Chjeng Lun Shieh

Impact of Chi-Chi earthquake on the occurrence of landslides and debris flows: Example from the Chenyulan River watershed, Nantou, Taiwan

Abstract The Chenyulan River watershed in central Taiwan was chosen for evaluating the impact of the Chi-Chi earthquake on the occurrence of landslides, and for exploring the initial conditions triggering debris flows. Interpretations of aerial photographs and SPOT images as well as field investigations were used to identify landslide and debris flow occurrences. Precipitation data were then utilized to investigate critical conditions leading to the debris flow. Comparison of five SPOT images taken between June 1996 (before the July 1996 Typhoon Herb) and September 1999 (before the Chi-Chi earthquake) shows that the landslide area increased from 7.1×106 to 7.5×106 m2 during that time. However, by January 2000 (after the Chi-Chi earthquake), the landslide area almost tripled to 20.8×106 m2. Measurement of images taken in March and August 2001 reveals that the landslide area had further expanded to 24.2×106 and 27.5×106 m2, respectively. Significant differences before and after the earthquake are also noticed in (1) the intensity and amount of precipitation required for triggering debris flow, (2) the size of contributing drainage basin in which debris flows occurred, and (3) the frequency of debris flows. After the Chi-Chi earthquake, maximum hourly rainfall intensity and critical accumulated precipitation necessary to initiate debris flow reduced to as low as 1/3 of the pre-earthquake figures. Prior to the Chi-Chi earthquake, most debris flows occurred in gullies with slopes greater than 10°, and in drainage basins larger than 0.1 km2. Conversely, after the earthquake, debris flows were observed even in gullies with effective drainage area smaller than 0.03 km2. Furthermore, before the earthquake, the debris flow recurrence time in the study area was greater than 5 years, whereas six debris flow events have been observed in the 2 years since the earthquake.

Quantity, distribution, and impacts of coastal driftwood triggered by a typhoon

Typhoon Morakot pounded Taiwan in 2009 with record-breaking rainfall, washing an unprecedented amount of driftwood into the sea that was partially deposited at the coastal areas. According to the satellite imagery analysis, more than three million trees fell and were washed away to occupy 83.2% of the Taiwanese coastline, including 52 fishing harbors. The amount cleaned-up was only 1/7 of the total coastal driftwood. It was found that the amount of coastal driftwood is not only related to the amount of precipitation but is also related to the distance from the location of the landslide to the river mouth and to the landslide area. The amount of accumulated coastal driftwood demonstrated log-profile declines with increasing distance to the river mouth. Nearshore current and wave motion are the critical factors for driftwood deposition. Much of the driftwood washed into the sea harmed the tourism and fishing industries, endangered navigation and oceanic activities, and impacted the marine environment and ecosystem.

Impulsive force of debris flow on a curved dam

Abstract Although Sabo dams are an efficient method for river and basin management, traditional Sabo dams have a great impact on ecology and landscape. Moreover, such dams are hit and often damaged by great impulsive force when they block the debris flow. Therefore, alternative shapes for Sabo dam deserve thorough investigation. In this investigation, a curved dam was designed by changing the upstream-dam-surface geometric shape to reduce the impulsive force of the debris flow, with enhanced stability and reduced concrete mass being the anticipated outcomes. In this study, the flume and laboratory facilities simulated the impulsive force of the debris flow to the Sabo dams. Three geometric forms, including vertical, slanted and curved Sabo dams, were used to determine the impulsive force. Impulsive force theories of the debris flow were derived from the momentum equation and the Bernoulli equation. In these, the impulsive force was balanced by the friction force of the Sabo dam and the opposite force of the load cell behind the dam as it was hit by the debris flow. Positive correlations were found when comparing the experimental data with the theoretical results. These findings suggest that our impulsive force theory has predictive validity with regard to the experimental data. The results from both theory and experimental data clearly show that curved dams were sustained less force than the other dams under the same debris flow. This comparison demonstrates the importance of curved geometry for a well-designed Sabo dam.

Effects of seismic ground motion and geological setting on the coseismic groundwater level changes caused by the 1999 Chi-Chi earthquake, Taiwan

The groundwater level changes induced by the 1999 Chi-Chi earthquake were well recorded at the monitoring wells in and around the Choshui River alluvial fan, Taiwan, which is adjacent to the focal region. We analyzed the coseismic groundwater level changes related to the geological setting and seismic ground motion. In a typical fan area, the groundwater levels coseismically rose and those amplitudes increased as the ground acceleration and hydraulic conductivity became larger. In the slope area near the earthquake fault, the groundwater levels coseismically dropped and those amplitudes increased as the ground acceleration became larger. The liquefaction and permeability enhancement, whose degrees depend on the geological setting and seismic ground motion, might explain the characteristics of the coseismic groundwater level changes in the Choshui River alluvial fan.

Classification of non-vegetated areas using Formosat-2 high spatiotemporal imagery: the case of Tseng-Wen Reservoir catchment area Taiwan

The occurrence of landslides in the catchment area is a potential threat to the water quality and the lifespan of a reservoir. Due to the limitations of spatial coverage in ground surveys and of temporal resolution in aerial photos, it is difficult to monitor such events in the entire catchment area at short intervals. Formosat-2 is the first commercial satellite dedicated to site surveillance with a high-spatial-resolution sensor placed in a daily revisit orbit (2 m in panchromatic and 8 m in multi-spectral). In this research, a new approach is proposed to identify the non-vegetated areas in the multi-temporal and multi-spectral images taken by Formosat-2 by integrating the Getis statistic, the spectral index and the unsupervised K-means classification. With this new approach, we analyse a total of 16 pairs of Formosat-2 images, taken in the catchment area of Tseng-Wen Reservoir from February to December 2006 at an interval of three to four weeks. The results show that newly developed non-vegetated areas are closely related to earthquakes and rainfall. Once the slump material is generated by an earthquake, a comparatively low amount of rainfall will trigger its flushing. However, once the slump material has gone, there are no significant changes in the non-vegetated areas, even with severe weather events such as typhoons or storms. This suggests that the most critical time for protecting the reservoir is right after an earthquake and before the next rain. If the slump material is not managed or removed during this crucial period of time, eventually it will fall into the reservoir. Since the catchment area of Tseng-Wen Reservoir is protected and restricted from access, most of the non-vegetated areas should be closely related to landslides caused by natural processes (such as rainfall or earthquake) rather than man-made processes (such as tree cutting or degradation of vegetation). This research demonstrates the potential of Formosat-2 imagery in monitoring the spatial and temporal variations of landslides in the catchment areas of reservoirs.

Rapid geometry analysis for earthquake-induced and rainfall-induced landslide dams in Taiwan

Stability analysis of the dam is important for disaster prevention and reduction. The dam’s geometry plays an important role in understanding its stability. This study develops a rapid landslide dam geometry assessment method for both earthquake-induced and rainfall-induced landslide dams based on nine real cases collected in Chinese Taipei and 214 cases collected worldwide. For simplification purposes, a landslide dam is classified into triangular or trapezoidal. The rapid landslide dam geometry assessment method in this paper uses only satellite maps and the topographic maps to get landslide area, and then analyze the dam geometry. These maps are used to evaluate the area of the landslide and the slope of the river bed. Based on the evaluation information, the proposed method can calculate dam height, the length of the dam, and the angles of the dam in both upstream and downstream directions. These geometry parameters of a landslide dam provide important information for further dam stability analysis. The proposed methodology is applied to a real landslide dam case at Hsiaolin Village. The result shows that the proposed method can be used to assess the landslide dam geometry.

Experiments on channel evolution caused by check-dam failure

A 10 m long, 0.2 m wide flume was employed to simulate the channel bed evolution of check-dam failure. The experiment longitudinal profiles, the gradient of channel bed, head-cutting propagation distance and deposition length were compared with the theoretical solution derived from a sediment transport diffusion equation. In contrast with the theoretical solution, two different gradients were obtained upstream and downstream of the check-dam. The theoretical solution provides a good description of the changes upstream of the check-dam. The ratio of clear water depth to sediment moving layer thickness in the experiment was analyzed and showed that high concentration sediment laden flow was taken in the incipient of check-dam failure, which may be the reason why the experiment result was slightly different from the theoretical solution in the downstream of check-dam.

Sediment control function of river notches

The purpose of this study is to investigate the control function and mechanisms of natural river notches. Physical and numerical experiments are analyzed in this study for two representative types of sediment events: high intensity and short duration Type A sediment disaster events, and low intensity and long duration Type B moderate non-disaster events. Two dimensionless parameters, sediment trapping rate and reduction rate of peak sediment transport, are defined to evaluate the sediment control function of river notches. Study results indicate that the contraction ratio of the notch has a significant influence on sediment control function, with high contraction ratios resulting in both high sediment-trapping and high reduction rates. River notches provide better sediment control during Type A events than Type B events. The sediment control mechanism of river notches is the result of multiple interactions among river flow, sediment transport, and riverbed variation. Analysis of these interactions supports the significant protection role of river notches on sediment control for disaster events