Ghana Reservoir Would Be Major Greenhouse Gas Emitter

Patrick McCully
Thursday, January 24, 2008

Bui Dam, now being built in Ghana with financial backing from China Exim Bank, is described by the project environmental assessment as having "minor" greenhouse gas impacts. In reality, it could end up becoming a major emitter of greenhouse gases, many times worse than a natural gas plant of a similar size.

Comments on Bui Dam EIA, sections on global warming

Greenhouse Gas Emissions

As a tropical dam which floods a very large area relative to its power generation capacity, the greenhouse gas (GHG) emissions from Bui are likely to be significant and of a magnitude similar to those of Brazilian dams that are known to be major emitters. Bui Dam will flood 440 km2 for an installed capacity of 400 MW, giving a "power density" of 0.91 W/m2. This is a very low power density, which can be taken as an indication of very high GHG emissions. This is similar to the average power density (0.90 W/m2) of three of the best studied reservoirs in Brazil (Tucuruí, Curuá-Una and Samuel). The average net emissions from these hydro plants is 2,154 g/CO2e/kWh, approximately 4 times the emission factor from a modern natural gas plant. It can be assumed that Bui's emissions would also be multiple times those of a natural gas power plant.

The ERM conclusion that the greenhouse gas impact will be "minor" appears to be a deliberate distortion of reality. This conclusion is based upon the erroneous and irrelevant assertion that the surface area of the reservoir is "not large in comparison to other man-made reservoirs in Africa." It must surely be obvious to the ERM consultants that what is relevant in terms of greenhouse gas emissions per kilowatt-hour is not the large surface area of the Bui reservoir, but the reservoir surface relative to its power generation. The power density figure given above shows that Bui fares very poorly on this metric.

There is no scientific evidence behind the assertion that partial clearance of reservoir vegetation would mean that emissions would be "relatively low." Brazilian dams with high measured emissions have usually had their reservoirs partly cleared as per Brazilian law. Even full reservoir clearance may have relatively little impact on long-term methane emissions which will largely result from carbon entering the reservoir after its initial filling. (And of course much of the vegetation cleared would likely decompose or burn which would also release GHGs).

An assessment of the GHG impacts of Bui Dam should at the very least use the methodology for CO2 and CH4 emissions from flooded land contained in the IPCC's 2006 guidance for compiling emissions inventories.

Susceptibility to Climate Change

ERM is correct to state that "a changing climate in turn holds major implications for the safety and performance of dams". However that statement that this will only happen "over the very long term (ie thousands of years)" is breathtaking in its ignorance. Numerous studies, including the 2007 report of the IPCC, clearly show that climate change is already impacting hydrological cycles. These changed hydrological cycles are in turn already impacting dam safety and performance. To just dismiss these extremely important issues as something for concern in thousands of year's time is shockingly irresponsible. A thorough sensitivity analysis showing the impact of realistic reduced flow scenarios upon Bui's performance should be an essential part of its feasibility assessment.

ERM implies that the impact of climate change on dam safety does not need to be considered as the dam designers have allowed for the "possible maximum flood" (PMF) event. This is standard practice in dam design, however it is based on a statistical analysis of historical hydrological data and does not reflect climate change. The PMF should be calculated taking into account potential future hydrologies.

More information: 

Visit our Bui Dam web page. 

Download the Bui Dam EIA, by UK consultants ERM

Learn more about reservoir emissions