Influence of salinity intrusion into water sources by a tsunami

Transcription

1 Influence of salinity intrusion into water sources by a tsunami K.Tagawa*, H.Sano*, M.Nakamura**, T.Hirabayashi*** *Osaka municipal Waterworks Bureau, 2--, Nanko-kita, Suminoe-ku, Osaka, Japan ** Osaka Municipal Waterworks Bureau Kunijima purification plant, -3-4, Kunijima, Higashi- Yodogawa-ku, Osaka, Japan *** Osaka Municipal Waterworks Bureau Water Examination Laboratory Niwakubo Branch, -3, Yodoe-cho, Moriguchi, Japan ( k-tagawa@suido.city.osaka.jp; h-sano@suido.city.osaka.jp; masnakamura@suido.city.osaka.jp; t-hirabayashi@suido.city.osaka.jp) Abstract An earthquake along the Nankai-Trough is predicted to occur in the near future. Therefore, it is necessary to evaluate the effect that such a tsunami would have on water sources, and the effect that the change in salinity of water sources would have on water treatment. To evaluate this, we carried out sophisticated 3-dimensional numerical analysis in order to comprehend the changes of salinity concentration over time due to water intake from the tsunami, and the magnification of salt being diffused in the river water. Moreover, we surveyed the influence extending to water treatment, undertaking testing at a pilot plant by producing mock-up salt water based on salt concentration data acquired from our analysis. The results of our numerical analysis found that chloride ion concentration in one location furthest downstream from the Yodo River water intake did not meet the drinking water quality standards for 3 hours in the case that the Yodo River Estuary Barrage is completely closed (it is closed during times of low water level). Additionally, the experiment at the pilot plant showed that in returning the mock-up salt water back to freshwater, organic substances adsorbed in granular activated carbon (GAC) were eluted, and yellow coloration occurred within the in outflowing water in approximately half a day. Keywords Nankai-Trough earthquake; numerical analysis; salinity; tsunami Introduction An earthquake along the Nankai-Trough, which sits in the Pacific Ocean off the coast of the Japanese archipelago (Figure ), is predicted to occur within the next 3 years with a 7% probability. In particular, in the instance that this earthquake sits along the wide focal area along the Nankai-Trough, a magnitude 9 class earthquake will occur (the Nankai-Trough Earthquake), and it is predicted to be followed by a large-scale tsunami. This particular configuration is the largest possible earthquake that could occur in the trough. With the Great East Japan Earthquake in 2, the tsunami that occurred caused serious damages to city infrastructure. For a long period, provision of Osaka Tokyo Nankai-Trough km Copyright(C) T-worldatlas All Rights Reserved Figure. Location of Nankai-Trough

2 things like drinking water was difficult due to the rise of chloride ion concentration within the raw water, which was caused from sea water going into the regional water sources. Osaka city is located about 2km from the Nankai-Trough, and in the instance that an earthquake originating at the trough occurs, lots of damage will be taken. In particular, in the Nankai-Trough Earthquake scenario, the tsunami caused by the earthquake is predicted to send water upstream the Yodo River, which is the city s water source (rd Nankai-Trough Earthquake disaster measures Working Group, 24). With that, the city s waterworks will not only be damaged from the earthquake, but there are concerns about the effect that salination will have on the raw water. The waterworks are an important lifeline to uninterrupted citizen life and as the infrastructure that supports city activities. Even in the instance that such an earthquake occurs, as much as possible it will be necessary to continue providing water. With that in mind, our predictions were carried out through sophisticated 3-dimensional numerical analysis looking at the influence of salinity intrusion from the tsunami caused by the Nankai-Trough earthquake. Additionally, even if the state of salinity concentration in the water source is high, it is assumed that water usage must continue in order to ensure firefighting use and domestic use. Therefore, in these instances of using water with a high salinity concentration, a survey was undertaken at the pilot plant looking at rapid sand filtration and GAC adsorption. Methods Prediction of influence of the tsunami-caused salinity intrusion on the water sources ) Location Figure 2 shows the location of the barrage and the location for water intake of the city s water treatment plant. The city carried out water treatment at three treatment plants, the Toyono and Niwakubo purification plants upstream from the city, and the Kunijima plant within the city. Water intake is installed in two locations for each purification plant, and all are taking water from the Yodo River. Additionally, a barrage is installed downstream from the water intake of Kunijima water treatment plant in order to prevent salination and maintain the water level upstream. Facility Yodo River Estuary Barrage Kunijima Water Niwakubo Water Placement Distance from river mouth height - 9.8km O.P.+3.8m Right bank.8km O.P.+2.m 2 Right bank.9km O.P.+2.3m 3 Left bank 6.84km O.P.+2.4m 4 Left bank 7.3km O.P.+2.6m Toyono Water Left bank 33.76km O.P.+3.64m 6 Left bank 33.82km O.P.+6.4m O.P. Is the lowest tide level of Osaka Bay. Kunijima Water Yodo River Niwakubo Water 6 Toyono Water Yodo River Estuary Barrage Osaka port River mouth Osaka City Figure 2. Location of Osaka city water intake and Yodo River Estuary Barrage

3 Tsunami water level (O.P.+m) 2) Analytical method The tsunami from the Nankai-Trough 4. Top of the barrage gate O.P.+3.8m earthquake will run up the Yodo River, 4 causing it to overflow and causing the water 3. to flow upwards, further upstream (Osaka 3 Prefecture Disaster Prevention Council, 24). 2. When overflow occurs from the tsunami, 2 fresh water mixes with the salty sea water,. and river sediment is mixed up. Then, based on the complex 3-dimensional flow and. (Location:Yodo River mouth) difference in concentration, changes in the river flow arise. In order to make quantitative predictions on this phenomenon, we Passage of time (hour) conducted 3-dimensional numerical analysis Figure 3. Tsunami waveform used in analysis of density flow using the Navier-Strokes Equation, which can make considerations for the density difference of freshwater and salt water (Yoneyama, 2). The details of this analysis are shown in Table. Additionally, using the inputs of this analysis, the waveform of the tsunami is shown in Figure 3. Table. Analysis conditions Item Flow analysis basic equation Turbulent behavior method of Water behavior analysis method Material balance basic equation Amount of sediment raised Sedimentation rate Analysis model River width Mesh Water depth construction Downstream Target scope of analysis Time interval of analysis Flow rate boundary conditions Analysis conditions Navia-Stokes equations for motion and continuity k-ε model VOF method Advection-diffusion equation Parker's equilibrium concentration distribution Tsuruda's equation 3-dimensional m pitch m pitch m pitch From the mouth up to about 28km upstream. seconds(highest) Logarithmic law (Blasius' /7 power law) Yodo River width(m) Water depth direction(m) Flow-down direction(m) 3) Yodo River Case Configurations The level of the Yodo River depends on factors like amount of rainfall, and so three different cases were assumed to calculate the level at the time of the tsunami: low water level, average water level, and high water level. Additionally, because there is a possibility that the influence of salinity intrusion can be reduced by opening the barrage gate after the tsunami impact, two cases were assumed with regards to the gate at the barrage: when the gate is half closed, and when the gate is totally closed. Based on these two assumptions, the total case configurations are combined for the Yodo River level and the barrage and shown in Table 2. Table 2. Case configurations of river flow rate and gate operation Operation of Yodo River Estuary Barrage gate Case River flow rate After the tsunami At time of tsunami arrival (2 hours after the earthquake) 76 m 3 /s 2 (low water level) Partially open 3 93 m 3 /s 4 (average water level) Partially open 8 m 3 /s (high water level)

4 Survey of the influence on water treatment in the instance of salt water intake Survey method This survey was undertaken in order to confirm the influence that GAC adsorption and rapid sand filtration Coagulo have in the case of dealing with salt water intake, and -Sedimentation was implemented within the pilot plant installed at the Intermediate Kunijima water purification plant. The experimental Ozon Contact flow is shown in figure 4 and the operating conditions are shown in table 3. Also of note, the treatment process Rapid sand filtration at the pilot plant shown in Figure 4 will be the same as the actual treatment process at the city water treatment Post plant. Additionally, in order to reproduce the Ozon Contact,mg/L concentration of chloride ions (a figure of which was acquired from the water source salination GAC adsorption influence analysis results) we diluted L of intermediate ozonated water with Tetra Marin Salt Pro made by Tetra Japan Co. Ltd. to prepare our mock-up salt water. Filtered water of Kunijima Water Table 3. Experimental conditions Coagulant (8% Aluminum sulfate) Surface loading O 3 Injection rate Contact time Air-wash flow rate Back wash flow rate O 3 Injection rate Contact time Reaction time Air-wash flow rate Back wash flow rate 27ml/m 3.4mm/min.mg/L Approx. min.4m/min.8m/min.4mg/l Approx. min Approx. min.3m/min.3m/min Raw water Coagulo -Sedimentation Intermediate Ozon Contact Rapid sand filtration Release Post Ozon Contact GAC Chlorine adsorption Contact Treated water Mock-up salt water Sand filtration column Mock-up salt water GAC column P Treated water P Treated water Figure 4. Experimental flow of pilot plant For the survey regarding the influence of rapid sand filtration, mock-up salt water using the intermediate ozonated water was put into an experimental column, and passed through a 7cm filter at a rate of m/d for to 24 hours. Afterwards, ozonated water that contained no salt water was once again passed through, and the change in water quality was surveyed. Additionally, for the survey regarding the influence from GAC adsorption, the same operation was undertaken using ozonated water at a space velocity of 36m/d. Washing of each column was performed after every 24 hours, with air-washing and backwashing taking place 24 hours after the salt water passage. In addition, measurements were done for turbidity, coloration, ph, electrical conductivity and chloride ions. Results and Discussion Prediction results of the influence of salinity intrusion on the water sources due to the tsunami ) Scope of salinity intrusion influence

5 Concentration of chloride ions (mg/l) Concentration of chloride ions (mg/l) Highest concentration of chloride ion (mg/l) Yodo River highest water level (O.P.+m) Figure shows data from case, showing the highest possible water level and its distance from the river mouth. Figure 6 shows the data for the highest chloride ion concentration and its distance from the river mouth. Due to the tsunami going up into the river, the water level raises to a point above stream of approximately 29km from the river mouth. On the other hand, the increase in chloride ion concentration occurs within approximately 4km from the river mouth. Despite a rise in river water level occurring at all intake locations in the city, only one place furthest downstream at the Kunijima intake location did not meet the drinking water quality standards for chloride ion concentration. Of note, the Guidelines for Drinking-Water Quality (fourth edition) do not set a fixed value, but the water quality standard in Japan is set at 2mg/L. It is thought that the reason for the differing scope of influence between the chloride ion concentration and the water level is due to a movement of substances in chloride ion concentration from the displacement of the wave. 2) Salinity Intrusion Influence Time Taken from Kunijima water intake, Figure 7 shows how the river flow rate for chloride ion concentration adjusts over passage of time after the Nankai-Trough Earthquake occurs. Figure 8 shows how adjusting operating procedures at the barrage affects these conditions. As the river flow rate is reduced, the time it takes the water source s chloride ion concentration to exceed water quality standards is extended. In particular, case has the most severe conditions, and lasted approximately 3 hours. However, it was found that the time was greatly compressed to approximately hours when the barrage gate is left partially open. This is thought to be a caused from chloride ions flowing into and residing on the upper river basin of the barrage, which then flow downstream quickly following the opening of the gate ,, Top of O.P.+3.8m the Barrage gate O.P.+2.2m (Case: Rate of flow 76m 3 /s) Highest water level of the tsunami O.P.+3.m Initial water level Distance from Yodo River (km) Figure. Relation between highest water level of the tsunami and distance from river mouth Yodo River Estuary Barrage (9.8km) (Case: Rate of flow 76m 3 /s) Drinking water quality standard (2mg/L or less) Distance from river mouth(km) Figure 6. Relation between highest chloride ion concentration and distance from river mouth Case3: Rate of flow 93m 3 /s Case: Rate of flow 8m 3 /s Case: Rate of flow 76m 3 /s Drinking water quality standard (2mg/L or less) Elapsed time from earthquake (hour) Figure 7. Chloride ion concentration change at water intake furthest downstream (separated by river flow rate),, Case2: partially open (Case: Rate of flow 76m 3 /s) Case: fully closed Drinking water quality standard (2mg/L or less) Elapsed time from earthquake (hour) Figure 8. Chloride ion concentration change at water intake furthest downstream (separated by conditions of gate operations) Water ntake

6 Color (CU) Electric conductivity (µs/cm) Survey results of the water treatment in instances of salt water intake ) Results of rapid sand filtration influence Figure 9 shows the change in electric conductivity when the salt water is changed back into fresh water and vice versa. The rapid changes of the electric conductivity at the time of transformation from salt water to fresh water can be seen. The reason for this is that because salt water s density is comparatively heavier than fresh water, so when the fresh water is added into the salt water containing filtration column, the layer of fresh water remains separated in the column, and does not mix with the salt water. In regards to other components, because there Fresh water salt water Salt wate fresh water were not any particular changes seen, it is Elapsed time (minute) thought that the influence of the salt water on Figure 9. Change in electric conductivity the rapid sand filtration is small, and the following for rapid sand filtration restoration is quick. 2) Results of GAC Adsorption influence The change in coloration of the processed water after the salt water passes through the GAC column is shown in Figure. The change of the color did not occur directly at the time of salt water passage, but turned yellow at the time the salt water changes into fresh water(after four and a half hours from the passage of salt water), and the change continues over the course of about half a day. Additionally, this colored water was not improved even after air-washing and backwashing. Of note, although the color is not specified in the Guidelines for Drinking-Water Quality (fourth edition), it is recognized in the color units (CU) of drinking water quality standards in Japan Pass through the salt water Pass through the fresh water Inflow water Outflow water - 2 Elapsed time (minute) Figure. Change in coloration of mock-up salt water into freshwater for GAC In order to investigate the cause of this coloration, the colored water underwent fluorescent spectrum analysis. The results show excitation wavelength levels of 2nm~29nm, and a fluorescent wavelength level between 43nm~nm. Due to these high levels, it is assumed to be a component derived from humic substances (Fukushima et al., 2). Additionally, a previously used GAC (used for a year period) was compared with a new GAC by the laboratory experimentation. When washing the salt water with purified water multiple times after immersion in the GAC, only the previously used GAC produced water with high fluorescence and coloration. Therefore, the results show that the coloration phenomenon only occurs with the previously used GAC, and that the substance absorbed by the GAC was eluted. Furthermore, after submerging the previously used GAC in 3% sodium chloride to substitute for the mock-up salt water, when changed with the purified water, over 8 CU at most were confirmed. It

7 is thought that the high concentration of substances such as sodium chloride produce this phenomenon. Conclusions Prediction results of salinity intrusion of water sources due to the tsunami Due to the tsunami caused by the Nankai-Trough Earthquake, the water level will increase up to 29km from the mouth of the Yodo River. Compared to the increase in scope of the water level, the increase in the scope of chloride ion concentration from the salt water diffusing to the river is smaller, reaching up 4km from the mouth of the Yodo River, and only failing to meet the concentration of drinking water quality standards at one location furthest downstream. It takes approximately 3 hours of time for salinity intrusion in instances where the barrage gate is closed and when the Yodo River flows at low water levels. It takes approximately hours for salinity intrusion in instances where the barrage gate is open after the tsunami arrival. Survey results of the influence of water treatment in instances of salt water intake Rapid sand filtration is not particularly affected by salt water passage, and the water is returned to fresh water and the water quality restored quickly. For GAC adsorption, when fresh water is returned from salt water, it elutes organic compounds derived from GAC-adsorbed humic substances. Because of this, yellow coloration occurs in the outflowing water, and the drinking water quality standards are exceeded in about half a day. Acknowledgments We would like to express our gratitude to Associate Professor Yoneyama at Kyoto University, who provided a great deal of cooperation in our analysis on the impacts of salt water on the water source. References rd Nankai-Trough Earthquake disaster deasures Working Group. (January 24). Damage Assumptions of Osaka Area. (accessed 24 June 2) Yoneyama, N. (2). Three Dimensional Numerical Analysis for Salt Water Behavior Caused by River-Runup of Tsunami in the Yodo River. Advances in River Engineering, Vol.6 pp , in Japan (accessed 24 June 2) Fukushima et al. (2). Characterization of Aquatic Dissolved Organic Matter Using EEMS. Journal of Japan Society on Water Environment, 24(),