August 20, 2017

Sedimentation: the way of all dams

Published as Chapter 16 of The Social and Environmental Effects of Large Dams: Volume 1. Overview. Wadebridge Ecological Centre, Worthyvale Manor Camelford, Cornwall PL32 9TT, UK, 1984. By Edward Goldsmith and Nicholas Hildyard.


In the previous sections, we have reviewed the ecological and social problems caused by large-scale water projects. We have seen the ecological havoc which results from salinisation; the human suffering inflicted by waterborne diseases; the destruction inflicted on fisheries; and the social upheavals caused by resettlement programmes. So too, we have seen how hydro-industrialisation has adversely affected food supplies; and how it is rarely the rural population of the Third World which ‘benefits’ from irrigation programmes.

In this chapter, we consider the final argument against building large-scale water projects. Even if the above problems would be solved by better management, through agrarian reform or through a more sensitive approach to development, the ‘benefits’ which a dam provided would still only be temporary. The reason is clear enough: sooner or later, the reservoir of a dam must fill up with the silt and other detritus which the dam prevents from flowing downstream. And when that happens, the dam must be decommissioned: indeed, without its reservoir, a dam is no more than a useless slab of concrete.

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Sedimentation rates in temperate areas

In temperate areas, the sedimentation of a reservoir is usually a slow process. A study by Dr. Cyberski of the State Hydrological-Meteorological Institute, Warsaw, for instance, reviewed sedimentation rates at 19 reservoirs in Central Europe. Cyberski found that their storage capacity (which ranged between 120 and 183,000 acre feet) was depleted by sedimentation at an average rate of 0.51 percent per annum. [1]

Another study, conducted by Dr. Dendy and his colleagues at the USDA’s Sedimentation Laboratory, looked at the sedimentation rates of small, medium and large reservoirs in the United States. That study found that:

  • The rate of sedimentation in 1,105 reservoirs with a capacity of less than 10 acre feet was approximately 3.5 percent a year.
  • In the case of medium-sized reservoirs (with a storage capacity of more than 100 acre feet) the annual storage loss was 2.7 percent per annum and the median rate of sedimentation was 1.5 percent.
  • For reservoirs with a storage capacity of more than a million acre feet, the rate of sedimentation was only 0.16 percent per annum, with the mean rate coming out at 0.11 percent a year. [2]
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Sedimentation rates in the tropics

If sedimentation rates – particularly for large reservoirs – are low in temperate areas, in the tropics the situation is very different indeed. That difference can be explained, principally, by the devastating effect which deforestation has had on tropical soils.

Clearly, the rate at which a reservoir ‘silts up’ depends on the amount of silt carried by the river which feeds it – and that, in turn, depends on the rate of soil erosion in the river’s catchment area. Where that catchment area is forested, the soil is not only held together by the elaborate network of roots which underlies the forest floor, but it is also protected from the effects of wind and rain erosion by the forest canopy above.

Under such conditions, the rate of soil erosion is thus very low: indeed, even steep slopes are protected. In those areas where forest cover has been depleted, however, the rate of soil erosion increases dramatically: the organically poor soils of the tropics are particularly vulnerable to erosion and although the Monsoons only last for a short time, they can quickly wash away the soils from deforested slopes. [see Chapter 10]

Given the present rate of deforestation in the tropics (10 hectares of rainforest are lost throughout the world every minute of the day) it is hardly surprising that rivers in the region carry enormous quantities of silt. Indeed, in many areas, the increased sediment load of rivers is clearly visible to the naked eye.

In Sri Lanka, for example, one can literally see the monsoon rains washing away the soil under the often semi-exposed roots of tea bushes on the less well-run tea estates in the Highlands – and one can equally well see that soil flowing down the Mahaweli River whose waters (like those of other rivers whose watersheds have been denuded of trees) are thick, brown and opaque.

The Indian Government has announced plans for the reafforestation of watershed. This is an encouraging move. Unfortunately, however, such reafforestation has – so far – only been carried out on a negligible scale compared to the extent of recent deforestation in critical areas.

The need for reafforestation is now generally accepted. The World Bank, for instance, fully realises its importance – although it still seems wedded to the counter-productive policy of planting pines and eucalyptus trees. Recently, in providing finance for a hydro-electric project in Malaysia, it has included money for the purchase in part of the watershed upstream, which it is to turn into a national park. Let us hope that the park will exist in reality as well as in name.

Linney and Harrison asked officials at TRDA and the EEC in Nairobi why funds could not be set aside so that a reafforestation programme might accompany the building of the Masinga Dam and other dams on Kenya‘s Tana River. They also suggested that a soil conservation programme be set up:

“TRDA thought this was a good idea, but preferred to let the Kenyan Government take care of soil conservation and they would take care of the dams. The EEC, a major financier of the Masinga Dam, was very much aware of the erosion problem but said that funding for soil conservation was unlikely because:

  1. they have never provided money for the watershed of a dam project;
  2. the funding of water development and soil conservation were in two different departments so they could not be included in the same project;
  3. economically, the dam would still benefit Kenya’s development and pay for itself, even if the dam silted up in 20 years.” [2a]

It goes without saying that the more sediment a river carries, the faster the reservoirs on that river will silt up. Predictably, therefore, the rate of sedimentation in the tropics in recent years has been nothing short of disastrous. Thus:

In India, the expected siltation rate of the Nizamsagar Dam in Andhra Pradesh was 530 acre feet a year. The actual rate was closer to 8,700 acre feet a year. Indeed, the dam’s reservoir is already estimated to have lost 60 percent of its storage capacity. [3] Other reservoirs in India have suffered similarly high siltation rate. In fact, few of the dams now operating in India (in 1978, there were 835, 26 of which provided more than two-thirds of the country’s storage capacity) have escaped siltation problems: more important still, many have experienced siltation rates way above those predicted by their planners.

In Haiti, the Peligre Dam on the Artibonite River was completed in 1956 as part of a plan to provide irrigation to the Artibonite River valley, Haiti’s main arable plain. It was built to last 50 years: in fact, its reservoir has silted up so quickly that the dam will probably be decommissioned in 1986 – after just 30 years of operation.

In China, the Sanmenxia Reservoir, which was completed in 1960, had to be decommissioned in 1964 due to premature siltation. Worse still, the Laoying Reservoir actually silted up before its dam was completed. [5]

Tragically, the lessons of premature siltation do not appear to have been learned by the governments of those nations where siltation poses the greatest threat. As we have seen, bigger and bigger dams are being planned in the tropics – and, even today, little regard is paid to the rate at which their reservoirs are likely to silt up.

Clearly, the premature sedimentation of reservoirs seriously affects their economics. Already, as we have seen, the final cost of constructing a large dam is – for various reasons – nearly always very much higher than estimated. If, therefore, the dam’s reservoir silts up several times more rapidly than predicted (or worse still, as at Laoying, before the dam even has a chance to function), the time over which the costs of the dam must be amortised is inevitably decreased – thus making nonsense of the calculations used to justify the darn’s construction.

As today’s dams silt up, so they will leave behind a vast muddy wasteland. Compacted by the weight of a reservoir’s waters, the fine particles of silt which have been deposited in the reservoir form a brick-hard pan as they build up. Even when the last waters of the reservoir have drained away, therefore, the land beneath will not be suitable for basin irrigation or rain-fed agriculture. Only a narrow strip close to the dam, where the courser and thereby less compacted particles of silt are likely to have accumulated, will be suitable for cultivation.

Moreover, as reservoir after reservoir is abandoned, it will become increasingly difficult to find sites for new dams. The number of sites where dams can be built is strictly limited – and, in many parts of the world, those sites have already been exploited. (Some of the worst dam failures have occurred as a result of building dams on geologically inappropriate sites).

On those grounds alone, it seems inevitable that large dams will prove to be a passing phenomenon in the history of human affairs. The devastation they will have caused, however, will be of a very much more permanent nature.

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1. Louis M. Glymph, “Summary: Sedimentation of Reservoirs”. In William C. Ackermann et al. (eds), Man Made Lakes, their Problems and Environmental Effects. American Geophysical Union, Washington DC, 1973, p.343.
2. F. E. Dendy, W. A. Champin and R. B. Wilson, “Reservoir Sedimentation Surveys in the United States”. In Willam C. Ackermann et al. (eds) . op.cit. 1973, p.353.
2a. See Warren Linney and Susan Harrison, Large Dams and the Developing World: Social and Environmental Costs and Benefits – A Look at Africa. Environmental Liaison Centre, PO Box 72461, Nairobi, 1981, pp.19-20.
3. The State of India’s Environment, 1982. Centre for Science and Environment, New Delhi,1982; pp.62-63. See also Bharat Dogra, “Big Dams in the Indian Subcontinent: Small Gains at High Costs”. In Edward Goldsmith & Nicholas Hildyard (eds) The Social and Ecological Effects of Large Dams, Vol. II: Case Studies. Wadebridge Ecologial Centre, Worthyvale Manor, Camelford, Cornwall, UK.
4. Earthscan, The Improbable Treaty: The Cartagena Convention and the Caribbean Environment. Press Briefing Documentation No. 34(a), 1983; p.60.
5. USCOLD Newsletter No. 69, November 1982; p.15. Quoted by Philip Williams, “Damming the World”. Not Man Apart, October 1983; p.11.
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