Urban Flooding

In News

• Many urban areas in India face flooding and water stagnation related issues during the monsoon season. 

Case Study: Bengaluru Urban Flooding

• Location & Formation of Lakes: 
  • Bengaluru is located at the top of a ridge which is the water divide between the watersheds of the Kaveri and the Ponnaiyar (Dakshina Pinakini) rivers.
  • Original settlements were concentrated on the ridges while the valleys were used for agriculture. 
  • To irrigate lands, bunds were erected to retain the water — creating lakes. Each lake had its own command area which it irrigated.
• Obstruction in Valleys:
  • The city has numerous valleys which act as conduits carrying water to these two major rivers.
  • The bulk of the flooding and stagnation in Bengaluru happened because of the obstructions in the valleys. There were very few instances of flooding outside the valley. 
• Blockage of Canals:
  • The older streams that once flowed were redesigned to create artificial canals (kaluve) which were used for irrigation of the command area of each lake and for carrying excess water downstream. 
  • Extreme amounts of engineering, sewage flow and clogging further hampered flow in the canals.
  • These existing canals, which were not created for inundation requirements, proved inadequate to the task of carrying excess rainwater. 
• Constructions Affected Topography: 
  • Ignoring the topography of the land, construction began in the valleys and ridges, which in turn altered the original topography, with minor drains disappearing.
  • The new structures not only impacted water infiltration into the soil but also began obstructing the movement of water in the valleys. 
• Huge Population: 
  • The city’s population, which stood at 1.6 lakh in 1901, is estimated to be more than one crore today. 
  • This rapid and extreme growth has triggered a massive demand for land and the city began sprawling out. 

What is Urban Flooding?

• Urban flooding occurs when stormwater flows into an urban area at a higher rate than it can be absorbed into the ground or moved to a water body (lake, river, etc.) or stored in a reservoir.

Causes of Urban Flooding

• Meteorological Factors:
  • India being a tropical country, has very heavy rainfall throughout the monsoon season. 
  • Global warming results in extreme weather conditions and is apparent to increase the flood risk significantly.
• Hydrological Factors: 
  • Flood risk arises when the surface runoff is more than the infiltration rate during precipitation. 
  • The infiltration rates depend upon the type of soil and their respective water retention capacity. 
• Human factors: 
  • Human intervention in the natural environment has increased the urban flood risk. 
  • These anthropogenic factors coupled with encroachments cause interference in the smooth flow of water in the drainage channels. 
• Urbanisation: 
  • Urbanization in India is directly linked with the increase in impervious surface. 
  • This reduces the speed and scale of percolation and increases surface runoff from buildings, roads, and other hard surfaces. 
  • By the very definition of urban floods – It can be stated that urbanization is directly linked with the risk of urban flooding. 
• Drainage Clogging: 
  • Population densities in urban centres are increasing at an alarming rate than designed for. 
  • The supporting infrastructure facilities such as solid waste disposal, sewer lines, stormwater drains etc are not being developed to adapt to the increased demand. 
  • This results in improper solid waste disposal into water bodies, unattended street waste clogging drainage channels. 
• Interference due to Infrastructure: 
  • The interference can also be in the form of the poorly planned construction of roads, bridges, railway tracks, and check dams, which hampers the flow of water resulting in a flood. 
• Unplanned release of water from dams: 
  • Unplanned and sudden release of water from dams and lakes lead to floods in an urban area, without giving the public enough time to respond. 
• Illegal mining activities: 
  • Illegal mining of river sand and quartzite for use in building construction deplete the natural bed of the rivers and lakes and have an irreversible damaging impact. 
  • This causes soil erosion and reduces the water retention capacity of the waterbody, increasing the speed and scale of stormwater flow and changes the natural course of water.
• Cultural or religious festivals:
  • People also misuse water bodies by throwing non-bio degradable matter into the rivers and lakes, reducing the water quality. 
  • In the event of floods, the suspended particles and pollutants overflow into the neighbourhood posing health risks.

Impact of Urban Flooding

• Affecting People: Urban areas are densely populated and people living in vulnerable areas suffer due to flooding, sometimes resulting in loss of life. 
• Destruction: Major cities in India have witnessed loss of life and property, disruption in transport and power and incidence of epidemics. 
• Health Issues: It is not only the event of flooding but the secondary effect of exposure to infection also has its toll in terms of human suffering, loss of livelihood and, in extreme cases, loss of life.
• Damages Infrastructures: Urban areas are also centres of economic activities with vital infrastructure which needs to be protected 24×7. In most of the cities, damage to vital infrastructure has a bearing not only for the state and the country but it could even have global implications.

Way Ahead

• The management of urban flooding has to be accorded top priority. Proper city master plans are required to be constructed and implemented.
• There is a need to have a deeper understanding of the growth patterns in the cities, their terrain, their infrastructure, and their ecosystems.
• Planting more vegetation and trees as they reduce the speed of the water, the presence of a previous surface helps in percolation and increases the infiltration rates, hence avoiding heavy damages to life and property. 

Source: IT