The construction of the consecutive weirs and excavation along the Nakdong River in Korea 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.
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 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.
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. 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. 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 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 m 3/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). (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 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. [table omitted]
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. (Abstract shortened by ProQuest.)
|Advisor:||Julien, Pierre Y.|
|Commitee:||Fontane, Darrell G., Rathburn, Sara L., Thornton, Christopher I.|
|School:||Colorado State University|
|Department:||Civil and Environmental Engineering|
|School Location:||United States -- Colorado|
|Source:||DAI-B 77/12(E), Dissertation Abstracts International|
|Keywords:||Dredging, Korea, Low-head dams, Nakdong River, Reservoir sedimentation, Trap efficiency, Weirs|
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