122
8 Sediment-Storage Effects of Check-Dam …
fluid flowed into the gully model, part of it was restored in the upstream of the checkdam and the rest arrived at the storing pool. To prevent the runoff from changing
the sediment concentration in the puddling pool, a valve was fixed at the end of the
storing pool. Thus, when an experimental run was completed, the valve could be
opened, and then the fluid in the storing pool could be released into the puddling
pool, which could be reused in the next experiment.
The gully model covered an area of 14 m by 3.5 m. The branch gully at the foreside
of the model was “V” shaped, while the main gully at the rear-side was “U” shaped.
The length of the branch gully was about 9 m. The dam-site at the main gully was
0.9 m away from the end of the branch, which stood in the entrance of the prototype
main gully. The effective height of the dam was 0.35 m before the first runoff. A
spillway was located on the left side of the check-dam, which allowed the floodwater
to overflow into the storing pool. Among the inlet pipes, an electric flowmeter was
fixed to measure the flux invading the gully. The two sidewalls, which were straddled
by a measurement bridge with a height finder, were parallel and their tops were of
approximately the same altitude. The bridge could move along the sidewalls and
the height finder could slide along the bridge, thus the altitude of any point on the
gully bed could be measured. The surface altitude was digitized for both horizontal
directions in 0.1 m grids.
Eighteen experimental runs were conducted with the same flux of 1.0 × 10
−3 m
3 /s
and the same duration of 12 min. The sediment concentration of the flow running to
the gully was kept at approximately 80–100 kg/m
3 . There was a 24 h break following
each runoff. The initial microtopography made according to the blueprint of the scale
model was designated as “N m (0)”, the one after the first runoff as “N m (1)”, and the
one after the second runoff as “N m (2)”, and so on. Each microrelief formed in 24 h
after the runoff became the initial landform of the next run. Before the next runoff,
muddy water stored in the reservoir in the upper reach of the check-dam was defecated
and pumped out, and the microrelief of the gully bed was measured by the height
finder.
In this experiment, the dam area after each runoff could be calculated by the
contour line of the gully bed. After the map of the gully bed had been converted into
an AutoCAD file with a ratio of 1 unit to 1 cm, the dam-land enclosed by the contour
line could be calculated by using the computer command “Area”.
8.3.2 Rainfall Simulation
Rainfall simulations were conducted in the model of a catchment to study the hydrosedimentologic process and geomorphological evolution in the gullies after the
check-dam system was built. The experiments were carried out in the laboratory
of the Yellow River Research Center, Tsinghua University, Beijing, China. The landscape simulator consisted of a rainfall simulator suspended above a flume in the
small watershed model. Nozzles with an inner rotor were screwed onto the ends of
the pipes in the spot, which could spray downwards to the landform 5.5 m below.
8 Sediment-Storage Effects of Check-Dam …
fluid flowed into the gully model, part of it was restored in the upstream of the checkdam and the rest arrived at the storing pool. To prevent the runoff from changing
the sediment concentration in the puddling pool, a valve was fixed at the end of the
storing pool. Thus, when an experimental run was completed, the valve could be
opened, and then the fluid in the storing pool could be released into the puddling
pool, which could be reused in the next experiment.
The gully model covered an area of 14 m by 3.5 m. The branch gully at the foreside
of the model was “V” shaped, while the main gully at the rear-side was “U” shaped.
The length of the branch gully was about 9 m. The dam-site at the main gully was
0.9 m away from the end of the branch, which stood in the entrance of the prototype
main gully. The effective height of the dam was 0.35 m before the first runoff. A
spillway was located on the left side of the check-dam, which allowed the floodwater
to overflow into the storing pool. Among the inlet pipes, an electric flowmeter was
fixed to measure the flux invading the gully. The two sidewalls, which were straddled
by a measurement bridge with a height finder, were parallel and their tops were of
approximately the same altitude. The bridge could move along the sidewalls and
the height finder could slide along the bridge, thus the altitude of any point on the
gully bed could be measured. The surface altitude was digitized for both horizontal
directions in 0.1 m grids.
Eighteen experimental runs were conducted with the same flux of 1.0 × 10
−3 m
3 /s
and the same duration of 12 min. The sediment concentration of the flow running to
the gully was kept at approximately 80–100 kg/m
3 . There was a 24 h break following
each runoff. The initial microtopography made according to the blueprint of the scale
model was designated as “N m (0)”, the one after the first runoff as “N m (1)”, and the
one after the second runoff as “N m (2)”, and so on. Each microrelief formed in 24 h
after the runoff became the initial landform of the next run. Before the next runoff,
muddy water stored in the reservoir in the upper reach of the check-dam was defecated
and pumped out, and the microrelief of the gully bed was measured by the height
finder.
In this experiment, the dam area after each runoff could be calculated by the
contour line of the gully bed. After the map of the gully bed had been converted into
an AutoCAD file with a ratio of 1 unit to 1 cm, the dam-land enclosed by the contour
line could be calculated by using the computer command “Area”.
8.3.2 Rainfall Simulation
Rainfall simulations were conducted in the model of a catchment to study the hydrosedimentologic process and geomorphological evolution in the gullies after the
check-dam system was built. The experiments were carried out in the laboratory
of the Yellow River Research Center, Tsinghua University, Beijing, China. The landscape simulator consisted of a rainfall simulator suspended above a flume in the
small watershed model. Nozzles with an inner rotor were screwed onto the ends of
the pipes in the spot, which could spray downwards to the landform 5.5 m below.