedarnath or Badrinath—faces the threat of increased precipitation,” he says.
According to the Jammu and Kashmir State Action Plan on Climate Change,2013, minimum temperatures in the Himalayan region are projected to rise by 1°C-4.5°C. The report also says that the number of rainy days in the region in 2030s may increase by five or 10.The intensity of rainfall is likely to increase by 1-2 mm per day. “What is increasing is sudden precipitation, which happened during the recent Kashmir floods,” Gupta says.
A report by the UN Intergovernmental Panel on Climate Change notes that every year there will be at least one extreme weather event in the Himalayas. “Last year, it was the Uttarakhand disaster. Before that we saw floods in Pakistan and cloudbursts in Jammu and Kashmir,” says Jatin Singh, chief executive officer of Skymet, a private weather forecast company.
In the course of such events, many urban areas are likely to be affected, the way Srinagar was caught unawares. The India Meteorological Department (imd) is yet to recognise that extreme weather events are a result of climate change. For the last four major natural disasters in India—Mumbai floods of 2005, Leh cloudburst of 2010, Uttarakhand disaster of 2013 and J&K floods of 2014—imd has cited disturbances in the wind current and monsoon as reasons. It used words like “unprecedented”, “unusual” and “unique”, but offered no explanation for why these events are happening at such high frequency.
In light of the complexity of urban floods, we need a comprehensive plan of action to reduce the damage thus caused.
The Army and National
Disaster Response Force were not familiar with the topography of Srinagar. This delayed the scaling up of rescue
Mumbai was plunged into chaos as the city received more than 900 mm of rainfall
in July 2005; (Right) Flood in Chennai the same year affected more than 500,000 people