Session

Session 12 2022

Start Date

10-27-2022 12:00 AM

Abstract

A partly unlined pressurized gallery supplies water to a hydropower plant. Since rocks and gravel may detach from the gallery walls, two gravel traps have been built on the gallery invert, downstream of the unlined sections. During gallery and powerplant inspections, it appears that the gravel traps are empty while gravel is found in the tail channel downstream of the turbines. Clear evidence of turbine damaging by solids impact is also observed. To understand why the gravel traps are ineffective and to define solutions to be implemented on site to increase their trapping efficiency, a 1:7.7 scale physical model of the gravel traps has been built and operated. The tests clearly demonstrated that the short-term trapping efficiency of the gravel traps is good since 98% of the gravel arriving upstream falls into the traps. However, the gravel traps are ineffective in keeping the gravel over long periods. This is due to two effects. In pressurized conditions, helicoidal vertical currents developing at the downstream extremities of the trap and turbulent velocity fluctuations extract the gravel from the trap. In addition, during gallery dewatering (for maintenance and inspection), temporary free surface flow can transport gravel downstream of the trap. Non-structural solutions to the problem have been identified, optimized and validated on the scale model. They consist in creating above the traps a pseudo-bottom aligned with the gallery invert, using horizontal bars placed transversally to the flow, and in compartmenting the traps with vertical walls. The paper presents in detail the physical model and all the tests and measurements performed to quantify the problem and assess the efficiency of the modification solutions.

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Oct 27th, 12:00 AM

Experimental Optimization of a Gravel Trap in a Gallery

A partly unlined pressurized gallery supplies water to a hydropower plant. Since rocks and gravel may detach from the gallery walls, two gravel traps have been built on the gallery invert, downstream of the unlined sections. During gallery and powerplant inspections, it appears that the gravel traps are empty while gravel is found in the tail channel downstream of the turbines. Clear evidence of turbine damaging by solids impact is also observed. To understand why the gravel traps are ineffective and to define solutions to be implemented on site to increase their trapping efficiency, a 1:7.7 scale physical model of the gravel traps has been built and operated. The tests clearly demonstrated that the short-term trapping efficiency of the gravel traps is good since 98% of the gravel arriving upstream falls into the traps. However, the gravel traps are ineffective in keeping the gravel over long periods. This is due to two effects. In pressurized conditions, helicoidal vertical currents developing at the downstream extremities of the trap and turbulent velocity fluctuations extract the gravel from the trap. In addition, during gallery dewatering (for maintenance and inspection), temporary free surface flow can transport gravel downstream of the trap. Non-structural solutions to the problem have been identified, optimized and validated on the scale model. They consist in creating above the traps a pseudo-bottom aligned with the gallery invert, using horizontal bars placed transversally to the flow, and in compartmenting the traps with vertical walls. The paper presents in detail the physical model and all the tests and measurements performed to quantify the problem and assess the efficiency of the modification solutions.