Several landslides which had resulted in serious property's damage in the Hoshe area, Nantou Hsien were triggered by heavy rainfalls of the typhoon Herb. In this paper, geological factors, mainly geological structures and lithology, that influenced the landslides along the New Central-Cross-Island Highway and its nearby area are discussed. Rock masses of the study area were highly fractured due to faulting and folding. Quaternary deposits are widely distributed along main rivers in the study area. Therefore, serious landslides and debris flows occurred when the typhoon Herb brought about two thousand mm rainfall within two days.
The Chenyulan Creek headed from northern slope of the Yushan mountain - the highest peak in Taiwan, runs from area of elevation more than 3000 meters down to junction at main course of the Choshui River, where elevation of riverbed is only about 300 meters. The average slope gradient reaches 7%. During Typhoon Herb event, a large and intense rainfall (larger than 200 year return period ) occurred, many landslides and debris flows were triggered in the area of high relief and poor geological conditions. It causes twenty seven persons died and a large of property loss.
The geological environmental processes are very active in Taiwan and they have caused extremely high erosion rate. The Chenyulan Creek is one of the very active rivers in Taiwan. As characterized by the development of alluvium fans, it may indicate that the creek is very wild. High denudation and high carry of sediments is the nature of the river. If we understand these causes, we will never feel the disaster is unexpected. Because we seldom pay attention to understand the history and natural hazard of a terrain from a geomorphological point of view, so that we can not prevent the attack of a natural event. The disaster in the Chenyulan Creek is a good example for us to learn about it, and this knowledge may be useful when we are doing land-use planning or engineering design.
From 31st July to 1st August 1996 when Typhoon Herb attacked Taiwan, the record breaking rainfall(max. 24-hours intensity was 1748.5mm) in the central Taiwan resulted in many disastrous landslides and debris flows along the New-Central-Cross-Island-Highway(NCCI-Highway). Heavy loss of human lives and properties has shocked the whole Country.
The topographic conditions, geological settings and drainage patterns of the mountain terrain where the NCCI-Highway passing through are always thought to be unfavorable for the construction of highway. Route selection and construction operation of the Highway were also poor in quality. Deforestration and over-cultivation after the completion of the Highway has long been recognized as the number one killer of the mountain slopes there. The disasters happened this August yield once again evidences of the grave influence of deforestation and over-cultivation on the slopelands in mountainous areas in Taiwan.
The effectiveness of forest in stopping a high speed debris flow has been observed in the National Taiwan University Experimental Forest.
The super typhoon, Herb, seriously damaged the middle region of Taiwan, especially, the areas along the New Central Cross-Island Highway. This paper systematically presents the major disasters occurred along this highway. There are a total of 20 disaster sites having been occurred, which includes the natural disasters, I.e., debris flow and slope sliding and the associated disasters including damage of roadway, breakdown of bridges and loss of human lives as well. The causes of these disasters, including the characteristics of the Herb typhoon, geomorphological features and geological settings are then discussed in this paper.
Widespread damage in slopeland areas along the New Central Cross-Island Highway resulting from Typhoon Herb in 1996 focused attention on risks to life and property from geologic hazards, including surficial slope failures, debris flows, floods and stream erosion. Both natural and artificial slopes underwent damaging surficial failures. However, foothill areas where alluvial fans had developed were covered by new debris flow deposits again.
Because of the rugged topography and limited time, SPOT images were employed for interpretation and analysis of the geologic hazards and their causes. It was found that steep slope, highly developed gullies, highly decomposed rocks, replacement of deep-rooted vegetation by shallow rooted one, and construction of highway and service roads had caused the slopes particularly susceptible to failure. And of course, the heavy precipitation with an intensity of greater than 100 mm/hr lasting for 8 hours had triggered the severe slope failures.
New Central Cross Island Highway is a major traffic line connecting the southern and central parts of Taiwan through mountainous areas. Due to the complex geological structure and uneven rain fall, rock falls and landslides are abundant along the highway.
The investigation results indicated that the slope failures along the highway were mainly caused by five factors: geology, groundwater seepage, runoff erosion, weathering of rock strata, and removing support at the toe of dip slopes during the highway construction. The investigation conducted by using GPS and GIS (ARC/INFO) concludes that there are 177 slope failures along the New Central Cross-Island Highway. Along Highway 18, slope failures mostly occurred at south-facing slopes with steepness of 30-70° and 1000-2000m in altitude, while slope failures occurred along Highway 21 are usually found on north-facing slopes with steepness of 30-70° and 500-1000m in elevation.
Typhoon Herb caused serious disasters in the areas along the New Central Cross-Island Highway. The disasters include block of highway, bridges, farming losses and a toll of human lives. The natural disasters including debris flow and land sliding may not be avoidable due to the heavy rainfall. However, some other disasters including damages of roads, bridges may not be necessary. The article aims at reviewing the engineering deficiencies exposed by the Herb Typhoon. The deficiencies include inadequate layout of highway, excavated roadway, bridge design and drainage pipes of the road way. Better designs which are able to accommodate the poor geological settings and weather conditions in the study area are then accordingly proposed.
A recent typhoon brought heavy damage to the highways in the mountainous area of Taiwan. This paper summarizes the previous research about the correlation between rainfall and landslide. Some simple classification method for the slopes would improve the prediction work for landslide occurrence by rainfall data. Probability concept will be also a choice for future prediction work.
Typhoon Herb attacked Taiwan and brought several disasters all around the island. The disasters occurred in hilly areas, especially the Ali-Shan highway, including slope failure, landslide and debris flow, etc. In order to understand the mechanism and find out the influential factors of the slope failure, the low elevation area on Ali-Shan highway was selected to promote this investigation work. This investigation was strengthened by the in-situ measurenont data acquired from extensometer and piezometer, which had installed in the slope at 26K+800m of Ali-Shan highway. These data are not only adopted to investigate the actual movement of slope, but also used to assess their suitability for predicting the time of slope failure.
The results of investigation showed that most slope failures occurred in low elevation areas of Ali-Shan highway being small scale and induced by the failure or erosion of heavily weathered layer. The slope angle of failed natural slope is greater than 40 degrees；however, quite a lot of man-developed slope were failed with a slope angle from 30°to 34°. A slope failure happened at 26K+800m, just few minutes after a significant movement of slope surface occurred. Although the movement was catched by an extensometer which had been installed there, it seems that only the extensometer without other instruments can not predict failure of a slope and alarm the danger perfectly. Therefore, the data regarding precipitation and variation of water table have to be included if the accuracy of slope failure prediction needs to be elevated.