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Effects of checkerboard sand barrier belt on sand transport and dune advance
Aeolian Research 42 (2020) 100546
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Effects of checkerboard sand barrier belt on sand transport and dune advance
T
Li Liua, Tianli Boa,
⁎
a
Key Laboratory of Mechanics on Western Disaster and Environment, Department of Mechanics, Lanzhou University, Lanzhou 730000, PR China
ARTICLE INFO
ABSTRACT
Keywords: Checkerboard sand barrier belt Dune speed Degradation length Wind speed Anti-desertification project
In this paper, experimental and numerical studies have been carried out in the design of the checkerboard sand barrier belt (CSBB) in anti-desertification project. Field observations show that the dune speed after CSBB was significantly reduced when checkerboard sand barriers were laid. The closer to the edge of CSBB, the more obvious the decrease of dune speed after CSBB is, the reduction rate of propagation speed of the edge of desertified land (δ = 1 − v/vno, here, v is the propagation speed of the edge of desertified land with CSBB, vno is the propagation speed of the edge of desertified land without CSBB) is about 0.7 at 200 m after CSBB. When the width and spacing are respectively 400 m and 700 m, CSBB can achieve the full paving effect, which confirms the effectiveness of the laying scheme of “strip patterns” (SP) proposed by Bo and Zheng (2013). Based on the model proposed by Bo and Zheng (2013), the optimal sizes of the spacing and width of CSBB (Smax and W0) were found, and the expression of degradation length (DL) of CSBB with time, annual wind regime and wind speed is given. Moreover, a laying method for CSBB is given, i.e., the width (W) and spacing (S) of CSBB should satisfy W>DL + W0 and S
1. Introduction Desertification is an important environmental issue for humanity. The problem of desertification in China is very serious. As of the end of 2009, the total area of desertified land nationwide was 2,623,700 square kilometers, which accounted for 27.33% of the country’s total land area. It was distributed in 508 counties of 18 provinces, such as Beijing, Tianjin, Gansu and Xinjiang. Among them, 861,200 square kilometers of mobile sand dunes (land) account for 50% of the country’s desertified land area. These sand dunes move to the oasis under the influence of the wind field, resulting in the desertification of the oasis. The desert area in China has grown at an annual rate of more than 1350 square miles, and many oases are disappearing. Therefore, the desertification prevention and control has attracted the attention of the government and scientific researchers. Semi-buried checkerboard sand barrier is a type of mechanical sand fixation commonly used in sand control projects (Qu et al., 2007). It is generally made of barriers such as firewood, straw, clay, and other materials on the sand surface. It is an effective engineering sand-fixing measure for eliminating the intensity of wind speed and fixing sand (Dong et al., 2000; Wang and Zheng, 2002; Gao et al., 2004). The purpose of the checkerboard sand barrier is to reduce the erosion of ⁎
sand particles on the surface. In general, the laying of checkerboard sand barriers is to be used in conjunction with the planting of vegetation (Li et al., 2004; Zhang et al., 2004). That is, first, checkerboard sand barriers are used to achieve sand fixation, and vegetation is then planted. During the growth of vegetation, the checkerboard sand barriers can ensure the normal growth of the vegetation. When the vegetation grows, it can achieve the effect of sand fixation. Ultimately, longterm control of desert expansion can be achieved. The existing studies on checkerboard sand barriers are focused on the following aspects: (1) the mechanisms by which these barriers provide protection (Zhang et al., 2010; Liu et al., 2011; Huang et al., 2013); (2) erosion and deposition in checkerboard barriers (Ding et al., 2009; Tian et al., 2015; Wang et al., 2013a; Wang et al., 2013b); (3) concave surface characteristics (Wang and Zheng, 2002; Zhou et al., 2009); (4) wind-blown sand and near-surface wind regime (Zhang et al., 2006; Qu et al., 2007; Zhang et al., 2012; Zhang et al., 2016); (5) the ecological restoration functions of the sand-barriers (Li and Lei, 2003; Qiu et al., 2004). These studies have achieved encouraging results. For example, Field observations of Qu et al. (2007) show that to ensure the formation of a vortex inside the grid the protective materials exposed above the sand bed must have a certain elasticity and permeability to air, and 1.0 × 1.0, 1.5 × 1.5, and 2.0 × 2.0 m sand barriers have good sand-
Corresponding author. E-mail address: [email protected] (T. Bo).
https://doi.org/10.1016/j.aeolia.2019.100546 Received 17 May 2019; Received in revised form 12 September 2019; Accepted 12 September 2019 1875-9637/ © 2019 Elsevier B.V. All rights reserved.
Aeolian Research 42 (2020) 100546
L. Liu and T. Bo
control effects. Field observations of Zhang et al. (2016) find that straw checkerboards make the aerodynamic roughness length increased, which is two to three orders of magnitude higher than the value of the bare sand bed. The sand flux profiles above these barriers resembled an ‘‘elephant trunk”, with maximum sand flux at 0.05–0.2 m above the bed, in contrast with the continuously and rapidly decreasing sand flux with increasing height above the bare sand. Numerical simulation of Huang et al. (2013) show that there are a series of unevenly distributed eddies in the checkerboard barrier, their strength decreases gradually with the increase of the transverse distance. Sand particles carried by the flow field deposits in the checkerboard barrier, and forming a vshaped sand trough, i.e., sand particles tend to deposit on two walls of checkerboard barrier. However, there have been relatively few studies on the checkerboard sand barrier belt (CSBB). For example, Xu et al. (1982) measured wind speed along the straw checkerboard barrier belt in the sides of the Baotou–Lanzhou railway at ShaPoTou desert region in China. Their results show that wind speed rapidly decreases along the wind direction and then becomes stable. Bo et al. (2015) studied the spatial distribution of wind speed along a straw checkerboard barrier belt by computational fluid dynamics. Their results show that the spatial distribution of wind speed along a straw checkerboard sand barrier belt can be divided into descent stage, stable stage and recovery stage, and the wind speed profiles satisfy three different log-linear functions. Bo and Zheng (2013) studied a straw checkerboard sand barrier belt how to affect the propagation speed at the edge of desertified land, where the effect of checkerboard sand barriers on the erosion of sand particles was introduced into the scale-coupled model of dune fields. The results revealed that due to the effect of the checkerboard sand barriers, the sand particles movement to the downwind direction of CSBB was limited. As a result, the propagation speed of the dune field at the downwind side is reduced. CSBB also affect the upwind side of the area, so sand particles on the surface can also be fixed to a certain extent. Therefore, vegetation can also be planted in this area. Based on this result, Bo and Zheng (2013) proposed a laying method of “strip patterns” (that is, there is no need to lay checkerboard sand barriers in a certain range between the two checkerboard sand barrier belts) instead of all laying methods for the laying of checkerboard sand barriers. From the above introduction, it can be seen that the effectiveness of the method of “strip patterns” for the laying of CSBB proposed by Bo and Zheng (2013) requires further experimental verification. Moreover, they only studied the width of the straw checkerboard area, and the distance between the straw checkerboard area in the case of frictional wind speed of 0.5 m/s, and the sand burial of CSBB has not been considered. In other words, how to determine the width and spacing of CSBB based on wind speed, duration and other conditions in the actual sand control process. A brief introduction of the model is given in Section 2. In order to understand these issues, the field experiment was carried out to study the effectiveness of the scheme of “strip patterns” for the laying of CSBB, as shown in Section 3. Furthermore, the variation of the propagation speed of desertified land and the degradation length of CSBB with frictional wind speed were studied based on the model proposed by Bo and Zheng (2013), see Section 4. The main conclusions are given in Section 5.
checkerboard barrier, i.e., Bn,ij equal to 1 indicates that there is a checkerboard sand barrier, and B equal to 0 indicates that there is no checkerboard sand barrier. BHn,ij denotes the height of the checkerboard sand barrier. Here, Hn,ij is the initial thickness of the sand bed. n is the time step, and ij is spatial location. 2) In the section of the calculation of the windblown sand flux for the eroded ‘SBE’: Windblown sand flux in CSBB and behind CSBB can be calculated by formula (15) in Bo et al. (2015), i.e.,
Qstraw = A u (u 2 g
(u , t / Rt ) 2)
(1)
Here, Rt = u*1/u*, u* is friction wind velocity of the inlet wind flow (namely, the flow before the belt which hasn’t been affected by CSBB), u*1 denotes friction velocity of wind near-surface. The calculation of u*1 along CSBB and behind CSBB see in Bo et al. (2015). The corresponding sand particles’ average saltation length l¯n, ij , saltation time t¯n, ij , and the average velocities v¯n, ij of impacting sand particles in steady-state windblown flux during Tn for the eroded ‘SBE’ are calculated. It should be noted that this process is different from Bo and Zheng (2013), which is closer to reality. 3) Evaluation of the thickness and the transportation length of the eroded ‘sand body element’ is the same with Bo and Zheng (2011). 4) In the section of the transportation and the relative position of the ‘sand body element’: If windblown sand movement makes the barrier buried, namely, once the thickness of deposition sand on the surface is larger than the height of sand barrier BHn,ij, the barrier will be buried, thereby the Bn,ij is zero. The processing of other processes is consistent with Bo and Zheng (2013). An open boundary condition was used to simulation, i.e., if an eroded ‘SBE’ leaves from the boundary of simulation area, it will be deleted from the simulation. 3. Experimental verification for the laying scheme of “strip patterns” In order to verify the effectiveness of the laying scheme of “strip patterns” (SP) proposed by Bo and Zheng (2013). Field experiments were carried out at Minqin, China in 2015 and 2016. The experiment in 2015 was mainly aimed at the feasibility of the laying scheme of SP for CSBB. In the experiments, we compared the windblown sand flux at the 5 m, 50 m, 100 m and 200 m behind CSBB under four different conditions. The four operating conditions are as follows (see Fig. 1): (1) Placing sand barriers in the 320 m area; (2) The width of CSBB is 100 m, and the spacing of CSBB is 120 m; (3) The width of CSBB is 100 m and the spacing of CSBB is 60 m; (4) No checkerboard sand barriers are laid in the area of 320 m. The length of CSBB is 100 m, and the material of checkerboard sand barriers is a nylon mesh. The actual photograph is shown in Fig. 1. According to the experimental data, the ratios between the windblown sand flux under the first three conditions and the windblown sand flux under the fourth condition at different downwind positions are given. The results are shown in Fig. 2. The relationship between dune speeds (vd) and dune heights (h) can be obtained from the advection equation and the mass conservation equation, i.e., vd = q/ (ρdh), Here, q is windblown sand flux and ρd is the density of sand surface materials. From this, it can be known that the dune speed is proportional to the intensity of windblown sand flux. Therefore, it can be seen that the ratio of windblown sand flux in Fig. 2 is approximately equal to the ratio of dune speed, i.e.,
2. An introduction to model Bo and Zheng (2013) realized the simulation of the propagation of desertified land based on the improvement of the scale-coupled model of dune fields which consider the influence of CSBB. In this paper, the spatial variation of wind speed along a CSBB and the sand barrier burial were introduced in the model. The following is a summary of the changes to the Bo and Zheng (2013) model:
q /( h) q vd1 = 1 d = 1 q2 /( d h) q2 vd2
(2)
The results in Fig. 2 show that if the checkerboard sand barriers were laid, the dune speed after CSBB was significantly reduced. Moreover, the closer to the edge of CSBB, the more obvious the
1) In the section of the discretization of the sand bed: For every single ‘sand body element’ (SBE), Bn,ij is used to describe whether there is a 2
Aeolian Research 42 (2020) 100546
L. Liu and T. Bo
Fig. 1. Feasibility experiment on the laying scheme of “strip patterns” in 2015. (a) schematic diagrams of experiments in the field under the three working conditions. In the first case, the sand barriers were laid in the 320 m area; in the second case, the width of CSBB is 100 m, and the spacing of CSBB is 120 m; in the third case, the width of CSBB is 100 m and the spacing of CSBB is 60 m. (b) and (c) are actual photographs in field experiments.
Fig. 3. The ratio of the windblown sand flux before and after laying CSBB at different positions when the width (W) and the spacing (S) of CSBB are respectively 400 m and 700 m. Here, the error bars indicate the uncertainty of the experimental results, i.e., the range of variation of the experimental results. The uncertainties are obtained from 3 sets of experimental results.
Fig. 2. The ratio of the windblown sand flux before and after laying CSBB at different locations under three conditions. Where, the abscissa represents 3 working conditions, as shown in Fig. 1. Here, the error bars indicate the uncertainty of the experimental results, i.e., the range of variation of the experimental results. The uncertainties are obtained from 4 sets of experimental results.
results. It indicated that the uncertainty of experimental results does not affect the conclusions reached in this paper, i.e., the laying scheme of SP is feasible. In summary, the results of field experiments confirm the effectiveness of the laying scheme of SP proposed by Bo and Zheng (2013). The reason, why the propagation of desertified land in the middle region of the two CSBB can be weakened, may be that the laying of CSBB limits the sand supplies which move to the downwind direction of CSBB, thereby affecting the propagation of desertified land. At the same time, the results of the field observations in this paper also indicate to a certain extent that the dune field model proposed by Bo and Zheng (2013) considering the effect of the checkerboard sand barriers is effective.
decrease of dune speed is, with a reduction of about 30% at 200 m, i.e., the reduction rate of propagation speed of the edge of desertified land (δ) is about 0.7, in which the δ is defined as 1 − v/vno, and it is a dimensionless parameter, here, v is the propagation speed of the edge of desertified land with CSBB, vno is the propagation speed of the edge of desertified land without CSBB. The larger the δ, the more obvious the attenuation of the propagation speed of the edge of desertified land. This shows that the laying scheme of SP proposed by Bo and Zheng (2013) is feasible. In Fig. 3, the ratio of the windblown sand flux before and after laying CSBB is given when the width (W) and the spacing (S) of CSBB are respectively 400 m and 700 m. The length of CSBB is 200 m. A field experiment was conducted in 2016. From Fig. 3, it can be seen that the variation of the ratio of the windblown sand flux along the downwind distance is consistent with the results of Fig. 2 qualitatively and quantitatively. It indicates that the width and spacing of CSBB given in Bo and Zheng (2013) can achieve the full paving effect. It should be noted that the variation of the ratio of the windblown sand flux before and after laying CSBB is larger than the uncertainty of experimental
4. Results Bo and Zheng (2013) proposed a laying scheme of SP instead of continuous laying for checkerboard sand barriers, and the width and spacing of CSBB with friction velocity of 0.5 m/s are discussed. However, in the actual laying process of checkerboard sand barriers, not 3
Aeolian Research 42 (2020) 100546
L. Liu and T. Bo
Table 1 Smax under different frictional wind speed conditions. u*(m/s) Smax(km)
0.35 0.7
0.4 0.7
0.45 0.7
0.5 0.7
0.55 0.72
0.6 0.72
0.65 0.72
Fig. 4. The variation of the δ with the spacing of CSBB under different frictional wind speeds.
only the effect of frictional wind speed on the width and spacing of CSBB must be considered, but also the degradation of CSBB, which is mainly induced by sand burial. Therefore, in order to be able to better guide the actual sand prevention and desertification engineering measures, the variation of the width and spacing of CSBB with wind speed, and the variation of the degeneration length of CSBB with wind speed and time were analyzed. Based on these results, a method for selecting the size of CSBB in the form of SP was proposed.
Fig. 6. The variation of the δ with the width of CSBB under different frictional wind speeds.
case of S < Smax, and can save the laying loss, that is, costs and time spent on the area within the region Smax-S can all be saved. Furthermore, it can be seen from Fig. 4 that when the spacing of CSBB is greater than 1.6 km, the checkerboard sand barriers have no effect on the propagation speed of the edge of desertified land, i.e., the effect of checkerboard sand barriers can be ignored.
4.1. The spacing of CSBB (S) The study on the spacing of CSBB was carried out using the numerical model. Using this model, the variation of the δ with the spacing of CSBB under different frictional wind speeds is obtained, as shown in Fig. 4. It can be seen from the Fig. 4 that the greater the wind speed is, the smaller the δ is, and the δ decreases exponentially as the spacing of CSBB increases, and it can be expressed by the following formula
=
A1 1 + e (S 990)/81
4.2. The width of CSBB (W) The study on the width of CSBB was carried out using the numerical model. Using this model, the variation of the δ with the width of CSBB under different frictional wind speeds is obtained, as shown in Fig. 6. It can be seen from the Fig. 6 that the greater the wind speed is, the smaller the δ is, and the δ increases exponentially as the width of CSBB increases, and it can be expressed by the following formula
(3)
Here, A1 is the steady reduction rate, which changes with the frictional wind speed, as shown in Fig. 5. A1 can be expressed by
A1 = 1
0.00016e (u
/0.08)
= A1(1
(4)
10e
(5)
W /82)
It can be seen from Fig. 6 that when the width of CSBB is less than the critical width (W0), the δ increases rapidly, but when the width of CSBB is larger than W0, the δ does not change significantly. The relationship between W0 and friction velocity is shown in Table 2. In other words, W0 is the optimal size of the width of CSBB, which can achieve the results of CSBB in the case of W > W0, and can save the laying loss, that is, costs and time spent on the area within the region W–W0 can all be saved.
At the same time, it can be seen from Fig. 4 that when the distance between two CSBB is greater than the critical distance (Smax), the δ decreases rapidly, but when the spacing of CSBB is less than Smax, the δ does not change significantly. The relationship between Smax and friction velocity is shown in Table 1. In other words, Smax is the optimal size of the spacing of CSBB, which can achieve the results of CSBB in the
4.3. Degradation length of CSBB For the study of degradation of CSBB, the sand burial of CSBB was introduced in Bo and Zheng (2013). That is, when the sediment volume at the position of the checkerboard sand barrier is higher than the height of the checkerboard sand barrier, the sand barrier will lose its effectiveness, and the degradation length of CSBB can be determined. Fig. 7 shows the variation of the degradation length of CSBB (DL) with time at different wind speeds. It can be seen from Fig. 7 that the larger Table 2 W0 under different frictional wind speed conditions. u*(m/s) W0(km)
Fig. 5. The variation of steady reduction rate A1 with wind speed. 4
0.35 0.27
0.4 0.27
0.45 0.27
0.5 0.28
0.55 0.28
0.6 0.30
0.65 0.30
Aeolian Research 42 (2020) 100546
L. Liu and T. Bo
4.4. Setting method of CSBB The laying process of CSBB needs to consider many factors, including the usage time of CSBB, the degradation length of CSBB, the inflow velocity, and the width and spacing of CSBB. In other words, in determining the width of CSBB, in addition to determining the propagation speed of the edge of desertified land based on the incoming wind speed, the degradation length of CSBB must be determined according to the usage time. Therefore, the final width of CSBB should be represented as
W> DL + W0
(8)
Fig. 9 shows the phase diagrams of the width of CSBB and the frictional wind speed under different usage times. According to the phase diagram, the width of CSBB used to achieve some protective effect under different frictional wind speed conditions and usage times can be determined. For example, when the usage time of CSBB is 3 years and the local frictional wind speed is 0.45 m/s, if the protective effect to be achieved is that the δ is greater than 0.8, and then we know that the width of CSBB should be greater than 360 m according to Fig. 9. In determining the spacing of CSBB, in addition to determining the propagation speed of the edge of desertified land based on the incoming wind speed, the degradation length of CSBB must be determined according to the usage time. Therefore, the final spacing of CSBB should be represented as
Fig. 7. The variation of the degradation length of CSBB (DL) with time at different wind speeds (φ = 5000 h).
S
DL
(9)
Fig. 9 The phase diagrams of the spacing of CSBB and the frictional wind speed under different usage times. The target variable is δ. 5. Conclusions Desertification is an important environmental issue affecting people’s production and life. Among them, the desert expansion prevention is a scientific and engineering problem that needs urgent solution. The checkerboard sand barrier is commonly used measures in the sand control project. The in-depth study of checkerboard sand barrier will help people to optimize the design of checkerboard sand barriers, which can achieve the purpose of saving laying costs and laying time. In response to this research topic, field observations were carried out on the influence of CSBB on desert expansion. Through the analysis of field observation data, it was found that if checkerboard sand barriers were laid, the dune speed after CSBB was significantly reduced. Moreover, the closer to the edge of CSBB, the more obvious the decrease of dune speed is, with a reduction of about 30% at 200 m, i.e., the reduction rate of propagation speed of the edge of desertified land (δ = 1 − v/vno, here, v is the propagation speed of the edge of desertified land with CSBB, vno is the propagation speed of the edge of desertified land
Fig. 8. The variation of parameter y1 with wind speed.
the wind speed is, the longer the degradation length is, and that it increases approximately exponentially with time, which can be expressed by the following equation
DL = (y1
215e
T /6.15)·(
/365)
(6)
Here, T is the time (in years) and φ is the duration of the wind in each year (in hours). The relationship between parameter y1 and friction velocity (u*) is shown in Fig. 8, which can be expressed by
y1 = 329
(1.12 × 105) e
u /0.06
(7)
Fig. 9. The phase diagrams of the width of CSBB and the frictional wind speed under different usage times. The target variable is δ. 5
Aeolian Research 42 (2020) 100546
L. Liu and T. Bo
Fig. 10. shows the phase diagrams of the spacing of CSBB and the frictional wind speed under different usage times. According to the phase diagram, the spacing of CSBB used to achieve some protective effect under different frictional wind speed conditions and usage times can be determined. For example, when the usage time of CSBB is 3 years and the local frictional wind speed is 0.45 m/s, if the protective effect to be achieved is that the δ is greater than 0.8, and then we know that the spacing of CSBB should be less than 680 m according to Fig. 10.
without CSBB.) is about 0.7. When the width and spacing are respectively 400 m and 700 m, CSBB can achieve the full paving effect, which confirms the effectiveness of the laying scheme of SP proposed by Bo and Zheng (2013), and indicate to a certain extent that the dune field model proposed by Bo and Zheng (2013) considering the effect of the checkerboard sand barriers is effective. At the same time, an in-depth study on the laying scheme of checkerboard sand barriers was carried out by improving the model proposed by Bo and Zheng (2013). The calculation results show that there is an optimal size (Smax) of the spacing of the CSBB, and the effect of CSBB is approximately the same in the case of S < Smax. There is an optimal size of the width (W0) of CSBB, and the effect of CSBB is approximately the same in the case of W > W0. Furthermore, the expression of degradation length of CSBB with time, annual wind regime and wind speed is given, i.e., DL = (y1 215e T /6.15)·( /365) . Based on the calculation results, a laying method for CSBB is given, i.e., the width and spacing of CSBB should satisfy W> DL + W0 and S
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was supported by a grant from the National Natural Science Foundation of China (No. 11490551), the authors express their sincere appreciation to the support. References Bo, T.L., Ma, P., Zheng, X.J., 2015. Numerical study on the effect of semi-buried straw checkerboard sand barriers belt on the wind speed. Aeolian Res. 16, 101–107. Bo, T.L., Zheng, X.J., 2011. Bulk transportation of sand particles in quantitative simulations of dune field evolution. Powder Technol. 214, 243–251.
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Effects of checkerboard sand barrier belt on sand transport and dune advance
T
Li Liua, Tianli Boa,
⁎
a
Key Laboratory of Mechanics on Western Disaster and Environment, Department of Mechanics, Lanzhou University, Lanzhou 730000, PR China
ARTICLE INFO
ABSTRACT
Keywords: Checkerboard sand barrier belt Dune speed Degradation length Wind speed Anti-desertification project
In this paper, experimental and numerical studies have been carried out in the design of the checkerboard sand barrier belt (CSBB) in anti-desertification project. Field observations show that the dune speed after CSBB was significantly reduced when checkerboard sand barriers were laid. The closer to the edge of CSBB, the more obvious the decrease of dune speed after CSBB is, the reduction rate of propagation speed of the edge of desertified land (δ = 1 − v/vno, here, v is the propagation speed of the edge of desertified land with CSBB, vno is the propagation speed of the edge of desertified land without CSBB) is about 0.7 at 200 m after CSBB. When the width and spacing are respectively 400 m and 700 m, CSBB can achieve the full paving effect, which confirms the effectiveness of the laying scheme of “strip patterns” (SP) proposed by Bo and Zheng (2013). Based on the model proposed by Bo and Zheng (2013), the optimal sizes of the spacing and width of CSBB (Smax and W0) were found, and the expression of degradation length (DL) of CSBB with time, annual wind regime and wind speed is given. Moreover, a laying method for CSBB is given, i.e., the width (W) and spacing (S) of CSBB should satisfy W>DL + W0 and S
1. Introduction Desertification is an important environmental issue for humanity. The problem of desertification in China is very serious. As of the end of 2009, the total area of desertified land nationwide was 2,623,700 square kilometers, which accounted for 27.33% of the country’s total land area. It was distributed in 508 counties of 18 provinces, such as Beijing, Tianjin, Gansu and Xinjiang. Among them, 861,200 square kilometers of mobile sand dunes (land) account for 50% of the country’s desertified land area. These sand dunes move to the oasis under the influence of the wind field, resulting in the desertification of the oasis. The desert area in China has grown at an annual rate of more than 1350 square miles, and many oases are disappearing. Therefore, the desertification prevention and control has attracted the attention of the government and scientific researchers. Semi-buried checkerboard sand barrier is a type of mechanical sand fixation commonly used in sand control projects (Qu et al., 2007). It is generally made of barriers such as firewood, straw, clay, and other materials on the sand surface. It is an effective engineering sand-fixing measure for eliminating the intensity of wind speed and fixing sand (Dong et al., 2000; Wang and Zheng, 2002; Gao et al., 2004). The purpose of the checkerboard sand barrier is to reduce the erosion of ⁎
sand particles on the surface. In general, the laying of checkerboard sand barriers is to be used in conjunction with the planting of vegetation (Li et al., 2004; Zhang et al., 2004). That is, first, checkerboard sand barriers are used to achieve sand fixation, and vegetation is then planted. During the growth of vegetation, the checkerboard sand barriers can ensure the normal growth of the vegetation. When the vegetation grows, it can achieve the effect of sand fixation. Ultimately, longterm control of desert expansion can be achieved. The existing studies on checkerboard sand barriers are focused on the following aspects: (1) the mechanisms by which these barriers provide protection (Zhang et al., 2010; Liu et al., 2011; Huang et al., 2013); (2) erosion and deposition in checkerboard barriers (Ding et al., 2009; Tian et al., 2015; Wang et al., 2013a; Wang et al., 2013b); (3) concave surface characteristics (Wang and Zheng, 2002; Zhou et al., 2009); (4) wind-blown sand and near-surface wind regime (Zhang et al., 2006; Qu et al., 2007; Zhang et al., 2012; Zhang et al., 2016); (5) the ecological restoration functions of the sand-barriers (Li and Lei, 2003; Qiu et al., 2004). These studies have achieved encouraging results. For example, Field observations of Qu et al. (2007) show that to ensure the formation of a vortex inside the grid the protective materials exposed above the sand bed must have a certain elasticity and permeability to air, and 1.0 × 1.0, 1.5 × 1.5, and 2.0 × 2.0 m sand barriers have good sand-
Corresponding author. E-mail address: [email protected] (T. Bo).
https://doi.org/10.1016/j.aeolia.2019.100546 Received 17 May 2019; Received in revised form 12 September 2019; Accepted 12 September 2019 1875-9637/ © 2019 Elsevier B.V. All rights reserved.
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control effects. Field observations of Zhang et al. (2016) find that straw checkerboards make the aerodynamic roughness length increased, which is two to three orders of magnitude higher than the value of the bare sand bed. The sand flux profiles above these barriers resembled an ‘‘elephant trunk”, with maximum sand flux at 0.05–0.2 m above the bed, in contrast with the continuously and rapidly decreasing sand flux with increasing height above the bare sand. Numerical simulation of Huang et al. (2013) show that there are a series of unevenly distributed eddies in the checkerboard barrier, their strength decreases gradually with the increase of the transverse distance. Sand particles carried by the flow field deposits in the checkerboard barrier, and forming a vshaped sand trough, i.e., sand particles tend to deposit on two walls of checkerboard barrier. However, there have been relatively few studies on the checkerboard sand barrier belt (CSBB). For example, Xu et al. (1982) measured wind speed along the straw checkerboard barrier belt in the sides of the Baotou–Lanzhou railway at ShaPoTou desert region in China. Their results show that wind speed rapidly decreases along the wind direction and then becomes stable. Bo et al. (2015) studied the spatial distribution of wind speed along a straw checkerboard barrier belt by computational fluid dynamics. Their results show that the spatial distribution of wind speed along a straw checkerboard sand barrier belt can be divided into descent stage, stable stage and recovery stage, and the wind speed profiles satisfy three different log-linear functions. Bo and Zheng (2013) studied a straw checkerboard sand barrier belt how to affect the propagation speed at the edge of desertified land, where the effect of checkerboard sand barriers on the erosion of sand particles was introduced into the scale-coupled model of dune fields. The results revealed that due to the effect of the checkerboard sand barriers, the sand particles movement to the downwind direction of CSBB was limited. As a result, the propagation speed of the dune field at the downwind side is reduced. CSBB also affect the upwind side of the area, so sand particles on the surface can also be fixed to a certain extent. Therefore, vegetation can also be planted in this area. Based on this result, Bo and Zheng (2013) proposed a laying method of “strip patterns” (that is, there is no need to lay checkerboard sand barriers in a certain range between the two checkerboard sand barrier belts) instead of all laying methods for the laying of checkerboard sand barriers. From the above introduction, it can be seen that the effectiveness of the method of “strip patterns” for the laying of CSBB proposed by Bo and Zheng (2013) requires further experimental verification. Moreover, they only studied the width of the straw checkerboard area, and the distance between the straw checkerboard area in the case of frictional wind speed of 0.5 m/s, and the sand burial of CSBB has not been considered. In other words, how to determine the width and spacing of CSBB based on wind speed, duration and other conditions in the actual sand control process. A brief introduction of the model is given in Section 2. In order to understand these issues, the field experiment was carried out to study the effectiveness of the scheme of “strip patterns” for the laying of CSBB, as shown in Section 3. Furthermore, the variation of the propagation speed of desertified land and the degradation length of CSBB with frictional wind speed were studied based on the model proposed by Bo and Zheng (2013), see Section 4. The main conclusions are given in Section 5.
checkerboard barrier, i.e., Bn,ij equal to 1 indicates that there is a checkerboard sand barrier, and B equal to 0 indicates that there is no checkerboard sand barrier. BHn,ij denotes the height of the checkerboard sand barrier. Here, Hn,ij is the initial thickness of the sand bed. n is the time step, and ij is spatial location. 2) In the section of the calculation of the windblown sand flux for the eroded ‘SBE’: Windblown sand flux in CSBB and behind CSBB can be calculated by formula (15) in Bo et al. (2015), i.e.,
Qstraw = A u (u 2 g
(u , t / Rt ) 2)
(1)
Here, Rt = u*1/u*, u* is friction wind velocity of the inlet wind flow (namely, the flow before the belt which hasn’t been affected by CSBB), u*1 denotes friction velocity of wind near-surface. The calculation of u*1 along CSBB and behind CSBB see in Bo et al. (2015). The corresponding sand particles’ average saltation length l¯n, ij , saltation time t¯n, ij , and the average velocities v¯n, ij of impacting sand particles in steady-state windblown flux during Tn for the eroded ‘SBE’ are calculated. It should be noted that this process is different from Bo and Zheng (2013), which is closer to reality. 3) Evaluation of the thickness and the transportation length of the eroded ‘sand body element’ is the same with Bo and Zheng (2011). 4) In the section of the transportation and the relative position of the ‘sand body element’: If windblown sand movement makes the barrier buried, namely, once the thickness of deposition sand on the surface is larger than the height of sand barrier BHn,ij, the barrier will be buried, thereby the Bn,ij is zero. The processing of other processes is consistent with Bo and Zheng (2013). An open boundary condition was used to simulation, i.e., if an eroded ‘SBE’ leaves from the boundary of simulation area, it will be deleted from the simulation. 3. Experimental verification for the laying scheme of “strip patterns” In order to verify the effectiveness of the laying scheme of “strip patterns” (SP) proposed by Bo and Zheng (2013). Field experiments were carried out at Minqin, China in 2015 and 2016. The experiment in 2015 was mainly aimed at the feasibility of the laying scheme of SP for CSBB. In the experiments, we compared the windblown sand flux at the 5 m, 50 m, 100 m and 200 m behind CSBB under four different conditions. The four operating conditions are as follows (see Fig. 1): (1) Placing sand barriers in the 320 m area; (2) The width of CSBB is 100 m, and the spacing of CSBB is 120 m; (3) The width of CSBB is 100 m and the spacing of CSBB is 60 m; (4) No checkerboard sand barriers are laid in the area of 320 m. The length of CSBB is 100 m, and the material of checkerboard sand barriers is a nylon mesh. The actual photograph is shown in Fig. 1. According to the experimental data, the ratios between the windblown sand flux under the first three conditions and the windblown sand flux under the fourth condition at different downwind positions are given. The results are shown in Fig. 2. The relationship between dune speeds (vd) and dune heights (h) can be obtained from the advection equation and the mass conservation equation, i.e., vd = q/ (ρdh), Here, q is windblown sand flux and ρd is the density of sand surface materials. From this, it can be known that the dune speed is proportional to the intensity of windblown sand flux. Therefore, it can be seen that the ratio of windblown sand flux in Fig. 2 is approximately equal to the ratio of dune speed, i.e.,
2. An introduction to model Bo and Zheng (2013) realized the simulation of the propagation of desertified land based on the improvement of the scale-coupled model of dune fields which consider the influence of CSBB. In this paper, the spatial variation of wind speed along a CSBB and the sand barrier burial were introduced in the model. The following is a summary of the changes to the Bo and Zheng (2013) model:
q /( h) q vd1 = 1 d = 1 q2 /( d h) q2 vd2
(2)
The results in Fig. 2 show that if the checkerboard sand barriers were laid, the dune speed after CSBB was significantly reduced. Moreover, the closer to the edge of CSBB, the more obvious the
1) In the section of the discretization of the sand bed: For every single ‘sand body element’ (SBE), Bn,ij is used to describe whether there is a 2
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Fig. 1. Feasibility experiment on the laying scheme of “strip patterns” in 2015. (a) schematic diagrams of experiments in the field under the three working conditions. In the first case, the sand barriers were laid in the 320 m area; in the second case, the width of CSBB is 100 m, and the spacing of CSBB is 120 m; in the third case, the width of CSBB is 100 m and the spacing of CSBB is 60 m. (b) and (c) are actual photographs in field experiments.
Fig. 3. The ratio of the windblown sand flux before and after laying CSBB at different positions when the width (W) and the spacing (S) of CSBB are respectively 400 m and 700 m. Here, the error bars indicate the uncertainty of the experimental results, i.e., the range of variation of the experimental results. The uncertainties are obtained from 3 sets of experimental results.
Fig. 2. The ratio of the windblown sand flux before and after laying CSBB at different locations under three conditions. Where, the abscissa represents 3 working conditions, as shown in Fig. 1. Here, the error bars indicate the uncertainty of the experimental results, i.e., the range of variation of the experimental results. The uncertainties are obtained from 4 sets of experimental results.
results. It indicated that the uncertainty of experimental results does not affect the conclusions reached in this paper, i.e., the laying scheme of SP is feasible. In summary, the results of field experiments confirm the effectiveness of the laying scheme of SP proposed by Bo and Zheng (2013). The reason, why the propagation of desertified land in the middle region of the two CSBB can be weakened, may be that the laying of CSBB limits the sand supplies which move to the downwind direction of CSBB, thereby affecting the propagation of desertified land. At the same time, the results of the field observations in this paper also indicate to a certain extent that the dune field model proposed by Bo and Zheng (2013) considering the effect of the checkerboard sand barriers is effective.
decrease of dune speed is, with a reduction of about 30% at 200 m, i.e., the reduction rate of propagation speed of the edge of desertified land (δ) is about 0.7, in which the δ is defined as 1 − v/vno, and it is a dimensionless parameter, here, v is the propagation speed of the edge of desertified land with CSBB, vno is the propagation speed of the edge of desertified land without CSBB. The larger the δ, the more obvious the attenuation of the propagation speed of the edge of desertified land. This shows that the laying scheme of SP proposed by Bo and Zheng (2013) is feasible. In Fig. 3, the ratio of the windblown sand flux before and after laying CSBB is given when the width (W) and the spacing (S) of CSBB are respectively 400 m and 700 m. The length of CSBB is 200 m. A field experiment was conducted in 2016. From Fig. 3, it can be seen that the variation of the ratio of the windblown sand flux along the downwind distance is consistent with the results of Fig. 2 qualitatively and quantitatively. It indicates that the width and spacing of CSBB given in Bo and Zheng (2013) can achieve the full paving effect. It should be noted that the variation of the ratio of the windblown sand flux before and after laying CSBB is larger than the uncertainty of experimental
4. Results Bo and Zheng (2013) proposed a laying scheme of SP instead of continuous laying for checkerboard sand barriers, and the width and spacing of CSBB with friction velocity of 0.5 m/s are discussed. However, in the actual laying process of checkerboard sand barriers, not 3
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Table 1 Smax under different frictional wind speed conditions. u*(m/s) Smax(km)
0.35 0.7
0.4 0.7
0.45 0.7
0.5 0.7
0.55 0.72
0.6 0.72
0.65 0.72
Fig. 4. The variation of the δ with the spacing of CSBB under different frictional wind speeds.
only the effect of frictional wind speed on the width and spacing of CSBB must be considered, but also the degradation of CSBB, which is mainly induced by sand burial. Therefore, in order to be able to better guide the actual sand prevention and desertification engineering measures, the variation of the width and spacing of CSBB with wind speed, and the variation of the degeneration length of CSBB with wind speed and time were analyzed. Based on these results, a method for selecting the size of CSBB in the form of SP was proposed.
Fig. 6. The variation of the δ with the width of CSBB under different frictional wind speeds.
case of S < Smax, and can save the laying loss, that is, costs and time spent on the area within the region Smax-S can all be saved. Furthermore, it can be seen from Fig. 4 that when the spacing of CSBB is greater than 1.6 km, the checkerboard sand barriers have no effect on the propagation speed of the edge of desertified land, i.e., the effect of checkerboard sand barriers can be ignored.
4.1. The spacing of CSBB (S) The study on the spacing of CSBB was carried out using the numerical model. Using this model, the variation of the δ with the spacing of CSBB under different frictional wind speeds is obtained, as shown in Fig. 4. It can be seen from the Fig. 4 that the greater the wind speed is, the smaller the δ is, and the δ decreases exponentially as the spacing of CSBB increases, and it can be expressed by the following formula
=
A1 1 + e (S 990)/81
4.2. The width of CSBB (W) The study on the width of CSBB was carried out using the numerical model. Using this model, the variation of the δ with the width of CSBB under different frictional wind speeds is obtained, as shown in Fig. 6. It can be seen from the Fig. 6 that the greater the wind speed is, the smaller the δ is, and the δ increases exponentially as the width of CSBB increases, and it can be expressed by the following formula
(3)
Here, A1 is the steady reduction rate, which changes with the frictional wind speed, as shown in Fig. 5. A1 can be expressed by
A1 = 1
0.00016e (u
/0.08)
= A1(1
(4)
10e
(5)
W /82)
It can be seen from Fig. 6 that when the width of CSBB is less than the critical width (W0), the δ increases rapidly, but when the width of CSBB is larger than W0, the δ does not change significantly. The relationship between W0 and friction velocity is shown in Table 2. In other words, W0 is the optimal size of the width of CSBB, which can achieve the results of CSBB in the case of W > W0, and can save the laying loss, that is, costs and time spent on the area within the region W–W0 can all be saved.
At the same time, it can be seen from Fig. 4 that when the distance between two CSBB is greater than the critical distance (Smax), the δ decreases rapidly, but when the spacing of CSBB is less than Smax, the δ does not change significantly. The relationship between Smax and friction velocity is shown in Table 1. In other words, Smax is the optimal size of the spacing of CSBB, which can achieve the results of CSBB in the
4.3. Degradation length of CSBB For the study of degradation of CSBB, the sand burial of CSBB was introduced in Bo and Zheng (2013). That is, when the sediment volume at the position of the checkerboard sand barrier is higher than the height of the checkerboard sand barrier, the sand barrier will lose its effectiveness, and the degradation length of CSBB can be determined. Fig. 7 shows the variation of the degradation length of CSBB (DL) with time at different wind speeds. It can be seen from Fig. 7 that the larger Table 2 W0 under different frictional wind speed conditions. u*(m/s) W0(km)
Fig. 5. The variation of steady reduction rate A1 with wind speed. 4
0.35 0.27
0.4 0.27
0.45 0.27
0.5 0.28
0.55 0.28
0.6 0.30
0.65 0.30
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4.4. Setting method of CSBB The laying process of CSBB needs to consider many factors, including the usage time of CSBB, the degradation length of CSBB, the inflow velocity, and the width and spacing of CSBB. In other words, in determining the width of CSBB, in addition to determining the propagation speed of the edge of desertified land based on the incoming wind speed, the degradation length of CSBB must be determined according to the usage time. Therefore, the final width of CSBB should be represented as
W> DL + W0
(8)
Fig. 9 shows the phase diagrams of the width of CSBB and the frictional wind speed under different usage times. According to the phase diagram, the width of CSBB used to achieve some protective effect under different frictional wind speed conditions and usage times can be determined. For example, when the usage time of CSBB is 3 years and the local frictional wind speed is 0.45 m/s, if the protective effect to be achieved is that the δ is greater than 0.8, and then we know that the width of CSBB should be greater than 360 m according to Fig. 9. In determining the spacing of CSBB, in addition to determining the propagation speed of the edge of desertified land based on the incoming wind speed, the degradation length of CSBB must be determined according to the usage time. Therefore, the final spacing of CSBB should be represented as
Fig. 7. The variation of the degradation length of CSBB (DL) with time at different wind speeds (φ = 5000 h).
S
DL
(9)
Fig. 9 The phase diagrams of the spacing of CSBB and the frictional wind speed under different usage times. The target variable is δ. 5. Conclusions Desertification is an important environmental issue affecting people’s production and life. Among them, the desert expansion prevention is a scientific and engineering problem that needs urgent solution. The checkerboard sand barrier is commonly used measures in the sand control project. The in-depth study of checkerboard sand barrier will help people to optimize the design of checkerboard sand barriers, which can achieve the purpose of saving laying costs and laying time. In response to this research topic, field observations were carried out on the influence of CSBB on desert expansion. Through the analysis of field observation data, it was found that if checkerboard sand barriers were laid, the dune speed after CSBB was significantly reduced. Moreover, the closer to the edge of CSBB, the more obvious the decrease of dune speed is, with a reduction of about 30% at 200 m, i.e., the reduction rate of propagation speed of the edge of desertified land (δ = 1 − v/vno, here, v is the propagation speed of the edge of desertified land with CSBB, vno is the propagation speed of the edge of desertified land
Fig. 8. The variation of parameter y1 with wind speed.
the wind speed is, the longer the degradation length is, and that it increases approximately exponentially with time, which can be expressed by the following equation
DL = (y1
215e
T /6.15)·(
/365)
(6)
Here, T is the time (in years) and φ is the duration of the wind in each year (in hours). The relationship between parameter y1 and friction velocity (u*) is shown in Fig. 8, which can be expressed by
y1 = 329
(1.12 × 105) e
u /0.06
(7)
Fig. 9. The phase diagrams of the width of CSBB and the frictional wind speed under different usage times. The target variable is δ. 5
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Fig. 10. shows the phase diagrams of the spacing of CSBB and the frictional wind speed under different usage times. According to the phase diagram, the spacing of CSBB used to achieve some protective effect under different frictional wind speed conditions and usage times can be determined. For example, when the usage time of CSBB is 3 years and the local frictional wind speed is 0.45 m/s, if the protective effect to be achieved is that the δ is greater than 0.8, and then we know that the spacing of CSBB should be less than 680 m according to Fig. 10.
without CSBB.) is about 0.7. When the width and spacing are respectively 400 m and 700 m, CSBB can achieve the full paving effect, which confirms the effectiveness of the laying scheme of SP proposed by Bo and Zheng (2013), and indicate to a certain extent that the dune field model proposed by Bo and Zheng (2013) considering the effect of the checkerboard sand barriers is effective. At the same time, an in-depth study on the laying scheme of checkerboard sand barriers was carried out by improving the model proposed by Bo and Zheng (2013). The calculation results show that there is an optimal size (Smax) of the spacing of the CSBB, and the effect of CSBB is approximately the same in the case of S < Smax. There is an optimal size of the width (W0) of CSBB, and the effect of CSBB is approximately the same in the case of W > W0. Furthermore, the expression of degradation length of CSBB with time, annual wind regime and wind speed is given, i.e., DL = (y1 215e T /6.15)·( /365) . Based on the calculation results, a laying method for CSBB is given, i.e., the width and spacing of CSBB should satisfy W> DL + W0 and S
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was supported by a grant from the National Natural Science Foundation of China (No. 11490551), the authors express their sincere appreciation to the support. References Bo, T.L., Ma, P., Zheng, X.J., 2015. Numerical study on the effect of semi-buried straw checkerboard sand barriers belt on the wind speed. Aeolian Res. 16, 101–107. Bo, T.L., Zheng, X.J., 2011. Bulk transportation of sand particles in quantitative simulations of dune field evolution. Powder Technol. 214, 243–251.
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