Sustaining the Geosphere to Manage Water

The conventional response to the threat of flooding is to control a river, particularly by the construction of raised banks called levees. In addition to raising the banks to contain a stream, the 

Stream channel may be straightened and deepened to increase the volume and velocity of water flow, a process called channelization. Although effective for common floods, these measures may exacerbate extreme floods by confining and increasing the flow of water upstream such that the capacity to handle water downstream is overwhelmed. 

An often-cited case of reservoir failure occurred with the Baldwin Hills Reservoir near Los Angeles, California, in 1963. This rectangular basin had been carved on top of a hill in 1951 and rested on poorly consolidated earthen material consisting primarily of silt, sand, and clay. Pumping of oil in the vicinity had led to ongoing settling of the land. Movement of faults beneath the reservoir dam led to an abrupt loss of water, erosion of material from beneath the dam, and an abrupt breach of the dam. The sudden rush of water from the reservoir that resulted killed 5 people and caused 15 million of damage. 

FIGURE 11.6 A dam placed across a stream impounds water in the stream valley above the dam, producing a reservoir. The maximum level of water in the reservoir is at the level of the spillway, and it can be lowered to the level of the sluiceway. 

As discussed in Chapter 3, Section 3.3, bodies of water tend to become stratified, with significant effects on aquatic chemistry. Impoundment of water in reservoirs leads to stratification such that reduced chemical species predominate in the bottom hypolimnion layer of the water. This can cause the production of relatively high concentrations of soluble Fe and Mn species, which can be undesirable water contaminants. Toxic hydrogen sulfide, HjS, and methane, CH4, a gas that can contribute to global warming, are also produced in reservoir sediments. 

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