Response of salinity intrusion to the hydrodynamic conditions and river mouth morphological changes induced by the 2011 tsunami

Journal of Science and Technology in Civil Engineering NUCE 2020. 14 (2): 1–16 RESPONSE OF SALINITY INTRUSION TO THE HYDRODYNAMIC CONDITIONS AND RIVER MOUTH MORPHOLOGICAL CHANGES INDUCED BY THE 2011 TSUNAMI Nguyen Xuan Tinha,∗, Jin Wanga, Hitoshi Tanakaa, Kinuko Itob aDepartment of Civil Engineering, Tohoku University, 6-6-06 Aoba, Sendai 980-8579, Japan bDepartment of Applied Aquatic Bio-Science, Graduate School of Agriculture, Tohoku University, 468-1 Aramaki Aza Aoba, Aoba-ku, Sendai, Miyagi, 980-8572, Japan Article history: Received 07/03/2020, Revised 29/03/2020, Accepted 31/03/2020 Abstract The 2011 Tohoku Earthquake and tsunami were one of the most devastating natural disasters in history. It caused significant ground subsidence and erosion along the Japan coastline. The Natori river mouth which is a habitat for both fishes and bivalves, as an important fishing ground, has been damaged by the tsunami because of the change of the process of salt transport in an estuarine system. In general, salinity intrusion into the river mouth can be affected by many factors such as river water discharge and tidal level, as well as estuarine morphology. In this study, the response of salinity intrusion to the river mouth morphological changes induced by the 2011 Tsunami is investigated. The topographical changes caused by the tsunami are mainly divided into two stages. The first is the direct action of the tsunami, which caused the severe scouring of the coast and the widening of the river. The results have clearly indicated that after tsunami the salt water can intrude much further upstream compare to the condition before the tsunami event. Another changes occurred during the restoration process after the tsunami. The sediment accumulation in the river channel prevented the saltwater from entering the river channel, which reduced the salt intrusion degree. However, the effect of the morphology change caused directly by the tsunami is far greater than the sedimentation of the river. Keywords: salinity intrusion; river morphology; tsunami impact; numerical simulation; EFDC model. https://doi.org/10.31814/stce.nuce2020-14(2)-01 c© 2020 National University of Civil Engineering 1. Introduction Salt intrusion is one of the important problems in estuaries because it affects the quality of surface water and groundwater as well as the aquatic habitat. Salinity has been used as an indicator of the water quality for organism distribution [1, 2]. The Natori River is an important fishing ground both for bivalves and fishes in central Miyagi prefecture. It is important to figure out the salinity distribution in this area, as it will prove invaluable in the maintenance of fishery resources in Miyagi. The effects of the Great East Japan Tsunami on fish populations and ecosystem recovery has been studied, which indicates that the distribution and abundance of bivalve can be affected by variations of salinity and depth of the water. The brackish area has extended upstream after the tsunami, presumably caused by ∗Corresponding author. E-mail address: nguyen.xuan.tinh.c5@tohoku.ac.jp (Tinh, N. X.) 1 Tinh, N. X., et al. / Journal of Science and Technology in Civil Engineering ground subsidence in this area [3]. The extension of brackish water area may increase the operation cost for the desalination processes such as using the nanofiltration technique for the drinking water treatment in the lower Thu Bon River Basin [4]. Based on this background, discussion about the salinity distribution in the Natori River mouth will be conducted. This research will reveal the spatial and temporal variations in salinity and the roles of river discharge, tidal period as well as morphology changes in regulating salt transport. Many kinds of complex processes such as tidal variation, hydrological flux, wind stress reflect changes in salinity. Numerous efforts have been made to understand the spatial and temporal dis- tributions of salinity under the external influences of these factors. The distribution depends on the estuarine response to river discharge, wind and tidal mixing over time scales from days to weeks and months. There is a consensus that salt intrusion is inversely correlated to river discharge. A high river flow results in a decreased salinity intrusion. The relationship between salt intrusion length and river discharge follows a power law with an exponent of n, which varies in different estuaries [5, 6]. And the response of salt intrusion to tidal mixing has also been studied extensively, while the relationship between salt and tidal mixing differs largely. For a well-mixed or salt wedge estuary, salt intrudes more landward during spring tides than during neap tides [7]. On the other hand, observations, analytical and numerical model results have indicated that larger upstream salt flux or salinity intrusion happens during neap tides in partially mixed estuaries. The difference has been attributed to the different salt transport mechanisms for different estuaries [5, 6, 8]. In addition, the salt transport process can be also affected by changes in some geometric charac- teristics. Such changes can alter both the hydrodynamics and the rate of mixing in the coastal ocean, thereby having a profound effect on salt transport in estuaries. Salt intrusion is generally caused by an imbalance between river and tidal flows but variation in seawater intrusion is also attributable to estuarine geometry. Morphological changes during tidal variation drastically affect the longitudinal salinity distribution [9, 10]. Journal of Science and Technology in Civil Engineering NUCE 2018 ISSN 1859-2996 3 estuaries. Salt intrusion is generally caused by an imbalance between river and tidal 72 flows but variation in seawater intrusion are also attributable to estuarine geometry. 73 Morphological changes during tidal variation drastically affect the longitudinal 74 salinity distribution [9, 10]. 75 Because the Great Tsunami which occurred on 11 March 2011, ma y coa tlin s 76 and river mouths has been greatly damaged. The serious coastal and estuarine 77 morphological changes due to the 2011 tsunami in Tohoku region have been reported 78 in the study by [11]. In addition, a detail study of the morphological characteristics of 79 Nato i River mouths after the 2011 tsunami and recov ry process have carried out by 80 [12]. Figure 1 shows the aerial photos of the river mouth taken between March 6, 81 2011 and March 4, 2013. Comparing Figs. 1 (a) and (b), it can be found that the 82 tsunami severely wash d away the estuary' lagoon area and the river channel was 83 also greatly expanded. After this event, the estuary has entered a slow recovery phase, 84 and the washed and broken coastline has gradually become complete again, after 85 2013, the shape of the river mouth maintaining a relatively stable state. However, 86 comparing Figs. 1 (f) with 1 (a), there is still a l rge difference between the form of 87 the estuary and that before the tsunami: a clear sediment accumulation inside the river 88 can be observed in 2013. 89 90 91 Figure 1. Aerial photographs of the Natori estuary morphological changes after the 92 2011 tsunami 93 (Các tiêu đề nhỏ (a), (b), để xuống dưới tranh, không chèn trong hình/ Font chữ 94 Times New Roman thường, không đậm—Phần chèn trong hình là phần đen khi rectify 95 ảnh do vậy đề nghị không thay đổi.) 96 As indicated above, the general understanding of estuarine dynamics and salt 97 intrusion has advanced greatly in recent decades. However, for a specific estuary, such 98 as Natori Estuary in particular, which was under the severe impact of the tsunami, the 99 morphology changed in a short period of time and continued to change in the 100 (a) 2011-03-06 (b) 2011-03-12 (c) 2011-06-08 (d) 2012-01-18 (e) 2012-09-07 (f) 2013-03-04 Figure 1. Aerial photographs of the Natori estuary morphological changes after the 2011 tsunami Because of the Great Tsunami which occurred on 11 March 2011, many coastlines and river mouths has been greatly damaged. The serious coast l and estuarine morphological changes due to the 2011 tsunami in Tohoku region has been reported in the study by [11]. In addition, a detail study of the morphological characteristics of Natori River mouths after the 2011 tsunami and recovery process have carried out by [12]. Fig. 1 shows the aerial photos of the river mouth taken between March 6, 2011 and March 4, 2013. Comparing Figs. 1(a) and 1(b), it can be found that the tsunami 2 Tinh, N. X., et al. / Journal of Science and Technology in Civil Engineering severely washed away the estuary’s lagoon area and the river channel was also greatly expanded. After this event, the estuary has entered a slow recovery phase, and the washed and broken coastline has gradually become complete again, after 2013, the shape of the river mouth maintaining a relatively stable state. However, comparing Figs. 1(f) with 1(a), there is still a large difference between the form of the estuary and that before the tsunami: a clear sediment accumulation inside the river can be observed in 2013. As indicated above, the general understanding of estuarine dynamics and salt intrusion has ad- vanced greatly in recent decades. However, for a specific estuary, such as Natori Estuary in particular, which was under the severe impact of the tsunami, the morphology changed in a short period of time and continued to change in the subsequent recovery process, the changes in salt transport have not been quantitatively evaluated so far. Therefore, several observation datasets (topographic survey data before and after tsunami, river discharge, water elevation, tidal level) are collected in this study. The verified model is used to investigate the impacts of morphology change, river discharge, and tidal level on salt transport in the Natori River Estuary. The purpose of this study is to quantitatively evaluate the changes in salinity distribution induced by factors with different time scales, from weeks (spring- neap tide) to months (seasonal river discharge change) and years (morphology change), then identify the extent to which each factor affects changes in salinity. The results obtained provide significant implications for the sustainable development of the estuarine system and the local fishery revival. 2. Materials and methods 2.1. Study area The Natori River is located in central Miyagi prefecture, in the Tohoku region of northern Japan, which is listed as a first-class river according to the River Act of Japan (Ministry of Land, Infrastruc- ture, Transport and Tourism (2013)). The Natori River is approximately 55 km in length, and has 13 branches. The basin area is about 939 km2, yearly averaged discharge is 16.32 m3/s. The Natori River Estuary is located on Japan’s east coast, and faces the Pacific Ocean (Fig. 2). The river divided into two branches about 5.5 km upstream from the river mouth, one of which is the Hirose River, whichJournal of Science and Technology in Civil Engineering NUCE 2018 ISSN 1859-2996 5 133 Figure 2. Location of the study area 134 2.2. Data collection 135 In this study, to achieve the above objectives, the required data sets are the 136 bathymetry data in different years before and after the tsunami, river discharge and 137 tidal elevation were specified as boundaries, water level and salinity were used for 138 model calibration and verification. Table 1 is the list of all data available from 2009-139 2016. 140 141 Table 1. Summary of the data collection from 2009-2016 (Black dots indicate the data 142 availability) [14] 143 Morphology Water level Tidal River discharge Salinity 2009 l l l l 2010 l 2011 l l 2012 l l l 2013 l l l l 2014 l l l l l 2015 l l l l l 2016 l l l l l JAPAN SENDAI Hirosebashi discharge St. Natoribashi discharge St. Fukurobara water level St. Yuriage water level St. 0 1 (km) Figure 2. Location of the study area 3 Tinh, N. X., et al. / Journal of Science and Technology in Civil Engineering passes through the city of Sendai. In the downstream close to the coast, there is the Idoura Lagoon on the north coast and Hiroura Lagoon on the south coast. The Great East Japan Earthquake and Tsunami in March 2011 were one of the most devastating natural disasters in history, affecting the society, economy, coastlines, infrastructure, and housing. In addition to affecting human life, the subsequent tsunami also struck organisms living in the water. Miyagi Prefecture is the second largest fishery landing region in Japan and as a result of the tsunami this fishery was heavily affected: many ships were lost; ports and jetties were destroyed [13]. The Natori River is an important fishing ground both for bivalves and fishes, various fish species live in brackish water areas, which are very important for the maintenance of fishery resources [3]. The tsunami resulted in significant ground subsidence and deposition of rubble and mud in the Natori River. 2.2. Data collection In this study, to achieve the above objectives, the required data sets are the bathymetry data in different years before and after the tsunami, river discharge and tidal elevation were specified as boundaries, water level and salinity were used for model calibration and verification. Table 1 is the list of all data available from 2009-2016. Table 1. Summary of the data collection from 2009-2016 (Black dots indicate the data availability) [14] Morphology Water level Tidal River discharge Salinity 2009 • • • • 2010 • 2011 • • 2012 • • • 2013 • • • • 2014 • • • • • 2015 • • • • • 2016 • • • • • a. Bathymetry data The topographic map of 2009 was used as the bottom elevation before the tsunami. From 2011 to 2015, the bottom elevation of shallow coastal terrain was measured every one kilometer along the coast of the Sendai Bay with the survey line which is perpendicular to the coastline, which was carried out by the Geospatial Information Authority of Japan. On the other hand, the Tohoku Regional Bureau, Ministry of Land, Infrastructure and Transport (MLIT) provided the bottom topography data of 4 sections, with the survey line which is perpendicular to the channel, within a distance of 0.6 km from the ocean side to the Natori River mouth as shown in Fig. 3. By combining these two data sets, the detailed topograpthic maps of the Natori estuary can be determined for each year by an interpolation process. b. Hydrodynamic data There are two river discharge measurement stations located in the upstream of the study area which are Hirosebashi station located on the Hirose river branch and Natoribashi station on the Natori river. These river discharge stations are located far enough to avoid the impacts by the tidal motion. In 4 Tinh, N. X., et al. / Journal of Science and Technology in Civil Engineering Journal of Science and Technology in Civil Engineering NUCE 2018 ISSN 1859-2996 6 144 a. Bathymetry data 145 The topographic map of 2009 was used as the bottom elevation before the 146 tsunami. From 2011 to 2015, the bottom elevation of shallow coastal terrain was 147 measured every one kilometer along the coast of the Sendai Bay with the survey line 148 which is perpendicular to the coastline, which was carried out by the Geospatial 149 Information Authority of Japan. On the other hand, the Tohoku Regional Bureau, 150 Ministry of Land, Infrastructure and Transport (MLIT) provided the bottom 151 topography data of 4 sections, with the survey line which is perpendicular to the 152 channel, within a distance of 0.6km from the ocean side to the Natori River mouth as 153 shown in Fig. 3. By combining these two data sets, the detailed topograpthic maps of 154 the Natori estuary can be determined for ach year by an interpolation process. 155 156 Figure 3. Natori river mouth transection measurement data before and after the 2011 157 tsunami [MLIT] 158 8 6 4 2 0 -2 -4 -6 0 200100 300 400 500 600 El ev at io n (m ) Section A 8 6 4 2 0 -2 -4 -6 0 200100 300 400 500 600 El ev at io n( m ) 0 200100 300 400 500 600 8 6 4 2 0 -2 -4 -6 Section B El ev at io n( m ) Distance (m) Distance (m) Distance (m) Section D Before tsunami 2011 (After tsunami) 2012 2013 2014 Mean sea level Figure 3. Natori river mouth transection measurement data before and after the 2011 tsunami [MLIT] addition, there are two water level stations where Fukurobara station is located upstream and Yuriage station is located downstream near the estuary respectively. Annual, monthly and hourly river dis- charge and water level data for 4 hydrodymanic stations are provided by the Japan Meteorological Agency (JMA) website [14]. The tidal levels used in this study are obtained from hourly measured data at Sendai Port station, provided by the JMA [14] The distribution of tidal phases in the Natori River estuary is mixed tide and the tidal range is from about 0.8 m to 1.6 m. The tidal amplitudes decrease gradually when the tide propagates upstream. c. Salinity data In this study, measured salinity data for the three years from 2013 to 2015 were used. This salinity data was provided by the College of Agriculture, Tohoku University. As shown in Fig. 4, there are three salinity measurement points, St.A, St.B., and St.C respectively. St.A as the basic setting point, located under the Yuriage Ohashi Bridge, with coordinates of 38◦10.949N, 140◦8.850E. St.B is lo- cated downstream which is very close to the estuary, St.C is located upstream of the Yuriage Ohashi Bridge, in the deep waters near the right bank. All of the measurement point is set 10-20 cm from the bottom of the river bed elevation. 5 Tinh, N. X., et al. / Journal of Science and Technology in Civil Engineering Journal of Science and Technology in Civil Engineering NUCE 2018 ISSN 1859-2996 7 (Chữ trong đồ thị Section D để Times New Roman thường, không đậm) 159 b. Hydrodynamic data 160 There are two river discharge measurement stations located in the upstream of 161 the study area which are Hirosebashi station located on the Hirose river branch and 162 Natoribashi station on the Natori river. These river discharge stations are located far 163 enough to avoid the impacts by the tidal motion. In addition, there are two water level 164 stations where Fukurobara station is located upstream and Yuriage station is located 165 downstream near the estuary respectively. Annual, monthly and hourly river discharge 166 and water level data for 4 hydrodymanic stations are provided by the Japan 167 Meteorological Agency (JMA) website [14]. 168 The tidal levels used in this study are obtained from hourly measured data at 169 Sendai Port station, provided by the JMA [14] The distribution of tidal phases in the 170 Natori River estuary is mixed tide and the tidal range is from about 0.8m to 1.6m. The 171 tidal amplitudes decrease gradually when the tide propagates upstream. 172 c. Salinity data 173 In this study, measured salinity data for the three years from 2013 to 2015 were 174 used. This salinity data was provided by the College of Agriculture, Tohoku 175 University. As shown in Fig. 4, there are three salinity measurement points, St.A, 176 St.B., and St.C respectively. St.A as the basic setting point, located under the Yuriage 177 Ohashi Bridge, with coordinates of 38°10.949N, 140°8.850E. St.B is located 178 downstream which is very close to the estuary, St.C is located upstream of the Yuriage 179 Ohashi Bridge, in the deep waters near the right bank. All of the measurement point is 180 set 10-20 cm from the bottom of the river bed elevation. 181 182 183 Figure