HGM01-01

Japan Geoscience Union Meeting 2016

Chemical weathering rates in granitic mountains in Japan: Estimation from cosmogenic
radionuclides and geochemical mass balance method*Tsuyoshi Hattanji1, Yuki Matsushi2, Masato Sato3, Chiaki T Oguchi4, Hiroyuki Matsuzaki5
1.Faculty of Life and Environmental Sciences, University of Tsukuba, 2.DPRI, Kyoto University,
3.Graduate School of Life and Environmental Sciences, University of Tsukuba, 4.Graduate School ofScience and Engineering, Saitama University, 5.MALT, the University of Tokyo
Recently denudation rates or exposure ages in mountains underlain by granite were estimated fromcosmogenic radionuclides. Many studies also combined this method with geochemical mass balance
approaches and estimated chemical weathering rates in granitic mountains, assuming steady stateconditions. We applied the modified methods to three mountainous areas (North Alps, Rokko
Mountains, and Abukuma Highland) in Japan with various tectonic and climatic conditions. The threesurveyed areas include small target basins; 9 basins in North Alps, 4 basins in Rokko Mountains, 2basins in Abukuma Highland. All these basins are underlain by biotite granite, and basin-wide
denudation rates were estimated from cosmogenic 10Be in stream sediment. We collected stream
sediment as weathered material in these basins and several fresh bedrock samples in each study
area. Stream sediment is sieved into three fractions; F: 0.063 - 0.25 mm, M: 0.25 - 2.0 mm, C: 2.0- 9.5 mm. We measured chemical compositions of fresh bedrock and the fractions of stream sedimentin each basin with X-ray fluorescence analysis (Phillips PW2400, Saitama Univ.), and calculated
chemical depletion fraction (CDF), which is defined as the ratio of chemical weathering rate tototal denudation rate. Both grain-size distribution of soil and chemical composition in the threefractions of stream sediment were used for calculating mean concentration of immobile elements inweathering materials. Titanium was treated as an immobile element for calculation of CDF. Chemical
weathering rates generally increase with increasing basin-wide denudation rates, although they are
more diverse for basins with high denudation rates (> 1000 mm/kyr). The values of CDF ranged from0.02 to 0.40 for North Alps, 0.12 - 0.33 for Rokko Mountains, and 0.43-0.67 for Abukuma Highland.
Contribution of chemical weathering to total denudation is relatively high in Abukuma Highlandwhere landscape is gentle and denudation rates are low (< 100 mm/kyr). The causes for decrease in
CDF in North Alps would be; (1) physical weathering processes due to cool climate in high elevation
zones, and (2) erosion of saprolite after landsliding in basins with high denudation rates.
Keywords: chemical weathering, cosmogenic radionuclides, denudation rate©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-01 -HGM01-02

Japan Geoscience Union Meeting 2016

High-resolution multibeam bathymetric mapping of barrier reef geomorphology of eastern
Kume Island, the Ryukyus*Hironobu Kan1, Masayuki Nagao2, Kazuhiko Fujita3, Hitoshi Hasegawa4, Yosuke Nakashima5, Nobuyuki
Hori6
1.Graduate School of Integrated Sciences for Global Society, Kyushu University, 2.ResearchInstitute of Geology and Geoinformation, National Institute of Advanced Industrial Science andTechnology, 3.Department of Physics and Earth Sciences, Faculty of Science, University of theRyukyus, 4.Department of Geography and Environmental Studies, Kokushikan University, 5.Ariake
National College of Technology, 6.Nara University
In contrast to the vast knowledge that has been accumulated regarding terrestrial landforms, few
previous studies have dealt with shallow seafloor landforms. Since a broadband multibeam
echosounder (Sonic 2022, R2 Sonic, LLC) and its accessory system were introduced to H. Kan’slaboratory in 2010 using JSPS Grant-in-Aid for Scientific Research A, we have been conducting
high-resolution multibeam bathymetric surveys around the Ryukyu Islands, southern Japan, andmapping the coral reef geomorphology with a horizontal grid size of 1–2 m.
Accurate descriptions of coral reef geomorphology improve our understanding of reef environments.
Here, we introduce our mapping project, which covers of a total of 39 surveyed areas across fiveislands, and present an example illustrating the barrier reef and island shelf geomorphology ofeastern Kume Island (1.8 ×6.5 km bathymetric area, with a depth range of 0.4–161.3 m) combined withSCUBA and VTR observations.
The bathymetric area comprises two contrasting reef edge formations: a double reef in the westernarea, and a deep breakwater reef in the eastern area. The latter shows the high-energy window
condition which is accompanied by spur and groove formation in the lagoon, formation of circularspits or bowl-shaped depressions behind the lagoon patch reefs, and traces of movement of coral
boulders in the lagoon following typhoon storm surges. These observations enable us to understandand reconstruct the paleo-geomorphology and sedimentology in reefs associated with the Holocene
high-energy window, and can also contribute to predictions of coastal environmental change
associated with future sea level rise.
On the island shelf, we observed several reef terraces at depths between 80–95 m, and isolated
patch reefs at 135 m depth. The discovery of these features may contribute to reconstructions ofpast sea level and reef growth in the northwestern Pacific.
Keywords: coral reef, multibeam ecosounder, lagoon, submarine terrace©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-02 -HGM01-03

Japan Geoscience Union Meeting 2016

The significance of landslide dams on large Chinese rivers*David Laurence Higgitt1
1.University of Nottingham Ningbo China
Landslides forming natural dams across rivers pose considerable hazards, particularly the risk ofcatastrophic flooding after dam failure. In China, documentary records of large earthquakes andriver blocking landslides have enabled inventories to be compiled. However, the record is far fromcomplete as the remote and rugged terrain shelters evidence both of recent landslide dams and ofancient landslides which occurred before documentary records. Geospatial technologies enable thedetection of anomalies in fluvial geomorphological systems which may indicate the past influence ofriver blocking landslides. Field evidence of sediment deposited behind landslide dams, enables thelocation of palaeo-landslides to be inferred. Recent discovery of an ancient landslide which dammed
the main stem of the Yangtze River, China (crest height 200 m; lake volume 11.4 ±1.3 km3) indicatesthat very large rivers have –and can be - impacted by persistent river-blocking landslides. As themagnitude of this landslide dam was much greater than examples in the historical record, it isargued that the potential for large earthquakes to generate enormous river-blocking landslides may
be underappreciated as a mega-hazard.
Keywords: landslide dam, flood hazard, Yangtze River©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-03 -HGM01-04

Japan Geoscience Union Meeting 2016

Solitary Step in Bedrock Rivers*Yan Ma1, Norihiro Izumi1
1.Hokkaido University
A solitary step is often observed to be formed in the mixed alluvium and bedrock rivers, which will
be described later as “bedrock river” for simplification. Once a step is formed on the river bed,due to bedrock erosion, it is wondered whether the step is maintained and propagates in theupstream direction, or change its profile continuously. Compared with the bed of alluvial rivers
which is easily eroded by flow, the mechanisms of bedrock erosion are more complicated, mainly
including abrasion by bedload, plucking and macroabrasion. Based on this complexity, in this paper,
we employ the Macro-Roughness Saltation Abrasion Alluviation model to analyze the evolution of aconvex solitary step, featuring a subcritical region upstream and supercritical region downstream,in abrasion-dominated bedrock rivers. From the results of the analysis, we obtain that the convex
solitary step cannot migrate in the direction of upstream with maintaining its profile. Inaddition, the erosion rate tends to be constant in the downstream reach, while the erosion rateincreases drastically in the upstream reach. From a numerical simulation, it is found that theslope of the upstream region becomes larger and the curvature of the step downstream reach ismaintained in relatively long time.
Keywords: Bedrock river, Solitary step

©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-04 -HGM01-05

Japan Geoscience Union Meeting 2016

Velocity and Sediment Concentratin Distributions of Turbidity Currents*Sytharith Pen1, Norihiro Izumi1
1.Hokkaido University
Abstract. In this work, we present a mathematical model to describe thedistribution of the velocity and the particle concentration of the turbidity
currents. The turbidity current is described by the Navier-Stokes equations
with the use of Boussinesq approximation to approximate the e ect of sediment concentration on theow, the di usion/dispersion equation for thesediment transport, and k- turbulence model for the problem closure. The
model is simpli ed by a roof-like con guration which allows a steady equilibrium state of theow. The numerical solution of the distribution of theow
velocity, the sediment concentration, the turbulence energy, and the dissipation rate are obtained by the nite-di erent numerical scheme. The study
shows that the entrainment capacity of sediment due to turbulence and thedampening characteristic of turbulence is aliated with Richarson number
and non-dimensional settling velocity. The model is capable of capturing thefundamental characteristics of the turbidity current which provides a vital
base state for the analysis of morphodynamics generated by the turbiditycurrent.
Keywords: turbidity current©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-05 -HGM01-06

Japan Geoscience Union Meeting 2016

Comparison of different kind disasters and find of the similarity*Masayuki Seto1, Akira Takagi2
1.Fukushima Future Center for Regional Revitalization, Fukushima University, 2.Kumamotogakuen
University
This study compared the damages and revival processes of Kawauchi village with Miyake Island.
Kawauchi village was damaged by the 2011 Off the Pacific Coast of Tohoku earthquake. On the otherhand, Miyake Island was damaged by volcanic eruption.
Kawauchi village is located in Fukushima prefecture, northeastern Japan. The population of 2010 was2997 people. In this village, depopulation and aging advanced. The main industry was tertiary.
Kawauchi village damaged by the 2011 Off the Pacific of Tohoku earthquake. The most serious damage
was nuclear hazard. This village evacuated outside in 16, March, 2011. The refuge continued untilJanuary, 2012.
Miyake village is the Island of active volcano belonging to Tokyo city. The population of 1999 was3872 people. The main industry before the volcanic eruption was sightseeing business. Miyake
volcano erupted in June, 2000. A large quantity of volcanic gas and volcanic ash were spouted out.
Landslide disasters were occurred frequently. All islanders of Miyake village evacuated outside ofMiyake Island in September, 2000. The refuge continued until February, 2005. The service industry,
the retail trade, the wholesale trade were decreased greatly. A village office moved under theinfluence of volcanic gas.
This study divided the damages of the two villages into two types. The first type affected theindustry. Another one affected the community. We found same kind of damages and revival processes
in different disaster.
Keywords: The Great East Japan Earthquake, Miyake Volcano, Disaster©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-06 -HGM01-P01

Japan Geoscience Union Meeting 2016

Study in Spatial Distribution of Sediment Strength in Omiya and north Musashino Uplands
Daishi Shitara2, *Chiaki T. Oguchi1, Shoichi Hachinohe3, Hasan Md. Imam4
1.Graduate School of Science and Engineering, Saitama University, 2.Department of Civil andEngineering, Saitama University, 3.Center for Environmental Sciences in Saitama, 4.Course inEnvironmental Science and Infrastructure Engineering, Graduate School of Science and Engineering,
Saitama University
In order to investigate the N-values of underground sediments in Omiya and north Musashino uplands,
isopleth maps were drawn using ArcGIS (ver 10. 3. 1, ESRI Japan Corporation). First, the summarized

N-values data1) were referred and picked up the values of 3-m depth intervals. Then, by using theGIS software, each boring point was plotted and made isopleth maps of each 3-m interval. As aresult, high N- value with the sediments composed by gravel is distributed in Musashino uplands,
whereas low N-values less than 10 with mud and silt sediments between 24-33m depth in Omiya
uplands. Sediment layers existing fossil shells are also pointed out in the original boring coredata1) in Omiya uplands. The low N-value area is corresponded to the distributed area of Kioroshi
formation, which deposited during Shimosueyoshi transgression. From these analyses of mapping data,
it is concluded that it is important to choose it by not only surface landscapes but also
considering the history of landscape development as well as geotechnical parameters such as N-values, when we decide the suitable low-cost construction site.
Keywords: N-value, Omiya upland, Musashino upland, Boring core sample, GIS©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-P01 -HGM01-P02

Japan Geoscience Union Meeting 2016

Response to the uplift of a single channel in mountain rivers: Laboratory experiments*SOTA IIJIMA1, Noritaka Endo11.Kanazawa university, Graduation school of natural science and technology
In considering the development of mountain rivers, it is very important to know about thedevelopmental process and the dynamics of downward erosion and lateral erosion, although these arepartially understood so far. A previous experimental study for channel networks described thatlateral migration of surface reliefs became dominant after the erosion rate reached the uplift rateon the whole. This report came from the observation though photograph of the entire basin, andstudies in individual channel scales are very few so far. The purpose of the present study is toelucidate relative strength of downward and lateral erosion of bedrock river when experienced
uplift, using model experiments. We targeted the observation of a single channel for measurementswith high accuracy.
Experiments were performed two times under almost the same condition (Experiments A & B). The
uplifts were realized by removing weirs of 1cm at the downstream end. Experiment A was carried out
for 300 minutes, during which the uplifts were generated when the river bed seemed stable.
Meanwhile, Experiment B was continued for 260 minutes, in which the uplifts were actualized at thesame timings as Experiment A regardless of the channel state.
The results, with few exceptions, showed that the elevation of the channel bed at any placescontinued to lower by downward erosion until a knick-point passed through, and thereafter lateralerosion occurred while downward erosion almost ceased.
Keywords: downward erosion, lateral erosion, profile©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-P02 -HGM01-P03

Japan Geoscience Union Meeting 2016

Transport processes of huge riverbed boulder and landforms of the Miyanoura River in theYaku Island, southern Japan*Hiroshi Shimazu1
1.Department of Geography, Faculty of Geo-Environmental Science, Rissho University
There are huge boulders on the riverbeds of most rivers in the Yaku Island, southern Japan. The
Miyanoura River is the most typical river and one of the largest rivers originated in the central
part of the island. This study aims to discuss the production and transport processes of such hugeriverbed boulders and relationships between the processes and landforms of the basin.
Most part of the Yaku Island, including the central part, is underlain by granitic rocks. The
Mitanoura River, whose length is 17 kilometers, flows down in granitic area except the lowest
section of 4 kilometers. Maximum diameter of the riverbed boulders is about 10 meters in the upper
reaches. It decreases downstream to several tens centimeters as the channel slope decreases.
Although uniform granite is underlain by this basin, maximum diameter of riverbed boulders
correlates with channel slope. This indicates such boulders are transported by river being affectedby sorting process. In the reaches of maximum boulder size being over 5 meters, relative height
between ridges and valley bottoms is as large as 500m. There are many landslide scars in thetributary basins which join the Miyanoura River in these reaches. At the junctions thesetributaries with steep channel slope develop alluvial cones on the valley floor. The deposits of
the riverbed and the alluvial cones include huge boulders with several meters in diameter. Alluvial
cone development across the Miyanoura River formed a dam. A dam break during a heavy rain event
caused torrential floods and huge boulder transportation.
There is a continuous terrace along the middle reaches of the Miyanoura River. The terrace deposits
consist of huge boulders of several meters in diameter. This terrace is not covered with the Koya
pyroclastic flow deposits, which was erupted at 7,300 y.B.P. and which covered the valley sideslopes around there. The cause of terrace is an important key to solve the cause of the hugeriverbed boulders.
Keywords: huge riverbed boulder, sediment transport process, landform, Miyanoura River, YakuIsland, southern Japan©2016. Japan Geoscience Union. All Right Reserved.

- HGM01-P03 -