A study of the characteristics of sedimentation in the Lower Yellow River during overbank floods

The stratigraphic structure of floodplains records the character of overbank floods occurred in the past, typically their magnitude, frequency and duration. For estimating and forecasting sediment load carried by overbank floods and for preventing from flood disasters and protecting wetland ecosystems, this study investigates the relationship between sedimentation and overbank floods in the lower Yellow River. Using the data from the Hydrological Yearbook of the Yellow River, our detailed analysis shows that during very large floods the channel of the river flow is erosional while the floodplain appears aggradational when S/Q (S is the sediment concentration and Q is the flow discharge) is smaller than 0.03. When S/Q is larger than 0.03, however, both channel and floodplain accumulate sediment. The amount of sedimentation in the floodplain is determined by the ratio of Q_max/Q_p (Q_maxis the peak flow discharge and Q_p is the bankfull flow discharge), the water volume over the floodplain and S. In contrast, the erosion in the channel is related not only to the water volume and sediment load of the floods but also to the sedimentation in the floodplain. During the normal overbank floods, both channel and floodplain accumulate sediment when S/Q is larger than 0.023, but when S/Q is smaller than 0.023, the channel is erosion while the floodplain appears aggradational. The channel erosion is related to S/Q and water volume of the floods while the sedimentation in the floodplain is mainly related to sediment concentration. Along the whole length of the lower Yellow River, the erosion in the channel and the sedimentation in the floodplain are concentrated mainly in the upstream reach of Sunkou, while the downstream reach of Sunkou experiences only slight erosion or minor aggradationt.