10th Asian Regional Conference of IAEG (2015)

A study on the relevance of geology and sediment discharge in mountain watershed ofShimanto accretionary complex
Ryunosuke NAKANISHI(1), Yasuhiro MITANI(1), Hiro IKEMI(1), and Hendra PACHRI(1)(1) Graduate School of Engineering, Kyushu University, Japan
E-mail:nakanishi@doc.kyushu-u.ac.jp
Abstract
Recently, comprehensive sediment management is necessary to solve sedimentproblem in whole river basin. However, sediment discharge in river has been measured inonly dam reservoirs, and sediment production from tributary and factors on sedimenttransport are not clearly in most cases.
In this study, we conducted sampling of sediment in Nanatsuyamakawa River, toclarify the relationship of sediment chemical composition and the geology.
Nanatsuyamakawa River is one of the tributaries that supplies more sediment to mainstream
than other tributaries in Mimikawa River Basin of Miyazaki Prefecture. It is also necessary
mission to clarify sediment volume from tributary.
First, we analyzed landform of the study area by GIS to select sampling locations inwhich sediment will be trapped due to the morphology of river-bed such as knickpoint,steps and pools etc. Chemical compositions of the sediment were revealed by XRF, andwere analyzed statistically. As a result, the chemical composition has been classified into
two components: one is characterized by SiO2, Na2O, Sr, and S, and the other ischaracterized by Fe2O3, Al2O3, and etc. Then, the sediment samples can be explained mix ofthese components, and the ratio of these components have relation to geology of slope, thatconsist of sandstone and mudstone of Shimanto Supergroup affect sediment transportation.
The ratio of these components has related to area ratio of Shimanto Supergroup sandstone
and mudstone. Consequently, the formula is defined to estimate sediment discharge ratio ofNanatsuyamakawa River according to the area ratio of sandstone and mudstone.
Keywords: sediment discharge, XRF, mudstone
1.

Introduction

completely measured. In addition, since tributaries ofMimikawa Basin are unlike alluvial river, measure ismore complicated.
Nanatsuyamakawa River has complex river shapebecause of bedrock and torrent. Nanatsuyamakawa
River is the biggest tributary of Mimikawa Basin.
Except conventional method it is highly required toapply different ways which make the result more
perfect. However, it is easy to grasp the river-bed
situation before and after the flood. So far, a river-bed
sediment survey in Nanatsuyamakawa River isconducted to get information about river-bed, then thesediment transportation considered as a result.
Moreover, sediment in the site where steep slopechanges to gentle slope is found and related with themovement of sediment.
In this study, sampling survey is implemented
where steep slope changes to gentle slope. The survey
sites are selected by GIS. Moreover, XRF is conducted
to clarify the chemical composition of the sediment,
and the relation between surface geology and sediment

In recent years, with frequent occurrence oflandslides and slope failures, preventing sedimentdisaster is getting more and more attention. Among all
the disaster, controlling sediment transportation is an
important issue especially in dam management (Wang,2014). Sediment increases the river-bed and the risk offlooding in the reservoir. On the other hand, thedecrease of sediment has bad influence on stability ofbridge pier and ecosystem in downstream of the dam(Nishida, 2011). Not only sediment management in
dam reservoirs but also comprehensive sedimentmanagement in whole basin is necessary to solve sucha sediment problem (Takahashi, 2006).
For this reason, it is important to measure theamount of moving sediment in a river and it is also asignificant issue in Mimikawa Basin. Although there isa method for measuring moving sediment such as
sediment measuring device and river-bed fluctuation
simulation, moving sediment in the river couldn’t be1

10th Asian Regional Conference of IAEG (2015)

in Nanatsuyamakawa River is statistically examined.
2.

Geological setting

Study area is a part of Mimikawa Basin in Miyazaki
Prefecture. As shown in Fig.1, study area contains fivebranches
suchasNanatsuyamakawa
River,
Yanabarugawa River, Masutanigawa River, etc.
Shimanto Supergroup mainly composes surface
geology of the area and it is different between northand south, in south side the geological age is younger.
Furthermore, Shimanto Supergroup distributes from thenortheast to the southwest, strata repeatedly exists
because of much strike fault.
In Nanatsuyamakawa Basin, surface geology
mainly consists of N.Shimanto Supergroup sandstoneand slightly N.Shimanto Supergroup mudstone.
Generally, sandstone and mudstone are affecting
sediment production. Shimanto Supergroup sandstone
is a rock containing much SiO2. The chemicalcomposition of Shimanto Supergroup is clarified in theresearch of Teraoka (2004). In Teraoka’s research,compared to sandstone, Shimanto Supergroup
mudstone is lack of SiO2, yet rich of Al2O3, Fe2O3 andMgO. Therefore, as a mixture of sandstone andmudstone, sediment sample in the river is expected to
be assessed by the ratio of two geology types.
3.

Fig. 1 Map of selected site by GIS and investigated siteof five branches

Site selection by GIS and river-bed sediment
surveyThe measurement of river-bed structure and theriver-bed sediment survey was conducted in
Nanatsuyamakawa River. The survey was carried out toclarify river-bed shape and sediment situation. As aresult, significant deposition has been confirmed in thesites where steep slope changes to gentle slope. Also,
the sediment was confirmed in the partial deposition
fields such as behind of large rock, it is considered tobe deposited by sediment transport during floods. As aresult of examining the grain size in these sites, it wasaround 2 mm. It is possible that sediment in the sitewhere steep slope changes to gentle slope can be said
as moving sediment.
The site of five branches in Mimikawa River wasselected by GIS. Then, the sediment samples weretrapped in selected site of each branch. Before thesampling survey, not only selected site but also surface
geology, river divergence, and sediment caused bytopographic condition are parameters for site
investigation. Moreover, survey site was limited
because investigation happened to be during typhoon.
The number of trapped sediment samples was twenty
two throughout five branches. Fig.1 is the map ofselected site by GIS and investigated site of fivebranches.

Fig. 2 The site of trapped sediment samples for XRF(Legend is same as Fig.1)

210th Asian Regional Conference of IAEG (2015)

Analysis results
sedimentofsurface

geology

andSecond principal component score

4.

4.1 X-ray Fluorescence Analysis (XRF Analysis)
The chemical composition of eight samples in
Nanatsuyamakawa River are clarified by XRF. XRF is
the method to identify element types and content ofelement by observing the wavelength of the fluorescent
X-rays emitted from the sample when irradiating thesample with X-rays. It is found that sediment contains
exactly thirty chemical elements. Fig.2 shows the siteof trapped sediment samples for XRF.
4.2 Principal Component Analysis (PCA)
Each chemical element is characterized by PCA.
PCA is one of statistical analysis, the method forelucidating characteristic of target factor. Fig.3 is theresult of PCA, direction and length of arrows explain
the chemical composition trend and strength in all
samples. Considering the major components such as
SiO2 and Fe2O3, Al2O3, each sample is represented as
the mixture of other components and SiO2. Therefore,relation between SiO2 and nine other major
components are compared. A comparison results
between SiO2 with Fe2O3, Al2O3, MgO are shown inFig.4 through Fig.6. MgO has the highest correlation
with SiO2, followed by Fe2O3 and Al2O3. Generally,among chemical elements of rock, alkaline metal (Na,K) and alkali earth metal (Ca, Mg) are easy to dissolve(Kimiya, 1991). Consequently, correlation coefficientof SiO2 with Na2O, K2O, CaO is considered to havelow value.
5.

First principal component score
Fig. 3 The result of principal component analysis

Fig. 4 The correlation between SiO2 and Fe2O3

Estimation of sediment discharge ratio

5.1 Relation between sediment and surface geology
in Nanatsuyamakawa Basin
On the basis of geological chemical composition, itis necessary to indicate the relation between thesediment and surface geology. Considering that eachsample is the mixture of Shimanto Supergroupsandstone and mudstone, WP008 sample must notcontain Shimanto Supergroup mudstone because
mudstone doesn’t exist in the upper stream. Although
there is little mudstone in the upper stream of WP009,
obviously, it has an influence on WP009 from theexistence of mudstone in the vicinity. Each sample isevaluated by the ratio of Shimanto Supergroupsandstone on the basis of WP008. Like Fig.7 shows, thearea ratio of Shimanto Supergroup sandstone is related
to the ratio of Shimanto Supergroup sandstone inriver-bed sediment. Except WP001, other samples havehigh correlation with surface geology. It is considered
the sediment comes from Shimanto Super group
mudstone because SiO2 ratio of sediment sample in site

Fig. 5 The correlation between SiO2 and Al2O3

Fig. 6 The correlation between SiO2 and MgO3

10th Asian Regional Conference of IAEG (2015)

WP001 is lower than other samples. Therefore, sample
WP001 doesn’t follow the correlation of Fig.7.
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5

5.2 Formulation of sediment discharge
According to Fig.7, in the case that about 65% area
is sandstone, the percentage of sandstone in sediment isaround 50%, so compared to sandstone, mudstone
plays a major role. The following model equation (1) isdefined to calculate contribution of basin for river-bed
sediment, since the chemical composition suggests
quantitative relation between surface geology andriver-bed sediment.
(Sediment discharge)= (Ratio of A in sediment) × (Area of A)+ (Ratio of B in sediment) × (Area of B)
A: Shimanto Supergroup sandstone
B: Shimanto Supergroup mudstone(1)

Fig. 7 The correlation between sandstone of river-bed
sediment and sandstone of surface geology

As a result of calculation, the area Group1
obviously including mudstone was identified as asignificant sediment discharge area like shown in theFig.8.
6.

Conclusions

In this study, river-bed sediment survey wasconducted in the site of significant sedimentation,
chemical composition of the sediment was clarified byXRF. As a result of XRF, the sediment was evaluatedas mixture of sandstone and mudstone. Moreover, thechemical composition of sediment was related to thechemical composition of surface geology in
Nanatsuyamakawa Basin. Particularly, the area ratio of
Shimanto Supergroup sandstone in the watershedinterrelates with the percentage of Shimanto
Supergroup sandstone in sediment. The area ofsignificant sediment discharge in the basin wasidentified by the relation. The result of this study
signified that mudstone plays a vital role in sediment
discharge. Additionally, it was suspected that moving
sediment was closely related to the sediment of the sitewhere steep slope changes to gentle slope.

Fig. 8 Sediment discharge ratio
Nishida, M., Azami, K., Ishizawa, N., Kumazawa, K.,
Nakazawa, S. (2011): Changes in physical habitat andbenthic animals in the Miharu Dam downstream riverin response to river regulation and sediment
replenishment after initial impoundment - The resultof 11 years monitoring -, Ecol. Civil. Eng.14 (1),pp.63-74.

Acknowledgements
This work was supported by JSPS KAKENHI
Grant Number 25350429.
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