1135

SELECTIVE MOVEMENTS OF SAND AND GRAVELS ANDRESULTING DYNAMIC MORPHOLOGY CHANGESOBSERVED AROUND THE TENRYU RIVER MOUTH
YOSHIMITSU TAJIMA, HIJIANG LIU, TOMOHIRO TAKAGAWA, SHINJI SATO
Department of Civil Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku,
Tokyo 113-8656, Japan. yoshitaji@coastal.t.u-tokyo.ac.jp.

Abstract:

This study focuses on the selective movements of sand grains andgravels around the sand spit of the Tenryu River mouth in Japan. Color sand andgravels were placed at the outer edge of the swash zone of both west and east sidesof the sand spit and their changing spatial distributions were traced afterward.
Observed different movements of sand and gravels were compared with the
dynamic deformation of the sand spit and surrounding hydrodynamic characteristics
such as waves and currents, all of which were continuously and quantitatively
analyzed based on succeeding snap shot images recorded by field cameras and Xbanded radar. Numerical model based on Boussinesq-type non-linear dispersive
wave with porous layer was finally applied to further investigate the hydrodynamic
characteristics observed around the river mouth where waves, currents and complex
river mouth morphology interact with each other.

Introduction
Morphology of a sand spit, developed at the river mouth, has significant impact
on surrounding hydrodynamic characteristics such as waves, currents, andresulting sediment transport. Due to such dynamic and active movement of thebottom sediments, it is virtually difficult especially around the river mouth toinstall ordinary measuring devices such as velocity meter and wave gauges toquantitatively capture such interactive features of hydrodynamics andmorphology deformations. Remote sensing techniques, instead, are often applied
to monitor physical processes around the surf zone and river mouth (e.g.,
Lippmann and Holman, 1989; Greidanus, 1997; and Bell, 1999). Authors
installed field cameras and X-band radar around the Tenryu river mouth in Japanand successfully captured various dynamic processes such as collapse andredevelopment of the sand spit under the flood, stormy waves, and the following
moderate waves (e.g., Liu et al., 2008 and Tajima et al., 2009). While these fieldobservations clarified various features of sand spit deformations, such asalongshore extension of the spit under the moderate waves and cross-shore
movement of the spit during either flood or stormy waves, it is not yet clear
about the detailed physical mechanisms of such deformation processes possibly
due to the lack of field data. This study therefore focuses on the actual

1136

movement of sand and gravels on the sand spit, which should directly
characterize the spit deformation, through unique sets of field survey using color
sand and gravels as tracers.
Field Survey
Continuous field surveys had been carried out around the Tenryu river mouth
through the year of 2009. This study focuses on the period from July to October
during which dynamic morphology change was observed around the sand spit
due to severe stormy waves and relatively large river discharge. Figure 1 shows
the plane view of the Tenryu river mouth with spatial allocations of newly
installed field cameras and an X-band radar. Field cameras recorded thesucceeding snap shot images of the Tenryu river mouth with time intervals of 1.2
seconds whereas the X-band radar recorded the spatial distributions of theintensity of the reflected signals with time intervals of 2 seconds. Besides thesecontinuous remote sensing data, we also carried out topographic measurementsof the sand spit for seven times during the focusing survey period.
In order to investigate the actual movements of the sediments on the sand spit,
this study also carried out the following tracing survey of color sands and gravels.
As shown in the Figure 1, color sands and gravels were placed in the swash
zones of both west and east sides of the spit at 7:00 on July 28, 2009 and their
movements were traced. Diameters of the color sand and gravels respectively
range from 0.2mm to 0.4mm and 1 cm to 3 cm and the amount of each color
sand and gravels placed at each site was 250kg. Movement of color sands andgravels were then traced by the following procedures. About 100g of sedimentson the ground within the 1-cm-thick surface layer was sampled and the numberof color sand included in the sample was counted by a device which

0

500m

TenryuRiver
X-bandradar

Field Camera
Ryuyowatergate

Points where color sands
and gravels were placedx

point where wave dir. is estimatedFig.1 Plane view of the Tenryu river and field survey setups

1137

automatically detects the color sand based on a number of digital images of thesampled sand surface (Niu et al., 2009). The amount of color sand contained ineach sample collected at different locations was then expressed in terms of ppm.
On the west side of the sand spit, sampling locations were selected along theshoreline and about 1 meter landward from the highest upper edge of the swash
zone at each sampling time. Since the ground level was relatively low around theeast end of the sand spit and therefore overtopping wave appeared to transport
sediments over the entire sand spit, sampling locations around the east tip of thespit were selected on the square grids with spatial intervals of 10 meters. Since
the number of gravels were much less than the color sand grains, tracer gravels
were directly searched by eyes at the site and the number of tracer gravels withina unit surface area was counted at the locations where the gravels were found.
Hydrodynamic characteristics during the field survey were analyzed based on theimages of field cameras and X-band radar. Besides these image-based
information, we also used the following quantitative hydrodynamic data
recorded by respective measuring instruments: (i) river discharge estimated fromthe water level data measured about 25km upstream from the Tenryu rivermouth; (ii) wave heights, periods and directions recorded at the Ryuyo wave
gauge, about 1000m offshore from the river mouth with water depth of 40m; and(iii) tidal water levels recorded at three different locations, i.e., at Kaketsuka,
about 3km upstream from the river mouth, at Ryuyo water gate, just behind theriver mouth, and at the Ryuyo wave gauge.
Deformation of the Sand Spit
Figure 2 shows the spatial deformation of the entire sand spit, in which shoreline
locations were recorded by a portable differential GPS device. As seen in the
figure, active deformation is observed mainly around the sea (south) side andeast end of the sand spit. Exceptionally significant deformation was observed
even around the land side (northside) of the spit from October 5th to 12th, during
which the typhoon T1018 hit the pacific coast of Japan and caused severe stormy
waves with significant wave height of 10 meters. From August 16th toSeptember 24th, it is observed seaward and landward movement of theshorelines respectively around the west side and east side of the sand spit. This
deformation somehow indicates significant westward littoral sediment transport
during this period.
Quantification of the Spit Deformation
As seen in Figure 2, active morphology change was observed especially aroundthe east tip of the sand spit. In order to investigate the more detailed
characteristics of such dynamic deformation around the east end of the sand spit,

1138

this study utilized succeeding digital snap shot images recorded by a fieldcamera. To avoid the influence of the tide level on the estimation of the shoreline
locations, we first selected images when the time-averaged water level at the
Ryuyo water gate was at TP+0.0m. Since the Ryuyo water gate is located justinside the river mouth, measured water level should already account for various
factors such as tides, river flow and wave setup. Fifty succeeding images with
1.2-second time intervals were then used to reproduce “1-minute-averaged
image,” which was then used to determine the time-averaged shoreline locations.
Finally, obtained time-averaged images were rectified to yield the images basedon the geographic XY-coordinate system. This study applied Tajima et al.’s(2009) rectification technique which, introducing weight functions, improves theaccuracy of the estimation of the XY coordinates in the far-camera field. Figure
3 shows an example of the original and rectified images around the east end of
the sand spit. Based on 107 similar rectified images around the east end of theJuly22
July29Aug 16
Sep 25

Sep 25
Oct 5
Oct 12

0500m

Fig.2 Spatial profiles of the sand spit at different time measured by GPS.
Original Image

Rectified ImageRiver flow

Sand Spitwave crest lines

Fig.3 Examples of original and rectified images

1139

sand spit, time-averaged shoreline profiles were extracted and compared witheach other on the same XY coordinate system (Figure 4). Four digit numbers in
the figure indicate the date when the images were recorded. For example, 1002
indicates October 2nd in 2009. As seen in the figure, shorelines are dynamicallychanging within a daily time scale. In order to quantitatively investigate thecharacteristics of the spit deformation, furthermore, this study divided thefocusing area in north and south or in east and west rectangular domains,respectively, and compared the time-varying surface ground areas in eachrectangular domain. Figure 4 shows the straight boundary lines which separate
the area into north and south, and in east and west. Obtained time-series of theground area are then directly compared with surrounding hydrodynamics such as
waves, river discharge, and tides. Figure 5 shows the time-series of the estimatedground areas in each domain and surrounding various hydrodynamics. In the
figure, wave directions were estimated at the surf zone in front of the sand spit,
which should show stronger correlations with the observed sand spit
deformations. Time-series of the images recorded by the X-band radar wasapplied to estimate the wave directions. At the location specified in Figure 1,
instantaneous velocity components of the moving wave crest lines in thenorthward and eastward directions were first estimated from two succeeding
images. Obtained time-series of the instantaneous wave directions were averaged
over 10 minutes to get averaged velocity components which eventually yield thetime-averaged wave directions. Figure 6 shows an example of the “snap shot”
image recorded by X-band radar. As seen in the figure, wave crest lines areclearly observed in the nearshore area where wave directions were estimated.
Relationship between Spit Deformation and Surrounding Hydrodynamics
Based on Figures from 2 to 6, this section discusses the relationship between
observed sand spit deformations and surrounding hydrodynamics.
Relatively large river discharge was observed around July 27th and, within a few
days after July 27th, the tip of the spit was clearly eroded. The affected area,
however, is limited only around the tip of the spit and the change of theestimated ground surface area was relatively small compared to the other period.
On August 9th, the typhoon T1009 caused relatively large waves with significant
wave heights of 2.5m propagating from SSW and largely decreased the groundsurface area in the domain S and vice-versa in the domain N. Accounting for thefact that the change of the ground surface area in domains E and W wererelatively small, it is deduced that the cross-shore northward sediment movement
was predominant compared to the alongshore movement during this period.

1140
West

East
North

South
0722
0727
0728
0731

0731
0809
0815
0821

0815
0821
0825
0901
0908

0908
0920
0928
1002
1023

100m

Fig.4 Time-varying profiles of the east tip of sand spit and Tenryu river and field survey setups

8/1

9/1

10/1

10 H (m)s

T0918(10/7)

T0909
(8/9)

4
0

Wave dir.(deg)

S12E
S
S12W

15 T s(s)1051Area S
Area N

0
1

Ground Area(ha)Area W

Ground Area(ha)Area E0tide(TP+m)
1
0
-1
2
1
0
8/1

Qriver(x103m3/s)
9/1

10/1

date

Fig.5 Time-series of various hydrodynamics and ground area in north, south, east and west area.

1141

Fig. 6 Example of a “snap shot” image recorded by X band radar (2:00, Aug. 8, 2009)

Milder waves after the storm increased the ground surface area around the sea
side within a relatively short period of time. Since this increase is observed bothin the east and west regions, this increase of the ground surface was deduced
mainly due to wave-induced onshore movement of the sediments which had beentransported seaward and deposited during the stormy waves.
From the middle of August to the beginning of September, wave directions
turned from SSW to SSE and this change of the wave directions surely supports
the westward sediment transport deduced from the shoreline changes observed in
Figure 2. While the clear evidence of the westward sediment movement wasobserved around the central and western part of the sand spit, east end of thesand spit was relatively stable and the relatively small changes of the groundsurface area observed in the area E appears to have stronger correlations withspring tides or with wave period rather than the wave heights. This observed
feature somehow indicates that east end of the spit has little westward sedimenttransport and this lower ground area was rather affected by overtopping waves
which were predominant especially during the high tide. After the middle ofSeptember, relatively mild waves continued but their directions again turned
slightly from west side and gradually increased the ground surface area in thedomains, E and N.
Finally, the typhoon T1018 hit the area on October 7th and caused severe stormy
waves with significant wave heights of 10 meters. Wave overtopping, observedon the entire sand spit, moved the entire sand spit in the landward direction.
According to Takagawa et al. (2011), alongshore distributions of the estimatedamount of the northward sediment transport during the storm showed fairly
strong correlation with the alongshore distributions of the highest ground levelon the spit. These observed facts also indicate that the characteristics of the sandspit deformation are highly affected by overtopping waves.

1142

Investigation of Selective Movement of Sand and Gravels
Figures 7 and 8 respectively show spatial distributions of the collected color
sands and gravels at different sampling times. The amount of the collected color
sand is expressed in terms of parts per million while the amount of tracer gravels
is expressed in terms of the actual number of gravels out of the unit groundsurface area of square meters. As shown in Figure 8, the tracer gravels were
initially placed along the line parallel to the shoreline with finite length of 30m
whereas color sands were placed at the points on east and west sides of the spit,
respectively.
As observed in Figure 7, all the blue color sands placed on the west side of thespit moved westward and their moving distance abruptly increases after 10 amwhen the tide level reached to its daily peak. While the increase of the tidal
water level from 7am to 10 am was just about 30cm, movement of the color sandplaced at the outer edge of the swash zone showed clear correlation with tide.
While the movement of the gravels were less active compared to the color blue
sand, their moving direction was also in the westward direction. Spatial
distributions of the wave crest lines recorded by the X-band radar clearly showed
the nearshore waves propagating from SSE and the estimated averaged wave
direction at the Ryuyo wave gauge was S12oE. The angle of the shoreline was 17
degrees in the clockwise direction from the east-west line and therefore the0400m

Initial locations ofcolor sand & gravels

Fig. 7 Alongshore distributions of color sands and tracer gravels placed at the west side of the spit.

1143

expected westward littoral sediment transport is also consistent with theobserved movement of the blue sands and gravels.
In contrast to the blue sands and tracer gravels placed on the west side of the spit,
the green sands and tracer gravels, placed in the east end of the spit, moved
eastward as shown in Figure 8. In Figure 8, a part of the ground area within 20meter from the east end of the spit was the place where the ground level was low
and frequent overtopping waves did not allow us to collect surface sand samples.
Instead, locations where no color sand was found in the collected samples wereindicated as solid circles in the figure. Difference between sands and gravels
were also clearly observed. While both sand and gravels moved in the northeastward direction, gravels tended to move more toward the north. Furthermore,
northward movement of the gravels appeared to be enhanced at 13:00 on the
29th after the placed gravels experienced the first high tide at 22:00 on the 28th.
These features will be discussed later with numerical analysis of thehydrodynamic characteristics around the sand spit. Observed northward
movement of sands and gravels surely corresponds to the northward deformationof the sand spit discussed in the previous section.

groundlevel (cm)gravelscolor sandFig.8 Spatial distributions of color sand and gravels placed at the east tip of the spit.

1144

Hydrodynamics around the Sand Spit
This section further investigates the hydrodynamic characteristics around thesand spit based on recorded digital images and numerical analysis. Following theobservations and discussions about the spit deformations and selective
movements of sand and gravels, this section focuses on wave overtopping andthe alongshore distributions of the angle between the wave and the shore-normal
directions, which, as seen in well-known CERC formula, should determine thealongshore directions of the sediment movements.
This study applied Tajima et al.’s (2006) Boussinesq-type weak non-linear
dispersive wave model for computations of wave-current interacting fields
including wave run-up around the swash zone. The model followed the conceptof Madsen et al.’s (1997) porous layer model, which enables efficientcomputation of the wave run-up in the swash zone. Applicability of the model
was verified through the comparisons with the experimental data in which
periodic waves were normally incident on a plane sloping beach with asubmerged breakwater placed around the surf zone. The model was applied to
the 5m-square-grids bathymetry data around the Tenryu river mouth and theincident wave conditions, Hs=0.8m, Ts=7.8s, and θ=S12oE, were applied forcomputations of the wave fields at 10:00 on July 28th, when tracing survey ofcolor sands and gravels were carried out. Either regular or unidirectional random
waves were applied. In order to investigate the influence of the wave-current
interactions, three different river discharge rates, Q=0, 140, and 280m3/s, wererespectively applied at the upstream boundary.
Wave Overtopping around the East Tip of the Spit
Digital snap shot images were firstly analyzed to evaluate the spatial
distributions of the inundation rate, the ratio of duration when the area wassubmerged and the entire observing time. Based on the RGB-values at each pixelof the original snap shot images, we first setup the threshold values which
determine whether the pixel is wet or dry. Based on the obtained thresholdvalues, original image was transferred to the binary images with 0 and 1
corresponding to dry and wet, respectively. The same procedures were applied tothe 750 succeeding snap shot images, which cover 15-minute-observation period.
The inundation rate was computed at each pixel as a ratio of the number ofimages which was wet at the same pixel out of the total number of images.
Finally, obtained images of the inundation rate were rectified to obtain thespatial distributions of the inundation rate based on the geographic XYcoordinate system. Figure 9 shows the obtained horizontal distributions of theinundation rates respectively at 7:00 and 10:00 on July 28th. Figure 9 also

1145

Fig. 9 Horizontal distributions of observed (top two) and predicted (bottom two) inundation rates.

shows the computed inundation rate, based on the same definition used in theimage analysis, when either periodic regular or unidirectional random waves
were respectively incident. As seen in the top two figures, observed spatial
distributions of the inundation rates are highly affected by the tidal water level.
Especially around the east end of the spit, where has lower and milder crosssection profiles, increase of the inundated area was more significant. As shownin two bottom panels of Figure 9, numerical model showed reasonable predictive
skills of the horizontal distributions of inundation rate when unidirectionalrandom waves were incident. Considering the observed fact that gravels placedaround the east end of the spit was transported to the north only when relatively
long wave ran up on the spit, it is deduced that randomness of the wave
frequency is one of crucial factors to determine the cross-shore movement of thesediment grains on the sand spit.
Wave Directions around the River Mouth
Numerical model was applied for estimation of the alongshore distributions of
the wave directions along the depth contour lines at T.P.-1.0m. Figure 10
compares instantaneous surface water profiles when either periodic regular or
unidirectional random waves were introduced with or without the river discharge.
The river discharge rate, Q=280m3/s, yielded the observed current velocity,
1.4m/s, at the river mouth where the acoustic Doppler velocity profiler was
installed. Besides the instantaneous surface water elevations, indicated in
shadings, these figures also show bathymetry used for the wave computation incontour lines. As compared in the first two panels of the Figure 10, the river flow
surely has certain impacts on wave refractions and waves appeared to beconcentrated more near the east tip of the spit when the river discharge was taken
into account.

1146

Figure 11 compares the alongshore distributions of predicted wave directions at
1-meter-deep-contour line, shore-normal directions, and the difference of thesetwo directions. In the figure, P0, P1 and P2 are predictions of regular wave casewith Q=0, 140, and 280m3/s, respectively whereas R2 is predictions with random
incident waves with Q=280m3/s. As seen in the figure, wave directions abruptly
changes within the distance of 100m from the east tip of the spit. As seen incomparisons of P0, P1 and P2, river discharge decrease the wave angle within
the horizontal range of 500m from the tip. This decrease of the wave angle
surely acts to increase eastward movement of the sediments near the tip of the(a) Regular Wave2
Q=0.0m /s

SWL(m)(b) Regular Wave
Q=280m2/s(c) Random Wave
Q=280m2/s

Fig. 10 Comparisons of predicted instantaneous surface water profiles.
SSE

P0(a) Wave dir.

P1
P2
R2

S

Image

SSE (b)Shore-normal dir.
S
SSW

π/8
0
-π/8

(c) = (a) − (b)-500

砂州東端からの距離(m)0Distance (m)Fig. 11 Alongshore distributions of (a) wave directions, (b) shore-normal directions, and (c) difference
between (a) and (b). Distance is from the east tip of the spit. (positive in eastward) P1, P2 and P3
denote the regular incident wave cases with Q=0, 140, and 280m3/s, respectively whereas R2 denotes
the unidirectional random wave case with Q=280m3/s.

1147

spit. It is also seen that the random waves tend to be refracted more due to thepresence of lower frequency wave components.
Conclusion
Based on the unique set of field survey, this study aimed to investigate themovement of sand and gravels and resulting deformations of the sand spit at the
Tenryu river mouth.
Daily deformation of the east tip of the sand spit was first analyzed based on the
succeeding snap shot images recorded by field cameras. Relatively large river
flow eroded only the north-east side of the sand spit. Relatively large waves
overtopped the east end of the spit and moved it landward. Moderate waves
following the stormy waves transported offshore sediments shoreward andincreased the land area only around the seaside of the sand spit. Significant
morphology change showed clear correlation with spring tide. Even when the
waves were incident from the south-east, sand spit was fairy stable.
Color sand and gravels were placed at the outer edge of the swash zone of bothwest and east sides of the sand spit and their spatial distributions were traced.
Sands and gravels placed on each side of the spit moved toward the opposite
alongshore directions. Especially at the east tip of the spit, gravels tended tomove in the cross-shore northward direction rather than the alongshore eastward
directions whereas sand moved north-east directions.
Finally, hydrodynamic characteristics were investigated through the analysis of
obtained images as well as the numerical analysis especially focusing on wave
overtopping on the spit and alongshore distributions of the wave angle against
shore-normal direction. Wave overtopping was strongly affected by tidal waterlevel as well as randomness of the incident waves. River discharge affected tochange the wave directions around the river mouth and induced the sediment in
the direction to which the sand spit was extended. All of these findings, based onimages, hydrodynamics, tracer surveys and numerical analysis, were consistentand supported with each other.
Acknowledgements
Authors sincerely acknowledge that this study is a part of Tenryu-Enshunada
Project financially supported by the Japan Science and Technology Agency(JST).

1148

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