Effect of Rock Fragments Cover on Distance of Rill Erosion Initiation and Overland Flow Hydraulics
Rock fragments on the soil surface protect the soil against erosive agents. However, the effect of rock fragments cover on rill initiation is not well documented. The objective of this study was to evaluate the effect of rock fragment cover and the rate of flow discharge on distance and time rill initiation. The investigation was conducted using a flume with 6 m length, 0.5 m width and 3% gradient. The treatments included rock fragment cover (0, 10, 20, 30%) and three levels of flow discharge (3, 6, 9 L min-1). The results showed that the surface cover of rock fragment had significant effect on different erosion processes, as well as rill initiation. Moreover, results indicated that velocity of water flow and the Froude number decreased but the Manning roughness coefficient increased (0.012-0.115 m-1/3 sec) with an increase in rock fragment cover, whereas the Reinholds number remained nearly the same with a small variation among different rock fragment cover percent. This variable was increased with increasing flow discharge. In addition, distance and time of rill initiation increased with a rise in rock fragment cover and diminished with increasing flow discharge.
Received: November 02, 2011;
Accepted: March 15, 2012;
Published: May 28, 2012
Rill erosion is a major participant to soil loss from crop land area. Other
erosion types, such as inter-rill, splash, or tillage erosion often lead to
translocation of soil within the field (Parsons et al.,
2004). Rill erosion has not only a serious importance for on-site effects
of erosion but also for off-site effects and environmental concerns. The critical
states for rill formation have been the focus of many researchers. Horton
(1945) conceptualized the idea of rill formation threshold condition and
later, Schumm (1956) used the concepts of the belt of
no erosion and constant of channel maintenance to describe the distance from
slope summit to the point of rill initiation. The threshold for rill initiation
was defined by Kirkby (1978) as when duration of runoff
exceeds this point, rill formation may take place. Torri
et al. (1987) set a criterion to define the rill formation as when
an incised rill is at least 5 cm long, 0.5 cm deep and 1 to 2 cm wide. Continuous
development of the rill after its formation depends on the flow type. During
the rill formation process, incised rills are formed as the result of detachment
and transport of soil particles by concentrated flow.
Soil surface conditions as roughness, vegetation, rock fragments cover (Guo
et al., 2010), rain characteristics and topography (Moghaddam
and Saghafi, 2008) play important roles in the control soil water erosion
and initiation rills and gullies. A number of studies have showed that rock
fragments cover has a significant effect on runoff and soil erosion. All these
studies reported that runoff and erosion decrease with increasing rock fragments
cover (Agassi and Levy, 1991; Chow
et al., 1992; Nyssen et al., 2001;
Martinez-Zavala and Jordan, 2008; Guo
et al., 2010). The soil surface can be protected by rock fragments
cover against the impacts of raindrops, surface sealing, detachment and transport
of soil particles (Martinez-Zavala and Jordan, 2008).
So, the rock fragment cover on the soil surface reduces the total sediment yield
(Rieke-Zapp et al., 2007). The effects of rock
fragments on eroding environment are: (1) protection of soil surface from direct
impact of raindrops and soil particles detachment (2) decreasing the physical
degradation of the soil surface and (3) increasing the surface roughness and
delaying overland flow and thus reducing detachment and transport capacity of
the run off (Poesen and Lavee, 1994; Abrahams
et al., 2001). Rock fragment surface cover can influence rill initiation
because of its effects on soil erosion processes. Studying the distance and
time of rill initiation can be useful in soil conservation practices, such as
determining of the appropriate intervals in plant works, terracing and other
conservation structures. The distance to rill initiation, on bare soil, decreases
with increasing the slope steepness (Yao et al.,
2008) and critical flow for rill initiation decreased with an increasing
slope degree (Farmanullah et al., 1998). Rieke-Zapp
et al. (2007) indicated Without rock fragments in the soil, rill
incision continued over time and headcutting increased for experiments with
few or no rock fragments in the soil. A few studies have focused on surface
cover and how it affects rill formation. The purpose of present study was to
investigate the effect of rock fragment surface cover on the distance and time
of rill formation and understanding the relationship between the flow discharge
and rock fragments cover in rill initiation.
MATERIALS AND METHODS
The experiment was conducted in the Laboratory of rainfall and runoff simulation,
Soil Conservation and Watershed Management Research Institute, Tehran, Iran.
A runoff simulator with a sloped plot (6x0.5 m) was used (Fig.
1). The plot was initially prepared in a horizontal position.
|| A view of flume
A 10 cm layer of coarse sand was uniformly placed in the bottom of the plot
box; drainage holes in the bottom provided free drainage. On the top of the
sand layer, a silty-loam soil (loess) was packed loosely and evenly to a depth
of 20 cm. The soil texture was 20% clay, 69% silt and 11% sand. The soil was
obtained from the root layer of a cultivated field located in the Loess Plateau
area in Golestan province, Iran. The soil was air-dried, crushed and sieved
with a 10 mm screen and then was packed in the plot to reach a bulk density
of about 1.3 g cm-3. During the packing process, a Rollers method
was used to pack the soil uniformly in the plot. After packing, the soil surface
was smoothed manually with a rake. Then rock fragments with 7 to 8 mm diameter
were randomly distributed on the soil surface. The soil was saturated from below
and allowed to equilibrate for 24 h, while the plot remained in a horizontal
position to ensure a uniform initial soil moisture profile.
The treatments included rock fragment cover (0, 10, 20, 30%), each with three levels of flow discharge (3, 6, 9 L min-1) that were tested at the 3% slope (the slope was same as the field). Each test was conducted 24 h after the saturation and pre wetting. The time of rill initiation was from runoff entry to plot until Primary rill formation moment. In each experiment, runoff and sediment samples were collected every minute until rill formation time. Runoff volumes and sediment mass were determined. Flow velocity was measured using a dye-tracing technique (potassium permanganate). The surface velocities (Vm) were converted to average velocity of flow profile (V) using the formula:
where, a is a coefficient equal to 0.67 (Li et al.,
Rill widths were measured with a ruler during the experiments. Each experiment
ended, when a rill channel (at least 5 cm long, 0.5 cm deep and 1 to 2 cm wide
was formed (Torri et al., 1987) (Fig.
2a, b). After each test, the rills locations were determined
throughout the profile accurately, by laser distance meter. The average distances
of all rills in a test were used as distance to rill initiation for subsequent
The critical point of rill formation is the time when a small pit appears on
the soil surface during the test that later develops into a rill. The critical
conditions of rill formation are related to hydraulic parameters of the surface
|| Rill initiation in surface rock fragment covers
When the strength and velocity of flow reaches a critical point, the soil particles
miss the ability to remain in place and are detached by the overland flow. Two
important hydraulic properties of the overland flow for rill formation are the
shear stress and velocity. The values of these two parameters can be defined
at the point of rill formation as τi and Vi, respectively.
Soil critical shear stress (τc) can be estimated assuming it
the same as the shear stress (τi) of the overland flow at the
point of rill formation. In reality, the shear stress (τi) of
overland flow at the point of rill formation is greater than but very close
to soil critical shear stress (τc) because soil detachment has
already occurred when the values related to the rill formation are measured
in the experimental run.
Soil critical shear stress:
where, τi is shear stress (pa),p is water density (kg m-3) with water temperature being taken into account, g is gravity acceleration (m sec-2), S is sin (α), in which α is slope angle and hi is flow depth (m):
where, q is unit flow discharge (m2 sec-1) and vi is average surface flow velocity (m sec-1).
The Reynolds (Re) and Froude numbers (Fr) were calculated by Eq. 3 and 4, respectively:
where, v is kinematic viscosity (1.01x10-6 m2 sec-1) and g is the gravity acceleration (m sec-2).
The Manning roughness coefficient (n) was calculated as follow:
where, n is manning roughness coefficient (m-1/3 sec), V is mean
surface flow velocity (m sec-1), S is average slope steepness (sine
of slope angle) and R is hydraulics radius (m).
RESULTS AND DISCUSSION
Water and sediment is trapped by rough surface because it contains many barriers
that decrease the flow velocity. Increasing the percentage of rock fragment
cover in this study increased the surface roughness and manning roughness coefficient
(Table 1). Poesen et al. (1990)
and Guo et al. (2010) reported similar results
at flume and field experiments, respectively. Increasing rock fragment cover,
decreased the Froude number but had not significant effect on Reynolds
number (Table 1). Also increasing the rate of flow discharge,
decreased the Froude number and increased the Reynolds number.
The soil critical shear stress values ranged from 0.18 to 1.55 Pa, with an
average of 0.87 Pa. (Fig. 3). A value of soil critical shear
stress for the rock fragment cover treatments was higher than blank. A trend
line added to Fig. 3 for the 6 L min-1 discharge,
indicates a high correlation. Figure 3 indicates that surface
roughnesss and hydraulic forces may be contributed to shear stresses of
increasing rock fragment cover. These results are comparable with some other
investigations conducted under various conditions. Despite differences in soil
type, plot size and methodologies, the critical shear stress values obtained
in this study were either within the range of that of the other reports. For
example, Laflen et al. (1991) studied 56 soil
types and obtained a shear stress range from 0 to 6.64 Pa. Yao
et al. (2008) reported a critical shear stress range from 1.33 to
2.63 Pa for loess soils at the moment of rill initiation.
Distance to rill formation values are presented in Table 2.
The rills developed when soil critical shear stress was exceeded the surface
roughness induced shear resistance. Figure 4 shows that distance
to rill initiation increased with increasing rock fragment cover and decreased
with increasing flow discharge rate. These results are similar to those reported
by Renard et al. (1997) which assumed that rill
erosion is insignificant in slope lengths shorter than 4.5 m at a field.
|| The flow hydraulic properties at different rock fragment
cover and flow discharge
|| Relationships of flow critical shear stress, rock fragment
and flow discharge
|| Relationship between distance to rill initiation and rock
|| Experimental data at point of rill initiation
Table 2 shows quantitatively that the changes in distance to rill initiation were different for varied rock fragment percents and flow discharge rates and that the changes due to the rock fragments were more than that of the flow discharge rates, within the ranges of this experiment. This can be more clearly demonstrated using the relative change rate, defined as:
where, r is the percent of distance change to rill formation, Δli is the change of rill initiation distance because of rock fragment and flow discharge ranges (0 to 30% rock fragment covers in a determined flow discharge rate, or 3 to 9 L min-1 flow discharge rates in a determined rock fragment cover) and L0 is minimum value (belong to distance to rill formation of 0% Rock fragment cover in different flow discharge rates, or 3 L min-1 flow discharge rate in different rock fragment covers. The results of this sensitivity analysis are given in Table 3. The distance to rill formation has a greater sensitivity to rock fragment covers relative to flow discharge rates, so the effect of rock fragment covers on distance to rill initiation (Li) is more significant than that of flow discharge rates.
|| Distance to rill initiation at different rock fragment covers
and flow discharge rates
Since time to rill initiation is the function of soil intrinsic characteristics, morphological conditions of soil surface and runoff, in each experiment, time were recorded when the rill initiation occurred. Table 2 indicates that time to rill initiation, generally increased with increasing the surface rock fragment cover and decreased with increasing flow discharge rate.
The general results of this investigation show that flow discharge rate and rock fragment cover percent, both affect the distance to rill initiation but the effect of rock fragment is more significant. Increased rock fragment cover decreases runoff velocity because of increasing the soil surface roughness and shear resistance. The rill initiation was retarded with increasing the rock fragment cover because of flow velocity and erosivity power. These results are very useful for understanding the mechanisms of rill formation, runoff impact, rock fragment cover effectiveness and help us in conservation practices such as determination of optimum distance for tree planting, channel terraces designing, etc.
These findings are usable for erosion control and soil and water resources conservation in sensitive areas of loess soils.
This study was supported by the Soil Conservation and Watershed Management Research Institute, Tehran and the Soil science department, University of Tehran, Iran. The authors thank Dr. M. Arab Khedri, R. Baiat, A. Zolphaghari, F. Asadzadeh and technical staff the Soil Conservation and Watershed Management Research Institute of Tehran, for their scientific and practical helps.
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