Nepal Dams Won’t Stop Indian Floods

Via China Dialogue, an interesting report on the need for Indian planners to radically rethink flood prevention strategies in the Ganges basin, given a new World Bank study that debunks old myths:

For decades, Indian planners working to harness the waters of the Ganges and its tributaries have believed building dams in Nepal will save Bihar and eastern Uttar Pradesh from the floods that occur almost every year. A recent comprehensive study led by the World Bank says this belief is no more than a myth.

The belief was that there were substantial upstream reservoir storage possibilities in the Ganges basin that straddles India, Nepal and Bangladesh. But the Ganges Strategic Basin Assessment (SBA) coordinated by the World Bank and carried out by experts in the region has found that the largest 23 dams that have been conceived would only hold an additional 13 percent of the annual flow of water.

The preliminary findings of the SBA have been presented to government officials in all three countries and were unveiled in public at the World Water Week in Stockholm.

The experts working with the South Asia Water Initiative (SAWI) coordinated by the World Bank found that in terms of flood control, there would be little basin-wide effect of upstream storage, and that effects were unlikely at the sub-basin level either. The models – developed by the Institute of Water Management in Bangladesh, the Indian Institute of Technology, Delhi and RMSI, a consultancy firm in India, and vetted by SAWI experts – showed that at the sub-basin level, the dams would reduce peak flows, but would not necessarily reduce floods. Most rivers in the basin are largely embanked, and local rainfall and embankment failures cause the most flooding, the study found.

Looking specifically at flooding in the Ganges delta, most of which is in Bangladesh, the experts found that the dams in the Himalayas would have a negligible impact on the main stem of the Ganges.

And in a finding that definitely counters the majority view in India, the study found that upstream water storage was not a robust strategy for flood control in Bihar, the Indian province immediately downstream of Nepal for most of the tributaries of the Ganges. Every monsoon when there is a flood in Bihar, including this year right now, local politicians blame Nepal and the lack of dams there.

But the experts found that most of the flooded area in Bihar is outside the basin of the Kosi river, the main Ganges tributary flowing from Nepal to Bihar in India. They also pointed out that most major tributaries of the Ganges in Bihar are embanked, and most floods are from direct rainfall and embankment breaches. In fact, experts have earlier pointed out that repeated embanking since the 1950s and silting of the rivers has created a situation in Bihar where most of the rivers actually flow at an altitude above the surrounding land. The result is that when the water overflows during the monsoon rains or when there is a breach in an embankment, the land acts like a bowl and is flooded, because the water has nowhere to drain out.

Supporters of building large dams in the Himalayas have also said that the reservoirs behind these dams can be used to augment low season flows. They have pointed out that there is huge seasonal variation of water flow in the Ganges basin, since South Asia gets around 85%of its annual rainfall during the four monsoon months of June to September.

But the SBA has cautioned against this line of argument. It points out that redistributing a small portion of the flood waters would make a big difference to low flows, but the appropriate use and economic value of this water is unclear. Current agricultural productivity in the Ganges basin is low anyway. In waterlogged areas additional low season water could actually be harmful, while the stress on ecosystems and municipalities that would have to cope with the extra water could be high.

So are there good alternatives or complements to reservoir storage in the Ganges basin? The perceived wisdom has been that there is not, but the SBA says yes. The experts say that natural underground water storage, strategically and sustainably managed, could be used in the basin on a scale comparable to the full suite of dams considered in the models. They say there are additional sustainable groundwater resources available in the Ganges basin, in contrast to other parts of India.

Specifically, they point out, there are significant opportunities for additional groundwater use in the basin, in conjunction with a well-managed surface water system in eastern Uttar Pradesh, Bihar and West Bengal. In the Ghaghra-Gomti basin – a sub-basin of the Ganges in eastern Uttar Pradesh – 2.5 million new tubewells can utilise additional groundwater storage of 20 billion cubic metres. There are around 1.75 million tubewells in this sub-basin now, used mostly for irrigation but also for drinking water.

There is another myth that the SBA has punctured. It has often been said that water stored in Himalayan reservoirs can be used to dilute pollution downstream. But the experts point out that any such release would join the Ganges downstream of its most polluted stretches.

There is yet another myth, which says watershed management and upstream storage can control sediment loads. But the experts point out that most dam engineers would want to pass the sediment through, as their reservoirs would get silted up otherwise. Hydroelectric stations also filter out as much of the sediment as they can, because it affects turbine operations.

But after all this, there is one big advantage of building dams in the Himalayas. They would generate a substantial amount of hydroelectricity, the study confirmed. The preliminary finding is that the 14 largest of the dams planned have an installed capacity of around 25,000 megawatts, valued at US$4-5 billion a year. The Ganges basin, with 650 million people the most populous in the world, suffers a chronic power shortage.

Climate change has arrived as an additional complicating factor in the Ganges basin, as elsewhere. Temperatures will increase, glaciers will melt faster, the sea level will rise, rainfall and snowfall scenarios vary widely. The SBA says there are great uncertainties on the scale of the effects, but opportunities to act now. Pointing out that a focus on managing current variability is a no-regrets strategy, the authors say more knowledge and coordination are needed to handle the effects of global warming.

The preliminary findings have four takeaway messages:

*  For regional floods, focus on warnings, not just water storage. Upstream storage infrastructure cannot protect the basin. Real, immediate benefits can, however, come from cooperative regional monitoring and warning systems, coupled with localised flood responses.

* For water storage to enhance low flows, look underground, not just upstream. Groundwater storage in the Ganges basin can provide the same scale of effective storage as upstream dams in Nepal, more immediately and at lower costs.

* Hydropower development and trade in the basin remain very promising. There is significant potential to deliver clean peaking power and improve trade imbalances.

* Climate adaptation can begin now, with enhanced and shared information, forecasting and warning systems; flood and drought management; and a major push to the use clean energy.

This entry was posted on Wednesday, August 24th, 2011 at 3:07 pm and is filed under India.  You can follow any responses to this entry through the RSS 2.0 feed.  Both comments and pings are currently closed. 

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