The entire country, be it Gujarat or Assam, is facing floods. In the times to come, this is only 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 duration over which it is spread out will get reduced. This means that we will need our rivers to have a much higher capacity in order to carry that intense rainfall to the sea. Instead, we have reduced their capacity to carry the flood waters by building a number of dams and, don’t be 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 carries these flows to the sea. Therefore, to prevent the rainwater from causing floods, it is necessary that the carrying capacity of Ganga is maintained, if not enhanced. But the Tehri Dam, abstraction for irrigation in Uttar Pradesh and the Farakka barrage have opposite effects.
We have contained the waters of Ganga behind Tehri Dam, in part, to “control” the floods and abstract large amounts of water of the river in Uttar Pradesh for irrigation. The result is that there is an immediate relief from current flood situations, but the plan lacks foresight. This is comparable to taking pain killers. There is an immediate relief from pain — which is an indication of the disease — upon medication, while the disease persists as the pain subsides and the patient suffers in the future. Speaking the language of rivers, the Ganga carries huge amounts of sediments from the Himalayas and part of these are deposited in the riverbed every year, causing a gradual rise. Once every 5 to 10 years, heavy rainfall in the Himalayas leads to massive flows in the river with great height and velocity. The sediments deposited by the river over the last 5 years or so are scoured away and carried to the sea. The riverbed sediments, height of river and carrying capacity are thus restored to ‘normal’ levels. This is how the river operates in Bihar. But today, Tehri Dam and abstraction for irrigation prevent the washing of the sediments from the riverbed.
The Tehri dam has two effects play out simultaneously that are opposite in nature. On one hand, large amounts of sediments are trapped in the Tehri reservoir reducing the flow of sediments downstream 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 into the Ganga. There is reduced scouring and lesser sediment carried to the sea. The combined impact of the two effects appears to be greater deposition of sediments and rising of the riverbed upstream, with reduced capacity to carry the rainfall to the sea. In short, floods.
The substantial abstraction of water for irrigation in Uttar Pradesh also adds to these circumstances. Major portion of scouring takes place during monsoons, and removal of water from the flow at this time reduces it resulting in lesser scouring and reduced capacity of the river to carry rainwater 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 form 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 lost 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 recognized 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 realizing 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.