SEDIMENT TRANSPORT STUDY ON THE TENRYU RIVER AND THE
ENSHU COAST BASED ON ANALYSES OF SURFACE SEDIMENTS
Takuya Yoshii1, Hirokazu Fujiwara2, Shinji Sato1, Guangwei Huang3,
Masaaki Shirai4 and Yoshimitsu Tajima1
We investigated the present condition of regional sediment transport and its temporalchange in Tenryu River and Enshu Coast in Japan. We focused on the distribution ofgeology in Tenryu River basin. By analyzing color and kinds of sediments, we tried to
identify the origin of sediments and the impact of each tributary, dams and rivervegetation on sediment transport. To obtain the sediment in old time (before dam
construction) for comparison, we also performed trench study on the old track of theTenryu River. We also investigated grain size distribution of sediments in the rivervegetation. Through those studies above, we recognized that fine sediments do notaccumulate on riverbed in lower Tenryu River except in the river vegetation but go
straightly to the river mouth. It is why the effect of dams in midstream of the Tenryu
River is not clear in riverbed in lower Tenryu River, but clear on coast around the rivermouth.

1. INTRODUCTION AND RESEARCH AREA

Enshu coast, located in the central district of Japan, has been eroded
significantly because of decrease in sediment supply from the Tenryu River,which has largest amount of sediment discharge in Japanese Rivers. It is mainly
caused by the sediment retention in dam reservoirs in the midstream of theTenryu River and sand mining for construction works. Among the dams, thelargest one is the Sakuma Dam, which was completed in 1957 and has
accumulated over 100 millions m3 of sediments up to now. Still now, thesediment transport is completely blocked by the Sakuma Dam. Therefore,
comprehensive sediment management to the whole fluvial system is needed tosolve this problem. First of all, to understand the present condition of sedimenttransport is important.
In terms of the attempt to investigate the sediment transport in the EnshuTenryu sediment system, Torii et al. (2004) performed numerical simulation ofriverbed and shoreline change, which also predicted the change in grain size
component. However, no studies of the regional sediment transport based on
analyses of surface sediment itself have been carried out so far.

1

Department of Civil Engineering, The University of Tokyo, 7-3-1 Hongo
Bunkyo-ku, Tokyo, 113-8656, Japan
2
SHIMIZU Corporation, 1-2-3 Shibaura, Minato-ku, Tokyo, 105-8007, Japan3
Department of Socio-Cultural Enviromental Studies, The University of Tokyo
4
Ocean Research Institute, The University of Tokyo1

2
In this study, we tried to understand the present condition of sedimenttransport process in the whole Enshu-Tenryu system by analyzing surface
sediments. With regard to geology, Tenryu River has the Median Tectonic Line
(MTL) in the middle of its basin. This tectonic line separates the geology
extensively. On the north-western side of MTL, granite and its metamorphic
rocks are dominant, therefore, whitish sand are dominant on the riverbed. On theother hand, on the south-eastern side, sedimentary rocks of Mesozoic or
Paleozoic era, schist, and blackish sand are dominant. Because of such
distribution of geology, every branch of Tenryu River has its characteristic in its
sediments. And the largest dam, the Sakuma Dam is located nearly on MTL.
Therefore, we tried to obtain the information of the geological component of thesediments at each place in Tenryu River and Enshu Coast, which tells us theorigin of sediments and contribution of each branch and dams on sedimenttransport.

(a)

(b)

GraniteSedimentaryRocks

MTL Schist

40km
Tenryu River(a) Geologic Map

Enshu Coast(b) Erosion at Enshu Coast

Figure 1: (a) Geologic Map of Tenryu River Basin (b) Lighthouse whose foot wasscouredTable 1: Dams in Tenryu River and Their Status (data from Okano, et al (2004))

Dam
Name

Distance From
the River Mouth

Capacity(1000㎥(1000㎥)

CompCompletion

Status of
Sedimentation

Sand/
Capacity

Yasuoka

130km

10761

1935

almost full

81.6%

Hiraoka

110km

42425

1952

almost full

84.5%

Sakuma71km

3268481956

not full/accumulating

34.6%

Akiha

47km

34703

1958

not full/decreasing

36.6%

Hunagira

30km

10900

1977

not full

8.3%

3
2. METHODOLOGY
2-1 Sampling of Surface Sediments and Analyses with Magnet

On May 12th and 13th in 2006, we collected surface sand in Enshu Coast at
each place indicated in Fig.2. Sampling points were arranged to be nearly on the
shoreline at high tide on that day. We collected samples which were locatedupper than the depth of 10cm from the surface.
Also, to obtain the sediments of riverbed in Tenryu River, on June 16th and17th in 2006, we collected surface sand and gravels of riverbed at each place in
Fig.2. We collected samples which located near the waterway, and upper thanthe depth of 10cm from the surface.
Fig.2 also shows the scanned image of sample sand which was pasted on two
sided tape glued on white paper.

Figure 2: Scanned Image of Sand Samples

As we discussed in the above, sediments from the Tenryu River containmany kinds of minerals and rock fragments, and it indicates the origin of thesediments. Therefore, we classified the samples according to their geology.
To classify the sand samples effectively, we utilized its magnetism. By
using magnets, we separated sand into magnet and non-magnet, whichcorrespond to blackish sand and whitish sand. First, to remove an effect of grainsize, we sieved the sand into same grain size class of 0.075-0.425mm. Next, welaid the sand in monolayer on a plate. As shown in Fig.3, we picked up sandgrains which were attracted by Ferrite magnet (normal magnet) and Neodymium
magnet (strong magnet) in this sequence. Sand which is attracted by Ferrite
magnet is Ferro magnet. On the other hand, sand attracted only by Neodymium

4magnet is Parra magnet. By using magnet, we succeeded in quantifying thefraction of whitish sand and blackish sand.
We classified gravels according to their kinds of rocks (sedimentary rocks,
granite, metamorphic rocks and others) by visual observation. Before theobservation, we sieved the rocks into grain size 4-16mm.

Figure 3: Procedure to Classification with Magnet

2-2 Trench Study and Optical Dating with OSL

Sediments on riverbed or coast are considered to be affected by dams or any
other artificial structures or human activities. To compare the collected sedimentwith the sediments deposited before the dam construction, we performed trench
survey on the old track of the Tenryu River shown in Fig.4, which was buried
before the completion of the Sakuma Dam. Now, the place is utilized as a park.
We selected investigation point in that park, which is indicated in Fig.4.
1947

2003
2003

Tenryu River
Offset (Buried in
1956)

(a)

Trench Point(b)

Figure 4: (a) Comparison of Aerial Photograph Around Tenryu River mouth(b) Location of the Trench

5First, we performed a preliminary survey by digging the ground about 1m.
At the bottom of that small trench, we found the “old” riverbed below thesurface soil. Then, we dug the trench of 4m length, 1m width and 2m depth.
Fig.5 shows the sediment structures of the old riverbed observed at the trench
wall. We collected sediment samples at every 10cm depth in the trench below
the surface soil. We also investigated these sediments (sand and gravels) in thesame methods with sediments sampled from coast and riverbed.
In addition, to confirm the
Sample for OSL
Measurement
difference of sediment from thetrench and one from the present
riverbed, we employed the dating
technique using Optically Stimulated
Luminescence (OSL). As introduced
by Aitken (1998) and others, OSL isgenerated when quartz or feldspar is
exposed to light. OSL intensity
Figure5: Sketch of Trench Wallincreases as the mineral grain absorbs
dose, and the OSL signal is reset withenough exposure to sunlight. This resetting process is called “bleaching”. OSL
is usually used to estimate the burial age of the sediments in the last hundred
thousand years. Shirai, et al. (2006) showed that by investigating the ratio of
bleached grains, which implies sunlight exposure condition of the grains inrecent years, information on the sediment transport process was extracted. We
compared sample’s OSL intensity between original condition (natural) and after
sunlight bleaching to get the percentage of the bleached sediment grains.
To investigate the difference of condition of exposure to sunlight between
sediments on old riverbed and present riverbed, we compared the bleaching
percentage among the 2 samples from the trench and 1 sample from the present
riverbed of the Tenryu River. Sampling points of the 2 samples of the trench arelocated at the depth of 160cm and 200cm from the ground respectively (seeFig.5). Sampling point of the riverbed is about 4km upstream from the Tenryu
River mouth. Because the OSL intensity is easily canceled by the sunlight, we
conducted sampling carefully to avoid the sample from being exposed to
sunlight. We obtained samples by setting in opaque chlorinated polyvinyl
chloride pipes capped with aluminum foil.
We followed Shirai, et al. (2008) in the process of the measurement except
in the grain size of the specimens. We chose feldspar as specimen for OSL
intensity investigation. All the processes were carried out in dark room with
subdued orange light. Procedures of preparation of the specimen are as follows.
First, we sieved washed samples to collect particles whose grain size is 0.30.5mm. Next, we removed organic material and carbonate from specimens withperoxide (10%) and hydrochloric acid (10%), respectively. After heavy liquid
separation using LST heavy liquid, we extracted feldspar by microscopic
handpicking.

6
After this preparation, we laid the individual feldspar grain on themeasurement disc with 1 cm diameter. Then, we start the measurement by an
OSL-TL reader (Risø). The process of measurement is as follows.
After removing unstable OSL signals by pre-heating (250 ℃ , 60s), we
measured natural OSL intensity (OSL intensity of original condition of theparticles) by irradiating infra-red (870nm). Next, we measured its sensitivity ofthe specimen to the artificial radiation for normalization. After irradiating β-ray(ca. 2 Gy), we performed cut-heating (250℃, 60s) to remove unstable signals
and then measured OSL intensity again with infra-red.
With the procedures above, we estimated the normalized OSL intensity of
every specimen. To obtain the OSL intensity of bleached samples, after
exposing the measured samples to sunlight enough (over 7 hours), we measured
OSL intensity again by the same procedure.
2-3 Investigation in River Vegetation

In recent years, river vegetation has developed significantly in the lower
part of the Tenryu River. It is mainly caused by degradation of the low water
channel compared to the high water channel because of the decrease in sedimentsupply from upstream and the reduction of floods by dam operations. We
considered that the sedimentation process in such vegetation area is different
from that in non-vegetated riverbed.
(a)

1

9

10

(b)

Point 8

2
7

6

3
4

8
5

A

(c)

B
200m

Tenryu River

Point B

Figure 6: (a) Location of Investigation Point (b), (c) Condition of Point 8 and B

To investigate the sedimentation process in river vegetation, we performed
grain size investigation on Jan. 14th and 15th in 2007. Fig.6-(a) shows the target
area of this survey. It is located about 14km upstream from Tenryu river mouth.
Points 1 to 10 were located inside the vegetation area (see Fig.6-(b)). To
investigate the relationship between condition of vegetation and the grain size,
we recorded the condition of vegetation (number of trees around the point,height and density of the grass) at every investigation point. Point A and B were7
selected as a point in non-vegetated riverbed for comparison. Point A was
located at the middle of riverbed, without any shields against river flow. On theother hand, Point B was just downstream of drift wood whose height was about
3 to 4m (see Fig.6-(c)).
We investigated grain size distribution of the surface sediments at every
investigation point. Because it is difficult to sieve wet samples, we sieved only
sediments larger than 8mm on site, and the remainings were sieved after drying
at laboratory.
To compare the magnetic component of sand in vegetation with that of othersamples, we collected sand in river vegetation on Aug. 9th in 2007. In this time,
because the river vegetation investigated above has been removed artificially,
we chose another vegetation area near the river mouth (see Fig.7).
1
2 3
4

Point 1

(b)

100m

5
6

Point 6(a)(c)

Figure 7: (a) Location of Investigation Points (b), (c) Condition of Point 1 and 6

3. RESULTS AND DISCUSSIONS
3-1 Results of Classification of Sediment Samples
Fig.8 shows the results of analyses on sand samples of coast with magnet.
When we see the results from east to west, samples near the Ooi River mouth
contains largest fraction of magnet, which means they are blackish. It is causedby the geology of the Ooi River basin, which is mainly composed of
sedimentary rocks of the Mesozoic era and the Tertiary period. As it become faraway from the Ooi River mouth, fraction of black sand in samples decreases
because of the effect of the Tenryu River which has granite belt in upstream.
However, at the points around Tenryu River mouth, fraction of black sandbecomes larger to some extent. And as it becomes far from Tenryu River mouth
westward, fraction of black sand decreases again. As a whole, except the areastrongly affected by the Ooi River, only the points around Tenryu River mouth
have blackish sand.
Recently, sediments from upstream of the Sakuma Dam do not reachdownstream because the Sakuma Dam continues to trap sediments. Therefore,
sediments discharged from the Tenryu River mouth should be affected such

8tributaries as the Misakubo River and the Keta River, whose sediments areblackish. Result of analyses on coastal sand suggests that because of the effectof the Sakuma Dam, sand discharged from Tenryu River mouth has become
blackish in recent years, and its effect is now spreading to the coast around theTenryu River mouth.
Ooi River
Lake Hamana
Tenryu River

Figure 8: Result of Classification of Coastal Sand with Magnet
Fig.9 shows the results of analyses by magnet on samples from Tenryu
riverbed, the trench (the depth of the sampling point was 190-200cm from theground) and the river vegetation. When we see samples from riverbed, first, it
appears that component of samples is strongly affected by geology of each
tributary. The Wachino River is located in the granite belt, therefore, most partis white sand. The Tooyama River, the Misakubo River and the Keta River are
located in the sedimentary rock area, therefore, it contains large amount of blacksand. In the upstream of Tenryu mainstream, most part is white sand because ofsuch rivers as the Wachino River, which comes from granite area. As it comes
downstream, mainstream meets several tributaries from sedimentary rock area,such as the Tooyama River and the Keta River. Because of the effect of suchtributaries, fraction of black sand becomes larger as it comes downstream.
However, samples downstream of the Funagira Dam show smaller content of
black sand (see the sample of 22km and 4km). The fraction of black sand in thesample from lowest part of the Tenryu River is smaller than that in the sample ofcoastal sand near the river mouth.

9
Samples of Tenryu Main Stream

Samples of Tributaries and TrenchDistance from River Mouth

Sakuma Dam

Funagira Dam

Tenryu River

Figure 9: Result of Classification of Sand of Riverbed with Magnet (Grain Size:
0.075-0.425mm)

With respect to the effect of the Sakuma Dam, it is expected that in thedownstream of the Sakuma Dam, sediments of the riverbed are strongly affectedby such tributaries as the Misakubo River and the Keta River, whose maincontents is black sand. Then, sand is expected to be blackish at every place
lower than the Sakuma Dam. However, result of the analyses of sand wasinconsistent with this prospect only in the downstream of the Funagira Dam.
As we explained in Section 2-3, we also investigated the component ofwhite and black sand of the samples in river vegetation. When we compare thoseresults with those of non-vegetated riverbed (the sample of 4km of mainstream),
there are significant difference between them. Fraction of black sand in thesample from vegetation is larger than that of samples not only from riverbed but
also from coast near the river mouth.
This result means not only that sand in the river vegetation are affected bysuch rivers as the Keta River and the Misakubo River which discharge blacksand, but also that in the lower part of the Tenryu River, sand from upstream
deposits only in vegetation, and on coast. The prominence of white sand in thesample of riverbed in lowest part of Tenryu River shows that riverbed is still
being eroded and the no fresh sediments are deposited on the present riverbed.
This is considered to be due to the Funagira Dam (see Fig.9) equipped withsediment discharge gates. Since the Funagira Dam flushes sediments in its
reservoir only during heavy floods, fine sediments, originated from the
Misakubo River and the Keta River, will be transported as washload andtherefore will not be deposited on the riverbed except for vegetation zones. This
scenario also accounts for the prominence of black sand on the coast around theriver mouth. In addition, there were no significant differences in the ratio of
black sand between sample from the trench and the present riverbed (4km). This

10
result means sample from the present riverbed is ‘old’ and the riverbed is eroded
now, which also supports our scenario.
Fig.10 shows the result of classification of gravels whose grain size is 416mm. In the most part, it shows the same tendency with the result of analysesof sand. Because of the difference of structural durability between granite andsedimentary rocks, the fraction of granite in these samples is smaller than that ofwhite sand in sand samples. Although the effect of the Funagira Dam is not soclear in this result, it is consistent with the scenario explained in the above,
because there is no significant difference between the components of riverbed
sample (4km) and trench sample.
Samples of Tenryu Main Stream

Samples of Tributaries and TrenchDistance from River Mouth

Sakuma Dam

Funagira Dam

TenryuRiver

Figure 10: Result of Classification of Gravels from Riverbed by Visual Observation(Grain Size: 4-16mm)

3-2 Results of OSL Measurements
Fig.11-(a) shows the OSL intensity of samples from the trench, which was
located at 160cm below the ground. Natural OSL intensity is higher than that ofbleached sample in every specimen, which means no particle has been exposed
to sunlight in recent years, which supported that we correctly found sediments ofold riverbed. The result of sample from 200cm depth showed almost same result.
Fig 11-(b) shows the result of sample from present Tenryu riverbed which islocated about 4km upstream from the river mouth. Most of the particles show nosign of exposure to sunlight in recent years. This result supports our scenariothat the riverbed of lower Tenryu River is still being eroded and sediments on
the riverbed is ‘old’, which are free from effect of Dams.

11

(b)

(a)

Figure 11: Comparison of OSL Intensity of Samples from Trench and Riverbed(a) Trench Sample (b) Riverbed Sample

3-3 Grain Size Distribution of Sediments in River Vegetation
Fig.12 shows the result of grain size investigation at the river vegetation
14km upstream from the river mouth. The pie chart indicates the weight ratio of
each grain size range. Compared with the result of point A and B which werelocated on the non-vegetated riverbed, grain size of the sediments inside thevegetation area is significantly finer. As shown in Fig.12-(b) and (c), as it
becomes far from the water channel or it is surrounded by more trees, thesediments would also become finer. Especially, the points far from water
channel such as point 7-10 contain considerable amount of silt (finer sediments
than 0.075mm).

9

1

10

Median Grain Size (mm)

(a)

2
6

7

3

100

((b)

10
1
0.1
0.01
0

A
5
B

100m

Median Grain Size (mm)

4
8

5

10

15

Number of Trees within 5m fromthe Point
100

((c)

10
1
0.1
0.01
0

50
100
150
Distance from the L ow Water
Channel (m)Figure 12: (a) Grain Size Distribution of Sediments in River Vegetation (b) Relation
between Number of Trees around the Point and Grain Size (c) Relation between
Distance from Water Channel and Grain Size

12

Fig.13 shows the spatial distributionof thickness of surface sediments layer
and ratio of beach sand trapped by thevegetation more than non vegetatedriverbed (point A). By counting the areaof each category multiplied by the depthof surface layer, we calculated the amountof trapped beach sand in this vegetation
area. It was estimated as 25,000m3.
Through the analyses of aerial
photographs in time series, we estimatedthe period of deposit of fine sediments inthis vegetation to be within 15years. Also,
this kind of river vegetation has
developed in whole area in the lower
Tenryu River, which is defined as the
Figure 13: Estimation of Volume
section between the river mouth and 25km
of Deposition of Fine Sediments
point. Taking those conditions into
in the River Vegetationaccount, the amount of annual deposit ofbeach sand in the vegetation zone in the whole lower Tenryu River was
estimated as 4.2x104m3. Torii et al. (2004) estimated annual discharge of
sediments whose grain size is between 0.106mm and 0.85mm is 10.8x104m3.
Compared to this amount, deposition of beach sand to the vegetation zone issignificant.

4. CONCLUSIONS

Analyses of surface sediments focused on their color and grain size werecarried out to understand the present condition of sediment transport process of
the Tenryu River-Enshu Coast system. We employed analyses technique using
magnet to quantify the ratio of color of sample sand, and OSL intensity toconfirm whether the sediment has been buried for a long time. We also
investigated grain size distribution of sediments in river vegetation area toestimate its effect to entrap fine sediments. Main conclusion is as follows.
1. Effect of dams in the Tenryu River, which is observed as the prominence of
blackish sand, is expanding on the coast around the Tenryu River mouth.
2. In the lower Tenryu River, fine sediments which compose beach sand will
not deposit on the riverbed except in the river vegetation, but go straightly
to the river mouth.
3. Measurements of OSL intensity support our scenario that the riverbed of thelower Tenryu River is still being eroded, and the present riverbed surface iscovered with ‘old’ sediments.
4. The amount of deposition of beach sand in the river vegetation was
estimated as 4.2x104m3/year in the whole lower Tenryu River, which is13significant compared with the amount of sediment discharged from the
Tenryu River mouth which amounts to 10.8x104m3/year.
ACKNOWLEDGMENTS

This study is a part of Tenryu-Enshunada Project financially supported by
the Japan Science and Technology Agency (JST).This research was also funded
by a grant from JSPS(Japan Society for the Promotion of Science) Global
COE(Center of Excellence) program entitled "Global Center for Sustainable
Urban Regeneration"
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Study of Tenryu Watershed and Enshu-Nada Coast. International
Conference on Coastal Engineering 2004, p3110-3122
Okano, M., Yasuda, Y, Mori, K., 2004. Study on the influence of the dam
reservoirs in Tenryu middle stream on the whole sediment-flow system inthe basin. Annual report of the Water Resources Environment Technology
Center, 2003, I-2-1 (in Japanese)
Aitken, MJ., 1998. An Introduction to Optical Dating:The Dating of Quaternary
Sediments by the Use of Photon-Stimulated Luminescence. London:Oxford
Science Publications.
Shirai, M., Tsukamoto, S., and Omura A., 2006. An attempt estimating
incorporation ratio of old grains into turbidity current using an opticallystimulated luminescence technique. Abstructs of 17th International
Sedimentological Congress 2006, Fukuoka (A), p319
Shirai, M., Tsukamoto, S. and Kondo, R., 2008. Transport-depositional
processes of present fluvial deposits estimated from OSL intensity of sandsized grains. The Quaternary Research, 47 (6), 377-386 (in Japanese withEnglish abstract, in press)