Landslides occurred on both sides of the mountains near the Swallow Grotto Trail (Yanzikou, 燕子口步道) in Taroko National Park in Hualien County, forming a barrier lake in the Liwu River (立霧溪). As water levels rose rapidly on Friday last week, the Hualien branch of the Forestry and Nature Conservation Agency issued a red alert — which has since been lifted — and evacuated more than 900 residents of downstream communities in Sioulin Township (秀林).
The Public Construction Commission also closed part of Provincial Highway No. 8 between the entrance to Taroko National Park and Tianxiang (天祥) and completely closed the park to the public.
Amid the crisis, attention should be paid to the geological and topographical environment, as well as the characteristics of naturally formed dam structures.
The outcrop geology in the area where the barrier lake formed consists of a lot of marble. That part of the gorge is narrow, limiting the flow of the river and making conditions favorable for the formation of the dam early on Friday last week. Within just one day, water levels rose to overflow through the Jin Heng Tunnel (靳珩隧道) before discharging back into the river channel below the dam.
As the lake’s area is relatively small, its capacity to accommodate upstream inflow is limited. If upstream runoff from heavy rain were to exceed the discharge capacity of the tunnel, there would be a significant likelihood that the barrier lake would top the dam’s crest, causing it to erode and potentially fail catastrophically.
Fortunately, the lake is adjacent to the highway — which is about 5m lower than the dam’s crest — and vented through the tunnel. The highway and tunnel functioned like a flood spillway, reducing the threat of dam failure and damage to downstream areas.
Given the relatively favorable situation that arose in which the tunnel acted to ease pressure on the natural dam, using machinery to open a spillway at the dam crest should not have been the top priority, given the safety risk that construction personnel face.
Water was seeping through at the Swallow Grotto Trail site through the landslide debris. The rate of seepage was a critical factor in determining whether the dam would experience piping failure — a phenomenon that occurs when water carries away sediment from a dam, creating natural pipes that expand and lead to the dam’s collapse. That scenario is difficult to prevent and impossible to rule out.
In addition to the potential effect on downstream areas, the risk of a piping failure should have kept construction personnel from being carelessly sent out to work on the dam crest.
Analyzing the sediment and silt accumulation in the middle and lower areas of the barrier lake is critical.
Academic institutions have previously provided relatively accurate forecasts of the potential for disaster from barrier lakes on Hualien County’s Guangfu Township (光復), and similar simulations should have been conducted for the Taroko barrier lake.
Scientific data could help more precisely determine the proper scope of downstream evacuation zones. The residents of Fushih Village (富世) and Sioulin live on terrain that lies just above the Liwu riverbed, without embankments and in an area where the river cross-section widens toward the estuary. Assessing the scope of potential flooding of residential areas in the event of dam failure was essential.
The temporary closure of Taroko National Park was a solid disaster prevention measure. People must always exercise caution in mountainous areas. When traveling through geologically sensitive zones, or areas prone to landslides and barrier lakes, the public is at risk before even reaching the upper sections of the mountain route. The overflow of the lake left parts of the highway impassable — so how could anyone safely enter or exit the park?
Johnson Kung is a civil engineer.
Translated by Kyra Gustavsen
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