DISSERTATION OPTIMIZATION OF SANGJU WEIR OPERATIONS TO MITIGATE SEDIMENTATION PROBLEMS Submitted by Hwa Young Kim Department of Civil and Environmental Engineering In partial fulfillment of the requirements For the Degree of Doctor of Philosophy Colorado State University Fort Collins, Colorado Summer 2016 Doctoral Committee: Advisor: Pierre Y. Julien Darrell G. Fontane Christopher I. Thornton Sara L. Rathburn Copyright by Hwa Young Kim 2016 All Rights Reserved ABSTRACT OPTIMIZATION OF SANGJU WEIR OPERATIONS TO MITIGATE SEDIMENTATION PROBLEMS The Four River Restoration Project (FRRP) in Korea was completed by the Korean Government in 2013. Through this project, eight weirs were constructed along the Nakdong River, which have altered the river regime from a natural environment to post-weir conditions. The construction of the consecutive weirs and excavation along the Nakdong River changed many channel characteristics such as longitudinal slope, cross-sectional area, water stage which led to changes in erosion and sediment transport patterns. It is now necessary to assess the sedimentation problem upstream of the weirs and seek a way to mitigate the problem. Firstly, sedimentation is partly linked to the current weir operation rules. There are multiple purposes, when it comes to reservoir operations, and the various interests include: (1) flood control, (2) dredging, (3) hydropower production, (4) water supply, and (5) environment, including fish passage, recreation, tourism and downstream turbidity. Thus, introducing optimization techniques for reservoir operations is expected to mitigate sedimentation problems for complex multi-purpose weirs. The study site of Sangju Weir has been selected for this study because it is a representative site for these types of problems and because sufficiency and availability of data. The purpose of this research is to: (1) estimate incoming sediment yield, define the trap efficiency and estimate the reservoir sedimentation rates of weirs and low-head dams using the Flow-Duration and Sediment ii Rating Curve (FD/SRC) method, along with the Series Expansion of the Modified Einstein Point Procedure (SEMEPP) to determine the long-term sediment yield for Sangju Weir from suspended load concentration measurement; (2) find the B/C ratio and break-even point between hydropower production revenues and sediment excavation costs using a Benefit and Cost Analysis (BCA); (3) suggest new operation rules for Sangju Weir and propose a systematic analysis procedure to find a better operation rules for mitigating sedimentation problems using the Multi- Criterion Decision Analysis (MCDA) method. The methodology used for this study includes the followings. First, in this study, the Integrated Reservoir Sedimentation Estimation Procedure (IRSEP) was proposed to estimate the reservoir sedimentation at weirs and low-head dams. IRSEP integrates all conventional methods related to reservoir sedimentation: (1) Flow-Duration (FD), (2) Sediment-Rating Curve (SRC), (3) Series Expansion of the Modified Einstein Point Procedure (SEMEPP), and (4) Trap efficiency (TE), in order to estimate reservoir sedimentation rate. Steps (3) and (4) constitute the new elements of this type of analysis. Through stream flow runoff modeling and channel geometry analysis, the trap efficiencies in accordance with certain stages were defined. The trap efficiency of Sangju Weir was calculated for each sediment size fraction as a function of the variables, sediment load at daily discharges and particle size distribution of the sediment transported by the Nakdong River. The trap efficiency is also dependent on channel geometry, inflow discharge and reservoir stage determined by the operation rules at Sangju Weir. Then, the amount of reservoir sedimentation was calculated by multiplying the annual sediment load with trap efficiencies, which vary with discharge and water stage. As a result, the reservoir filling rates were examined. iii Second, the B/C ratio and break-even point between hydropower production revenues and sediment excavation costs were estimated using a Benefit and Cost Analysis (BCA) method based on daily-modeling. Since the operation rules of Sangju Weir significantly affect the trap efficiency and the volume of dredging, it is essential to determine the effects of the reservoir operation rules using the Multi-Criterion Decision Analysis (MCDA) technique in order to seek improvement in the operation rules to mitigate sedimentation costs, simultaneously considering the other decision makers’ interests. Finally, new operational rules for Sangju Weir and a systematic analysis procedure were proposed. The conclusions of this research with respect to the sediment problems and reservoir operation rules are summarized as follows: (1) the total incoming sediment load and the average trap efficiency (TE) at the lowest (EL37.2m) and highest (EL47.0m) stages were estimated as 425,000 tons/year, 50.1 % and 78.1%, respectively; and the maximum annual amount of reservoir sedimentation at Sangju Weir was estimated as 332,000 tons/year (207,000 m3/year) which corresponds to 0.76 %/year of the total reservoir storage of Sangju Weir. On the contrary, the minimum reservoir sedimentation rate was 0.49 %/year, which was come when the water stage was the lowest (EL37.2m). According to this result, it was proved that reservoir operation rules like management water stage considerably affects reservoir sedimentation. (2) Since the operation rules of Sangju Weir affects significantly the trap efficiency, a Benefit and Cost Analysis (BCA) based on daily-modeling was performed in order to find the break-even point between hydropower production revenues and sediment excavation iv costs. For daily sediment transport modeling, historical and predicted reservoir stream flow data (2015-2034) were generated by Tank model using seventy-six daily precipitation data. The B/C ratio obtained 20 years of daily simulation was calculated as 2.28. Also, the discharge and stage thresholds, balancing both hydropower production revenues and sediment excavation costs, are found as 600 m3/s and EL43.6m, respectively. (3) based on the daily Multi-Criterion Decision Analysis (MCDA) modeling, the most favorable Sangju Weir operation rules to mitigate reservoir sedimentation were found in Alternative 5 after including seasonal management of stage control according to the magnitude of the upstream inflow. Proposed operation rules are shown below: Season Inflow (m3/s) Stage (EL. m) Non-flood season (October-May) Any 47.0 below 50 47.0 Flood Season (June-September) 50 ~ 600 46.0 above 600 44.5 The systematic analysis procedure, the combination of the Integrated Reservoir Sedimentation Estimation Procedure (IRSEP) and the Multi-Criterion Decision Analysis (MCDA) method, are deemed useful to find optimum operation rules of weirs and low- head dams, which can mediate the disputes among various decision makers who have different interests related to reservoir operation. The proposed methodology could be applied to the other weirs of the Four River Restoration Project, and elsewhere. v ACKNOWLEDGEMENTS I would like to thank Dr. Pierre Y. Julien who has inspired me to keep studying during the Ph. D program since 2013. Also, I give great thanks to my committee members: Dr. Darrell G. Fontane, Dr. Christopher I. Thornton, and Dr. Sara L. Rathburn, who gave me nice guide and comments for my dissertation and papers during my studies at Colorado State University. And, I also thank CSU faculties including Dr. Peter A. Nelson, Dr. Subhas Venayagamoorthy, Dr. Jeffrey Niemann, Dr. Jorge Ramirez, and Dr. Ellen Wohl. I learned a lot from their wonderful classes. I also would like to thank other CSU members including Linda Hinshaw, Laurie Alburn, and Karleene Schindler for their patience and kindness. I am very thankful to my company, Korea Water Resources Corporation (K-water). I would like to give great thanks to Dr. Nahm-Chung Jung, Jin Ho Lee, Tai Hyun Park, Pyung-rock Park, Jae Ok Jang who supported me and provided the opportunity to study at CSU. I specially thank to Dr. Kee Uk Cha for giving great motivation for the advanced study in 2002. Special thanks to members of Four River Restoration Project including Dr. Myung-pil Shim, Sikwon Ahn, Hee Kyu Jung, Sung Chul Park, Sang Soo Park for the encouragement and providing materials. Also, I am grateful to my own professors at Chungbuk National University, and particular to my former advisor Dr. Jin Soo Kim and life-time mentor Dr. Seung Jin Maeng. I am grateful to members of Dr Julien’s Rising Stars: Dr. Ladislav Rousar, Jon Rainwater, Olga Matyusheva, Haider Addab, Patrick O’Brien, Patrick Ndolo Goy, Aaron Orechwa, Shukran Sahaar, Nick Koutsunis, Kennard Lai, Marcos Cristiano Palu, Neil Andika, Mosaed Alrashidi, vi Weimin Li, Woochul Kang, Corinne Horner, Christoph Matzke, Dr. Jose Anderson Batista, Dr. Jai Hong Lee, Dr. Young Ho Shin, Dr. Hyeon Sik Kim, Dr. Kiyoung Park, Dr. Sang Do An and Dr. Un Ji for the support and friendship. I would also like to extend my appreciation to Dr. Kyung Seop Shin, Jongseok Cho, Youngjai Jung, Young Ho Seo, Dr. Kyung Hak Hyun, Dr. Sung Hoon Kim, Dr. Kyung Sik Ryu, Cheol Kyun Shin, Sung Phil Jang for tremendous support. Special thanks to Dr. Jai Hong Lee not only for the support throughout my study but also for his friendship. And I really appreciate to Randall & Jan Babcock, Evan Rambikur, Alex & Mary Bricker, Steve Eckles, Dr. Hosam-Aldeen Ahmad, Vesna & Vlado Basovska, Seongyun Kim, Sunah Kim, Jungho Kim, Seonggyu Park, Soojung Hong and Heechan Han. Special thanks to Jung Hee Kim, Sulyoung Shin and their family. They have supported me and my family a lot since we arrived in Fort Collins, Colorado. They have made our life at Fort Collins happy and successful. Thanks to all colleagues and friends who I met in Fort Collins. Finally, my deepest thanks extend to my family members including my parents-in-law, my son, Myong Seob (Charlie), who provided me the opportunity to get up early every morning for giving him a ride to Poudre High School, which eventually accelerated the process of my study, and especially my wife, Ho Sun Lee, for her endless support, patience, encouragement, and unconditional love.