Flood Defense and Water/Sediment Management—With Particular Reference to The Yellow River - River Dynamics and Integrated River Management - page 379

was associated with the second ascending period of Tongguan's Elevation. The ascending and descending
of Tongguan's Elevation generated erosion and sedimentation waves, which propagated retrogressively
along the Weihe River, at a speed of about 10 km per year.
Sedimentation has caused flood stage to rise escalating flood hazards in the lower Weihe River.
Figure. 6.48 shows the stage rise at the Huaxian Hydrological Station for flood discharges at 250 and
3,000-5,000 m 3 /s. The stage rise is defined as the difference of the present flood stage minus the stage of
the same discharge before the impoundment. For a discharge at 250 m 3 /s the flow is confined in the main
channel and the stage rise reflects only the sedimentation and shrinking of the main channel. For a flood
discharge in the range 3,000-5,000 m 3 /s the stage rise is due mainly to sedimentation on the floodplain.
During the ascending periods of Tongguan's Elevation in the 1960s both the flood stage and low flow
stage sharply increased by 4 m and 3 m respectively, because sediment deposition and channel reshaping
increased the resistance. During Tongguan's Elevation descending periods 1970-1975 and 1980-1985,
however, erosion occurred in the channel and the flood stage rise reduced by 1-2 m. In the mid 1990s the
enhanced floodplain had been not flooded for several years and invasive and ruderal vegetation had
developed, which increased the flow resistance greatly. As a result the flood stage rise increased sharply
from 3-4 m to 6 m for floods flowing over the floodplain. At present the low flow stage is 4 m higher than
that before the impoundment of Sanmenxia Reservoir and the flood stage is now 6 m higher, which poses
a severe flood risk to the lower reaches of the Weihe River.

Flood stage rise due to sedimentation at Huaxian station for discharges 250 m 3 /s and 3,000-5,000 m 3 /s

Fig. 6.48

6.5.4 Equilibrium Sedimentation Model
Two questions to be answered about the fluvial processes in the Weihe River induced by Sanmenxia Dam
are: Is there any equilibrium of sedimentation in the Weihe River? And whether the sedimentation has
reached the equilibrium? The authors proposed a simple model to answer the questions (Wang et al.,
2004). Assume there is an equilibrium sedimentation volume, V e , for a given increment of Tongguan's
Elevation. If the real sedimentation volume, V, is much less than V e , the rate of sedimentation in the river
is high. The rate of sedimentation is proportional to the difference between the equilibrium and real
sedimentation volume:

d V

t

K VV ( ) (6.3)

e

d

In which K is a constant with dimension of [1/T]. The solution of the equation is ˖

Kt

³

Kt

V e[ KV ed ] t (6.4)

e