Environmental Assessment
Volume E-2
Tailings Facility
Project Description
Ambatovy Project
16
January 2006
The marine outfall will be assembled on land, then floated out to sea, flooded and
sunk onto the sea bottom. In developing the design of the outfall, the designer
will need to decide whether the pipeline is to be trenched and buried over its full
length, or whether the seaward portion can be exposed on the seabed. In the
former case, burial depth, together with any armouring provided over the pipe,
should be designed for stability against scour and liquefaction under the design
loading. In the latter case the pipeline will require sufficient weight to remain
stable under the maximum wave and current loading.
Storm Water Management
The tailings facility has been designed to contain a 1 in 50 year storm above the
maximum normal operating level (MNOL). Based on a preliminary estimate for
the 1 in 50 year storm (500 mm in 24 hours – to be updated once site-specific
data is available), it is estimated that 3.0 Mm
3
of storage will be required in the
water basin above the MNOL. The decant weirs will have capacity to transfer
the water quite quickly to minimize the length of time the tailings basin is
flooded. Based on the proposed excess water removal rate of 4,000 m
3
/hr it is
estimated that it will take up to three months to return the water levels in the
tailings basin to normal operating range within the water basin should a 1 in
50 year event occur.
To handle flows in excess of a 1 in 50 year storm, an overflow spillway system
will be constructed to pass the inflow design flood from the tailings basins to the
water return pond and then to the environment. The overflow spillways will
consist of rip-rap and gabion-lined channels up to 200 m wide so that the
probable maximum flood can be passed with 1 m or less of head. The final
discharge from the water basin will be into the downstream portion of the TA-10
valley. Analyses of downstream areas that could be impacted by the spillway
discharge have been conducted to determine required mitigations.
2.8
OPERATION OF THE TAILINGS FACILITY
2.8.1
Embankment Raising
Ongoing raising of the tailings embankments will be required for the duration of
operations. Most of the earthworks will need to be completed within the dryer
period of the year when conditions are suitable for fill placement operations.
Generally, this will involve an annual construction cycle in the second half of
each year with some minor items being addressed over the rest of the year. In
addition to embankment raising earthworks, new decant weirs will be constructed
each year and overflow spillways will be expanded or new spillways will be
constructed as required.
