Environmental Assessment
Volume E-3.8
Tailings Facility
Hydrology
Ambatovy Project
82
January 2006
Erosion that occurs within the tailings impoundment will be contained within the
pond for all events up to the 1:50 year design storm. Flows in excess of the
1:50 year storm will discharge to the environment; however, the water ponds will
still act as sedimentation basins and will help reduce sediment concentrations to
receiving streams. The discharges will mix with downstream water from
undisturbed basins that will also be high in sediment due to the naturally high
erosion rates associated with these types of rainfall and runoff conditions.
During construction there will be a temporary increase in sediment derived from
the outer surfaces of the newly constructed tailings embankment, before
revegetation and stabilization of the embankment. New construction phases will
occur throughout the life of the project as new embankments are built to contain
the tailings. Mitigations to minimize the amount of sediment transported off-site
and protect water quality will include sediment control structures, a storm water
management system, best management practices and reclamation measures.
Closure of the tailings impoundment will entail construction of permanent
spillways that are designed to resist erosion and that will convey water to natural
streams. These spillways will have mild channel slopes to minimize the
possibility of erosion. Closure plans also call for revegetation or alternate
erosion control of the final tailings surfaces.
3.8.6
Impact Analysis
3.8.6.1 Residual Impacts
Residual impacts associated with hydrology are shown in Table 3.8-11. Results
of the impact analysis indicate that, during operation, streamflows downstream of
the tailings embankment will be reduced considerably due to the runoff diversion
in the upper basin and the resulting reduction in drainage area at downstream
locations. The expected changes along the main stems of the rivers are
considered high in magnitude (greater than 30%) in all basins except Basin B3,
which is not affected by the development. Other upper tributary areas, including
a large portion of Basin C1, are also unaffected by the diversion. The high
magnitude impact extends to the boundary of the local study area to where a
large tributary river joins the Ambolona River system.
Based on the high magnitude effect, local extent, and medium-term duration, the
environmental consequence during operation is considered high. This
classification relates to changes in stream hydrology, however, endpoints for
other biophysical or social disciplines are assessed separately. A high
consequence in terms of hydrology does not necessarily equate to a high
