**4. Conclusions**

It is necessary to minimise sediment erosion to increase the turbine's lifespan. Threedimensional models based on the Tonstad power plant sand trap were created. Versions of the model include various upgrades to determine their effect on sediment settling. The numerical domain was discretised by a combination of hexahedral and tetrahedral mesh. Steady-state and transient multiphase simulations were performed on the models, using water and sand with a variable grain size. The objective was to investigate how installing rake- or rib-type structures affect particle sand trap efficiency and head loss.

By investigating the results, it was found that the sand trap with ribs at the outlet reduces the total weight of sediments exiting the sand trap by 24.5%, while increasing the head loss by around 1.8%. Installing rakes in the diffuser, although showing signs of increasing settling speed for larger sediments, was found to increase the total weight of sediments leaving the sand trap by 48.5%. This led to a reduced sand trap efficiency. In addition, the rakes caused an increased head loss of 12.7%. It is shown that in all models, sediments escaping the sand trap have a diameter smaller than one millimetre. These findings are supported by physical scale experiments on the sand trap [4,5,13]. The results show that installing ribs at the outlet of the sand trap will reduce sediment transport to the turbine and increase sand trap efficiency, thus prolonging turbine lifespan at the Tonstad power plant.

The main novelties from this work are the analysis results for the tested design of ribs and v-shaped rakes. The tested rib design has previously not been installed in a hydropower plant, and can be recommended based on the results in this work. The tested v-shaped rakes have been installed in existing hydropower plants previously, but this practice should be reconsidered.

For further work, running two-dimensional simulations along the centre line of the sand trap with the Large Eddy Simulation turbulence model could give a more accurate representation of the turbulence and sediment settling in the plane. Additional variants of the rakes should be tested to verify that they still have an adverse effect. The effects of extending the rakes to reach the crown of the tunnel so that the whole flow area is covered should also be investigated. Additionally, a model allowing for sediment resuspension should be explored to better represent sediments bouncing on or being resuspended from the bed. Further, experimental measurements of the inlet velocity profile are needed to create realistic and accurate inlet boundary conditions.

**Author Contributions:** Conceptualization, M.M.I., C.T. and K.V. methodology, M.M.I.; software, M.M.I.; formal analysis, M.M.I.; investigation, M.M.I.; resources, C.T.; data curation, M.M.I.; writing original draft preparation, M.M.I.; writing—review and editing, C.T. and K.V.; visualization, M.M.I.; supervision, C.T. and K.V.; project administration, C.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** No funding is received for this work.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict interest.

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