Natural disasters in Taiwan.

Taiwan is an island that lies on the western edge of the Pacific-rim earthquake belt, which is an extremely active tectonic region with seismic activities among the highest in the world. The boundary of the Eurasian and Philippine Sea Plates is near the island of Taiwan. Earthquake occurs frequently, and a recent catastrophic event, the Chi-Chi earthquake, is known to the world. Taiwan also is located in the subtropical climate region with high average temperature and precipitation. May and August are periods influenced by continuous seasonal rains that occasionally come with an extraordinary intensity. From summer to fall, Taiwan is affected by typhoons. Some extreme events would accumulate significant rainfalls within a relatively short period of time and result in disastrous consequences such as floods. Given the topography of Taiwan, which is covered by slopelands and high mountains over two third of the island, landslides and debris flows often were triggered after heavy rainfalls. The remaining quarter of the land is plains, but they have been densely inhabited and developed. Due to excess extraction of groundwater for fish farming, significant portion of the plains have suffered severe ground subsidence over the past decades. As a result, Taiwan has to cope with various hazardous situations caused by the Mother Nature. Most significant natural disasters in this island include earthquakes, floods, landslides, debris flows, and land subsidence. The government and people in Taiwan have long being aware of the disastrous consequence of these natural hazards, and continuous efforts have been placed on prevention and mitigation of these natural disasters. This paper discusses these natural disasters that confront the people in Taiwan. Studies and investigations carried out to understand the characteristics of these natural hazards are discussed, and development of programs and approaches in preventing and mitigating natural disasters in Taiwan are also addressed.

INTRODUCTION

Since the end of the 20th century, natural disasters started to take heavier tolls than ever on human lives and properties around the world owing to the unprecedented global climate variation and accelerating urbanization in mankind history. Among all kinds of natural disasters in the world, the earthquakes, followed by floods and fierce winds (typhoons, hurricanes, tornados, etc.), are the most catastrophic and have taken the most monetary loss in urban areas. The damages caused by earthquakes, floods, and fierce winds have totaled almost 90% of the monetary loss from all the natural disasters worldwide in the past decade (Guy Carpenter & Company, 2000). In addition, the death tolls of earthquakes ranked the highest and were the major part of human live loss in natural disasters in the world history. Due to the continuous increase of live and monetary losses from natural disasters, prevention and mitigation of natural disasters became an urgent issue for governments and people in areas with high natural hazards potentials.

Taiwan is located along the west rim of Pacific Ocean with Tropic of Cancer across the middle. It is at all times under the threats of earthquakes, typhoons, and torrential rains. In 1999, the Chi-Chi earthquake declared a death toll of 2,434, 12,029 wounded, and a total loss of about 11.4 billion US dollars. In 2001, the Toraji typhoon claimed 214 lives including the missing. Later in September of the same year, the flood caused by the extremely heavy precipitation (425mm/day, record high in Taipei City) brought by the Nari typhoon cast a severe hit on northern Taiwan with a monetary loss estimated to 5.7 billion US dollars (Chang, 2002). The monetary loss in the event of Chi-Chi earthquake alone was approximately 3% of the Taiwan GDP in 1999 and human life loss was approximately one-ten thousandth of the total Taiwan population.

Along with the earthquakes and typhoons, landslides and debris flows in Taiwan seem to have higher occurrence rates after the Chi-Chi earthquake (Chin et al., 2005). The loss of lives and properties from debris flows was the highest in 2001 with a death toll of 129. Damages from landslides and debris flows on landscapes, water resource systems, transportation and lifeline systems are much more often reported during heavy precipitation.

In light of the significant increase in damages caused by natural disasters island-wide and around the world, many studies initiated by government and private sectors have been carried out in various aspects for the prevention and mitigation of natural disasters in Taiwan. Some resolutions have been implemented. As the effort for prevention and mitigation of natural disasters continues, it is of supreme importance to understand the characteristics of these natural hazards and their interaction with local environment. The dynamic interaction between unpredictable natural events of earthquakes, typhoons, precipitation and changing local environment by continuous urbanization as well as civilization has significant impact on the outcomes of natural hazards.

NATURAL ENVIRONMENT OF TAIWAN

Tectonic formations, geology, and topography

The island of Taiwan is located approximately 150 km off the southeast coast of China mainland on the convergent boundary of the Eurasian and the Philippine Sea tectonic Plates. It is spindle-shaped, with the longitudinal axis extending roughly north-south for a length of 385 km and an east-west width of 143 km (Moh and Ou, 1979). The island is composed of geosynclinal deposition of Tertiary sediments of the most recent era (the Cenozoic) to a thickness of more than 10,000 meters on a metamorphic base formation (Figure 1). As the result of northwestward movement of the Philippine Sea Plate, the tectonic interactions between the Philippine Sea Plate and the Eurasian Plate are extremely complex in the vicinity of Taiwan. There has been apparent northward subduction of the Philippine Sea Plate beneath the Eurasian Plate and an eastward subduction of the Eurasian Plate underneath the Philippine Sea Plate (Figure 2). This active tectonic interaction and collision have produced the Central Mountain Range in Taiwan (Penglai Orogeny) and the prevailing structural pattern of rock formations on the island aligning in long narrow belts of elongated arcs with their convexities facing the west. All the major geological structures, including the faults and fold axes, align fairly well with this structure throughout the whole island and roughly parallel to the longitudinal axis of the island. The Penglai Orogeny has been continuously elevating the surface of Taiwan. In term of geological time, Taiwan is still a young and developing land (Lin and Chou, 1978).

[FIGURES 1-2 OMITTED]

The topography and geological condition of Taiwan are closely related to the above tectonic activity. The rock formations are often fractured. In the island of approximately 35,960 sq. km, there are more than 200 peaks of over 3000 m and occupy approximately 1.3% of the island surface (Table 1). The plains with elevations below 100 m take about 31.3% of the island surface and lie mostly on the western side of island, which leaves approximately two-third of the island covered by slopeland merging from mountains to plains (Figure 3). Considering the dimensions of Taiwan, the heights of mountains, and the area of slopeland, the slopes in slopeland area are relatively steep and pose high threats to development in these areas.

[FIGURE 3 OMITTED]

Central Mountain Range generally serves as the water divide of river system on the island of Taiwan. Conforming to the topography, the rivers are generally short with steep slopes and possess high scouring capacity in the upstream due to high gradients (Figure 4). Moreover, the flow rates are highly seasonal and fluctuated with the precipitation.

As the result of tectonic interaction and collision of the Eurasian and Philippine Sea Plates, the seismicity in Taiwan is among the highest in world (Figure 5). More than 200 earthquakes with tremors sensible to human (usually with Level 2 intensity and above) each year in Taiwan were recorded in the past century. In the last 15 years, more than 30 earthquakes of Ritcher's scale of 5.0 and above were recorded (Figure 6).

[FIGURES 5-6 OMITTED]

Climatic features

Located in western Pacific Ocean with Tropic of Cancer across the middle, Taiwan is warm and humid. The average annual precipitation in Taiwan over the past 30 years is around 2,400mm, which is almost 3 times of the world average annual precipitation, 800 mm. Approximately 80% of the precipitation is cumulated from May to October (Figure 7). With the topographical and geological conditions of Taiwan, approximately 71% of the precipitation becomes surface runoff, 24% evaporation, and 5% infiltration. Only about 15% of the precipitation can be retained in surface water bodies. The amount of fresh water stored in the island is around 20% of annual precipitation including 5% of infiltration into groundwater.

[FIGURE 7 OMITTED]

In the past, Taiwan was subjected to the attack of typhoons mostly from May to October at an average rate of 3.7 times a year with dominant routes in northwestward direction (Figure 8). Heavy precipitation is usually associated with typhoons visiting Taiwan and constitutes one of many major parts of precipitation in addition to monsoon in May and June.

As a result of global climate variation, Taiwan has also been experiencing a gradual climate change. The number of rainy days is decreasing with higher rainfall intensity (Figure 9) even though the annual precipitation remains fairly constant in the past few years.

[FIGURE 9 OMITTED]

NATURAL DISASTERS IN TAIWAN

Earthquakes

The tectonic setting and dynamics of the Eurasian and Philippine Sea Plates are the major triggering mechanism of seismic activities in vicinity of Taiwan. Although occurring often, most of the seismic activities caused very minimal damage on the island. However, the catastrophic ones from time to time rock the island and induce significant loss of lives and properties.

In the Chi-Chi earthquake, the total loss in Taiwan was roughly 11.4 billion US dollars including an asset loss of more than 8 billion dollars and indirect loss of approximately 3 billion dollars. Three areas, respectively the vicinity of Chelungpu fault, the densely populated Taipei City and County area, and Yuanlin area, were identified later in study with major building and infrastructure damages (Figure 10) in the earthquake. In the area within a 6 km distance from Chelungpu and Shuangtung Faults, the damages were caused primarily by high seismic forces (a recorded horizontal peak acceleration of 989 gals in east-west direction at Station TCU084, and a vertical peak acceleration of 716 gals at Station CHY080) and large relative displacement at surface (average fault dislocations of 1.5 m horizontally and 3 m vertically). Most of the structures near the fault were severely damaged even for buildings designed in compliance with modern design codes. The bridges in central Taiwan suffered severe damages from collapsed spans, cracked piers, and distressed components (Figure 11). The damages on dams, lifelines and critical facilities, and power systems were also reported. In Hsinchu Industrial Park along, an estimated loss of 400 million US dollars was reported and most of which was incurred in semiconductor and silicon wafer production facilities.

[FIGURES 10-11 OMITTED]

In Yuanlin Town and Changhwa County, soil liquefaction seriously affected an area of approximately 60 sq. km in the Chi Chi earthquake (Moh et al., 2002). Sand boils were found at locations at which the clay covers are either too thin or totally missing. At some locations where the clay covers are thick, the settlements of buildings up to 1 m or so as a result of ground subsidence were observed. The buildings in these areas suffered severe damages from soil liquefaction. Soil liquefaction also caused damages on levees and resulted in lateral spreading at several locations along Koniaokeng Creek near Chelungpu Fault. Moreover, over the reclaimed land of Taichung Port liquefaction, damaged 4 of its 45 docking wharves (Figure 12).

[FIGURE 12 OMITTED]

Located approximately 150 km north of the epicenter, Taipei area suffered damages more than anticipated in the Chi Chi earthquake. The result of study indicates that the geological condition with thick soft and poorly cemented young sediment deposits could have adverse site response effect in bowl shape base formation of Taipei basin and be rationally argued to contribute to this consequence (Tsai, 1987). The adverse site response effects in Taipei basin include amplification of earthquake magnitudes and extended period of maximum design spectral response acceleration (Chen and Chen, 1995). In 1986, two earthquakes with magnitudes of 6.2 and 6.8 and epicenters approximately 15 and 10 km east of Taiwan respectively have caused damages on buildings in Taipei predominantly of 10 to 16 stories (estimated predominant structural periods between 1.0 and 1.6 sec). In light of the above findings, the seismic design code revised in 1989 included the site response effect and extended the predominant period with revised design response spectra for Taipei area. In July 2005 the design response spectrum was further modified with higher design seismic force (Figure 13).

[FIGURE 13 OMITTED]

Landslides were also reported during the Chi Chi earthquake. Almost all of the slope failures occurred east of Chelungpu Fault on the hanging wall with shallow slips in residual soils of depths between 1 m to 5 m. At Tsao Lin, the debris from a massive landslide slammed into the valley of Ching Shui River, blocked the river, and formed a reservoir behind (Moh et al., 2002).

Landslides and debris flows

Landslides and debris flows are two major natural disasters that threaten the people and environment in Taiwan, and the later has caused greater disasters in this island over the past few years. The geological settings, topographical features, and climatic conditions of Taiwan provide the inherent factors for potential cause of these disasters. In addition, the population congregation, excessive land development, and rapid economy growth over past decades contributed more potential adverse effects to the environmental condition. The precipitation statistics of Taiwan indicate the tendency of decreasing total raining days per year but increasing precipitation intensity per rainy day. The change in raining pattern suggests that the intensity of a raining event is becoming higher, which can be easily related to the increased possibility of landslide and debris flow hazards.

In 1999, the Chi-Chi earthquake triggered over 20,000 landslides island-wide (Chin et. al., 2005). The deposited landslide masses of critically stable conditions became the source materials for debris flows during or after intensive precipitation. As a result, approximately 250 hazardous debris flow events have been reported during or after the earthquake. This hazardous threat of landslide and debris flow is aggravated with a higher occurrence rate and greater intensity after the Chi-Chi earthquake, and debris flows were easily triggered than ever during an event of heavy rain or typhoon (Table 2).

Typhoons and floods

Taiwan is located in the area with primary passages of typhoons originated in northwest Pacific Ocean. The average number of visitation of typhoons to the island is approximately 3.7 per year, which is roughly 15% of the typhoons originated in the region of northwest Pacific Ocean and South China Sea. When visiting Taiwan (Figure 8), the typhoons with tracks through either northern or southern Taiwan (Tracks 1, 2, 4 and 9) tend to pose higher threats to the island.

The damages by typhoons in Taiwan are primarily incurred by the strong winds, storm surges in the coastal area, and heavy precipitation. In addition to strong winds, the foehn and salty winds associated with typhoons have caused significant damages on agricultural produces and plants. The ground subsidence induced by excessive groundwater extraction has worsened the harm of storm surges to coastal areas, and the drainage of areas with seawater encroachment takes longer to complete. The precipitation further can easily trigger the landslides and debris flows. Moreover, the concentrated precipitation can cause extremely high flow rates in water channels and serious scouring problems on riverbeds and bridge foundations.

Nevertheless, the floods generated by heavy precipitation associated with typhoons have been the predominant cause of damage in Taiwan. In 2000 and 2001, the Xangsane, Troaji, and Nari typhoons caused severe damages on agriculture and transportation systems, and triggered landslides and debris flows island-wide. The flood in Taipei caused by the Nari typhoon severely damaged the MRT system at Taipei Station and it took several months for restoration to complete.

Ground subsidence

Ground subsidence can be triggered by many factors such as seismic activities, volcano eruptions, dissolution of limestone, etc. and exploitation of underground resources (groundwater, oil, coal, etc.) as well as underground space excavation. In Taiwan, the primary reason for regional ground subsidence is excessive exploitation of groundwater. Until now, more than 11% of the land in Taiwan has subsided, which was increased from 7.4% in 2001 and mostly along west coast of the island (Figure 14).

[FIGURE 14 OMITTED]

In 1960s, the ground subsidence in Taipei City was recognized as a result of excessive usage of groundwater for industry and domestic consumption. A surface settlement of more than 2 m was recorded near the center of city (Figure 15). Regulatory restriction on groundwater pumping activity was later reinforced within the city limit and had successfully stopped the subsidence in 1980s. With the gradually elevated groundwater levels, rebound of ground surface was observed from the elastic portion of soils due to decreased effective overburden stresses (Figure 15).

[FIGURE 15 OMITTED]

However, continuous subsidence with extending territory still occurs along the southwest coast of Taiwan, where the land has long been used for agricultural purposes and fishery farms. Due to limited precipitation, relatively level topography, predominately granular alluvial formation, and unstable flow rates of rivers in the western plains of Taiwan, the surface water resources are rare and insufficient to provide steady supply of fresh water for consumption. Groundwater became the only economical and steady water supply in this area. Extensive pumping of groundwater for usage in fishery farm and agricultural purposes further aggravates ground subsidence and causes the intrusion of salt water into the groundwater systems.

Ground subsidence over the western plains of the island leads to irreversible consequences of infrastructure settlements and regional drainage difficulties in addition to the loss of land. Floods in these areas have occurred more often and the retention times become longer than ever.

Water deficiency

The average annual precipitation of Taiwan is roughly 2,400 mm/year, approximately three times of the world average. However, with concentrated precipitation over the period from May to October and with regional discrepancy, topography of steep slopes, and rivers of limited lengths, only one-sixth of the precipitation was able to be retained as surface water for consumption in addition to 5% of infiltration into groundwater. The surface water features and geological as well as topographical conditions in the island provide very limited locations for construction of water storage facilities. Retention of fresh water has long been a challenging task in Taiwan. In the densely populated western plains of Taiwan, sufficient and steady supply of fresh water for agricultural, industrial, and domestic consumptions has been sustained by continuous exploitation of groundwater, which is the main cause of ground subsidence in this area.

After the Chi-Chi earthquake, significantly increased events of landslides and debris flows, mostly in areas of watersheds, have stripped, to certain degrees, the slopeland and damaged hydraulic facilities. Surface erosion during heavy precipitation is aggravated on non-vegetated and steep slopes, which leads to large quantities of sediments in reservoirs, and generates high turbidity in surface water and causes much difficulty in water treatment processes.

In the past, the precipitation incurred from visitation of typhoons has been a major replenishing source of water storage in the island. However, in recent years, the high turbidity of surface water after typhoons significantly reduced the treatment capacity of water treatment plants and created water supply shortage in certain areas.

NATURAL DISASTERS MITIGATION

Disaster warning and monitoring systems

The issues regarding natural hazards started to receive more attention in Taiwan since the disastrous events of the Chi Chi earthquake in 1999 and the floods in 2000 and 2001. The issues regarding natural hazard forecast, prevention, and mitigation have been intensively discussed and studied by government agencies and private sectors. The government has taken initiatives to integrate loosely kept information in various different government agencies and to enhance capabilities in tasks of hazard warning and monitoring.

The Central Weather Bureau (CWB), Ministry of Transportation and Communications, is the primary government agency responsible for collecting and interpretation of climatic and seismic information in Taiwan. As part of the island-wide climatic monitoring system, it implements a total of 362 automated rain gauges island-wide (Figure 16) to collect real-time precipitation data, which is now incorporated with geographical information system (GIS) to provide early warnings on floods and debris flows. The CWB also operates one of the most advanced systems in the world on seismic monitoring in Taiwan area. The system includes 75 short period seismometers, 680 strong motion seismometers, 12 broadband seismometer stations, and 68 strong motion seismometers in major cities on buildings and bridges (Figure 17). With this system, the CWB made crucial information of the Chi Chi earthquake available within 102 seconds from the moment when earthquake occurred. The information included location of hypocenter, magnitude, and island-wide intensities.

[FIGURES 16-17 OMITTED]

Also in the Chi Chi earthquake period, the groundwater monitoring system (Figure 18) implemented and maintained by the Water Resources Agency (WRA), Ministry of Economic Affairs, supplemented invaluable information on groundwater variation in elevations and quality before, during and after earthquakes. The system, consisting of 492 monitoring wells mostly in the western plains of Taiwan, is set up to provide continuous monitoring data on groundwater levels and quality, and additional reference for ground subsidence in these areas.

[FIGURE 18 OMITTED]

Risk management and mitigation strategy formulation

To enhance the disaster response capability as part of the disaster mitigation resolution, the government enacted a Disaster Prevention and Response Act (DPRA) in 2000 (Bylaws in 2001) soon after the Chi Chi earthquake. The National Disasters Prevention and Protection Commission of the Executive Yuan was also established to cope with the natural disaster related issues in the central government. DPRA is basically structured on four stages of disaster management, i.e., mitigation, preparedness, response, and recovery. With respect to different types of natural disasters, different government agencies are in charge of the matters regarding various stages of disaster management according to DPRA (Table 3).

The Taipei City is the most populated area in Taiwan and had suffered serious damages in various events of natural disasters. In response to the urgent need for disaster mitigation in the city, the Taipei Disaster Prevention and Rescue Committee was established under the city government to develop and promote hazard mitigation work on earthquakes, typhoons and inundations, and landslides as well as debris flows (Figure 19).

[FIGURE 19 OMITTED]

The earthquake risk assessment model using HAZ-Taiwan and TELES (Taiwan Earthquake Loss Estimation System) based upon the framework of HAZUS (developed by the Federal Emergency Management Agency, FEMA, U.S.A.) is adopted by the Taipei City government to provide necessary information in forming the earthquake mitigation plans. Integrated with geological GIS information, demographic data, structure and building information, lifelines and critical facilities in the city, the assessment model can provide estimation on direct physical damages due to earthquakes and associated social losses (Figure 20). The city emergency preparedness and response plans are then developed in combination with transportation networks. With the city geotechnical boring data and geological GIS information, liquefaction potential maps can also be developed for reference of earthquake hazard and land use as part of mitigation plans in the city.

[FIGURE 20 OMITTED]

The Taipei City is located in the center of Taipei basin and surrounded by Tanshuei River, Keelung River and Hsintein Creek. The city is protected by levees and pumping facilities along these rivers against floods during heavy precipitations. The flood mitigation plan of Taipei City is primarily constituted on the basis of the inundation potential maps. These maps are constructed in consideration of precipitation with various intensity and elapsed times, land development conditions, topography, drainage capacity from the sewer system and pumping facilities. Different inundation hazard scenarios are formulated with different return periods of rainfall intensity and elapsed times and presumed drainage capacity with fully operational or partly failed pumping facilities. The inundation potential maps with inundated areas and depths of flood can be generated for mitigation plans and decision making process of evacuation (Figure 21). With the transportation GIS information, the routes of evacuation and shelter information can also be supplemented.

[FIGURE 21 OMITTED]

To form the landslide/debris flow mitigation plan, the Taipei City government built a comprehensive geological and natural environment database for the slopes in the area. The database includes 49 areas of potential debris flow rivers, situations of the existing slope management by the administration, documentation of historical landslides, the relationship between precipitation and landslide potential, and geological, hydrological, as well as geomorphic information of potential landslide areas. In addition to the rain gauges installed by the CWB, many other rain gauges were established by the Taipei City government to collect real-time precipitation data as a crucial index for issuing early warning of debris flows. In addition to the warning issued during precipitation in areas with high risk of landslides and debris flows, long-term resolutions of slope management and relocation of villages along the potential paths of debris flows were implemented as part of the city mitigation plans.

Latest technical development

The CWB is conducting the Climate Variation and Severe Weather Monitoring and Forecasting Development Project, which is a multi-year project to establish monitoring and warning capabilities for climatic disaster prevention, to develop techniques of short-term climate analysis and prediction, and to strengthen the capabilities of very short-term severe weather analysis and forecasting. The capability of the CWB in typhoon monitoring and prediction, precipitation monitoring and forecasting, and marine climate forecast can be significantly improved. With the Third-Phase Strong-Motion Observing Project to develop an earthquake real-time warning system, the CWB will be able to reduce the earthquake rapid reporting time from the currently 60 seconds to 30 seconds by 2009 and even provide an early warning of major earthquakes in matter of seconds to organizations and facilities in pre-established system.

[FIGURE 22 OMITTED]

After the Chi Chi earthquake, the building seismic design codes in Taiwan were revised immediately in 2000 according to recorded seismometer data and field observations. Two seismic zones were adopted for seismic design of buildings and infrastructure in Taiwan (Figure 23). However, based upon further analysis of seismic data and new findings in earthquake, the latest revision of building seismic design codes was issued in July 2005. The major changes include larger seismic design loads in most areas, additional requirement on structure ductility under maximum considered earthquake with return period of 2,500 years, supplement of near fault effects and special consideration on site response characteristics of soft deposits, addition of provisions on seismic retrofit, isolation, and damper design guidance.

[FIGURE 23 OMITTED]

Modeling techniques of debris flows is also advanced in predicting the flow paths. A numerical solution using rheological model with two-layer system consisting of a plug region and the bottom boundary layer has provided better simulation on debris flow propagation (Liu and Huang, 2005). This advancement enables a physics based model to estimate the extent of influence of a given debris flow event. The result can be implemented to provide useful information for slopeland management and necessary information for urgent evacuation in debris flow areas.

[FIGURE 24 OMITTED]

CLOSING REMARKS

Typhoons, earthquakes, and precipitations are natural events occurring in Taiwan all the time. It is, however, the interaction between those natural events and the natural as well as man-made environments that determines the outcomes and consequences of these events. Even though the occurrences of these natural events are inevitable, the extent of preparedness beforehand and action taken afterwards certainly decide the cost incurred at each event. Taiwan has encountered many different types of natural hazards. The unique natural environment brings out the complexity of natural hazards. Systematic resolutions in many aspects to mitigate the disasters caused by natural events are needed. Even though the natural hazards may evolve with changing environments, knowledge of fundamental issues can always be exercised to provide valuable insights to the trends and resolutions to these disastrous events.

ACKNOWLEDGEMENT

The writers wish to acknowledge the assistance and advices provided by their colleagues in the MAA Group, particularly Dr. Chung-Tien Chin, Dr. Richard N. Huang, Dr. Jie-Ru Chen, Mr. Chi-Hung Chao, and Mr. Cheng-Chen Wu.

REFERENCES

Guy Carpenter & Company (2000). "Natural Hazard Review of the Year 1999", Marsh and McLennan Companies, New York, New York, U.S.A.

Lin, C.H. and Chou, R.D. (1978). "Taiwan Geology", 1st edition, Maw Chang Book Co., Taipei, Taiwan.

Liu, K.F. and Huang, M.T. (2005). "Debris flow disaster mapping using numerical simulation", Proc. International Conference on Environmental Fluid Mechanics, Guwahati, Assam, India. (in printing)

Chang, C. C. (2002). "On the establishment of risk mitigation system for natural disasters and risk management", Proc. Conference on Weather Analysis and Forecasting, Taipei, Taiwan, 13-18.

Chen, C.H. and Chen, L.Y. (1995) "Ground period of Taipei Basin", Earthquake Geotechnical Engineering Proceeding of IS-Tokyo, Tokyo, 543-548.

Chin, C.T., Liu, K.F., and Chen, J.R. (2005). "Landslide and debris flow in Taiwan", Proc. 3th International Conference on Geotechnical Engineering combined with 9th Yearly Meeting of the Indonesian Society for Geotechnical Engineering, Semarang, Indonesia, 59-68.

Lee, C.T. (1991). "Earthquakes and disaster prevention", Website: http://140.115.123.30/earth/edu98/edu98.htm. (in Chinese)

Moh, Z.C. and Ou, C.D. (1979). "Building foundation practice in Taiwan", Proc. 6th Asian Regional Conference on Soil Mechanics and Foundation Engineering, Singapore, 205-209.

Moh, Z.C., Huang, R.N., Ueng, T.S., and Lin, M.L. (2002). "921 Chi Chi earthquake of Taiwan", Proc. 17th Australasian Conf. on the Mechanics of Structures and Materials, Gold Coast, Australia.

Ou, C.D. (2004). "Sustainable development and natural Hazard mitigation in Taipei City", Proc. The 3rd Civil Engineering Conference in Asian Region, Special Forum 2, Seoul, Korea, 51-60.

Tsai, I.C. (1987). "Review on normalized design coefficient, C, of Building Seismic Design Code from the strong motion seismic data at Taipei from earthquake on November 15, 1986", Journal of Structural Engineering, Vol. 2, No. 4, 13-20. (in Chinese).

ZA-CHIEH MOH

MAA Group Consulting Engineers,11th Floor, No. 3, Tunhwa South Road, Section 1 Taipei 105, Taiwan

DANIEL T. C. YAO

MAA Group Consulting Engineers,11th Floor, No. 3, Tunhwa South Road, Section 1 Taipei 105, Taiwan

Table 1. Topographical distribution of Taiwan (from Lin and Chou,
1978).

Elevations
(m)          0/100   100/500  500/1000   1000/2000   2000/3000   3000+
(From/To)

Area (%)      31.3     23.5       13.7        19.7        10.5     1.3

Figure 4. Rivers of Taiwan.

                      Elevation     River    Average
River Name                  (m)    Length      Slope
                    (origin-end)     (km)        (%)

Danshuei River           3529-0      158.7       0.82
Fongshan River           1320-0       45.5       0.44
Toucian River            1913-0       63.0       0.53
Jhonggang River          2616-0       54.0       0.67
Houlong River            2616-0       58.3       0.63
Da-an River              3488-0       95.8       1.33
Dajia River              3740-0      124.2       1.67
Wu River                 2596-0      119.1       1.09
Jhuoshuei River          3220-0      186.6       0.53
Beigang River             516-0       82.0       1.69
Puzih River              1421-0       75.9       1.89
Bajhang River            1940-0       80.9       2.38
Jishuei River             550-0       65.0       0.85
Zengwun River            2609-0      138.5       0.50
Yanshuei River            350-0       41.3       0.34
Eren River                460-0       61.2       0.13
Agongdian  River          300-0       38.0       0.14
Gaoping River            3330-0      171.0       0.67
Donggang River           1166-0       44.0       0.20
Sihchong  River          1062-0       31.9       1.69
Lanyang  River           3536-0       73.0       1.82
Heping  River            3536-0       48.2       2.70
Hualien River            1200-0       57.3       0.35
Siouguluan River         2360-0       81.2       2.94
Beinan River             3293-0       84.4       0.61

Figure 8. Major tracks and statistics of typhoons visiting Taiwan.

Track      Visits     %

1           50       13.4
2           45       12.3
3           40       10.7
4           35        9.3
5           76       20.3
6           53       14.2
7           30          8
8           16        4.3
9           28        7.5

Total      374        100

From 1897 to 1996

Table 2. Losses from recent debris flow disasters in
Taiwan (from Chin et al., 2005)

                                              People
             Typhoon         Location        Missing
Time          Event     (Township/County)    and Dead   Property Loss

6/23/1990     OFELIA    Sioulin / Hualien       35      35 houses
                                                        damaged

7/31/1996      HERB       Sinyi / Nantou        2       21 houses and
                                                        roadways
                                                        damaged

7/31/1996      HERB      Shueili / Nantou       8       17 houses
                                                        damaged

10/29/2000   XANGSANE   Rueifang / Taipei       8       20+ houses,
                                                        roadways,
                                                        and bridges
                                                        damaged

7/28/2001     TORAJI    Fonglin / Hualien       6       3 houses and
                                                        a wastewater
                                                        treatment
                                                        plant damaged

7/28/2001     TORAJI    Guangfu / Hualien       43      150+ houses
                                                        damaged

7/28/2001     TORAJI      Sinyi / Nantou        16      67 houses,
                                                        roadways, and
                                                        bridges damaged

7/28/2001     TORAJI     Shueili / Nantou       1       21 houses and
                                                        roadways
                                                        damaged

7/28/2001     TORAJI     Shueili / Nantou       17      55 houses
                                                        damaged and
                                                        all public
                                                        infrastructures
                                                        fully destroyed

7/28/2001     TORAJI      Sinyi / Nantou        46      20+ houses,
                                                        roadways,
                                                        and bridges
                                                        damaged

6/30/2004    MINDULLE   Heping / Taichung       3       30 houses and
                                                        roadways
                                                        damaged

6/30/2004    MINDULLE   Heping / Taichung       1       40 houses
                            Dongshih /                  damaged

6/30/2004    MINDULLE                           1       Roadways
                             Taichung                   damaged

6/30/2004    MINDULLE     Puli / Nantou         2       17 houses
                                                        damaged

6/30/2004    MINDULLE     Hezuo / Nantou        1       6 houses
                                                        damaged

8/23/2004      AERE      Wufong / Hsinchu       28      20+ houses
                                                        damaged

9/11/2004      AERE     Jianshih / Hsinchu      4       1 house
                                                        damaged

Table 3. Administrative agencies for natural disasters
in Taiwan (DPRA).

Agency                            Types of Natural Disasters

Ministry of the Interior          earthquakes and strong winds

Ministry of Economic Affairs      floods and droughts

Council of Agriculture,           landslides and debris flows
Executive Yuan