Investigating The Flood History Of The Yellow River:from Analyzing Historical Records To Computer Modeling

According to historical documents, the lower Yellow River has flooded>1000times in the historical times. In the last millennium, the river has shifted its lower course every-25years, breached its levees once a year, and during the mid seventeenth century the breach frequency was as high as three times a year. The elevated channel bed has contributed to many epic disasters, making river control an arduous, costly job. The Yellow River floods were the most destructive geomorphological disasters in the environment history of China. Investigating the causes of the flood history of the Yellow River, will not only benefit today’s river management, a harmonious human-river relationship, and our commitment to sustainability, but also increase our understanding of anthropogenic geomorphology and the dynamics of Coupled Natural and Human (CNH) Systems.My thesis is guided by the theories of complex system and Lyell’s principle of geology "the present is the key to the past". Theories and findings in hydraulics, channel morphodynamics, fluvial geomorphology and physical geography are applied, integrated with many conclusions contributed by historians. The analyses of historical records and computer modeling are combined to study the dynamical4000-year history of Yellow River floods. Three studies in the thesis use deductive, inductive and abductive reasoning, respectively. They span different times and spaces, employ qualitative or quantitative methods, and thus explore the causes of the history of Yellow River floods from different perspectives.I first analyze the4000-year flood history of the lower Yellow River and the history of agricultural development in the middle river by investigating historical writings and quantitative time series data of environmental changes in the river basin. I find that flood dynamics are characterized by positive feedback loops, critical thresholds of natural processes, and abrupt transitions caused by socio-economic factors. Technological and organizational innovations were dominant driving forces of the flood history. The popularization of iron plows and embankment of the lower river in the4th century BC initiated a positive feedback loop on levee breaches. The strength of the feedback loop was enhanced by farming of coarse-sediment producing areas, steep hillslope cultivation, and a new river management paradigm, and finally pushed the flood frequency to its climax in the seventeenth century. The co-evolution of river dynamics and Chinese society is remarkable, especially farming and soil erosion in the middle river, and central authority and river management in the lower river.I subsequently use the theory of channel morphology and four time-series reconstructed from historical records to establish a multiple linear regression model for the10-year averaged daily probability of levee breach on the lower Yellow River over1550-1855. It turns out that significant prediction of levee breach probability relies on four variables:total annual runoff, super-elevation, bankfull discharge, and levee construction works of the previous year. Uncertainty assessment shows the model efficiently predicts levee breach probability to an average determination coefficient of-0.7. Goodness of fit, and significance of fitting equations are most sensitive to changes in total outflow ratio. When the ratio is0.2-0.4, the model performs better. The regression model suggests two mechanisms of variations in breach frequency. Firstly, water depth was the dominant forcing for the decade-to-decade variations in breach frequency. Secondly, when technology was limited, embankment of a sediment-laden river created a positive feedback loop, forcing, though not significantly, the long-term change in breach frequency.The third study develops a reduced-complexity model, enabling to combine the generation of daily water and sediment discharges, the yearly calculations of channel changes and the reach-averaged simulations of breach morphologic evolution. The combined model calculates numbers of breach-years and avulsions, breach widths and outflow ratios, avulsion thresholds (super-elevations) of the first200-km reach of the lower Yellow River in a period of time. The model is used to quantitatively reconstruct the flood history of the Yellow River and to explore the long-term influences of climate, land use and river control on the flood behavior of the Yellow River. Sensitivity analysis indicates that when sediment load is low, super-elevation is the sole factor that can significantly influence numbers of breach-years and avulsions. However, when sediment load becomes very high, long-term averaged total annual precipitation of the river basin, super-elevation, critical shear stress of bank materials in weak sections, and number of days between breach initiation and breach repair have to be combined so as to exert significant influences. The sediment-laden river has a shallow channel, and thus a small near-bank flow shear stress. Hence, numbers of breach-years and avulsions become sensitive to critical shear stress of bank materials. When human-induced basin erosion is intense, the sensitivity of sediment load to precipitation significantly increases. As a result, shallowness of channel becomes more sensitive to precipitation, which in turn increases the sensitivity of numbers of floods to precipitation. Using uncertainty analysis, I explore the most likely values of the dominant factor (or combination of factors) in8historical periods during850BC and1839AD; on these basis, a hypothetical history of the Yellow River floods is quantitatively constructed. The model has two limitations. First, parameterizing the process of super-elevation accumulation obscures the influence of anthropogenic erosion factor on flood frequency. Second, the process of headward erosion can not be included in the model; as a result, the feedback mechanism that avulsion will impact on flood frequency can not be fully represented.All studies show that the flood history of the Yellow River is the outcome of interactions among multiple factors, such as the inherent characteristics of the Loess Plateau and lower channel, climate, land use, river control, and time. Anthropogenic factors played an increasingly influential role in the historical time.