Experimental study on the structural parameters of an optimized sloping roof grill barrier for the prevention of debris flows in Yanmen gully
Introduction
Permeable barriers have been adopted extensively for preventing and controlling debris flows in small watersheds (Wehrmann et al., 2006; Lin et al., 2007; Mizuyama, 2008; Takahara and Matsumura, 2008; Liang et al., 2018). These barriers can effectively block large debris flow particles while enabling the transport of finer particles, thereby reducing the reservoir pressure of a dam body (Fukawa et al., 2002; Wendeler et al., 2006; Canelli et al., 2012; Xu et al., 2012; Liu et al., 2014; Chen et al., 2015; Fan et al., 2018). However, existing permeable barriers (e.g. silt dams and grille dams) often lose water-sediment separation function over time and/or during a debris-flow event because of blockage by sediment and other debris (Xie et al., 2014, Xie et al., 2016). Hence, preventing blockages of openings of permeable debris-flow barriers and reducing sediment accumulation within the reservoir area have become important research topics in the use of permeable retaining structure to mitigate debris flows.
The structural parameters of permeable barriers play important roles in engineering structures used to separate water from sediment. Thus, these parameters have been analysed using physical experiments and numerical simulations (Kim et al., 2012; Mizuyama, 2008; Wendeler et al., 2008; Yong et al., 2015; Shen et al., 2018). Nevertheless, there is not any efficient solution to prevent blockage of a permeable retaining structure by debris-flow particles. Xie et al. (2014) proposed a novel horizontal water-sediment separation structure that maintains sediment separation function after the passage of a debris flow. Subsequently, Xie et al. (2016) developed a formula for the grid design length by neglecting the friction between the debris- flow particles and the structural surface. Brunkal and Santi (2016) recommended that the d85 value for grain-size distribution of the debris flow be used as the grid spacing to allow water to drain freely, while ensuring that fine sedimentary particles do not become too compacted and coarse particles are retained. Liang et al. (2018) modified the novel permeable structure proposed by Xie et al. (2014) to include a horizontal entrance grid and a global gradual grid. This improved structure is known as a sloping roof grill barrier (SRGB) and ensures good contact between debris-flow particles and a structural surface, thereby reducing blockage of barrier openings. Liang et al. (2018) used the SRGB to conduct flume experiments. Both the retention efficiency of the permeable barrier and the attenuation rate of the particle velocity are maximized when the size of the opening is equal to the d60 particle size. This result differs from the conclusions of other studies (e.g. the optimal opening size was reported as d95 by Silva et al., 2015 and as dmax by Lien, 2003). Compared with vertical debris dams (including vertical-rigid barriers and vertical-flexible barriers), the SRGB has a high impact resistance, a high permeability and excellent retention and water-sediment separation capacities (Jaeggi and Pellandini, 1997; Huebl and Fiebiger, 2005; Xie et al., 2014; Xie et al., 2016). However, limited storage capacity in a narrow gully can result in blockage of SRGB openings, and the subsequent impairment of water-sediment separation.
Previous studies have shown that the SRGB structural parameters play key roles in trapping sediments and facilitating slurry passage in the early stages of a debris flow. However, the structural dimensions of the SRGB and the relationship between them are not clear, and there is no a solution to the structural failure problem of the horizontal permeable structure associated with the blockage of openings. Moreover, the limited storage capacities of previously proposed structures affects the permeability of these structures (Xie et al., 2014, Xie et al., 2016; Liang et al., 2018). In this paper, these issues are addressed by expanding the lateral area of the horizontal permeable retaining structure developed by Liang et al. (2018), and performing a series of physical model tests to quantify the effects of the structural parameters on controlling debris flows. The barrier length is correlated with the structural parameters. This result of this study serves as technical references for the engineering design of horizontal permeable retaining barriers.
Section snippets
Novel structure for mitigation of debris flow
The study site is located in Yanmen gully, Sichuan Province, China. The width of the gully varies, but the gully is usually narrow in the upstream source area and relatively wide in the downstream transport area (Fig. 1a). Irrespective of whether the horizontal permeable retaining structure is placed in the narrow or wide section of the gully, different degrees of openings for siltation and blockage occur because of an insufficient deposit area. A debris flow mitigation structure with a lateral
Experimental apparatus
As shown in Fig. 2, the experimental flume is 7.3 m long and 2.5 m high. The rectangular cross section of the flume has a width of 0.4 m, a depth of 0.5 m and a slope gradient of 20°. A hopper is arranged at the top of the flume for mixing solid materials and water. A sluice is set at a distance of 7.3 m, 6.8 m, or 6.3 m from the bottom outlet along the direction of the flume to initiate the debris flows at different heights for a series of comparative tests. The SRGB is 1.0 m long and 0.4 m
Retention effect
Given that the water content is a key factor in facilitating the motion of a debris flow (Shakesby and Matthews, 2002; Wendeler et al., 2006), the SRGB retains debris flow particles by drainage and deceleration of the flow. The mass of the retained particles in the lateral deposit areas can be used to evaluate the barrier performance under different structural parameters. The retention effect depends mainly by the spacing and inclination degree of the grid. The grid spacing is related to the
Discussion
The motion of a debris flow is easily affected by gully topography and various origins of debris flows, which makes it difficult to prevent and control debris-flow disasters (Zhou et al., 2015; Huo et al., 2017, Huo et al., 2018). In this paper, the lateral deposit areas of a SRGB are expanded based on the terrain of Yanmen gully, and a global gradual grid is set in the structure. A series of flume experiments are designed to study the responses of the SRGB structural parameters for debris-flow
Conclusion
A novel horizontal permeable barrier is presented in this paper. Such barriers have previously been proven to be effective for debris-flows prevention. A series of flume experiments with different structural parameters is conducted to evaluate the performance of the optimized SRGB in terms of the retention ability and separation efficiency of the barrier. Relationships between these structural parameters and the length of the optimized structure are obtained. The following conclusions are drawn.
Declaration of Competing Interest
The authors declare no competing financial interests.
Acknowledgements
This research was supported by the National Key Technology Research and Development Program of China (Grant No. 2018YFC15086).
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