LONG-TERM EVOLUTION OF SAND AND GRAVEL BEACHESON THE MIYAZAKI COAST
Shinji Sato1, Shun Kishimoto2 and Haruna Hiramatsu2
Long-term evolution of Miyazaki Coast was investigated in terms of four aspects, geology and geography,
comparison of shoreline in available maps and photographs, sediment retention analysis in watershed scale andsediment size as well as luminescence measurements of foreshore sediments. Geological and geographical analysis
revealed steep topography in northern part and fluvial plain on the southern part. The total rate of sediment retention
in reservoirs was found to be as much as 1.9 million m3/year. Historical shoreline retreat in the recent 200 years wassignificant in the northern region whereas severe erosion was developed in the last decades on the southern region
close to the rivermouths of the Hitotsuse River and the Ooyodo River. The sand grain size and thethermoluminescence intensity were both found to decrease from north to south, implying the dominant direction oflongshore sand transport is from north to south.
Keywords: long-term morphology; longshore transport; thermoluminescenceINTRODUCTION

Miyazaki coast is located on the south part of Japan, facing the Pacific Ocean. It is a 70km beach,running almost from north to south, covered with gravels in the northern part and fine sand in thesouthern part as shown in Fig. 1. Beach sediments are supplied from four major rivers, Mimigawa
River, Omaru River, Hitotsuse River and Ooyodo River. Severe coastal erosion has been observed inrecent years especially on the Sumiyoshi Beach located in the southern region near the rivermouth ofthe Ooyodo River.

Figure 1 Miyazaki Coast and major four rivers

12Department of Civil Engineering, The University of Tokyo, 7-3-1, Hongo, Tokyo, 113-8656, JAPAN
Former Graduate Student, Department of Civil Engineering, The University of Tokyo

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Figure 2 shows aerial photos taken in 1971 and 2005 around the rivermouth of the Ooyodo River.
Coastal morphology around the rivermouth is characterized by a slightly protruding shoreline andlagoons on the both sides of the river. The lagoon areas have been utilized for port and airport. Coastalerosion becomes accelerated since 1990’s with the introduction of many structures such as coastal road,port breakwaters and extended airport runways. Photo 1 shows the situation of the eroding Sumiyoshi
Beach at the north end of the coastal revetment introduced to protect the coastal road. The picture wastaken half year after the attack of a typhoon. Tentative protection structures were introduced in front of
the coastal road. Looking at the shape of eroded beach, it is considered that the direction of longshoretransport is from south to north, since the north end of structure is severely eroded. However, it isdifficult to determine the direction on the long-term basis since the incident wave angle is highlyvariable on this coast.

Figure 2 Comparison of aerial photographs in 1971 and 2005

Figure 3 illustrates the distribution of incident wave angle in ten years from 1992 to 2002, in which
the horizontal axis denotes the incident wave angle from the north and the vertical axis represents thefraction of wave energy flux. Wave data are obtained at the tip of the breakwater of the Miyazaki Port
located on the left side of the Ooyodo rivermouth (see Fig. 2). The angle normal to the shoreline is 105
degrees east from the north. Although the mean wave angle is almost normal to the shore, two peaks
are noticed representing typhoon waves (incident from south) and winter storm waves (incident fromnorth) respectively. From the analysis of wave data alone, it is hard to determine the direction of thelongshore sand transport on the long-term basis.
In order to understand the long-term morphology of the Miyazaki Coast, analyses were made in thisstudy on the following four aspects; (1) geography and geology, (2) sediment retention in reservoirs, (3)shoreline change revealed from existing maps and aerial photographs, and (4) beach sediment size andluminescence.

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Photo 1 Erosion at the north end of coastal revetment, Sumiyoshi Beach (March 29, 2006)

Figure 3 Distribution of incident wave angle at the Miyazaki Port in a period from 1992
to 2002 (Miyazaki Prefecture).

GEOLOGY AND GEOGRAPHYThe Miyazaki plain is characterized by the presence of the late Tertiary fluvial terraces created byuplifting tectonic movement, bounded by mountains and hills in the northern region and by a fluvial
plain in the south end as illustrated in Fig. 4. The coast is covered with gravels in the northern part andfine sand in the southern part. The north end and the south end of the coast are bounded by rocky capes,
so that longshore sediment fluxes across both boundaries can be ignored. Beach sediments are supplied
from four major rivers, Mimigawa River, Omaru River, Hitotsuse River and Ooyodo River. Offshore
bathymetries at the depth of 60 to 70m are supposed to represent the shoreline in the last glacier age,4COASTAL ENGINEERING 2010

when the sea level was 60 to 70m lower than the present sea level for a relatively long duration of morethan 30 thousands years. Although wave climate in the glacier age is different from that of the present
age, the dominant wave direction can be estimated from the shape of the corresponding contour lines
since the sea level then was stable for a relatively long period. They indicate that the shoreline at thatage runs from north to south whereas the present shoreline is concave compared from the equilibrium
shape. This suggests that the concave region on the south will be filled by sediments if the sea level is
kept unchanged. The longshore sand transport on the present non-equilibrium beach is therefore
considered to be from north to south. In order to demonstrate such long-term sedimentary processes,
further analysis will be required on the dating of sedimentary processes in the offshore zone by using
OSL/TL techniques.

Figure 4 Geology and geography of the Miyazaki CoastCOASTAL ENGINEERING 2010

5SEDIMENT RETENTION IN RESERVOIRS

The watersheds of the four rivers are characterized by huge production of sediments. Many dams
were constructed in the tributaries of the four rivers in the 20th century, interrupting the sediment flow.
Photo 2 shows a typical land slide developed by a typhoon in 2005 in the watershed of the Mimigawa
River. The land slide was developed on the downstream of the Tsukabaru Dam. In most of dams in thewatershed, like Tsukabaru Dam in Photo 2, a large amount of sediments has been accumulated in thereservoir. Figure 5 illustrates the sediment retention for two high dams in the Omaru River watershed.
Sediments are accumulated steadily for both Matsuo Dam and Dogawa Dam. The rate of sediment
retention for the two dams in recent years is calculated at 300 thousand cubic meter per year. Figure 6
shows the specific sediment yields qs calculated by the rate Qs of sediment retention in the reservoir
divided by the catchment area A. The straight line in the figure indicates a relationship between qs andA, which was proposed by Ashida and Okumura (1974). The specific sediment yields for Tenryu Riverand Kurobe River were also shown in Figure 6, which were located in the highest sediment yield zone
in Japan. The specific sediment yields for the four rivers are found to be at a high level categorized as
the second largest sediment production group in Japan. Because of steep slope of the river profile asshown in Figure 7, the interruption of sediment flow is significant especially for rivers in the northernregion. The total sediment retention rate in 23 reservoirs existing in the watersheds of four rivers wasfound to be 1.9 million m3/year, which is of the same order with the sediment retention in the Sakuma
Dam reservoir, which is the largest dam in Japan located in the Tenryu River watershed. Owing to theinterruption of sediment flow at dams, more than 3m degradation of riverbed was observed on thedownstream of the Omaru River and sand bar at the river mouth has been disappeared for the Ooyodo
River as confirmed in Figure 2. Sediment discharge through the four rivers is thus considered todecrease considerably.

Photo 2 Typical landslide developed near Tsukabaru Dam, Mimigawa River (Miyazaki Prefecture,September 2005).

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COASTAL ENGINEERING 2010Figure 5 Sediment retention in reservoirs of Matsuo Dam and Dogawa Dam, located in the Omaru
River watershed (Miyazaki Prefecture).

Figure 6 Specific sediment retention rate in reservoirs of watersheds of four rivers (MiyazakiPrefecture).

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7Figure 7 Riverbed profiles and sediment retention in reservoirs for Omaru River, Hitotsuse Riverand Ooyodo River (Miyazaki Prefecture).

SHORELINE CHANGE IN 200 YEARS

Shoreline change in the last 200 years was investigated by comparing geographical maps andaerial photographs. The available oldest map in Japan is ‘Inouzu’ chart, compiled in 1810 based onprimitive but scientific survey techniques. In order to compensate for the distortion of the map,geometric correction was conducted for the Inouzu Chart for every 20km section using several
benchmarks identified along major streets, such as shrines and bridges. Maps surveyed in 1902, at thebeginning of the Meiji Restoration, and those surveyed in 2000 to 2003 and photographs taken in 1947
and 1948 were also used for the analysis. Geometric distortion for the modern maps after the MeijiRestoration 1868 was confirmed to be negligibly small. Figure 8 shows the shoreline change of a town
Tsuno located in the northern region, in which significant shoreline retreat as much as 100m isidentified. The shoreline change in the whole region is illustrated in Figure 9. Significant retreat isobserved in the northern region whereas slight advancement is observed in the southern region. Thissuggests the direction of the longshore sand transport has been from north to south. Assuming thesediment volume lost in the northern part was transported to the south, we can calculate the mean rateof longshore sand transport in 200 years to be about 100 thousand cubic meter per year. The depth ofclosure in the beach profile was assumed to be 15m in the calculation. On the average, however, theretreat is larger than the advancement, indicating the long-term deficit in the sediment budget in thewhole area in spite of large sediment supply from the rivers.

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Figure 8 Shoreline retreat at Tsuno Town, northern part of the Miyazaki Coast, in 200 years illustratedon Inouzu Chart.

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9Figure 9 Shoreline change of the Miyazaki Coast in 200 years (reference to Inouzu Chart, 1810).

BEACH SEDIMENT SIZE AND LUMINESCENCE

Foreshore sediments at the high tide level were sampled on February 1, 2009, every 2km in theregion from the Mimigawa River to the Aoshima Island. Gravels were dominant on the northern coast
between the Mimigawa River and the Omaru River, and sand was dominant on the southern coast.
Figure 10 shows the size of gravel on the northern coast. The straight part of the scale in thephotographs is 40cm. It is confirmed that the size of gravels is as large as 10 to 20 cm on the northerncoast and gradually tends to become smaller on the southern coast. Sand grains 10cm below the surface
were taken by an opaque film container as shown in Figure 11. Sediments on the riverbed within 2km
from the river mouth were also sampled in the same manner. Sediment size was investigated for thefraction of sediment diameter d smaller than 16mm. Figure 12 illustrates longshore distributions ofmedian diameter. Blue data indicates the present study and the red ones are by Yoshitaka (1978). No
significant change is observed in sediment diameter in 30 years. Since sediment size generally tends todecrease with longshore transport, overall direction of longshore sand transport is considered to be from
north to south, although some peaks are noticed near river mouths.

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Figure 10 Gravel size on the northern coast (One straight part of the scale in thephoto is 40cm).

Figure 11 Sand sampling by an opaque film container and feldspar grain isolation forluminescence measurement.

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11Figure 12 Longshore distribution of grain size for sand finer than 16mm.

The sediment samples were treated in a dark room to avoid exposure to extra light. After sieving
0.18–0.3 mm grain fraction and separation by heavy liquid, the potassium feldspar grains (K-feldspar;
density less than 2.58 g/cm3) were isolated (Liu et al., 2009). The isolated grains were pasted on a 1cm
diameter stainless disk to form a monolayer aliquot. The disk was then installed in the Risoe OSL/TL
reader, which measured the Thermo-Luminescence (TL) intensity for natural and irradiated conditions.
At least four disks were measured for one sample.
The luminescence intensity is an index of the duration of light exposure of quartz and feldspar
particles. During the fluvial transport, sand particles decrease their luminescence intensity by theexposure to sunlight. The luminescence intensity can be measured by stimulating sand particles bythermal or optical energy. The luminescence intensity measured by thermal stimulation is called
Thermo-Luminescence (TL) and that measured by light stimulation is called Optically Stimulated
Luminescence (OSL). The sensitivity to light exposure is different in OSL and TL. Figure 13 indicatesthat the decay response of TL is of the order of several days, which is much longer that that of OSL(Liu et al., 2010). In this study, we utilized TL since we focus our attention to long-term movement ofsediment particles.
Figure 14 illustrates the longshore distribution of TL intensities. TL intensities of riverine samples,illustrated by blue marks, are larger than those of coastal samples. This is because of short exposure
time for sunlight in riverine transport processes. Oscillatory motion with strong mixing of sediments insurf zone and swash zone is considered to be effective in decreasing TL intensity. TL intensity is large
in the northern beach, reflecting the short exposure to sunlight during the relatively short duration in thetransport processes. TL intensity decreases to the south, indicating the direction of longshore transport
is from the north to the south, which is found to be consistent with previous analyses based on
geography, shoreline change and sediment size distribution. In the vicinity of the river mouth, however,
TL intensity tended to decrease in the both directions, suggesting that the sediments discharged fromthe river are transported to the both sides around the river mouth. TL intensity becomes minimum at
the Sumiyoshi Beach which is a severely eroding beach, where sand is considered to have been supplied
from both sides. The decrease of sediment supply to the Sumiyoshi Beach is considered to be thereason of ongoing severe erosion. The sediment supply from the north has been decreased owing to thedecrease in sediment supply from northern rivers, Mimigawa River, Omaru River and Hitotsuse River.
The sediment supply from the south has been blocked by the breakwater of the Miyazaki Port, which
was extended to the depth of 16m in 1990. Furthermore, TL intensity is large at the location ofnourishment, suggesting the possibility of tracing sediments by TL measurements.

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Figure 13 Decay time of luminescence intensities for feldspar grains (Liu et al., 2010).

Figure 14 Longshore distribution of TL intensities.

COASTAL ENGINEERING 201013CONCLUDING REMARKS

Comparisons of a series of geographical maps for the Miyazaki Coast revealed the significant
shoreline retreat in the northern region and slight advancement in the southern region. The sediment
discharge from the rivers is considered to be decreased rapidly by sediment retention in reservoirs andrapid urbanization around the Miyazaki City. The longshore distribution of TL intensity and sedimentsize indicated that the direction of longshore transport is from north to south.
ACKNOWLEDGMENTS

The authors would like to express their sincere gratitude to Miyazaki Prefecture and Miyazaki
Work Office of the Ministry of Land, Infrastructure, Transport and Tourism, for providing valuable
data.
REFERENCES

Liu, H., S. Kishimoto, T. Takagawa, M. Shirai and S. Sato. 2009. Investigation of the sediment
movement along the Tenryu–Enshunada fluvial system based on feldspar thermoluminescence
properties, J. Coastal Research, 25, 5, 1096–1105.
Liu, H., A. Hamamoto and S. Sato. 2010. Investigation on the nourished sand movement in terms ofluminescence measurements, J. Coastal Engineering, JSCE, 66, 1, 666–670 ( in Japanese ).
Yoshitaka, M., 1978, Sediment movement on the Hyuuganada Coast, Proc. 25th conference on Coastal
Engineering, JSCE, 274-278 ( in Japanese ).
Ashida, K. and T. Okumura, 1974, Study on sedimentation in reservoirs, Annual Report of Disaster
Prevention Research Institute, Kyoto University, Vol. 17B, 555-570.