Geology of Reservoirs

Humans have stored so much water in reservoirs that it has been suggested to have subtly altered the planet's rotation. Water impoundment is thought to have shortened the length of the day and shifted the Earth's axis by tiny amounts. No other human activity has been big enough to cause any appreciable alteration in these global phenomena. Large reservoirs can also cause earthquakes because of their addition of heavy mass. The largest reservoir-induced earthquake occurred in 1967 in India and had a magnitude of 6.3 or 6.8 on the Richter scale. Reservoirs are also recognized for their ability to collect sediments. All rivers erode their beds and carry sediments downstream. Large reservoirs trap most of these sediments because currents slow down and drop their sediment loads when entering the reservoirs. Relatively sediments-free water is then released from the reservoir, in turn eroding new sediments from the channel below the dam and carrying them downstream. Because no sediments are deposited where this erosion is happening below the dam, the erosive capacity of the water may lower the riverbed by several meters until it is armored by stones and boulders and free from available fine-grade material. All reservoirs are, however, not effective sediment sinks. For example, in small, narrow reservoirs with low water residence time sediment deposition may be negligible.

The continuous addition of sediments to reservoirs successively reduces their storage capacity. Small reservoirs in sediment-rich regions can be completely filled within a few decades whereas large reservoirs in rivers running over coarse substrates have an expected life span of several centuries. It has been estimated that almost 30% or more than 100 billion metric tons of the global sediment load is trapped behind dams. This development is one of the factors eliciting demands for construction of new reservoirs. Nutrients, pesticides, and heavy metals that are transported with sediments are also trapped in the reservoirs. Such loadings make it more difficult to restore a river stretch should the dam and the reservoir be regarded as no longer necessary at some time in the future.

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