Tue, Mar 07, 2023 page8

• Reservoirs need innovative upgrade

  • By Chang Yen-ming 張炎銘
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  Given the dry conditions in southern Taiwan, the water situation indicator for the Chiayi region — Chiayi City and Chiayi County — has since Wednesday last week been set to “yellow,” signifying reduced water pressure.

  Meanwhile, Tainan’s has been set to “orange,” meaning reduced water supply.

  This is certain to inconvenience major water consumers, but most people living in houses or apartments with water storage towers are unlikely to be affected.

  The Ministry of Economic Affairs has halted irrigation of rice paddies in the Chiayi and Tainan regions for the first quarter of this year to save water. If this had not been enforced, the dry conditions over the past few months might have made the situation even worse.

  Water conservancy departments have been combating shortages by developing subsurface water and drought-relief wells.

  However, possibly out of concern for environmental conservation pressures, plans for reservoirs — which could greatly increase the nation’s water storage capacity — are rarely mentioned.

  The only large-scale reservoir project approved by the Water Resources Agency is the Niaozueitan (鳥嘴潭) artificial lake, which was approved under the “environmentally correct” principle of “reducing land subsidence by using surface water instead of groundwater.”

  Climate change is making drought and flooding more extreme, so there is an even greater need to expand the nation’s already insufficient water storage capacity.

  As well as building new reservoirs, great efforts have been made in the past few years to dredge the existing ones.

  The annual volume of sediment dredged in Taiwan is about 17 million cubic meters. This process should be continued, but dredging is not cheap.

  During the past three years, the average cost of dredging has been NT$90 to NT$100 per cubic meter of sediment, bringing the annual cost to more than NT$1 billion (US$32.71 million).

  Heightening reservoirs is another way to achieve the same goal. The old Jianshanpi Reservoir (尖山埤水庫) has been heightened twice. The main and secondary dams of the Lantan Reservoir (蘭潭水庫) have been heightened by 3m, increasing the reservoir’s capacity by nearly 30 million cubic meters.

  The second phase of the Sinshan Reservoir (新山水庫) in Keelung aims to increase its capacity by 6 million cubic meters. The most recent alteration was in 2009, when the impervious core wall of the Zengwen Reservoir’s (曾文水庫) dam was heightened by 3m, thus increasing its capacity by about 55 million cubic meters for a cost of just NT$200 million.

  There should be several other reservoirs in Taiwan whose impounded level could be raised, and even some whose capacity could be increased by modifying some key points of reservoir operation — with safety concerns considered. These possibilities should be comprehensively reviewed.

  The only way for the amount of silt discharged from a reservoir to balance the amount that flows into it is by dredging, combined with management of the catchment area, as well as other methods such as sediment bypassing and hydraulic silt discharge when the reservoir is filled to capacity.

  However, it is still extremely difficult or impossible to restore a reservoir to its original capacity.

  The total volume of sediment in the Zengwen Reservoir is about 180 million cubic meters, but in recent years the maximum annual dredging volume has been a little less than 4.8 million cubic meters.

  This amount of dredging can only achieve a balance with the volume of silt inflow, thus preventing a further increase in the total volume of sediment. Even if the dredging capacity and site constraints could be overcome to double the dredging volume, it would still take 38 years to restore the reservoir to its original capacity.

  Unless the reservoir’s operation can be suspended, restoring the original capacity of Tainan’s Wushantou Reservoir (烏山頭水庫) could take 300 to 400 years by dredging alone.

  Dredging is therefore only a necessary means of maintaining reservoirs’ existing storage capacity.

  Apart from building and heightening reservoirs, promoting the production and use of reclaimed and desalinated seawater is an effective means to develop water resources. In particular, promoting reclaimed water not only meets the requirements of environmental protection, but also provides a stable supply.

  Unfortunately, only a limited amount of reclaimed water can be produced, so it can only be used by industries that are located near water reclamation plants.

  Demand for water is likely to keep increasing. If the means of saving and transferring water have been exhausted, the public can be expected to support water resource development plans, including building new reservoirs to avoid water shortages.

  Chang Yen-ming is a former Water Resources Agency section head, and an author of a book about Taiwan’s reservoirs.

  Translated by Julian Clegg
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