DISSERTATION THE SEDIMENT YIELD OF SOUTH KOREAN RIVERS Submitted by Chun-Yao Yang 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 Spring 2019 Doctoral Committee: Advisor: Pierre Y. Julien Robert Ettema Peter Nelson Sara L. Rathburn Copyright by Chun-Yao Yang 2019 All Rights Reserved ABSTRACT THE SEDIMENT YIELD OF SOUTH KOREAN RIVERS South Korea is experiencing increasing river sedimentation problems, which requires a reliable method to predict the sediment yield. With the recent field measurements at 35 gaging stations in South Korea provided by K-water, we quantified the sediment yield by using the flow dura- tion curve and sediment rating curve. The current sediment yield models have large discrepancies between the predictions and measurements. The goal of this dissertation is to provide better un- derstanding to the following questions: (1) How much of the total sediment load can be measured by the depth-integrated samplers? (2) Can we predict the sediment yield based only on watershed area? (3) Is there a parametric approach to estimate the mean annual sediment yield based on the flow duration curve and sediment rating curve? With 1,962 sediment discharge measurements from the US D-74 sampler, the total sediment discharge is calculated by both the Modified Einstein Procedure (MEP) and the Series Expansion of the Modified Einstein Procedure (SEMEP). It is concluded that the SEMEP is more accurate because MEP occasionally computes suspended loads larger than total loads. In addition, SEMEP was able to calculate all samples while MEP could only compute 1,808 samples. According to SEMEP, the ratio Qm=Qt of measured sediment discharge Qm to total sediment discharge Qt is a function of the Rouse number Ro, flow depth h, and the median grain size of the bed material d50. In Korean sand and gravel bed rivers, the materials in suspension are fine (silt or clay) and Ro ≈ 0. The ratio Qm=Qt reduces to a function of flow depth h, and at least 90% of the total sediment load is measured when h > 1 m. More than 80% of the sediment load is measured when the discharge Q is larger than four times mean annual discharge Q¯ (Q=Q¯ > 4). The ratio Qs=Qt of suspended sediment discharge Qs to total sediment discharge can be also analyzed with SEMEP and the result shows that Qs=Qt is a function of h=d50 and Ro. When Ro ii ≈ 0, the ratio Qs=Qt increases with h=d50. The suspended load is more than 80% of the total sediment load when h=d50 > 18. The relationship between specific sediment yield, SSY , and watershed area, A, is SSY = 300A−0:24 with an average error of 75%. Besides the specific sediment yield, the mean annual dis- charge, the normalized flow duration curve, the sediment rating curve, the normalized cumulative distribution curve, and the half yield discharge vary with watershed area. From the normalized flow duration curve at an exceedance probability of 0.1%, small watersheds (A < 500 km2) have 42 < Q=Q¯ < 63, compared to large watersheds (A > 5000 km2) which have 14 < Q=Q¯ < 33. In terms of sediment rating curves, at a given discharge, the sediment load of small watersheds is one order of magnitude higher than for large watersheds. From the normalized cumulative dis- tribution curves, the half yield (50% of the sediment transported) occurs when the discharge is at least 15 times the mean discharge. In comparison, the half yield for large watersheds corresponds to Q=Q¯ < 15. The flow duration curve can be parameterized with a^ and ^b by using a double logarithmic fit to the flow duration curve. This parametric approach is tested with 35 Korean watersheds and 716 US watersheds. The value of a^ generally increases with watershed area. The values of ^b are consistently between 0.5 and 2.5 east of the Mississippi River and the Pacific Northwest. Large variability in ^b is found in the High Plains and in Southern California, which is attributed to the high flashiness index in these regions. A four-parameter model is defined when combining with the sediment rating curve. The four parameters are: a^ and ^b for the flow duration curve, and a¯ and ¯ ¯ ¯ ¯b ^¯ b for the sediment rating curve. The mean annual discharge Qs is calculated by Qs =a ¯a^ Γ(1+bb). The model results are compared to the flow-duration/sediment-rating curve method. The average error of this four-parameter model is only 8.6%. The parameters can also be used to calculate the cumulative distribution curves for discharge and sediment load. iii ACKNOWLEDGEMENTS I would like to thank everyone who helped me both directly and indirectly with my dissertation. The support that I’ve received in both a scholarly sense and in my personal life has greatly helped me in my pursuit a PhD degree at Colorado State University. I thank Dr. Pierre Julien for being an amazing advisor. You are always inspiring, encouraging, and patient. I learned A LOT from you. Thanks also to my committee members, Drs. Rob Ettema, Sara Rathburn, and Peter Nelson for their positive and helpful comments. I’d also like to give special thanks to Nick Grieco and Larry Thayer for their friendships and the helps on editing my dissertation. Thanks to my coworkers from Dr. Julien’s Dream team: Marcos Palu, Neil Andika, Weimin Li, Dr. Jai Hong Lee. Thanks to Woochul Kang for working with me on the Korean project. Thanks to Kristin LaForge and Sydney Doidge for working with me on the Middle Rio Grande project. It has been a pleasure working with all of you. I am grateful for the discussions in Friday seminars. They helped me improve my thinking and direct my thoughts. A special thanks to Haw Yen for suggesting that I study at CSU. I also appreciate the friends I’ve met during my study at CSU: Irene Hsu, Alice Lin, Da-Wei Lu, Noriaki Hosoya, Yejian Huang, Yishu Zhang, Yangyang Wu, Dustin Lance, Sam Shih, Alan Li, Jordan Deshon, Ryan Rykhus, and Noah Gustavson. Life is much more joyful with your friendships and companionship. Thanks to Jen and Brian in Berkeley for their hospitality when I just arrived the US and taking me to explore the Sierra Nevada mountains. Shout out to Ching-Yu Wang and Randy Babcock. They are doing an amazing job helping international students like me adjusting to our new lives in the US. Thanks to Dr. Mazdak Arabi, K-water, and US Bureau of Reclamation for providing the fund- ing over the past four years. Thanks to the kind donors of the following scholarships: Whit- ney Borland Advanced Student Graduate Scholarship, Tipton-Kalmbach/Stantec Fellow, and Jeng Song Wang Memorial Scholarship. iv I am also grateful to my friends and family in Taiwan. Thanks to Samuel Huang for visiting. Thanks to Jin Hsueh, Hung-ta Chien, Esther Chang and Ian Lin for checking on me every once in a while. Last, thanks to my dad and mom for their unconditional love and support. v TABLE OF CONTENTS ABSTRACT........................................... ii ACKNOWLEDGEMENTS................................... iv LIST OF TABLES ....................................... ix LIST OF FIGURES....................................... x Chapter 1 Introduction................................... 1 1.1 Problem Statement.............................. 1 1.2 Research Objectives............................. 5 Chapter 2 Literature Review................................ 6 2.1 Total Sediment Load............................. 6 2.1.1 Measurement of Suspended Load..................... 8 2.1.2 Measurement of Bedload......................... 10 2.2 Estimating Total Load from Measurements................. 11 2.2.1 Empirical Approaches........................... 12 2.2.2 Einstein’s Approach............................ 15 2.2.3 Modified Einstein Procedure (MEP) ................... 17 2.2.4 Bureau of Reclamation Automated Modified Einstein Procedure (BO- RAMEP).................................. 18 2.2.5 Series Expansion of the Modified Einstein Procedure (SEMEP) . 24 2.3 Sediment Rating Curves........................... 30 2.4 Computing the Sediment Load........................ 31 2.4.1 Time-Series Summation Method..................... 31 2.4.2 The Flow-Duration/Sediment-Rating Curve Approach.......... 32 2.5 Parametric Analysis of Runoff and Sediment Transport........... 32 2.5.1 Graphical Method............................. 35 2.5.2 Method of Moments............................ 35 2.5.3 Interpretation of the Exponent Parameter ^b . 37 2.6 Statistical Analysis.............................. 37 Chapter 3 Sediment Yield in South Korea......................... 41 3.1 Study Site................................... 41 3.2 Previous Sediment Yield Studies in South Korea .............. 42 3.3 Available Data for this Study......................... 49 3.3.1 River Data in South Korea ........................ 49 Chapter 4 The Ratio of Measured to Total Sediment Load ................ 51 4.1 MEP vs SEMEP............................... 51 4.1.1 MEP Computation Example ....................... 52 4.1.2 SEMEP Computation Example...................... 56 4.1.3 All Korean Data.............................. 58 vi 4.2 Ratio of Measured to Total Load Qm=Qt . 59 4.3 Ratio of Suspended to Total Load Qs=Qt . 63 4.4 Discussion and Conclusion.......................... 66 Chapter 5 Sediment Yield and Watershed Area...................... 69 5.1 Flow-Duration/Sediment-Rating Curve Method............... 69 5.1.1 Flow Duration Curve ........................... 69 5.1.2 Sediment-Rating Curve.......................... 69 5.1.3 Flow-Duration/Sediment-Rating Curve Method ............. 70 5.2 Water and Sediment Discharge........................ 74 5.2.1 Water Discharge.............................. 74 5.2.2 Sediment Rating Curve for Total Load.................. 77 5.2.3 Sediment Yield .............................. 81 5.2.4 Cumulative Distribution Curves for Flow and Sediment......... 82 5.3 Discussion and Conclusion.......................... 86 Chapter 6 Parametric Analysis of the Sediment Yield................... 88 6.1 Parametric Analysis ............................. 88 6.1.1 Definition of the Four Parameters..................... 88 6.1.2 Mean Annual Flow and Sediment Yield ................. 90 6.1.3 Cumulative Distribution Curves.....................