A COMPREHENSIVE STUDY ON SEDIMENTMOVEMENT IN TENRYU RIVER WATERSHEDAND ENSHU COAST
Shinji SATO1 and Haijiang LIU21Fellow of JSCE, Professor, Dept. of Civil Eng., The University of Tokyo

(7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan)
E-mail:sato@civil.t.u-tokyo.ac.jp
2Member of JSCE, Research Associate, Dept. of Civil Eng., The University of Tokyo
E-mail:hjliu@coastal.t.u-tokyo.ac.jp

A comprehensive study on surface sand quality in the Tenryu River watershed and the Enshu Coast wasconducted with an aim at describing long-term sediment movement dynamics in a wide region. The study
area exhibits a variety of sedimentary problems such as retention of sediments in reservoirs, riverbeddegradation, dune erosion and coastal erosion. Sediment representation by magnetism and measurement ofOSL (Optically Stimulated Luminescence) intensities was found to provide valuable information on themechanisms of sand movement.
Key Words : coastal erosion, regional sediment movement, reservoir sedimentation, OSL/TL
measurements

temporal and spatial distribution of sediment
movement is of great importance for proposing anefficient countermeasure to coastal erosion.
The implementation of regional sedimentmanagement has been initiated by the Ministry ofLand, Infrastructure and Transport, Japanesegovernment. A frontier plan is compiled for theTenryu River watershed, where riverine and coastal
sediment management is coupled with discharge ofsediments from reservoirs. In parallel to the practicalimplementation, a five year research project, entitled“Dynamic sediment management and coastal
disaster prevention based on advanced technology”,
has been started in 2006 aiming at collaborative
monitoring of sediment movement based on
advanced technologies. In the present paper, resultsof ongoing studies will be described for the Tenryu
River watershed and the Enshu Coast, in which theregional movement and long-term evolution of thealluvial deposits in a relatively wide area, areinvestigated on the basis of the qualitative andquantitative analysis on surface sediments.

1. INTRODUCTION
Coastal erosion has been accelerating worldwide
in the last century. The rate of decrease in land area
in Japan, estimated by the comparison of old and newgeographical maps, is reported to be 24 km2 in recent
15 years (Tanaka, et al., 1993). The erosion has beendeveloped by various mechanisms, such as
construction of ports and harbors, marine sandexploitation and decrease in sediment supply fromthe river. Since coastal erosion gradually develops ina wide area with a relatively long duration, thecountermeasures should be considered for a large
area with long time scales. Considering a fluvial
system composed of a river watershed and asediment cell of the adjacent coast, the estimation ofsediment discharge from the river is essential for theregional sediment analysis. The sediment supplyfrom the river is characterized by the watershed area
and the climate but also influenced by various
anthropogenic impacts such as construction of dams
and weirs (Syvitski, et al., 2005), river route change
and sand dredging from the riverbed. Estimation of1

2. SEDIMENTARY PROBLEMS IN THE
TARGET AREA
The target area of the present study is a fluvial
system composed of the Tenryu River watershed andthe Enshu Coast, which is located at the west ofShizuoka Prefecture, Japan (Fig. 1). The Tenryu
River, which originates from Lake Suwa with adrainage area of 5090 km2 and a route length of 213km, flows to the south and feeds into the Pacific
Ocean. The Enshu Coast is a long sandy beach and it
has a double-arc concave shape separated in themiddle where the Tenryu River delta is formed. In
this paper, sediment movement characteristics will
be described along the Tenryu River starting fromthe Yasuoka Dam to the river mouth with a total
length about 130 km as well as on the coastal area
stretching from the Ooi River mouth to the Atsumi
Peninsula.
Sediment movement characteristics
elucidated in the existing studies are summarized asfollows:

Fig. 1 Tenryu River and Enshu Coast

the dams has been degraded by 1-1.5 m.
(d) In the last several decades, serious coastal
erosion has developed around the Tenryu Rivermouth and along the Enshu Coast. The terrace infront of the river mouth has been eroded. In orderto mitigate this problem, various shore protection
structures have been constructed, such as seawall,
detached breakwaters and groins. The erosion ispropagated to both sides of the river mouth,
reaching the Nakatajima Sand Dune located 4 kmwest of the river mouth.

(a) The amount of sediment discharge from theTenryu River is estimated at about 38×106 m3/year(Ashida et al., 1983), which is the largest in Japan.
This is due to the sharp terrain and the fragile
geology of the catchment area, where the Median
Tectonic Line (hereafter referred to as MTL)
passes though.
(b) The geology of the northwest part of the MTL,
i.e., the upstream region of the Tenryu watershed,is characterized by the igneous rock zone whereas
the southeast of the MTL, i.e., east of thedownstream of the watershed, is composed of asedimentary rock zone. Different geological rockzones are characterized by specific rocks andmineral components, e.g., the granite rocks withquartz and feldspar being two typical mineral
constituents are representative in the igneous rockzone.
(c) A series of dams have been constructed along theTenryu River as well as its branches. In the mainriver route, five dams were constructed (Fig. 1) inwhich the Sakuma Dam, constructed about 70 km
upstream of the river mouth in 1956, is one of thebiggest dams in Japan and interrupts a hugeamount of sediments inside the reservoir. The rateof the sediment retention at the Sakuma Reservoir
amounts to two million cubic meters per year.
Together with huge amount of sand exploitationfrom the riverbed during the rapid economic
growth period in 1960’s, it significantly reduced
the sediment supply to the downstream zone and tothe sea. The riverbed on the downstream of

3. QUANTITATIVE DESCRIPTION OFSEDIMENT MOVEMENT
Figure 2 illustrates nearshore bathymetry of theHamamatsu Coast, located on the west side of theTenryu River mouth. The bathymetry survey has
been conducted by the Ministry of Construction andthe Shizuoka Prefecture since 1962. The comparisonof bathymetries of 1971 and 2005 demonstrates
significant coastal erosion and shore protection
efforts by a series of detached breakwaters. Figure 3
shows temporal variation of total sand volume in thenearshore area surrounded by the dashed line in Fig.
2. The arrow on the bottom of the figure indicates theoccurrence of Tenryu River flood greater than 7000
m3/s. In Fig. 3, sudden increase in the total sandvolume between successive data is observed in theperiods 1981-2, 1983-4, 1991-4 and 1999-2000, allof which correspond to the occurrence of large flood.
By excluding these four periods, the sand volume in
Fig. 3 shows gradual steady decrease at a rate of
3×105 m3/year. This amount is considered to bebalanced by the westward longshore sand transport
rate.
Figure32Fig. 4 Topographies of Nakatajima Sand Dune (1957and 2005)

Fig. 2 Nearshore bathymetries of Hamamatsu Coast(1971 and 2005)

Fig. 5 Temporal variation of mean elevations onNakatajima Sand Dunealso indicates that the sand supply from the Tenryu
River is intermittent in the sense that it is dependent
on the occurrence of large flood. However, when we
assume that the formation of the river delta
topography is dynamically in equilibrium, theaverage sand supply from the river should bebalanced by the longshore sand transport rate. It istherefore concluded that the average sediment supply
from the Tenryu River is estimated at 6×105 m3/year,

Fig. 3 Temporal variation of total sand volume on
Hamamatsu Coast3

Qualitative analyses on the collected samples
were conducted in terms of the sediment magnetism,
which is directly related to the particle mineral
constituents. Collected samples were sorted into
three components, i.e., particles with strong
magnetism (mainly containing mineral constituents
such as magnetite), particles with general magnetism(mainly containing mineral constituents such as
biotite, amphibole and pyroxene) and particles
without magnetism (mainly containing mineral
constituents such as quartz and feldspar). By
evaluating the weight ratio of every magnetic
component and considering the geological features,quantitative discussion was carried out, throughwhich we tried to achieve the sediment movement
mechanism in the corresponding area. The sandcomponent without magnetism is mainly
white-colored; whereas the sediment component
with magnetism, including both the strong andgeneral magnetism, is mainly black-colored.
The scanned sand images are presented in Fig. 6.
Images listed on the left column are from the mainriver route, whereas ones on the right column arefrom branches. At the same time, the weight ratio of
three magnetic constituents for every sample isillustrated using pie chart.
Along the main river route, the sand color and themagnetism clearly reflect the local geology. The
upstream region of the Tenryu River watershed, e.g.,the Wachino River area, is mainly composed of an
igneous rock zone in which granite rocks are
dominant. For other three main branches, Tooyama
River, Misakubo River and Keta River, all of thempass though the sedimentary rock zone which aremainly composed of black-colored sediments. Aftermerging with Tenryu River, the influence frombranches can be observed. For Samples 2, 3 and 4 in
the main riverbed, sand gradually changes color fromwhite to black due to the sand supply from branches.
Another important factor is that the presence of dams
along the main river route also changes the sedimenton the riverbed. The effect of the Sakuma Dam is themost significant. Considering the relevant geology,
the entrapped sediments are mainly white-coloredparticles originated from igneous rocks. Hence, after
the Sakuma Dam, the contribution of sediments from
Keta River becomes dominant, which plays asignificant role to black the Tenryu riverbed. Thiscan be verified from Sample 4, which is collectedjust downstream of the Tenryu-Keta river
conjunction. However, further looking at the sandsamples in the downstream area, i.e., Samples 5 and6, both are located after the Funagira Dam, the sandcolor becomes somewhat bright comparing with thehalf of which can contribute to the sediment supply
for the Hamamatsu Coast estimated at 3×105
m3/year.
The decrease of the amount of sand is observedon coastal dunes as well. Figure 4 illustrates
topographies of the Nakatajima Sand Dune located
4km west of the Tenryu River mouth. The erosion of
the dune is a major concern to the local governmentand residents since the dune is utilized for many
purposes. The elevation of the dune has been
lowered especially in the west part of the dune.
Figure 5 shows the mean ground elevation in the six
sub-regions of the Nakatajima Sand Dune indicatedby the dashed line in Fig. 4. The rate of decrease in
the ground level in the west part of the dune isestimated at 5.8 cm/year, projecting the loss of thedune in less than 20 years.

4.

INVESTIGATIONQUALITY

ONSEDIMENT

Sediment movement in the watershed scale will
be influenced by many impacts in a wide area.
Quantitative analysis on the influence of these
impacts is difficult since most of them are exerted
within the economic growth period and interact witheach other along the temporal and spatial scales. Dueto the superposition of various processes in a rather
complex manner, a sediment budget analysis only
based on the topography survey data sometimes fails
to describe the complicated picture of sediment
movement. Further investigation on the sedimentquality will be useful for these situations. Suchsediment quality is represented by certain physicaland chemical properties, e.g., sand size, specificdensity, grain color, shape, magnetism, radioactivity,
the mineral components and the luminescencecapability.
In this study, investigation wasconducted on the basis of sand mineral (magnetism)
constituents and luminescence features, which arefurther specified in the following two subsections,respectively.
(1) Mineral (magnetism) analysis
In order to investigate the mineral (magnetism)
characters of sediments, sediments were sampled atvarious locations in the watershed. Surface sandsamples 10cm below the ground surface werecollected. The collected sand samples were dried in
the oven, which were then sieved to isolate grain size
less than 2 mm. Then, sand particles were pasted on awhite paper (3.0 cm by 1.5 cm) and designated usinga scanner.
4rather dark color of Sample 4. More discussions canbe found in the trench study.

5

Fig. 6 Surface sand samples on the Tenryu watershed

Fig. 7 Surface sand samples on the Enshu Coast6

In order to understand sediment movement processes
before the dam construction, a trench study wasconducted at Iida Park (5 km from the river mouth),where the original river route before the constructionof Sakuma Dam was located. Sediment samples inthe trench were used to compare with the sandsample collected in the present riverbed at Kaketsuka(Sample 6 in Fig. 6). It was found that the magnetic
constituents are almost the same for the Iida sample
and the Kaketsuka sample, which demonstrates thatthe sediment contribution from the Keta River to theTenryu riverbed sand constitution along thedownstream river route is negligible. The deposit
constituents from the present riverbed are nearly thesame as ones from the old riverbed and there is nosediment accumulation in the present downstreamriverbed. This considered to be ascribed to thescenario that the recent sediment supply from Keta
River is washed away into the coastal area andcannot stay in the present riverbed. Along the Riverroute, the effect from Keta River is limited just to thedownstream of Tenryu-Keta river conjunction. To
justify this scenario, OSL intensities for sandparticles were measured.
Figure 7 illustrates surface sand samples
collected along the Enshu Coast. The same magnetic
sorting procedure was applied for sample
investigation. It is found in Fig. 7 that the sandsample is black at the Ooi River mouth, which isconsistent with the geology of the Ooi Riverwatershed. Going to the south, sand samples become
white. The magnetism analysis also supports thisobservation. Grains without magnetism are about
30% in Sample 1 but about 70% in Sample 7. Thismay be ascribed to the significant sediment discharge
from the Tenryu River, a certain part of the alluvial
deposits from the Tenryu River can bypass theOmaezaki Cape and affect the coastal beachconstitution over there. Considering that it may takea rather long time for the particles to move such long
distance, it is assumed that these sands are oldTenryu discharge before the construction of theSakuma Dam. Hence, they contain a large amount ofwhite-colored components from the igneous rockzone, which bleach the coastal beach south of the Ooi
River. After going around the Omaezaki Cape,
Sample 12, to the Tenryu River mouth, Sample 23,

the black sand component increases and the sandcolor turns to be dark. Passing though the rivermouth and going further west, the black sandcomponent decreases again as demonstrated fromSamples 29 and 34. This is considered to be due tothe change of the sediment delivery from Tenryu
River. After the construction of Sakuma Dam,sediment discharge into the Enshu Coast iscomposed of a predominantly black-colored
constituent owing to the contribution from Keta
River in the sedimentary rock zone. After theTenryu-carried sediments pour into ocean, thesesands are gradually transported on both sides alongthe Enshu Coast owing to the longshore sedimenttransport.
(2) OSL/TL analysis
Quartzandfeldspargrainsexhibitlaboratory-stimulated
luminescenceasaconsequence of their natural radiation exposure
while buried in a sediment environment (Aitken,1998). Nuclear radiation is emitted when a nucleus
undergoes radioactive decay. Such natural radiationinduces the electron ionization in the grain crystal
lattice. Luminescence is the emission of light fromthe particle crystal after being stimulated by certain
external energy through the detrapping agency, suchas heating or absorption of a photon of light. Theformer is termed thermoluminescence (TL) and thelatter is known as optically stimulated
naturalluminescence(OSL). The magnitude of TL/OSL
emission in a
bleached
grain is proportional to the number of trappedelectrons and hence to the accumulated energy thathas been stored from the flux of nuclear radiation. In
general, natural erosion and transportation of
sedimentary particles from the upstream riverwatershed to the downstream area, and to the rivermouth, finally carried in the nearshore region, areaccompanied by solar radiation exposure. Such sandmovement reduces the Sample
particleIDTL/OSL signals bydepopulating the trappedelectronic charges from the(a) Iida Trench
lattice defects where they were stored during burial.
This illuminates the possible use of TL/OSL as a tool
naturally bleached particles
to describe
the temporal and spatial sediment
movement processes.
naturalbleached

7Sample ID(b) Kaketsuka riverbed
Fig. 8 OSL intensities of sand samples collected at(a) Iida trench and (b) Kaketsuka riverbed

sediment movement mechanisms in a wide area.

Figure 8 shows OSL intensities for sand samples
at Iida trench (Fig. 8(a)) and Kaketsuka riverbed(Fig. 8(b)). Natural OSL intensities are comparedwith OSL intensities after complete sunlightbleaching. The sand particle is judged to be exposedto the sunlight in modern era if the two intensities are
similar. No particles are judged to be exposed to thelight for Iida trench samples. For Kaketsuka sample,
very few sand particles are judged to be exposed tothe light. This result shows that few sand particles
are accumulated on the present riverbed, supporting
the sediment movement scenario that most ofsediment particles are directly flushed to the sea.

ACKNOWLEDGMENT: This paper describes
preliminary results of Tenryu-Enshunada Project
financially supported by the Japan Science andTechnology Agency (JST). The authors are gratefulto all the members of the Project, Toyohashi
University of Technology, Hamamatsu Photonics,
Honda Electronics, Public Works Research Centerand Hamamatsu City for stimulating discussions.
REFERENCES
1) Aitken, M.J.: An Introduction to Optical Dating: the datingof quaternary sediments by the use of photon-stimulated
luminescence, Oxford Science Publications, 267p., 1998.
2) Ashida, K., Takahashi, T. and Michiue, M.: River deposit
disaster and its countermeasure – sediment transport,
landslide, reservoir sedimentation and change of river route,
Morikita Press, Disaster Prevention Series, 5. 260pp., 1983
(in Japanese).
3) Syvitski, J.P.M., Vorosmarty, C.J., Kettner, A.J. and Green,
P. : Impact of humans on the flux of terrestrial sediment tothe global coastal ocean. Science, 308: 376-380, 2005.
4) Tanaka, S., Koarai, M. and Fukazawa, M.: Measurement on
the long-term change of the coastline in Japan by using
1/50,000 topographic maps. Proc. of Coastal Engineering,JSCE, 40: 416-420, 1993 (in Japanese)

5. CONCLUDING REMARKS
A comprehensive investigation was conducted on
the long-term evolution of sediments in a fluvial
system composed of the Tenryu River watershed andthe Enshu Coast. Investigation of sediment quality,
combined with analysis of sediment quantity, wasfound to provide valuable information on the8