Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 433

Dams trap immense quantities of river sediment. An estimated 5% of the world's total reservoir storage
capacity is lost annually to sediment accumulation. In addition to creating problems for existing dams,
sediment poses challenges during dam removal. Sediment removal is likely to represent the most costly
and technically intensive aspect of decommissioning large dams.
Specific sediment removal techniques vary depending upon the amount of sediment, reservoir
characteristics, project age and the effectiveness of periodical flushes, if at all feasible, to pass trapped
sediment downstream. Sediment removal must be conducted carefully, as excessive release can damage
sensitive downstream habitats. On Washington's Elwha River, for example, experts propose gradual,
incremental drawdown to transport sediment without harming spawning habitats or juvenile salmon.
A potential result of sediment flushing is the release of accumulated contaminants into fisheries or
water supplies. Following removal of a 9 m high dam on New York's Hudson River in 1973, tons of trapped
toxins were suddenly exposed in the old riverbed or flushed downstream. Hazardous waste in sediment
poses significant health risks, degrades water quality, and ultimately requires extensive cleanup efforts.
Thus, thorough sediment an analysis and prior assessment of the foreseeable effects of releasing sediment
must be included in decommissioning studies.
A key aspect of dam removal planning is early identification of alternative sources of hydropower,
irrigation and public water supply, or other dam functions. Dam removal often entails trade-offs between
competing river functions. However, American experience with dam removal demonstrates that replacement
can often be accomplished with minimal difficulty. For example, a single hydropower dam may contribute
only a fraction of a region's overall power—alternate sources are often available, and conservation
measures can eliminate demand for this electricity altogether. In other cases, such as in the removal of 12
small dams on California's Butte Creek in 1998, dismantling the dams only had negligible effects on
water supplies due to mitigation measures (e.g., improving efficiency of irrigation systems). Developing
a comprehensive management plan that accounts for displaced dam functions minimizes the negative
impacts of removal. Where changes or impacts are unavoidable, society may accept them as the price of
long-term river restoration.
There are different dam removal methods: 1) Complete removal is often accomplished by first temporarily
diverting the river, then using heavy equipment (e.g., wrecking ball, backhoe, and hydraulic hammer) to
dismantle the dam. The removal of the 7 m high, 280 m long Edwards Dam on Maine's Kennebec River
was accomplished in a matter of days using this technique. 2) Breaching of dams allows the river to flow
around existing dam structures. Heavy machinery is typically used to breach earthen portions of dams
located in relatively wide river corridors. Breaching is recommended for partial dam removal, such as the
Lower Snake River dams, and represents a relatively inexpensive decommissioning option for larger
structures, when feasible. 3) In the case of concrete dams, controlled explosives are occasionally used for
demolition. Explosives were used to remove dams on the Clearwater (1963), Clyde (1996), Loire (1998)
and Kissimmee (2000) rivers, among others. 4) A combination of explosives and heavy machinery may
be required, especially with larger projects.
In summary, dam removal may be regarded as an option for the dams which have more negative impacts
than benefit. Nevertheless, dam removal is not a good choice for many impounded rivers. Careful
comparison should be undertaken before dam removal is used to end the life of the reservoir.
7.4 Construction and Management of the Three Gorges Project
The Three-Gorges Project (TGP) is well known due to its great scale and far-reaching influence in hydro-
power development and river management in China. The main purposes of the TGP are flood control,
power generation and navigation. The construction and management of the TGP are discussed in this session.