Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 383

more meanders have generally developed after the impoundment. Figure 6.52 shows the numbers of
meanders with different wavelength in the reach in the 1970s, 1980s, and 1990s.
The meander wavelength is defined as the distance from one turning point of the channel on one side
of the valley to the next turning point at the same side. As shown in Fig. 6.52, there were 17 small
meanders in the reach in the 1970s. Some meanders were separated by straight sections and some other
meanders connected with each other and form small meandering sections. Between two small meandering
sections was a section with straight channel. Late the reservoir operation became stable, more meanders
developed and the meandering sections became longer. In the 1980s, however, 22 meanders with a
wavelength from 3 to 30 km had developed in the reach. In the 1990s, the number of meanders continued
increasing and the meanders became regular; 31 of them have meander wavelength within the range of
6-15 km. The river became more and more meandering. In the process, human constructed spur dykes
affected, more or less, the development of meanders.
Fig. 6.52 Numbers of meanders with different wavelengths in a 400 km long reach downstream of Sanmenxia
Reservoir in the 1970s, 1980s, and 1990s
6.6 Proposed New Strategies
6.6.1 Reclamation of the Floodplain
In general, most alluvial river valleys are narrowed and the channels are stabilized following social and
economic development. The lower Yellow River was several tens of kilometers wide, which provided
enough space for the channel to migrate and sediment deposition. Now it is much narrow and will be
narrower. The Mississippi River valley was wide and has been narrowed for more than half century.
Humans narrow the river valley for various purposes: urban development, land reclamation, flood control
and navigation. In Europe, most large rivers, such as the Rhine, Rhone, Elbe and Danube, have been
channelized, mostly for to gain land for development, to eliminate diseases such as malaria, to prevent
floods, and to open waterways for shipping (Kern 1994). The Danube River near Vienna has been
changed from a braided to a single-thread channel in 1859 (Humpesch, 1994), providing a large area of
land for urban development. The river Rhone was channelized for shipping and land reclamation
(Bloesch, 2002).