Environmental Assessment
Volume E-3.8
Tailings Facility
Hydrology
Ambatovy Project
84
January 2006
Table 3.8-11 Residual Impact Classification for Hydrology
Phase
Direction
Magnitude
Geographic
Extent
Duration Reversibility Frequency Environmental
Consequence
Issue: Changes in Streamflows and Water Levels in Receiving Water Bodies
construction/
operations
negative
high
local
medium-
term
yes
high
(average
conditions)
high
closure
negative
average
conditions:
negligible to
low
dry month
conditions:
moderate for
main valleys
and negligible
along the
main stem of
the ambolona
river
local
long-
term
no
high
(average
conditions)
average
conditions:
low
dry month
conditions:
low
Issue: Changes in Sediment Levels in Receiving Water Bodies
construction
negative
high
local
short-
term
yes
high
(average
conditions)
moderate
operations
negative
low to
moderate
local
medium-
term
yes
high
(average
conditions)
low
closure
negative
negligible to
low
local
long-
term
no
high
(average
conditions)
low
3.8.6.2 Prediction Confidence
With the exception of evapotranspiration, the primary climate variables affecting
baseline hydrology are well understood. Hydrology in terms of rainfall response
and runoff generation is understood to the extent of the limited amount of site-
specific data that are available. Similarly, the interaction between surface water
and groundwater in the project area is difficult to quantify, particularly during the
dry season in locations where surface flows may be dominated by groundwater
inflows.
The impact ratings are based on a hydrologic model that assesses average
monthly conditions based on a reasonable estimate of annual rainfall and runoff.
For these average conditions, the prediction confidence is considered medium.
For dry monthly conditions, however, flow estimates are based on a runoff depth
derived from a single dry season of hydrometric monitoring and the estimated
interaction with groundwater. Based on the uncertainties associated with these
