Environmental Assessment
Volume C-3.6
Slurry Pipeline
Hydrology
Ambatovy Project
55
January 2006
3.6.4.5 Results for Key Question H-1: What Effect Will the Slurry
Pipeline Have on Flows and Water Levels in Water Bodies?
Pipeline Right-of-Way
Construction and installation of the pipeline will involve land disturbance
including removal of vegetation, excavation and backfilling, and compaction.
These activities will result in higher runoff rates from the disturbed areas
compared to baseline conditions. However, the percentage of disturbed area
within a given basin is typically very small relative to the overall drainage area.
Changes in flows and water levels in receiving streams are not expected to be
measurable provided best management practices are used and natural drainage
paths are maintained. Any increases in runoff that occur during construction will
decrease once the disturbed areas are reclaimed and as vegetation is re-
established through the operations and closure phases.
Pipeline Watercourse Crossings
Construction of the slurry pipeline will involve about 100 watercourse crossings
of a variety of sizes, along the route. Initial assessments of the crossings have
already been conducted to assist in pipeline routing. Prior to construction, an
environmental team will conduct a field assessment of the crossing locations and
identify any areas with high sensitivities. Where feasible, the pipeline route will
be adjusted through design or reroute to avoid or minimize impacts on the
sensitive areas.
Based on the initial crossing assessment, aerial crossings are expected to be
required at two rivers (K.P. 68.6 and 70.3) as well as several smaller
watercourses in deep and narrow valleys. Subsurface directional drilling may be
applied in some instances to avoid stream disturbances, in which case no changes
in flows, water levels, or sediment concentrations are expected.
The majority of crossings will be buried and will involve excavating across the
channel, laying the pipe, and backfilling the trench. Construction of the
crossings will typically be conducted during low flow conditions to minimize
water depths and the amount of suspended sediment generated and transported to
downstream reaches. Where possible, water will be diverted to one side of the
stream to enable a relatively dry work space on the opposite side. Several
techniques may be used to isolate areas for construction to occur. On larger
watercourses, coffer dams may be used. On smaller watercourses, berms may be
established across the channel both upstream and downstream of the construction
zone, and water either conveyed by gravity or pumped across the construction
site. Effects on water levels are expected to be high during construction;
