Matai'an Creek lake breach involved 403 meter slope collapse: Agency

Taipei, Oct. 1 (CNA) The main cause of the Matai'an Creek barrier lake overflow was several mountain slope collapses, the largest of which was 403 meters, the Forestry and Nature Conservation Agency said Wednesday.

A comparison with pre-overflow topographic data provided by the Ministry of the Interior showed that large-scale terrain changes occurred from the upstream mountains to the downstream creek bed, the agency's Aerial Survey and Remote Sensing Branch (ASRS) said in a news release.

The steepest collapse occurred at the top of the barrier lake, according to figures provided by the ASRS.

Large amounts of earth and rock have piled up in the midstream and downstream sections of the Hualien County creek, raising creek bed levels by 40 to 50 meters, the agency said.

The observations were based on a digital elevation model with a precision of one meter, produced using an airborne Light Detection and Ranging (LiDAR) scanner that surveyed the area on Sept. 25, two days after the barrier lake breach, according to the ASRS.

The breach, triggered by heavy rains from Typhoon Ragasa, caused severe flooding that left 18 dead and six missing as of Wednesday morning, with search and rescue missions ongoing, according to the Central Emergency Operation Center (CEOC).

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 downstream creek, the CEOC said.

Meanwhile, about 800 meters of levee reconstruction have been completed in Guangfu Township, the area most affected, accounting for roughly 27.9 percent of the total work, according to the CEOC.