Integrating the geographic information system and predictive data mining techniques to model effects of compound disasters in Taipei

Located in the circum-Pacific seismic zone, Taiwan is continually threatened by such natural disasters as typhoons, floods, landslides, and earthquakes, which increase risk of property loss and severely endanger public safety. Taipei City, the political and economic capital of Taiwan, must address disaster prevention and relief operations for compound disasters and extreme climatic events in addition to existing metropolitan disaster prevention operations. This study formulates 48 compound-disaster scenarios based on threats to Taipei City due to heavy rainfall and surrounding faults. Hydrology and flood analysis results and the Taiwan earthquake loss estimation system are utilized to assess the potential for compound disasters and the number of people they would displace in Taipei’s administrative districts. Analytical results can be used to create a pre-disaster static potential diagram and a refuge or shelter capacity assessment table. The disaster potential diagram is adopted to conduct geographic information system spatial and data analysis, and temporary refuges or shelters planned by the city government are integrated for shelter capacity comparison. Furthermore, a dynamic assessment curve for the number of displaced people during a disaster is plotted using data mining and attribute filtering. Subsequently, a cross table is obtained and employed to predict the number of refugees in the various administrative districts. Finally, conclusions and recommendations are provided for making disaster prevention and relief decisions simultaneously concerning earthquakes and flooding.     

Flash flood routing modeling for levee-breaks and overbank flows due to typhoon events in a complicated river system

There has been a yearly increase in precipitation in Taiwan, consistent with trends seen across the world. In the summer and fall, typhoons or tropical cyclones with torrential rainfall frequently occur as a result of Taiwan’s subtropical climate. Flash floods may cause a levee-break and/or the overtopping of banks at narrow neck locations in a river system, which may in turn produce inundation in urban areas. Therefore, a model that predicts flash floods is of vital importance for river management. The present study is based on a flash flood routing model, which incorporates levee-break and overbank functions to calculate the discharge hydrographs in the complicated Danshuei River system of northern Taiwan. The numerical model was calibrated and verified against observed water stages using three typhoon events. The results indicate reasonable agreement between the model simulations and the observed data. The model was then used to calculate the levee-break and overbank flow hydrographs due to Typhoon Talim (2005) and Typhoon Nari (2001), respectively. The simulated results indicate that several parameters significantly affect the flow hydrograph during a levee-break and should be carefully monitored when levee-break events occur in the river system. The simulated water stages at several stations are consistent with observed data from Typhoon Nari. The simulated overbank flow results quantitatively agree with reported information. The data also confirm that most of the overbank events occurred at the upper reaches of the Keelung River, consistent with the low levee height protection.     

Decision-tree analysis on optimal release of reservoir storage under typhoon warnings

The wet and dry seasons are distinctive in Taiwan as the amount of precipitation in wet seasons accounts for over three-fourth of the total rainfall. And the water-resources management relies pretty much on the rainfall brought in by typhoons as it accounts for a significant portion of the precipitation during wet seasons. Furthermore, as the storage of reservoirs is limited due to topographical factors, the management of typhoon rainfall has always been an important issue in Taiwan. The technique of decision-tree analysis is applied in this article to determine the optimal reservoir release in advance upon the issuance of a typhoon warning by the Central Weather Bureau (CWB), and the proposed methodology may provide solution to the trade-off judgment of reservoir operations between flood control and water supply according to economic efficiency. In this article, the economic loss functions of flooding damage and water-supply shortage are assumed in linear and nonlinear conditions, and the respective expected optimal releases based on the predicted precipitation as issued by CWB are derived. The proposed methodology has been applied to the Shihmen Reservoir System, and the capabilities of the model as an aid to real-time decision-making as well as the evaluation of the economic worth of forecasts is presented.     

Potential hazard analysis and risk assessment of debris flow by fuzzy modeling

Taiwan is a mountainous country, so there is an ever present danger of landslide disasters during the rainy seasons or typhoons. This study aims to develop a fuzzy-rule-based risk assessment model for debris flows and to verify the accuracy of risk assessment so as to help related organizations reduce losses caused by debris flows. The database is comprised of information from actual cases of debris flows that occurred in the Hualien area of Taiwan from 2007 to 2008. The established models can assess the likelihood of the occurrence of debris flows using computed indicators, verify modeling errors, and make comparisons between the existing models for practical applications. In the establishment of a fuzzy-based debris flow risk assessment model, possible for accounting it on the basis of far less information regarding a real system and the information can be of an uncertain, fuzzy or inexact character, the influential factors affecting debris flows include the average terrain slope, catchment area, effective catchment area, accumulated rainfall, rainfall intensity, and geological conditions. The results prove that the risk assessment model systems are quite suitable for debris flow risk assessment, with a resultant ratio of success 96?% and a normalized relative error 4.63?%.     

Impact of Flood Disasters on Taiwan in the Last Quarter Century

The increasing natural disasters, especially floods during the last quarter century, are raising the economic losses in Taiwan. The most severe hazard in Taiwan is flooding induced by typhoons and storms in summer and autumn. By comparing the rivers around the world, the ones in Taiwan have the steepest slopes, the largest discharge per unit drainage area, and the shortest time of concentrations. Rapid urbanization without proper land uses managements usually worsen the flood problems. Consequently, flood hazards mitigation has become the most essential task for Taiwan to deal with. Although the government keeps improving flood defense structures, the flood damage grows continuously. In this article, possible flood mitigation strategies are identified for coping with complex environmental and social decisions with flood risk involved.     

Coupling typhoon rainfall forecasting with overland-flow modeling for early warning of inundation

Taiwan suffers from an average of three or four typhoons annually, and the inundation caused by the heavy precipitation that is associated with typhoons frequently occurs in lowlands and floodplains. Potential inundation maps have been widely used as references to set up non-structural strategies for mitigating flood hazards. However, spatiotemporal rainfall distributions must be addressed to improve the accuracy of inundation forecasting for emergency response operations. This study presents a system for 24-h-ahead early warning of inundation, by coupling the forecasting of typhoon rainfall with the modeling of overland flow. A typhoon rainfall climatology model (TRCM) is introduced to forecast dynamically the spatiotemporal rainfall distribution based on typhoon tracks. The systematic scheme for early warning of inundation based on the spatiotemporal downscaling of rainfall and 2D overland-flow modeling yields not only the extent of inundation, but also the time to maximum inundation depth. The scheme is superior to traditional early warning method referring to the maximum extent and depth of inundation determined from conditional uniform rainfall. Analytical results show that coupling TRCM with an overland-flow model yields satisfactory inundation hydrographs for warning of the extent and peak time of inundation. This study also shows that the accuracy of forecasting spatiotemporal rainfall patterns determines the performance of inundation forecasting, which is critical to emergency response operations.     

基于力学过程的蓄滞洪区洪水风险评估模型及应用

为定量评估分蓄洪工程启用过程中蓄滞洪区的洪水风险等级,创建了基于力学过程的蓄滞洪区洪水风险评估模型。该模型采用二维水动力学模块计算蓄滞洪区的洪水演进过程,利用洪水中人体跌倒失稳公式及洪水中房屋、农作物损失的计算关系式,评估各类受淹对象的洪水风险等级。然后将二维水动力学模块计算的洪水要素与两个物理模型试验值进行对比,表明二维水动力学模块的计算精度良好。最后计算了荆江分洪工程启用时分洪区内洪水的演进过程,并评估洪灾中群众的危险等级和财产损失。计算结果表明:洪水演进至140 h时,蓄滞洪区群众、房屋、水稻和棉花的平均损失率分别为85%、59%、63%和72%。模型中提出的采用基于受淹对象失稳机制的洪水风险分析方法,比以往经验水深法划分风险等级的适用性更好,不仅能为洪水风险管理及蓄滞洪区启用标准制定提供参考,也能推广应用于溃坝或堰塞湖溃决等极端洪水风险评估。     

The storage potential of different surface coverings for various scale storms on Wu-Tu watershed,Taiwan

An impervious surface cover is continuously spreading over the Wu-Tu upstream watershed due to the concentrated population and raised economical demands, while that area also frequently suffers from heavy storms or typhoons during the summer season. The increased flood volume due to this extended imperviousness causes a greater potential hazard than that of the past. In order to evaluate the urbanized impacts on the watershed, a set of methods were used to estimate the changes of the watershed storage. This research chose 51 observed events from three raingauges on the Wu-Tu upstream watershed, Taiwan, to study the volume characteristic of abstracted rainwater. In the study, the block Kriging method was used to estimate the area rainfall and the hourly excess was derived through the non-linear programing (NLP). A total of 40 samples were calibrated through the hydrological model and the Soil Conservation Service (SCS) model using the optimum seeking method in order to search out and establish the best parameters that illustrate the hydrological and geomorphic conditions at that time. Eleven cases were used to examine the established relationship of the parameters and the impervious coverings. A design storm approach was used to view the changes of the volume for various scale storms/typhoons because of the different degrees of urbanization. Then, a diagram was designed to show the relationships that exist among the runoff coefficient, return period, and impervious surface. The satisfactory results show that storage capability of rainwater for various scale storms on the Wu-Tu watershed would be respectively reduced about 42–156 cms in different decrements up to now.     

Identification and assessment of heavy rainfall–induced disaster potentials in Taipei City

With increasing threat to lives and properties, identifying and assessing disaster potentials has become necessary and prior for effective disaster preparation and rescue planning. This study first introduces practical methods currently used in Taipei City, Taiwan, to identify and assess heavy rainfall–induced potential risks on flood, debris flow, and landslide. The identified disaster potential information is further applied to a series of deterministic and probabilistic risk analyses using Shilin District of Taipei City as a case study. The deterministic risk analyses are conducted to evaluate the impact of various heavy rainfall intensities on the residents. The probabilistic risk analyses are performed to establish risk curves for the population affected by heavy rainfall–induced hazards. The risk curve represents the relationships between the affected population and the annual exceedance probability. This study found the annual exceedance probability is very sensitive to the assumed coefficients of variation of the affected population. It is recommended historical statistical data on the correlation between affected population and rainfall intensity should be recorded and compiled in order to assess the actual probability distribution function of the affected population. Risk analysis results are further applied to assess the community evacuation capacity in this district. Last, short-term and long-term mitigation strategies and recommendations are discussed.     

An Assessment of Structural Measures for Flood-prone Lowlands with High Population Density along the Keelung River in Taiwan

Midstream of the Keelung River Basin in Northern Taiwan has become highly urbanized and densely populated area. Flood inundation along riversides frequently occurred during typhoons or rainstorms. Three protection measures, including constructions of high-level protection levees, a diversion channel, and a detention reservoir, were proposed for flood mitigation. The main purpose of this study is to evaluate the flood mitigation performance of the three proposed structural measures by using combined hydrologic analyses and hydraulic routings. A semi-distributed parallel-type linear reservoirs rainfall-runoff model was used for estimating the surface runoff. Furthermore, a 1-D dynamic channel routing model was coupled with a two-dimensional inundation model to simulate the hydraulic characteristics of river flooding and overland flow. Simulation results of flood stages, runoff peak discharges, and inundation extent under design rainfall scenarios were chosen as the criteria for evaluation. The results showed a diversion channel is superior to the other two measures for flood mitigation of the study area. After the process of environmental impact assessment, a revised diversion channel approach has been approved for construction as the major structural measure.     

A macro-scale flood risk model for Jamaica with impact of climate variability

Floods account for more than half of the global hydrometeorological risks. Severe floods cause significant economic shocks and loss of lives, particularly for developing countries such as Jamaica. There is need for more information on the present and projected flood risks to justify macro-scale planning for climate change adaptation and facilitate the decision-making processes. In this study, a catalogue of 198 flood events occurring between 1678 and 2010 is compiled for Jamaica and used to examine the climatology, occurrence, trends, causes and duration of the island’s severe events. The annual flood risk is estimated to be a loss of life rate of 4 persons and estimated annual damage of USD96.3 million per annum in 2010 values and approximately 0.84 % of GDP per annum. Macro-scale models for flood risks (deaths and damages) are also developed using data from the flood catalogue and maximum precipitation at the town and parish level. The models examine the relationship between flood risks (death and damages) and extreme rainfall depths and intensities. Future climate risks of loss of lives and damages are predicted to increase 11 and 9 %, respectively, to 4.4 persons and USD105.2 million per annum.     

Integrated application of the analytic hierarchy process and the geographic information system for flood risk assessment and flood plain management in Taiwan

Flooding is one of the major natural hazards in Taiwan, and most of the low-lying areas in Taiwan are flood-prone areas. In order to minimize loss of life and economic losses, a detailed and comprehensive decision-making tool is necessary for both flood control planning and emergency service operations. The objectives of this research were (i) to develop a hierarchical structure through the analytic hierarchy process (AHP) to provide preferred options for flood risk analysis, (ii) to map the relative flood risk using the geographic information system (GIS), and (iii) to integrate these two methodologies and apply them to one urban and one semi-rural area in central Taiwan. Fushin Township and the floodplain of Fazih River (1 km on either side of the channel) in Taichung City were selected for this study. In this paper, the flood risk is defined as the relative flood risk due to broken dikes or the failure of stormwater drainage systems. Seven factors were considered in relation to the failure of stormwater drainage, and five to that of broken dikes. Following well-defined procedures, flood maps were drawn based on the data collected from expert responses to a questionnaire, the field survey, satellite images, and documents from flood management agencies. The relative values of flood risk are presented using a 200-m grid for the two study areas. It is concluded that integration of AHP and GIS in flood risk assessment can provide useful detailed information for flood risk management, and the method can be easily applied to most areas in Taiwan where required data sets are readily available.     

Relationships between typhoon types and debris flow disasters in Taiwan

Frequent debris flow disasters caused by heavy precipitation during the annual typhoon season are some of the most serious disasters in Taiwan. This study is on the debris flow disasters associated with the typhoons that hit Taiwan between 1986 and 2004. Typhoon data and records of debris flow disasters available for Nantou and Hualien counties in Taiwan were analyzed. The paths and rainfall characteristics of typhoons were found to have a great effect on the debris flows at these locations. Accordingly, the typhoons were grouped into four major types based on their paths and related disasters. The relationships between rainfall intensity and accumulation and debris flow are discussed for the four major typhoon types. The information may form the basis for providing useful indicators for disaster management.     

Earthquake risk assessment and optimal risk management strategies for Hi-Tech Fabs in Taiwan

In this paper, we build an event-based seismic hazard assessment and financial analysis model for Hi-Tech Fabs in Taiwan. As we know, the low occurrence rate, tremendous loss and high uncertainty are characteristics of earthquake disasters. To handle the above issues, the model integrates knowledge from many fields including earth science, seismology, geology, risk management, structural engineering, the insurance profession, financial engineering and facility management. The portfolio of data from the site survey indicates that the model can be used to calculate the event losses (including buildings, contents and business interruption losses); furthermore the average annual loss and loss exceeding probabilities also can be calculated. The total earthquake risk cost, which includes earthquake insurance premiums, average annual retained loss and equivalent annual retrofit cost, is defined as an indicator for selection of optimal risk management strategies.     

山区小流域洪水分布模式模拟与危险性评估

流域暴雨山洪过程时空异质性强,准确评估雨洪变化特性和洪水危险性对山洪灾害防治具有重要意义。以7个降雨特征指标和6个洪水特征指标刻画流域场次雨洪特性,采用中国山洪水文模型和洪水频率指标相结合,模拟和评估口前流域洪水过程及其危险性。结果表明：场次洪水洪峰模数、洪峰时间偏度、高脉冲历时占比、涨落洪速率与降雨总量、平均雨量、最大雨强、雨峰位置系数、基尼系数等降雨特征指标显著相关,三场致灾洪水过程的降雨均呈现量级大、强度大、历时短、暴雨中心偏中下游的特点;率定期和验证期的平均径流深相对误差均在9%以内,平均洪峰流量相对误差均在11%以内,平均峰现时间误差均在1.7 h以内,平均Nash-Sutcliffe系数为0.80和0.76;各场次洪水有0.0%～93.3%的河段流量达到一般危险及以上等级,三场致灾洪水过程的危险性等级最高,分别有80.0%、35.0%和1.7%的小流域河段流量达到高危险及以上等级。研究可为山区小流域暴雨洪水危险性评估、灾害响应和复盘等提供技术支撑。     

Evolution of landslide hotspots in Taiwan

Among the disasters facing Taiwan, earthquakes and typhoons incur the greatest monetary losses, and landslide disasters inflict the greatest damage in mountainous areas. The nationwide landslide susceptibility map gives an indication of where landslides are likely to occur in the future; however, there is no objective index indicating the location of landslide hotspots. In this study, we used statistical analysis to locate landslide hotspots in catchments in Taiwan. Global and local spatial autocorrelation analysis revealed the existence of landslide clusters between 2003 and 2012 and identified a concentration of landslide hotspots in the eastern part of Central Taiwan. The extreme rainfall brought by typhoon Morakot also led to the formation of new landslide hotspots in Southern Taiwan. This study provides a valuable reference explaining changes in landslide hotspots and identifying areas of high hotspot concentration to facilitate the formulation of strategies to deal with landslide risk.     

A Climatology Model for Forecasting Typhoon Rainfall in Taiwan

The continuous torrential rain associated with a typhoon often caused flood, landslide or debris flow, leading to serious damages to Taiwan. Thus, a usable scheme to forecast rainfall amount during a typhoon period is highly desired. An analysis using hourly rainfall amounts taken at 371 stations during 1989–2001 showed that the topographical lifting of typhoon circulation played an important role in producing heavier rainfall. A climatology model for typhoon rainfall, which considered the topographical lifting and the variations of rain rate with radius was then developed. The model could provide hourly rainfall at any station or any river basin for a given typhoon center. The cumulative rainfall along the forecasted typhoon track was also available. The results showed that the R² value between the model estimated and the observed cumulative rainfall during the typhoon period for the Dan-Shui (DSH) and Kao-Ping (KPS) River Basins reached 0.70 and 0.81, respectively. The R² values decreased slightly to 0.69 and 0.73 if individual stations were considered. However, the values decreased significantly to 0.40 and 0.51 for 3-hourly rainfalls, indicating the strong influence of the transient features in producing the heavier rainfall. In addition, the climatology model can only provide the average conditions. The characteristics in individual typhoons should be considered when applying the model in real-time operation. For example, the model could give reasonable cumulative rainfall amount at DSH before Nakri (2002) made landfall on Taiwan, but overestimated the rainfall after Nakri made landfall and weakened with significant reduction in convection.     

Basinwide disaster loss assessments under extreme climate scenarios: a case study of the Kaoping River basin

This study examined the Kaoping River basin, Taiwan, an area severely destroyed by Typhoon Morakot in 2009. Dynamically downscaled data were applied to simulate extreme typhoon precipitation events for facilitating future preparation efforts (2075–2099) under climate change conditions. Models were used to simulate possible impacts in upstream and downstream areas for basinwide disaster loss assessment purposes. The Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability and FLO-2D models were applied to simulate slope-land disaster impacts and sediment volume in the upstream area. The sediment delivery ratio was used to calculate the valid sediment amount delivered downstream and the riverbed uplift altitude. SOBEK was used to build a flood impact model for the Kaoping River basin, and the model was used to simulate potential flooding caused by future extreme typhoon events. The Taiwan Typhoon Loss Assessment System established by the National Science and Technology Center for Disaster Reduction was used to evaluate the potential loss associated with extreme events. The property loss calculation included 32 land-use categories, including agriculture, forestry, fishery, and animal husbandry losses; industrial and commercial service losses; public building losses; and traffic and hydraulic facility losses. One of the Kaoping River basin townships, Daliao District, had the highest flood depth increase ratio (12.6%), and the losses were 1.5 times the original situation. This was much worse than were the losses suffered during Typhoon Morakot. These results also show that sediment delivered from the upstream areas had a significant influence on the downstream areas. This is a critical issue for future flood mitigation under climate change conditions.     

Rainfall thresholds for flood triggering. The case of Marathonas in Greece

Basins across Mediterranean coast are often subject to rapid inundation phenomena caused by intense rainfall events. In this flash flooding regime, common practices for risk mitigation involve hydraulic modeling, geomorphic, and hydrologic analysis. However, apart from examining the intrinsic characteristics of a basin, realistic flood hazard assessment requires good understanding of the role of climatic forcing. In this work, peak rainfall intensities, total storm accumulation, average intensity, and antecedent moisture conditions of the 52 most important storms in record, during the period from 1993 to 2008, in northeast Attica, in Greece, are examined to investigate whether there is a correlation between specific rainfall conditions and flood triggering in the area. For this purpose, precipitation data from a network of five rain gauges installed across the study area were collected and analyzed. Storms totals, average intensity, antecedent moisture conditions, and peak intensities variations were calculated and compared with local flooding history. Results showed that among these rainfall measures, only peak storm intensity presents a significant correlation with flood triggering, and a rainfall threshold above which flooding becomes highly probable can be defined.     

Limitations of real-time models for forecasting river flooding from monsoon rainfall

Very intense rainfall during the southwest and northeast monsoons causes severe river flooding in India. Some traditional techniques used for real-time forecasting of flooding involve the relationship between effective rainfall and direct surface runoff, which simplifies the complex interactions between rainfall and runoff processes. There are, however, serious problems in deducing these variables in real time, so it is highly desirable to have a real-time flood forecasting model that would directly relate the observed discharge hydrograph to the observed rainfall. The storage routing model described by Baba and Hoshi (1997), Tanaka et al. (1997), and Baba et al. (2000), and a simplified version of this model, have been used to compute observed river discharge directly from observed hourly rainfall. This method has been used to study rainfall–runoff data of the Ajay River Basin in eastern India. Five intense rainfall events of this basin were studied. Our results showed that the Nash–Sutcliffe efficiency of discharge prediction for these five events was 98.6%, 94.3%, 86.9%, 85.6%, and 67%. The hindcast for the first two events is regarded as completely satisfactory whereas for the next two events it is deemed reasonable and for the fifth it is unsatisfactory. It seems the models will yield accurate hindcast if the rainfall is uniform over the drainage basin. When the rainfall is not uniform the performance of the model is unsatisfactory. In future this problem can, in principle, be corrected by using a weighted amount if rainfall is based upon multiple rain-gauge observations over the drainage basin. This would provide some measure of the dispersion in the rainfall. The model also seems unable to simulate flooding events with multiple peaks.