Keywords: 
• overbank flow /
• hyperconcentrated flood /
• definition of channel and floodplain /
• channel shaping and floodplain silting /
• threshold flow and sediment load /
• the lower Yellow River

Abstract: A relatively narrow and deep channel can be formed, therefor visible change in channel width can be observed, at most of cross sections due to intensive channel adjustment during a hyperconcentrated flood in the lower Yellow River. The spatial and temporal distribution of sediment deposition was calculated for overbank hyperconcentrated floods by comparison of channel width after an overbank flood with that of non-overbank flood. Results shown that 59% of sediment that would be deposited in the main channel during non-overbank floods was removed by overbank hyperconcentrated floodsto the lower floodplain or floodplain. This indicated that overbank hyperconcentrated floods can reduce sediment deposition in the main channel comparing to non-overbank ones. Based on analysis of measured data, the relationship of channel contraction ratio, ranging from 15.5% to 44.0%, at overflow sections after floods to the channel width before floods was established. Correlation between sediment volume deposited on floodplain and the main hydraulic factors during overbank flooding period, as well as their physical meanings, was analyzed. Responding function of sediment volume deposited on floodplain to hydraulic factors was determined. A threshold index of flow and sediment load for floodplain deposition and main channel reforming during overbank hyperconcentrated floods was proposed, which was between 0.025 and 0.040. This research is significant to regulation of hyperconcentrated floods in the lower Yellow River.