8.4 Results and Discussion
129
originating from the slopes and gullies, thus causing the smaller magnitude of floods
and smaller sediment concentrations.
Scour/deposition balance is a common phenomenon of gully evolvement which
has been testified by investigations on other watersheds. The rate of gully incision is
controlled by the flow rate, depth, turbulence and temperature of the water flow, as
well as by soil texture, soil mechanical pattern, and the level of vegetation protection (Sidorchuk 1999). Foster (1982) has found that gullies were eroded downwards,
until a layer of low erosion susceptibility was reached. After this, the gullies would
become widened, and the erosion rate would decrease. Eventually, the gullies would
“equilibrate” with the overland flow. Novak (1985) has developed equations for calculating the time required to reach equilibrium for rill and channel erosions. Recent
studies have indicated that gully formation can be divided into two stages (Sidorchuk
1999). The initial stage takes up about 5% of a gully’s lifetime during which the morphological characteristics of a gully, such as length, depth, width, area and volume,
are developing, and stability cannot be attained. After this, a stable stage will be
reached, and the gully will attain a maximum in size. In the initial stage, hydraulic
erosion is predominant at the gully bottom, and rapid mass movements occur on the
gully sides, whilst in the stable stage, sediment transport and sedimentation become
the main processes at the gully bottom, and the gully width increases limitedly due
to lateral erosion. The mass movements go on slowly in this stage. In our natural
world, if the local conditions are permitting, some streams can maintain their relatively stable channels over even a century, despite that there have been great floods
in such a long period of time (Warburton et al. 2002; McEwen 1994; Warburton et al.
1993).
As check-dams have been built in small watersheds on the Loess Plateau, the
relieves of the gullies were changed. Substantial sediment caused by rainfalls and
floods have been stored on the dam-land for decades, and the mean slope gradients of
the gullies became gentler. Consequently, erosion in the gullies has been alleviated,
and the gullies have attained a relative stability at last. On the other hand, even though
the amount of sediments from the upper reaches has not been reduced, the rising rate
of the dam-land becomes slower due to the enlargement of the dam-land area. Hence
a critical height of a check-dam exists for a given gully having definite conditions
of soil, water, geology and physiognomy. If the check-dam in a gully is higher than
the critical value, and there are depositions in the upper reach of the check-dam, the
mean slope gradient of the gully would be gentle enough to diminish the erosion
greatly. As a result, if the annual sedimentation thickness is very little in the later
stage of the dam-land formation, the workload for heightening the dam is so little that
it can be afforded by the local people (Zeng et al. 1999). Although other factors also
play their roles in maintaining the relative stability of the check-dam systems (Fang
1995), a equilibrium of soil and water in the gullies is the most important criterion.
In fact, many natural check-dams, such as the Balihe Gully and the Sanshilihe
Gully in Shannxi Province and the Laobatou and the Qianqiuzi in Gansu Province,
all have achieved a good relative stability. Huangtuwa in the Zizhou County of
Shannxi Province, another famous natural check-dam covering 40 hm
2 of lowlands,
has been working favorably for more than 400 years. Some check-dam systems
129
originating from the slopes and gullies, thus causing the smaller magnitude of floods
and smaller sediment concentrations.
Scour/deposition balance is a common phenomenon of gully evolvement which
has been testified by investigations on other watersheds. The rate of gully incision is
controlled by the flow rate, depth, turbulence and temperature of the water flow, as
well as by soil texture, soil mechanical pattern, and the level of vegetation protection (Sidorchuk 1999). Foster (1982) has found that gullies were eroded downwards,
until a layer of low erosion susceptibility was reached. After this, the gullies would
become widened, and the erosion rate would decrease. Eventually, the gullies would
“equilibrate” with the overland flow. Novak (1985) has developed equations for calculating the time required to reach equilibrium for rill and channel erosions. Recent
studies have indicated that gully formation can be divided into two stages (Sidorchuk
1999). The initial stage takes up about 5% of a gully’s lifetime during which the morphological characteristics of a gully, such as length, depth, width, area and volume,
are developing, and stability cannot be attained. After this, a stable stage will be
reached, and the gully will attain a maximum in size. In the initial stage, hydraulic
erosion is predominant at the gully bottom, and rapid mass movements occur on the
gully sides, whilst in the stable stage, sediment transport and sedimentation become
the main processes at the gully bottom, and the gully width increases limitedly due
to lateral erosion. The mass movements go on slowly in this stage. In our natural
world, if the local conditions are permitting, some streams can maintain their relatively stable channels over even a century, despite that there have been great floods
in such a long period of time (Warburton et al. 2002; McEwen 1994; Warburton et al.
1993).
As check-dams have been built in small watersheds on the Loess Plateau, the
relieves of the gullies were changed. Substantial sediment caused by rainfalls and
floods have been stored on the dam-land for decades, and the mean slope gradients of
the gullies became gentler. Consequently, erosion in the gullies has been alleviated,
and the gullies have attained a relative stability at last. On the other hand, even though
the amount of sediments from the upper reaches has not been reduced, the rising rate
of the dam-land becomes slower due to the enlargement of the dam-land area. Hence
a critical height of a check-dam exists for a given gully having definite conditions
of soil, water, geology and physiognomy. If the check-dam in a gully is higher than
the critical value, and there are depositions in the upper reach of the check-dam, the
mean slope gradient of the gully would be gentle enough to diminish the erosion
greatly. As a result, if the annual sedimentation thickness is very little in the later
stage of the dam-land formation, the workload for heightening the dam is so little that
it can be afforded by the local people (Zeng et al. 1999). Although other factors also
play their roles in maintaining the relative stability of the check-dam systems (Fang
1995), a equilibrium of soil and water in the gullies is the most important criterion.
In fact, many natural check-dams, such as the Balihe Gully and the Sanshilihe
Gully in Shannxi Province and the Laobatou and the Qianqiuzi in Gansu Province,
all have achieved a good relative stability. Huangtuwa in the Zizhou County of
Shannxi Province, another famous natural check-dam covering 40 hm
2 of lowlands,
has been working favorably for more than 400 years. Some check-dam systems