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The ocean surface waves during Super Typhoons Maria (2018), Lekima (2019), and Meranti (2016) were reproduced using hybrid typhoon winds and a fully coupled wave-tide-circulation modeling system (SCHISM-WWM-III). The hindcasted significant wave heights are in good agreement with the along-track significant wave heights measured by the altimeters aboard the SARAL (Satellite with ARgos and ALtiKa) and Jason-2 satellites. Two numerical experiments pairing Super Typhoons Maria (2018) and Meranti (2016) and Super Typhoons Lekima (2019) and Meranti (2016) were conducted to analyze the storm wave characteristics of binary and individual typhoons. Four points located near the tracks of the three super typhoons were selected to elucidate the effects of binary typhoons on ocean surface waves. The comparisons indicate that binary typhoons not only cause an increase in the significant wave height simulations at four selected pints but also result in increases in the one-dimensional wave energy and two-dimensional directional wave spectra. Our results also reveal that the effects of binary typhoons on ocean surface waves are more significant at the periphery of the typhoon than near the center of the typhoon. The interactions between waves generated by Super Typhoons Maria (2018) and Meranti (2016) or Super Typhoons Lekima (2019) and Meranti (2016) might be diminished by Taiwan Island even if the separation distance between two typhoons is

Taiwan was successful in containing the spread of the novel coronavirus (COVID-19) in 2020. One major factor in this success was the compilation and provision of comprehensive information about the pandemic. The present study proposes a pandemic intelligence system that provides data on the number of epidemic prevention professionals in each county and city, as well as daily confirmed cases, the demographics of the confirmed cases, and available resources (negative-pressure room beds and artificial ventilation apparatuses) in hospitals. Furthermore, the system provides the location of pharmacies selling masks and their current inventories, as well as the distribution of crowds at popular tourist destinations and social-distance monitoring. The most frequently used map layer in the thematic map of the pandemic is that of crowd distribution during the study period from March 2020 until the end of the same year. The case study used in this investigation for applying the system is represented by the 4-day weekend for Tomb-Sweeping Day of 2020. Through the real-time analysis of dynamic data and the integration of intelligence, the system offers a clear insight into changes in relevant information and, thus, enables the preemptive deployment of control measures by the county/city governments regarding pandemic management.

This study investigated the effects of social determinants and resource distribution on trust in government after a disaster. This involved an analysis of four waves of the Social Impact an...

Regional monitoring of rock slope failures by the seismic technique is rarely studied due to significant source location errors, and it still lacks the signal features needed for understanding events of this type because of the complex mass movement involved. To better understand events of this type, ten known events along highways in Taiwan were analyzed. First, a hybrid approach (GeoLoc) composed of cross-correlation-based and amplitude-attenuation-based approaches was applied, and it produced a location error of maximum 3.19 km for the ten events. Then, we analyzed the ratio of local magnitude (ML) and duration magnitude (MD) and found that a threshold of 0.85 yields successful classification between rock slope failure and earthquake. Further, the GeoLoc can retrieve the seismic parameters, such as signal amplitude at the source (A0) and ML of events, which are crucial for constructing scaling law with source volume (V). Indeed, Log(V) = 1.12 ML + 3.08 and V = 77,290 A00.44 derived in this study provide the lower bound of volume estimation, since the seismic parameters based on peak amplitudes cannot represent the full process of mass loss. Second, while video records correspond with seismic signals, the processes of toppling and sliding present column- and V-shaped spectrograms, respectively. The impacts of rockfall directly link directly to the pulses of seismic signals. Here, all spectrogram features of events can be identified by event volumes larger than 2,000 m3, corresponding to the farthest epicenter distance ~2.5 km. The previous results were obtained using the GeoLoc scheme for providing the government rapid reports for reference. Finally, a recent event on 12th June 2020 was used to examine the GeoLoc scheme’s feasibility. We estimated the event's volume by the two scalings: 3,838 m3 and 3,019 m3, which was roughly consistent with the volume estimation of 5,142 m3 from the digital elevation model. The physical processes, including rockfall, toppling, and complex motion behaviors of rock interacting with slope inferred from the spectrogram features were comprehensively supported by the video record and field investigation. We also demonstrated that the GeoLoc scheme, which has been implemented in Sinwulu catchment, Taiwan, can provide fast reports, including the location, volume, and physical process of events of this type to the public soon after they occur.

Landslide-generated seismic waves (landslide-quakes), exhibiting distinctive waveforms and frequency characteristics, can be recorded by nearby seismometers. Implementing an automatic classifier for landslide-quakes could help provide objective and accurate initiation times of landslides with efficiency. This study collected and analyzed the time information of 214 landslide seismic records due to 33 documented landslide events, from the Broadband Array in Taiwan for Seismology (BATS). In addition, equal numbers of earthquake and noise signals were also incorporated. The 642 seismic signals and time information were carefully examined using the random forest algorithm to create an automatic landslide-quake classifier. By validating the signal attributes of the landslide, earthquake, and noise events, specifically in the time and frequency domains, it was shown that the proposed classifier can reach an accuracy (the proportion of all correctly classified events to the total number of events) of 91.3%. To further evaluate the applicability of the automatic classifier, landslide-quakes generated during the devastating Typhoon Morakot (2009) and Typhoon Soudelor (2015) were also verified, showing that the sensitivity of the classifier is higher than 98%.

Physically based numerical models can predict scour depth at embankments located in bend reaches. However, methodologies for utilizing these numerical models to assess the risk of reinforced concrete embankment failure are rarely investigated. Therefore, a new assessment methodology is proposed to predict the riverbank failure caused by bend scour. The methodology is primarily based on a bend scour simulation model that integrates a one-dimensional (1D) hydraulic model, a two-dimensional (2D) hydrodynamic finite-volume model, and an empirical equation of bend scour prediction. The model was first applied to the Shuiwei Embankment located in a river bend reach of Da-An River in Taiwan and verified against results from the 1D hydraulic model and field data. The model was then used to simulate 2D flow field and the temporal evolution of bend scour depth under different return period flood events to examine the relationships between river discharge, water level, shear stress, and bend scour depth. The influence of shear stress on the stability of toe protections was also investigated. The field data (from two events) and numerical solutions (four scenarios) were assessed to conceive two empirical equations for predicting shear stress and bend scour depth. A new assessment methodology was proposed using these two equations to predict the risk of river embankment failure during flood periods. The proposed methodology can be easily applied in other disaster-prone regions to mitigate the effects of disasters caused by bend scouring.

Typhoon-induced waves threaten the coastal areas of Taiwan each year. The roaring waves caused by typhoon Meranti (2016) even destroyed a lighthouse in a fishing port on the southwestern coast of Taiwan. Therefore, there has been increased interest in creating potential risk maps for typhoon-induced waves along the coast of Taiwan. In this study, the highest intensity and lowest intensity typhoons (HITs and LITs, respectively) for each category from 1977 to 2016 were selected. A fully coupled tide-surge-wave model was utilized to map the distributions of maximum significant wave heights for 18 typhoon events. Each map was classified into one of 5 levels and employed to generate maximum and minimum potential risk maps for typhoon-induced waves. Our results demonstrate that the northern and the eastern coasts of Taiwan are threatened by violent waves (significant wave heights (SWHs) range from 7 to 11.5 m) over a coastline length of 236.4 km and roaring waves (SWHs exceeding 11.5 m) over a coastline length of 298.1 km under HIT conditions. The lengths of coastline threatened by big waves (SWHs ranging from 3 to 6 m) and small waves (SWHs less than 3 m) are 637.7 km and 553.4 km, respectively, under LIT conditions.

Rainfall-induced landslides number among the most devastating natural hazards in the world and early warning models are urgently needed to reduce losses and fatalities. Most landslide early warning systems are based on rainfall thresholds defined on the regional scale, regardless of the different landslide susceptibilities of various slopes. Here we divided slope units in southern Taiwan into three categories (high, moderate and low) according to their susceptibility. For each category, we established separate rainfall thresholds so as to provide differentiated thresholds for different degrees of susceptibility. Logistic regression (LR) analysis was performed to evaluate landslide susceptibility by using event-based landslide inventories and predisposing factors. Analysis of rainfall patterns of 941 landslide cases gathered from field investigation led to the recognition that 3 h mean rainfall intensity (I3) is a key rainfall index for rainfall of short duration but high intensity; in contrast, 24 h accumulated rainfall (R24) was recognized as a key rainfall index for rainfall of long duration but low intensity. Thus, the I3–R24 rainfall index was used to establish rainfall thresholds in this study. Finally, an early warning model is proposed by setting alert levels including yellow (advisory), orange (watch) and red (warning) according to a hazard matrix. These differentiated thresholds and alert levels can provide essential information for local governments to use in deciding whether to evacuate residents.

This research looked for social determinants that shaped choices of shelter during a natural disaster. The choices consisted of hotels/rental houses, finding relatives/friends, contacting the government for help, and checking for information about public shelters online. Social determinants examined included age, disability, education, income, social network, trust in the government, and previous disaster experience. The 2013 Taiwan Social Change Survey data and logistic regression were used for analysis. The findings were as follows: (1) compared with other groups, the richest favored hotels or rental houses for sheltering. (2) Compared with other groups, people with lower levels of education had a tendency to contact the government for help regarding shelter. (3) Compared with other groups, young people (20–34 years old), people with larger social networks (informal social capital), and people with more than one previous disaster experience preferred to contact relatives or friends for shelter. (4) People with at least a senior high school education were more likely to search for sheltering information online than their counterparts. This study provides new contributions to the literature in its investigation of the influences of social networks and previous natural disaster experience on shelter choice and of education on the choice of checking sheltering information online.

A state-of-the-art, third-generation, spectral wind wave model was implemented for the surrounding waters of Taiwan. The model was driven by 10-m winds above sea level from ERA-Interim reanalysis data. Medium-term (ten-year) hindcasts covering the period from 2007 to 2016 were conducted to assess the wave power resources in Taiwanese waters. The lowest monthly mean wave power density occurs in May and June, while the most energetic month is December. The sea area with the highest wave power density was observed near Lanyu. Four energetic areas located off the northern coast (H1), the southeastern coast (H2 and H3), and the southern coast (H4) of Taiwan serve as the appropriate sites (hotspots) for deploying wave energy converters. The annual mean wave power densities over ten years are approximately 8.72 kW/m, 10.97 kW/m, 13.72 kW/m, and 13.02 kW/m for H1, H2, H3, and H4, respectively. The annual total wave energy outputs were estimated to be 48.80 MWh/m, 76.59 MWh/m, 94.20 MWh/m, and 92.56 MWh/m at H1, H2, H3, and H4, respectively. The wave power is primarily driven by the northeast monsoon and the bulk of the wave energy appears to be generated by local wind waves in the surrounding waters of Taiwan.

Debris sourced from landslides will result in environmental problems such as increased sediment discharge in rivers. This study analyzed the sediment discharge of 17 main rivers in Taiwan during 14 typhoon events, selected from the catchment area and river length, that caused landslides according to government reports. The measured suspended sediment and water discharge, collected from hydrometric stations of the Water Resources Agency of Taiwan, were used to establish rating-curve relationships, a power-law relation between them. Then sediment discharge during typhoon events was estimated using the rating-curve method and the measured data of daily water discharge. Positive correlations between sediment discharge and rainfall conditions for each river indicate that sediment discharge increases when a greater amount of rainfall or a higher intensity of rainfall falls during a typhoon event. In addition, the amount of sediment discharge during a typhoon event is mainly controlled by the total amount of rainfall, not by peak rainfall. Differences in correlation equations among the rivers suggest that catchments with larger areas produce more sediment. Catchments with relatively low sediment discharge show more distinct increases in sediment discharge in response to increases in rainfall, owing to the little opportunity for deposition in small catchments with high connectivity to rivers and the transportation of the majority of landslide debris to rivers during typhoon events. Also, differences in geomorphic and geologic conditions among catchments around Taiwan lead to a variety of suspended sediment dynamics and the sediment budget. Positive correlation between average sediment discharge and average area of landslides during typhoon events indicates that when larger landslides are caused by heavier rainfall during a typhoon event, more loose materials from the most recent landslide debris are flushed into rivers, resulting in higher sediment discharge. The high proportion of large landslides in Taiwan contributes significantly to the high annual sediment yield, which is among the world's highest despite the small area of Taiwan.

Rainfall intensity-duration (I-D) characteristics are widely used to study landslide triggering. Recent studies also propose that rainfall kinetic energy (ek) can be used to quantify the rate of soil erosion induced by strong precipitation events. In the present study, the Joss-Waldvogel Disdrometers (JWD) was utilized to measure ek data, and the relationship between rainfall kinetic energy and rainfall intensity was determined as the regression results of ekN = 32.19 × (1–0.725e (− 0.029I)) for northern Taiwan and ekS = 32.23 × (1–0.643e(− 0.022I)) for southern Taiwan. Additionally, by examining data from 76 landslide events from the landslide inventory for the period of 2006–2012, the study established an empirical power law I-D relationship, which is crucial for determining the rainfall threshold needed in forecasting landslides. Two rainfall thresholds, i.e., functions of rainfall intensity (I) and rainfall duration (D), were established for study areas in northern and southern Taiwan to be IN = 13.81 D− 0.31 and IS = 66.44 D− 0.57 respectively. Landslides in northern Taiwan usually occurred when D was over 10 h and such events in southern Taiwan took place in two different D groups whose gap was between 16 h and 20 h. Two mechanisms were operative in triggering landslides in the two areas—water infiltration and high rainfall intensity. However, ek also proved to be an important rainfall parameter, and the present study further established that not only rainfall intensity-duration but also kinetic energy can provoke landslide triggering, thus allowing the compiling of more comprehensive information for more accurate landslide forecasting.

This study analyzed 267 landslide landforms (LLs) in the Kumamoto area of Japan from the database of about 0.4 million LLs for the whole of Japan identified from aerial photos by the National Research Institute for Earth Science and Disaster Resilience of Japan (NIED). Each LL in the inventory is composed of a scarp and a moving mass. Since landslides are prone to reactivation, it is important to evaluate the sliding-recurrence susceptibility of LLs. One possible approach to evaluate the susceptibility of LLs is slope stability analysis. A previous study found a good correlation (R 2 = 0.99) between the safety factor (F s ) and slope angle (α) of F s = 17.3α −0.843. We applied the equation to the analysis of F s for 267 LLs in the area affected by the 2016 Kumamoto earthquake (M j = 7.3). The F s was calculated for the following three cases of failure: scarps only, moving mass only, and scarps and moving mass together. Verification with the 2016 Kumamoto earthquake event shows that the most appropriate method for the evaluation of LLs is to consider the failure of scarps and moving mass together. In addition, by analyzing the relationship between the factors of slope of entire landslide and slope of scarp for LLs and comparing the results with the Aso-ohashi landslide, the largest landslide caused by the 2016 Kumamoto earthquake, we also found that morphometric analysis of LLs is useful for forecasting the travel distance of future landslides.

Rock hazard is a common geohazard event that occurs in the orogenic mountain belt and often causes the destruction of road and casualties. The steep topography, fractured bedrock and frequent earthquakes favor to happen. Those are usually fast and unpredictable, leading a lack of direct observation of physical process. Recent seismological studies highlighted the rock hazard induced seismic signals could improve understanding of its dynamics. This study focuses on the three provincial highways that cross the Taiwan Island from east to west. The regions along the highways have the complexity in tectonic structure and extreme climate-forced erosion, causing the hazard frequently occurred. In order to understanding seismic features and physical process of rock hazard, we conducted a series of seismic analyses using the seismic records collected form regional seismic network for ten events, which were reported by the government agency. Four of them have the video recordings, which would be helpful to understanding the relationships between physical process (falling, rolling, bouncing and fragmentation), movement type (fall, topple, slump, slide, avalanche or complex) and seismic features. We developed the hybrid method of determination of geohazard event location (GeoLoc) that combines the cross-correlation-based method and the amplitude-attenuation-based approach. We apply the GeoLoc scheme to locate the events recorded by the seismic station with epicentral distance ranging from 2 to 56 kilometers (km) and it helps to reduce the location error. The leading seismic signals of the mass detachment linked to the crack propagation or slope response can be observed, and we also found that the seismic feature caused by fragment of rock block exhibits the higher frequency than the seismic signals corresponding to impaction of rock particles. Our results highlight the possibility of the seismic technique for locating rock hazards distributed along highways in a regional scale and further understanding its physical process. The aforementioned results would be helpful to build the near-real-time monitoring system along the highways for hazard mitigation of events.

Typhoon-induced storm surges are catastrophic disasters in coastal areas worldwide, although typhoon surges are not extremely high in Taiwan. However, the rising water level around an estuary could be a block that obstructs the flow of water away from the estuary and indirectly forms an overflow in the middle or lower reaches of a river if the occurrence of the highest storm surge (HSS) coincides with the highest astronomical tide (HAT). Therefore, assessing the highest storm tide (HST, a combination of the HSS and HAT) hazard level along the coast of Taiwan is particularly important to an early warning of riverine inundation. This study hindcasted the storm surges of 122 historical typhoon events from 1979 to 2018 using a high-resolution, unstructured-grid, surge-wave fully coupled model and a hybrid typhoon wind model. The long-term recording measurements at 28 tide-measuring stations around Taiwan were used to analyze the HAT characteristics. The hindcasted HSSs of each typhoon category (the Central Weather Bureau of Taiwan classified typhoon events into nine categories according to the typhoon&rsquo, s track) were extracted and superposed on the HATs to produce the individual potential HST hazard maps. Each map was classified into six hazard levels (I to VI). Finally, a comprehensive potential HST hazard map was created based on the superposition of the HSSs from 122 typhoon events and HATs.

Ex-ante and ex-post assessments are both essential in risk management, although ex-post assessments of engineering measures have not been widely reported. In this study, an ex-post benefit-cost assessment method was developed to enable the direct impact of remediation engineering measures for debris flows to be quantified. We propose three categories of parameters for calculating benefit and cost: (1) predicted benefits (comprising predicted benefits of saving lives, at the household level, and of infrastructure protection); (2) actual loss (of losing lives, at the household level, and of infrastructure); (3) actual cost of the engineering. Actual benefit was defined as predicted benefit minus loss after completion of the engineering measure. The engineering measures were regarded as effective when benefits outweighed costs. Government data, survey results based on past disasters, and interview results with specialists were used. This methodology was applied and tested in ten villages located in the Gaoping River Basin in southern Taiwan. The result showed that the range of the benefit and cost ratio was between 1.26 and 7.99. We recommend this ex-post assessment method for rapid screening of districts with low benefit-cost ratios.

This study analyzed the size of 172 rainfall-induced landslides in Taiwan during 2006–2012. Comparing the landslide size with rainfall conditions, this study found that large and deep landslides usually occurred due to long-duration and moderate-intensity rainfall (11.5–31.0 mm/h; 26.5–62.5 h), whereas small and shallow landslides occurred in a wide range of rainfall intensity and duration (8.5–31.0 mm/h; 4.0–62.5 h). This observation is ascribable to the fact that large and deep landslides need a high ground water level caused by a prolonged rainfall. Concerning the area of landslides, their frequency–area distribution correlates well with a power law relation having an exponent of −1.1 ± 0.07, over the range 6.3 × 102 to 3.1 × 106 m2. The slope of the power law relation for the size–frequency distribution of landslides in Taiwan is lower than those for other areas around the world. This indicates that for the same total area or total number of landslides, the proportion of large landslides is higher in Taiwan than in other areas.

We present precise deformation velocity maps for the two year period from September 2011 to July 2013 of the northern Taiwan area, Taipei, by using persistent scatterer interferometry (PSI) technique for processing 18 high resolution X-band synthetic aperture radar (SAR) images archived from COSMO-SkyMed (CSK) constellation. According to the result, the highest subsidence rates are found in Luzou and Wuku area in which the rate is about 15 mm/yr and 10 mm/yr respectively in the whole dataset. However, dramatic change from serve subsidence to uplift in surface deformation was revealed in the Taipei Basin in two different time spans: 2011/09–2012/09 and 2012/09–2013/07. This result shows good agreement with robust continuous GPS measurement and precise leveling survey data across the central Taipei Basin. Moreover, it also represents high correlation with groundwater table. From 8 well data in the Taipei basin, the storativity is roughly constant across most of the aquifer with values between 0.5 × 10 − 4 and 1.6 × 10 − 3 in Jingmei Formation and 0.8 × 10 − 4 and 1.4 × 10 − 3 in Wuku Formation. This high correlation indicated that one meter groundwater level change could induce about 9 and 16 mm surface deformation change in Luzou and Wuku area respectively, which is about eight times faster the long-term tectonic deformation rate in this area. Thus, to access the activity of the Shanchiao Fault, it is important to discriminate tectonic movement from anthropogenic or seasonal effect in the Taipei Basin to better understand the geohazards and mitigation in the Taipei metropolitan area.

This study examined landslides caused by typhoon events in the upstream area of the Tao-Cheng River in northwestern Taiwan during a 13-year period, 1996 to 2008. Data from six typhoon events were compiled to analyze the relevant characteristics of landslides, vegetation, and changes in sediment discharge with particulate carbon in rivers in this area. The landslide ratio, the ratio of the landslide area to the total study area, was determined to be between 0.60% and 1.29%, and a high new generation ratio and low reactivated ratio of landslides were noted. Analysis of different bands of satellite images taken over a 10-year period disclosed that the Normalized Difference Vegetation Index (NDVI) in the upstream area of the Tao-Cheng River ranged from 0.47 to 0.63 before typhoon events and from 0.38 to 0.46 after typhoon events. The low landslide ratio and high new generation ratio in this area indicated that sporadic landslides tend to occur in new areas and do not have a great impact on vegetation conditions. Thus, the decrease in NDVI after typhoon events was caused by the seasonal effect with withering of vegetation. On the other hand, landslides in this area tend to occur on nonforest land because of the interventions of external forces, such as human developments and reclamation. Geological materials produced by landslides and vegetation flushed by heavy rainfall during typhoon periods have a great effect on the amount of sediment discharge and particulate carbon discharge in the rivers. Furthermore, during typhoon periods, the amount of accumulated rainfall has a greater impact than the peak rainfall intensity on the increase of sediment and particulate carbon in rivers and on the decrease in the NDVI. Analysis of the particulate carbon from the rivers in this area revealed that in the rainy season, high strength and accumulated rainfall will lead to more sediment discharge and particulate carbon discharge in rivers. However, the particulate carbon is limited with further increases in rainfall because of the limited supply of particulate carbon in the upstream of the Tao-Cheng River.

Time-series prediction of a river stage during typhoons or storms is essential for flood control or flood disaster prevention. Data-driven models using machine learning (ML) techniques have become an attractive and effective approach to modeling and analyzing river stage dynamics. However, relatively new ML techniques, such as the light gradient boosting machine regression (LGBMR), have rarely been applied to predict the river stage in a tidal river. In this study, data-driven ML models were developed under a multistep-ahead prediction framework and evaluated for river stage modeling. Four ML techniques, namely support vector regression (SVR), random forest regression (RFR), multilayer perceptron regression (MLPR), and LGBMR, were employed to establish data-driven ML models with Bayesian optimization. The models were applied to simulate river stage hydrographs of the tidal reach of the Lan-Yang River Basin in Northeastern Taiwan. Historical measurements of rainfall, river stages, and tidal levels were collected from 2004 to 2017 and used for training and validation of the four models. Four scenarios were used to investigate the effect of the combinations of input variables on river stage predictions. The results indicated that (1) the tidal level at a previous stage significantly affected the prediction results, (2) the LGBMR model achieves more favorable prediction performance than the SVR, RFR, and MLPR models, and (3) the LGBMR model could efficiently and accurately predict the 1–6-h river stage in the tidal river. This study provides an extensive and insightful comparison of four data-driven ML models for river stage forecasting that can be helpful for model selection and flood mitigation.

A pluvial flash flood is rapid flooding induced by intense rainfall associated with a severe weather system, such as thunderstorms or typhoons. Additionally, topography, ground cover, and soil conditions also account for the occurrence of pluvial flash floods. Pluvial flash floods are among the most devastating natural disasters that occur in Taiwan, and these floods always /occur within a few minutes or hours of excessive rainfall. Pluvial flash floods usually threaten large plain areas with high population densities, therefore, there is a great need to implement an operational high-performance forecasting system for pluvial flash flood mitigation and evacuation decisions. This study developed a high-performance two-dimensional hydrodynamic model based on the finite-element method and unstructured grids. The operational high-performance forecasting system is composed of the Weather Research and Forecasting (WRF) model, the Storm Water Management Model (SWMM), a two-dimensional hydrodynamic model, and a map-oriented visualization tool. The forecasting system employs digital elevation data with a 1-m resolution to simulate city-scale pluvial flash floods. The extent of flooding during historical inundation events derived from the forecasting system agrees well with the surveyed data for plain areas in southwestern Taiwan. The entire process of the operational high-performance forecasting system prediction of pluvial flash floods in the subsequent 24 h is accomplished within 8–10 min, and forecasts are updated every six hours.

Antecedent rainfall plays an important role in rainfall-induced mass movements. However, it is difficult to define the appropriate period of antecedent rainfall for mass movement assessment. To solve this problem, this study provides a simple approach that combines calibrated antecedent rainfall (CAR) and 24-h rainfall for 283 mass movements that occurred in Taiwan from 2006 to 2013. The 24-h rainfall at the time of each mass movement was compared with the total cumulative rainfall for various periods preceding the event. The lowest correlation was found for the total cumulative rainfall from 15 to 20 days before a mass movement day. The 24-h rainfall was compared with the cumulative CAR values for various days of antecedent rainfall. The effect of cumulative CAR on mass movements increased from 22.0 to 39.7% when the number of days considered was increased from 3 to 30 days. However, the increase became gradual after 15–18 days. In addition, the critical antecedent rainfall conditions occurred within 18 days before mass movements in all cases. These results suggest that the antecedent rainfall of 15–18 days is useful for mass movement assessment in Taiwan. This study also established a critical antecedent rainfall threshold for mass movements in Taiwan that is useful for early warnings: Ia = 28.7 D a −1.24 , where Ia is critical mean rainfall intensity during the antecedent rainfall period up to 18 days (mm/day) and Da is the length of the antecedent rainfall period. According to the relationship between 24-h rainfall and the critical antecedent rainfall conditions, low antecedent rainfall intensity that continues for a long time leads to a gradual increase in soil moisture such that a small amount of 24-h rainfall can trigger mass movements. On the other hand, high antecedent rainfall intensity for a short time is not enough to increase soil moisture, and a large amount of 24-h rainfall is needed to flush surface materials and cause mass movements.

Fluctuations in the sediment yielding rate within mountainous areas are found after extreme precipitations. These changes are associated with rainfall-driven landslides and can be evaluated through observations of sediment transported in river water. In this study, we assess the differential impact level and duration of an extreme rainfall event caused by the 2009 Typhoon Morakot in Taiwan. Using a time series of suspended sediment data and landslide inventories within three affected catchments, our results found that the proportion of the typhoon-generated sediment discharges reached 58% of the 2009 annual sediment discharges, and the annual sediment discharges could be 15-fold higher than the average annual sediment discharges prior to the typhoon. The impact of the typhoon on suspended sediment discharge lasted for around four months. It is further revealed that a significant increase in the number of landslides was attributed to the impact of the extreme typhoon event, and the critical rainfall condition triggering landslides, based on the relationship between rainfall and landslide number and the rainfall intensity-duration thresholds, declined for 4–5 years.

The tidal current power (TCP) resource, the impact of TCP extraction on hydrodynamics and the influence of sea-level rise (SLR) on TCP output in the coastal waters of Kinmen Island (Taiwan) are investigated using a state-of-the-art unstructured-grid depth-integrated numerical model. The model was driven by eight tidal constituents extracted from a global tidal prediction model and verified with time series of measured data for tide level and depth-averaged current. The simulations showed reasonable agreement with the observations; the skill index was in the excellent (0.71–0.93) range with regard to simulating tide level and currents. Model predictions indicated that the channel between Kinmen and Lieyu serves as an appropriate site for deploying the tidal turbines because of its higher tidal current and deeper water depth. The bottom friction approach was utilized to compute the average TCP over a spring-neap cycle (i.e., 15 days). Mean TCP reached its maximum to 45.51 kW for a coverage area of 0.036 km2 when an additional turbine friction coefficient (Ct) increased to 0.08, and a cut-in speed of 0.5 m/s was used. The annual TCP output was estimated to be 1.08 MW. The impact of TCP extraction on the change in current is significant, with a maximum reduction rate of instant current exceeding 60%, and the extent of influence for the average current is 1.26 km in length and 0.30 km in width for the −0.05 m/s contour line. However, the impact of TCP extraction on the change in tide level is insignificant; the maximum change in amplitude is only 0.73 cm for the K2 tide. The influence of SLR on the TCP output in Kinmen waters was also estimated. Modeling assessments showed that due to SLR produces faster tidal current, the annual TCP output increased to 1.52 MW, 2.01 MW, 2.48 MW and 2.97 MW under the same cut-in speed and coverage area conditions when SLR 0.25 m, SLR 0.5 m, SLR 0.75 m and SLR 1.0 m were imposed on the model.

On August 31st, 2013, high intensity rainfall caused a severe rockfall disaster in Badouzih, Keelung. This study combines the results of detailed field investigation and process-based as well as empirical-based method to point out unsafe road sections. The mechanisms and the characteristics of motions of rockfall are analyzed in the field. Two intersecting joints and the bedding plane divided bedrock into blocks. Weathering and the infiltration of rainfall accelerated the erosion process and precipitated ruptures. When the rainfall reached its highest intensity which is 94.5 mm/h in this event, a great deal of water on the slope surface eroded the matrix, causing instability and leading to the failure of the rock blocks. The process of rockfall took 23 s and can be divisible into 4 stages including roll, fall, bounce, and roll. Process-based simulations are adopted in this case and several critical road sections to understand the characteristic of movements. With the empirical-based energy line angle analysis, potential road sections which may affect by rockfall are showed and their energy are also estimated for the purpose of mitigating rockfall impact, reducing similar disasters in this area.

Wind field data from the National Centers for Environmental Prediction Climate Forecast System Version 2 (CFSV2), the European Centre for Medium-Range Weather Forecasts ERA-Interim and ERA5 and the ERA5 blended into the parametric typhoon model (ERA5H) from September 1 to 30, 2016, were adopted to drive a fully coupled circulation-wave model. The best performance of the maximum storm tide and significant wave height (SWH) and mean wave period simulation for typhoon events was found by applying the ERA5H. The contributions of nonlinear interactions to the storm tide induced by Super Typhoon Meranti in 2016 were quantified using seven scenario simulations. The results indicated that the wave setup had the most significant nonlinear interaction with the storm tide, which was up to 0.97 m both with and without tidal effects. The wave-tide interaction was relatively minor for the total water level and contributed only ± 0.1 m to the storm tide. Significant differences in the tide-surge interactions were detected with and without wave effects, as these interactions provided approximately 0.06–0.08 m and ± 0.02 m of the storm tide, respectively. The spatial distributions of the maximum SWH for typhoon events were similar when coupled and decoupled models were employed. In this paper, it is concluded that storm surge miscalculation can be avoided if wave setups are included when predicting typhoon-generated storm tides.

This study examined how to effectively implement the Sendai Framework for Disaster Risk Reduction 2015–2030 by inspecting disaster governance strategies for persons with disabilities in Taiwan. We identified priorities of the Sendai Framework that especially mention persons with disabilities or related stakeholders, including Priorities 24(a), 24(g), 24(h), 27(g), 27(h), 30(j), 30(k), 33(b), and 33(i). A set of questions was designed according to these priorities and distributed to all central ministries, agencies, and local governments related to disaster governance in Taiwan. The results showed that strategies currently cover the issues of evacuation, shelter operation, facility operation for persons with disabilities, electricity backup during emergency, strategies for schools, and disaster recovery and reconstruction. We found that the present disaster governance strategies for persons with disabilities in Taiwan are focused on the preparedness and emergency response phases, and most strategies utilize a top-down approach. Based on these findings, we suggest the development of more strategies for the recovery and reconstruction phases and the incorporation of bottom-up mechanisms such as stakeholder participation, dialogue and cooperation, coordination forums, empowerment, and consultation with stakeholders, as discussed in Priorities 24(h), 27(g), 30(j), and 33(i) of the Sendai Framework.

Large waves generated by typhoons can pose a serious threat to human life and property in the coastal areas of Taiwan. We hypothesize that an accurate hindcast of typhoon-generated significant wave heights (SWHs) is highly dependent on an accurate typhoon wind field. This paper evaluated the performance of SWH modeling for typhoons on the northeastern coast of Taiwan using different wind fields and a fully coupled tide-surge-wave model. Wind products from the Climate Forecast System Reanalysis (CFSR), ERA-Interim (ERA), and Cross-Calibrated Multi-Platform V2 (CCMPV2) were blended into the parametric typhoon models (hybrid wind models) to serve as the main forcing in the fully coupled model. The results of model-data comparison for SWHs reveal that the hindcasts only using the CFSR, ERA and CCMPV2 own the excellent skill indexes ranging from 0.92 to 0.94. The use of hybrid wind models indicated a consistent improvement in SWH hindcasting, and the CCMPV2 combined with the parametric typhoon model has the best overall performance. However, the extremely large typhoon-induced SWHs (exceeding 11 m) can only be well-matched when the CFSR merged with the parametric typhoon model was imposed on the fully coupled model.

The study collected water samples from three rivers in southern Taiwan to understand the temporal and spatial variations of major dissolved ions, including Na + , K + , Mg 2+ , Ca 2+ , Cl - , SO 4 2- -, HCO 3 - and H 4 SiO 4 , and to explore the relationships between chemical and physical weathering in an orogenic and tropical area. During 2006-2007, total dissolved solid discharge (TDS discharge) from the investigated rivers was calculated to be around 3.27 Mt/yr, and 60 % of dissolved ions were attributable to the outcrop of silicate. However, TDS discharge occupied just 3 % of total delivered material in the river water. Accordingly, physical transport plays a decisive role in weathering process with only a small fraction capable of going through chemical dissolution in reaching the river. The investigation results also show that the huge amount of water discharge in rainy seasons dilute ion concentrations, and that sea spray delivered by monsoon and compositions or properties of source rocks also induce spatial variations of dissolved ions. Additionally, areas with low strength rock have more dissolved material in the river water.

Summary The combination of abundant precipitation and frequent earthquakes in Taiwan, an orogenic, subtropical island, results in huge amounts of sediment discharge delivered into river channels and provides high potential for turbidity current, a kind of extreme hydro-phenomenon characterized by high turbidity and flow velocity. This study collected hydrometric data from 11 rivers around Taiwan, and applied statistical methods to estimate the recurrence interval ( RI ) of hyper-concentration flow. Analysis results revealed that RI of hyper-concentration flow was dominated mainly by the magnitude of the drainage area and the geological environment within the catchments. The catchments in southwestern Taiwan with relatively larger areas and/or frequent landsliding have comparatively shorter RI s of RI of daily rainfall with the RI of hyper-concentration flow reveals the possible rainfall threshold for hyper-concentration flow, and the application of rainfall threshold for hyper-concentration flow is well demonstrated in the catchments. The average rainfall threshold for the whole of Taiwan is 510 mm/day, and is positively related with geomaterial properties within the catchments.

[1] Distributed activity of geomorphic processes with different spatiotemporal scales is hard to monitor in detail with conventional methods but might be detected with seismometers. From July to September 2010, we deployed 14 seismometers to evaluate the ability of a two-dimensional array with small interstation distances (11 km) to continuously monitor geomorphic processes in a mountain catchment (370 km2) in Taiwan. Spectral analysis of seismic records highlights different sources with high-frequency content (>1 Hz), consistent with hillslope and river processes. Using a common detection algorithm and a location technique based on the timing of seismic amplitude, we have located 314 near-surface events, most of which (69%) occurred during daily convective storms. Event activity was positively correlated with the precipitation intensity, but this relation was not uniform in the catchment. High-resolution satellite images and air photos did not show geomorphic change during the study, which did not have any episodes of extreme precipitation. A majority of events (61%) were collocated with preexisting geomorphic features (landslide scars, gullies) within the uncertainty on location (9% of interstation distance). The combination of event location and timing suggests a geomorphic source of recorded signals and most events had the seismic characteristics of rockfall, debris avalanches, or slides. Reactivation of prior erosion sites by such processes is difficult to detect with imagery, but can possibly be resolved by seismic monitoring. When proven, this approach will allow a spatially comprehensive survey of geomorphic activity at the catchment scale, with temporal detail sufficient to evaluate the exact (meteorological) conditions under which process events occur.

Hazards from gravity-driven instabilities on hillslope (termed ‘landquake’ in this study) are an important problem facing us today. Rapid detection of landquake events is crucial for hazard mitigation and emergency response. Based on the real-time broadband data in Taiwan, we have developed a near real-time landquake monitoring system, which is a fully automatic process based on waveform inversion that yields source information (e.g., location and mechanism) and identifies the landquake source by examining waveform fitness for different types of source mechanisms. This system has been successfully tested offline using seismic records during the passage of the 2009 Typhoon Morakot in Taiwan and has been in online operation during the typhoon season in 2015. In practice, certain levels of station coverage (station gap 106 m3 and area > 0.20 km2) are required to ensure good performance (fitness > 0.6 for successful source identification) of the system, which can be readily implemented in other places in the world with real-time seismic networks and high landquake activities.

Summary Heat-pulse flowmeter can be used to measure low flow velocities in a borehole; however, bias in the results due to measurement error is often encountered. A carefully designed water circulation system was established in the laboratory to evaluate the accuracy and precision of flow velocity measured by heat-pulse flowmeter in various conditions. Test results indicated that the coefficient of variation for repeated measurements, ranging from 0.4% to 5.8%, tends to increase with flow velocity. The measurement error increases from 4.6% to 94.4% as the average flow velocity decreases from 1.37 cm/s to 0.18 cm/s. We found that the error resulted primarily from free convection and frictional loss. Free convection plays an important role in heat transport at low flow velocities. Frictional effect varies with the position of measurement and geometric shape of the inlet and flow-through cell of the flowmeter. Based on the laboratory test data, a calibration equation for the measured flow velocity was derived by the least-squares regression analysis. When the flowmeter is used with a diverter, the range of measured flow velocity can be extended, but the measurement error and the coefficient of variation due to friction increase significantly. At higher velocities under turbulent flow conditions, the measurement error is greater than 100%. Our laboratory experimental results suggested that, to avoid a large error, the heat-pulse flowmeter measurement is better conducted in laminar flow and the effect of free convection should be eliminated at any flow velocities. Field measurement of the vertical flow velocity using the heat-pulse flowmeter was tested in a monitoring well. The calibration of measured velocities not only improved the contrast in hydraulic conductivity between permeable and less permeable layers, but also corrected the inconsistency between the pumping rate and the measured flow rate. We identified two highly permeable sections where the horizontal hydraulic conductivity is 3.7–6.4 times of the equivalent hydraulic conductivity obtained from the pumping test. The field test results indicated that, with a proper calibration, the flowmeter measurement is capable of characterizing the vertical distribution of preferential flow or hydraulic conductivity.

In tropical mountainous areas, typhoon-induced rainstorms trigger extensive landslides and deliver huge amounts of sediment to river basins. Two mountainous catchments in central Taiwan, with 0.8–13.5% of their catchment areas disturbed by landsliding induced by a series of typhoons, were selected for the purpose of quantifying the impact of typhoon-induced rainfall kinetic energy on landslides and sediment transport during 1994–2008. Distribution analysis of the rainfall kinetic energy required to trigger landslides revealed the threshold at which rainfall kinetic energy Ek induces landslides in the mountainous catchments of central Taiwan to be approximately 2000 J m− 2. The present study demonstrates that higher rainfall kinetic energy causes more sediment to be delivered to rivers, and finds a positive relationship between Ek and sediment transfer during typhoon periods. We postulate that real-time monitoring of rainstorms can be used to predict the impact of typhoon events by applying the threshold of rainfall kinetic energy needed to trigger landslides and transport of huge amounts of sediment.

From 1996 through 2007, several heavy typhoons and earthquakes precipitated landslide hazards in southeast Taiwan. In the present study, we analyze the impact of topography, lithology, rainfall and earthquakes on landsliding and sediment transport by quantifying the landslide ratio (ratio of landslide area to catchment area) and the sediment discharge for the Sinwulu and Luye upstream catchments of the Peinan River. The steep topography in these two catchments causes a large proportion of landslide ratios on slopes to exceeding 50°, a condition which accelerates the delivery of landslide debris to the downslope channel. The landslide ratios of 0.84%–2.30% are obviously related to the rock strength and discontinuity density of the terrain, but we find the magnitude of landsliding and sediment discharge to vary with the occurrence of earthquakes. Typhoon-induced rainstorms generate the bulk of sediment discharge in the study catchments, though the landslide debris is transported by the huge amounts of runoff produced by rainstorms. A contributing factor is the recent occurrence of severe earthquakes which weakened the geomaterial, leading in turn to further increases of fluvial sediment during subsequent typhoons. Additionally, the finding that eventual reduction of unit sediment concentration after high magnitude earthquake events reveals that the downslope transport of landslide debris associated with earthquake activity and subsequent typhoon events precipitates a relatively short term, 1–3 year, increase of erosion rates in these two study areas in southeastern Taiwan.

Typhoon-triggered landslides deliver huge amounts of sediment to the upstream channel of the Shihmen Reservoir in northern Taiwan. Observation data regarding landsliding, sediment discharge and water turbidity following five major typhoon events from 1985 to 2006 demonstrated that each time water turbidity in the reservoir area rapidly increased up to ten-fold from the river catchment drainage, and the weight of landslide debris exceeded total sediment discharge five-fold. The fact that huge amounts of landslide debris still remained on upstream slopes and water turbidity suddenly increased in the reservoir area but not in upstream channel implied that the increasing water turbidity in the Shihmen Reservoir was indirectly related to the large landslides occurring in the upstream catchment. The main cause of high turbidity in the reservoir area was that, during a typhoon event, high water discharge flowing into the reservoir scoured the fine fraction sediment at the bottom of the reservoir and formed hyperpycanl flow with high turbidity, which then ascended to contaminate the reservoir surface water.