When Tropical Storm Mindulle crossed the nation on July 2 and caused the Chenyulan River to rise suddenly, Wu Chia-mao (吳佳懋) was very surprised not to hear the roar of rocks being swept downstream.
"In Typhoon Herb in 1996 and Typhoon Toraji in 2001, we always heard the roaring of stones, some weighing one or two tonnes, when the heavy rains came," said Wu, 37, the owner of Shang'an Village's Echin Farm (
Wu has been back in his hometown since 1994 after giving up his job as a radiotherapist at Chang Gung Memorial Hospital in Kaohsiung.
PHOTO: CHIANG YING-YING, TAIPEI TIMES
But most of the time since then, it seems, Wu has been fighting natural disasters.
The village, made up of about 500 agricultural families, lies next to where the Sanpukeng Stream flows into the Chenyulan River. But ever since the mid-1990s until recently, the name Shang'an has been synonymous with devastation. The village has been buffeted by mudflows, landslides, river debris, flooding and earthquakes. Grapes grown by persistent farmers on slopes around 450m above sea level are often washed away.
Wu says that after Typhoon Herb in 1996, landslides and mudflows left locals without running water or electricity. For two months they were sustained only by canned food and rice transported to them by air. After the devastating 921 Earthquake in 1999, they were isolated for 40 days. In the summer of 2001, heavy rains brought by Typhoon Toraji claimed more than 17 villagers' lives, destroyed more than 50 buildings and drowned 80 hectares of crops. Survivors ate instant food for a month.
So Shang'an's residents abandoned their crops and houses built in the Sanpukeng Stream danger zone and agreed that government engineers should broaden the river.
The results were encouraging.
no damage
"Surprisingly, Mindulle brought no damage to us at all. We just returned the space to the river so it could flow naturally," Wu said.
In two years, engineers with the Soil and Water Conservation Bureau widened the river from 8m to 17m. When Mindulle inundated the area with 760mm of rainfall within three days, the village suffered no flooding or damage from river debris.
According to bureau official Chuang Chih-hong (莊志宏), a comprehensive investigation of the river, which has a 400-hectare catchment area, resulted in the adoption of ecological engineering methods.
One method saw the river's embankment packed with large rocks and stones. According to engineers, such a structure enables the water to press the stones further into the soil, increasing its stability. Another method uses "comb dams" (梳子壩) to block large rocks from rolling downstream without restricting water flow.
"Public safety can be ensured by adopting ecological engineering methods in river-treatment projects," said Kuo Ching-chiang (
On ordinary days, river water flows slowly because dams are built to mediate river steepness. Reducing the angle of the slope generally reduces the velocity of the water running across it.
"Advantages of slow water flow include the replenishing of aquifers, minimizing runoff and reducing the erosion of embankments," Kuo said.
Kuo stressed that other reconstruction projects involving ecological engineering methods proved to be effective in the aftermath of the flooding caused by Mindulle.
At the foot of one verdant hill behind two dilapidated houses ruined by Typhoon Toraji, there are no signs that this slope was once in a state of collapse.
Kuo said the slope remained stable during Mindulle because a permeable structure consisting of large stones wrapped in gabions, or metal cylinders, prevented any landslides. By employing wooden palings, building channels to divert rainwater and cultivating indigenous vegetation, the engineers claim to have demonstrated that Mother Nature can be nursed with "ecological therapy."
However, just across Linpeng Bridge on the Chenyulan River, the main entrance to this remote area, Hsinglung Village was almost overwhelmed during Mindulle by the huge rocks that tumbled down from untreated slopes.
"This suggests that mountainous land damaged by natural disasters needs to be carefully treated in order to avoid future catastrophes," Kuo said.
Kuo said that other persuasive examples demonstrating the necessity of adopting ecological engineering methods exist in Nantou County.
For example, the Sitou Forest Recreation Area in Luku township was unscathed after Mindulle brought more than 830mm of rain.
The aftermath differed remarkably from that of Typhoon Toraji, as demonstrated by the ruined but still-tottering, seven-floor Le Midi Hotel, one of the structures to fall victim at the time.
Jeng Fu-shu (鄭富書), a professor at the Department of Civil Engineering at National Taiwan University (NTU), said that disaster prevention structures in Sitou helped spare locals the nightmare of river debris.
Jeng said the structures' designs were based on the principle of maximizing guidance for debris flow according to slope, flow rate and location of facilities used by people.
"Preventive measures that allow debris to flow past without causing too much damage will be our priority," Jeng said.
Jeng estimated that more than 50,000m3 of debris could be washed downriver when there next is heavy rain in the area.
STABILITY
To prevent landslides, retaining walls have been built in the last two years using wooden grids interwoven with a diverse series of endemic plants. Chang Cheng-sheng (張振生), an assistant researcher with the Experimental Forest pro-ject at NTU's Agricultural College in Sitou, said that the structure had been effective in stabilizing slopes.
"In the future, when plants thrive, such retaining walls will become part of the environment without sacrificing their function," Chang said.
Kuo, a champion of ecological engineering, said that the recent debate over engineering methods used in disaster areas to prevent debris flow gives the public a chance to learn what ecological engineering methods are and why the environment needs them.
Floods resulting from Tropical Storm Mindulle killed 29 people and injured 16, while 12 people are still missing.
The government evacuated 9,478 people in the central and southern parts of the nation and rescued 4,732 people from remote mountain areas. Among these were 2,132 people in isolated villages who were lifted out by helicopter.
Abnormally high rainfall readings in excess of 1,000mm over three days were measured at several stations throughout the disaster zone.
Total financial losses in all sectors is expected to pass NT$12.2 billion.
"We have to seize the chance to review our existing constructions using these ecologically friendly methods to hone our skills to live in harmony with Mother Nature," Kuo said.
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