Before Typhoon Morakot made landfall in Taiwan, everyone was hoping it would bring enough rain to end the drought and avert imminent water rationing. When it left, the drought had indeed been ended, but it also brought with it large scale flooding, as happens almost every year. The government debates how to approach the situation every year, but that does not seem to have had any effect on flood prevention.
Taiwan sees plum rains in May and June and a dry season in July, as reflected in the Taiwanese saying “scorching July.” The typhoon season between August and October then brings plenty of rain. It is therefore normal for water levels in water reservoirs to be low toward the end of July. Despite this, the government’s water resource agencies always make a big deal about promoting water conservation because of the shortage.
While there is nothing inherently wrong with preventive measures, it nevertheless shows that the authorities lack expertise and do not understand Taiwan’s water situation. In August, flood and disaster prevention — not drought relief — should be the overriding engineering concern. Failure to realize this shows that water resource agencies have their priorities all wrong.
The severe rainfall brought by Typhoon Morakot was heaviest on the Ailiao River (隘遼溪) at Sandimen (三地門) and the upper reaches of Linbian River (林邊溪), both in Pingtung County, which received 1,826mm on Aug 8 and 1,072mm on Aug 7 respectively. These figures almost surpassed Taiwan’s normal annual rainfall and by far surpassed all protective river drainage standards, making it all but certain that floods would break through or overflow river and drainage embankments.
In rivers with large water reservoirs, most of this extreme rainfall was collected by those reservoirs and only smaller volumes flowed downstream. The Feitsui Reservoir (翡翠水庫) and the Shihmen Reservoir (石門水庫) in the upper reaches of the Tamsui River received almost 400 million cubic meters, saving downstream Taipei and Taoyuan. The Tsengwen Reservoir (曾文水庫) in the upper reaches of the Tsengwen River received almost 500 million cubic meters, thus saving the downstream sections of the river and Tainan City from flooding. This accumulation of floodwater is both a necessary and an effective function of reservoirs.
For more than 100 years, Taiwan’s water conservation agencies have approached flood prevention by building embankments to control flooding and it can’t be denied that this has been effective — to a certain degree. During Taiwan’s early agro-industrial development, this could be tolerated, but it is not sufficient at the present stage.
As the world is dealing with climatic change, population growth and socioeconomic development, developed countries approach river control based on concerns for national land management, comprehensive watershed planning and overall flood prevention. The government’s water conservation agencies are intimately familiar with these concepts, but implementation is slow and nothing has happened in the past dozen years. And yet, the government still only builds embankments and pumping stations.
An analysis of flooding in the hardest hit areas around Linbian, including Linbian River, the Xinpi (新埤) drainage on its right bank and the Dawuding (大武丁), Qiangyuan (羌園) and Wenfeng (塭豐) drainages on its left bank, shows that the embankments and pumping stations that for years seem to have been the only solution are completely useless — if it rains, it floods. In fact, more than 75 percent of the land along the Linbian River and the adjacent drainage and catchment areas is higher land. When it rains, the water flows from these areas down to the lower lying areas and it is thus only natural that people living in the lower areas will experience severe flooding.
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