Ada Kaleh buried by Danubian sediments

MAGYARUL

It's been more than fifty years since the island of Ada Kaleh, with its adventurous past, monuments and fort, was buried under the Danube after the construction of the Iron Gate I hydroelectrical power plant. Since then, it has been lost to sight, sleeping in its eternal slumber at a depth of about 30 metres. But a recently discovered sonar image gives us a rough idea of the decades the island has spent in the depths, and also reveals that, contrary to legend, the tower of the Turkish minaret will no longer emerge, even if the reservoir level drops.

Ada Kaleh in a watery grave (source) 

In the first days of December, Ada Kaleh's underwater sonar image was posted on several Facebook groups. However, the post, which included a source tag and information on the image, was later deleted, but by then the image had been downloaded. The image was also published in the Romanian media on 13 December, but aktual24.ro only reported the story of the island and the imaging process in a paragraph. According to this article, the image was taken with a multibeam echosounder (MBES) at an unknown time. The MBES is based on a sonar mounted on a floating structure, which emits sound waves in the direction of the seabed in the shape of a fan and calculates the distance from the reflection time. The sound waves from Ada Kaleh's deep-hidden ramparts and buildings return sooner because they are closer to the surface than the deeper forms of the Danube bed. The distance data are represented by a colour scale, where blue represents deep and red shallow. Unfortunately, the image does not provide any specific data in the form of a legend.

The sonar image shows only the western part of Ada Kaleh, lying like a wreck in the Danube. The sparsely built-up garden area to the east has been left out, probably because the survey was limited to a small area. It is worth comparing with the sketch of the island as a whole. The two-street settlement itself was located within the inner ramparts. In addition, there was an outer wall system, but the western tip of the island was also fortified with bastions with eaves in the mid-18th century. The Turkish settlement also included a mosque with a slender minaret expanding from it, which stood roughly in the middle of the island, above the eastern gate. If it is true that the fortress was demolished by the Romanian state before the flooding to build a replica of the bastions from the bricks on Simian Island, relatively minimal work was done, as the fortress walls are still sharply defined in the sonar image. Although the sketch does not show the topography, contemporary photographs or postcards show that the moat system between the outer and inner ramparts was standing water, while a small forested island grew in the southern inner curve of Ada Kaleh.

A sketch of the island before the flooding

From a hydrological point of view, the sonar image can be described as extraordinary. It contains extremely important information about the sedimentation processes that took place after the flooding. So far, the blog has published two large-scale articles on the sedimentation of the Iron Gates, one on the three towers of Trikule and the other on the Crown Chapel near Orsova, which illustrated the process of filling in the section of the river that was backwatered by the Iron Gate I power plant. The same can be done for the island of Ada Kaleh, where the sonar image gives a very detailed picture of the altered flow conditions and the associated sediment movements.

Farewell. Soon the ramparts will be swallowed by the Danube reservoir.

Let's take a look at the sonar image, especially the longitudinal positive shape extending to the right of the fortress. At first glance it looks like a long dune in the desert. Several similar forms can be observed within the fort area and in the western foreground. All of them take the same direction and are characterised by being 'lee shaded', i.e. they are formed behind a large projecting wall section or bastion. They do not follow the course of the Danube branches that surrounded the island of Ada Kaleh from two directions, but appear to run straight through the longitudinal axis of the island. It is also revealing that, apart from the ramparts and one or two houses, the settlement's street network is not visible. One reason for this is that the Romanian state has done a thorough job of systematically destroying the houses of the Turks, extracting as much building material as possible. Another is the aforementioned filling of the reservoir.

Once the island was submerged, the flow conditions changed fundamentally. The drift lines that had been bypassing the island in two directions merged just above the island, as evidenced by the direction of the sediment plumes. As Ada Kaleh remains in the centre line of the estuary, unaffected by the construction of the streamside alluvial cone, the sediment deposited in the fort moves in the direction of the Iron Gates dam in the centre line of the estuary. Where the water flow encounters an obstacle, such as along the line of the ramparts, it first deepens the bed by breaking through the obstacle and deposits the sediment washed out in quieter areas such as the western foreshore, the interior of the fort, or even the eastern extension of the island. The sonar image shows that the most significant erosion is at the base of the wall of the fort in the south-west corner of the island. This outcrop probably formed a significant depression in the riverbed before the flooding.

This suggests that the sediment conditions of the former island are not primarily determined by the sediment that is being deposited from the filling reservoir, but by the sediment that is being washed locally due to the changed flow conditions in the bed, and to a lesser extent by the trapping of transported sediment from further away. And it is the demonstration of this that gives the sonar image its importance, and we can only hope that measurements will be taken at regular intervals so that the data can be compared and possible trends in sediment accumulation and leaching can be identified, preferably for the whole island.