Characteristics of the Turkish Straits and Their Importance

Kpt. Alpertunga Anıker

The Turkish Straits consist of the Dardanelles Strait, which is 37 nautical miles long, the Sea of Marmara, which is 110 nautical miles long, and the Bosphorus Strait, which is 17 nautical miles long, all under the sovereignty of the Republic of Turkey. There is no alternative to this waterway, which stretches a total length of 164 nautical miles between the Black Sea and the Aegean Sea, and it is of great importance for the economies of all countries, especially those bordering the Black Sea.

With its geographical structure, narrowness, strong currents, sharp turns, and variable climatic conditions, the Bosphorus Strait is one of the most important natural narrow waterways in the world. The most significant issue is that the more than 15 million people living in Istanbul are constantly exposed to great dangers that may arise from maritime traffic.

With the signing of the Montreux Convention on July 20, 1936, while the principle of freedom of passage for commercial vessels through the straits was preserved, the regime for passage through the straits was reorganized with consideration for Turkey's security. The Montreux Convention is one of the rare multilateral agreements that have survived since its signing. For 90 years, Turkey has implemented the Montreux Convention with complete neutrality and diligence, creating a reasonable and applicable balance of interests not only for our country but also for the coastal states of the Black Sea and all third countries.

The Montreux Convention did not establish regulations for ensuring safety of life, property, environment, and navigation during passages through the Turkish Straits. However, navigation safety constitutes an integral part of the freedom of passage envisaged in the Montreux Convention. Therefore, Turkey has the authority to regulate passage safety within the framework of international law or generally accepted agreements and treaties. In other words, Turkey believes that the principle of "freedom of passage" in the Turkish Straits under its sovereignty cannot be interpreted as "free and unregulated" passage.

The Republic of Turkey controls and conducts the passage of all domestic and foreign vessels through the straits in accordance with the procedures defined in the "Turkish Straits Maritime Traffic Regulation" published in 1998.

Formation and Structural Features of the Bosphorus Strait

The geography of Istanbul and the Bosphorus Strait were formed during the 4th geological period. There is no universally accepted view that can fully answer the question of how the Bosphorus Strait was formed.

The prevailing scientific consensus resulting from studies conducted to date is that the Bosphorus Strait is a fault depression filled with seawater from a geological perspective. Accordingly, between 20,000 and 18,000 years B.C., the Ice Age came to an end, and the ice masses covering much of the world began to melt. As a result of a melting process that lasted for millennia, the waters of the Mediterranean rose approximately 150 meters higher than their original state around 8,000 to 7,000 B.C. Due to this significant rise in sea level, the waters of the Mediterranean flooded the Sea of Marmara, and as a result of ongoing rises, the waters of the Sea of Marmara merged with those of the Black Sea. The decrease in the depth of the strait from north to south supports the argument that the elevations in the southern part once acted as a barrier against the waters of the Marmara, which were surpassed by the rise in sea level.

The Bosphorus Strait is situated at a lower elevation from the Black Sea and at a higher elevation from the Sea of Marmara.

Another theory suggests that a very large river flowed through the area where the Bosphorus Strait is located in ancient times. The geographical formations that today appear as bays in the Bosphorus, particularly the Golden Horn, were the confluence points of the branches of this river with the main waterway during that period. When the Ice Age ended and the melting of glaciers around the world began, the water level of this river also rose like all waters, taking on its current form.

Surface and Bottom Currents in the Bosphorus Strait

Under normal conditions, the prevailing surface current in the Bosphorus Strait flows from north to south. The water level of the Black Sea is, on average, 30–35 cm higher than that of the Sea of Marmara throughout the year. This difference in level is due to the discharge of high-flow rivers such as the Danube, Dniester, Dnieper, Don, Kızılırmak, and Sakarya into the Black Sea. A scientific measurement recorded that the difference in water levels between the two ends of the Bosphorus Strait from February to July ranged from 28–56 cm (average 40 cm ± 3 cm), while during the rest of the year, it ranged from 19–35 cm (average 23 cm ± 3 cm).

As the snow begins to melt toward the spring and summer months, the discharge of rivers flowing into the Black Sea increases, causing the sea level in the Black Sea to rise relatively. This situation also increases the level difference between the two ends of the strait, and during this period, with the dominance of northern winds, the current in the strait becomes even stronger. Strong northern winds push the waters of the strait toward the Sea of Marmara, and the drop in water levels at the docks towards the southern end of the strait is one of the signs that the current will strengthen.

Under normal conditions, the current speed in the strongest areas of the Bosphorus Strait averages 4–5 knots, while the average for the entire strait is 2–3 knots.

The current speed generally averages around 0.5–1 knot at Rumeli Cape, 1.5–2 knots near Yeniköy, 2–3 knots at Kanlıca, 3–4 knots at Rumelihisarı, 4–5 knots at Akıntı Cape, and 3–4 knots at Sarayburnu.

Approximately 20 meters below the surface current, there is a bottom current flowing northward, contrary to the surface current. This current is primarily caused by the difference in salinity between the two seas. The salinity level of the Black Sea is around 1.7–1.8%. The Sea of Marmara, on the other hand, has a salinity of about 2.2–2.5% at the surface and 3.8–3.9% at depth. Since saltwater is heavier than freshwater, the saline water from the Marmara flows as a bottom current toward the Black Sea. It is known that in ancient times, ships would lower a basket filled with stones 20–25 meters below the water to reduce the speed-reducing effect of the current while heading north in the Bosphorus Strait.

Even deeper, below 50 meters, there is a third bottom current flowing from the Black Sea to the Sea of Marmara in the same direction as the surface current.

One of the strong currents unique to the Bosphorus Strait is the orkoz. The orkoz current is formed primarily due to strong southern winds, particularly from the southwest, piling the waters of the Marmara at the southern entrance of the strait. During these times, the waters at the entrance of the Bosphorus can rise by half a meter. This unusual rise also alters the current regime of the strait, creating counter currents at the surface known as orkoz currents. The speed of this current can sometimes reach up to 6–7 knots, blocking the surface current from the Black Sea to the Marmara and completely changing the direction of the prevailing current, especially in the southern parts of the strait. Orkoz currents can generally be observed several times a year from October to March and can be strong enough to cancel the voyages of city ferryboats. According to the Turkish Straits Maritime Traffic Regulation, when the orkoz current exceeds 7 knots, the Bosphorus Strait is closed to vessel traffic.

According to data from 2018, southern winds dominated Istanbul for 11% of the year, while orkoz currents were observed for 4% of the year. The orkoz current does not form immediately with the onset of a southwest wind; it can develop only after a strong southwest wind has prevailed in the region for at least one or two days. Generally, when the southwest wind begins to blow in the Marmara, it can dominate the area for 4–5 days or sometimes even longer. The longer the southwest wind lasts, the greater the likelihood that the orkoz current will reach the northern parts of the strait.

As a vessel descends from north to south and approaches Kandilli, instead of expecting its speed to increase, it begins to slow down, and if mirror-like formations and eddies are seen on the water, it indicates that the effect of the orkoz current has begun to manifest. The current can make it difficult for the vessel to turn from Kandilli toward Bebek Bay.

The orkoz current can sometimes unexpectedly affect the maneuverability of vessels negatively. Therefore, it is advisable to stay close to the centerline as much as possible rather than navigating very close to the shore when conditions allow.

When the southwest wind is strong for 2–3 days or more, it has been observed that the orkoz current can reach as far north as the Kavaklar area. The orkoz current quickly loses its effect after the southwest wind diminishes.

Kandilli Turn and the Devil Current

The narrowest part of the Bosphorus Strait is between Kandilli and Aşiyan, measuring 698 meters.

In the strait, depths tend to increase from south to north. The deepest point is in front of Kandilli Cape at 109 meters. It is not incorrect to say that the narrowest part is also the deepest and that the current is strongest at this point. The strong current that curves from outside Küçüksu Bay, brushing past Kandilli Cape and heading towards Akıntı Cape is referred to as the Devil Current. This section of the strait, which we can call the Kandilli Turn, is considered one of the three most critical points for navigation.

Especially for vessels navigating from south to north, it is crucial to adjust their turns gradually and in advance as they approach Kandilli Cape. If they prefer to make the turn late, the strong current affecting the starboard bow will make it extremely difficult, if not impossible, to turn to starboard, causing the vessel to drift to the opposite side and potentially run aground near Aşiyan Cape. The first point to pay attention to here is to plan the turn in advance and to start using the port helm without delay as the vessel begins to turn to starboard. Vessels exposed to a current of around 4 knots near Kandilli will see that the strength of the current decreases as they progress northward, dropping to around 2 knots.

Another important point to consider is to avoid cutting the course as much as possible to maximize the effectiveness of the helm and not to reduce engine power.

The vessel shown in Figure 3 has delayed planning its turn towards Kandilli Cape and, at the point where it intends to make the complete turn, has encountered the strong current affecting the starboard bow, dangerously close to Aşiyan. At the Kandilli point, vessels typically make a course change of approximately 45 degrees.

Another critical navigation point in the strait is the Yeniköy Turn. Especially vessels descending from north to south must make a turn of approximately 80 degrees under the influence of the current coming from the stern.

The north-south passage through Yeniköy is generally smooth up to Köybaşı Cape, but deep-draft loaded vessels may still encounter difficulties at the Umuryeri turn. At the Köybaşı Cape turn, vessels change their course by approximately 80° and this area has the potential to cause the most serious problems during the north-south passage.

Here, while the current affects the vessel from the starboard quarter, the counter current can also affect it from the port bow, accelerating the turn to starboard and making it difficult to turn to port in time. If this turn is not skillfully planned, there is a risk of the vessel running aground on the Yeniköy shore. In this area, vessel operators must plan the turn in advance and not delay applying the port helm once the turn to starboard begins. As much as conditions allow, it is also important to avoid cutting the course during this turn and to strive to use the helm as effectively as possible. If northern winds are also present at this point, it should be taken into account that the current will be much stronger. Many vessels have run aground in this part of the Bosphorus in both recent and distant history.

However, there are also shallows in this area, and vessels with a draft of over 10 meters generally run aground about 125 meters from the shore. However, there have also been accidents where vessels with a shallower draft have caused damage to the houses on the shore. Planning the turning angles early and making the turn gradually, while applying the counter helm to stop the turn without letting the vessel be swept away by the current, is one of the key points for these turns. In accidents that have occurred in this area, it has generally been observed that the failure to properly plan the Yeniköy turn was the main reason. Instead of carrying out the turn gradually, they preferred a sharp turn and likely applied the port helm too late. As a result, the main current affecting the starboard quarter and the counter current affecting the port bow accelerated the vessel's turn to starboard, and failing to turn to port in time, the vessel ran aground or came to a stop by contacting the shore.

Eddies and Mirrors

Sometimes vessels may be affected by eddies referred to as "Mirrors" by experienced mariners. In the Bosphorus Strait, eddies are formed by the water flowing against the coastal structure as it enters the curves of bays and capes along the path of the main current. Whirlpools form at the points where these eddies mix back into the main surface current. We generally refer to these whirlpools as "Mirrors." Skilled vessel operators warn their crews in such areas by saying "watch out for the mirrors." Because a vessel caught off guard can suddenly lose control.

In the Bosphorus Strait, a similar situation occurring between Kandilli – Bebek – Akıntı Cape is known as the Devil Current and can sometimes create serious difficulties for vessels navigating north or south during their turns. This area is also the narrowest part of the Bosphorus Strait.

Another area where the current can pose significant challenges is in front of Sarayburnu and near the Maiden's Tower.

The surface current hits the Rumeli shore at Akıntı Cape after flowing from Kandilli. The current is felt most strongly in the center of the strait between Defterdar and Çengelköy. In front of Beylerbeyi Palace, the current speed reaches 4–5 knots and flows toward Sarayburnu, brushing past Üsküdar. In this area, the speed of the main current from Vaniköy to the Maiden's Tower is around 3–4 knots.

For vessels heading south, the turn at the Maiden's Tower is another critical point that requires great caution. There have been vessels that lost control and ran aground south of the Maiden's Tower after making the turn. At this point, the port turn should be executed carefully, and unnecessary speed reduction should be avoided. If a vessel reduces speed excessively in this area, it may uncontrollably be swept toward Sarayburnu.

A vessel heading north may navigate closer to the European side after the Maiden's Tower to avoid the very strong current in front of the Haydarpaşa breakwater. In this part of the strait, the current is stronger on the Anatolian side.

Utilizing the counter current extending between Beşiktaş and Ortaköy can relatively facilitate the vessel's ascent northward. However, to correctly plan such an unusual course change, detailed local knowledge and years of experience are required.

Formation and Structural Features of the Dardanelles Strait

The Dardanelles Strait was formed by the flooding of an ancient river valley, similar to the Bosphorus Strait, during the 4th geological period. The most emphasized thesis regarding the formation of the Dardanelles Strait is as follows:

During a time when the Sea of Marmara was still a lake and the southern Marmara was still land, a river originating from the east of present-day Gelibolu flowed where the current strait extends and passed through a land area that was still dry between Imbros-Lemnos-Bozcaada, emptying into the Saros Gulf between Gökçeada and Lemnos.

Subsequent flooding created the valley of this river, forming the foundation of the Dardanelles Strait. In the Dardanelles Strait, there is an inverse current flowing from the Sea of Marmara to the Aegean Sea from above and from the Aegean Sea to the Sea of Marmara from below.

The length of the Dardanelles Strait, measured from the centerline, is approximately 32 miles.

The depths are generally quite ample, posing little problem. The widest part is 3,200 meters on the northern side and 3,600 meters on the southern side. The narrowest part is between Çanakkale and Kilitbahir, measuring 1,380 meters. The deepest point of the strait is also one of its narrowest points, located in the central channel near Nara Cape, with a depth of 104 meters.

Currents Affecting the Dardanelles Strait

There is a 20-centimeter level difference between the northern mouth of the Dardanelles Strait and the Aegean Sea mouth. The surface current flows from the Marmara to the Aegean, while the bottom current flows from the Aegean to the Marmara. Surface currents can be considered more regular compared to those in the Bosphorus Strait. The current flows at a speed of 1.5–2 knots from Gelibolu to the front of Nara Cape.

The current speed between Nara Cape and Uzun Cape averages around 1.5 knots. Eddies and whirlpools can form near Akbaş Cape. Between the northern entrance and the Nara passage, the current flows west-southwest (WSW) and is effective throughout the passage. Its effect decreases toward Poyraz Bay. Between Eceabat and Çanakkale, the direction turns south-southwest (SSW) and is effective.

After Nara, the current begins to flow even faster. In front of Kilitbahir, the current speed can sometimes reach up to 4–5 knots. Near Kepez Cape, the current speed is around 2.5–3 knots. The highest average currents are observed toward Nara Cape, while the lowest average is observed between Hisarlık Cape and Karanfil Cape.

Large eddies and whirlpools can be observed southeast of Kumkale Cape. The west-southwest (WSW) current, which is effective throughout the strait, can be seen between Kumkale Cape and Seddülbahir, with a speed between 1.5–3 knots.

Mutual Passage at Nara Cape

The Nara

Source: SeaNews Türkiye