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Simulating Bathymetric Changes in Reservoirs Due to Sedimentation

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Simulating Bathymetric Changes in Reservoirs Due to Sedimentation Examensarbete TVVR 12/5018 Simulating bathymetric changes in reservoirs due to sedimentation Application to Sakuma dam, Japan 2012 __________________________________________________________________ Qaid Beebo Raja Ahmed Bilal Division of Water Resources Engineering Department of Building and Environmental Technology Lund University Avdelningen för Teknisk Vattenresurslära TVVR-12/5018 ISSN-1101-9824 Simulating bathymetric changes in reservoirs due to sedimentation Application to Sakuma dam, Japan 2012 Qaid Beebo Raja Ahmed Bilal Abstract Currently sedimentation is one of the major issues to deal with, for professionals, as it causes continuous loss of storage and hinder the intended purposes of a dam. More research and case-specific studies are required to understand the behavior of sediment transport mechanism in order to propose remedial measures. Sakuma dam on Tenryu River in Japan is one of the largest dams in Japan and it is rapidly losing its storage capacity due to sedimentation. The dam started its operations in 1957. To study the bathymetric changes upstream of the dam, a mathematical modeling approach was selected using HEC RAS to simulate the existing changes and predict future trends. Before going into the detailed modeling, a literature review has been made about different sediment- related studies on the river and Sakuma dam to get deeper insight and to build a conceptual model. Then a reach of about 32 km of Tenryu River, from Hiraoka dam to Sakuma dam was modeled. Google Earth and AutoCAD were used to extract the geometrical data of Tenryu River to be used as input in HEC-RAS. Other data about flow and sedimentation were obtained from the Department of Civil Engineering, University of Tokyo. As necessary, simplifications and assumptions were also made, and sometimes data was extracted indirectly. After initial data input, it was required to do calibration and validation of the model from 1957 to 2004, when the data were available. Once the model was validated, prediction of future bathymetric changes was also made through model simulation. For this prediction it was assumed that the existing flow data could be recycled. Predictive simulation shows that the dam would probably not serve its intended purpose after year 2035 ± 5. Therefore it is recommended to employ some suitable remedial measures to remove sediment and to prevent loss of storage in order to increase the useful age of the dam. Some prospective study options are also identified at the end. Key words: HEC-RAS, sediment transport, bathymetric change, Sakuma dam, Tenryu River. i Acknowledgements: First of all we are very pleased to express our deepest gratitude to our supervisor, Prof. Dr. Magnus Larson, for his valuable important guidance and advice, generous support, help, suggestions and precious friendship with patience throughout whole study. This study would not have been possible without the support of many people. Some of them to whom we want to express thanks include: Professor Shinji Sato and Dr. Haijiang Liu from University of Tokyo as they provided initial data to get started with the case study and modeling. Gregory Morris for his valuable comments about choice of modeling software and case study. Fabio Farias for his kind attitude, when he was guiding and helping us to overcome all difficulties and problems related to the use of the modeling software (HEC-RAS). Chris Goodell, Paul DeVries and Maria Stefansdottir for their comments and support for the use of HEC-RAS. We express many thanks to our families, for sparing their time during the making of this report. Finally, we are also thankful to all those who are not mentioned here but have helped and supported us in order to achieve this project successfully. ii Table of Contents Abstract ..................................................................................................................................................... i Acknowledgements: ................................................................................................................................. ii List of figurers .......................................................................................................................................... v List of tables ............................................................................................................................................ vi 1 Introduction ....................................................................................................................................... 1 1.1 Background ................................................................................................................................ 1 1.2 Purpose of Study ........................................................................................................................ 2 1.3 Procedure ................................................................................................................................... 2 2 Sediment Transport and Reservoir Sedimentation ........................................................................... 3 2.1 General Overview ...................................................................................................................... 3 2.2 Sediment Transport.................................................................................................................... 4 2.2.1 Sediment Transport in Watersheds .................................................................................... 5 2.2.2 Sediment Transport in Rivers ............................................................................................. 5 2.2.3 Sedimentation in Reservoirs ............................................................................................... 5 2.3 Controlling Reservoir Sedimentation ........................................................................................ 6 3 Modeling Reservoir Sedimentation .................................................................................................. 8 3.1 Modeling of Reservoir Sedimentation ....................................................................................... 9 3.2 Basic Procedure of Mathematical Modeling ............................................................................. 9 3.3 Types of Numerical Models .................................................................................................... 11 4 Case Study: Sakuma Dam and Tenryu River ................................................................................. 12 4.1 Background .............................................................................................................................. 12 4.2 Sakuma Dam............................................................................................................................ 13 5 HEC-RAS ....................................................................................................................................... 16 5.1 Steady flow .............................................................................................................................. 17 5.1.1 Water surface profiles calculations .................................................................................. 17 5.1.2 Steady Flow Boundary Conditions .................................................................................. 19 5.2 Sediment Transport Calculations............................................................................................. 19 5.2.1 Sediment Transport Boundary Conditions ....................................................................... 20 5.2.2 Transport Functions .......................................................................................................... 21 5.2.3 Fall Velocity ..................................................................................................................... 23 6 Model Input Data ............................................................................................................................ 24 iii 6.1 Geometrical Data ..................................................................................................................... 24 6.2 Flow Data ................................................................................................................................ 25 6.3 Sediments Data ........................................................................................................................ 27 6.4 Assumptions ............................................................................................................................ 29 6.5 Limitations ............................................................................................................................... 29 7 Simulation and Results ................................................................................................................... 30 7.1 Sensitivity Analysis ................................................................................................................. 30 7.2 Calibration ............................................................................................................................... 30 7.3 Validation ................................................................................................................................ 33 7.4 Predictive Run Based Upon Prospective Flow ........................................................................ 35 7.5 Results and Discussions..........................................................................................................
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