Field monitoring and modelling of sediment transport, hydraulics and hydro-abrasion at the Mud Mountain Sediment Bypass Tunnel in the USA

Sediment Bypass Tunnels (SBTs) reduce or even stop reservoir sedimentation by routing sediment-laden flows around reservoirs, mainly in mountainous regions with gravel bed rivers. High sediment loads and flow velocities can cause hydro-abrasion on SBT inverts, leading to costly maintenance and structural risk. Minimizing hydro-abrasion is crucial for reducing costs and ensuring long-term functionality. Selecting cost-effective, abrasion-resistant materials is thus essential. Current options include high-strength concrete, cast basalt plates, steel armoring, and granite pavers, but their long-term field performance remains poorly validated and thus uncertain despite recent research. In this study, we investigate the hydro-abrasion resistance of the 25.4 cm thick granite lining of the Mud Mountain 9-foot SBT using inspection data from 2018 to 2023, provided by the US Army Corps of Engineers (USACE). The SBT is located on the White River in Washington State, USA. We determined spatially averaged, maximum, and minimum hydro-abrasion depths, as well as hydro-abrasion patterns from 3D invert scans. Bed-load particle size distribution and transport rates, hydrological data, and operational data were also analyzed. This data was used to validate a mechanistic saltation abrasion model developed at VAW, ETH Zurich. The calibrated model yielded an abrasion coefficient of kv = 2.22 ± 1.55 × 105 for the granite invert of the SBT, about three times higher than the quasi-constant value of kv = 4.4 ± 2.2 ×104 proposed in the literature for brittle material such as concrete. This highlights the fact that the hydro-abrasion resistance of granite is significantly higher than that of concrete. Finally, the analysis of invert scans and hydraulic conditions revealed the presence of Prandtl’s first and second kinds of secondary currents in the bends and straight sections of the SBT, respectively. The results provide valuable knowledge for selecting cost-effective invert materials and designing SBTs, and contribute to the sustainable use of hydraulic infrastructure exposed to sediment-laden flows and hydro-abrasion.