Those Dammed Lakes - Hydro-Environmental Analysis: Freshwater Environments - page 314

Dams by completion date

18,717

12,910

11,517

5,189

4,393

4,115

3,930

2,681

2,321 2,290

2,165

1,964

FIGURE 10.40 U.S. dams by their completion date. (From U.S. Army Corps of Engineers National Inventory
of Dams, National data available at: http://geo.usace.army.mil/pgis/f?p=397:5:0::NO.)

With age, dams like any other infrastructure deteriorate. As illustrated in Figure 10.40, many
U.S. dams were constructed between 1950 and 1980, so the average age of U.S. dams is 51 years.
A commonly used design life expectancy for dams in the United States is 50 years. As a result, a
large number of U.S. dams have exceeded, or will soon exceed, their design life. FEMA (2001, cited
by Conyngham et al. 2006) estimates that 58,000 large dams (85% of dams in the NID) will have
exceeded their design lifespan by 2020.
Only 11% of dams are federally owned, thus the responsibility for ensuring the safety of the
remaining 89% of all dams falls on state agencies, which often do not have the resources to ade-
quately inspect dams within their states and take appropriate corrective actions (ASCE 2009). For
example, in 2009, Texas had only seven engineers and an annual budget of $435,000 to regulate
more than 7400 dams, so that each inspector was responsible for more than 1050 dams (ASCE
2009). Since 1998, the American Society of Civil Engineers (ASCE) has issued report cards on
the condition of America's infrastructure, and America's dams have consistently received a D
grade. The number of dams in the United States that are deicient, in terms of age or structure, is
increasing.

10.6 DECOMMISSIONING/REMOVING DAMS

A recent trend related to large dams in the United States and elsewhere has been to decommis-
sion dams that no longer serve a useful purpose, are too expensive to maintain safely, or have
unacceptable levels of impact (World Commission on Dams 2000). This trend is particularly
common in the United States where over 500 dams have been decommissioned and the rate for
decommissioning large dams has overtaken the rate of dam construction (World Commission on
Dams 2000).
Dam removal may have positive environmental and social impacts, such as the removal of barri-
ers to migrating ish as part of river restoration projects or in reducing hazards. Dam removal may
have negative environmental and economic impacts, so the process must be evaluated in terms of
the risk and hazards as well as the cost and beneits of the dam and its removal.