In July, heavy rainfall brought by Tropical Storm Wipha triggered a large-scale landslide in the upper reaches of the Mataian River (馬太鞍溪) in Hualien County. The ground fractured and shifted, forming a natural dam nearly 200m high, with an estimated water storage capacity of about 86 million cubic meters — equivalent to the Nanhua Reservoir (南化水庫).
In just two months, repeated heavy rainfall caused the water level to rise to nearly the top of the barrier. As the outer bands of Super Typhoon Ragasa approached on Tuesday, the barrier lake burst, and muddy floodwaters flowed downstream, devastating the townships of Guangfu (光復), Wanrong (萬榮) and Fonglin (鳳林).
This disaster is a reminder that barrier lakes — which form when debris, landslides or natural blockages create a temporary dam along a river — are not merely a flood-related issue, but a complex challenge involving geology, engineering and disaster management.
Since Caoling Lake (草嶺潭) — a barrier lake in Yunlin County’s Caoling Township (草嶺) — formed in 1862, Taiwan has experienced 75 similar events. The most devastating occurred during the earthquake on Sept. 21, 1999, which triggered a massive landslide in Nantou County’s Jioufenershan (九份二山) mountain that formed two barrier lakes and resulted in 41 fatalities.
In 2009, Typhoon Morakot — the most damaging typhoon to hit Taiwan in decades — caused widespread landslides and mudslides across the nation, forming at least 16 barrier lakes that caused severe flooding and landslides in downstream areas. These incidents highlight the significant threat posed by the collapse of barrier lakes to downstream areas, and illustrate an urgent need to strengthen disaster prevention and emergency response measures.
The government’s current standard operating procedure for responding to landslide-formed barrier lakes on state-owned forest land provides guidelines for handling such incidents. Experience from the Jioufenershan and Caoling barrier lakes has proven that a combination of overflow, dam reinforcement and monitoring is an effective model for mitigating risk.
These lessons are equally applicable to the recent Mataian River incident — however, remote terrain, challenging construction conditions and unstable weather made on-site response significantly more difficult. Fortunately, the monitoring system proved effective, issuing early warnings that enabled the timely evacuation of residents to reduce casualties.
Statistics indicate that barrier lakes have a high potential for recurring. The Caoling barrier lake, for example, has formed multiple times due to major landslides triggered by typhoons or earthquakes — in 1862, 1941, 1942, 1979 and 1999. Taiwan’s steep mountainous terrain and the frequency of typhoons and earthquakes cause barrier lakes to reappear almost every two to three years. Each disaster serves as a reminder of the necessity of long-term preparedness beyond just emergency response.
First, medium-to-long-term management should take priority. After such events, actions must be taken to dredge river channels, construct check dams, stabilize slopes and restore vegetation to reduce the risk of future landslides and the reforming of barrier dams.
The second step is to systematize geological monitoring. Tools such as aerial photography, remote sensing and geological surveys can be employed to conduct long-term monitoring of mountain terrain and landforms. This would ensure that potential large-scale landslides are detected as early as possible and promptly addressed.
Furthermore, it is essential that a centralized database be established and regular drills conducted. Creating a unified database and holding regular emergency response exercises would significantly shorten the time required for decisionmaking when disaster strikes, and maximize the protection of lives and property.
The Mataian River incident reminds us yet again that barrier lakes are not solely a flood issue, but a comprehensive test of geological conditions and human response capabilities. In Taiwan’s rainy environment, such lakes are unlikely to exist for a long time — therefore, the most practical strategy is to quickly assess, drain and evacuate.
The effective integration of geological surveys, engineering measures and community disaster preparedness would help to minimize damage when the next natural disaster strikes.
Yu Wei-yao is president of the New Taipei Engineering Geologists Association.
Translated by Kyra Gustavsen
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