Field Investigation on Hydroabrasion in High-Speed Sediment-Laden Flows at Sediment Bypass Tunnels

Abstract

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...

Bedload transport and hydro-abrasive erosion at steep bedrock rivers and hydraulic structures

“…The mean (i.e. spatially-averaged) abrasion depths increased approximately linearly with increasing cumulative bedload mass for the four-year period of measurement data at SBT Pfaffensprung [25]. This agrees with the observations of [6,16].…”

“…In the present contribution, we address this question, extend the application range of the models to further invert materials, calibrate the abrasion coefficient and give insight into the abrasion patterns related to hydraulic conditions. To this end, hydro-abrasive erosion on the concrete and granite inverts implemented at the Pfaffensprung SBT in Switzerland was investigated under various hydraulic and sediment transport conditions from 2012 until 2015 [25]. In the following, test set-up and procedure are described, followed by the field results and conclusions.…”

“…The Pfaffensprung reservoir is located on the Reuss River near Wassen, Canton Uri, in the Swiss Alps and has a capacity of 0.17•10 6 m 3 , which is quite small compared to the mean annual runoff of 645•10 6 m 3 [26]. The mean particle size of the bed material is dm = 0.25 m, and the mean annual bedload transport =amounts to about 350'000 tons based on the estimations [25,27]. The reservoir is equipped with a 282 m long and 4.4 m wide SBT (Fig.…”

“…Bedload transport in the Reuss River was simulated with a 1D hydro-morphological numerical model [25]. Between 2012 and 2015 the mean annual bedload transport mass (BL) in the Pfaffensprung SBT amounted to 350•10 3 tons (±50% due to model uncertainty).…”

Effects of Secondary Currents on Turbulence Characteristics of Supercritical Open Channel Flows at Low Aspect Ratios

“…Sediment availability and transport, and reservoir sedimentation tend to increase both in the Alpine region and worldwide under the impact of climate change [1][2][3]. High transport rates of bed load particles in combination with high flow velocities cause bedrock incision in high gradient mountain streams, and hydro-abrasion of invert materials and cavitation damages at hydraulic structures including dam outlets, weirs, and particularly sediment bypass tunnels (SBTs), and sediment flushing channels (SFCs) [4][5][6][7][8][9]. Therefore, a realistic and mechanistic abrasion model is of fundamental importance for both river and landscape evolution and for the sustainable design and operation of hydraulic structures.…”