The Mataian River (馬太鞍溪) barrier lake overflow was mainly caused by a massive landslide, with a maximum depth of 403m, the Forestry and Nature Conservation Agency said yesterday.
A comparison with topographic data provided by the Ministry of the Interior before the barrier lake collapsed showed large-scale terrain changes from the upstream mountains to the downstream riverbed, the agency’s Aerial Survey and Remote Sensing Branch (ASRS) said in a statement.
The steepest collapse occurred at the top of the barrier lake, figures provided by the ASRS showed.
Photo: CNA
Vast amounts of earth and rock have piled up in the midstream and downstream sections of the Mataian River, raising the riverbed by 40 to 50m, the agency said.
The observations were based on a digital elevation model with a precision of 1m, produced using an airborne Light Detection and Ranging, or LiDAR, scanner that surveyed the area on Thursday last week, two days after the barrier lake breach, the ASRS said.
The breach, triggered by heavy rains from Super Typhoon Ragasa, caused severe flooding that left 18 dead and six missing as of yesterday morning, with search-and-rescue missions ongoing, the Central Emergency Operation Center said.
While there are no signs of abnormality in the lake dam and its flow, the region remains under a red alert due to the large amount of mud and sand clogging the river, the center said.
Meanwhile, about 800m of levee reconstruction have been completed in Guangfu Township (光復), the area most affected, accounting for 27.9 percent of the total work, the center said.
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