Nature cannot take more abuse. Hence the degradation caused to the environment of the city has to respond in terms of its wrath. Floods in Delhi are not natures wrong doing, it is invariably the irresponsibility of the authorities and those who are to tally insensitive of human life blinded by the economics of haves and have nots. This is very clear from the recurring phenomenon of floods in the mighty river Yamuna and flash floods caused by rains due to choked drains of Delhi.
Keeping in view the topography, Yamuna catchments upto Delhi is divided in two parts - (1) The upper catchment from source in Himalayas to Kalanaur in Haryana - which com[rises parts of Himachal Pradesh and hills of West Uttar Pradesh and (2) the lower catchment from Kalanaur to odl Delhi rail bridge which consists of West Uttar Pradesh and Haryana.
The flow of Yamuna within Delhi is by and large influenced by discharge from Tajewala Headwork 240 kms upstream. In the event of heavy rain in the catchment area excess water is released from Tajewala. Depending upon the river flow level down stream, it takes about 48 hours for Yamuna level in Delhi to rise. The rise in water level also causes backflow effect on the city's drains. The city also experiences floods due to its network of 18 major drains having catchment areas extending beyond the city's limits.
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.
Four major floods : profile
1977: Najafgarh drain experienced heavy floods due to discharge from the Sahibi River. The drain breached at six places between Dhansa and Karkraula, marooning a number of villages in Najafgarh block. Six human lives were lost due to house collapse. 14 persons died in a boat mishap. Crop damage was estimated at Rs 10 million.
1978: (September) River Yamuna experienced a devastating flood. Widespread breaches occurred in rural embankments, submerging 43 sq km of agricultural land under 2 meters of water, causing total loss of the kharif crop. In addition to this, colonies of north Delhi, namely, Model town, Mukherjee Nagar, Nirankari Colony etc. suffered heavy flood inundation, causing extensive damage to property. The total damage to crops, houses and public utilities was estimated at Rs 176.1 million.
1988: (September) River Yamuna experienced floods of very high magnitude, flooding many villages and localities like Mukherjee Nagar, Geeta Colony, Shastry Park, Yamuna Bazzar and Red Fort area, affecting approximately 8,000 families.
1995: (September) The Yamuna experienced high magnitude floods following heavy runs in the upper catchmen area and resultant release of water from Tajewala water works. Slow release of water from Okhla barrage due to lack of coordination between cross state agencies further accentuated the problem. Fortunately, the flood did not coincide with heavy rains in Delhi, and could be contained within the embankments. Nonetheless, it badly affected the villages and unplanned settlements situated within the river-bed, rendering approximately 15,000 families homeless. These persons had to be evacuated and temporarily housed on roadsides for about two months, before they went back to living in the river-bed. (Source : Sharma, 1996).
Settlement Patter in Flood Plain
A close analysis of the flood zoning pattern reveals that the high risk zones are the areas that have earlier been identified as unplanned or poorly planned areas having high population densities and sub standard housing structures. These include areas of North Delhi, and Trans Yamuna Area. Some of the colonies that have come up in these areas are at levels 3 to 4 meters below the 1978 flood level.
The community exposed to the highest risk from floods comprises the families living in the villages and unauthorized colonies within the river-bed. There are over 15,000 such families, having over 75,000 persons. Situated on the wrong side of the embankments, these people live on the edge of the floods, and are the first ones to find their homes washed away.
Direct effect of floods in the river Yamuna and the city's network of drains. These affect the population living in the Yamuna River-bed and on the banks of the river and drains.
Local flash floods and water logging increased surface run-off due to high ratio of hard surfaces leading to flash floods. This in turn badly affects the low lying areas, particularly the unplanned colonies which get water logged.
Risk of break in embankments
Protection from the river by embankments lead to a false sense of safety and development starts taking place in the shadow of these embankments. In the event of failure of these protective works, as has been seen in the form of breaches during past floods, the effect is devastating because the pressure of the entire embanked stretch is released at one point, and it takes the people by surprise.