Knickpoint retreat is one of the essential denudation processes in pyroclastic plateaus with partially welded ignimbrites. However, knickpoints located in the inner part of such plateaus have been less studied than those formed in the edge of plateaus, as one of dissecting fronts in ignimbrite plateaus. This study estimated the recession rates in the drainage basin of the Ogawa River, southern Kyushu, Japan, to demonstrate how knickpoints have retreated. This paper discusses the contribution of those knickpoints in the denudation processes. Recession rates of 6 knickpoints were obtained from the difference of the longitudinal profiles of riverbed between at present and 110 ka B. P., restored by interpreting the elevation measured at the top of Ata ignimbrite as the paleo riverbed. The recession rates of knickpoints were estimated at 0.005-0.013 m per year in the inner part of plateau, and 0.026-0.037 m per year in the plateau edge. Recession rates for such same knickpoints were also calculated by the empirical equation for estimating recession rates. Computed recession rates of knickpoints were 0.004-0.006 m per year in the inner part of plateau, and 0.016 m per year in the border of plateau. The results from two methods were consistent with each other. Such knickpoints located in inner part of plateaus have played a significant role of bedrock erosion in pyroclastic plateaus, because their horizontal recession rates are much more rapid than the rates of downcutting (less than 0.1 mm per year) there so that the major part of denudation is debt to horizontal retreat of knickpoints.

This study aims to elucidate the geomorphic development processes of alluvial fans and their catchments in the Japanese Islands, paying special attention to smaller alluvial fans, poorly investigated so far. We employed the following approaches as, interpretation of aerial photographs, measurement and correlation analysis of basic geomorphic quantities, as such fan area (A_f), fan slope (S_f ), catchment area (A_d ) and relief ratio of catchment (Rr), using digital elevation models, and geomorphic interpretation of fan development with information of sediment transport from catchment, geology of catchment, periglacial condition during the last glacial period and tectonic condition (effect by active faulting). Consequently, strong positive exponential relationship for A_f with A_d , and strong negative exponential relationship for S_f with A_d were observed also for the Japanese small-scale fans. In particular, those above relationships were variable according to different type of sediment transport from catchments to fan surfaces. The results offer, therefore, the probable geomorphic development processes for small-scale fans in the Japanese islands, which are chronologically ordered from Stages I to III, based on sequential transition of dominant sediment transport mechanism from debris f low to fluvial traction, related to the long-term course of fan-drainage development. The scenario is summarized as; smaller and steeper catchments and fans, characterized by common occurrence of debris flows (Stage I); larger and gentler catchments and fans, particularly characterized by comparatively drastic decrease of fan slope due to the combined sediment transport of fluvial traction and debris flow from the catchment to fan surface (Stage II); and sufficiently larger and gentler catchments and fans (no longer “small” in this stage), controlled by the fluvial traction over the drainage system (Stage III). For the development of smaller fans, we also need to keep in mind that additional effects by the catchment geology and tectonic condition (vertical active faulting) were still observed from the correlation analyses of basic geomorphic quantities.

Topographic changes along the Idoura coast in Miyagi Prefecture following the Tohoku-oki tsunami in 2011 were investigated, using digital elevation models (DEMs), digital surface models (DSMs) produced by structure-from-motion multi-view stereo (SfM-MVS) photogrammetry, and bathymetric data. The river-mouth bar of the Natori River was divided by the tsunami, and the upper part (2 m or more) disappeared. After the bars were reconnected, the resulting bar initially expanded in width and then increased in height. Remarkable bathymetric changes occurred in water of 12 m depth in the nearshore and offshore zones. The steep profile became the smooth, before reverting back to the original profile. The rate of decrease in volume of the river-mouth bar caused by tsunami erosion was higher than in area. It took approximately two and a half years for the area to recover to its pre-tsunami state, and about four years and four months for volume to do the same. The amount deposited in the nearshore and offshore zones by the tsunami was 87.7% of the amount eroded on land above sea level in the study area. The amount recovered on the river-mouth bar accounted for 30.9% of the loss in the nearshore and offshore zones for approximately two and a half years after the tsunami.

Field observation was carried out at a comet-tail spit extending in the lee of Mashima Island on Sone tidal flat facing the Suonada Sea on April 25, 2017. The formation of a comet-tail spit was predicted using the BG model (a model for predicting three-dimensional beach changes based on Bagnold’ concept). In the calculation, a triangular island was placed on a mud flat of 1 m depth, and waves with significant wave height of 0.25 m and wave period of 2 s were assumed to be incident to the island. Two cases of the calculations were carried out; the island is composed of sand in Case 1, and the island is surrounded by solid wall and sand source is located offshore of the island in Case 2. As a result of the calculation, a couple of sand spits obliquely extended from both corners of the island in Case 1, resulting in no formation of a comet-tail spit. In contrast, in Case 2, a slender comet-tail spit extended behind the island. The formation of a comet tail spit strongly relates to the available volume of sand, and a comet-tail spit can develop only if sand for the formation of a comet-tail spit is slowly supplied from upcoast without rapid deformation of island itself.

Relief maps and topographic analysis derived from DEM are becoming more advanced because LIDAR or photogrammetry provide high resolution digital elevation models (DEMs). Typical relief map tools include ELSAMAP (Mukouyama and Sasaki, 2007) and red relief image map (Chiba and Suzuki, 2004). However, microtopographic features cannot be analyzed if positive and negative curvatures in ELSAMAP are excluded. Red relief image map does not include elevation information; therefore, it cannot convey the height of the features. This study aims to resolve these issues and create a new relief map that uses simple colorization to convey information about factors, such as elevation and slope, and identifies ridges and valleys from curvature calculations. The open-source DEM from the Geospatial Information Authority of Japan, and the processing open-source programming language are used, and the development environment is integrated. The software reads out elevation data from DEM, and calculates elevation, slope, and curvature. After these factors are calculated, the program substitutes the magnitudes of elevation, slope and positive or negative curvature for the magnitudes of hue, saturation and value, which are the dimensions of the HSV color model. The changes in elevation and slope of mountain areas can be easily understood from the terrain representations based on the HSV color model. As the elevation changes, the color on the relief map changes. As slope becomes steeper, the saturation increases. Mountain ridges appear light, and valleys appear dark. However, this relief map cannot describe minute elevation changes of terrain with a gentle slope, such as a plain. Most gentle-slope terrains appear gray. Overlaying other maps, such as a contour map, over the HSV relief map is one way to overcome this problem. For this purpose, a new software program to create a rainbow elevation map from the same area of DEM is also examined. The rainbow elevation map includes color change per 1 m, and the colors change in a 360-m cycle. After the rainbow elevation map is created, the HSV relief map is overlaid on it. The effect of the rainbow elevation representation allows understanding the minute elevation changes in a plain. The elevation change of mountains is more dramatic compared with that of a plain; therefore, the microtopography of mountains is difficult to understand using the rainbow map. The color representation for understanding elevation, slope and micro topography of both mountains and plains in one map is a subject for future study.