Last week, Nantou County’s Renai Township (仁愛鄉) was hit by torrential rainfall as the outer bands of Typhoon Khanun passed, causing large flows carrying debris in many places. Many roads, including Provincial Highway No. 14, were blocked by soil and rock, and debris flows struck Nanfong (南豐) and Chunyang (春陽) villages.
In the Lushan (廬山) hot spring area, the foundations of some buildings were washed away, leaving them precariously suspended.
Quite some time has passed since this area was hit by debris-laden flows. Faced with the threat of extreme weather, the features of last week’s disaster should be studied to improve debris flow prevention.
Renai Township has 34 streams that are susceptible to debris flows, with 10 near Nanfong Village, the Agency of Rural Development and Soil and Water Conservation said in a statement.
This explains why the most serious flows affected the 68km to 80km section of Provincial Highway No. 14.
By counting the places where soil and rocks were piled on roads, debris flows affected at least 20 locations, including downhill from small landslides and downstream from artificial or natural gullies.
Cumulative rainfall of about 1m over three days washed debris from landslides over the past few years into streams, which added to the material dislodged in the storm.
Although traffic along Provincial Highway No. 14 was quickly restored, piles of soil and rocks — some as tall as a vehicle — were left along the road.
The courses of natural gullies are generally uneven, providing relatively strong resistance that impedes debris flows, but artificial gullies, such as concrete check dams, provide smooth surfaces that allow faster flows.
The rain from the typhoon also created several temporary streams.
In view of this, the focus of work to reduce the scale of debris flows should be on mimicking nature’s way of dispersing debris runoff and easing flows.
In Chunyang, the surface runoff from the huge catchment area upstream of the village might have been more than artificial gullies could cope with; that is, even without the soil and rocks in the water.
If a catchment area is developed, consideration should be made for debris runoff. Runoff should not be channeled into a limited number of gullies.
The debris flows in and around Chunyang explain why the silt level in the Wushe Reservoir (霧社水庫) rose to 75 percent. More advanced and effective ways to manage reservoir catchment areas should be developed, including their spatial planning.
Another important factor contributing to the scale of the debris flow in Chunyang was a bridge that obstructs the artificial channel there. The bridge restricts the river’s capacity to discharge floods and expel debris.
Furthermore, the channel is in a downstream section, where greater volumes flow. The debris flow in Chunyang was concentrated in the lower parts of the village, close to the highway, leaving parts of the village further upstream.
Appropriate dispersal of runoff channels is one practical method that could reduce the impact of flows and reasonable enlargement of the downstream section is another.
From a policy perspective, control and spatial planning of developed upstream areas might succeed if they are implemented in time.
Johnson Kung is a civil engineer and a board member of the Taiwan Professional Civil Engineers’ Association.
Translated by Julian Clegg
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