- Email: [email protected]
Large bedform generated by the 2011 Tohoku-oki tsunami at Kesennuma Bay, Japan
Marine Geology 335 (2013) 200–205
Contents lists available at SciVerse ScienceDirect
Marine Geology journal homepage: www.elsevier.com/locate/margeo
Letter
Large bedform generated by the 2011 Tohoku-oki tsunami at Kesennuma Bay, Japan Tsuyoshi Haraguchi a, Kazuhisa Goto b,⁎, Masataka Sato c, Yuichi Yoshinaga d, Naofumi Yamaguchi e, Tomoyuki Takahashi f a
Graduate School of Sciences, Osaka City University, Osaka, Japan International Research Institute of Disaster Science, Tohoku University, Japan Arc Geo Support Co., Ltd., Japan d CTI Engineering Co. Ltd., Japan e Center for Water Environment Studies, Ibaraki University, Japan f Graduate School of Safety Science, Kansai University, Takatsuki, Japan b c
a r t i c l e
i n f o
Article history: Received 29 February 2012 Received in revised form 6 November 2012 Accepted 16 November 2012 Available online 7 December 2012 Communicated by J.T. Wells Keywords: 2011 Tohoku-oki tsunami Japan dune Kesennuma tsunami deposit
a b s t r a c t The 11 March 2011 MW 9.0 Tohoku megathrust earthquake off the Pacific coast of Japan was a salient event in the history of Japan. The resultant huge tsunami (the 2011 Tohoku-oki tsunami) inundated a vast coastal area of northeastern Japan, causing widespread devastation. Twenty days after the tsunami, we analyzed the impact of the tsunami on the sea bottom of the Kesennuma inner bay using side-scan sonar to explore the damage and bathymetric change in the harbor. Herein we present the first direct evidence that the sea bottom sediments of around 10–15 m were largely reworked by the tsunami to thickness of a few meters, and that large dunes were formed by the tsunami. Considering that the sea wave influence is as weak as it is inside the inner bay, the potential exists that even meter-thick paleo-tsunami deposits are preserved in shallow sea bottoms with large bedforms. This finding will be a stepping-stone to future geological studies of tsunami effects in shallow sea regions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction A large tsunami is well known to be capable of changing coastal morphology radically (e.g. Kon'no, 1961; Goto et al., 2011). In fact, the recent 2011 Tohoku-oki tsunami strongly affected the coastal morphology of the Pacific coast of Tohoku, Japan (e.g., Goto et al., 2012; Tanaka et al., 2012; Udo et al., 2012; Tappin et al., 2012). Different from well-studied onshore tsunami impacts, few reliable observational data of the submarine processes of sediment transport and deposition by tsunamis are available (e.g., Dawson and Stewart, 2007). In fact, few reports of studies based on sediment samples have described offshore tsunami deposits left by historical tsunamis on a shallow sea bottom (e.g., Sugawara et al., 2009; Sakuna et al., 2012) or bathymetric data taken before and after a tsunami (e.g., Kawamura and Mogi, 1961; Goto et al., 2011). These studies revealed that fewmeter-scale erosion and sedimentation has sometimes occurred in very shallow sea bottoms of up to 10 m water depth. However, although possible offshore tsunami deposits have been reported (e.g., Kastens and Cita, 1981), it remains uncertain whether a tsunami can affect much deeper areas or create a bedform of any type such as dunes and ripples on the sea floor. ⁎ Corresponding author. E-mail address: [email protected] (K. Goto). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2012.11.005
Herein we present the first direct evidence that sea bottom sediments of around 10–15 m water depth were reworked by the 2011 Tohoku-oki tsunami at Kesennuma Bay, Japan (Fig. 1) to a thickness of a few meters, and that large bedforms were formed by the tsunami.
2. Local setting Kesennuma Bay, approximately 90 km northeast from the Sendai megacity, is a drowned valley at the Sanriku ria coast. The bay entrance is about 2.6 km wide. The area is about 15.4 km2, with maximum depth of 29 m. The tidal range in the bay is about 1.3 m. The inner bay usually has calm conditions, so it is used as a port of refuge during typhoon events. Sediments at the bottom of the bay are silt to sand; the average grain size was 70–240 μm (Suzuki and Chiba, 2003). The bay has been affected frequently by large tsunamis, according to historical documents. For example, the 1960 Chilean tsunami inundated Kesennuma Bay and destroyed part of the city. Onshore and offshore tsunami sedimentation and erosion were studied soon after the event (Kon'no, 1961; Kawamura and Mogi, 1961). Kawamura and Mogi (1961) studied the bathymetric change of the inner bay using bathymetric data obtained 4 years before and 1 month after the tsunami. They reported that approximately 4 m of sedimentation or erosion occurred at some places in the bay.
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205
201
Fig. 1. (a and b) Location maps of Kesennuma Bay. The shipping route and water depths are presented in (a). The tsunami inundation area (blue), and run-up height (yellow bar) are also shown (after Haraguchi and Iwamatsu, 2011). The background aerial photograph was provided by the Geospatial Information Authority of Japan. (c) Bathymetric contour map (10 m interval) of the bay.
We investigated the bathymetry and sea bottom conditions using side-scan sonar and a depth sounder (see Methods for details) in February 2009 and in March 2010 to analyze the annual change of bathymetry in Kesennuma Bay (Haraguchi et al., 2012). During this period, the 2010 Chilean tsunami of February 27, 2010, which had less than 2 m run-up height (Tsuji et al., 2010), struck the bay and partly inundated the city. Haraguchi et al. (2012) reported that no remarkable bathymetric change occurred in the ocean bottom during this tsunami event.
About one year later on March 11, 2011, the 2011 Tohoku-oki tsunami, which was generated in accompaniment with Mw = 9.0 earthquake off the coast of Japan's Tohoku region about 130 km east of Sendai (Simons et al., 2011), struck this bay. Local subsidence by the earthquake at this bay was about 0.6 m (Haraguchi et al., 2012). According to Fritz et al. (2012), the tsunami arrived as sea levels passed through low tide. An approximately 9–12 m high tsunami run-up was recorded at the bay entrance (Haraguchi and Iwamatsu,
202
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205
Fig. 2. (a) Depth map around the large bedforms. Side-scan image of the bedforms at the bottom of Kesennuma Bay is also shown. (b) Sounding survey result on February 2009 along the A–B line in Fig. 2a, which shows no bedform.
2011; The 2011 Tohoku Earthquake Tsunami Joint Survey Group, 2011). The maximum tsunami height of 20 m was measured at the headland to the west of the Kesennuma Bay entrance (Fritz et al., 2012). Harbor facilities were destroyed by this tsunami. To examine harbor functions––whether large ships could safely come alongside the pier––we conducted an emergency field survey to analyze the geomorphologic impact of the tsunami on the sea bottom 20 days after the tsunami in the bay, the earliest time to conduct the survey after sufficient daily life commodities had been supplied to most of the tsunami-affected area residents. As an initial report, Haraguchi et al. (2012) reported the bathymetric change by the 2011 Tohoku-oki tsunami over Kesennuma Bay. They reported that approximately 7 m erosion occurred at the narrow area of approximately 8 m water depth before the tsunami (Fig. 1).
Based on this finding, they reported that a tsunami can erode a substantial amount of sediment on a very shallow sea bottom. 3. Methods We used three-dimensional side-scan sonar (C3D), which has a swath range of about 10 times wider than the depth, to investigate the bathymetric change and sea floor imaging. The sonar was a rig transported on a ship, with positioning measured using a differential GPS system. The survey ship oscillation was corrected using a motion sensor (DMS-05) and the direction was corrected using a GPS gyrocompass (Vector Crescent VS110; Hemisphere GPS). Depth data were collected using a salinometer and a water temperature gauge (CTD). Using the “Computed Angle-of-Arrival Transient Imaging
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205
203
Fig. 3. (a) Close-up image of the isobath with 20 cm interval showing around the bedform. (b) Cross section of the bedforms along the C–D line in Fig. 3.
(CAATI)” algorithm, the C3D can measure high-resolution bathymetric data similarly to a multi-beam depth sounder. The measured data were analyzed using Hypack-Max software to remove errors. The measurement accuracy is 5 cm horizontal distance and 10 cm vertical distance (Haraguchi et al., 2012). 4. Results At the harbor entrance, with approximately 10–15 m water depth, the water depths before and after the tsunami were not significantly different (ca. 1 m). However, large bedforms were clearly visible on the sea floor (Fig. 2a). The wavelength and heights of these dunes were highly variable but, for example, they were 20 m long and 1.8 m high along the transect depicted in Fig. 3. The scale of the dune becomes smaller, with shorter length toward the shallower area (Fig. 2). The dunes have mainly straight or sinuous long crest lines with bifurcations, perpendicular to the direction of the slope. At deeper areas, short-crested dunes with slightly lunate geometries were observed (Figs. 2a and 3). The dunes had asymmetric profiles: the landward slopes were steeper than the seaward (Fig. 3b). In Fig. 3b, for example, the angles of the landward slopes are ca. 20 to ca. 33°, whereas the seaward slope angles are ca. 7 to ca. 10°. We also took sediment samples to ascertain the grain size and dune composition. As portrayed in Fig. 4, sediments are formed mainly by silt to sand, whereas the top ca. 25 cm was filled with well-rounded gravel (ca. 5 cm diameter) and shell fragments (Fig. 4).
5. Discussion No dune was observed in our survey on February 2009 (Fig. 2b), which is plausible because the sea waves and tidal current in the inner bay are probably insufficient to create such dunes. Moreover, it is noteworthy that no remarkable bathymetric change was observed after the 2010 Chilean tsunami (Haraguchi et al., 2012). Therefore, even the tsunami with a ca. 2 m high run-up was insufficient to create such a large bedform at the certain area. For that reason, we infer that these dunes were very likely formed during the 2011 Tohoku-oki tsunami. A tsunami as large as the 2011 event would be necessary to create such a large bedform on the shallow sea bottom of Kesennuma Bay. Dunes comprise silt to sand but were covered with gravel, the latter of which might have been transported from the surrounding gravel beaches along the Pacific coast, suggesting they were probably transported by the run-up waves. The wavelength, height, and the ratio of the dunes were consistent with those previously reported in marine and fluvial environments with similar water depths (Allen, 1982; Ashley, 1990) and those reported for gravel dunes (Carling, 1999), although further careful investigation of the composition, grain size, and inner sedimentary structures of these dunes is required. These dunes with lunate short crests suggest that movable sediment was limited there (e.g. Bridge and Demicco, 2008). The dunes in the observation area were asymmetrical with high-angle landward slopes that are close to angle of repose (Fig. 3). These characteristics of the
204
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205
6. Conclusion Two critically important findings related to the deposition of large bedforms by the 2011 Tohoku-oki tsunami at Kesennuma Bay were reported. First, although some reports have described bathymetric changes in very shallow seas by the tsunami (e.g., Goto et al., 2011), the large bedforms at Kesennuma Bay constitute the first direct evidence that sea bottom sediments of around 10–15 m were reworked by the tsunami with thickness of a few meters. The other important finding is the fact that the tsunami can truly leave a large bedform on the shallow sea bottom. Considering that the sea wave influence is weak because it is inside the inner bay, the potential exists that even sandy or silty paleo-tsunami deposits are preserved in the shallow sea bottom with large bedforms in the inner bay. In urban areas, the tsunami deposits on land are usually unpreserved because of the artificial land reformation. In such areas, the inner bay can serve as a site to explore paleo-tsunami deposits (Fujiwara et al., 2000) if suitable places exist. Monitoring the interannual variation of the dunes at Kesennuma Bay is crucial to ascertaining the preservation potential of the tsunami deposits in the inner bay setting. Acknowledgments This research was supported by the Japan Nuclear Energy Safety Organization and by a research grant from Tohoku University for an emergency field survey following the 2011 Tohoku-oki tsunami. We thank D. Tappin for his valuable suggestions and comments. References
Fig. 4. Core sample of the bedform. See Fig. 1 for location.
dune profile indicate that substantial sediment transport over the dunes occurred as bedload (e.g. Kostaschuk and Villard, 1996). The landward high-angle slopes also suggest that, during the landward incoming flows, flow separation was generated over the onshore dune slopes (e.g. Kostaschuk, 2000). The presence of flow separation over the high-angle asymmetric dunes possibly affected sediment transport (e.g. McLean et al., 1994; Bennett and Best, 1995; Best, 2005). Asymmetric shapes of the dunes at the bay (Fig. 3b) show that they were formed (or deformed) by the landward incoming flow. The geometries of lunate-crested dunes at the deeper area also suggest the effect of a landward incoming flow. However, Fritz et al. (2012) estimated that the return flow velocity at Kesennuma Bay was as great as 11 m/s, based on video footage taken near our studied site. Our preliminary modeling of the tsunami inundation suggests that the maximum current velocity of the first run-up wave at the bay was about 8 m/s. This flow velocity is sufficiently high to move the sea bottom sediments (e.g. Takahashi et al., 2000). Preservation of the landward asymmetric shapes of the dunes is probably explainable as follows: the dunes were formed after the passage of first few strong waves, but the landward incoming flows were generally dominant at the latest stage of the tsunami waning. Additional numerical analysis might require proof of this point, but it is certain that such large dunes were formed at 10–15 m water depth and that they are preserved on the sea floor as tsunami markers despite the passage of multiple tsunami waves.
Allen, J.R.L., 1982. Sedimentary Structures: Their Character and Physical Basis, vol. 1. Elsevier, Amsterdam, p. 593. Ashley, G.M., 1990. Classification of large-scale subaqueous bed-forms: a new look at an old problem. Journal of Sedimentary Petrology 60, 160–172. Bennett, S.J., Best, J.L., 1995. Mean flow and turbulence structure over fixed, twodimensional dunes: implications for sediment transport and bedform stability. Sedimentology 42, 491–513. Best, J.L., 2005. The fluid dynamics of river dunes: a review and some future research directions. Journal of Geophysical Research 110, F04S02. Bridge, J.S., Demicco, R.V., 2008. Earth Surface Processes, Landforms and Sediment Deposits. Cambridge University Press, New York, p. 815. Carling, P.A., 1999. Subaqueous gravel dunes. Journal of Sedimentary Research 69, 534–545. Dawson, A.G., Stewart, I., 2007. Tsunami deposits in the geological record. Sedimentary Geology 200, 166–183. Fritz, H.M., Phillips, D.A., Okayasu, A., Shimozono, T., Liu, H., Mohammed, F., Skanavis, V., Synolakis, C.E., Takahashi, T., 2012. The 2011 Japan tsunami current velocity measurements from survivor videos at Kesennuma Bay using LiDAR. Geophysical Research Letters 39, L00G23. Fujiwara, O., Masuda, F., Sakai, T., Irizuki, T., Fuse, K., 2000. Tsunami deposits in Holocene bay mud in southern Kanto region, Pacific coast of central Japan. Sedimentary Geology 135, 219–230. Goto, K., Takahashi, J., Oie, T., Imamura, F., 2011. Remarkable bathymetric change in the nearshore zone by the 2004 Indian Ocean tsunami: Kirinda Harbor, Sri Lanka. Geomorphology 127, 107–116. Goto, K., Sugawara, D., Abe, T., Haraguchi, T., Fujino, S., 2012. Liquefaction as an important source of the 2011 Tohoku-oki tsunami deposits at Sendai Plain, Japan. Geology 40, 887–890. Haraguchi, T., Iwamatsu, A., 2011. Detail map of the Great Eastern Japan tsunami (Aomori, Iwate, and Miyagi). Kokon-Shoin . (167 pp. (In Japanese)). Haraguchi, T., Takahashi, T., Hisamatsu, R., Morishita, Y., Sasaki, I., 2012. A field survey of geomorphic change on Kesennuma bay cased by the 2010 Chilean tsunami and the 2011 Tohoku tsunami. Proceedings of the Coastal Engineering JSCE 67–2, 241–245 (In Japanese). Kastens, K.A., Cita, M.B., 1981. Tsunami-induced sediment transport in the abyssal Mediterranean Sea. Geological Society of America Bulletin 92, 845–857. Kawamura, B., Mogi, A., 1961. On the deformation of the sea bottom in some harbours in the Sanriku coast due to the Chile tsunami. Report on the Chilean Tsunami, Field Investigation Committee for Chilean Tsunami, pp. 57–66 (In Japanese). Geological observation of the Sanriku coastal region damaged by the tsunami due to the Chile earthquake in 1960. In: Kon'no, E. (Ed.), Contribution of Institute of Geology and Paleontology, Tohoku University, 52, pp. 1–45 (in Japanese with English abstract). Kostaschuk, R., 2000. A field study of turbulence and sediment dynamics over subaqueous dunes with flow separation. Sedimentology 47, 419–531. Kostaschuk, R.A., Villard, P., 1996. Flow and sediment transport over large subaqueous dunes: Fraser River, Canada. Sedimentology 43, 849–863.
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205 McLean, S.R., Nelson, J.M., Wolfe, S.R., 1994. Turbulence structure over two-dimensional bed forms: implications for sediment transport. Journal of Geophysical Research 99 (C6), 12,729–12,747. Sakuna, D., Szczucinski, W., Feldens, P., Schwarzer, K., Khokiattiwong, S., 2012. Sedimentary deposits left by the 2004 Indian Ocean tsunami on the inner continental shelf offshore of Khao Lak, Andaman Sea (Thailand). Earth Planets Space 64, 931–943. Simons, M., Minson, S.E., Sladen, A., Ortega, F., Jiang, J., Owen, S.E., Meng, L., Ampuero, J.-P., Wei, S., Chu, R., Helmberger, D.V., Kanamori, H., Hetland, E., Moore, A.W., Webb, F.H., 2011. The 2011 magnitude 9.0 Tohoku-Oki earthquake: mosaicking the megathrust from seconds to centuries. Science 332, 1421–1425. Sugawara, D., Minoura, K., Nemoto, N., Tsukawaki, S., Goto, K., Imamura, F., 2009. Foraminiferal evidence of submarine sediment transport and deposition by backwash during the 2004 Indian Ocean tsunami. Island Arc 18, 513–525. Suzuki, K., Chiba, M., 2003. Annual change of the sediments and water quality in the Kesennuma Bay. Report of Marine Research (Miyagi Prefecture) 3, 53–62 (In Japanese). Takahashi, T., Shuto, N., Imamura, F., Asai, D., 2000. Modeling sediment transport due to tsunamis with exchange rate between bed load layer and suspended load layer. Proceedings of the International Conference of Coastal Engineering, ASCE, pp. 1508–1519.
205
Tanaka, H., Tinh, N.X., Umeda, M., Hirao, R., Pradjoko, E., Mano, A., Udo, K., 2012. Coastal and estuarine morphology changes induced by the 2011 Great East Japan Earthquake Tsunami. Coastal Engineering Journal 54. http://dx.doi.org/ 10.1142/S0578563412500106 (25 pp.). Tappin, D.R., Evans, H.M., Jordan, C.J., Richmond, B., Sugawara, D., Goto, K., 2012. Coastal changes in the Sendai area from the impact of the 2011 Tōhoku-oki tsunami: interpretations of time series satellite images and helicopter-borne video footage. Sedimentary Geology 282, 151–174. The 2011 Tohoku Earthquake Tsunami Joint Survey Group (299 authors), 2011. Nationwide field survey of the 2011 off the Pacific coast of Tohoku EARTHQUAKE and tsunami. Journal of Japan Society of Civil Engineers, Series B2 (Coastal Engineering) 67, 63–66. Tsuji, Y., Ohtoshi, K., Nakano, S., Nishimura, Y., Fujima, K., Imamura, F., Kakinuma, T., Nakamura, Y., Imai, K., Goto, K., Namegaya, Y., Suzuki, S., Shirosita, H., Matsuzaki, Y., 2010. Field investigation of the 2010 Chilean earthquake tsunami along the comprehensive coastal region in Japan. Proceedings of the Coastal Engineering, JSCE, 55, pp. 1346–1350 (In Japanese). Udo, K., Sugawara, D., Tanaka, H., Imai, K., Mano, A., 2012. Impact of the 2011 Tohoku earthquake and tsunami on beach morphology along the northern Sendai coast. Coastal Engineering Journal 54 (125009-1-15).
Contents lists available at SciVerse ScienceDirect
Marine Geology journal homepage: www.elsevier.com/locate/margeo
Letter
Large bedform generated by the 2011 Tohoku-oki tsunami at Kesennuma Bay, Japan Tsuyoshi Haraguchi a, Kazuhisa Goto b,⁎, Masataka Sato c, Yuichi Yoshinaga d, Naofumi Yamaguchi e, Tomoyuki Takahashi f a
Graduate School of Sciences, Osaka City University, Osaka, Japan International Research Institute of Disaster Science, Tohoku University, Japan Arc Geo Support Co., Ltd., Japan d CTI Engineering Co. Ltd., Japan e Center for Water Environment Studies, Ibaraki University, Japan f Graduate School of Safety Science, Kansai University, Takatsuki, Japan b c
a r t i c l e
i n f o
Article history: Received 29 February 2012 Received in revised form 6 November 2012 Accepted 16 November 2012 Available online 7 December 2012 Communicated by J.T. Wells Keywords: 2011 Tohoku-oki tsunami Japan dune Kesennuma tsunami deposit
a b s t r a c t The 11 March 2011 MW 9.0 Tohoku megathrust earthquake off the Pacific coast of Japan was a salient event in the history of Japan. The resultant huge tsunami (the 2011 Tohoku-oki tsunami) inundated a vast coastal area of northeastern Japan, causing widespread devastation. Twenty days after the tsunami, we analyzed the impact of the tsunami on the sea bottom of the Kesennuma inner bay using side-scan sonar to explore the damage and bathymetric change in the harbor. Herein we present the first direct evidence that the sea bottom sediments of around 10–15 m were largely reworked by the tsunami to thickness of a few meters, and that large dunes were formed by the tsunami. Considering that the sea wave influence is as weak as it is inside the inner bay, the potential exists that even meter-thick paleo-tsunami deposits are preserved in shallow sea bottoms with large bedforms. This finding will be a stepping-stone to future geological studies of tsunami effects in shallow sea regions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction A large tsunami is well known to be capable of changing coastal morphology radically (e.g. Kon'no, 1961; Goto et al., 2011). In fact, the recent 2011 Tohoku-oki tsunami strongly affected the coastal morphology of the Pacific coast of Tohoku, Japan (e.g., Goto et al., 2012; Tanaka et al., 2012; Udo et al., 2012; Tappin et al., 2012). Different from well-studied onshore tsunami impacts, few reliable observational data of the submarine processes of sediment transport and deposition by tsunamis are available (e.g., Dawson and Stewart, 2007). In fact, few reports of studies based on sediment samples have described offshore tsunami deposits left by historical tsunamis on a shallow sea bottom (e.g., Sugawara et al., 2009; Sakuna et al., 2012) or bathymetric data taken before and after a tsunami (e.g., Kawamura and Mogi, 1961; Goto et al., 2011). These studies revealed that fewmeter-scale erosion and sedimentation has sometimes occurred in very shallow sea bottoms of up to 10 m water depth. However, although possible offshore tsunami deposits have been reported (e.g., Kastens and Cita, 1981), it remains uncertain whether a tsunami can affect much deeper areas or create a bedform of any type such as dunes and ripples on the sea floor. ⁎ Corresponding author. E-mail address: [email protected] (K. Goto). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2012.11.005
Herein we present the first direct evidence that sea bottom sediments of around 10–15 m water depth were reworked by the 2011 Tohoku-oki tsunami at Kesennuma Bay, Japan (Fig. 1) to a thickness of a few meters, and that large bedforms were formed by the tsunami.
2. Local setting Kesennuma Bay, approximately 90 km northeast from the Sendai megacity, is a drowned valley at the Sanriku ria coast. The bay entrance is about 2.6 km wide. The area is about 15.4 km2, with maximum depth of 29 m. The tidal range in the bay is about 1.3 m. The inner bay usually has calm conditions, so it is used as a port of refuge during typhoon events. Sediments at the bottom of the bay are silt to sand; the average grain size was 70–240 μm (Suzuki and Chiba, 2003). The bay has been affected frequently by large tsunamis, according to historical documents. For example, the 1960 Chilean tsunami inundated Kesennuma Bay and destroyed part of the city. Onshore and offshore tsunami sedimentation and erosion were studied soon after the event (Kon'no, 1961; Kawamura and Mogi, 1961). Kawamura and Mogi (1961) studied the bathymetric change of the inner bay using bathymetric data obtained 4 years before and 1 month after the tsunami. They reported that approximately 4 m of sedimentation or erosion occurred at some places in the bay.
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205
201
Fig. 1. (a and b) Location maps of Kesennuma Bay. The shipping route and water depths are presented in (a). The tsunami inundation area (blue), and run-up height (yellow bar) are also shown (after Haraguchi and Iwamatsu, 2011). The background aerial photograph was provided by the Geospatial Information Authority of Japan. (c) Bathymetric contour map (10 m interval) of the bay.
We investigated the bathymetry and sea bottom conditions using side-scan sonar and a depth sounder (see Methods for details) in February 2009 and in March 2010 to analyze the annual change of bathymetry in Kesennuma Bay (Haraguchi et al., 2012). During this period, the 2010 Chilean tsunami of February 27, 2010, which had less than 2 m run-up height (Tsuji et al., 2010), struck the bay and partly inundated the city. Haraguchi et al. (2012) reported that no remarkable bathymetric change occurred in the ocean bottom during this tsunami event.
About one year later on March 11, 2011, the 2011 Tohoku-oki tsunami, which was generated in accompaniment with Mw = 9.0 earthquake off the coast of Japan's Tohoku region about 130 km east of Sendai (Simons et al., 2011), struck this bay. Local subsidence by the earthquake at this bay was about 0.6 m (Haraguchi et al., 2012). According to Fritz et al. (2012), the tsunami arrived as sea levels passed through low tide. An approximately 9–12 m high tsunami run-up was recorded at the bay entrance (Haraguchi and Iwamatsu,
202
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205
Fig. 2. (a) Depth map around the large bedforms. Side-scan image of the bedforms at the bottom of Kesennuma Bay is also shown. (b) Sounding survey result on February 2009 along the A–B line in Fig. 2a, which shows no bedform.
2011; The 2011 Tohoku Earthquake Tsunami Joint Survey Group, 2011). The maximum tsunami height of 20 m was measured at the headland to the west of the Kesennuma Bay entrance (Fritz et al., 2012). Harbor facilities were destroyed by this tsunami. To examine harbor functions––whether large ships could safely come alongside the pier––we conducted an emergency field survey to analyze the geomorphologic impact of the tsunami on the sea bottom 20 days after the tsunami in the bay, the earliest time to conduct the survey after sufficient daily life commodities had been supplied to most of the tsunami-affected area residents. As an initial report, Haraguchi et al. (2012) reported the bathymetric change by the 2011 Tohoku-oki tsunami over Kesennuma Bay. They reported that approximately 7 m erosion occurred at the narrow area of approximately 8 m water depth before the tsunami (Fig. 1).
Based on this finding, they reported that a tsunami can erode a substantial amount of sediment on a very shallow sea bottom. 3. Methods We used three-dimensional side-scan sonar (C3D), which has a swath range of about 10 times wider than the depth, to investigate the bathymetric change and sea floor imaging. The sonar was a rig transported on a ship, with positioning measured using a differential GPS system. The survey ship oscillation was corrected using a motion sensor (DMS-05) and the direction was corrected using a GPS gyrocompass (Vector Crescent VS110; Hemisphere GPS). Depth data were collected using a salinometer and a water temperature gauge (CTD). Using the “Computed Angle-of-Arrival Transient Imaging
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205
203
Fig. 3. (a) Close-up image of the isobath with 20 cm interval showing around the bedform. (b) Cross section of the bedforms along the C–D line in Fig. 3.
(CAATI)” algorithm, the C3D can measure high-resolution bathymetric data similarly to a multi-beam depth sounder. The measured data were analyzed using Hypack-Max software to remove errors. The measurement accuracy is 5 cm horizontal distance and 10 cm vertical distance (Haraguchi et al., 2012). 4. Results At the harbor entrance, with approximately 10–15 m water depth, the water depths before and after the tsunami were not significantly different (ca. 1 m). However, large bedforms were clearly visible on the sea floor (Fig. 2a). The wavelength and heights of these dunes were highly variable but, for example, they were 20 m long and 1.8 m high along the transect depicted in Fig. 3. The scale of the dune becomes smaller, with shorter length toward the shallower area (Fig. 2). The dunes have mainly straight or sinuous long crest lines with bifurcations, perpendicular to the direction of the slope. At deeper areas, short-crested dunes with slightly lunate geometries were observed (Figs. 2a and 3). The dunes had asymmetric profiles: the landward slopes were steeper than the seaward (Fig. 3b). In Fig. 3b, for example, the angles of the landward slopes are ca. 20 to ca. 33°, whereas the seaward slope angles are ca. 7 to ca. 10°. We also took sediment samples to ascertain the grain size and dune composition. As portrayed in Fig. 4, sediments are formed mainly by silt to sand, whereas the top ca. 25 cm was filled with well-rounded gravel (ca. 5 cm diameter) and shell fragments (Fig. 4).
5. Discussion No dune was observed in our survey on February 2009 (Fig. 2b), which is plausible because the sea waves and tidal current in the inner bay are probably insufficient to create such dunes. Moreover, it is noteworthy that no remarkable bathymetric change was observed after the 2010 Chilean tsunami (Haraguchi et al., 2012). Therefore, even the tsunami with a ca. 2 m high run-up was insufficient to create such a large bedform at the certain area. For that reason, we infer that these dunes were very likely formed during the 2011 Tohoku-oki tsunami. A tsunami as large as the 2011 event would be necessary to create such a large bedform on the shallow sea bottom of Kesennuma Bay. Dunes comprise silt to sand but were covered with gravel, the latter of which might have been transported from the surrounding gravel beaches along the Pacific coast, suggesting they were probably transported by the run-up waves. The wavelength, height, and the ratio of the dunes were consistent with those previously reported in marine and fluvial environments with similar water depths (Allen, 1982; Ashley, 1990) and those reported for gravel dunes (Carling, 1999), although further careful investigation of the composition, grain size, and inner sedimentary structures of these dunes is required. These dunes with lunate short crests suggest that movable sediment was limited there (e.g. Bridge and Demicco, 2008). The dunes in the observation area were asymmetrical with high-angle landward slopes that are close to angle of repose (Fig. 3). These characteristics of the
204
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205
6. Conclusion Two critically important findings related to the deposition of large bedforms by the 2011 Tohoku-oki tsunami at Kesennuma Bay were reported. First, although some reports have described bathymetric changes in very shallow seas by the tsunami (e.g., Goto et al., 2011), the large bedforms at Kesennuma Bay constitute the first direct evidence that sea bottom sediments of around 10–15 m were reworked by the tsunami with thickness of a few meters. The other important finding is the fact that the tsunami can truly leave a large bedform on the shallow sea bottom. Considering that the sea wave influence is weak because it is inside the inner bay, the potential exists that even sandy or silty paleo-tsunami deposits are preserved in the shallow sea bottom with large bedforms in the inner bay. In urban areas, the tsunami deposits on land are usually unpreserved because of the artificial land reformation. In such areas, the inner bay can serve as a site to explore paleo-tsunami deposits (Fujiwara et al., 2000) if suitable places exist. Monitoring the interannual variation of the dunes at Kesennuma Bay is crucial to ascertaining the preservation potential of the tsunami deposits in the inner bay setting. Acknowledgments This research was supported by the Japan Nuclear Energy Safety Organization and by a research grant from Tohoku University for an emergency field survey following the 2011 Tohoku-oki tsunami. We thank D. Tappin for his valuable suggestions and comments. References
Fig. 4. Core sample of the bedform. See Fig. 1 for location.
dune profile indicate that substantial sediment transport over the dunes occurred as bedload (e.g. Kostaschuk and Villard, 1996). The landward high-angle slopes also suggest that, during the landward incoming flows, flow separation was generated over the onshore dune slopes (e.g. Kostaschuk, 2000). The presence of flow separation over the high-angle asymmetric dunes possibly affected sediment transport (e.g. McLean et al., 1994; Bennett and Best, 1995; Best, 2005). Asymmetric shapes of the dunes at the bay (Fig. 3b) show that they were formed (or deformed) by the landward incoming flow. The geometries of lunate-crested dunes at the deeper area also suggest the effect of a landward incoming flow. However, Fritz et al. (2012) estimated that the return flow velocity at Kesennuma Bay was as great as 11 m/s, based on video footage taken near our studied site. Our preliminary modeling of the tsunami inundation suggests that the maximum current velocity of the first run-up wave at the bay was about 8 m/s. This flow velocity is sufficiently high to move the sea bottom sediments (e.g. Takahashi et al., 2000). Preservation of the landward asymmetric shapes of the dunes is probably explainable as follows: the dunes were formed after the passage of first few strong waves, but the landward incoming flows were generally dominant at the latest stage of the tsunami waning. Additional numerical analysis might require proof of this point, but it is certain that such large dunes were formed at 10–15 m water depth and that they are preserved on the sea floor as tsunami markers despite the passage of multiple tsunami waves.
Allen, J.R.L., 1982. Sedimentary Structures: Their Character and Physical Basis, vol. 1. Elsevier, Amsterdam, p. 593. Ashley, G.M., 1990. Classification of large-scale subaqueous bed-forms: a new look at an old problem. Journal of Sedimentary Petrology 60, 160–172. Bennett, S.J., Best, J.L., 1995. Mean flow and turbulence structure over fixed, twodimensional dunes: implications for sediment transport and bedform stability. Sedimentology 42, 491–513. Best, J.L., 2005. The fluid dynamics of river dunes: a review and some future research directions. Journal of Geophysical Research 110, F04S02. Bridge, J.S., Demicco, R.V., 2008. Earth Surface Processes, Landforms and Sediment Deposits. Cambridge University Press, New York, p. 815. Carling, P.A., 1999. Subaqueous gravel dunes. Journal of Sedimentary Research 69, 534–545. Dawson, A.G., Stewart, I., 2007. Tsunami deposits in the geological record. Sedimentary Geology 200, 166–183. Fritz, H.M., Phillips, D.A., Okayasu, A., Shimozono, T., Liu, H., Mohammed, F., Skanavis, V., Synolakis, C.E., Takahashi, T., 2012. The 2011 Japan tsunami current velocity measurements from survivor videos at Kesennuma Bay using LiDAR. Geophysical Research Letters 39, L00G23. Fujiwara, O., Masuda, F., Sakai, T., Irizuki, T., Fuse, K., 2000. Tsunami deposits in Holocene bay mud in southern Kanto region, Pacific coast of central Japan. Sedimentary Geology 135, 219–230. Goto, K., Takahashi, J., Oie, T., Imamura, F., 2011. Remarkable bathymetric change in the nearshore zone by the 2004 Indian Ocean tsunami: Kirinda Harbor, Sri Lanka. Geomorphology 127, 107–116. Goto, K., Sugawara, D., Abe, T., Haraguchi, T., Fujino, S., 2012. Liquefaction as an important source of the 2011 Tohoku-oki tsunami deposits at Sendai Plain, Japan. Geology 40, 887–890. Haraguchi, T., Iwamatsu, A., 2011. Detail map of the Great Eastern Japan tsunami (Aomori, Iwate, and Miyagi). Kokon-Shoin . (167 pp. (In Japanese)). Haraguchi, T., Takahashi, T., Hisamatsu, R., Morishita, Y., Sasaki, I., 2012. A field survey of geomorphic change on Kesennuma bay cased by the 2010 Chilean tsunami and the 2011 Tohoku tsunami. Proceedings of the Coastal Engineering JSCE 67–2, 241–245 (In Japanese). Kastens, K.A., Cita, M.B., 1981. Tsunami-induced sediment transport in the abyssal Mediterranean Sea. Geological Society of America Bulletin 92, 845–857. Kawamura, B., Mogi, A., 1961. On the deformation of the sea bottom in some harbours in the Sanriku coast due to the Chile tsunami. Report on the Chilean Tsunami, Field Investigation Committee for Chilean Tsunami, pp. 57–66 (In Japanese). Geological observation of the Sanriku coastal region damaged by the tsunami due to the Chile earthquake in 1960. In: Kon'no, E. (Ed.), Contribution of Institute of Geology and Paleontology, Tohoku University, 52, pp. 1–45 (in Japanese with English abstract). Kostaschuk, R., 2000. A field study of turbulence and sediment dynamics over subaqueous dunes with flow separation. Sedimentology 47, 419–531. Kostaschuk, R.A., Villard, P., 1996. Flow and sediment transport over large subaqueous dunes: Fraser River, Canada. Sedimentology 43, 849–863.
T. Haraguchi et al. / Marine Geology 335 (2013) 200–205 McLean, S.R., Nelson, J.M., Wolfe, S.R., 1994. Turbulence structure over two-dimensional bed forms: implications for sediment transport. Journal of Geophysical Research 99 (C6), 12,729–12,747. Sakuna, D., Szczucinski, W., Feldens, P., Schwarzer, K., Khokiattiwong, S., 2012. Sedimentary deposits left by the 2004 Indian Ocean tsunami on the inner continental shelf offshore of Khao Lak, Andaman Sea (Thailand). Earth Planets Space 64, 931–943. Simons, M., Minson, S.E., Sladen, A., Ortega, F., Jiang, J., Owen, S.E., Meng, L., Ampuero, J.-P., Wei, S., Chu, R., Helmberger, D.V., Kanamori, H., Hetland, E., Moore, A.W., Webb, F.H., 2011. The 2011 magnitude 9.0 Tohoku-Oki earthquake: mosaicking the megathrust from seconds to centuries. Science 332, 1421–1425. Sugawara, D., Minoura, K., Nemoto, N., Tsukawaki, S., Goto, K., Imamura, F., 2009. Foraminiferal evidence of submarine sediment transport and deposition by backwash during the 2004 Indian Ocean tsunami. Island Arc 18, 513–525. Suzuki, K., Chiba, M., 2003. Annual change of the sediments and water quality in the Kesennuma Bay. Report of Marine Research (Miyagi Prefecture) 3, 53–62 (In Japanese). Takahashi, T., Shuto, N., Imamura, F., Asai, D., 2000. Modeling sediment transport due to tsunamis with exchange rate between bed load layer and suspended load layer. Proceedings of the International Conference of Coastal Engineering, ASCE, pp. 1508–1519.
205
Tanaka, H., Tinh, N.X., Umeda, M., Hirao, R., Pradjoko, E., Mano, A., Udo, K., 2012. Coastal and estuarine morphology changes induced by the 2011 Great East Japan Earthquake Tsunami. Coastal Engineering Journal 54. http://dx.doi.org/ 10.1142/S0578563412500106 (25 pp.). Tappin, D.R., Evans, H.M., Jordan, C.J., Richmond, B., Sugawara, D., Goto, K., 2012. Coastal changes in the Sendai area from the impact of the 2011 Tōhoku-oki tsunami: interpretations of time series satellite images and helicopter-borne video footage. Sedimentary Geology 282, 151–174. The 2011 Tohoku Earthquake Tsunami Joint Survey Group (299 authors), 2011. Nationwide field survey of the 2011 off the Pacific coast of Tohoku EARTHQUAKE and tsunami. Journal of Japan Society of Civil Engineers, Series B2 (Coastal Engineering) 67, 63–66. Tsuji, Y., Ohtoshi, K., Nakano, S., Nishimura, Y., Fujima, K., Imamura, F., Kakinuma, T., Nakamura, Y., Imai, K., Goto, K., Namegaya, Y., Suzuki, S., Shirosita, H., Matsuzaki, Y., 2010. Field investigation of the 2010 Chilean earthquake tsunami along the comprehensive coastal region in Japan. Proceedings of the Coastal Engineering, JSCE, 55, pp. 1346–1350 (In Japanese). Udo, K., Sugawara, D., Tanaka, H., Imai, K., Mano, A., 2012. Impact of the 2011 Tohoku earthquake and tsunami on beach morphology along the northern Sendai coast. Coastal Engineering Journal 54 (125009-1-15).