Journal of Coastal Research

255

1096–1105

West Palm Beach, Florida

September 2009

Investigation of the Sediment Movement along the
Tenryu–Enshunada Fluvial System Based on Feldspar
Thermoluminescence Properties
Haijiang Liu†, Syunn Kishimoto†, Tomohiro Takagawa†, Masaaki Shirai‡, and Shinji Sato†
Department of Civil Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku
Tokyo 113-8656, Japanhjliu@coastal.t.u-tokyo.ac.jp†‡
Ocean Research Institute
The University of Tokyo
1-15-1 Minamidai, Nakano-ku
Tokyo 164-8639, Japan

ABSTRACT
LIU, H.; KISHIMOTO, S.; TAKAGAWA, T.; SHIRAI, M., and SATO, S., 2009. Investigation of the sediment movement
along the Tenryu–Enshunada fluvial system based on feldspar thermoluminescence properties. Journal of Coastal
Research, 25(5), 1096–1105. West Palm Beach (Florida), ISSN 0749-0208.
The thermoluminescence (TL) properties of feldspar were used to investigate the sediment movement processes in a
fluvial–coastal system. Field samples were collected at various locations along the Tenryu River and the Enshunada
Coast in Japan. After a series of pretests, an appropriate TL measuring sequence was proposed for this study. Applying
this approach, the natural TL intensity of feldspar grains was measured. Owing to the young age of the researcharea, it was assumed that the natural TL difference was ascribed to the sample depositional environment under which
different possibilities of sunlight exposure exist. Surface sediment particles in the target area were classified into
three groups in terms of TL intensities, i.e., river sand with large TL signals, coastal sand with medium TL signals,
and dune sand with small TL signals. Stratified configuration of the Nakatajima Coastal Dune was observed fromthe underground-sample TL glow curves with a top, windblown, dune-sand layer; a bottom, wave-induced, coastal
sand layer; and a mixing layer in between. A rather complex sediment-movement pattern in front of the Tenryu Rivermouth was revealed after investigation of the underwater samples. Because of the seawater influence, acting as anultraviolet filter to sunlight, underwater samples present a larger, high-temperature TL peak than the low-temperature peak; whereas these two are almost the same for ground samples.
ADDITIONAL INDEX WORDS: River, coast, dune, surface sand, underground sediment, underwater sample.

INTRODUCTION
Accurate and reliable measurements of sediment transport
are necessary for both developing efficient countermeasures
to coastal-erosion problems and proposing better predictive
tools in coastal-engineering applications. In a case study ofthe nearshore sediment processes, identification of sedimentsources, sinks, transport paths, and historical evolution areof crucial importance. Therefore, a comprehensive insight isrequired of the entire fluvial system, composed of both a riverwatershed (sediment source) and coastal sediment cells (sediment sink) in the nearshore area.
Traditionally, early field works, mainly using fluorescent
tracers to investigate the nearshore sediment movement,
have involved a variety of techniques. White and Inman(1989) studied longshore sediment transport using dyemarked tracer sands. Net sediment movement over periods
ranging from one to five tidal cycles was investigated using
fluorescent tracers in Voulgaris et al. (1998). Later, McComb
and Black (2005) used two colors of artificial fluorescent tracers to monitor the sediment entrapment/bypassing of a port
entrance as well as the sand movement from a nearshore
DOI: 10.2112/08-1065.1 received 30 June 2008; accepted in revision
28 July 2008.

dredged-sand dump mound. However, sand tracer studies
suffer from many practical problems in sampling the tracer
distribution and in elaborate methods of determining tracer
concentration in samples, e.g., counting. Nevertheless, the artificially injected sand tracers are different from the natural
subaqueous bedload in physical and chemical properties, e.g.,grain size, sand size distribution, shape, specific density, andmineral components, which may introduce inaccurate or unreliable field measurements. At the same time, other methods for investigating sediment movement in the field wereintroduced, such as traps (Dean, 1989), mineral composition
analysis (Sato et al., 2004), and magnetism analysis (Liu etal., 2007). Among those methods, using large-scale sedimenttraps, such as inlets or groins, is a rather passive approach,in which transported sediment is accumulated for periods ofdays to years. Mineral or magnetism analysis is strongly related to the local geology, and in general, such techniques are
fairly laborious. Intercomparison among various methods forstudy of coastal sediment transport, including each technique’s theory, problems/solutions, advantages/limitations,and practical tricks was summarized and presented in White(1998).
Recently, Rink (1999, 2003) suggested the use of the thermoluminescence (TL) properties of quartz and feldspar grains
as a transport indicator in the study of various coastal de-Sediment Movement Study Based on Thermoluminescence

positional processes. Shirai and Omura (2007) and Shirai,
Tsukamoto, and Kondo (2008) proposed the use of the percentage of sand particle bleaching to investigate the sediment
movement process, based on the optically stimulated luminescence approach. In these methods, in contrast to the traditional injected-tracer methods in which an artificial material is added at a particular moment, the sediment particle
constituting the investigated object (natural sand), itself, isused as a tracer to monitor the sand movement. This would
allow more facility and reliability in experimental design ofcoastal process studies. Investigations of the longshore andcross-shore sediment movement, and the tracing of coastal
sands sourced in areas of river deltas, local coastal sequences,
and anthropogenic deposits, are all potential applications forthis new technique. Pioneering studies have been conductedon storm deposits along a specified cross-shore beach profilein Canada (Rink, 1999) and on the littoral sediment movement along the Israel coast (Rink, 2003). Only surface samples were considered in these studies. However, more studies
are needed to establish the degree of TL homogeneity andheterogeneity within surface, underground, and underwatersand bodies. Further research and field applications areneeded to verify the feasibility and applicability of the TL
property as a useful tool for studying nearshore sediment
movement.
In this article, we applied feldspar TL measurements to an
investigation of the sediment transport along the Tenryu–
Enshunada fluvial system. The regional sand transport andhistorical evolution of the alluvial deposits in the corresponding area were investigated by considering TL signal differences among collected samples to outline characteristics ofthe sediment movement in the fluvial system. After a seriesof pretests, an appropriate TL measurement sequence wasproposed for this study. Applying this approach, the grain’s
natural TL signals were measured for the collected field samples. Intercomparison and discussions of the measured, normalized, natural TL signals, together with comparison between a natural and a test dose of the TL integration over atemperature range of 200⬚–400⬚C display a comprehensive
feature regarding the spatiotemporal distribution of sediment
movement in the target area.

THERMOLUMINESCENCE
Quartz and feldspar grains exhibit laboratory-stimulated
luminescence as a consequence of their natural radiation exposure while buried in a sediment environment. Nuclear radiation is emitted when a nucleus undergoes radioactive decay. Such natural radiation is mainly from potassium (40K),
thorium (232Th), uranium (238U), and their daughter products(there are also certain contributions from cosmic rays, as well
as a minor contribution from rubidium, 87Ru), which induces
the electron ionization in the grain crystal lattice. Then, theionized (detached) electrons are diffused and trapped in thelattice defects (Aitken, 1998). This irradiation and storage
process is presented in Figure 1a, through an energy-level
diagram. Luminescence is the emission of light from the particle crystal after being stimulated by certain external energies through the detrapping agency, such as heating or ab-

1097

Figure 1. Energy-level diagram for TL and OSL processes (modified after Aitken, 1998).

sorption of a photon of light. The former is termed thermoluminescence (TL), and the latter is known as optically stimulated luminescence (OSL). The light emission occurs during
the detrapping–recombination of electrons between traps (donors) and luminescence centers (acceptors) in the defectstructure of the quartz or feldspar crystal lattice. This electron eviction and light emission mechanism is illustrated in
Figure 1b.
The magnitude of the luminescence emission in a grain is
proportional to the number of trapped electrons and hence tothe accumulated energy that has been stored from the fluxof nuclear radiation. The more prolonged the exposure to nuclear radiation, i.e., the longer the burial time, the greater
the number of trapped electrons. In general, natural erosion
and transportation of sedimentary particles from the upstream river watershed to the downstream area, to the rivermouth, and finally, to the nearshore region, are accompanied
by solar radiation exposure. Such sand movement reduces
the particle luminescence signals by depopulating thetrapped electronic charges from the lattice defects where they
were stored during burial. Considering the extent of the incoming light flux, it is thought that the more significant thelight exposure, the smaller the luminescence signal. This illustrates the possible use of TL/OSL as a tool for describing
the temporal and spatial sediment-movement processes.
Nevertheless, the same sunlight exposure induces different
amounts of reduction (bleaching) of the TL and OSL intensities because of the different sets of donor–acceptor systems(Aitken, 1998; Duller, 1997; Godfrey-Smith, Huntley, andChen, 1998). Donor–acceptor systems for TL have long response time to light exposure (hours to days), whereas the
OSL system is much more sensitive, in terms of response
time, on a scale of seconds to minutes. Rink (1999) found no
OSL signal remained in the storm-induced samples collected
at the subaqueous, swash, berm, and dune areas, whereas
there existed a significant difference with respect to the TL
signals after measuring the same samples. At the same time,
the sample collection requirements are much less stringent
in the case of the TL study. In contrast to OSL, in which
extreme caution must be used to prevent light exposure during sample collection, using TL allows a minimal amount ofadditional exposure in the field. Taking these items into account, the present study applied the TL technique for further

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Figure 2. Research area and sampling locations. The 䢇 mark represents
the sampling location, and the 円 symbol represents the geo-slicer survey
line along which underground samples G1, G2, and G3 were extracted.

investigation of sediment movement in the case study fluvial
system.

RESEARCH AREA AND SEDIMENT SAMPLING
In this study, the target area is a fluvial system composedof the Tenryu River watershed and the Enshunada Coast,
which is located to the west of the Shizuoka Prefecture in
Japan (Figure 2). There are several typical characteristics related to this region, such as the largest amount of sediment
discharge in Japan from the Tenryu River, the anthropogenic
impacts from dam construction, sand exploitation, and so on.
Coastal sands within this area predominantly originate within the Tenryu River watershed. A couple of hundred years
ago, the large amount of sediment discharge from the Tenryu
River caused the shoreline of the corresponding area to progress seaward. However, during the economic development of
the past several decades, a series of dams was constructedalong the Tenryu River, which along with sand dredging,have significantly reduced the sediment discharge to the Enshunada Coast. This causes severe erosion problem along the
Enshunada Coast. Based on topographical survey data, it
was found that, just in front of the Tenryu River mouth, thepresent seabed bathymetry, with a 10 m water depth, wasonly 5 m deep about 20 years ago. After analysis of a seriesof historical maps, Hiramatsu et al. (2008) concluded that the
Nakatajima Coastal Dune developed in the past 200 years.
Therefore, the present research area is considered to be arather dynamic region, with a relatively young age from thegeological viewpoint.
Sampling locations are also marked in Figure 2. To clarify
the spatial distribution of TL signals, i.e., the development of
sediment transport processes, surface samples (about 10 cm
below the surface) were collected at the subaerial–subaque-ous areas, which include two nearby samples extracted at
Kaketsuka, S1/S2 (about 3 km upstream from the Tenryu
River mouth); one sample from the swash zone at the Tenryu
River mouth sand spit, S3; and one sample from the swash
zone of Nakatajima Coast, S4 (about 4 km west of the rivermouth). Sample S1 was collected from one side of the lowwater river channel bank, and sample S2 was from a riversand dune in the high-water channel. In the field, these locations emerge and submerge intermittently, owing to thechanges in river water levels (S1/S2) or influences by tidelevels and wave climates (S3/S4). Furthermore, surface sands
were also sampled at the inland of Nakatajima Coastal Dune,
S5. This location is an aeolian environment. Hence, the surface sand at S5 is windblown, with a high probability of being
exposed to sunlight. To sample surface sands, an opaque,polyvinyl chloride tube that was open at one end was used.
After digging a sampling hole, the tube was inserted into thesand, then, taken out, and immediately put into a dark bag
to avoid further light exposure.
To specify the vertical structure as well as the provenanceof the Nakatajima Coastal Dune, underground TL samples
at different depths were collected along the extracted sandcores in a geo-slicer study, which included three different locations (G1, G2, and G3). G1 and G2 are located along thedune ramp, and G3 is at an interdune area. Details of thegeo-slicer study can be found in Takagawa et al. (2008). Furthermore, an underwater sample (C1) was collected as a 10m-long boring core, which was extracted at an offshore point
just in front of the Tenryu River mouth at a water depth of
10 m. Sample C1 was used to reveal the sediment moving
pattern in a rather dynamic nearshore zone with frequentchanges in the bottom bathymetry. During the geo-slicer andboring-core sample collections (the G series and C1 samples),the whole sediment core was covered by an opaque shelter atthe beginning, and photographic, dark film cases were used
to extract the TL samples at various designated depths. As awhole, great effort was taken to prevent significant, naturallight exposure during field samplings.

SAMPLE PREPARATION AND
EXPERIMENTAL METHODOLOGY
Sample Preparation
After field collection, all sample pretreatments were conducted in a dark room. In the laboratory, it was of crucial importancethat all pretreatment processes were done in semidarkness to
avoid the severe risk of underestimating the luminescence signals (Aitken, 1998). Taking into account that luminescence signals are less sensitive to low energy photons (red to green) thanto higher energy ones (blue to ultraviolet), laboratory preparation was carried out under subdued, orange light (600 nm),which had little influence on the TL signals. Initially, the collected samples were wet-sieved to obtain the 180–300 ␮m grain
fraction. Subsequently, that fraction was treated with 15% hydrogen peroxide and 18% hydrochloric acid to remove organic
matter and carbonate minerals, respectively. Then, the potassium-feldspar–rich fraction (K-feldspar; density less than 2.58
g/cm3) was isolated by lithium–heteropolytungstate heavy liquidusing a separating funnel. All drying was at the room temper-

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Sediment Movement Study Based on Thermoluminescence

ature to avoid effects from thermal transfer. Using this approach, following Huntley and Baril (1997), the resulting grains
were typically half K-feldspar and half quartz and plagioclase
feldspar.
There are many practical advantages of using feldspar inluminescence applications (Duller, 1997; Huntley and Lamothe, 2001), such as the great luminescence intensity of the
K-feldspar (suitable for young samples), the significant proportion of potassium, and the avoidance of using concentratedhydrofluoric acid etching. Taking into account that the collected samples in this study were geologically young, theK-feldspar fraction of each sample was used.
Nevertheless, when using feldspars, the difficulty of anomalous fading (Wintle, 1973) has to be considered because, if
present, it will introduce an age shortfall into the dating ofsamples, despite apparently adequate preheating of samples
before measurement. Widely discussed mechanisms foranomalous fading include the localized transition model andquantum mechanical tunneling (Aitken, 1998; Duller, 1997).
Visocekas et al. (1994) proved that the quantum mechanicaltunneling was responsible for anomalous fading in feldspars
after detecting the tunnel afterglow. They concluded thatmost feldspars show anomalous fading. Mejdahl (1983), however, showed that many feldspars extracted from sediments
and archaeological samples did not show a significant degreeof anomalous fading. Later, Huntley and Lamothe (2001) argued that this phenomenon appears to be ubiquitous for mostfeldspars. Feldspars that form by rapid cooling display anomalous fading because they are disordered, whereas those thatform during slow crystallization and have an ordered lattice
structure do not. This is also specified in Aitken (1998). Hütt
et al. (1993) found that the K-feldspars of granitic origin, did
not display TL fading. K-feldspars used in the present study
originated from the Tenryu River’s upstream, igneous rockzone (Liu et al., 2007), which formed gradually with an ordered lattice structure. Hence, we postulate that the K-feldspars collected in this study should not be affected by anomalous fading. If this is not the case, the effect of anomalous
fading would still be minor because the collected samples areyoung, and Huntley and Lamothe (2001) mentioned thatanomalous fading is more severe in older samples.
Luminescence measurements were performed using both aHarshaw Model 3500 Manual Thermoluminescent Detector(TLD) Reader and a Risø 48-sample automated TL/OSL
Reader with an internal 90Sr/90Y ␤-irradiation source (delivering 0.12 Gy dose/s). Four aliquots of each sample were
mounted on 6 mm (TLD reader) or 10 mm (TL/OSL reader)diameter, stainless-steel discs using silicone spray as an adhesive. To avoid the heterogeneous effect from individual
grains (grain-to-grain variation), the monolayer technique
was applied, so that each disc contained approximately several hundred feldspar particles. To produce a TL glow curve,
samples were measured using a heating ramp rate of 5⬚C/s(up to 500⬚C) and were detected though the photomultipliertubes (TLD reader) or a photomultiplier using a Schott BG39
(infrared absorbing) and a comparatively narrow glass filter,
Corning 7-59, combination with a transmission window between 320 and 480 nm. Such a combination effectively ex-

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Figure 3. Delayed test for determining the preheating conditions, using
thermally bleached sample, S3. The solid line is the TL measurement
immediately after 7.2 Gy laboratory irradiation; the dashed line is the TL
measurement with a 30 min delay after laboratory irradiation (a) without
preheating, and (b) with preheating at 180⬚C for 10 s.

cluded luminescence from any quartz and from most plagioclase feldspar from being measured.

Experimental Methodology
Before proceeding to the TL measurement, an adequate TL
test sequence was examined. In this sequence, a series of TL
pretests were investigated first. Among these, the preheating
test, used to decide a suitable preheating temperature andduration, was of great importance to drawing a correct luminescence response curve (Aitken, 1998). There are a number of feldspar TL peaks, corresponding to a series of TL
traps, which reflect variations in the mineralogy (Duller,
1997). When feldspar is artificially irradiated in the laboratory, many shallower traps are filled, which produces lowtemperature TL peak as shown in Figure 3a at around 110⬚C.
A 110⬚C, TL peak for K-feldspar was also detected by Strickertsson (1985). In general, such low-temperature TL peakdoes not exist in natural TL signals because of the long storage times in the field. Shallow-trap–related low-temperature
TL peaks are unstable, and their signals gradually decay
with time. Figure 3a illustrates a delayed test result at two
delaying time intervals (0 and 30 min) after a 60 second artificial-dose irradiation on sample S3, which was thermallybleached before the delayed test. It is obvious that with a 30
minute detention, the TL signals decrease dramatically in the
low-temperature range, whereas there is no clear TL signal
reduction in the high-temperature region. Preheating to acertain temperature can help to remove such unstable artificial TL signals in the low-temperature region. The boundary
between low- and high-temperature regions (unstable andstable TL signals) is located around temperature of 180⬚C, as
highlighted by the vertical line in Figure 3a. Hence, a 180⬚C
preheating temperature was chosen for this study. Applying
a longer preheating duration might remove useful TL signals,
even in the high-temperature region. A preheating durationof 10 seconds was used, which is the same condition that wasapplied by Wallinga, Murray, and Wintle (2000) and by Blair,
Yukihara, and McKeever (2005) in their feldspar tests. Figure 3b shows the same delayed test result as in Figure 3a,but with preheating of 180⬚C for 10 seconds before the TL
measurement. In this Figure, the role of the preheating isclearly demonstrated because there is no distinct variance

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Figure 4. Difference between the natural TL signal (solid line) and the
7.2 Gy test-dose TL signal (dashed line) for several samples S2, S4, S5,
and C1-30.
Figure 5. General TL test protocol in this study.

between the TL measurements taken immediately and thosetaken after a 30 minute delay, which suggests the low-temperature, unstable TL signals were efficiently removed after
preheating.
To compare the sample aliquots collected at different locations in the field, normalization of the natural TL signal
was needed to compensate for the differences that are inherent in the samples, such as aliquot mass. Therefore, after
each natural TL measurement, the sample discs were firstheated to 450⬚C for 30 seconds to remove any residual TL
signal (Duller, 1992). Then, each aliquot was irradiated at alaboratory test dose of 60 second (7.2 Gy), and measurements
were carried out again to obtain the test dose–induced TL
response curve for normalization. The following equation wasapplied for the natural TL normalization,
Normalized natural TL ⫽Natural TL
Test dose TL.

(1)This equation was performed after taking into account thedifference between natural and the test-dose TL glow curves.
Figure 4 presents a comparison of natural and test-dose TL
glow curves for several test aliquots from river sample S3,coast sample S4, dune sample S5, and the underwater sample
C1-30 (30 cm below the core surface). There exist clear differences between these natural and test-dose TL curves, suchas the curve shape and the peak location, which implies thatthe natural TL signal cannot be reproduced by simply adjusting the artificial irradiation-dose magnitude. Such differences are assumed to be to the result of the complex natural
sediment-transport process. In the field, grain particles undergo repetitive processes of erosion, transportation, and deposition, which induce continuous storage and bleaching ofgrain TL signals. Hence, the measured, natural TL signal
may come from several such cycles. The complex, natural processes can not be reproduced from a single laboratory testdose irradiation. Shirai, Tsukamoto, and Kondo (2008) alsomentioned the influence of such complicated natural process

after their OSL measurements. To minimize the effect fromthe natural/test-dose TL difference, a relatively wide temperature range (200⬚–400⬚C) was considered as the average forthe test-dose TL signals. Figure 4 shows that this temperature range covers the predominant TL signals. A similar temperature range (250⬚–400⬚C) was also applied by Zhang, Li,
and Tso (2001) in their TL studies.
Figure 4 also reveals that, for the collected samples, in general, the natural TL signals are much larger than the 7.2 Gy
test-dose–induced values. Within these results, dune sample
S5 presents the smallest natural TL signal with a magnitude
almost equal to the test-dose one. This indicates that the apparent natural dose is about 7.2 Gy for dune sands. If suchan apparent natural dose came from natural electron accumulation in the defects as a result of environmental radiation
after burial, the age of burial would be about 7,000 years
after considering the fairly weak natural radioactivity in Japan, which has a feldspar dose rate of about 0.6–2.1 Gy every
thousand years (Watanuki, Murray, and Tsukamoto, 2005).
However, that is not the case in this study. As previously
mentioned, the target area is a geologically young region,
with a formation age of only several hundred years. Hence,
we hypothesize that the TL signals of the collected samples,
as well as the differences among them, do not come from thelimited variance in burial time, but rather from the differences in the sample depositional environments, i.e., aeolian,
subaerial, and subaqueous, under which different amounts ofsunlight exposure possibilities exist. This subject is discussed
further in the ‘‘Results and Discussions’’ section.
Based on the pretests, a generalized TL test sequence wasproposed for this study. Figure 5 presents the corresponding
test sequence, together with explanations for each step.

RESULTS AND DISCUSSIONS
Based on the proposed test protocol, TL measurements
were carried out for all collected field samples. In the follow-

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Sediment Movement Study Based on Thermoluminescence

Figure 6. Normalized natural TL glow curves for collected samples at
S1 and S2 from Kaketsuka, S3 from the river mouth sand spit, S4 fromthe Nakatajima Coast, and S5 from Nakatajima Coastal Dune.

ing sections, the normalized TL glow curves (the average offour discs for each sample) are presented, and the integrationof the signals over the 200–400⬚C temperatures, together
with their standard deviations were calculated for data interpretation and intercomparison. Results are categorized
into three groups with respect to their sampling locations, i.e.,surface samples (S1–S5), underground samples (G1, G2, andG3), and underwater samples (C1).

Surface Samples
To clarify the spatial distribution of TL properties in the
research area, all surface samples, S1–S5, are considered inthis subsection. Figure 6 illustrates the normalized TL glow
curves for these samples. In this Figure, it is clear that the
TL signal is large along the river route (S1/S2) and small at
the coastal area (S3/S4). There is no clear difference between
S1 and S2. Similar results were also found for the S3 and S4
samples. Compared to the unidirectional river flows (S1/S2),
sand particles may be suspended and bleached by sunlight
more easily and efficiently under repeated wave motions, especially in the dynamic swash zone (S3/S4), where sedimentparticles are frequently mobilized by the wave uprush anddownwash process. Such TL signal reduction along the coastal region can be clearly observed by comparing the TL curvesof S1/S2 and S3/S4. Nevertheless, Rink (2003) observed,
along the coastal line, a trend of decreasing residual TL intensity with increasing distance from the sand’s primarysource. In the present study, and taking the results of S3(near the source) and S4 (away from the source) into account,
there is no obvious difference between them. This is considered to be due to the short longshore distance between S3and S4 (about 4 km). In Rink (2003), the study area covereda longshore distance of around 150 km. Therefore, in thisstudy, and within the studied area, the measured TL difference should not come from the transport distance of the original source. Figure 6 shows that the TL signal at Nakatajima
Coastal Dune (S5) is the minimum, but not at zero. This isdue to sufficient solar exposure for many years, determined

1101

Figure 7. TL intensity (the ratio between the natural and test-dose TL
integration over temperature range of 200⬚ to 400⬚C) and the standard
deviations (error bar) for the surface samples (S1–S5).

by considering that the sampling position is located at acoastal dune, where the surface sand is mainly transported
in air by blowing wind. Compared with the other surface samples, sample S5 was much less protected from the sunshine
because particle movement was induced by aeolian transport.
These grains would be expected to be well bleached (Ballarini
et al., 2003). However, even after long sunlight exposure,these sand particles did not lose all their TL signals (trappedelectrons), and certain natural TL signals remained, which
are called the natural residual TL in Zhang, Li, and Tso(2001) and in Rink (2003).
To further specify the insight into the grain’s inherent natural TL features, the average of the normalized TL glow
curve over temperature range of 200⬚–400⬚C was calculated.
In fact, considering the normalization equation applied here,
this approach presents the comparison (the ratio) between
the 200⬚–400⬚C channel integrations of the natural and test
dose TL because normalization was also carried out in thesame temperature range. Such a ratio provides a clear indication of the apparent dose. Here, we named this ratio the
TL intensity. Figure 7 shows the estimated TL intensities,
together with their standard deviations, for the surface samples. Similar to the results shown in Figure 6, there weredecreases in the TL intensities from S1 to S5. In general, we
can classify the data into three groups with respect to the TL
intensity, i.e., river (S1/S2), coast (S3/S4), and dune (S5) as
specified using three circles. In river group, the TL intensity
was about 6, indicating that the river sample had an apparent natural residual dose of around 6 ⫻ 7.2 ⫽ 43.2 Gy; TL
intensity of coastal group was around 3, with an apparent
dose of 21.6 Gy; whereas the TL intensity was only about 1
for the dune group (with an apparent dose of about 7.2 Gy).
Such difference in the TL intensity represents the typicalgrain properties in terms of its unique depositional environment, under which different light-exposure possibilities existed. A further look at the standard deviations shows thatthe variance for the dune sample was small, which suggests
a relatively uniform distribution of TL signal. This is due tothe windblown mechanism by which sand particles are trans-

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Underground Samples

Figure 8. Normalized natural TL glow curves at six different verticallocations along the underground geo-slicer sample G2.

ported in air, providing an equal opportunity to be bleached.
On the contrary, sunlight exposure for river particles is more
occasional and irregular (mainly during flood events), which
results in a large deviation of the measured TL signals. The
situation for coastal grains, under repeated wave actions, isin between the other sample groups.

All underground samples collected in the geo-slicer study
at Nakatajima Coastal Dune, i.e., samples G1–G3 were investigated in this subsection to illuminate the coastal dune
evolution process. Figure 8 shows the TL glow curves for six
vertical locations along sample G2 as illustrated in Figure 9.
The line marked G2-30 represents the measured TL values
30 cm below the surface of G2. In this Figure, the TL signal
gradually increases with the burial depth. The maximum TL
value is found at the deepest location at G2-330, with a peakvalue around 4, whereas the minimum TL is located at G230, with a peak value of around 1.5. Taking Figure 6 into
account, it is found that G2-330 presents a similar result as
the coastal samples S3/S4, and G2-30 presents a similar
trend as the dune sample S5. This implies that the whole
coastal dune may consist of sands from two different origins,i.e., the upper windblown sand and the lower wave-induced
sand. In between, a mixing layer, with a medium TL signal(G2-90, G2-130, and G2-175), exists. Within the mixing layer,
the TL intensity is large in cases in which the coast sandcontribution predominates and vice versa.
Figure 9 shows the vertical distribution of TL intensities
for all geo-slicer samples (left panel). In total, 12 TL samples
were measured for various depths at three locations. T.P.
stands for the average water level of Tokyo Bay, which is a

Figure 9. Integrated TL intensity for geo-slicer samples G1 (䡵), G2 (⽧), and G3 (䉱), together with the schematic diagram, which specifies the fieldsampling locations, on the right panel.

Journal of Coastal Research, Vol. 25, No. 5, 2009

Sediment Movement Study Based on Thermoluminescencestandard datum level in Japan. Taking Figure 7 into account,the corresponding TL bands for the dune and coastal sand,with relatively narrow and wide widths, are overlapped inthis Figure to facilitate our understanding. Considering thefield sampling locations, presented in the right panel of Figure 9, three subareas with different TL intensities can beobserved, i.e., an upper aeolian zone, a middle mixing zone,
and a lower coastal zone. The horizontal dash-dot line presents a schematic interface between the aeolian and mixing
regions, whereas the horizontal dashed line shows a roughboundary between the mixing and coastal regions. Threesamples located in the upper layer of samples G1 and G2 arein the aeolian zone, which has a small TL intensity (about1), with a value similar to that of S5. Therefore, we conclude
that these sands are composed of the well-bleached dune
sand, which was blown there by the wind. In the lower layer,
sample TL intensities are around 3. Considering the sorting
criterion presented in Figure 7, this layer consists of relatively poorly bleached coastal sands, which were transported
there by wave motion. In between, there exists a mixing layer. The relevant sample TL intensity is within the dune sandand coastal sand scales. Consequently, the vertical configuration of Nakatajima Coastal Dune can be described as stratified, with dune sand and coastal sand at two ends and amixing layer in between. Such stratified distribution was also
observed by Takagawa et al. (2008) in their ground-penetrating radar images as well as in their subsequent particle-size
analysis. They found that the high-tide level at Nakatajima
was around T.P. 90 cm, which is located just within the mixing-layer range proposed by these TL measurements. Taking
wave actions into account, the mixing-layer range specified
by the TL test is verified.
Considering the historical evolutionary process of the Nakatajima Coastal Dune, as mentioned previously, with its sediment source in the Tenryu River watershed, the sand wastransported alongshore by nearshore waves and currents.
During the process of dune generation over about 200 years,
certain sands were relocated to the top layer through thewind blowing, during which, their TL signals decrease;
whereas other sands remained at the bottom and keep their
TL signals almost unchanged. The stratified structure of
Nakatajima Coastal Dune, again, demonstrates that themeasured, natural TL signals probed the various sedimentdepositional environments.

Underwater Samples
Up to now, the measured TL samples were all ground ones,
either subaerial or underground. As for surface samples, S1–
S4, they may be covered by water during flooding or swashing. However, such water coverage is intermittent, and thesesand particles may be directly exposed to sunshine at mosttimes. In this subsection, the underwater samples will be investigated to illuminate the formation process in front of theTenryu River mouth and to demonstrate how the water bodyplays its role as a filter on sample TL signals.
Figure 10 shows the corresponding results for samples
from a 10-m-long boring core (C1) extracted at a 10 m water
depth in front of the Tenryu River mouth. Four positions

1103

Figure 10. Normalized natural TL glow curves for underwater sample
C1 (four positions at 30, 260, 640, and 980 cm below the surface), together
with surface samples, S1–S3.

were considered for TL studies, i.e., 30 cm, 260 cm, 640 cm,
and 980 cm beneath the core surface. It was found that the
TL intensities of C1-30 and C1-260 were almost the same,with values larger than that of the river samples, S1/S2. Considering that the water body plays the role of a light attenuator, the underwater samples may present a larger TL value
than that of surface samples because subaerial exposure produces a more efficient bleaching than that of underwater exposure. At the same time, samples C1-640 and C1-980 havethe similar TL values, which are in between that of S1/S2and S3. This may be ascribed to a fairly complex sedimentmovement process in front of the river mouth, which caused
a rather dynamic evolution of the coastal bathymetry as mentioned previously. The boring-core constituent may comefrom various sand contributions under different depositional
environments, such as river discharge, with a large TL intensity, and coastal sand, with a small TL signal. At the sametime, offshore/longshore sand movement may also need to beconsidered. The development of the seabed stratum is accompanied by a mixing process of these sand fractions. At thesame time, influence from the collapse/deformation of the river mouth sand spit/shoreline should also be taken into account. Such an event is not unpredicted after a typhoon,
which struck in the summer, with accompanying river flooding (Liu, Tajima, and Sato, 2008). Therefore, the TL signal of
C1 samples may reveal such mixing processes from different
sands with various TL intensities. Therefore, it is assumed
that the formation of the sediment stratum at C1 is very complex, and such processes are somewhat unpredictable andneed to be investigated carefully and comprehensively.
A further look at the shape of all TL glow curves in Figures 6, 8, and 10 shows a clear difference between the underwater samples and other ones. In general, two TL glowcurve peaks can be observed in the temperature range ofthis study. One is around the 290⬚C temperature, and theother is at 350⬚C. For the underwater samples, the hightemperature TL peak (HTLP) has a larger value than the
low-temperature TL peak (LTLP); whereas a different result, with HTLP being more similar to the LTLP, is found

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1104

Liu et al.

for ground samples. Spooner (1987) and Aitken (1998)
demonstrated that water was particularly effective at filtering ultraviolet (UV) radiation and that the UV components are only weakly present in the solar radiation spectrum after passing through seawater. As for quartz particles, they found that visible light only reduces the LTLP.
On the contrary, UV wavelength can diminish both. In thepresent investigation with feldspar, similar results wererevealed. During the natural sediment-transport process,loss of TL signals mainly comes from exposure to light, as
mentioned formerly. Such eviction process is also related
to the light wavelength, i.e., the quantum energy (Aitken,
1998): the shorter the wavelength, the further toward the
UV end of the spectrum, the more rapid/effective the eviction. Different TL peaks may also relate to the different
trap deepness, e.g., deep traps, corresponding to the HTLP,
demand high quantum energy (UV components) to bebleached. Because the seawater acts as a UV filter, only
allowing visible sunlight components to penetrate down to
the seabed, the bottom particles are only bleached by visible light to evict the relatively shallow traps, i.e., reducing
the LTLP, but leave the HTLP almost unchanged. Hence,a high HTLP, together with a low LTLP, is observed forunderwater samples. As for the ground samples, they are
exposed to both visible and UV light, which decreases
LTLP and HTLP simultaneously. Therefore, relatively uniform LTLP and HTLP are observed.

CONCLUSIONS
Investigation of the river/coastal sediment transport was
carried out based on the TL properties of natural feldspar.
After a series of pretests, an appropriate TL measurement
protocol was proposed for this study. Subsequently, that protocol was applied to the study of the sediment movement process along the Tenryu–Enshunada fluvial system throughcollected field samples at various locations. In the target
area, the source of sediments is the Tenryu River watershed.
In view of the young age of the investigated area, natural TL
difference in the present study does not come from the limited
burial time variance, which is within several hundred years.
Rather, the natural TL difference is caused by the sample
depositional environment under which different possibilities
of sunlight exposure exist. The main conclusions of this study
are summarized as follows:(1) After analysis of the surface samples, three groups of sediment particles were categorized in the research areawith different natural TL intensities, i.e., river sand, witha large TL signal; coastal sand, with a medium TL signal;and dune sand, with a small TL signal. This demonstrates the influence of the sample environment. That is,poor bleaching for river sand (unidirectional transport
due to river flow), medium-bleaching for coastal sand(multidirectional movement due to wave/current actions),
and complete bleaching for dune sand (full-light exposure
by aeolian transport).
(2) Investigation of the underground TL samples from a geoslicer study implies that the vertical structure of the Nakatajima Coastal Dune can be described as a stratified con-figuration, with windblown dune sand and wave-induced
coastal sand located at the top and bottom of the dune,
respectively, together with a mixing layer in between.
(3) After considering the underwater samples extracted in
front of the Tenryu River mouth, a rather complex sediment movement pattern is revealed, which is affected byvarious factors and needs to be investigated carefully andcomprehensively. Influence from the seawater, acting as
an ultraviolet filter, can be observed from the difference
in TL spectra. The high-temperature TL peak is larger
than the low-temperature TL peak for underwater samples, whereas these two peaks are almost the same inground samples.
As mentioned at the beginning, nearshore sediment-movement investigation, based on the grain’s TL properties, is stillin its early research stage for coastal-engineering applications. Further studies on particle natural TL signals areplanned for a broad area, which covers both the upstream
river area and the coastal area away from the river mouth.
To acquire a quantitative insight into the sediment movement process in a fluvial–coastal system, sediment dating,e.g., using OSL technique, will also be considered.

ACKNOWLEDGMENTS
This study was financially supported by the Grants-in-Aid for
Scientific Research Program (Project 19656121) through the Japan Society for the Promotion Science (JSPS). Partial field work
was also conducted under the Tenryu–Enshunada Project funded by the Japan Science and Technology Agency (JST). The underwater boring core was offered by the Hamamatsu Office ofRiver and National Highway, Chubu Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism,
Japan. We also appreciate the comments from Dr. Jakob Wallinga and another anonymous reviewer, which helped to improve the manuscript considerably.

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