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7.1.2 Tailings dams and lagoonsA particular hazard is represented by tailings dams and storage lagoons.
International concern regarding the safety of such facilities has been
heightened as a result of a number of serious failures, e.g. Stava (Italy
(1985)), Aznalc ́ollar (Spain (1998)) and Baia Mare (Romania (2000)).
The Stava failure released 0.19 106 m^3 of tailings slurry from a fluo-
ride mine and led to 269 deaths (Berti et al., 1988). Two embankments in
series, the upper one 29 m high and the lower 19 m, collapsed in cascade,
the slurry travelling up to 4 km. The initiating mechanism was rotational
instability of the upper dam following raising of the phreatic surface as a
result of problems with the decanting culvert.
Aznalc ́ollar, with a height of 28 m at time of failure, released some
7 106 m^3 of acidic tailings which inundated 2.6 103 ha of farmland,
causing very considerable environmental and economic damage (Olalla
and Cuellar, 2001). Baia Mare, newly constructed with tailings from an old
impoundment employed as fill, had a surface area of c. 80 ha. The retaining
embankment failed by local overtopping, and the cyanide plume following
the release of c. 1 105 m^3 of contaminated liquid was detectable four
weeks later some 1500 km away at the mouth of the Danube (Penman,
2002).
The potential hazard associated with tailings dams and lagoons is
raised very significantly by several factors which distinguish them from
conventional embankment dams and reservoirs (see also Section 2.11):
- the risk of tailings dams breaching may be much enhanced by inade-
quate levels of supervision and surveillance; - the environmental and economic consequences of serious breaching
are frequently heightened by the toxic nature of the outflow; - a tailings dam or lagoon may be discontinued and/or abandoned, and
left unsupervised, following the cessation of active operation.
The management of tailings dams in the UK and the legislation applicable
to their construction and operation are discussed in DoE (1991a), and an
international perspective on the safety of tailings storage dams has been
published in ICOLD (2001).
7.2 Instrumentation
7.2.1 Application and objectivesThe provision of monitoring instruments is accepted good practice for
all new dams of any magnitude. In parallel with this, a basic level of