Field Investigation on Regional Sediment Movement Around the Shounan Coast

FIELD INVESTIGATION ON REGIONAL SEDIMENT MOVEMENT AROUND THE SHOUNAN COAST

SHINJI SATO Department of Civil Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku Tokyo, 113-8656, JAPAN

TAKAKO FUKUYAMA Kajima Co. Ltd., Tokyo, JAPAN

TAKEHISA MATSUDA Department of Civil Engineering, The University of Tokyo, Tokyo, JAPAN

SUSUMU TANAKA Mikuniya Co. Ltd., Tokyo, JAPAN

Regional sand movement and long-term beach deformation were investigated for the fluvial system composed of the Shounan coast and three major rivers flowing into the coast. Anthropogenic impacts, such as dams and weirs, sand dredging, fishery harbors and shore protection structures on long-term beach erosion were discussed on the basis of the comprehensive analysis on nearshore sand volume, mineralogical properties of surface sediments and decadal dating of the sand layer based on Pb-210 radioactivity.

1. Introduction The Shounan coast, located on the southeastern side of Japan facing the Pacific Ocean, has experienced significant erosion in recent 40 years. The erosion has been influenced by the interruption of longshore sand transport by harbor and coastal structures, and by the shortage in sand supply from three major rivers flowing into the Sagami Bay, that is, the Hayakawa River, the Sakawa River and the Sagami River. It is essential to understand the regional sediment movement in the fluvial system composed of the three rivers and the coast since the sedimentary processes on the Shounan Coast are strongly influenced by the sediment supplies from the three major rivers. Figure 1 illustrates a map of the watersheds of the three rivers. The sediments supplied from the three rivers are considered to reflect the geology of the individual watershed. The Hayakawa River originates in the Hakone Volcanoes and rushes into the sea with a steep slope. The Sakawa River originates in the Tanzawa Mountains while the Sagami River has a relatively

1 2

Sagami dam(1947) Sand dredging (1944-1962) Numamoto dam(1944) Doushi dam Shiroyama dam(1965) (1955) Isobe weir (1928) Miyagase dam(2000) Sagami River

Miho dam(1979) Samukawa weir (1964) Sakawa River Iizumi weir (1973) Shounan Coast Hayakawa River 10km

Old Headland Shore harbor (1990) Headland protection (1998--) (1990--) Jetty Fishery harbor Fishery harbor (2000) (1984) Oiso 1km Port(1972)

Fig. 1 The Shounan Coast and modern anthropogenic impacts

（×10 6ｍ3） 8

7 dredged sand & gravel

6 accumulated sand & gravel 5

2 1

0 198019851990 1995 2000

Fig. 2 Accumulation of sand and gravels in the Miho Dam reservoir

3 large watershed originated in Mt. Fuji. It is noticed in Fig. 1 that the regional sediment movement has been influenced by various anthropogenic impacts in the last 40 years such as construction of dams and weirs on rivers, sand exploitation from riverbeds and construction of fishery harbors and shore protection facilities on the coast. A comprehensive investigation is considered to be necessary including analyses on quantitative sand budget as well as on sediment quality, such as grain diameter, mineralogical property and historical process of sedimentation. Figure 2 shows the amount of sediment accretion in the reservoir of the Miho Dam constructed on the Sagami River in 1979. The amount of siltation in the reservoir reached 5.5 million m 3 in 24 years. Considering for the continuous operation of dredging from the reservoir, the average rate of the interruption of sediment by the Miho Dam is estimated at 2.8x10 5 m3/year.

2. Spatial and Temporal Change in Nearshore Sand Volume Quantitative analysis of nearshore sand volume is made on the basis of survey data obtained by the Kanagawa Prefecture with a spacing of 200m. Figure 3 shows the movement of nearshore contour lines of T.P.0m and T.P.-10m in the region around the Sagami river mouth which is illustrated at the bottom of Fig. 1 . It is noticed that the contour line has been retreated significantly in the last 30 years especially in the region around the Sagami river mouth. The large retreat in the offshore contour line is partly due to the relatively mild bed slope (1/60) compared with the steeper slope (1/20) at the shoreline. Figure 4 shows the temporal variation in nearshore sand volume in five sub- regions. The sand volume was estimated in the region from the sea dikes to 1km offshore, and plotted as the difference from that of 1971. The total sand volume decreased significantly during the period from 1970 to 1985. Although it recovered in the period from 1985 to 1995, the total loss of sand in 30 years is estimated at 5x10 5 m3. It is also noticed that the main contribution to the loss of sand is found in the region around the river mouth. Figure 5 shows the cumulative amount of nourishment in this region. It is considered that the recovery of sand volume is partly due to the nourishment.

3. Distribution of Gravel Properties Surface sediments were sampled at 50 points during the period from June to October 2002. One hundred gravels with diameter from 1cm to 5cm were randomly sampled on the riverbed as well as on the shoreline. The gravels were classified according to the mineralogical property. 4 No.10 No.8 No.6 No.7 No.9 No.31 No.5 No.28 No.30 No.4 No.29 No.3 No.2 No.1 No.27 No.0 No.26 No.25 No.24 No.23 No.22

1972年1972 1982年198 2 1992年199 2 2002年200 2 o.22N o.23N o.24N o.25N o.26N o.27N o.28N o.29N o.30N o.31N o.10N No.0 No.1 No.2 No.3 No.4 No.5 No.6 No.7 No.8 No.9

80 堆積advance 60 T.P.0m 40 20 0 (m) -20 -40 -60 1972年 1982年 1992年 2002年 -80 侵食retreat No.22 No.23 No.24 No.25 No.26 No.27 No.28 No.29 No.30 No.31 No.10 No. 0 No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9

40 堆積advance 20 0 -20

(m) -40 -60 -80 T.P.-10m -100 侵食retreat

Fig. 3 Variation in nearshore contour lines

5 No.10 N o.8 N o.7 N o.6 No.31 N o.9 N o.5 N o.4 No.2 8 No.2 9 No.3 0 N o.3 N o.2 N o.1 No.2 7 N o.0 o25No.2 No.2 6 o24No.2 No.2 3 No.2 2 Chigasaki headland

Chigasaki rivermouth Chigasaki Hiratsuka Harbor Ooiso

大Ooiso 磯平Hiratsuka 塚相River 模川左mouth 岸茅Chigasaki ヶ崎西茅ヶ崎漁港～ヘッドランドChigasaki harbor 全Total 体 1970 1975 1980 1985 1990 1995 2000 0.5 0 ) 3 -0.5 (m 6 -1.0

×10 -1.5 -2.0 -2.5

Fig. 4 Temporal variation in sand volume in sub-regions

平塚海岸Hiratsuka 茅ヶ崎海岸柳島地区River mouth 茅ヶ崎海岸漁港～ヘッドランドChigasaki harbor 茅ヶ崎海岸ヘッドランド周辺Chigasaki headland 茅ヶ崎海岸ヘッドランド東側East of Chigasaki headland 1985 1990 1995 2000 1.5 ) 3

(m 1.0 5

×10 0.5

Fig. 5 Cumulative amount of sand nourishment 6

quartz diorite Sagami andesite Dam green tuff

miscellaneous

Sagami River

Miho Dam

Sakawa River 20 25 Hayakawa 15 River 10 5 Sagami Bay 1 1 5 10 15

16 20 25 29

The number refers to the sampling point on the coast

Fig. 6 Distribution of gravel properties in watersheds and coast

Figure 6 illustrates the distribution of mineralogical properties of gravels. The properties in watersheds were plotted at the sampling points and those on the coast are illustrated on the bottom, in the sequence from west to east. It is noticed that characteristic gravels were found in the individual watershed, that is, andesite predominant in the Hayakawa watershed reflecting the geology of the Hakone Volcanoes, quartz diorite and green tuff predominant in the Sakawa watershed representing the Tanzawa Mountains. It is also noticed that the ratio of quartz diorite, a representative rock in the upstream of the Sakawa River, decreases significantly across the Miho Dam.

100% 80% 60% Aramaki & Suzuki (1958)

40%

20% 0% 1Hayakawa 2 3 4 5Sakawa 6 7 8 9 10111213141516171819Port Ooiso 202122232425Sagami 100% present study(2002) 80% 60% 40%

20%

0% 1 3 5 7 9 11 13 15 17 19 21 25 28 30 33

quartz diorite andesite green tuff

Fig. 7 Change in the distribution of gravel properties along the coast

Figure 7 shows the longshore distribution of gravels. The top figure indicates the measurements by Aramaki and Suzuki (1958) and the bottom is those by the present study. Andesite is predominant near the Hayakawa river mouth and gradually decreases on the eastern coast, indicating the eastward longshore transport. The eastward longshore transport was also confirmed from the gradual decrease in the maximum size of gravels on the shoreline, which changed from 20 to 30 cm at the Hayakawa river mouth to 3 to 4 cm at the Sagami river mouth. Quartz diorite and green tuff, which are representative rocks in the Sakawa River watershed, are predominant near the river mouth of the Sakawa River. However, the ratio of quartz diorite was found to be decreased significantly in the recent 40 years. This is considered to be due to the decrease in the sediment supply from the Sakawa River. The decrease of the sediment supply was partly due to the interruption of sands and gravels at the Miho Dam, which was also confirmed by the sudden decrease of the quartz diorite across the Miho Dam in Fig. 6. 8

4. Sand Bar Decline at the Sagami River mouth Figure 8 shows the profiles of the riverbed in the downstream of the Sagami River. Overall erosion is noticed in the last 40 years. Figure 9 shows the size of surface sediment of the riverbed. It is noticed that the surface sediments tend to become finer in 1998 compared with that in 1972.

distance from河口からの距離（km） the river mouth (km) 0 1 2 3 4 5 6 7 5 4 1961年1961 3 1971年1971 2 1986年1986 1 2001年2001

T.P.(m) 0 -1 -2 -3

Fig. 8 Change in the riverbed profile in the downstream Sagami River

2 10

1 10 ） 0 mm 10 （ 60 d

-1 10 1972 （30cm ） 1998 （0 - 30cm ） 1998 （30 - 60cm ） -2 10 02 4 68 10 12 14 16 distance from the rivermouth （km ）

Fig. 9 Distribution of the size of riverbed sediments in the downstream Sagami River

500m 1946 Jun 1961

Feb 1988 May 2002

Fig. 10 Topography change around the Sagami river mouth

contor lines T.P.(m)

sand bar volume

terrace volume

400m

Fig. 11 Sub-regions defined for sand volume analysis around the river mouth

Intensive field measurements were also performed at the river mouth of the Sagami River. The sand bar topography was surveyed in July and October, 2002. The position and the volume of the sand bar were estimated from survey data for recent twenty years. Vertical core samples were taken by using PVC pipes with diameter 4.6cm and length 1.5m. Figure 10 illustrates the landward movement of the sand bar. The area of the wetland developed behind the sand bar decreased considerably and tended to be lost. Detailed quantitative analysis of sand volume was made by using survey data obtained by the Ministry of Land, Infrastructure and Transport. Figure 11 shows the area of the survey data. The temporal variation in the shoreline position, the sand bar volume and the terrace volume was estimated and illustrated in Fig. 12 . The volumes of the sand bar and the terrace were estimated for the region illustrated in Fig. 11 . The rate of the retreat in the sand bar position is estimated at 5m/yr during the period from 1980 to 1990 and 30m/yr after 1990. The shoreline position at No. 0 line retreated with the same speed until 1990. The shoreline retreat at No. 0 was stopped since it hit the seawall in 1990. The decrease in the volume of the sand bar and the terrace became significant after 1990 and the total amount of sand loss reached 0.1 million m 3 for the sand bar and 0.2 million m 3 for the terrace. The change in the amount and the quality in sand supply from the river, caused by various anthropogenic impacts, is considered to exert essential influence on the sediment dynamics around the river mouth. The sedimentary processes in the decadal scale were also confirmed by the radioactivity due to Pb-210 fallout flux (half life time 22.3 years). Figure 13 shows the vertical profiles of radioactivity due to Pb-210 for core samples obtained on the sand bar. It is common for steadily accreted sand layer that the radioactivity of Pb-210 shows an exponential decay. Figure 13 shows, however, no trend of exponential decay, indicating that the accretion process is not steady. In order to estimate the age of the sand layer, the radioactivity was compared with that of samples taken at the river mouth of the Samegawa River, Fukushima Prefecture (Sato et al., 2004). The radioactivity due to Pb-210 for samples at the Samegawa River was strong near the ground surface and weak at elevation deeper than 1m, showing that the surface sand layer experienced steady accretion in the time scale of the lifetime of Pb-210. The dashed line and the dotted line in Fig. 13 respectively represent the radioactivity corresponding to the old sand layer and the fresh sand layer sampled at the Samegawa River. It is noticed that the surface sediments on the Sagami river mouth were mostly old sediments in the time scale of 20 years. This is considered to be because of the sediment budget deficit resulted from long-term decrease in the sediment supply from the Sagami River.

50 0 汀線位置比較図Shoreline position -50 -100

（m） -150 -200 No0汀線位置No.0 shoreline (1971(reference年基準 1971)) -250 河口砂州汀線位置sand bar shoreline (refere(1971nce1971)年基準 ) -300 2.0 河口砂州土砂量Sand bar volume 1.5 ） 3

（m 1.0 5

×10 0.5 Volume higher than TP0m以上T.P. 0m TP1m以上T.P. 1m 0 TP2m以上T.P. 2m 2 1 ） 3 0 （m

5 -1 -2 ×10

-3 河口テラス土砂量Terrace volume (reference(1988 年基準1988) ) -4 1970 1975 1980 1985 1990 1995 2000

Fig. 12 Temporal variation in the shoreline position, sand bar volume and terrace volume for the Sagami river mouth 12

2.02.02.0 ② ④ ⑤ ⑥

old fresh

1.51.51.5

）））） m m m m 1.01.01.0 （（（（

T.P. T.P. T.P. T.P. G.L. G.L.G.L. 0.50.50.5

000

---0.5-0.50.50.5 000555 101010 000555 101010 000555 101010 000555 101010

radioact ivity ×××10 -5 count/s/g

Fig. 13 Vertical profiles of radioactivity due to Pb-210 (46.5 keV)

5. Conclusions The main conclusions obtained in the present study are summarized as follows: (1) The prevailing direction of the longshore sand transport on the Shounan Coast is eastward, which was confirmed from the maximum size as well as the mineralogical properties of gravels on the shore. (2) Distribution of gravels in the Sakawa River watershed suggested the interruption of gravel transport at the Miho Dam. Comparison of the alongshore distribution of gravels with that of 40 year ago revealed significant decrease in sand supply from rivers. (3) A serious erosion is observed especially at the Sagami river mouth where irreversible processes of landward migration of sand bar, reduction in the sand bar volume as well as in the terrace volume are in progress. The radioactivity measurements of Pb-210 indicated the surface sand layer of the sand bar is relatively old in the decadal timescale.

Acknowledgments The authors acknowledge Dr. Kimio Inoue, Nippon Koei Co. Ltd. and Mr. Daiji Hirata, Kanagawa Prefectural Museum of Natural History, for technical advices on mineralogical properties of gravels.

References Aramaki, M. and T. Suzuki 1958 The Prevailing Direction and Mechanics of Beach Drift Inferred From Variation Series of Beach Sediments Along the Sagami Bay Coast, Japan, The Geographical Review of Japan , Vol. 35, No.1, pp. 17-34. Sato S., T. Kajimura, M. Abe and M. Isobe 2004 Sand Movement and Long- Term Beach Evolution in a Fluvial System Composed of the Samegawa River and the Nakoso Coast, Coastal Engineering Journal , JSCE, Vol. 46, No. 2, pp. 219-242. 14

KEYWORDS – ICCE 2004

PAPER TITLE Field investigation on regional sediment movement around the shounan coast Authors Shinji SATO, Takako FUKUYAMA, Takehisa MATSUDA and Susumu TANAKA Abstract number 443

KEYWORDS Regional sediment movement Sedimentary processes River mouth Beach erosion

Sagami dam(1947) Sand dredging (1944-1962) Numamoto dam(1944) Doushi dam Shiroyama dam(1965) (1955) Isobe weir (1928) Miyagase dam(2000) Sagami River

Miho dam(1979) Samukawa weir (1964) Sakawa River Iizumi weir (1973) Shounan Coast Hayakawa River 10km

Old Headland Shore harbor (1990) Headland protection (1998--) (1990--) Jetty Fishery harbor Fishery harbor (2000) (1984) Oiso 1km Port(1972)

Fig. 1 The Shounan Coast and modern anthropogenic impacts

（×10 6ｍ3） 8

7 dredged sand & gravel

6 accumulated sand & gravel 5

1 0 198019851990 1995 2000

Fig. 2 Accumulation of sand and gravels in the Miho Dam reservoir

16 No.10 No.8 No.6 No.7 No.9 No.31 No.5 No.28 No.30 No.4 No.29 No.3 No.2 No.1 No.27 No.0 No.26 No.25 No.24 No.23 No.22

1972年1972 1982年198 2 1992年199 2 2002年200 2 o.22N o.23N o.24N o.25N o.26N o.27N o.28N o.29N o.30N o.31N o.10N No.0 No.1 No.2 No.3 No.4 No.5 No.6 No.7 No.8 No.9

40 堆積advance 20 0 -20

(m) -40 -60 -80 T.P.-10m -100 侵食retreat

Fig. 3 Variation in nearshore contour lines

No.10 N o.8 N o.7 N o.6 No.31 N o.9 N o.5 N o.4

No.2 8 No.2 9 No.3 0 N o.3 N o.2 N o.1 No.2 7 N o.0 o25No.2 No.2 6

4No.2 No.2 3

No.2 2 Chigasaki headland

Chigasaki rivermouth Chigasaki Hiratsuka Harbor Ooiso

×10 -1.5 -2.0 -2.5

Fig. 4 Temporal variation in sand volume in sub-regions

(m 1.0 5

×10 0.5

Fig. 5 Cumulative amount of sand nourishment 18

quartz diorite Sagami andesite Dam green tuff miscellaneous Sagami River

Miho Dam

Sakawa

River 20 25 Hayakawa 15 River 10 5 Sagami Bay 1 10 15 1 5

20 25 16 29

The number refers to the sampling point on the coast

Fig. 6 Distribution of gravel properties in watersheds and coast

100%

80%

60% Aramaki & Suzuki (1958) 40%

20%

0% 1Hayakawa 2 3 4 5Sakawa 6 7 8 9 10111213141516171819Port Ooiso 202122232425Sagami 100% 80% present study(2002) 60%

40% 20% 0%

1 3 5 7 9 11 13 15 17 19 21 25 28 30 33 quartz diorite andesite green tuff

Fig. 7 Change in the distribution of gravel properties along the coast

distance from河口からの距離（km） the river mouth (km) 0 1 2 3 4 5 6 7 5 4 1961年1961 3 1971年1971 2 1986年1986 1 2001年2001

T.P.(m) 0 -1 -2 -3

Fig. 8 Change in the riverbed profile in the downstream Sagami River

2 10

1 10

） 0 mm 10 （ 60 d

-1 10 1972 （30cm ） 1998 （0 - 30cm ） 1998 （30 - 60cm ） -2 10 02 4 68 10 12 14 16

distance from the rivermouth （km ）

Fig. 9 Distribution of the size of riverbed sediments in the downstream Sagami River

500m 1946 Jun 1961

Feb 1988 May 2002

Fig. 10 Topography change around the Sagami river mouth

contor lines T.P.(m)

sand bar volume

terrace volume

400m

Fig. 11 Sub-regions defined for sand volume analysis around the river mouth 22

50 0 汀線位置比較図Shoreline position -50 -100

（m） -150 -200 No0汀線位置No.0 shoreline (1971(reference年基準 1971)) -250 河口砂州汀線位置sand bar shoreline (reference1971)(1971 年基準 ) -300 2.0 河口砂州土砂量Sand bar volume 1.5 ） 3

（m 1.0 5

×10 0.5 Volume higher than TP0m以上T.P. 0m TP1m以上T.P. 1m 0 TP2m以上T.P. 2m 2 1 ） 3 0 （m

5 -1 -2 ×10 -3 河口テラス土砂量Terrace volume (reference(1988 年基準1988) ) -4 1970 1975 1980 1985 1990 1995 2000

Fig. 12 Temporal variation in the shoreline position, sand bar volume and terrace volume for the Sagami river mouth

2.0 ② ④ ⑤ ⑥ 2.02.0

old fresh 1.51.51.5

）））） m m m m 1.01.01.0 （（（（

T.P. T.P. T.P. T.P. G.L.G.L.G.L. 0.50.50.5

000

---0.5-0.50.50.5 000555 101010 000555 101010 000555 101010 000555 101010 radioact ivity ×××10 -5 count/s/g

Fig. 13 Vertical profiles of radioactivity due to Pb-210 (46.5 keV)