Experimental study on the structural parameters of an optimized sloping roof grill barrier for the prevention of debris flows in Yanmen gully

Highlights

• A flume experiment was carried out on a horizontal permeable retaining structure with an expanded lateral area.

• The influences of the structural parameters on the retention and separation abilities of the sloping roof grill barrier were evaluated.

• The relationship between the length of the barrier and its structural parameters was established.

Abstract

A horizontal permeable retaining structure (i.e., a sloping roof grill barrier) is an efficient way to separate water from sediment. However, the effect of the structural parameters and lateral deposit area on separation efficiency has not been investigated. Taking Yanmen gully as an example, this paper first expands the lateral deposit area of the retaining structure, and then conducts a series of flume experiments to study the influences of the structural parameters (grid spacing and grid inclination degree) on the separation efficiency. The experimental results indicate that the retention ability of the structure with the optimized lateral deposit area is much higher. The grid spacing is the main factor affecting the grain-size distribution of the debris flow beyond the barrier and should be higher than the d₈₀ value of grain-size distribution of the debris-flow. The retention capacity is directly proportional to the grid inclination degree, while the separation efficiency is inversely proportional to the grid inclination degree. The steeper the grid inclination is, the smaller the displacement of debris flow particles along the barrier surface is. The experimental results are analysed to correlate the barrier length with the structural parameters. This paper serves as a reference for the structural design of horizontal permeable barriers.

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 d_max 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.