Thu, Dec 31, 2009 - Page 8 News List

Dredging reservoirs will get us nowhere

By Ko Hai-sheng 柯海生

Is it possible to increase reservoir volume by removing excess sediment? While possible in theory, there have been very few successful cases.

For example, the mid and upstream regions of the Yellow River in China have several reservoirs with equipment designed to remove excess sediment from the bottom, but the effects have been limited. The Zengwun Reservoir (曾文水庫) and the Shihmen Reservoir (石門水庫) in Taiwan have similar equipment, but it has proven impossible to eliminate excess sediment in these reservoirs and increase the amount of water stored.

In the past, dredge boats were used in the Shihmen Reservoir to dredge excess sediment, but because of environmental and site-specific issues, there was insufficient space to store it and large-scale dredging was not performed.

As soon as the volume of water that can be stored in a reservoir decreases, it is nearly impossible for it to be replenished. Once large amounts of excess sediment gather in a reservoir, the most important thing is to stop further sedimentation, rather than dredge reflexively.

Wushantou Reservoir (烏山頭水庫) has the slowest rate of increase of excess sediment of all of Taiwan’s reservoirs. The reason for this is that those in charge of operating the reservoir eradicated illegal logging and cultivation in the reservoir’s catchment area. The Wushantou and Zengwun reservoirs are located near each other and share similar geological structures. The pattern of rainfall in the catchment areas of the reservoirs during Typhoon Morakot was also similar, but the average surface area of sediment that ended up in the reservoirs differed greatly. This was a direct result of the quality of soil and water conservation.

The government’s current pre-emptive measures against drought in southern Taiwan in the short term involve leaving the Zengwun Reservoir irrigation area fallow during the first crop season of next year. This could very well work; however, if over the next six months we see a repetition of extremely low levels of annual rainfall — that is, 30 percent less rainfall in the reservoir’s catchment area than the annual average, which has happened three times in the last 90 years — then we will be in serious trouble.

We can expect to see industrial users of water that can afford transport costs of NT$50 to NT$100 per tonne transporting clean water by road to address shortage problems. This is what the Hsinchu Science Park did about five years ago.

However, it is harder to imagine what will happen with regard to the general population. Things could get so bad that we will have to transport water to northern Taiwan via truck or even purchase water from China and have it delivered here.

This scenario is not an exaggeration, and given these dire circumstances, it is no wonder that many people have suggested holding a national conference to discuss measures dealing with the drought we could face next year.

The measures currently under way or under discussion will not ease a possible drought. A conference is necessary to discuss medium to long-term solutions to drought in southern Taiwan.

If a conference is held, it should at least involve discussion of the following points: (1) carrying out soil and water conservation in catchment areas and eradicating illegal logging and cultivation; (2) immediate restarting of the Zengwun reservoir diversion project and (3) the feasibility of desalination.

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