Hydro-abrasion processes and modelling at hydraulic structures and steep bedrock rivers: 1. Hydro-abrasion and cover effect

• This study enhances hydro-abrasion model for sustainable hydraulic design and predicting river and landscape evolution. • Experiments explore hydro-abrasion depth and patterns under varied hydraulic, sediment transport, and bed material conditions. • Results reveal increased abrasion rates with higher sediment supply and material hardness. • Abrasion patterns are governed by the flume width-to-water depth ratio and the homogeneity of bed lining material. • Peak abrasion rates occur when the sediment transport rate reaches sediment transport capacity of the flow. Hydro-abrasion refers to the gradual loss of material on the surface of a solid body, caused by mechanical stress, mainly from the impacts of sediment saltation in flowing water. Hydraulic structures like weirs, spillways, diversion tunnels, and especially sediment bypass tunnels (SBTs) experience significant hydro-abrasion due to high flow velocities and elevated sediment transport rates. The hydro-abrasion process is critical in hydraulic engineering, where material loss can lead to structural damage and costly repairs, and in geomorphology, where it drives bedrock incision and shapes landscape evolution over time. This study aims to advance the knowledge of hydro-abrasion mechanics (part I, present paper) and to enhance a mechanistic saltation hydro-abrasion model (part 2) for predicting river and landscape evolution and hydro-abrasion at hydraulic structures. To this end, hydro-abrasion tests of polyurethane foams and weak mortar mixtures as bed materials were systematically conducted in a 0.20 m wide, 0.7 m deep and 13.5 m long laboratory flume at VAW at ETH Zurich, under supercritical flow conditions. The study investigates the effect of flume width-to-flow depth aspect ratios, approach flow Froude numbers, particle diameter and hardness and sediment supply rate on hydro-abrasion rate and pattern. The focus is on the latter two parameters, which were not previously and systematically investigated in flume studies at low aspect ratios. The abrasion depths were measured using a 3D high precision laser scanner. Results revealed that abrasion rate increases with sediment supply rate with maximum abrasion occurring when sediment transport capacity is reached. Harder sediment leads to higher abrasion rates. Abrasion patterns depend on the aspect ratio, causing the formation of one or two incision channels. Three cover effect functions, namely, linear, exponential, and probabilistic were compared to the data. The exponential cover function provides the best representation of the present data. These findings provide new insights into the physical mechanisms of hydro-abrasion under varying hydraulic, sediment, and bed material conditions. This research contributes to the enhancement of a well-known mechanistic saltation abrasion predictive model by incorporating the proposed hardness and cover equations, which is detailed separately in the accompanying paper as Part 2.