Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
POWER GENERATION
Plant Load Factor (No, h/day, h/yy) 0.46, 11, 4010
Max. Net Head (m) 211
Average Net head (m) 189
Design flow (m 3 /sec) 950
Installed power (MW) 1870
Average energy production (GWh/year) 6500
Firm energy production (95 %) (GWh/year) 5400
3.3 Alternative layouts analysis
The final layout of the project has been selected basing on the analysis of various alternative solutions.
The investigations and the design of several layouts have been carried out during the first project phases
(Preliminary, Basic, Level 1 design) to assess the comparison among the most promising alternatives.
The construction period has been one of the most relevant parameters for the selection of the layout.
The main characteristics of the reservoir (i.e. dam elevations) and the installed power of the plant have been
assessed by means of a cost-benefit analysis and basing on the preliminary environmental studies.
Since the majority of the materials for the dam construction (rockfill, aggregates, and natural pozzolans)
were found on site the analyzed dam types did not show substantial differences for environmental impacts.
In the preliminary phase the design of an arch-gravity structure has been prepared. This layout, while being
technically feasible, has not been maintained because of the risk of delays related to the required
investigations and foundation treatments in the right abutment.
Similarly the classical earth rock dam has been found feasible but time constraints due to the zoning of the
embankment (in particular impervious core and filters) could not guarantee the respect of the construction
schedule.
Eventually the two most promising alternatives have been investigated and designed in detail during the
Basic and Level 1 design phases:
• Bituminous Facing Rockfill Dam (BFRD)
• Roller Compacted Concrete gravity dam (RCC)
The majority of the very high dams constructed in the recent years are either rockfill dams (homogeneous
and permeable embankment) or RCC gravity dams.
The detailed analysis of the BFRD layout showed that:
• the rockfill embankment is feasible using materials available on site (basalts, trachyte, alluvium);
• some risks of delays due to exceptional floods during construction or reservoir drawdown after the
first impounding cannot be avoided.
Eventually the RCC dam alternative has been selected having the following key positive aspects:
• the gravity structure is extremely safe where an exceptional flood occurs during construction and in
all potential extreme conditions as earthquakes, terrorism, etc.;
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