Floods in Delhi
The city has been experiencing floods of various magnitudes in the past due to floods in the Yamuna and the Najafgarh Drain system. The Yamuna crossed its danger level (fixed at 204.83m) twenty five times during the last 33 years (table 3.1). Since 1900, Delhi has experienced six major floods in the years 1924, 1947, 1976, 1978, 1988 and 1995 when peak level of Yamuna river was one meter or more above danger level of 204.49m at old rail bridge (2.66m above the danger level) occurred on sixth September 1978. The second record peak of 206.92m was on twenty seventh September 1988.
In the recent part, the city experienced high magnitude floods in 1977, 1978, 1988 and 1995, causing misery and loss of life and property to the residents of the city. A profile of these four floods (table 1) indicated the extent of damage caused by these calamities.
In Delhi Environment Status Report: WWF for Nature-India (1995), it has been pointed out that since 1978, the flood threat to Delhi has increased. In 1980, a discharge of 2.75 lakh causes at Tajewala resulted in flood level of 212.15 meters at the bund near Palla cillage in Delhi.
The flood situation is projected in the flood atlas map prepared by central water commission.
As per the map of the flood prone areas of Delhi has been classified into thirteen zones based on the flooding risk in relation to incremental rise in the water level of the Yamuna (DDA, 1993). These cover a range from 199m to 212 m level of water in the Yamuna. This zoning map covers part of North Delhi on the West bank of the Yamuna and almost the entire Trans Yamuna Area on the East bank. Besides this, the Delhi Flood Control Order also the NCTD into four Flood Sectors, namely Sectors, namely, Shahadra, Wazirabad - Babrapur, Alipur and Nangloi - Najafgarh sectors.
Although the unprotected flood prone area is only 1.7% or 25km only towards the south east and about 5% or 74 sq km in the north eastern parts which is protected by earthern embankments every year water level ruses in Yamuna above danger level and large population has to be evacuated to the top of the bunds and Delhi highways.
As already stated, main reasons for this rise of water level is not natural but release of excess water from Tajewala headworks upstream to the two canals one on left and other on the right bank of the river. Rise in water levels also cause back flows in the connecting drains and have effect on the city drain network causing overflow cause of many monsoon related diseases.
A significant phenomenon which has been increasing during recent years is that of local flooding. Urban areas are characterized by a high area under impervious surfaces (Roads, pavements, houses etc). High rates of development along with the resultant loss of soft landscape has led to high surface water sun-off rates. This results in flash floods in the low lying areas even after moderate precipitation. Another factor adding to this effect is that of river because the river is already flowing at a higher level within its embankments. Thus, the water gets logged in the city areas and it takes several days to mechanically pump it out and bring the situation under control. Similarly, during the past few years, flooding due to the city's 18 major drains has also become a common phenomenon. Already under the pressure of the city's effluent discharge, these drains experience reverse flow from the Yamuna, which is in spate, and as a result they tip their banks, flooding the neighbouring colonies.