Flume experiments on an abrasion countermeasure for sediment bypass tunnels by gravel self-lining

Abrasion of a tunnel invert by large gravel is a crucial problem in the sustainable use and maintenance of sediment bypass tunnels (SBTs). Coating the entire tunnel invert with a durable material such as steel for protection might be an effective countermeasure, but impractical in terms of cost consideration. Against these backgrounds, a potential method called ‘self-lining’ has been proposed as an abrasion countermeasure for SBTs. In this method, artificial roughness elements are placed on the tunnel invert to promote gravel deposition between them and to coat the invert with the gravel sediment itself, even in the supercritical flow condition. In this study, we performed flume experiments for flow velocity fields and sediment deposition in open-channel flows with strip roughness elements to verify the effectiveness of the self-lining method. To clarify optimal setups of strip roughness elements for self-lining, we considered various experimental conditions with different Froude numbers (greater than 1 for all cases), cross-sectional shapes of roughness elements (square/triangle) and roughness intervals. First, flow velocity distributions between and over the roughness elements were measured by particle image velocimetry (PIV). Next, model sediment particles (corresponding to gravel in real scale) were added from the flume inlet, and the deposition processes between roughness elements were observed to evaluate the coverage rate of the channel bottom with the glass beads using image analysis. Results of the experiments show that with proper shapes and intervals of strip roughness elements, the channel bottom (tunnel invert) can be completely covered with gravel sediment deposition.