Hyperconcentrated flows in the slope-channel systems in gullied hilly areas on the loess plateau, China

This study is based on the data from Zizhou and Wangjiagou experimental stations on the Loess Plateau in the major sediment-producing areas of the middle Yellow River drainage basin. It deals with characteristics of hyperconcentrated flows in the slope-channel systems in the gullied hilly areas on the Loess Plateau. The results show that the formation of hyperconcentrated flows is closely related to the vertical differentiation of landforms. Based on data from 21 rainfall events in the period 1963-1970, event-averaged suspended sediment concentration for hilltop, upper hillslope, lower hillslope and gully slope was calculated as 36 kg/m(3), 89 kg/m(3), 304 kg/m(3) and 505 kg/m(3), and the frequency of hyperconcentrated flows was 0.0, 0.17, 0.74 and 1.0, respectively. Thus, hyperconcentrated flows form on the lower part of hillslopes and on the gully slopes, and develope well in gully channels of various orders. There exists a sediment storing-releasing mechanism, resulting from different behaviours of sediment transport by non-hyperconcentrated and hyperconcentrated flows. When water flows are nonhyperconcentrated, the relatively coarse fractions of sediment from the slopes are deposited in the channel. When hyperconcentrated flows occur, the previously deposited coarse sediment may be eroded and released from the channel. A close relationship is found between rainstorms and the formation of hyperconcentrated flows, and some thresholds of rainfall and runoff for the occurrence of hyperconcentrated flows have been identified