URBAN FLOODING....... ........CHENNAI FLOODS

Urban flooding is significantly different from rural flooding as urbanisation leads to developed catchments which increases the flood peaks from 1.8 to 8 times and flood volumes by up to 6 times. Consequently, flooding occurs very quickly due to faster flow and sometimes in a matter of minutes. The un-even distribution of rain fall coupled with mindless urbanization, encroaching upon and filling up natural drainage channels and urban lakes; to use the high-value urban land for buildings are the cause of urban flooding.

CAUSES OF URBAN FLOODING:

• Heavy Rainfall / Flash floods 
• Encroachment of  Lakes and lakes become smaller to hold the water
• The drains carry large amounts of sediments which are deposited in the lower courses making beds shallower thus reducing channel capacity. 
• Population pressure causes overgrazing, over cultivation and soil erosion which increases the risk of flooding.
• Deforestation of Large areas of forests near the rivers/catchment causing overflow and in turn lead to urban flooding.
• Trespassing on water storm drains leading to obstruction of water flow and thus contributing immensely to the fury of floods.
• Urbanization leads to paving of surfaces which decreases ground absorption and increases the speed and amount of surface flow. This results in inadequate channel capacity causing urban flooding.
• Un - authorised colonies have been developed by the local colonizers on the agriculture land, earlier being used for crop has been purchased at lucrative prices from farmers, without consideration to the city plans ,drainage, sewerage etc. and thus subjected to flooding during heavy rain falls.
• Poor Water and Sewerage Management 
• Lack of attention to the natural of hydrological system.
• Lack of flood control measures.
• Multiple authorities in a city but owning of responsibility by none.

FLOOD IN CHENNAI

• Chennai receives on an average approximately 1300 mm of rainfall per year – most of this (~800 mm) falls during the northeast (NE) monsoon in the months of October through December. Due to the plain terrain Chennai lacks natural gradient for free run-off. 

• The sewage system in Chennai was originally designed for a population of 0.65 million at 114 litres per capita per day of water supply; it was further modified during 1989–1991, but is now much below the required capacity. Cooum and Adyar rivers in Chennai city are almost stagnant and do not carry enough water.

• The green cover reduced rapidly across the city between 1997 and 2011. In some wards almost 99% of the green cover has been replaced by non-vegetative development. As a result, the water-holding capacity of the city’s surface has gone down drastically.
  
  
• Chennai has witnessed a steady deterioration of and decease in water bodies and open spaces .It is estimated that in Chennai city more than half of the wetlands have been converted for other uses. Chennai had about 150 small and big water bodies in and around the city, but today the number has been reduced to 27.
  
  
• According to a report from the Chennai Metro Development Authority, there are over 1.5 lakh illegal structures in the city which have been responsible for the disappearance of over 300 water bodies.
  
  
• Chennai has a host of expensive infrastructure aimed at ushering in a “Make in Chennai” boom – a brand-new airport built on the floodplains of the River Adyar, a sprawling bus terminal in flood-prone Koyambedu, a Mass Rapid Transit System constructed almost wholly over the Buckingham Canal and the Pallikaranai marshlands, expressways and bypass roads constructed with no mind to the tendency of water to flow.
  
  
• Not acting on time, neglecting advance notice of possible flooding, and overburdened drains, reservoirs, rivers and rivulets has blighted Chennai for years.
  
  
• Before the deluge in Chennai earlier in the  month, after being warned of heavy rainfall, the civic authorities decided to release water from the Chembarambakkam reservoir on the outskirts into the Adyar river. Since more than 500 mm rainfall was predicted over 1 and 2 December, bringing down the level of water in the reservoir from 22 to 18 feet — so that it could absorb the downpour — appeared to be a viable solution. But the proposal became mired in bureaucratic wrangling and the sluice gates could not be opened before the rain started and when it began to rain, the reservoir overflowed within hours. Panicking officials opened the sluice gates, hoping Adyar would absorb the gushing water. But soon its embankments were overrun. The swollen river soon inundated the city.

Urban storm water management may not be efficient unless it was handled in conjunctive manner with protect areas for groundwater recharge and utilize conjunctive management to enhance groundwater storage. A multidimensional approach is necessary to solve this problem with the help of Urban planning which is of critical importance today to plan out the waterways, the road infrastructure etc. Experts believe it is critical to have external agencies assess drainage systems and waterways and this disaster serves as a stark example of why it is essential for project planning to go hand in hand with city planning. Measures to better cope with current climate variability and Integrated Flood Risk Management are important today.