De-aeration of a diversion tunnel of a large-scale hydroelectric scheme

Due to the influence of a varying water level in the main reservoir, the Jokulsa Tunnel as diversion tunnel from a secondary intake is subjected to a transition from free-surface to pressurized flow. As to avoid damage in the main conduit as well as of the hydraulic equipment, the air entrained into and transported downstream by the pressurized flow has to be removed before the junction of the two tunnel branches. A novel concept of de-aeration, basically consisting of vertical drill holes distributed along the Jokulsa Tunnel, was developed and technical feasibility ascertained by means of two physical hydraulic models.A concise description of the concept itself as well as of the modeling procedures is given.