When waves are obliquely incident at a large angle over 45° to the direction normal to the shoreline in a slender water body with a large aspect ratio, a small perturbation on the shoreline may develop owing to the high-angle wave instability mechanism (Ashton et al., 2001), resulting in the formation of sand spits along the shoreline. Serizawa et al. (2012) explained the development of a group of sand spits and cuspate forelands owing to this mechanism using the BG model (a model for predicting 3-D beach changes based on Bagnold’s concept), when waves were obliquely incident at an angle over 45° to the direction normal to the shoreline. However, the examples of the development of sand spits owing to the high-angle waves in the field were lacking, because the angle of incident waves relative to the direction normal to the shoreline generally has a value within ±20° at most. Lingayen Bay in Philippine, which faces the South China Sea, is a bay of a large aspect ratio, so the shoreline instability owing to the high-angle waves occurs along the coastline in the bay, and it significantly affects the shore protection along the coast. In this study, the shoreline instability along the river delta coasts around the Balili and Aringay Rivers flowing into Lingayen Bay, and the sand spit in the bay was investigated using satellite images with the field observation on February 23, 2018. The observed shoreline changes south of the Aringay River mouth were compared with the previous study regarding the development of a sand spit by Uda et al. (2013).

As an introductory lecture on the role of geomorphology in the prevention of sediment disasters, we first demonstrate the map reading of the geomorphic settings for main sediment disasters triggered by cut-slope failure, rockfall, slump, and debrisflow. Second, we outline the reference system called EADaS, which is a software to predict geomorphic disasters at a given site in Japan by examining the combination of Environment, Agent, Disaster mode and Structure at a target site. EADaS lists 225 questions and 1,450 answers for specific qualities of the environment. It correlates 1,450 choices and the frequency of the 90 kinds of possible disaster agents at each environment, i.e. 133,470 combinations of E and A. The user of the EADaS, however, does not have to answer all the questions, because questions with no relevance to the site are skipped automatically. The result of the EADaS is shown as a list of the potential risk points of 90 kinds of disaster agents at the target site.

Though the contour map is nothing but a quantitative presentation of landform, morphometry has been applied it to obtain such particular features of landform as the slope gradient, the relief energy, the summit level, the valley density, etc. Importance of these quantitative properties given by morphometry have been remarked in Sabo branch in Japan, as these quantities concern the mass movement which often brings serious disaster. From this point of view, it’s appreciated now to apply a new method of morphometry covering the equivalent slope length and the texture or roughness of land surface, in addition to the slope gradient and the Laplacian, to the detailed contour map or DEM by LiDAR measurement. Especially the equivalent slope length concerns the concentration of surface water and the thickness of weathered zone on the slopes. The elevation change not limited to the gigantic but in the usual magnitude is also detected using the detailed DEM before and after the unit landmass movement, and the result serves the new quantitative parameters, mean travel distance and standard diffusivity, characterizing the unit mass movement under consideration.

Basic principles of theoretical geomorphology in application to the analysis of debris hazards are described, following the footsteps of the late Dr. Setsuo Okuda, a renowned theoretical geomorphologist and the first president of JGU, who died in 2017. The foundation of the Laboratory of Applied Geomorphology in the Disaster Prevention Research Institute, Kyoto University and the research works in theoretical geomorphology therein is first reviewed, followed by the inspection of the relationship between the debris movements and the topographic changes, the description of the relationships between debris hazards and topographic changes in terms of mechanics, and the expansion of such description with the catenation of elementary mechanical geomorphic processes in account. Finally, a note is given concerning the attitudes of scientists asked in treating natural hazards, on the basis of recent theories of disaster.

A review of geomorphic hazards in the Hida Mountains, specifically related to landslides and debris flows, reveals: (1) Severe disasters have occurred repeatedly and are related mainly to rainstorms and earthquakes. In the late 1960s, landslide and debris flow processes and hazards were not well understood. (2) To obtain better knowledge about debris flows, a field monitoring program began in 1970 at Mount Yakedake. This monitoring program yielded substantial insights. For example, it showed that not only the size but also the characteristics of debris flows including composition are strongly controlled by rainstorm pattern. (3) Rainfall-triggered debris flows in August 1979 forced scientists and engineers to recognize that alluvial fans are at severe risk of debris-flows. (4) The August 1998 Nagano-Gifu Border earthquake swarms reminded us that heavy rainstorms are not the only way to trigger landslides and rock falls. (5) Pyroclastic flows caused by volcanic eruptions are also hazard in the Hida Mountains where tourism is popular year round. These different hazards have variable probabilities of occurrence and consequences. Pyroclastic flows are low probability events here, whereas rainfalltriggered landslides and debris flows are high probability events. Owing to population encroachment on alluvial fans and within the Hida Mountains, any of these natural processes can have severe consequences.

falls also occurred near the epicenter. Thick tephra layers interleaved with andosol soils cover the hillslopes of the area in which these landslides occurred. The tephra layers are derived from the Shikotsu caldera and surrounding volcanoes, approximately 50 km west of the town of Atsuma. The main tephra layers that can be found on the slopes are Spfa1 pumice (42 ka) from the Shikotsu caldera, En-a pumice (20 ka) from the Eniwa volcano, and Ta-d pumice (9 ka) from the Tarumai volcano. In the northern part of the study area, the En-a layer is thick, at > ca. 100 cm, while in the southern part, the Ta-d layer is thick, at > ca. 50 cm. These tephra layers were the main constituents of the landslides caused by this earthquake. Slope surficial deposits affected by solifluction caused by freeze–thaw cycles actively moved during the last glacial period ( ~10 ka ago). As a result, the Spfa1 and En-a could not remain on the slopes, except at valley head hollows and at the feet of those slopes, where surficial deposits accumulated, or on crest slopes. However, the lower parts of the slopes were eroded by increased fluvial activity related to the warmer and wetter climate during the late glacial period. Therefore, Spfa1 and En-a deposited during the last glacial period do not cover the lower valley-side slopes, but the Ta-d cover the slope deposits on the weathered rock. Therefore, in this area, most slopes are covered with Ta-d. An En-a covering of a slope, for example at a valley head hollow, is underlain by the reworked Spfa1. The sliding surface was on the lower volcanic soil, mixed with glassy Spfa1, beneath the En-a. The volcanic soil containing Spfa1 has high water retention and becomes slippery. However, Ta-d is often underlain by reworked En-a on the valley-side slopes. These reworked tephras and the lower unit of a Ta-d layer with a high water retentivity would become the slide layers in this earthquake. Some of the sliding surface were formed in a fine-pumice layer containing large amounts of moisture at the lowest part of Ta-d layer. The water-rich fine pumice often adheres to the slip surface with striations on the slopes. In the southern part of the disaster area, most slopes except for the steep parts are covered with a thick layer of Ta-d. The main source of landslides caused by the earthquake was this thick Ta-d. Therefore, many landslides occurred on those slopes covered by the thick Ta-d layer. In contrast, in the northern part of the disaster area, the landslide of the Ta-d layer was rare, because the layer was thin. Meanwhile, landslides of the thick En-a in the valley head hollows were unevenly distributed.