İstanbul Boğazı'nın 3 boyutlu hidrodinamik modeli
3D hydrodynamic model of strait of İstanbul
Yazar: MEHMET MELİH KOŞUCU
Danışman: DOÇ. DR. MEHMET ÖZGER
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Hidrolik ve Su Kaynakları Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: İstanbul Boğazı = Bosphorus ; İstanbul Boğazı = Bosphorus

Özet:
Türk Boğazlar Sistemi'nin önemli bir kısmı olan İstanbul Boğazı, Marmara ve Karadeniz'i birbirine bağlamaktadır. 31 km uzunluğunda olan Boğaz'ın, en dar kısmı 700 m, en geniş kısmı ise 3500 m'dir. İstanbul Boğazı'nın sınırları kuzeyde Anadolu Feneri ile Rumeli Feneri hattı, güneyde ise İnciburnu Feneri ile Ahırkapı Feneri hattıdır.
Dünyada nadiren görülen bir özelliğe sahip olan İstanbul Boğazı, birbirine çok yakın olan iki kara parçasını ayırması ve bilhassa Tabakalı Akım durumuna sahip olmasıyla bilinmektedir. Boğazdaki akıntı durumunu inceleyecek olursak üst ve alt tabaka olmak üzere hız vektörlerinin birbirine zıt yönde olduğu bir tabakalaşma hali mevcuttur. Karadeniz'deki su seviyesinin Marmara Denizi'ndeki su seviyesine göre daha yüksek kotlarda yer alması, ve Marmara Denizi'nin Karadeniz'e göre daha tuzlu olması bu tabakalaşmada en etkin faktör olarak gösterilebilir. Bu iki faktör sonucunda Karadeniz'den Marmara Denizi'ne doğru bir üst akıntı, ve Marmara Denizi'nden Karadeniz'e doğru bir alt akıntı oluşmaktadır. Atmosferik koşulların (Lodos gibi) aksi tarzda cereyan etmesi durumunu hariç tutarsak Karadeniz'den Marmara Denizi'ne doğru seyr eden üst tabaka akımı, Tuna Nehri debisinin yüksek olduğu ekstrem koşulları yok sayarsak alt tabaka akımı yılın tamamında mevcuttur.
Bu çalışmada yapılmak istenen ve yapılan ise İstanbul Boğazı'ndaki bu tabakalı akım durumunun gerçeğe uygun bir şekilde modellenmesi oldu. Delft3D adlı bir Hesaplamalı Akışkanlar Dinamiği (HAD) programı vasıtasıyla İstanbul Boğazı'nın 3 boyutlu bir Hidrodinamik modeli kuruldu. 
Model sonuçları incelendiğinde, uygun sınır şartı, başlangıç şartı ve fiziksel parametreler girildiğinde, hidrodinamik ve hidrografik yapının gerçek durumdaki gibi davrandığı görülmüştür. Sıcaklık, tuzluluk ve hız profilleri, ölçüm profilleri ile uyuşmakta olup, üst tabaka ve alt tabaka debileri de ölçülen debilere yakın değerler almıştır. Buradan, modelin tutarlı ve güvenilir olduğu sonucuna ulaşılmıştr.

Abstract:
Strait of Istanbul is a waterway, which connects Black Sea and Marmara Sea and seperates Rumelia and Anatolia. It is one of the most important parts of Turkish Straits System. Its North Border is a line between Rumelia and Anatolia Lighthouses and its south border is a line between Ahırkapı Lighthouse and İnciburnu Lighthouse. Istanbul Strait's length is 31 km, and its width is changing about 700 m and 3500 m. There are two different humps at the south and north part of the strait. South hump's depth is about 30 m. North hump's depth is approximately 45-50 m. In addition, there is a constriction at the north side of south hump. In the constriction section, width is 700 m.
Water circulation along Istanbul Strait, includes very complicated and comprehensive processes, because of differences between North and South part of Istanbul Strait. North part of Istanbul Strait is an exit to Black Sea. Black Sea is an interior sea, which has lower salinity and higher water level. Especially, Danube River's freshwater discharge towards Black Sea, leads to low salinity (approximately 18 ppt), and high water levels. South part of Istanbul Strait is an entrance to Marmara Sea. Because of brackish water discharge from Mediterranean Sea, Marmara Sea is saltier than Black Sea. Freshwater flow and Brackish Water flow pass the Strait of Istanbul, without mixing. Freshwater flow routes the upper side of the Strait, and brackish water flow routes the lower side. This process is named as "Stratified Flow". 
Istanbul Strait has a very complicated bathimetry and morphology. From the plan, Strait of Istanbul has a zig-zag shape. Its maximum depth is 110 m at the narrowest section which is between Kandilli and Bebek. Its minimum depth is on the south hump, which is 28 m. Average Depth is approximately 50-70 m, whole the strait.  There are four islets in the strait. Most important of these islets are Kızkulesi and Galatasaray islets. 

Hydrographic structure of Istanbul Strait has two layers. Upper layer is –except summer- cold and less salty. Lower layer is –except summer- warmer and saltier. In the Black Sea side of the strait, upper layer depth is 50-60 m. In the Marmara Sea side, upper layer is nearly 20-25 m. Therefore, there is an intermediate layer gradient between two borders of the strait. The intermediate layer's position changes seasonally. When upper layer flow rate is high, intermediate layer moves downwardly, and upper layer depth increases. When lower layer flow rate gets rise, intermediate layer moves upwardly and upper layer depth decreases. 
Before the modelling process, a description about obtained data is required. In the model, it is used 4 different type data. 1. Bathymetric Data 2. Hydrographic Data, 3. Water Level Datas 4. Meteorological Data. Bathymetric data is obtained by Turkish Naval Forces Office of Navigation, Hydrography and Oceanography. The data has high definiton, and reflects the straits general and critical bathymetric conditions. Hydrographic data is obtained by General Directorate of Istanbul Water and Sewerage Administration. In the hydrographic data, there are temperature and salinity parameters with depth-changing format. This data is observed by monthly periods. In a year, there are 12 different hydrographic observations.  Water Level datas are observed by Turkish Naval Forces. Hourly observations is occured for water levels. Water level observations are executed at two different stations, which are north and south part of the strait. Meteorological datas are obtained by ECMWF and AKOM. From ECMWF, wind speed and mean sea level air pressure observations are obtained. From AKOM, Temperature, Relative Humidity and Cloudiness datas are delivered. 
To establish hydrodynamic model, Delft3D-Flow is used. Delft3D-Flow is a computational fluid dynamics software, which uses finite differences method to solve problems. Momentum, Continuity and Hydrostatic Pressure Equations are solved by Delft3D-Flow. In order to start modelling, computational grid is setted up. After the establishing of grid, bathymetry is made compatible with Delft3D-Flow format. Time frame, physical parameters, initial conditions, boundary conditions, processes, observation points are adjusted. Because of the stratified flow phenomena, 3D model establishment is required. Thus, for 3D model, 20 vertical layers are determined. Boundary conditions are Water Level, Salinity and Temperature. Initial conditions parameters are same with Boundary Conditions parameters. 
After the running of model, salinity, temperature and velocity profiles are extracted. It is observed that, salinity and temperature profiles are similar with observational profiles of the strait. Velocity profiles are also realistic. In the hump and constriction zones, velocities are risen. In the mentiones zones, there is another different event, which is exchange between layers. Especially on the hump and constriction, two layers are being mixed, and a mass transfer occurs. Velocities magnitudes and vectors are differentiate month by month. In February, upper layer velocities are taken high values. On the other hand, in October month, upper layer velocities are getting lower values, moreover, rarely, upper layer velocity vectors' direction is changed 180o. This event is blockage of upper layer. In February, January and December, lower layer blockage event is occured uncommonly. These blockage events are related with magnitude of discharge of layers.
Discharge values are one of the most important parameters of model's output. After the comparison of monthly averaged discharge values with measured discharge valuıes, it is determined that model results are reliable and confident. High correlation numbers are achieved, from comparison of model and measurment discharge values. Another remarkable result of model is, high correlation numbers between upper layer discharges and water level differences (between North and South sides of th strait) are found. It means, water level differences and upper layer discharges are convenient for regression analysis. When regression analysis is done, a regression equation is found, and this equation's R2 value is sufficiently high.
Ultimate aim of the hydrodynamic modelling of Istanbul Strait is establishing a reliable and consistent model. From the consistent and harmonious results with observations, the goal of this study is achieved.