Wear due to sediment particles in fluid flows, also termed 'hydroabrasion' or simply 'abrasion', is an omnipresent issue at hydraulic structures as well as in bedrock rivers. However, interactions between flow field, particle motion, channel topography, material properties and abrasion have rarely been investigated on a prototype scale, leaving many open questions as to their quantitative interrelations. Therefore, we investigated hydroabrasion in a multi-year field study at two Swiss Sediment Bypass Tunnels (SBTs). Abrasion depths of various invert materials, hydraulics and sediment transport conditions were determined and used to compute the abrasion coefficients k v of different abrasion models for high-strength concrete and granite. The results reveal that these models are useful to estimate spatially averaged abrasion rates. The k v -value is about one order of magnitude higher for granite than for high-strength concrete, hence, using material-specific abrasion coefficients enhances the prediction accuracy. Three-dimensional flow structures, i.e., secondary currents occurring both, in the straight and curved sections of the tunnels cause incision channels, while also longitudinally undulating abrasion patterns were observed. Furthermore, hydroabrasion concentrated along joints and protruding edges. The maximum abrasion depths were roughly twice the mean abrasion depths, irrespective of hydraulics, sediment transport conditions and invert material.Water 2020, 12, 469 2 of 27 prevailing hydraulics and sediment transport conditions in the SBTs, i.e., high-speed sediment-laden flows, can cause severe invert hydroabrasion, which provokes high maintenance cost and, in the worst case, endangering the tunnel stability [17].Hydroabrasion is a common phenomenon occurring not only in SBTs and at other hydraulic structures, e.g., weirs, flushing channels, bottom outlets and diversion tunnels, but also in high-gradient bedrock rivers. Hence, it plays an import role in river incision and landscape evolution. Hydroabrasion is defined as continuous material loss from a fixed, submerged surface caused by the contacts of solid particles transported in the flow [18,19]. Depending on the hydraulic conditions and sediment properties, such as size and shape, sediment particles can be transported in sliding, rolling, saltation, or suspension mode causing grinding, rolling, or saltation impact stresses on the surface, respectively. Among these, impacts of saltating bedload particles govern not only material loss at SBTs and hydraulic structures, but also long-term bedrock incision and landscape evolution by driving hydroabrasion and macroabrasion-a process of fracturing bedrock into pluckable sizes mediated by particle impacts [19][20][21][22][23].Prediction of hydroabrasion is crucial for design service life analysis of invert materials used in hydraulic structures as well as for landscape evolution studies with potential applications in steep bedrock channels and channel knickpoint evolutions of exposed bedrock, such as waterfall...
