Flow through the Bosphorus

Abstract

This paper presents the results of a numerical study on a two-layer flow system in the Bosphorus, the strait connecting the Black Sea and the Marmara Sea. The numerical model is a three-dimensional (3D) model incorporated with the large Eddy simulation model for the horizontal turbulence closure and a modified mixing-length model for the vertical turbulence closure. The model, tested and validated against field data and other numerical models, was used to study hydrodynamic processes associated with the two-layer flow system in the Bosphorus. It was found from the 3D model calculations that the length-averaged flow rates are 18.55 × 103 m3/s for the upper layer and 6.55 × 103 m3/s for the lower layer for the mean flow conditions, corresponding to the mean head difference between the Black Sea and the Marmara Sea of 33 cm, in good agreement with the field observations. It was also found that, for the mean flow conditions, a net amount of transfer of water of 2.9 × 103 m3/s from the lower layer into the upper layer occurs, and this is due to mixing between the two layers, also in good agreement with the field observations. It was further found that the present model captures the location of the density interface between the layers and its variation with the longitudinal distance quite well. Given the time variation of the head difference between the Black Sea and the Marmara Sea over a time span of a year, the time variations of the flow rates in the upper and lower layers, calculated through the present model, compared favorably well with the field data. The present model solution indicated that the flow in the lower layer essentially stops when the head difference between the Black Sea and the Marmara Sea approaches approximately 40 cm, and, by contrast, the upper layer, likewise, stops when the head difference approaches approximately 10 cm. However, the model indicated that, for truly stagnant water in the upper layer, the head difference needs to approach nil. Additionally, hydrodynamics of the water body in the Marmara Sea adjacent to the Marmara Sea end of the Bosphorus and that in the Black Sea adjacent to the Black Sea end of the Bosphorus were unveiled by the implementation of the present 3D model.