Environmental Assessment
Volume B-3.8
Mine
Hydrology
Ambatovy Project
97
January 2006
Changes in streamflows at locations further downstream of the ponds were also
evaluated. These were derived based on the expected pond outflows and
additional tributary flow that enters the channel downstream of the ponds.
Changes in water depths, flow area and velocity were estimated at the
hydrometric monitoring locations based on changes in flows at those locations
and on available cross-sectional survey information. Water levels for various
flows were calculated using the available rating curves, and were then translated
to water depths and flow areas. Average velocities were calculated based on the
flow rates, derived flow areas, and surveyed widths of the streams.
Changes in sediment levels were evaluated qualitatively based on expected
changes in drainage areas and streamflows.
3.8.5.3 Impact Description Criteria
The assessment criteria used for hydrology are presented in Table 3.8-6.
Table 3.8-6 Impact Description Criteria and Numerical Scores for the Ambatovy
Project – Surface Water Hydrology
Resource
Direction
(a)
Magnitude
(b)
Geographic Extent
(c)
Duration
(d)
Reversibility
(e)
Frequency
(f)
surface
water
hydrology
positive
,
negative
or
neutral
for the
measurement
endpoints
negligible
: <5%
change
low
: 5 to 10%
change
moderate
: 10 to
30% change
high
: >30%
change
local
: effect restricted
to the LSA
regional
: effect
extends beyond the
LSA into the RSA
beyond
regional
:
effect extends beyond
the RSA
short-term
:
<3 years
medium-term
: 3 to
30 years
long-term
:
>30 years
reversible
or
irreversible
low
: occurs
once
medium
: occurs
intermittently (1
to 10 times per
year)
high
: occurs
frequently (>10
times per year)
(a)
Direction: positive or negative effect for measurement endpoints, as defined for the specific component.
(b)
Magnitude: degree of change to analysis endpoint.
(c)
Geographic Extent: area affected by the impact.
(d)
Duration: length of time over which the environmental effect occurs. Considers a three-year construction period and a 27-year
operations period.
(e)
Reversibility: effect on the resource (or resource capability) can or cannot be reversed.
(f)
Frequency: how often the environmental effect occurs.
3.8.5.4 Mitigation
To mitigate the effects on flows, water levels and sediment levels, the Ambatovy
Project will implement the following measures:
•
Monitor downstream flows to verify there is sufficient water for the
receiving environment, and that any excess does not result in channel
