Toyohiko Miyagi

Coastal erosion due to long-term human impact on mangrove forests

A coast in southern Vietnam, which is located in a wide and flat alluvial fan and neighbors tidal rivers fringed by wide mangrove swamps, has been eroded continuously by approximately 50 m/year since the early 20th century. Based on field observations and numerical experiments, it is inferred that this large scale erosion is caused by the transition of mangrove vegetation resulting from the long-term impact of humans since the late 19th century. This eroded coast is not in direct contact with mangrove swamps, but is strongly affected by the existence of mangrove forests through the intermediation of neighboring tidal rivers. Thus, with a view to coastal protection, it is argued that the mangrove vegetation in adjacent areas should be managed more sensitively.

Mangrove habitat formation and response to Holocene sea-level changes on Kosrae Island, Micronesia

Mangrove habitats on Kosrae are divided into three types, i.e., an estuary or delta type, a backmarsh or lagoon type and a coral reef or tidal-flat type. Most of the mangrove forests of Kosrae have been developed during the last 2000 years by accumulating mangrove peat with the gradual sea-level rise of 1 to 2 mm/yr except the landward part of the estuary or delta type. On the other hand, during the period of rapid sea-level rise of about 10 mm/yr between 4100 and 3700 yr B.P., the mangrove forests ceased peat accumulation and retreated landward. Until 3500 yr B.P., mangrove forests were distributed only in narrow bands in the inlets. Therefore, the critical rate of mangrove peat accretion with sea-level rise is estimated at more than 2 mm/yr and less than 10 mm/yr. If the anticipated sea-level rise exceeds this critical rate, all of the mangrove forests of Kosrae will retreat landward and reduce rapidly.

The Simulation of a Deep Large-Scale Landslide Near Aratozawa Dam Using a 3.0 MPa Undrained Dynamic Loading Ring Shear Apparatus

The translational block glide of deep and large-scale landslide near the Aratozawa dam that occurred in the middle of 2008 is of great importance for detailed study. Aratozawa landslide had resulted from landform deformation and the subsequent change of watershed geomorphology at the upstream part of the Aratozawa reservoir. The evidence of this phenomena was revealed during a site investigation in November 2012, as several natural reservoirs (lakes) formed in the cavities between ridges and the depression zone in main block slide. Aratozawa landslide located in the Ohu Mountains basically was triggered by the earthquake which had a peak ground acceleration of more than 1,000 gal. In addition, the possibility of reactivated landslides in surrounding terrains near Aratozawa dam has resulted in the hypothesis that the 2008 event was one sequence of the dynamic-geomorphological activity in this mountainous area within a period of hundred years. In this paper, the mechanism of the deep and large-scale landslide near Aratozawa dam is analysed through a physical laboratory experiment. Deep landslide simulation is conducted by applying high normal stress to address the assumed slip surface of 150 m depth in the Aratozawa case. The 3.0 MPa undrained dynamic loading ring shear apparatus with the high pore-water pressure controlled is used to meet the criteria of deep-seated landslide of Aratozawa. The effect of groundwater fluctuation and the inter-linkage with the reservoir in the Aratozawa dam was found to be the main concern besides the peak ground acceleration based on the 2008 landslide event. Results also show that there was no significant deformation in the Aratozawa dam area when the large Tohoku earthquake, magnitude 9.0, in 2011. Indication is, that the slide blocks, ridges and mass depression due to the 2008 event are already stable. However, the slope and soil mass movement are still possible in the future.

Landslide Inventory in the Area of Zagreb City: Effectiveness of Using LiDAR DEM

Preliminary results of landslide mapping in the City of Zagreb (Croatia), obtained in the frame of the Japanese-Croatian scientific project, are presented in this paper. The aim of this research is to develop a method for landslide delineation in order to enable land use officials to implement this data to create more useful measures for landslide risk management. Selected landslides in the hilly zone of Mt. Medvednica were identified visually using LiDAR bare-earth DEMs. The results of data analysis will be implemented to perform a more comprehensive study of landslides in the entire pilot area (total area is 180km2).

Detection of Active Landslide Zone from Aerial Photograph Interpretation and Field Survey in Central Provinces of Vietnam

Landslides are destructive and an annually recurring phenomena which cause disruption of traffic and fatalities along transport arteries in Vietnam, especially in central provinces of Vietnam. These landslides are caused by deep weathering processes, high precipitation and cut slopes. This paper presents a summary of our findings on some landslides in central provinces of Vietnam based on aerial photography interpretation and field surveys. It covers: (1) landform deformation features, (2) types, sizes and dynamic characteristics of moving masses,(3) geologic structure, (4) active zonation.

Landslide Risk Evaluation in Central Provinces of Vietnam

Open image in new window In central provinces of Vietnam, landslides are hazardous phenomena that occur frequently, destroying human life, damaging structures, infrastructure and adversely affecting living conditions. Assessing landslide risks (probability of landslide re-occurrence) poses a difficult challenge for all Vietnamese scientists and civil managers. For risk evaluation, we applied Japan Landslide Society’s Inspection sheet to study area with 261 landslide units were chosen for evaluation and developed 6 sheets of risk map. But this sheet shows geomorphological factors only, it does not mention geologic structures and weathering. The study area has a varied geologic composition with many stratigraphic unit and strata, original rocks have been found from Precambrian to Quaternary. Fieldwork shows that geologic conditions (such as geology age, bedrock lithology, bedrock structure, level of weathering) must play an important role in landslide occurrences in Vietnam. The authors propose and presume a new integrated inspection sheet that will combine two components: the first is geomorphology (as mentioned in the Japan Landslide Society’s Inspection sheet); the second is geology. With this component, it is also classified into large, medium and small categories, similar to the Japan Landslide Society’s Inspection sheet. A standard score system was created for the new integrated inspection sheet. Five landslide locations were trial application. Results present few differences when applying Japan Landslide Society’s Inspection sheet and new integrated inspection sheet. The new inspection sheet is the initial stage of development, presenting some limitations and should be discussed much more.

Risk Evaluation of Landslide Topographic Area by Aerial Photointerpretation

Landslide topographies are widely distributed in Japan and amount to more than 300,000 sits. The area has potential for various kinds of slope disaster such as reactivation, and high potential of slope failure because of the weak geology. Some landslide deposits are stable already.

Analysis of a Deep-Seated Landslide in the Phan Me Coal Mining Dump Site, Thai Nguyen Province, Vietnam

A large landslide occurred at the Pha Me coal mining dump site at 4:20 AM on 15 April 2012, buried a huge area, including tens of houses and seven persons. There was no abnormal weather or seismic activity at the time of the landslide. A joint work between Vietnamese and Japanese experts was carried out to investigate characteristics and reasons of the landslide. The achieved results show that coal mining wastes are disposed of on low hill sites where granitic bedrock was intensively crushed due to tectonic activity. Weathering crusts include rich clays of over 15–20 m in thickness. The landslide has a volume of about 2.5 million m3, with a slip surface cut through weathering soils at a depth of about 10 m. The scarp of the landslide departs at an approximate elevation of 85–100 m. Travel distance is 300–350 m. Sliding materials are primarily mining wastes. However the sliding surface is defined to be situated at the depth of 12–15 m in the residual soils. There are two significant causes of the disaster. Firstly, the waste dump site plays a role as a water-storage layer which keeps residual soils permanently saturated. The second cause of the deep-seated landslide is over-loading of mining wastes. Prior evidences of the landslide such as cracks at the top, heave at the trough of the dump site were recognized a week before, however they were not seriously considered.

TXT-tool 1.081-2.1: Landslide Mapping Through the Interpretation of Aerial Photographs

Topography created by landslides is widely distributed. Landslide risk evaluation is very important for mitigating or preventing these natural disasters, and finding facts can be very difficult. We here describe the techniques for aerial photo interpretation and explain the results. Sometimes, we “can’t see the forest for the trees.” This manual will clearly and practically explain the following: (1) why stereo pair interpretation is necessary; (2) how to perform stereo pair interpretation with aerial photos etc. to determine the landslide area; (3) what can be learned using this method of interpretation; (4) how landslide motion characteristics can be ascertained once interpretation of landslide micro-topography is performed; (5) once this is ascertained, how to rationalize landslide surveys and countermeasures.

TXT-tool 1.081-2.2: Landslide Mapping Through the Interpretation of Aerial Photographs and Topographic Maps

Processes of formation of topography and slopes are the result of a comprehensive combination of climatic change and crustal movement. When examining long time-scales, it is necessary to examine variations in internal and external stresses to understand the formative history of the topography. In this manual, the steps for understanding landslides will be described. The concept and frameworks for identifying unstable slopes (landslide topography) using aerial photos and topographic maps are introduced in details.