The Bosphorus, a sea strait which runs between the Black Sea and the Sea of Marmara, forms part of the boundary between Asia and Europe. It is an internationally significant waterway, located in Istanbul, Türkiye. There exists a two-layer flow system in the Bosphorus: The head difference between the Black Sea and the Marmara Sea (and therefore the favorable pressure gradient created by this head difference in the North-South direction) drives the flow in the upper layer, from the Black Sea to the Marmara Sea, while the density difference (and therefore the resulting adverse pressure gradient between the Marmara Sea and the Black Sea) drives a counter flow in the opposite direction, in the lower layer. River runoffs to the Black Sea are mainly responsible for the head difference, while the difference in the salinity between the Marmara Sea and the Black Sea is responsible for the density difference between these two water bodies.

Although a substantial amount of knowledge has accumulated over the years on the flow through the Bosphorus, comparatively little is known about the detailed flow processes, and therefore its impact on water quality in this waterway and its neighboring seas.

Istanbul Water and Sewerage Administration, Municipality of Istanbul, supported a five-year (2018-2023) research project on this topic entitled “Hydrodynamic Modelling and Water Quality Assessment of the Bosphorus and Golden Horn for the Master Plan of ISKI (Istanbul Water and Sewerage Administration)”, commissioned to Yuksel Proje, Istanbul. The Guest Editors were the key investigators of this project. A “consortium” of five research and consultancy entities undertook this project, namely, BM SUMER Consultancy & Research, Istanbul Technical University (ITU) Civil Engineering, ITU Environmental Engineering, ISKI (Istanbul Water and Sewerage Administration), and Yuksel Project. The objectives of the project have been twofold: (1) to investigate the hydrodynamics of the flow through the Bosphorus by use of 3D hydrodynamic modelling; and (2) to conduct a comprehensive study for water quality assessment of the Bosphorus (including the neighboring water bodies), based on the knowledge of the hydrodynamics obtained in the first part of the project.