Environmental Assessment
Volume E-3.8
Tailings Facility
Hydrology
Ambatovy Project
79
January 2006
Reductions in long-term post-closure streamflows vary by location, and typically
range from 3 to 5% for average conditions and from 8 to 15% for dry month
conditions. Reductions are slightly greater in the shorter term post closure (e.g.,
Year 42 when interceptor pumping ends) at 4 to 7% and 10 to 21% for average
and dry conditions, respectively. Flow reductions typically decrease with
increasing distance downstream. The exception of Basin A6, with a slight
increase compared to Basin A5, is a result of tributary flow from Basin B. The
reductions in flows for long-term post-closure conditions reflect decreased
infiltration rates through the tailings, and the resulting reduction in groundwater
movement to downstream areas.
Changes in flow areas, water depths and velocities were estimated at the three
hydrometric monitoring locations based on expected changes in flows.
A summary of the changes is presented in Tables 3.8-9 and 3.8-10. During
operation, the estimated 30 to 40% reductions in flow would result in 5 to 31%
reductions in both flow area and average depth. Estimated flow velocity
reductions are 15 to 33%, although the changes are only 0.1 to 0.2 m/s in terms
of magnitude. For post-closure conditions the reductions in flow are relatively
small, resulting in negligible changes in hydraulic characteristics for both
operations (0 to 6%) and post-closure (0 to 4%).
As described previously, operation of the tailings facility will divert flow from
the upper watersheds of the tailings basins and has also been designed to contain
the 1:50 year 24-hour storm (402 mm of precipitation). As a result of diversions
to the water pond, peak flows and water levels in Basin A and C downstream of
the embankments will be reduced compared to baseline conditions. Peak flows
and water levels will also be reduced in Basin B where, for storms up to the 1:50
year event, all runoff is held in the water pond and subsequently discharged to
the ocean. For larger events, the facility will still contain the volume associated
with the 1:50 year storm, and runoff in excess of this volume will be released to
the downstream area of Basin B. Excess runoff volumes will be considerably
less than if no storage was provided, therefore, peak flows and water levels for
this scenario will also be reduced compared to baseline conditions. For example,
the total precipitation associated with the 1:100 year 24-hour event is 465 mm;
however, the tailings facility provides storage for approximately 400mm of
rainfall which means only about 65 mm contributes the downstream peak flows.
Peak flows and water levels following closure of the tailings facility are expected
to be similar to baseline conditions once drainage paths are returned to their
natural receiving waterbodies.
