120
8 Sediment-Storage Effects of Check-Dam …
Theory for deposition in reservoirs with hyperconcentrated flow is practicable and
has been applied to several practical projects (Zhang et al. 2001, 2002), so that some
of their technical approaches could be used as references in this study.
The experiments were carried out in the Laboratory Hall of the Yellow River
Research Center, Tsinghua University, Beijing, China. Due to the limitation of the
experimental ground, the length scale λ L was restricted to 60, which implied that the
gully dimension and the dam height were shrunk to 1/60 of the prototype ones. The
matrix loess stacked near the experimental spot, which was homologous to the loess
in the Yangdaogou Catchment, was used as the erodible material in this experiment.
The material had passed through a sieve with 1 cm sieve pores for removing large
clods, debris and grassroots. To ensure a regular original microrelief, the soil was
prepared by hand-patting for generating a smooth roughness. The diameter of 50%
soil particles, d 50 , was 30.8 µm, and the dry bulk density ρ d was 1.44 × 10
3 kg/m
3 .
No matter it is a field survey or a laboratory experiment, the gravitational effect
is more decisive than the viscous one for the turbulent flow caused by rainfall, so the
duration of each rainfall is demanded by Froude’s Law of Time Scale (Jiang et al.
1994):
t m =
t p
λ t
=
t p
√ λ L
=
10 − 40
√
60
= 1.3 − 5.2 (min)
(8.4)
where t is time, min; λ t is the time scale; λ L is the length scale. Since the objective
of this study is to validate the trend of check-dams in attaining a relative stability, the
reliability of the experimental results would not be degraded if several runoffs were
performed continuously. Consequently, the duration of each experimental runoff, t
m ,
was prolonged to 12 min without making any changes in the sediment concentration
and the flow rate, so that it was possible to observe the evolution process of the gully
landform in greater details and to complete the experiments more quickly.
In practice, the perennial annual runoff rate and erosion rate are usually taken
as design indices for soil and water products in programming the sequence of dam
constructing for the check-dam system in small watersheds (Zheng 2003). The flow
rate and concentration of the mixture flowing into the reservoir, which were accurately calibrated before the formal experiments, was 10
−3 m
3 /s and 80–90 kg/m
3 ,
respectively. Thus the soil product pouring into the reservoir in each runoff of the
experiment is shown as follows:
S
m = Q
m × C
m × t
m = 10
−3
× 85 × 12 × 60 = 61.2 (kg)
(8.5)
where S
m is the soil product, kg; Q
m is the flow rate of the runoff, m
3 /s; C
m is the
concentration of the runoff, kg/m
3 ; t
m is the duration of the runoff, m
3 /s. All the
parameters are measured/calculated in the experiment.
Thus the scale of cumulative sediment for each rainfall can be obtained as follows:
λ S =
S p
S m
=
4.29 × 10
6
61.2
= 7.01 × 10
4
(8.6)
8 Sediment-Storage Effects of Check-Dam …
Theory for deposition in reservoirs with hyperconcentrated flow is practicable and
has been applied to several practical projects (Zhang et al. 2001, 2002), so that some
of their technical approaches could be used as references in this study.
The experiments were carried out in the Laboratory Hall of the Yellow River
Research Center, Tsinghua University, Beijing, China. Due to the limitation of the
experimental ground, the length scale λ L was restricted to 60, which implied that the
gully dimension and the dam height were shrunk to 1/60 of the prototype ones. The
matrix loess stacked near the experimental spot, which was homologous to the loess
in the Yangdaogou Catchment, was used as the erodible material in this experiment.
The material had passed through a sieve with 1 cm sieve pores for removing large
clods, debris and grassroots. To ensure a regular original microrelief, the soil was
prepared by hand-patting for generating a smooth roughness. The diameter of 50%
soil particles, d 50 , was 30.8 µm, and the dry bulk density ρ d was 1.44 × 10
3 kg/m
3 .
No matter it is a field survey or a laboratory experiment, the gravitational effect
is more decisive than the viscous one for the turbulent flow caused by rainfall, so the
duration of each rainfall is demanded by Froude’s Law of Time Scale (Jiang et al.
1994):
t m =
t p
λ t
=
t p
√ λ L
=
10 − 40
√
60
= 1.3 − 5.2 (min)
(8.4)
where t is time, min; λ t is the time scale; λ L is the length scale. Since the objective
of this study is to validate the trend of check-dams in attaining a relative stability, the
reliability of the experimental results would not be degraded if several runoffs were
performed continuously. Consequently, the duration of each experimental runoff, t
m ,
was prolonged to 12 min without making any changes in the sediment concentration
and the flow rate, so that it was possible to observe the evolution process of the gully
landform in greater details and to complete the experiments more quickly.
In practice, the perennial annual runoff rate and erosion rate are usually taken
as design indices for soil and water products in programming the sequence of dam
constructing for the check-dam system in small watersheds (Zheng 2003). The flow
rate and concentration of the mixture flowing into the reservoir, which were accurately calibrated before the formal experiments, was 10
−3 m
3 /s and 80–90 kg/m
3 ,
respectively. Thus the soil product pouring into the reservoir in each runoff of the
experiment is shown as follows:
S
m = Q
m × C
m × t
m = 10
−3
× 85 × 12 × 60 = 61.2 (kg)
(8.5)
where S
m is the soil product, kg; Q
m is the flow rate of the runoff, m
3 /s; C
m is the
concentration of the runoff, kg/m
3 ; t
m is the duration of the runoff, m
3 /s. All the
parameters are measured/calculated in the experiment.
Thus the scale of cumulative sediment for each rainfall can be obtained as follows:
λ S =
S p
S m
=
4.29 × 10
6
61.2
= 7.01 × 10
4
(8.6)