Current Forecast for Tunnel-Element Immersion in the Bosphorus Strait, Turkey

The Bosphorus Tube Tunnel beneath the Bosphorus Strait is under construction in Istanbul, Turkey, using the tunnel-element immersed method. It is well known that the Bosphorus Strait has a two-layer current system with a rapidly changing flow. To maintain quality and safety of tunnel-element installation, it is important to accurately forecast the current-flow condition for periods of 48h to identify windows of opportunity for the element-immersion process. In this study, a forecast system is developed based on the information collected and analyzed over an immediate past five-day period (120h) to provide a two-day (48h) period current prediction required for the element-immersion process. The system consists of a water-level forecast model and a current forecast model. The water level is forecasted from air pressure and wind information obtained from on-line monitoring data and meteorological service forecasts. The current is then forecasted from the water-level data taking the two-layer density interface effect and the immediate preceding 120-h current data (including its concurrent value) into consideration. This selected data-analysis period is determined based on our operational experience at the site. Both models are statistically constructed and calibrated using a year-long observation data set (October 2004–September 2005). It is found that the proposed forecast system gives sufficiently accurate current information for safe ongoing marine construction operation.