Artificial gas supersaturated water from small hydropower plants: methods to detect air entrainment at intakes
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- Master theses 
Hydropower plants with submerged intake and Francis turbine can release artificial Total Dissolved Gas (TDG) supersaturated water from the plants outlet, if air is drawn into the pressure pipe at intake and dissolved in water at high pressure. Air entrainment at intake can be initiated by debris covered trash rack . Fish exposed to TDG supersaturated water can develop Gas Bubble Disease (GBD), which is harmful and can result in fish death. A common used small hydropower plant design consist of submerged intake, intake air vent, pressure pipe and Francis turbine. The report, "Gassovermetning i vassdrag – en kunnskapsoppsummering" , published in 2018, recommend pressure loss measurements at intake trash rack in combination with TDG monitoring in the plants outlet river as mitigations, where air entrainment is initiated by covered trash rack. The mitigation principle is described to be promising but not concluded, and in general, several topics related to supersaturation in rivers is relevant for further research . This thesis have investigated ifmeasurement of air entrainment through intake air vents can be an alternativemethod to indirectly detect supersaturation fromthe plants outletwhere air entrainment is initiated by covered trash rack. Two field tests have been performed at Grønhaug Kraftverk AS to investigate the measurement principles and plant behavior at covered trash rack with the following air entrainment at intake. Coverage of trash rack have been done manually to simulate situations where trash rack is highly attached by e.g. debris and leaves. The first field tests have been performed with abnormal operation conditions at low discharge. The second test utilized higher discharge to gain tests result from plant behavior at normal operation conditions. Results from field tests at Grønhaug plant indicate that measurement of air flow through intake air vent can be an alternative and simple method to detect release of TDG supersaturation from the plants outlet, with submerged intake as a prerequisite. Further, test result is used to assess air ventmeasurement principle against head loss measurements at trash rack and TDG monitoring in the plants outlet. Obtained results is also used to investigate possible design improvements to avoid air entrainment at intake, and in more detail parameters that initiate and stop air entrainment at intake.