A diversion tunnel is planned for flood protection in the City of Lyss, Switzerland. This tunnel includes a junction, from which water from a side creek is added to the tunnel flow. The latter is sensitive to choking, so that the junction had to be optimized in a hydraulic laboratory investigation. Its setup finally consists of a drop shaft and an injector generating annular flow in the tunnel. Free surface tunnel flow then occurred for all relevant discharges. This setup is also of interest in sewer systems as well as in spillway and diversion tunnels, where similar problems may occur. The concept was optimized for a specific prototype. General recommendations may be derived for similar situations