Yuichi S. Hayakawa

The Climatic Signature of Incised River Meanders

Messy Mountain Meandering Predicting the influence of climate on landscapes is sometimes straightforward; for example, river deposits might grow with increased rainfall because erosion rates and sediment transport increase. However, long-term tectonic processes complicate the geomorphic signatures of more gradual climate-related phenomena that reconfigure landscapes. By correlating a decades-long record of typhoon rainfall in Japan with digital elevation models, Stark et al. (p. 1497) show that climate directly influences the extent of river meandering. When expanded to a larger region of the western North Pacific, this analysis revealed a strong climatic imprint on the landscape of humid mountainous areas. The region-wide analysis also revealed that underlying bedrock strength, as opposed to tectonic uplift, acts as a secondary control. Typhoon frequency and bedrock strength influence river meandering in mountain environments. Climate controls landscape evolution, but quantitative signatures of climatic drivers have yet to be found in topography on a broad scale. Here we describe how a topographic signature of typhoon rainfall is recorded in the meandering of incising mountain rivers in the western North Pacific. Spatially averaged river sinuosity generated from digital elevation data peaks in the typhoon-dominated subtropics, where extreme rainfall and flood events are common, and decreases toward the equatorial tropics and mid-latitudes, where such extremes are rare. Once climatic trends are removed, the primary control on sinuosity is rock weakness. Our results indicate that the weakness of bedrock channel walls and their weakening by heavy rainfall together modulate rates of meander propagation and sinuosity development in incising rivers.

GIS-Based Landslide Susceptibility Mapping Using a Certainty Factor Model and Its Validation in the Chuetsu Area, Central Japan

The principal aim of this study is to assess the landslide susceptibility in the Chuetsu area, Niigata Prefecture, Central Japan, using a certainty factor model in a GIS environment. The landslide inventory data used in this study were obtained from the National Research Institute for Earth Science and Disaster Prevention (NIED). The data were divided into two groups: one for training the model and the other for its validation. Seven relative factors, elevation, slope angle, slope aspect, density of geological boundary, density of drainage network, plan curvature, and lithology were utilized for this susceptibility analysis. Based on the aforementioned correlative factors, a landslide susceptibility map was produced and then verified using receiver operating characteristics (ROC). The value of area under the ROC curve (AUC) of the constructed CF model is 0.82. A model with such a high AUC value is considered good and therefore acceptable in predicting landslides. The landslide susceptibility map prepared in this study can hence be used to mitigate risks associated with landslides in the study area.

New tin mines and production sites near Kültepe in Turkey: a third-millennium BC highland production model

Abstract An unexpected new source of tin was recently located at Hisarcık, in the foothills of the Mount Erciyes volcano in the Kayseri Plain, close to the Bronze Age town of Kültepe, ancient Kanesh and home to a colony of Assyrian traders. Volcanoes in Turkey have always been associated with obsidian sources but were not known to be a major source of heavy metals, much less tin. X-ray fluorescence analyses of the Hisarcık ores revealed the presence of minerals suitable for the production of complex copper alloys, and sufficient tin and arsenic content to produce tin-bronze. These findings revise our understanding of bronze production in Anatolia in the third millennium BC and demand a re-evaluation of Assyrian trade routes and the position of the Early Bronze Age societies of Anatolia within that network.

POST-VOLCANIC EROSION RATES OF SHOMYO FALLS IN TATEYAMA, CENTRAL JAPAN

Abstract. Post‐eruptive fluvial erosion of welded pyroclastic flow deposits often depends on the recession of waterfalls because of their rapid erosion involved. We examine the recession rate of Shomyo Falls, which consists of Pleistocene welded pyroclastic flow deposits in Tateyama, north‐central Japan. The mean recession rate of the waterfall obtained from lithological and topographical evidences is 0.08‐0.15 m/a for 100000 a. However, the recession rate estimated by means of an empirical equation comprising physical parameters of erosive force and bedrock resistance is 0.006‐0.011 m/a with small uncertainties. The discrepancy between the geology‐based and equation‐based recession rates indicates that some factors, not taken account of in the equation, significantly influence the recession rate. We suggest that a factor in the rapid erosion of the waterfall is a large amount of transported sediment acting as abrasive material, which is supplied from high mountains in the watershed above the waterfall.

Effects of tsunami wave erosion on natural landscapes: Examples from the 2011 Tohoku-oki tsunami

The 2011 Tohoku-oki Tsunami affected approximately 600 km of the northeastern coast of the Japanese Honshu Island, leaving traces of destruction on man-made buildings and depositing mud- to boulder-sized sediment. Our field observations at Aneyoshi along the Sanriku “ria” coast, where a maximum run-up height of 39.2 m was recorded, add to the limited number of studies of tsunami wave effects on natural landscapes. We found evidence for (1) tsunami wave erosion that exposed bare rock by stripping basal hillslopes of regolith and vegetation, including trees, (2) transport and deposition of coarse gravel, and (3) scour-hole generation around a large boulder and a large sea wall fragment. Computer simulations indicate that the highest first wave reaching the Aneyoshi coast may have been about 20 m high, that the combined duration of the first three waves was tens of minutes to 1 h, and that the maximum wave velocity on land reached over 10 m/s and probably exceeded 20 m/s in the lower, wide reach of the Aneyoshi valley. We hypothesize that hillsides along the Sanriku Coast have been stripped by erosion of numerous ancient tsunami events recurring at century or even decadal scales, since at least the mid-Holocene. The cumulative effects of tsunami erosion on the hillslopes and their long-term evolution are important potential topics for future studies.

Spatial correspondence of knickzones and stream confluences along bedrock rivers in Japan: implications for hydraulic formation of knickzones

Abstract Knickzones, defined here as locally steep reaches including distinct knickpoints, in bedrock river morphology, have often been investigated in relation to local anomalies in lithology, tectonics, hydraulics, climate and associated base‐level change, and/or deformation of valley‐side slopes. However, exact formative causes of many knickzones in a humid, tectonically active island arc remain unclear. Using databases of geology, streams and knickzones, we examine knickzone distribution across the apanese rchipelago to evaluate the effects of the stream network structure and rock type boundaries on knickzone formation. Knickzones are frequently found just upstream and downstream of major stream confluences along mainstreams, whereas knickzones are less frequent around major rock type boundaries. While the major confluences do not form hanging valleys due to similar catchment size, this observation suggests that many knickzones have been formed by the long‐term effect of flow turbulence scouring bedrock at the confluences. Such a hydraulic control on bedrock erosion in the steep apanese mountains under humid climate conditions indicates that the formative cause of many knickzones therein can be autogenic by means of stream hydraulics.

Application of terrestrial laser scanning for detection of ground surface deformation in small mud volcano (Murono, Japan)

We perform terrestrial laser scanning (TLS) to detect changes in surface morphology of a mud volcano in Murono, north-central Japan. The study site underwent significant deformation by a strong earthquake in 2011, and the surface deformation has continued in the following years. The point cloud datasets were obtained by TLS at three different times in 2011, 2013 and 2014. Those point clouds were aligned by cloud-based registration, which minimizes the closest point distance of point clouds of unchanged ground features, and the TLS-based point cloud data appear to be suitable for detecting centimeter-order deformations in the central domain of the mud volcano, as well as for measurements of topographic features including cracks of paved ground surface. The spatial patterns and accumulative amount of the vertical deformation during 2011–2014 captured by TLS correspond well with those previously reported based on point-based leveling surveys, supporting the validity of TLS survey.

Hydrogeomorphic Effects of Basin Lithology on Development of Channel Steps in First-Order Basins of the Ashio Mountains, Japan

This study focuses on lithological contrasts of headmost channel morphology of two mountainous areas underlain by chert or sandstone. The areas have contrasting runoff processes: subsurface storm flow predominates in the chert area, whereas groundwater flow recharges stable stream flow in the sandstone area. Manual field measurements from 84 sections of first-order streams revealed that channel steps develop along 26% of all sections in the chert area, while only along one section (3%) in the sandstone area. High-resolution analysis of long profiles using terrestrial laser scanning detected a weaker pattern of channel steps in a colluvial reach of the sandstone basin. The contrast in runoff processes, as well as grain-size distributions of weathering products between the two areas, influences dissimilar bed-load transport regimes and promotes spatial variations in channel profiles between headwater streams. These contrasting hydro-geomorphic processes affect channel-step morphology, indicating that lithology plays a key role in the formation of channel steps and in variations between headwater streams.

RATES OF SLOPE DECLINE, TALUS GROWTH AND CLIFF RETREAT ALONG THE SHOMYO RIVER IN CENTRAL JAPAN: A SPACE‐TIME SUBSTITUTION APPROACH

Abstract. On the basis of a space–time substitution, rates of cliff retreat and talus development overthe pasttens ofthousands of years were quantified for valley‐side slopes along the Shomyo River with a known rate of waterfall recession. Detailed profiles of the valley‐side slopes were obtained at 19 sites along the river by map reading and field survey, and the morphologic characteristics of the slopes were then measured. By combining the locations of the slopes with the recession rate of Shomyo Falls, it is estimated that the growth rate of the talus slope is 3.9–7.2 mm/yr, the rate of decline of the cliff is 2.0–4.0 × 10−4 deg/yr, and the retreat rate of the cliff top is 6.2–11.6 mm/yr.

Towards Long-Lasting Disaster Mitigation Following a Mega-landslide: High-Definition Topographic Measurements of Sediment Production by Debris Flows in a Steep Headwater Channel

Mega-landslides usually cause long-lasting subsequent sediment production, and long-term strategies for disaster mitigation are necessary in the case of such extreme events. The Ohya-kuzure landslide in central Japan is typical of sites where hillslope erosion and sediment yield have been continuously active since its formation in 1707. Sediment production is particularly active by debris flows in the headwater channels formed within the landslide. However, the dynamics of such debris flows in steep headwater channels have not been fully examined compared to those in gentler downstream reaches. To investigate the changes in headwater channel bed sediments remobilized mainly by frequent debris flows, repeated high-resolution measurements were carried out using terrestrial laser scanning. Freeze-thaw weathering in the surrounding slopes, which are composed of deformed shale and sandstone layers, delivers quantities of small particles onto the valley floor. Measurements in spring, summer, and autumn conducted over two years provided high-definition (0.1 m resolution) topographic datasets, revealing the seasonal amount of erosion and deposition to be on the order of 1000–5000 m3. Erosion and deposition along the reach also showed contrasting spatial patterns according to the sections bounded by knickpoints and valley narrows. These basic estimates of sediment production in headwater channels can be utilized for further mitigation of possible sediment-related disasters in downstream areas.