Seth

The initial greenhouse gas production would be the anaerobic
decomposition of all the buried vegetable matter, as you state. This
will produce vast amounts of methane, which has the same effect as
something like 40 times that of CO2 (I've seen figures quoted between 20
and 50, but the consensus is 35-40), weight for weight. Both molecules
contain just one carbon atom. However, it does not stop there. China's
biggest city, Chongquing, with a population of 27 million, (and various
smaller ones) will lie near the head of the retention lake. Currently,
the raw sewage is channelled into the fast-flowing Yangtze, which is
well-oxygenated, and it is whisked downstream and decomposes
aerobically, producing CO2. When the lake is filled, this sewage will
simply drop to the anaerobic depths of the relatively still waters:
result, methane again. I'm not saying that it is 100% CO2 before and
100% CH4 after, but you can imagine that if the ratio is 75:25 CO2:CH4
before and 25:75 after, considering the daily mass of excrement from the
35 million people in the catchment area, the difference will be
considerable. The same applies, of course, to all the vegetable silt
brought down from higher reaches into the lake. I suspect that the lake
may become somewhat noisome in places, as well!

Another possibility is that it is known that organic silt constantly
releases methane from the surface and, as the depth builds up, some
methane becomes trapped in very large bubbles (up to tens of m3). These
occasionally burst randomly but, if there is a minor earthquake - and
this is a seismic area - the agitation could cause many of them to burst
simultaneously, releasing vast amounts of methane at once. Methane is
lighter than air, so it would quickly dissipate, but not before it could
cause problems in populated areas, if the proportions were right.

As you say, HE is cheap power, almost costless once the infrastructure
is amortised in, typically, 10-15 years, but it is not cheap in human
lives. On an average, since 1945, > 2,000 people throughout the world
lose their lives each and every year from dam failures, overspills,
penstock bursts and so on. This makes Chernobyl and the lesser nuclear
accidents together look like a child's toy, in comparison, but an HE dam
burst hits the media for just one day. I can say this very sincerely:
I'd much rather live next door to a modern nuke station than downstream
from a dam.

My personal feeling is that the Chinese would have been better off with
a number of smaller HE projects rather than one big one. Putting their
eggs into one basket is not, IMHO, a very good idea, especially when the
basket is 2 km across and 180 m high. However, they are also investing
heavily in nuclear power and this seems sensible, on condition that the
reactors are built to the latest safety standards.

Incidentally, having seen the section of the dam and remembering 11
Sept, I should imagine that a 747 (or A380) crashing into the main body
would have very little effect. I think the most vulnerable parts would
be the two passages for fluvial traffic: a lift for smaller craft and a
series of locks for larger ships.

Brian



Brian Ellis wrote:
>
> The TV stations were showing a clip last night on various newscasts of
> the preparations taking place for flooding the Yangtze valley upstream
> from the dam. I think that there is a strong risk of this being the
> worst environmental and human disaster in the world, having visited the
> site a few weeks ago. It will certainly be a major producer of
> greenhouse gases (possibly more per kWh produced than fossil fuel power
> stations). It will also put at risk the lives of millions of Chinese and
> will promote a much higher level of chemical pollution downstream.
>
> On the positive side, it will produce 1/6 of the country's current (no
> pun!) needs of electricity, assuming there are no major hitches, by
> 2009, by which time the demand will be up over 25%, mainly because of
> our industry (the polluting coal-fired stations will not be closed down,
> therefore, because of the dam coming on line.
>
> I have purposely not justified any of my assertions above, but can
> substantiate every one of them.
>
> The question is: is such a massive project justified today?
>
> Brian