Field Investigation on Regional Sediment Movement Around the Shounan Coast
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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 4 3 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. retreat advance retreat advance 堆積 侵食 侵食 No.10 No.10 堆積 No.10 No.9 2 No.9 No.9 No.8 200 2002年 No.8 No.8 No.7 2002年 No.7 T.P.0m No.7 No.6 T.P.-10m No.5 No.6 No.6 No.5 No.5 No.4 No.3 2 No.4 No.4 199 No.2 No.3 No.3 1992年 No.1 No.2 1992年 No.2 No.0 No.1 No.1 No.0 No.0 No.31 No.31 2 No.31 No.30 No.30 No.30 198 1982年 No.29 1982年 No.29 No.29 No.28 No.28 No.28 No.27 No.27 Variation in nearshore contour lines No.27 No.26 No.26 No.26 No.25 No.25 Fig. 3 No.25 No.24 No.24 1972 1972年 No.24 No.23 1972年 No.23 No.23 No.22 No.22 0 No.22 0 80 60 40 20 40 20 -20 -40 -60 -80 -20 -40 -60 -80 (m) -100 (m) 4 5 No.10 No.8 No.7 No.6 No.31 No.9 No.5 No.4 No.28 No.29 No.30 No.3 No.2 No.1 No.27 No.0 No.25 No.26 No.24 No.23 No.22 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 0 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. 7 100% 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.