Elusive Flood Control
| Date :08-Aug-2019 |
We have arrested the Ganga behind Tehri Dam, in part, to “control” the floods and we have abstracted large amounts of water of the river in Uttar Pradesh for irrigation. The result is that the floods are reduced immediately but future floods are made much more severe.
THE entire country from Gujarat to Assam is facing floods. In the times to come, this is likely to get worse. One reason is that global warming is leading to intense rainfall in short periods. The total rainfall during a year is expected to remain the same but the number of days in which it will fall is likely to get much reduced. This means that we will need higher capacity of our rivers to carry the intense rainfall to the sea. On the other hand, we have reduced the capacity of our rivers to carry the flood waters by building a number of dams and, don’t get surprised, by reducing the yearly floods. Let us consider the floods in Bihar. All the rainfall in Bihar flows through Baghmati, Kosi, Gandak and other rivers into Ganga which alone carries these flows to sea.
Therefore, for the rainfall to be removed quickly to avoid the floods, it is necessary that carrying capacity of the Ganga to carry the flood water is maintained, if not increased. But the Tehri Dam, abstraction for irrigation in Uttar Pradesh and the Farakka barrage have done the opposite. We have arrested the Ganga behind Tehri Dam, in part, to “control” the floods and we have abstracted large amounts of water of the river in Uttar Pradesh for irrigation. The result is that the floods are reduced immediately but future floods are made much more severe.
This is like taking pain killers. The immediate pain is reduced but the disease increases and the patient suffers in the future. Let me explain. The Ganga carries large amounts of sediments from the Himalayas and deposits a part of these on her riverbed every year. The riverbed rises. Then once in 5 or 10 years, there is a huge rainfall in the Himalayas leading to a massive flow of water in the river.
This flow has both higher height and velocity. The sediments that have been deposited by the river in the last 5 years or so are scoured at this time and carried to the sea. The riverbed again attains its normal low level. The capacity of the river to carry the rainfall in Bihar to the sea is thus restored. This scouring of the sediments in the riverbed is no longer taking place due to the Tehri Dam and abstraction for irrigation.
The Tehri dam has two simultaneous and opposite effects. On the one hand, large amounts of sediments are trapped in the Tehri Reservoir which reduces the flow of sediments downstream and also to less deposition of sediments in the 5-year cycles. On the other hand, the Tehri dam arrests all the monsoon flows of the Bhagirathi River and reduces the flood flows in the Ganga. This, in turn, leads to reduced scouring and carrying of the sediments to the sea. The combined impact of the two effects appears to be more deposition of sediments, rising of the riverbed, reduced capacity of river to carry the rainfall to the sea and to more floods. The large abstraction of water for irrigation in Uttar Pradesh also adds to the problem. Water is removed during the monsoon period as well.
This again leads to less flow of water during the monsoons, less scouring and reduced capacity of the river to carry rainfall to the sea. The Farakka barrage has made things worse. A huge pond of about 50 to 100 kilometers long has been created upstream of Farakka. The velocity of water in this pond is less. The reduced velocity in the pond leads to large amounts of sediment settling down in the pond. This has led to the reduction in the depth of the water in this 50-100 kilometers stretch of river-pond. Let us say, previously the ponded river had a depth of five meters. Now it may be three meters because two meters has been filled up by the sediments. This reduction in depth means that the amount of the flood waters that the river can carry is reduced.
In this way Farakka Barrage has led to increase in floods from Patna to Farakka. It could be asked why the waters of the floods due to high rainfall in Bihar are not scouring the riverbed? There could be two reasons though I am not clear about this. One, the flood flows coming from upstream flow in a linear manner and have greater velocity while those joining in the plains are like a side road joining the highway. The tributaries joining in the plains may not add to the velocity that is required to scour the riverbed. This may be like a traffic barrier. The passage is quick if all vehicles remain in one. However, the passage is slower if vehicles but in from all sides. Similarly, the velocity of the river is less if the linear flow from upstream is less. Another reason could be that the rainwater trying to enter the river in the plains is spread thin over a large area and loses its velocity.
This aspect requires a deeper study. The main cause of the floods we are facing across the country is that we have reduced the capacity of our rivers to carry the flood waters. This is, in part, because of our penchant to encroach upon the riverbed for making houses. This problem is recognised by the engineers and it is mostly a problem of implementation. A number of High Courts have ordered that the riverbeds must be cleared of the encroachments. The difficult part is that our engineers do not understand the sediment dynamics of our rivers.
They are good persons. But they think that the river is only bringing water. They want to control the amount of water so that the floods are less and people are saved. They are not realising that the reduction of flood flows is leading to the opposite result because of reduced capacity of the river to scour its bed and to a reduction in her capacity to carry flood water. In order to get rid of the smaller problem of flood in the river, our engineers have unintentionally increased the floods across the country. The situation will only get worse in the coming years as the rainfall becomes more intense. The solution is to restore the sediment dynamics of all our rivers. This may require removing dams like Tehri, reducing the abstraction of water for irrigation, and redesigning the likes of Farakka Barrage. Unless we do this, the whole country will continue to be increasingly affected by such floods.