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Keeping the water Guanabara bay salty will cost €21 million annually?
The article below discusses sea level rise and what this would mean for Guanabara Bay in Rio de Janeiro. It could be a solution to close off a part of the bay with a closure dam, but this will affect the water quality behind the dam. In this article you can read what solutions have been devised to keep the water behind the dam of equivalent quality when looking at the salt content in this water. In addition, the annual costs of keeping the bay salty are calculated, which raises the question of whether it is realistic. Two different propositions have been made. One method is to create an artificial tide in which the entire volume of water displaced by the current tide is pumped out, so that the amount of water refreshed remains the same as in the current situation. the other method is to draw up a water balance in which the salt content can be determined, so that the amount of water that has to be pumped out can be considered more accurately in order to maintain the desired salt content and water level.
The problem of rising sea levels is a fact. According to the Dutch metrology institute KNMI, this could be 1 to 2 meters in 2100 (Pieterjan Huyghebaert, 2021). But how much it will eventually rise, can still be influenced. When the global warming can be limited the sea level rise will be limited as well. If no action is taken, a sea level rise of 3 meters by the year 2100 needs to be taken into account. When this is the case it will require extreme measures to keep our feet dry. The delta cities in particular will be hit hardest by the rising sea level. With these thoughts in mind, the Rotterdam University of Applied Sciences started the delta floods project. During this project, groups of students studied the problems that will arise in various delta cities around the world if the sea level rises by 3 metres. This article will look at Rio de Janeiro, with a focus on the Guanabara Bay. Plans with possible solutions for this region will be discussed. However everyone will need to hope it will never come to the point where these solutions need to be applied. Because they are extreme measures for an extreme situation.
During the previous project, preventing delta floods, a design was created for the Guanabara bay. In this plan the bay is closed off with a closure dam near the Rio-Niteroi bridge. The main advantage is improvement of approximately 195 kilometres of quay construction will be saved, because these areas will be protected by the 9.5 kilometre closure dam. By installing the dam at this location, the docks in front of the dam remain accessible for shipping. The docks behind the dam can remain accessible by means of a sluice in the dam. In the Bay behind the closure dam the current water level will be maintained by a number of pumping stations. Due to the life span of the bridge, which is generally a hundred years for these type of structures, the dam could serve as a replacement for the bridge. In addition, the dam can facilitate a metro line, which could give the public transport connection in Rio de Janeiro a boost because now the public transport connection is by
boat. The downside of placing a closure dam is that it will have a major impact on the environment, because a part of the bay will be closed off and there is no longer a direct connection with the Atlantic Ocean. As a result, there will be no more inflow of salt water, which ensures that the part behind the dam becomes fresh water over time due to the inflow of fresh water from the rivers. As can be seen in previous projects, this will have a huge impact on the current ecosystem behind the dam. In what way can the water behind the dam maintain the same salinity, so that it has as little impact as possible on the current ecosystem?
To find out, we look at the current quality of the water at the back of the bay where the inflow of freshwater is greatest. Figure 1 shows the salinity of the water in the bay. It can be seen that the salt content at the level of the closure dam is 31 to 32 ups and behind the dam it is 29 to 30 ups. Because of this tide, a large amount of salt water will come into the bay every 6 hours. When the tide is low, it will flow back to the Atlantic Ocean. Because of this, the inflow of fresh water has little impact on salt levels at the back of the bay.
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After the dam is built, an inflow of salt water from the Atlantic Ocean is needed to keep the current ecosystem intact. To keep the ecosystem intact, two options are considered: creating an artificial tide that fully imitates the current tide, or calculating the amount of salt water that needs to be inserted to keep the salinity the same behind the dam. As a result, the water behind the dam will not change into fresh water over time and the current ecology will not be affected. The inflow of fresh water through the rivers has a flow of 150 m3/s in the normative month.
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In order to fully imitate the current tide and create an artificial tide, a water level difference of 80 centimetres must be achieved over an area of 300 square kilometres in six hours. The inflow of water must therefore have a flow rate of over 12,000 m3/s and an outflow of 12,400 m3/s in order to carry not only the water that has flowed in from the Atlantic Ocean but also the water from the rivers. To get the water back, a large number of pumps have to be used. To create the flow with the current maximum pumping capacity of 50 m3/s, which are also used in other projects, a total of 250 pumps are needed. This would mean that over a length of 2.5 km there would be pumps in the dam. However, there is a catch, because pumping water costs a lot of energy. To simulate the tide, the pumps will use an amount of electricity of 4800 MWH per day, which is equal to the consumption of 1800 Dutch households in one year (NIBUD, 2021). The costs for the amount of power to be supplied would be enormous. One advantage is that if the water level in Guanabara Bay maintains approximately the same as the current, there is a 3-meter difference. This offers the possibility of generating energy from the kinetic energy of the water as it enters the bay. This generated energy can suppress the costs that are made when pumping out the water. However, there will always be a loss of energy and external energy needs to be added for pumping out the water. The cost of adding this energy will be around EUR 21.2 million per year.
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To ensure that the electricity grid in Rio de Janeiro is not overloaded by the used electricity of the pumps, the peak and off-peak hours in Rio de Janeiro will be used. For example, during the night less electricity is used so if you let the pump run during this period, you ease the system. During the day, however, a lot of electricity is used, which makes it more convenient to let the water flow into the bay behind the dam in order to generate energy for the city. To take into account the peak and off-peak hours of the city's electricity consumption, the tide will have to be adjusted. The tide, which currently has a cycle time of 12 hours, will have a cycle time of 24 hours in the future. This means that over a period of 12 hours water will flow into the bay behind the dam and for 12 hours water will be pumped from this bay to the water that has a direct connection with the Atlantic Ocean. Adjusting the tide in this way will increase the water level differences during a tide, but the electricity grid will not be overloaded and the electricity generated can be used in Rio de Janeiro. The water levels during the artificial tide will be both 40 centimetres lower and higher than with the current tide. The difference between the water level in front of the dam and behind the dam will be a maximum of 3.4 metres and a minimum of 1.6 metres. For now, this only applies to the option where the tide is to be fully imitated.
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The other option is to maintain the salt level in the bay. This can be done by setting up a water balance. This will show how much fresh water enters the bay as a result of the rivers and precipitation. The evaporation of the water in the bay is also taken into account. Cycles can then be set up, in which the inlet and outlet of the water are chosen. As a starting point, it was decided in this study that the intake and outlet of water can never take place simultaneously. As a result, a difference in water level will occur during the cycles. To be able to carry out this calculation, it is important to choose limit values for the minimum and maximum water level. What matters in the end is the salinity of the bay. The wish is for the bay to remain saline. However, it is not possible to keep the salinity on a constant level this way. Therefore, it is also important to choose a limit for the minimum and maximum salinity. The normative month of January has been taken as the starting point for calculating this option. In this month, 15 million cubic metres of fresh water enters the bay within 24 hours from both rivers and precipitation. In addition, 330 thousand cubic metres of fresh water evaporates. In order to keep the salt water in the bay above a salinity of 28,5% an inflow of 389 million cubic metres of salt water from the Atlantic Ocean is required. Figure 2 shows the salinity over time in the month of January. This figure shows the course of salinity in the bay during the month of January. It shows that at the beginning of the month the salinity drops rapidly because of the relatively large amount of freshwater inflow. It can also be seen that in the course of time the decrease in salinity stagnates. This can be explained by the fact that the difference in salinity becomes greater, which means that the seawater increasingly influences the salinity in the bay.
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Figure 1, Salinity water (Scielo Brazil, sd)
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Figure 2, Salinity Guanabara Bay
90 turbines with a capacity of 50 cubic metres per second are needed to a sufficient flow rate. This will generate an average of 2912 MW of electricity. To keep the water in the bay up to standard, 403 million cubic metres of water must be pumped out of the bay 36 hours after the 24-hour inflow cycle. This requires 66 pumps. In total, 66 pumps with turbines in 1 are needed and 24 turbines. The annual cost of keeping the pumps running comes to 11.45 million euros.
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The energy costs between the different options do not differ much, but the construction costs, on the other hand, differs greatly. The option of creating an artificial tide requires about 250 pumps and the other option only 90 pumps. This is a huge difference and will certainly be reflected in the implementation costs.
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In order for the water behind the dam to have the same salt content after construction, drastic measures and a lot of money is needed. Two options have been developed that differ greatly in this respect. To make the current ecosystem work completely the same, an artificial tide can be created to solve the problem. The construction costs for creating an artificial tide are considerably high and it does not seem very realistic. The other option of allowing the water to have the same salt content is a slightly more realistic option because the construction costs are many times less than the previous option. However, a follow-up study will have to examine the impact of creating the tidal range with the higher water level differences on the ecosystem behind the dam. The feasibility of this option and what needs to be done to allow the water to rise by 40 centimetres to prevent houses from being flooded must also be examined. In addition, only the second option takes the evaporation into account of the fresh water at the surface and the precipitation in the bay. In addition, future pumps and turbines can be considered to increase both capacity and efficiency. This can save costs on the construction of the pumps, and the annual costs can be reduced if the efficiency of the pumps and turbines is increased. The cost that will be saved annually in the second option turned out to be only about 10 million euro. This is probably due to the limit values chosen for salinity in this study. Further research should be done to find out if and how much the costs can be reduced if a lower salinity is allowed in the bay. If this is the case, it would be interesting to have ecologists find out what salinity this would be. Also during this study, only the normative month of January was taken into account. This has the disadvantage that the large quantity of fresh water has a great impact on the salt water in the bay. Further through the year, it can be seen that the inflow of fresh water and precipitation decreases. If this is the case, the input of salt water can also be reduced, which in turn will reduce the annual costs. Another possibility is to restore the lost salt content in the months when there is little fresh water in the bay to the original percentage of 29.5% in and around January.
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To us, it seems better to determine the amount of salt water on the basis of the fresh water to save costs. Because more than half of the pumps are needed than when an artificial tide has to be created. In addition, it is possible to reduce the costs in further research because only the normative month of January has been taken into account. It is already questionable whether Brazil will spend the money for the cheaper option. In addition, the appearance of the dam will be better when there are 90 pumps in it than when there are 250 pumps in it.