Effects of checkerboard sand barrier belt on sand transport and dune advance

Abstract

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/v_no, here, v is the propagation speed of the edge of desertified land with CSBB, v_no 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 (S_max 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 $\text{W}>DL+{\text{W}}_0$ and $\text{S}<S_{\text{max}}-DL$. This laying design can save the laying cost and laying period to some extent, which provide guidance for the optimal design of CSBB in anti-desertification project.

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, long-term 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-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 v-shaped 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.