Modeling Flood Dynamics Along the Super-Elevated Channel Belt of the Yellow River, China, over the Last 3,000 Years

Abstract:

During the much of the 20^th Century, the Yellow River, China, carried between 1.1 and 1.6 Gt y^-1 of sediment derived from the over-used Loess Plateau. A portion of this sediment load accumulates inside the artificial levees, reducing the accommodation space and subsequently building up the modern channel-belt >10m above the surrounding floodplains. Historical levees often failed along the older Yellow River courses resulting in >1000 floods in 3000 yrs. In the last millennium, the river has shifted its lower course every ~25 years, breached its levees once a year; in mid 17th century up to 3 breaches occurred per year.

A novel methodology is employed to quantitatively reconstruct and interpret flood dynamics on the Yellow River. A reduced-complexity model is developed to explore how climate change and human activity affect levee breaches and river avulsions. The model integrates yearly morphological change along a channel belt with daily river fluxes, and hourly evolution of levee breaches. The model calculates breach characteristics at the scale of 100yr and 200km. To cope with the sparseness in historical records and to incorporate the complex and uncertain nature of flood behavior, 17,118 experiments are conducted to explore dominant factors controlling flood frequency and their likely values in historical times. Model sensitivity analyses reveal that under natural conditions, super-elevation of the channel belt dominates flood frequency. However, when there is significant human-accelerated basin erosion and breach repair, the dominant factors shift to a combination of mean annual precipitation, super-elevation, critical shear stress of weak channel banks, and the interval between breach initiation and its repair. With human perturbation, breaching became more sensitive to precipitation and channel bank strength. Applying uncertainty analyses, the most likely values of the dominant factors for six historical periods between 850BC and 1839AD are explored and used to quantitatively reconstruct the history of Yellow River floods. During 850BC-1839AD when the sediment load increased fourfold, the major breach recurrence interval was shortened from more than 500 years to less than 6 years, and the breach outflow rate increased ~27 times.