Sediment Movement in Alluvial Rivers - River Dynamics and Integrated River Management - page 301

sediment concentration around 2,690 m 3 /s and 2,450 m 3 /s and 346 kg/m 3 and 156 kg/m 3 , respectively.
The sediment of the hyperconcentrated flood was fine and the median diameter was around 0.008 mm. The
stage-discharge relation for a man-made flood is shown in the figure for comparison. The man-made
flood was created with the Xiaolangdi Reservoir to scour the lower Yellow River, on July 4 - 13, 2004.
The sediment concentration of the man-made flood was zero at Xiaolangdi and increased to 12 kg/m 3 at
the Aishan station. The hyperconcentrated flood waves scoured the riverbed down and the stage had been
clearly reduced by about half a meter.

5.2.3.6 Development of Narrow and Deep Channel
Hyperconcentrated floods change the channel to narrow and deep but low concentration floods cause
collapse of the banks and make the channel wide and shallow. Figure 5.41 shows the comparison of the
sedimentation and erosion along the course of the North Luohe River, which is a tributary of the Weihe
River, after a hyperconcentrated flood and a low concentration flood. Sanmenxia Dam is located about
150 km downstream of the reach on the Yellow River. The horizontal axis is the distance to Sanmenxia
Dam, which also shows the number of the measurement cross sections. ' and ' represent the

changes in cross sectional area, width of the channel, and elevation of the deepest point of the channel
due to sedimentation or erosion during the floods. A positive value of

' ,

' means sedimentation and
negative value means erosion, and positive values of ' and ' imply enlargement in width and depth,
respectively, and negative values mean reduction in width and depth respectively. The upper diagram
shows the changes in the cross sectional area after a low concentration flood and a hyperconcentrated
flood. The low concentration flood caused no sedimentation on the flood plain, but the hyperconcentrated
flood caused sedimentation on the floodplain in the lower section of the river. The cross sectional area of
the main channel increased largely after the hyperconcentrated flood because the hyperconcentrated
flood caused bed erosion in the main channel. The low concentration flood also increased the cross
sectional area of the main channel but much less than the hyperconcentrated flood. The width of the main

Fig. 5.41 Changes in the area of the floodplain due to sedimentation, and in the area, width, and elevation of the
deepest point of the main channel resulting from sedimentation and erosion during a low concentration flood and a
hyperconcentrated flood along the course of the North Luohe River. (Note: A positive value of ' means
sedimentation and negative value means erosion; positive values of ' and ' imply enlargement in width and
depth, respectively, and, negative values mean reduction in width and depth, respectively)