Dams: The sediment problem

Jawaharlal Nehru, India’s first Prime Minister called Dams the temples of Modern India.

This was a time when dams were a novelty, and everything was hunky dory. People could see only benefits and the high points that the dam offered (figuratively and literally). It was “the solution” to the modern problems of water requirement, agriculture and electricity.

Much has changed since then. We have monitored the impact that dams have on us, and more importantly on the ecology in a lot more detail in the last few decades. The results have been….unexpected, to say the least. There are many hazardous impacts dams are having on the ecology today, and I would like to highlight something that is probably not talked about as often as it should.

According to WCD (2000), 46 per cent of the water in the 108 most important rivers of the world are first flowing into a reservoir before it continues its way to a natural lake and/or to the sea. That makes sense. Dams need to be, and are, built on high ground in a valley, for it to be used effectively. But here’s what we didn’t anticipate. While dams capture and store the water in rivers marvelously, it also captures and stores other things that the river carries as well: sediments. The efficiency of reservoirs at trapping sediment is frequently reported as 70 -90 per cent of the sediment volume delivered from the watershed. And while we do let out water when the water level gets too high, we only remove the uppermost of sediments, and throw it to the side.

This keeps up to 30-40% of the sediment load of the river from reaching where it is supposed to: the seas. This simple sedimentation has the potential to cause enormous implications, and some of them are already underway.

• The lack of sedimentation transport to deltas has changed the behaviour of deltas. Instead of growing, it is now receding. The coastline is now progressing further inland, and this amplifies the sea-level rise happening around the world.

 

• There is a particular phenomenon called tectonic subsidence. In simplest terms, it says that the weight of the things on the crust keeps the entire crust (and it’s parts) stable. The heavier the material on the crust, the more the crust sinks into the mantle and remains stable. Deltas play a major role here, because of the huge load of sediments that it supports. Now that this support is going, the crust in these parts of the world will rise (because the weight is reducing).

 

• Sediments are deposited along the river in the floodplains, and it is there sediments that are most suitable for agriculture. Less sediments, less food for you and me.

 

• The huge load of sediments in the reservoir of the dam will reduce the capacity of the dam. The purpose for which the dam was built, will then not be met to it’s full potential.

 

• The entire nutrient cycle of the floodplain of the river on which the dam is built will change. This change occurs in a cycle that has taken tens of thousands of years to establish.

It is hard to think of countermeasures for this, but some things need to be implemented in order to counter these extremely adverse, long-term effects of sedimentation in dams.

1. Ensure the reservoirs are regularly desilted. And not the half hearted process people do these days. Complete desiltation.
2. Developing checkdams at regular intervals instead of one large, multipurpose dam everywhere.
3. Ensuring environmental flows in the river channel are maintained.

---

References:

1. http://parra.sdsu.edu/roberson_chapter06.html
2. World commision of Dams report- Dams and development: A new framework for decision making (2000)