Flood is an ecological process which is a characteristic of every monsoon river. This ensures the stability of a complex network of ecosystems, such as floodplains, riparian and free-flowing water ecosystems, which all form a river system. The flood washes away the sediments in various channels and maintains the flood plains. Man-made flooding occurs through the upstream storage of large amounts of water from catchment areas in dams or barrages, which are then released when these reservoirs are full, resulting in flooding downstream. The first activity (i.e. water storage) prevents natural flooding and the second (release of massive amounts of water) can cause man-made flash floods, causing not only ecological disruption to the river system, but also ecological disaster for people living in affected low-lying areas.
A flash flood in the Yamuna River ravaged Delhi from July 10 to 17 following the release of 350,000 cusecs of water from the Hathnikund Barrage located in the foothills of the Himalayas after incessant rains across northern India, resulting in the river’s water level rising to 208.66 metres, submerging low-lying areas on both sides and also causing significant back-flow in storm drains carrying sewage, which worsened the situation.
There was also a flash flood in 1978, when more than 700,000 cusecs of water was released from the Hathinikund Barrage in Haryana, but the Yamuna’s water level rose only by 207.49 metres.
So, what has been happening to the expansion of the Yamuna in Delhi during the past 45 years – especially the downstream section of the Wazirabad barrage – that has contributed to a sharp rise in river levels this time with less than half the water released than in 1978?
Several human-mediated activities in the river system have resulted in increased water levels in the river. These include the following:
One, the width of the floodplain of the Yamuna has reduced from about 5 km at one point to a mere 300 meters from the channel, with no floodplain in some stretches. This resulted in a significant reduction in the flood-carrying capacity of these floodplains, which normally act as flood-ways for floodwaters. This reduction in floodplain coverage is due to the increase in the number of dams and their ‘guide dams’ built to ensure the safety of new human settlements and other built structures, and the presence of large solid waste dumps.
Two, the loss of flood plain catchment area wetlands that can absorb large amounts of flood water. Wetlands that have disappeared over the years have also contributed to the rise in flood water levels this year.
Three, increasing the number of bridges (road, railway and metro) over the river between Wazirabad and Okhla Barrage. It is known that each pillar of the bridge contributes to the water-flow resistance and this increases the flood water level. Delhi Metro Railway Corporation, City Public Works Department and National Highway The Indian authorities together have increased the number of bridges on the Yamuna.
Fourth, the concretization of flood plains in the name of river ghats. Due to this, the path of flood has also reduced. More dams/barrages on the upper reaches of the Palla River also contribute to frequent floods downstream.
Above Wazirabad, extending up to Palla, the flood plains of the river are broad and up to 5 km wide. In some parts, this width was reduced by dams, but it is sufficient to carry flood water without a significant rise in river level. In addition, there is no bridge on this stretch and there are many floodplain wetlands, including the wetlands of the Yamuna Biodiversity Park, all of which contribute to the smooth flow of flood water without increasing the level of the Yamuna.
One of the indirect effects of flooding is the backflow of sewage which has submerged many residential areas. This situation was aggravated by the reduction in the capacity of these drains (which arrest the flow) due to filling of open spaces on either side of the stream with solid waste and their use for road expansion and bus parking areas. The wetlands that are supposed to intercept surface runoff from residential areas are also filled with solid waste, so roads in these stretches easily get blocked during heavy rains.
What lesson should we learn from the recent floods in Delhi?
We should not disturb the ecological integrity of whatever is left in our Yamuna floodplain. In addition, efforts should be made to reclaim some of the floodplains destroyed due to dumping, encroachment and dams built. There should be a ban on bridge construction on the river. Dense plantation of trees/woody plants on flood plains should be avoided, as trees provide resistance to flood water. Restoration of floodplain catchment wetlands that store flood water can also be used to augment water supply in Delhi. The city can develop reservoirs underground or outside the open river to contain flood water and provide the city with low water season.
In fact, a project proposal on creation of off-river mini reservoirs to arrest flood water and store flood water in flood plain catchment areas was prepared and approved by the present Chief Minister and former Lt. Governor of Delhi.
Development of a network of Biodiversity Parks along the flood plains on the lines of the Yamuna Biodiversity Park will also not only contribute to the revitalization of the river, but will also reduce the velocity of flow and help prevent flood water. National Green Tribunal has ordered setting up of such parks in the floodplains of Indian rivers including the Ganges.
All wetlands should be restored to store surface water to prevent water logging and should not be used for dumping solid waste. Open spaces along major drains should be left empty to prevent flow. Natural drains should not be converted into sewers.
Finally, it is important for the people at large and governments at all levels to respect the country’s rivers and streams as urban lifelines, and act against their casual treatment as dumping grounds for waste. There should be a single authority to monitor the sustainability and protect the ecological integrity of the Delhi stretch of the Yamuna. Man-made destruction can be prevented.
CR Babu is Professor Emeritus at the Center for Environment Management of Degraded Ecosystems, University of Delhi