Study on the relation of river morphology and tsunami propagation in rivers
- 243 Downloads
The relations of river morphology and tsunami propagation in rivers were studied at several rivers in the Tohoku region during The Great Chilean Tsunami of 2010 and The Great East Japan Tsunami of 2011. It was found that river mouth morphological features play an important role in the intrusion of low magnitude tsunamis in which the geological and geographical conditions are an important factor. Nevertheless, the effects of these features were not found in the case of an extreme tsunami wave. As the wave enters the river, the propagation depends on other factors. It was found that the intrusion distance correlates well to the riverbed slope. The measurements of water level and riverbed slope were analyzed to propose an empirical method for estimating the damping coefficient for the tsunami propagation in rivers based on the tsunami of 2011. The proposed empirical method was used to approximate the length of the tsunami intrusion into a river by assuming that the furthest distance is given for the ratio of local tsunami wave height to the tsunami wave height at the river entrance of 0.05 (5 %). The estimated intrusion length from the proposed method in this study shows a good comparison with measurement data.
KeywordsTsunami Propagation River morphology Bed slope
Tsunami waves are one of the natural disaster events that may, depending on the magnitude, cause severe damages to coastal areas. A massive tsunami wave may affect and cause damage to a larger area since the wave may propagate further in land. Past tsunamis have shown the tremendous damage that a tsunami wave may cause to the coastal area. In the past decade, several major tsunami events were recorded globally. They are The Great Indian Ocean Tsunami of 2004, The Great Chilean Tsunami of 2010, and The Great East Japan Tsunami of 2011. In the last two events, the effect of both tsunami waves was felt in the northeast coast of Japan.
The Chilean Earthquake occurred on 27 February 2010 with a moment magnitude (Mw) of 8.8 causing tsunami waves that reached as far as Japan. The epicenter of the earthquake was 3 km off the Chilean Coast. Some areas in Chile suffered from the tsunami wave that reached up to 30 m, causing tremendous damage (Fritz et al. 2011). The tsunami wave traveled a great distance and reached the North East Japan coast at approximately 1 m in height. There were no significant damages to the coastal area in Japan. However, the tsunami wave intruded into some rivers and propagated the upstream. This phenomenon was efficiently recorded and reported by Tanaka et al. (2011).
The Great North East Japan Earthquake occurred on 11 March 2011 with a moment magnitude (Mw) of 9.0. The epicenter was approximately 70 km east of the North East Japan coast at an underwater depth of approximately 32 km. It generated a huge tsunami wave that was the biggest ever recorded in Japan. The wave hit the north east coast of Japan, affecting the Tohoku and Kanto districts, with devastating effects, which significantly changed the coastal morphology, especially in estuaries (Suppasri et al. 2012; Tanaka et al. 2012; Tappin et al. 2012). In Iwate, Japan, a tsunami height of approximately 40 m was recorded (Mori et al. 2012). As well, the generated tsunami wave and resulting debris were reported to travel as far as California in the USA.
In general, the river mouth is more vulnerable to a tsunami threat than the seashore. Additionally, the tsunami propagation in a river has higher celerity than the tsunami propagation over land. The tsunami intrusion in the river may maintain and propagate the wave energy further upstream (Adityawan et al. 2012a). Thus, tsunami intrusion in a river may cause damage to the infrastructure along the river and to the area far from the shoreline. Viana-Baptista et al. (2006) had simulated the tsunami propagation in the river at the Tagus estuary, Lisbon, and showed that the tsunami propagation in the river causes severe flooding in the upstream area, far from the shoreline. Yeh et al. (2011) conducted a similar study, in which a simulation of a hypothetical tsunami propagating in the Columbia River may travel as far as 173 km from the ocean. A detailed study by Tanaka et al. (2007) on Sri Lankan rivers, in the tsunami event of 2004, discovered that the tsunami intrusion along a small river that flowed inside a city had caused flooding and local damages, extending the impact of the tsunami to the upstream area. Thus, it is important to understand the effects of tsunami propagation along a river to take proper disaster prevention measures in case of a future tsunami.
The fundamental function of a river is to serve as drainage. A river collects and carries the excess water on land to the downstream end. An attempt to reduce the tsunami wave propagation in a river may cause conflict with this function. Therefore, tsunami disaster prevention in a river must be studied in detail. There have been various studies on the tsunami propagation process over land. However, there were only a few studies related to the tsunami propagation process in a river. Abe (1986) analyzed The Central Sea of Japan Earthquake of 1983 with respect to the propagation of the tsunami wave in five large rivers. He concluded that various factors, i.e., meandering, flow branching, and so on, may complicate the observed wave profiles. Tsuji et al. (1991) found that the tsunami height in a river might be amplified by a factor of 1.5, based on laboratory experiment and theoretical approach. Yasuda et al. (2010) showed that, in general, the propagation of the tsunami wave in the rivers did not reach the upstream area during The Great Indian Ocean Tsunami on 2004 in Sri Lanka. Their study stated that the tsunami propagation in the Japanese rivers might extend upstream due to the rivers-specific characteristics.
2 Tsunami height and river morphology
2.1 Water level data
The measurement data of the water level in the rivers for The Great East Japan Tsunami of 2011 were acquired from the Ministry of Land, Infrastructure and Transport, Japan. In the latest tsunami event, most of the water level measurement devices were destroyed due to the massive force of the tsunami. Nevertheless, some of them were in good condition and continued to measure the water level during the tsunami. They are located in the Sunaoshi River, the Naruse River, and the Kitakami River (see Fig. 1). The stations measured the water level variation at 10 min intervals. The water level data in the Kitakami River were not used for further analysis due to the existence of a weir, which will be explained in detail at the end of this section. The water level data in the Sunaoshi River and the Naruse River were used for further analysis in this study.
The Great East Japan Earthquake of 2011 caused land subsidence in many places (Udo et al. 2012). The subsidence may cause uncertainty in the measurement of the water level and the tidal level in some places due to the possible vertical shifting of the measurement device. It may be difficult to estimate the tsunami height based on the normal water level. Therefore, for the 2011 tsunami, the wave height was defined as the water level difference from the first peak recorded to the water level drop prior to this peak without tidal correction.
2.2 Tsunami traces
2.3 2010 water level data
2.4 River mouth classification and the effect on tsunami reduction
3 The tsunami damping coefficient in the river
The value of the damping coefficient (k) was estimated for each data set and each tsunami event. In the case of the tsunami in 2011, the tsunami height based on survey and the water level data may yield slightly different values of k due to the reason stated in the previous section. The water level data were recorded at 10 min intervals, and it may not be short enough intervals to capture the exact tsunami height, which leads to the deviation from the survey data.
4 Relation between the riverbed slope and the damping coefficient
The relation between the riverbed slope (S) and the damping coefficient (k) in the event of the 2011 tsunami was investigated. The riverbed slope was estimated by calculating the distance and the bed level difference between the farthest downstream and upstream measurement points, based on the 2011 tsunami. The data were provided by the Ministry of Land, Infrastructure and Transport, Japan.
The tsunami trace data has more points than the water level data. Thus, the k value obtained from the tsunami trace data was used for further analysis. However, there was no survey data for the Sunaoshi River, thus the k value in this river was estimated from the water level data.
5 Relation between the riverbed slope and the tsunami intrusion distance
This study investigated the relation of the river mouth morphology and the riverbed slope to the tsunami intrusion into the rivers. The study analyzed tsunami height based on the water level data as well as the tsunami trace survey for the rivers in Miyagi, Japan, based on The Great Chilean Tsunami of 2010 and The Great East Japan tsunami of 2011.
The effect of a weir to the tsunami propagation in a river was observed in the Kitakami River and the Abukuma River for the 2011 tsunami. In both rivers, the weir significantly reduced the tsunami wave height at the upstream part of the weir. However, the weir also caused wave reflection that increased the wave height at its downstream area. It was also reported that the weir suffered damage due to the tsunami since it was not designed to sustain its force coming from the downstream area. Thus, concerning future disaster prevention, it is recommended that a weir in a river that is located within the range of tsunami intrusion should be designed to sustain the force of a tsunami. In addition, the river weir may cause a significant water level increase in the downstream area due to the effect of the tsunami reflection. The affected river length may require further protection.
It was found that the river mouth morphological features have a significant effect on the wave height when entering the river. River mouth with a buffer, i.e. sand formation, at its entrance tends to reduce the wave energy higher than the river mouth with no buffer. Hence, the tsunami intrusion and wave height are smaller. However, the protective buffer has a high chance of being swept away in an extreme tsunami event, negating its effect. The tsunami propagation in the river itself will depend on the river morphologies.
The tsunami wave height decreases exponentially to the distance from the river mouth in the 2010 tsunami as well as in the 2011 tsunami. It was found that the damping coefficient is closely related to the riverbed slope. However, river meandering may have different affect on this parameter depending on the tsunami event. In addition, the 2010 tsunami flowed in the main channel and the 2011 tsunami flowed in the flood plain, which also affect the value of the damping coefficient in each tsunami event.
The relation was further utilized to propose a new method to estimate the tsunami intrusion distance based on the riverbed slope for the case of the 2011 tsunami. The estimated intrusion distance showed a good comparison to the measured value. The proposed method in this study can be used to estimate the required protection length of a river in relation to future mega tsunami prevention. However, it should be further verified with more combination of rivers and tsunami magnitude based on real case, numerical experiments, and with laboratory experiment.
The Ministry of Land, Infrastructure and Transport Tohoku district maintenance office and Miyagi Rivers Division provided the water level data and cross section in the rivers. The authors would like to thank the financial supports from Grant-in-Aid for Scientific Research from Japan Society for Promotion of Science (No. 2503364). This research was also funded by the Grant-in-Aid for Specific Research Project, International Research Institute of Disaster Science, Tohoku University, the Grant-in-Aid for Scientific Research from the River Environmental Fund (REF) in charge of the Foundation of River and Watershed Environmental Management (FOREM), and Assistance for Technological Development, Tohoku Construction Association.
- Adityawan MB, Roh M, Tanaka H, Farid M (2012b) The effect of river mouth morphological features on tsunami intrusion. Proceedings of The International Conference on Disaster Management: 75-83Google Scholar
- Dalrymple RA (1992) Water wave propagation in jettied channels. Proceedings of 23rd Conference on Coastal Engineering: 3040-3053Google Scholar
- Fritz HM, Petroff CM, Catalán PA, Cienfuegos R, Winckler P, Kalligeris N, Weiss R, Barrientos SE, Meneses G, Valderas-Bermejo C, Ebeling C, Papadopoulos A, Contreras M, Almar R, Dominguez JC, Synolakis CE (2011) Field survey of the 27 February 2010 Chile Tsunami. Pure Appl Geophys 168:1989–2010. doi:10.1007/s00024-011-0283-5
- Suppasri A, Koshimura S, Imai K, Mas E, Gokon H, Muhari A, Imamura F (2012) Damage characteristic and field survey of the 2011 Great East Japan Tsunami in Miyagi Prefecture. Coast Eng J 54(1):30. doi:10.1142/S0578563412500052
- Tanaka H, Ishino K, Nawarathna B, Nakagawa H, Yano S, Yasuda H, Watanabe Y, Hasegawa K (2007) Field investigation of disasters in Sri Lankan rivers caused by the 2004 Indian Ocean Tsunami. J Hydrosci Hydraulic Eng 66:91–112Google Scholar
- Tanaka H, Tinh NX, Dao NX (2011). Field measurement and numerical studies on the tsunami propagation into upstream of rivers. Proc. of 34th IAHR Congress: CD-ROMGoogle Scholar
- Tanaka H, Nguyen XT, Umeda M, Hirao R, Pradjoko E, Mano A, Udo K (2012) Coastal and estuarine morphology changes induced the 2011 Great East Japan Earthquake Tsunami. Coastal Engineering Journal 54(1):25 pages. doi:10.1142/S0578563412500106
- Tappin DR, Evans HM, Jordan CJ, 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, helicopter-borne video footage and field observations. Sediment Geol 282(30):151–174CrossRefGoogle Scholar
- Viana-Baptista MA, Soares PM, Miranda JM, Luis JF (2006) Tsunami propagation along Tagus estuary (Lisbon, Portugal) preliminary results. Sci Tsunami Hazards 24(5):329–338Google Scholar
- Yeh H, Tolkova E, Jay D, Talke S, Fritz H (2011) Tsunami hydrodynamics in the columbia river. J Disaster Res 7(5):604–608Google Scholar