Environmental Assessment
Volume E-3.8
Tailings Facility
Hydrology
Ambatovy Project
67
January 2006
The impact pathways between project activities, environmental changes, key
questions, and assessment results are shown in Volume H, Appendix H-9.
3.8.5
Impact Assessment
3.8.5.1 Impact Pathway Evaluation
Project activities during construction, operation and closure may result in the
following: i) changes in flows and water levels in receiving water bodies; and
ii) changes in sediment levels.
During construction there will be disturbance to the landscape as a result of site
clearing and the development of infrastructure. Surface runoff characteristics
and natural drainage patterns will be affected. Landscape disturbance, along with
the exposure of soil to erosion, may increase sediment yield and transport to
receiving water bodies.
During operation, the tailings facility will divert natural runoff in the upper
watersheds and will result in reduced flows at downstream locations. Reduced
groundwater contribution in downstream reaches will also occur due to lower
infiltration rates in the tailings facility and groundwater interception by the wells
located downgradient of the embankments. Diversion of runoff from the upper
watersheds will also restrict sediment movement from the upper basin to lower
reaches. The tailings embankments, however, could act as a sediment source and
could result in increased sediment yield to the lower basin.
Long-term changes in flows and water levels, in conjunction with changes in
sediment supply, could also affect channel morphology as the channel attempts to
reach new equilibrium conditions. However, as discussed in Section 3.8.5.5.,
impacts on flows, water levels, and sediment levels will only occur during
operation of the tailings facility and will therefore be of medium-term duration.
The changes in key indicators also involve reductions in flows which could result
in sediment deposition within channels and channel aggradation (in contrast to
the erosion and channel degradation that can occur with increased flows). It is
important to note that the channels downstream of the tailings facility have been
heavily altered by local farmers, and are not natural, equilibrium streams. It is
therefore difficult to predict the morphological changes that could occur in these
streams as a result of flow and sediment supply changes. Furthermore, most
changes are expected to be reversed at closure when natural flow conditions are
re-established.
