Figure 11 Width changes in the main channel and floodplains in the different reaches.

4.3 Mode of environmental management in floodplain

During the operation of the XLD reservoir, the riverbed elevation in the LYR has been effectively controlled by the implementation of the WSRS, resulting in an increase in the bank-full discharge and sediment transport capacity of the main channel, which has reduced the probability of large-scale floods and ensured life and property safety on the floodplain. However, sediment resources on the floodplain have been decreasing since 2000, and the unfavourable phenomenon of a secondary suspended river still exists in local reaches. Moreover, the impact on floodplain environmental management is negative because of the shortage of sediment resources. Sediment is the primary source of a sustainable agro-ecosystem in the LYR, which not only influences riverbed morphology development but also may counteract floodplain subsidence and soil erosion. How to optimize sediment resource allocation in the lower reaches of the Yellow River to achieve positive outcomes for both environmental management and economic development has become an urgent problem.
Under the new requirements of water control and ecological urban construction, we propose a new design idea for floodplain environmental management; some floodplains of the LYR can be reconstructed by measures such as pipeline sediment transport and river dredging. From the levee to the main channel, the floodplain can be sequentially reconstructed into different regions (see Figure 12), including the “high floodplain”, “secondary floodplain” and “new floodplain”; among these regions, the new floodplain together with the main channel could perform the tasks of flood control and sediment transport in the LYR. Through reconstruction of the floodplain domain and in coordination with environmental management measures, different functional zones (such as residential areas, efficient agricultural areas and ecological wetland areas) can be formed. This mode of floodplain reconstruction can solve the unfavourable morphology of the secondary suspended river in local reaches and guarantee flood control safety in the LYR, promote the development of floodplain areas and achieve positive results both in river management and in benefits to people.