Author(s): Zhaoyin Wang; Chunhong Hu
Keywords: Fluvial process; Reservoir; River pattern; Channelization; Water diversion
Abstract: A large-scale hydro-project is defined as one or a chain of engineering structures, whose operation may obviously change the hydrological or hydraulic conditions of the river on which it is constructed. This paper studies the fluvial processes in the upstream reaches and tributaries and downstream reaches affected by dams, and the fluvial impacts of channelization and water diversions. Damming of rivers not only causes sedimentation in the reservoir but also creates additional backwater and deposition even further upstream. The Three Gorges Dam on the Yangtze River may change the upstream reaches from a braided channel into a single thread channel, and Sanmenxia Dam has changed the Weihe River from meandering to wandering-meandering. Downstream reaches of dams experience the following fluvial processes: (1) channel incision; (2) variation in channel width; (3) reduction in bank erosion and channel migration; and (4) changes in river patterns. The study reveals that dams on rivers with low sediment concentration reduce the channel migration remarkably, but dams on rivers with hyperconcentrated sediment have little effect on channel migration. Dams change downstream river patterns from braided to wandering, or from wandering-braided to wandering-meandering. Defining the channelization degree as the ratio of the length of the hardened banks to the length of the channel, we found that if the degree is within the range of 0.8-1.3, the highest probability of bank failure occurs, because the natural fluvial process tends to break the constraint of the channelization. Water diversion has becomes an important stress causing fluvial processes. The lower Yellow River is a perched river with its riverbed 10 m higher than the surrounding land, which poses a flooding risk but also provides flowing potential for water diversion to farmland and cities and towns. At present, more than 10 billion m 3 of water is diverted from the Yellow River, which has caused shrinkage of the channel and readjustment of the bed profiles. If the quantity of water diversions along the course is more than the inflow from tributaries, the riverbed profiles will develop toward a concave profile in the upper reach and a convex profile in the lower reach.