SMALL DAMS PETITS BARRAGES

emergency management organizations and take additional actions necessary to safeguard
life, health and property.
4.5 DECOMMISSIONING
Decommissioning is defined as the full or partial removal of an existing dam and its
associated facilities or significant changes to the operations thereof. There are many reasons
for removing a dam - obsolescence, environmental concerns, economics, safety criteria, risk
reduction, and operation and maintenance costs [7].
According to the Michigan Department of Natural Resources (MDNR), several
abandoned small dams have been washed out during storms in recent years. These failures
have caused extreme erosion, excessive sediment deposition and destruction of aquatic
habitat accompanied by the loss fisheries.
A small dam could be decommissioned when:
a) It no longer meets required safety standard and it is economically not viable to incur in
the expenses to make it safe;
b) It can no longer fulfill its functions and it is economically not justifiable to repair it.
Other reasons could be the water is no longer required or better manages via alternative
sources, increased environmental flow requirements, ecosystem or catchment restoration or
the potential legal or financial liabilities incurred.
In the USA, in an analysis of 417 case studies Pohl found that environmental reasons
were most commonly cited for dam removal (39%), followed closely by safety (34%). Also in
the USA, some observations are apparent from data on removed hydro dams: they were of
moderate height (5m to 18m), had a small installed generating capacity (0.4 to 10MW), were
reasonably old (average age 87 years) and had already been retired at the time of removal
(86%) [8].
It is very important to consider the costs of decommissioning (such as sediment removal,
landscaping, approval process, safe demolition etc) that may, in some cases, be even higher
than the costs of repairs and upgrades. Also the impact on upstream and downstream
infrastructure and replacement of benefits, such as water supply and power generation. The
economic aspect here must be thoroughly examined. Each situation is different and must be
considered on a case by case basis.
The decommissioning of small dams is usually done by full or partial removal in such
manner that the remaining structure does not store water and lets it through without retention.
In the case of small dams with floodgates, one can choose between raising the floodgates or
removing them completely.
The main objective of decommissioning a dam is to restore the natural flow of water,
avoiding the concentration of sediments, and preventing or eliminating all actual and potential
unsafe situations regarding people, property and the environment that could arise out an
abandoned or unsafe dam. In some cases, the dam may be removed to improve upstream fish
passage.
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