Environmental Assessment
Volume C-3.6
Slurry Pipeline
Hydrology
Ambatovy Project
57
January 2006
Table 3.6-5 Typical Erosion and Sediment Control
Type
Application
Examples
site management
minimum exposure of soils; perimeter control; site access
management; stockpile management
procedural
scheduling
optimize construction sequence; install BMPs early; restore
and reclaim early
water
management
various
diversions; proper design of drainage channels; management
of shallow groundwater
protection of exposed
surfaces
mulching; tree and shrub planting; riparian zone preservation
erosion control
runoff control
grading; drains; diversion ditches and berms
infiltration
buffer strips; hay bales
sediment control
settling
silt fences, rock berms, sediment traps and basins
Source:
adapted from TAC 2005.
Pipeline Watercourse Crossings
The majority of watercourse 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. As with water levels, the effects on sediment levels are
expected to be high during construction and to last only a short time, i.e., on the
order of hours for small streams and up to a few days for very large streams. An
initial flush of sediment is also expected at the onset of the wet season when high
flows transport previously disturbed sediments to downstream locations. At
locations where aerial crossings are used, construction activities will not affect
the stream channel; therefore, no changes in sediment concentrations are
expected.
No effects on sediment levels are expected in the streams during operation or
post-closure provided revegetation and erosion control is applied and stream
banks are stabilized. Design and construction specifications, e.g., minimum
burial depths, size of armour material and sag-bend setbacks, will also minimize
the possibility of channel instability and geomorphic change.
Pipeline burial depths beneath streams will be a minimum of 1.2 m below the
bottom of the stream bed in rock material and 2 m below the stream bed in earth
material. Site-specific scour depth calculations will be conducted for streams
where maximum scour depth associated with the 1:100 year flood is expected to
be greater than the minimum covers noted above. These estimates rely on-site
specific characteristics including descriptions of substrate, channel cross-
