Sedimentation of reservoirs.A method to estimate reservoir sedimentation: a case study of the 'Nga Moe Yeik'reservoir, Myanmar

Kats, H. (2016) Sedimentation of reservoirs.A method to estimate reservoir sedimentation: a case study of the 'Nga Moe Yeik'reservoir, Myanmar.

Abstract:Myanmar’s irrigation and domestic water supply is very dependent on the storage capacity of their fresh water reservoirs. Due to forest fires, deforestation and bad governmental policies, erosion is posing a big treat to the life time of their reservoirs. Erosional processes cause reservoirs to silt up, resulting in major capacity losses. Myanmar does possess more than 200 reservoirs, but there is still very little know about the sedimentation rates affecting them. To improve the reservoir and irrigation management it is important to make estimations about the reservoir life expectancy. A good method to estimate the sedimentation of a reservoir is by performing a bathymetric survey. But, these surveys can be expensive and time consuming. Therefore it will not always be possible to perform them on a regular basis. A supplementary method is demanded to predict sedimentation in an easy and cheap way and to help interpreting the bathymetric survey’s results. This may be possible by modelling the catchment area of a reservoir. By using the InVEST model, based on the Universal Soil Loss Equation (USLE) and a method to predict the trap efficiency of the reservoir, it is possible to make predictions about the accumulated sediment in the reservoir. This prediction can be compared to the results of a bathymetric survey to see if they show agreement. The goal of this research is too see if the combination of InVEST and trap efficiency has potential to be used in Myanmar as a reliable method to estimate the capacity loss of reservoirs. The area of study in this research is the ‘Nga Moe Yeik’ reservoir and its catchment area. ‘Nga Moe Yeik’ is situated 100 kilometres to the north of Yangon City. The reservoir also fulfils an important function within the water supply of Yangon City. The catchment area is 414 square kilometres, it has a capacity of 222 million cubic meters and the dam was finished in 1995. The dam has two supplementary dams: ‘Paung Lin’ and ‘Ma Hu Yar’. Those dams lie upstream of the ‘Nga Moe Yeik’ dam and are constructed in 2003. The research is conducted at the Irrigation Technology Centre (ITC), Bago. There has been an intensive cooperation with the staff of the ITC to make this research into a success. Several field survey trips from ITC, Bago, have been performed to execute a bathymetric survey and to study the reservoirs and their catchment area. Three research questions were stated to achieve the research goal. Every research question has its own section within the research methodology and results. Firstly, the spatially explicit InVEST model is used to make average annual predictions for the watersheds sediment yield. Sediment yield is the total amount of sediment that will flow into the reservoir after erosion and deposition within the watershed. Rainfall, land cover and soil characteristics are very important parameters influencing the sediment yield and is therefore demanded input for this InVEST model. The total sediment yield accumulation for the ‘Nga Moe Yeik’ catchment during the past 21 years is estimated to be between 44,5 *106 m3 and 64,4 *106 m3. Annual erosion rates are estimated to be between 14,5 *103 ton/km2 and 42,3 ton/km2, which are very high compared to results within the literature. Secondly, when the eroded material enters the reservoir, some of it will deposit and some of it will flow out. The ratio of sediment inflow and outflow is called the trap efficiency and is determined by multiple factors. The trap efficiency of a reservoir can be estimated by using empirical equations. By using data about daily inflow and stored volume, it is possible to estimate the trap efficiency on basis of the residence time of water within the reservoir. The longer water stays in the reservoir, the more sediment will deposit and that will increase the trap efficiency. The average trap efficiency for the ‘Nga Moe Yeik’ reservoir is 97,65%. Thirdly, a bathymetric survey has been performed to assess the capacity loss of the reservoir. The measurements of the bathymetric survey were used to build a digital elevation model (DEM) of the reservoir bed using ArcGIS. An old map of the ‘Nga Moe Yeik’ area from 1995 was present to model the before dam situation, also with the use of ArcGIS. The differences between the two DEM’s resulted in a capacity loss that represent the real capacity loss during the last 21 years. An error assessment has been performed to estimate the error propagation of the used measurement and model techniques, namely the georeferencing, the interpolation, the gab filling with Landsat and the measuring setup. The sediment accumulation is between 14,74 * 106 m3 and 27,66 *106 m3. Resulting in a capacity loss between 11,94 and 6,36%. Comparing the InVEST predictions and the trap efficiency with the results of the bathymetric survey, show that there is some agreement between them. However, they differ with a ratio between 2 and 3. This difference is too vast for making proper predictions about reservoir sedimentation. As explained in the conclusion, improvements on the parameterization of the InVEST model will have to be made to develop this model into a proper tool to estimate reservoir sedimentation. In the future, this method may have the potential to develop into an easy and low costing tool to better understand the erosion processes and the impact of individual parameters on the sediment yield.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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