**1. Introduction**

Sediment handling and erosion in hydropower plants have been long-standing engineering challenges. Hydropower plants are often upgraded and refurbished to improve performance and plant capacity. Sediment handling in hydropower plants can be done through catchment manipulation, dam and intake design, tunnel lining, sand traps, and turbine design. The motivation of the work is the upgrading and refurbishment of existing hydropower plants. After upgrading their installed capacity, several large Norwegian hydropower plants experienced operational problems associated with sediments entering the penstock and causing erosion to the turbine [1]. Moreover, a higher variability in power demand in recent years has led to some plants converting from base-load to peak-load production. This results in variable discharge through the tunnels, which stirs up sediments from the tunnel bed in unlined tunnels with remaining rock material on the invert, which is typical in Norwegian hydropower plants. Currently, the power plant does not have a system to indicate how much sediment actually passes through the sand trap and is transported through the turbine. To the authors experience, this is the typical situation for hydropower sand traps. For comparison, the records of removed sediments since 2015 are

**Citation:** Ivarson, M.M.; Trivedi, C.; Vereide, K. Investigations of Rake and Rib Structures in Sand Traps to Prevent Sediment Transport in Hydropower Plants. *Energies* **2021**, *14*, 3882. https://doi.org/10.3390/ en14133882

Academic Editors: John M. Cimbala and Bryan J. Lewis

Received: 1 June 2021 Accepted: 23 June 2021 Published: 28 June 2021

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in the range from 150 m<sup>3</sup> to 15 m<sup>3</sup> per year, with a decreasing trend. Sediments can cause damage to the turbine through abrasion on the turbine surfaces. Larger particles may also cause larger dents on the turbines. Such damage results in reduced turbine efficiency and reduced structural integrity of the turbine components. The damage may, in turn, trigger increased cavitation that accelerates the degradation [2,3].

Tonstad hydropower plant, located in the mountains of southwestern Norway, is currently experiencing such challenges. In 1988, the plant upgraded its capacity from 640 to 960 MW by installing a new 320 MW Francis turbine. An additional penstock, surge tank, and sand trap were built. However, the unlined headrace tunnel was left untouched. It was expected that the higher discharge required to run all three turbines would lead to an increased amount of sand and rocks to be flushed into the turbines. However, the actual amount of sediments being transported with the flow was surprisingly large. One of the reasons for the increased amount of sediment transport is the poor design of the sand trap, which was designed in the 1960s and is now underperforming.

Figure 1 shows the layout of a typical high-head hydropower plant. A sand trap is a section of the water way, typically located immediately upstream of the penstock. This allows the headrace tunnel, with the remaining stone and sand material after its construction, to be unlined. Sand traps are designed to reduce flow velocity, which allows sediments to settle easier. The velocity is typically reduced by 30 to 50% [1]. This is done by increasing the cross-sectional area of the tunnel. The main challenge is poor performance of sand trap, which is intended to trap sediments before they reach the penstock. A proposal to improve sand trap performance is to cover sections of the bed with concrete ribs. This has been shown in physical experiments to create a low-velocity zone beneath the ribs, protecting sediments on the bed from being stirred up, while also allowing sediments to fall through the gaps between ribs [4,5]. Another proposal is to install rows of v-shaped rakes in the diffuser at the sand trap inlet. Increased levels of turbulence, such as those induced by these v-shaped rakes, has been shown to increase sediment settling speeds for certain particle sizes in numerical studies by Maxey in 1987 [6] and Wang and Maxey in 1993 [7]. This was later confirmed in physical experiments by Aliseda et al. in 2002 [8]. In these studies, sediments were found to settle in the peripheries of local vortex structures, which is coupled to a sweeping of sediments in directions normal to the flow.

**Figure 1.** Layout of a typical high-head hydropower plant. The sand trap is marked with red color.

Several scientific studies have been conducted on the topic of sediment transport in hydropower sand traps. Olsen and Skoglund modelled the flow of water and sediment in a three-dimensional sand trap geometry using the *k* −  turbulence model [9]. After including modifications to the turbulence model, both the flow field solution and sediment concentration calculations were in agreement with experimental procedures. Kjellesvig and Olsen modelled the bed changes in a sand trap using the transient convection–diffusion equation for sediment concentration and an adaptive grid adjusting for changes in the bed [10]. Large amounts of sediments could be seen being moved through the geometry in the simulations. The results compared well to physical model tests. Bråtveit and Olsen used 3D computational fluid dynamics (CFD) simulations to calibrate horizontal acoustic Doppler current profilers (H-ADCP) in the Tonstad sand trap [11]. The study found that the 3D CFD simulations could accurately calibrate the H-ADCP while also assessing the flow conditions at the locations of installation. Almeland et al. computed water flow in a model of the Tonstad sand trap using different versions of the *k* −  turbulence model [12]. Depending on the discretisation scheme, grid resolution, and turbulence model, the computations showed the flow field to follow both the left side, right side, and centre of the diffuser. Field measurements showed that the main current followed the centre of the diffuser.

The present work is part of an ongoing sand trap research project. In previous work by Richter et al., it was found that implementing ribs just upstream of the penstock increased sand trap efficiency dramatically, as sediments were trapped in the low-velocity zone underneath the ribs [4,5]. Havrevoll et al. performed PIV analyses on the flow around ribs in the sand trap to investigate sediment settling characteristics. They showed that ribs successfully separate the flow field [13]. Daxnerova performed experiments on a physical scale model of the sand trap at NTNU to determine the effects of installing various calming flow structure designs in the diffuser [14]. The best performing design from Daxnerova's research, a v-shaped rake type structure, will be further studied in this work.

The objective of the present work is to assess the changes in sand trap efficiency when including ribs in the downstream end of the sand trap. The effects on the turbulence dissipation and sediment trajectories by including v-shaped rakes in the diffuser will be investigated. The work aims to reproduce results from experiments on physical scale models of the sand trap in order to gain further confidence in the experimental results.
