The river bed and flood level in the estuary are rising continuously, which is mainly the result of the continuous sediment deposited on the front slope of the estuary, which leads to the continuous extension of the estuary to the sea. Therefore, the river dredging should be carried out at the tail end of the estuary first, from the front end of the tail end to the upstream, so as to make the front end of the tail end retreat, make the lower boundary control the base surface to move inward, shorten the process, and make the longitudinal slope of the river steep, the riverbed scour and the water level drop. On the other hand, compared with the reduction of erosion base, it can produce long-distance tracer erosion and slow down river siltation, which is one of the effective measures to realize the elevation-free riverbed in the lower Yellow River. Thirdly, from the analysis of recent siltation in the estuary, the tailrace should also be dredged.
The transition section between curved river sections is also the focus of river digging.
There are many river regulation projects on both sides of the narrow curved river; General projects are built on curved sections, and there is a transition section between two adjacent projects. The deep flood point of the bend section is lower than that of the transition section, especially after the flood season, the bend section is about 2m lower than the adjacent transition section. The different cross-sectional shapes of rivers determine the different evolution of river scouring and silting during flood and low water. When the flow velocity is low, the bend is narrow and deep, and the riverbed is low, while the riverbed in the transition section is high, which causes the backwater in the transition section, reduces the slope of the bend, and makes the outflow not smooth, thus forming convex bank siltation in the dry season of the bend and further narrowing the cross section of the bend channel. During the flood period, due to the high water level rise rate in the narrow and deep section and the low water level rise rate in the wide and shallow section, the transition section in the dry season rushed out of the water tank, and the slope of the bend section increased during the flood period, thus increasing the sediment transport capacity of the bend section and making it washed away. On the other hand, the evolution of the transition section is just the opposite, and the scouring occurs in the dry season, which shows that a small river channel is pulled out on the river bed to reduce the deep flood point; During the flood season, due to the narrow channel in the bend section, the water level rises at a high rate, the water level rises, the water surface is wide, the water level in the transition section is relatively low, and the outflow is not smooth, which aggravates the siltation in the transition section, raises the river bottom, and tends to be wide and shallow.
According to the above evolution characteristics, if the river channel in the transition section is excavated, the outflow of the bend section in the dry season will be smooth, the gradient will be increased, and the siltation of the convex bank of the bend section in the dry season will be reduced, so that the bend section will not be too narrow when it enters the flood season, thus reducing the water blocking and backwater effects of the bend section on the transition section in the flood season, which is beneficial to reducing the siltation of the transition section in the flood season. If the convex beach is excavated in the bend section, it will also reduce the siltation in the upstream transition section in flood season, but the excavation of the bend section will widen the originally narrow and deep river section, which will not adapt to the low water, thus increasing the siltation in the bend section in the low water period. In addition, for the bend section, the riverbed is low, and then the excavation is reduced, the height difference between the bend section and the transition section is further increased, which is not conducive to sediment transport and will increase siltation in the dry season. Therefore, the choice of the excavated river section in the transition section of the downstream narrow river section should be ranked second.