Geometry and water height-area-volume curves of the reservoirs in the semiarid Madalena basin in Northeast Brazil

Grin, Sido (2014) Geometry and water height-area-volume curves of the reservoirs in the semiarid Madalena basin in Northeast Brazil.

Abstract:The water scarcity in the Northeast of Brazil due to high variability of rainfall, evaporation rates that can reach up to 2,500 mm/year, and the occurrence of shallow soils, has led to the construction of thousands of small dams. In this high-density reservoir network no direct monitoring of small reservoirs takes place. Remote sensing has been pointed out as a feasible way of monitoring seasonal changes in order to understand the magnitude of the hydrological processes involved. To support remote sensing development and thereby efficient water management and reservoir planning in the semiarid Northeast of Brazil, it is needed to determine area-depth-volume relations for this region. The study site is the semiarid Madalena Basin, it is an experimental catchment that has been monitored regarding basic hydrological variables since 2008. The objective for the assignment was to determine the geometry and the water height-area-volume curves for the main reservoirs in the semiarid Madalena Basin by performing a bathymetric survey and data-analysis. The research focused on three of the five main reservoirs. These are the reservoirs Raiz, São Nicolau and Marengo. The reservoir São Nicolau is the youngest reservoir in the Madalena basin, it is constructed in 1995. The reservoirs is very shallow in most areas and there is many vegetation present in the water. When the field surveys took place the reservoir was almost completely filled. The reservoir Raiz, constructed in 1958, was almost dry at time of the field surveys. The reservoir is shaped with two major side branches coming together at the dam. In the reservoir are some minor obstacles located like fences and tall grass. The reservoir Marengo is the biggest reservoir in the Madalena basin. It is built in 1934 and enlarged in 1956. There is no vegetation present in the water and the shores are well accessible. Approximately 22% of the maximum surface area was covered with water in Marengo. The reservoirs only have runoff when they are completely filled and the water passes the spillway, there is no other discharge channel. The research method is divided into three parts; data collection, reservoir modelling and data analysis. The data is collected by a bathymetric survey consisting of measurement on water and measurement on land. Multiple points with water depths were taken by using an echo-sounder with GPS antenna attached to a boat. To verify that the data from the echo-sounder is correct, multiple points are also measured by a handheld depth meter and handheld GPS. The water surface area was measured by walking around the water and register multiple points with a handheld GPS. To complete the data, the height from the water surface to the spillway is measured by the total station. As the reservoirs of Raiz was almost completely dry, the terrain of this reservoir was mostly measured by the total station. The collected data for each reservoir is modelled into a 3D-model. The process of creating a model for each reservoir can be divided into four steps; add data, create surface areas, create dataset, and gridding data. After spatial adjustment of the total station data by using GPS control points, all the data contain coordinates. For the reservoirs Raiz and Marengo the water surface area is not equal to the maximum area. Therefore the maximum area was determined by using Landsat 5 images from July 2004, when the reservoirs were completely full. A useful dataset to create the model was formed by removing and adding points. The removal of points was necessary due to errors in the depth values, and the adding of points was needed to make sure a correct surface is created through interpolation. The final dataset and the maximum area was used to create a 1 by 1 meter grid that represents the geometry of the reservoir. An important aspect hereby was choosing the right interpolation method. Of course each of the above steps slightly differs for each reservoir. For example, for Raiz it was needed to use Landsat 5 images to estimate unmeasured areas and for Marengo there was data used from a previous study to make a complete model. The models extracted from the 1 by 1 meter grids give an overview of the bathymetric shape and the characteristics of each reservoir, which correspond with the observation in the field and approach the reality. With the developed models, the area and volume for certain depths are calculated for each reservoir, the chosen step value for the water level is 0.1m. The output from the computation allows the creation of logArea-logVolume and logDepth-logVolume curves for each reservoir. As the water in the reservoirs is extracted at every water level, it is necessary to find a curve that is correct for all the water levels. In other words, the dataset used for the log-log curves contains all points. By taking the log-log curves it is possible to make a linear line that represents the relation. The function of the linear line is rewritten to a power function using basic logarithm principles. The derived power function represents the area-volume relation (
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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