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The influence of change in reservoir shape on water availability : an assessment of the shape and water availability - in the past and future - of the Pentecoste water reservoir in Ceará, Brazil

Daggenvoorde, Roy (2015) The influence of change in reservoir shape on water availability : an assessment of the shape and water availability - in the past and future - of the Pentecoste water reservoir in Ceará, Brazil.

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Abstract:The Pentecoste reservoir is a large water reservoir approximately 85 kilometer west from Fortaleza, Ceará, Brazil. The reservoir is located within the basin of the river Curu and is fed by two rivers: The Rio Canindé and the Rio Capitão Moore. Due to the drought of the last 3 years in the semi-arid region of northeastern Brazil the amount of water in the Pentecoste reservoir is at only 2% of its maximum capacity. It is expected that the drought is not the only reason behind the low water level in the reservoir. The expectation is that the changed shape of the reservoir due to sedimentation also influences the water availability. This is caused by the fact that the regulated outflow of the reservoir is calculated using this shape. The shape which is used in this calculation is outdated since the reservoir will have a different shape now than it had when it was constructed in 1957. The shape of a reservoir is determined with a bathymetric survey. Bathymetric surveys are normally conducted when the reservoir is full. At that moment the depth is measured at various points in the reservoir. With the corresponding coordinates these depth measurements can be used to model the entire reservoir. This method is very time-consuming; it took 16 days to survey the Pentecoste reservoir in 2009. Another method of bathymetric survey makes use of satellite images. Within this method the bathymetric survey is conducted when the water level is low and satellite images are used to investigate the surface elevation above the water level. This method is less time consuming but it is unknown how accurate the results of this methodology are. So the water availability is influenced by the shape of the reservoir and the shape of the reservoir can only be determined by time-consuming or inaccurate bathymetric surveys. These facts are the basis for the goals set in this research. The first goal concerns the way how the shape changes and influences the water availability and the second goal concerns the accuracy of the simplified bathymetric methodology. The goals are Analyze the influence of the change in shape due to sedimentation of the Pentecoste reservoir –in the past and future– on the water availability and analyze and predict how this shape has and will change in the past and future. Analyze the measurement and propagation errors of the Landsat bathymetric survey methodology in comparison to the real reservoir shape determined by the extensive bathymetric survey in 2009. If the first goal can be achieved it can make frequent repetition of bathymetric surveys unnecessary because the reservoir shape can be predicted, the second goal will give insight into the accuracy of the simplified bathymetric methodology. To achieve these goals a research design has been made. This design consists of three parts: the theoretical framework, the methodology and the data collection. The theoretical framework introduces and explains the different theoretical concepts used in this research, such as: reservoir shape, depth-area-volume relations, reliability levels for regulated outflow and water availability. The methodology introduces the methods and tools which will be used to operationalize the concepts and achieve the goals. It introduces the methods which are used to find the relations between the various parameters and time and how the parameters will be used to assess the water availability over time. After these parts which concern the first research goal, the different bathymetric surveys are explained more extensively. The third and last part of the research design shows how the required information to perform the introduced methods is collected, the data collection. After the research design is presented and the research has been conducted the results are presented. The results are split up in three parts, first the reservoir shape, second the water availability and last the comparison of the different bathymetric surveys. In the first part of the results the reservoir shape is presented. The four parameters, maximum depth, maximum surface area, maximum volume and the shape-factor are plotted against time. These graphs show different relations between the shape-parameters and time. The maximum depth and maximum volume show a linear decrease over time. The maximum surface area does not show any change over time while it is made plausible that the surface area determined at the construction in 1957 contains a large measurement error. The fourth shape-parameter, the shape-factor, shows an exponential increase over time. The reservoir shape is also presented by the depth-area-volume relations. The equations of these relations are determined for 1957, 2009 and 2064 (50 years after the start of this research). The equations have and will change over various decades. The changes in the depth-area-volume equations show that the Pentecoste reservoir has and will become shallower and that the slopes of the reservoir become more concave. This change can be explained easily by the fact that sediment is transported into the reservoir and settles down within it, particularly onto the lower-lying parts of the bed. The second part of the results concerns the water availability. The water availability is shown to not change linear over time. This is explained by the increase in evaporation when a reservoir becomes shallower and the increase in spillway loss when the reservoir volume decreases. This effect only becomes visible when the reservoir volume has decreased significantly. Since this is not the case in the first 100 years after construction it is possible to represent this period using a linear function. This linear function represents the yield loss per year, which is 0.14%, 0.13% and 0.12% of the initial volume for respectively the 99%-, 95%- and 85%-reliability levels. The last part of the results is about the measurement and propagation errors of the simplified bathymetric methodology. The measurement errors are 7.6% and 10.4% for respectively the maximum surface area and the maximum volume. The results of the maximum depth did not show any deviation between both methodologies. The propagation errors, errors in volume and yield reductions per year are larger, this is caused by the propagated error of the maximum volume. The outcomes are still usable to some extend because they have the same order of magnitude. Another conclusion is that the bathymetric survey conducted in 2009 resulted in a more detailed bed elevation map of the entire reservoir, which can be explained by the difference in input data. All these results add up to the possibility to predict the change of shape of the Pentecoste reservoir. This prediction is possible to make, but it is unknown how accurate it is so it is recommended to do a bathymetric survey in the future, 20 to 50 years from now, to assess the real shape of reservoir and to compare it to the predicted values. Some remarks have to be made about the validity of this report. In the discussion, several assumptions are pointed out which will need attention in a further report. After these remarks some possibilities for additional research are introduced. Ideas for additional research are for example a bathymetric survey in the future, more comparisons between the two survey methods and a further analysis of the volume decrease under influence of the erosivity factor.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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