Debris flow disaster mitigation on early warning and evacuation after the Chichi Earthquake in Taiwan

The Australasian Journal of Disaster
and Trauma Studies
ISSN:  1174-4707
Volume : 2010-2

Debris flow disaster mitigation
on early warning and evacuation
after the Chichi Earthquake in Taiwan

Huei-Long Wu, Honorary President, Chinese Soil and Water Conservation Society, Taipei 100, Taiwan, R. O. C.
Su-Chin Chen, Professor, Department of Soil and Water Conservation, National Chung-Hsing University, Taichung 402, Taiwan, R.O.C. Email: scchen@nchu.edu.tw
Tien-Ying Chou, Professor, Department of Land Management, Feng Chia University, Taichung 407, Taiwan, R.O.C.
Bo-Tsung Huang, Assistant Engineer, Taitung Branch, Soil and Water Conservation Bureau, Taitung 95055, Taiwan, R.O.C.

Huei-Long Wu Honorary President Chinese Soil and Water Conservation Society Taipei 100 Taiwan R. O. C.

Su-Chin Chen Professor Department of Soil and Water Conservation National Chung-Hsing University Taichung 402 Taiwan R. O. C.

Tien-Ying Chou Professor Department of Land Management Feng Chia University Taichung 407 Taiwan R. O. C.

Bo-Tsung Huang Assistant Engineer Taitung Branch Soil and Water Conservation Bureau Taitung 95055 Taiwan R. O. C.

Abstract

Since the Chichi Earthquake (M L=7.3) of 1999, the frequency of sediment-related disasters, such as landslides and debris flows, in Taiwan have increased dramatically. Because structure regulations cannot be fully implemented promptly, the government has initiated non-structure disaster mitigation actions. The debris flow evacuation drills in communities were promoted in 2000, advocating safety and security through hazard mitigation. Typhoon Toraji of 2001 caused 181 casualties and many debris flows, which indicated that the evacuation before the occurrence of a debris flow is closely related to disaster prevention. Thus, the Taiwanese government has expanded debris flow evacuation drills. In addition, the foundation of the early warning system developed after the Chichi Earthquake promotes Taiwan’s performance on debris flow prevention projects. The rainfall threshold for debris flow warnings of different areas are revised according to the onsite observation data collected from debris flow monitoring stations. Real-time information is gradually integrated to construct a debris flow disaster response system, which announces debris flow hazard-prone zones. The two-level-warning signs of evacuation mechanisms, red and yellow, as presented in 2005 with the execution of mandatory evacuation, are able to evacuate the public before disasters. The number of casualties in communities equipped with the evacuation system has decreased among the six debris flow disasters in the four presented communities in this paper. Overall, the decreased casualties from debris flow disasters during decade after the Chichi Earthquake is not due to a decrease of sediment related disasters, but the gradually improved early warning and evacuation systems.

Debris flow disaster mitigation
on early warning and evacuation
after the Chichi Earthquake in Taiwan

Introduction

Debris flows are mainly caused by steep riverbed, abundant sediment, and heavy rainfall. Taiwan is located at the interface between the Philippine Sea Plate and the Eurasia Plate. In addition, it is in the pacific-rim seismic zone, has young topography, and fragile geology, with almost 1,000 sensible shocks annually (Central Weather Bureau (CWB), 2008). Thus, bare lands and landslides are common in Taiwan. Mountainous areas, steep hills and torrents slopes account for 72% of the total area of Taiwan (Soil and Water Conservation Bureau (SWCB), 2004a), with a mean annual rainfall of 2,515 mm. Obviously, Taiwan has natural conditions for the occurrences of debris flow hazards and their probability is relatively high. On September 21, 1999, the Chichi Earthquake, measuring 7.3 on the Richter scale, struck central Taiwan and was followed by 3,228 aftershocks (CWB, 2008). Such strong shocks aggravated mountain slope surfaces in affected areas. These regions are prone to slope disasters, and face a greater probability of disaster.

Debris flow evacuation

People in debris flow disaster prone areas lacked hazard awareness before the promotion of debris flow evacuation route planning and practice drills. There are few records of debris flow disasters in Taiwan before the Chichi Earthquake of 1999. Yet, due to the lack of preventive measure s, severe casualties may occur. For example, Typhoon Herb of 1996 caused many debris flow disasters in central Taiwan, and 106 casualties. After the Chichi Earthquake, as the number of debris-flow- hazard-prone areas increased sharply, evacuation drills have been carried out in those areas since 2000. The content of evacuation drills includes planning for evacuation routes and shelters, community evacuation maps, and scenarios of operational procedures for residential evacuation in disasters. Relief organizations learn from practice drills resulting from disaster warnings through their role in instructing evacuation and organizing disaster preparedness, response and measures before and after disasters.

Typhoon Toraji of 2001 caused debris flows in many locations and 181 casualties. Of all those affected areas, 12 communities had carried out drills shortly before the disaster attacked the island. Community residents from Shenmu and Jyunkeng took evacuation drills seriously. When the disaster attacked their communities, they followed the evacuation instructions and moved away from dangerous areas beforehand. As a result, they only lost one person to that particular disaster. Unfortunately, other affected communities suffered relatively severe casualties. Such results stress the importance of the increased awareness of debris flow disaster and evacuation drills to residents of disaster prone areas. Subsequently, a number of communities perform debris flow evacuation drills have increased annually since 2002. A total of 453 drills were carried out by 2008 (Fig.1). In 2004, based on the original evacuation drills and with the concept of community-based disaster management, "disaster resistant communities" of community-based hazard mitigation planning by residents were successively promoted. When the hazard perception of residents is developed, residents can participate in the research on community disaster environment, planning evacuation routes, evacuation shelters, discussing mitigation strategies, and reach a target for self-aid and mutual assistance in disasters, and quick recovery after disasters (SWCB, 2007).

Fig. 1 Statistics of debris flow evacuation planning meetings (SWCB, 2008a)

Early warning system

1. The phase of monitoring station system (before 2003)

In the past, Taiwan had no warning system for debris flow disasters caused by typhoons. Even when the concept of evacuation was gradually promoted in 2000, the government had no debris flow warning system, and residents had to judge the time for evacuation themselves. The first stage involved developing debris flow monitoring stations. These were successively constructed, beginning in 1996, and real-time observation data are transmitted to the Debris Flow Disaster Emergency Action Team of the SWCB through satellite communications, and 13 monitoring stations continue to work at the present (Fig .2a,b).

However, the torrents that are likely to cause debris flows in Taiwan are mostly 500~ 2000 m long. Consequently, when a debris flow occurs, the time for residents' response after the warning from the monitoring station's instrument is less than 10 minutes (Chen, 2002). Therefore, the response time for early warning instruments of field monitoring is too short, and systematic evacuations cannot be carried out (Chen, 2002). In addition, not all debris flow torrents can be included in the current monitoring stations due to limited funds. Therefore, the main task of monitoring stations is not the release of real-time warnings, but gathering field observation data of debris flows in Taiwan, which would enable knowledge of the movements and occurrence mechanisms of debris flows. Moreover, in order to improve the monitoring mobility of debris flows, two mobile debris flow monitoring stations are developed (Fig.2[c]), which can be sent to debris flow torrents that are in the predicted route of a typhoon.

2. The phase of early warning system at initial stage (2003~2005)

To predict potential debris flow disasters during periods of heavy rainfall in typhoons, the Debris Flow Disaster Emergency Action Team (DFDEAT) was established to provide comprehensive planning of data analyses and warning releases. In addition, the Debris Flow Disaster Response System, as officially implemented in 2002, integrates all real-time observation data in an online mode, including typhoon information, rainfall accumulation, live video, and information of debris flows, and can be synchronously used by the DFDEAT as the single window for the release of all debris flow warnings during heavy rainfall of typhoon (SWCB, 2008b).

The debris flow warning is released according to the rainfall threshold value for debris flow warning of different zones. At the initial stage, it is determined one by one, using the concept of cumulative probability and analyses of the effectives of rainfall accumulation after each rainfall event according to the geologic features, hydrological conditions, and historical data of debris flows of different zones. The warning value is between 200 ~ 550 mm, and is divided into eight levels (Fig.3[a]). After the Chichi Earthquake, the rainfall threshold value for warnings in the primary influence areas is set at a low standard, since loose soil materials are likely to cause debris flows (Fig.3[b]). The real-time observation data from 153 automatic rainfall stations of CWB were synchronously obtained online, beginning in 2002, and integrated into the Debris Flow Disaster Response System.

Once the rainfall condition of an area reaches the threshold value for a warning, the DFDEAT will broadcast a warning message to the relevant areas by fax and text message through the Debris Flow Disaster Response System (SWCB, 2008b). At this stage, the release of a debris flow warning can only advise residents of evacuation. However, the residents decide whether to evacuate, and there is no executive force of mandatory evacuation.

3. Early warning system time at near stage (2005~2008)

Since 2005, the release of debris flow warnings took the community as a unit. However, some areas have no rainfall station, thus, residents of communities nearby debris flow torrents have been recruited to form local professional volunteers in order to obtain local real-time rainfall data. The professional volunteers help with observations of local real-time rainfall, using a simple rain gauge and communication equipment. In addition to complementing rainfall data in areas lacking rainfall stations, they take part in announcing situations of disaster and evacuating residents. The Quantitative Precipitation Estimation and Segregation Using Multiple Sensors (QPESUMS) was bought in 2006 (Fig.4) for calculating and analyzing the predicted rainfall within next 1~3 hours in order to provide information of future rainfall trends for the response team, and strengthen the accuracy of debris flow warning analysis (SWCB, 2008b).

Fig.4 The estimate method of QPESUMS system, (SWCB, 2008b)

The mode of the debris flow warning system in Taiwan was changed into two levels in 2005, which are classified as the alarm levels of yellow and red (Fig.5). When the predicted rainfall is greater than the threshold value for debris flow warning, a yellow sign is released to the possible-affected community, and the local government should advise residents of evacuation. When the actual rainfall accumulation reaches the threshold value for a debris flow warning, it will be a red sign, and the local government must enforce public evacuation. Therefore, rapid notification is required to inform the public of evacuation. A red sign announcement or when the rainfall accumulation observed by the community reaches the debris flow warning value also mobilizes local police and rescue units for assistance. Those who refuse to evacuate will be forced to evacuate to safe shelters. As the procurement of rainfall information and warning release methods have been improved since 2005, the early warning system for debris flows of Taiwan enters into a relatively perfected stage. The process can be illustrated in the following case study.

Fig. 5 Current debris flow warning modes in Taiwan

Case Study

1. Shenmu Community

Shenmu Community is located in Shinyi Township, Nantou County; it has 969 residents in 338 households, as well as 13 debris flow torrents. In 1996, Typhoon Herb caused many debris flow disasters, damaging several houses and caused the roads to crumble. Many debris flows occurred in May 1998, and May and July 1999, which destroyed bridges and besieged residents. Shenmu Community carried out a debris flow evacuation drill in June 2001 in order that residents and related units were familiar with the evacuation process, and thus, it became one of the rare debris flow resistant communities to experience evacuation drills in Taiwan. Figure 6 is the map for evacuation that made by residents and experts. Typhoon Toraji of 2001 and Typhoon Mindulle of 2004 caused heavy casualties all over Taiwan; however, there were no deaths in Shenmu Community, even though there were numerous debris flow torrents (Fig.7), because the residents were evacuated in advance.

Fig.6 The map for evacuation of debris flow of Shenmu community (Chen, et al., 2003)

There was no rainfall threshold for debris flow and evacuation mechanisms in the early years, thus, community leaders would decide the actions. During Typhoon Toraji of 2001, houses, roads, and bridges in five locations were destroyed by debris flows in Shenmu Community. However, there were no casualties due to the residents evacuating before the debris flow (Fig.8). As for Mujiliao Community, six persons were buried in their homes by the debris flow because they refused to evacuate, the evacuation of residents of Shenmu Community reduced possible loss of life.

Typhoon Mindulle of 2004 caused debris flow disasters in several locations, and damaged houses and bridges. However, there were no casualties due to the debris flow warning, allowing residents to be evacuated beforehand (Fig.9).

2. Songhe Community

Songhe Communityis located in Heping Township, Taichung County, it has two debris flow torrents, 848 residents in 394 households. The communitycarried out debris flow evacuation drills in 2000 and 2003. Typhoon Mindulle that severely hit Songhe Community on July 1, 2004 caused debris flow disasters (Fig.10) .

The hourly rainfall in Songhe Community from July 1 to 5, 2004 is shown in Fig.11, local residents took the initiative and began to evacuate in the early morning of July 2, and all residents were in evacuation shelters by 11:00 pm. The debris flow caused two casualties who returned to the dangerous areas. The debris flow disaster damaged 30 houses in Songhe Community. However, the proper evacuation measure minimized the casualties.

3. Minduyou Community

Minduyou Community is located in Wufong Township, Hsinchu County, where severely attacked by Typhoon Aere on August 25, 2004. The catastrophe caused debris flow disasters (Fig.12), and buried two houses and caused six casualties. This community had not implemented the debris flow evacuation drill at that time, the residents suffered severe casualties without warning and evacuation.

The hourly rainfall in Minduyou Community from August 23 to 25, 2004 is shown in Fig.13. The threshold value was estimated as reached at 7: 00 a.m. on August 24, according to the rainfall threshold value for debris flow warning. However, the actual time of the occurrence of debris flow was at 9: 55 a.m. on August 25. The timing was enough for residents to evacuate; however, the residents did not completely evacuate on that day. Based on the case of Minduyou Community, the government reviewed whether previous promotions of community evacuation planning and drills led by the government and followed by residents could effectively increase hazard awareness of residents in disaster prone areas. Therefore, the emergency planning for disaster resistant community operations that was led by residents and assisted by the government started in 2004 (as Fig.1).

4. Jyunkeng Community

Jyunkeng Community is located in Shuili Township, Nantou County, where has one debris flow torrent and about 113 residents. On July 31, 1996, Typhoon Herb caused debris flows, killed eight people, and damaged 17 houses. The community evacuation route and shelter were planned in May 2001, and Typhoon Toraji on July 31 in the same year caused debris flow again, one person was killed, 21 houses were damaged and 60 ha of farming land were lost.

Evaluation of hazard mitigation effect

The debris flow early warning system in Taiwan has been improved gradually during the past decade, and as the evacuation drills were promoted, the casualties in debris flow disasters reduced year after year. Among the six debris flow disasters in four communities presented in this paper, four debris flow disasters in Shenmu Community, Songhe Community, and Jyunkeng Community in 2001 and 2004, with evacuation prior to debris flow disasters caused only three casualties. However, the debris flow disaster in Jyunkeng Community in 1996 and Minduyou Community in 2004 caused 14 casualties due to lack of hazard awareness and evacuation planning.

Jyunkeng Community, which has suffered from debris flow disasters before and after the Chichi Earthquake, can be used as an example to describe the effect of evacuation on reducing casualties. About 7.1% of 113 residents around debris flow torrents in the community were killed without evacuation before the debris flow disaster caused by Typhoon Herb of 1996; while 0.9% of the residents were killed in the debris flow disaster caused by Typhoon Toraji of 2001 due to the residents evacuation beforehand, it is obvious that evacuation can reduce casualties.

If the analysis is based on the mean casualty in debris flow disasters in Taiwan, each debris flow disaster caused 8.26 casualties before the Chichi Earthquake, it is much higher than 2.36 mean after the Chichi Earthquake (Fig.14). The key factor for the decreased casualties is the debris flow evacuation drills promoted in Taiwan since 2000, as well as the gradually improved debris flow early warning system, developed and enacted after the severe casualties caused by Typhoon Toraji of 2001. The concept of preparedness and response in disaster management is carried out, thus, the number of casualties is effectively reduced.

The Chichi Earthquake occurred 10 years ago, according to the deposited sediment survey of main reservoirs in central Taiwan, the past seven years have universally higher historical mean value, which is 1.4~3.6 times higher (Fig.15), showing that sediment-related disasters are still high. In average, Taiwan had been attacked by 7.11 typhoons per year between years 2000 to 2008, which is respectively higher than 4.45 typhoons before the year of 2000 (Fig.16). The number of rain days with rainfall intensity of 10~ 50 mm/hr during the period of typhoon was increased by about 100%, in comparison to the past 45 years (Shiu et al., 2006; Liu et al., 2008). As the natural environment was not improved, there were 10.22 debris flow events per year between years 2000 to 2008, and this number is much higher than 1.42 times before the Chichi Earthquake (Fig.16). These environmental conditions indicate that the decreased casualties in debris flow disasters since 2000 is not due to a decrease of disasters, nor improvements of the natural environment, but the gradually improved early warning system, which allows residents to evacuate in advance of disasters, resulting in reduced casualties in disasters.

According to the distribution of funds for debris flow disaster mitigation between years 2002 to 2008 (Table 1), the ratio of funds for non-structure mitigation projects, such as evacuation and the early warning system, is far less than the funds for structure ones, and accounts for only 1.06%~3.64% of annual total funds. However, the frequency of debris flow disasters in the same time period was not obviously decreased. It indicates that the aforementioned natural environment has not been improved yet; in addition, it shows that current hazard mitigation works cannot fully reduce the occurrence of debris flow disasters.

Although the ratio of funds for non-structure hazard mitigation projects is relatively low, the mean casualty in each debris flow disaster is far below the 8.26 persons that occurred prior to the Chichi Earthquake. It means that the funding invested in evacuation and the early warning system has notable benefits in securing life in debris flow disasters.

Table 1 Analysis of debris flow disaster mitigation funds and effect (SWCB, 2008b)

Conclusions

Because of the Chichi Earthquake of 1999, debris flow disasters during heavy rainfall in typhoon are common in Taiwan, and often lead to deaths and property losses. A total of 453 evacuation drills have been conducted since 2000. These evacuation drills have increased the hazard awareness of residents, and evacuation planning has been improved greatly. In addition, as Typhoon Toraji caused severe casualties, the Taiwanese government established an early warning system for debris flow disasters, by mounting 13 debris flow monitoring stations to procure observation data, resulting in the rainfall threshold of different debris flow hazard-prone areas being revised, and the overall mitigation and response operations become mature. Thus, residents of the warned areas can be evacuated effectively in advance and to prevent the occurrences of disasters.

References

Central Weather Bureau. (2008) . FAQ for Earthquake. Taipei, Taiwan: Author.

Central Weather Bureau. (2009). [Typhoon Database]. Unpublished raw data.

Chen, S. C. (2002). Knowledge about Debris Flow Hazard mitigation, Presented at the meeting of Natural Hazard of Slope-land Conference for High School Teachers, Nantou, Taiwan. (in Chinese)

Chen, S. C., Lien, H. P., Chen, Z. E., Dung, S. P., Chen, L. K., Wang, S.Y., Lin, C. C., Wu, C. H., Peng, C. P., & Shih. (2002) . Investigation and Analysis of Significant Debris Flow Disaster and the Measures Discussion of Hazard Mitigation (Tech. Rep. SWCB-91-046) . Nantou, Taiwan: Soil and Water Conservation Bureau, Council of Agriculture, Executive Yuan. (in Chinese)

Chen, S. C., & Lien, H. P. (2003). Planning of Debris Flow Evacuation of 921 Earthquake Reconstruction Area (Tech. Rep. SWCB-92-060) . Nantou, Taiwan: Soil and Water Conservation Bureau, Council of Agriculture, Executive Yuan. (in Chinese)

Chen, S. C., Lin, C. P., & Feng, J. W. (2004). Discussion on Slopeland Community with Significant Sediment-related Disaster in 2004 . Paper presented at the meeting of 2004 R & D Results of Slopeland Hazard Mitigation, Taipei, Taiwan. (in Chinese)

Fang, Y. M., Lee, B. J. Chou, T. Y. Lie, H. P. Chang, Y. H. Hsiao, T. C. Lin, Y. I. Lien, J. C., & Yin, H. Y. (2007, November), Analysis and Maintenance of Debris Flow Monitoring System- A Case Study of Events in Aiyuzih River. Paper presented at the 2nd International Conference on Urban Disaster Reduction, Taipei, Taiwan.

Lin, C. W., Shieh, C. L., & Wang, W. N. (2002). I mpact of Chi-Chi Earthquake on the Occurrence of Landslides and Debris Flows in Central Taiwan , Paper presented at the meeting of Active Faults and Earthquake Disaster, Taichung, Taiwan. (in Chinese)

Soil and Water Conservation, ( 2004a). [Statistics of the slope land in Taiwan]. Unpublished raw data.

Soil and Water Conservation Bureau. (2004b). 2003 DEBRIS FLOW ANNUAL REPORT. Nantou, Taiwan: Author. (in Chinese)

Soil and Water Conservation Bureau. (2006). 2005 DEBRIS FLOW ANNUAL REPORT. Nantou, Taiwan: Author. (in Chinese)

Soil and Water Conservation Bureau. (2007). 2006 DEBRIS FLOW ANNUAL REPORT. Nantou, Taiwan: Author. (in Chinese)

Soil and Water Conservation Bureau. ( 2008a) . 2007 DEBRIS FLOW ANNUAL REPORT. Nantou, Taiwan: Author. (in Chinese)

Soil and Water Conservation Bureau. (2008b). The Achievement of Debris Flow Hazard mitigation. Nantou, Taiwan: Author. (in Chinese)

Tsai, C. H. & Shu, S. Y. (2005). Innovative E-government policy into practice. Communications of National GIS, 53, 2-12.

Water Resources Agency. (2008). The management of water storage facilities in Taiwan for 2007. Taipei, Taiwan: Ministry of Economic Affairs. (in Chinese)

Wu, H. L. (2006). Multi-strategies of debris flow mitigation in Taiwan. Paper presented at the meeting of Debris Flow Hazard Prevention and Technology in Taiwan & Austria, Taipei, Taiwan.

Copyright