29 April 2021

Measuring the Babacai cliff

Stupidity is contagious, especially on the internet, where people copy nonsense from each other. The question of the height of the Babakai rock on the Al-Dunan is like the same wrong answer in a school test example. We don't know who copied it from whom, but I think it's important to have at least one correct answer, so we're going to measure the Babacai rock, located near Coronini.  

"Babakai Rock used to be a 50-metre high cliff in the Danube, but since the construction of the Iron Gate power plant, only the top of the cliff has been sticking out of the river." 

Whether you read (the Hungarian) wikipedia or other online descriptions of the Lower Danube, the Babacai rock was 50 metres high before the construction of the Iron Gate power plant. The same descriptions estimate the height of the current cliff at 6-7 metres. This Jurassic limestone cliff, which in Turkish means grandfather (wise old man), still defines the image of the Danube, which narrows from two kilometres to 400 metres between Coronini and Golubac. Now imagine how a 15-storey cliff would define the Danube?

At 50 metres, the height would most closely resemble a telecommunications tower made of stone. Since it only rises six or seven metres above the Danube, it is easy to calculate that the Danube must have risen by 43 metres in 1972, when the Iron Gate I power station, located about 100 river kilometres from Babakai Rock (1,041 km), was completed. Except that the Iron Gate I power plant has caused a dam to rise 33 metres, up to 35 metres, and this value is decreasing as we move away from the dam upstream. If the Babacai rock had been somewhere around Ada Kaleh there would still have been 15-17 metres left instead of 6-7. It is therefore easy to see that a height of 50 metres is, to put it mildly, a baroque exaggeration.

But how can we determine the true height of the rock? 

We need a contemporary geological description of it, or if not at hand at least a picture to help us. Of course, it wouldn't hurt to have a scale next to the rock to give an indication of the true height. It would take a rare stroke of luck to find one, but for once we were lucky. The picture you want is available on the Hungaricana website: 

Judging by the style of the picture, it was undoubtedly taken before the creation of the reservoir. We still have the base of the cliff, where it could have been moored. It shows a tree braving the elements and, directly next to it, two "castaways", just the right scale. If we estimate their height at 1.7 to 1.8 metres, we can determine the height of the Babacai cliff for a given water level by a simple pair of ratios.

Based on this calculation, the height of the Babakai rock is between 14 and 15 metres.

If in doubt about the accuracy of the calculation, validation can be done with the towers of the castle of Golubac, located on almost the same river-kilometre section, which have been flooded by the Danube since the construction of the dam. 

Naturally, the rock appears lower at high tide and higher at low tide. That's no small feat; 4-5 storeys. But by no means 50 metres.

It would be nice if everyone corrected it for the next lesson! ;)

Translated with www.DeepL.com/Translator (free version)

27 April 2021

100 years of the (Czech)Slovak Danubian border

In 1910 there were only two Slovak majority settlements on the banks of the Danube. One of them remained in Hungary after the Trianon decision, the other was annexed to Yugoslavia. The fact that there were practically only Hungarians on both sides of the Danube was not a factor in the drawing of the (Czech) Slovak-Hungarian Danube border in 1920. In today's post, we use a series of border maps recently posted on mapire.eu to explore what has changed along the 142-kilometre-long Danube border over the past 100 years. 

Before we start to discuss the changes in the course of the river and its surroundings, it should be noted that since 4 June 1920 the border between Hungary and Czechoslovakia has changed several times. It has changed so much that by the end of this post we may want back the absurd, unrealistic 'Trianon' borders along this stretch.

Before 1920, the Hungarian section of the Danube stretched from the mouth of the Morava river to Ada Kaleh, initially over a thousand kilometres in length. By 1893, this figure had fallen to 997 kilometres as a result of river regulation. Today, the Hungarian section of the Danube is 417 kilometres long (637 kilometres periodically between 1941 and 1944), of which only 275 kilometres are exclusively in Hungarian territory.

The Slovak-Hungarian section of the Danube by right and left bank (source: wikipedia)

On 4 June 1920, at 4:30 p.m., Hungary signs the peace treaty ending the first world war. Under this treaty, the main shipping route on the Danube between the mouth of the Ipoly and the northern administrative boundary of Horvátjárfalu (Jarovce) village south of Bratislava automatically becomes the border between Czechoslovakia and Hungary. The islands are divided between the two countries according to which side of the shipping route (Thalweg) they fall on. This line is not the same as the river's centre line, which connects points equidistant from both banks. And it did not necessarily coincide everywhere with the drift line of the river, which is officially defined as the imaginary line connecting the points of maximum velocity of successive cross-sections of the river.

The demarcation did not take ethnic considerations into account at all. There were Slovaks living along the Danube, but nowhere did their proportion exceed 25% (Dévény, Devín), and in the free royal city of Pozsony (Bratislava) it was only 10%. On the Slovak side, the closest Slovak-majority settlement to the river was Pozsonyhidegkút (Dúbravka) between Dévín and Bratislava, 3.2 km north from the banks as the crow flies. On the Hungarian side, Mogyorósbánya was slightly closer to the Danube, at 3.1 km.

A further ethnic geographical curiosity was that 88% of the Slovak population living in Esztergom County, cut in two by the Danube border, remained in Hungary, as most of them lived south of the Danube in the district of Esztergom. The same was true to a lesser extent in Komárom county, where 58% of the Slovak population remained in Hungary. Perhaps this is why the Entente negotiating delegation seriously considered leaving the Csallóköz, with its 98% Hungarian population, to Hungary. In the end, this was only an idea, as strategic considerations weighed more heavily. Under the Trianon decision, a short stretch of the Danube flowed exclusively through Czechoslovak territory, as Czechoslovakia was given the village of Pozsonyligetfalu (Petržalka), which belonged to the Transdanubia region.
On 2 November 1938, the First Vienna Award largely abolished the Danube border, except for a short section. Since Germany also received two settlements from Slovakia, such as Dévény and Pozsonyligetfalu, the Slovak Danube border was reduced to a short stretch between Bratislava and Szemet (Kalinkovo), which village was returned to Hungary. The capital of the independent Slovakia, Bratislava remained the only exit to the Danube. The border, measured on the basis of a pharmacy scale and taking ethnic relations into account as much as possible, remained in force until 1945. (you can browse this border here)

On 10 February 1947, under the Paris Peace Treaty, the Danube border between Hungary and the re-created Czechoslovakia was restored with one modification: three Transdanubian villages, the so-called Bratislava bridgehead, were transferred to Czechoslovakia. Dunacsún, Horvátjárfalu and Oroszvár were again separated by a strategic idea, as the majority of the inhabitants (mainly in Oroszvár) were ethnic Germans, while the other two settlements were Croatian-majority, with a significant Hungarian minority and a negligible Slovak population. There were plans to include Rajka and Bezenye in Czechoslovakia, but this was not supported by the Peace Conference. In 1947, the length of the Danube River within Slovak territory nearly doubled. As it later turned out, the decision had a disastrous effect on the Danube, which later allowed the construction of the Bős (Gabčíkovo) power station and the unilateral diversion of the Danube in October 1992. After the construction of the "Variant C", many in Hungary believed that since the border between the two states was still the shipping route, Doborgaz, Vajka and Nagybodak had effectively been transferred to Hungary through the diversion of the river. Unfortunately, as we will see, this is not the case, as the Danube border is not like the border of the Ipoly or the Maros rivers, where the border has to be redrawn at regular intervals to follow the changes in the river's channel.
After the long introduction, let's see what changes have taken place along the border in terms of islands. The map sections for the Danube show a close relationship with the 1:5000 scale site plan of the Danube of 1911. The inscriptions have been replaced by Slovakian ones, but there is still a typical phonetic transcription of Hungarian nomenclature, e.g. sihoť instead of ostrov. Since the 1925 version focuses mainly on the boundary line, all other distractions such as drift lines and inscriptions have been removed. 

The Muzslai-island, below Nyergesújfalu on the Slovak side, was still there. Its closed upper side branch has been almost completely silted up in the last 100 years. It will be worth visiting the buried enclosure one day.

The island of Nyergesújfalu was placed on the Slovak side, despite being closer to the Hungarian side. At that time it was much smaller, and the stone dyke to the Slovak coast did not yet exist. Over the last century, the island has grown upstream, and the forest that has grown on the gravel bar is still well marked by vegetation.

The island of Süttő is also on the Slovak side. Here, the stone dyke was already in place. Fortunately, it was later opened up, and the intervention created some interesting alluvial deposits. There are two islands in the Karva bay that have disappeared, one of them was the island of Karva, the other one I could not find a Hungarian name for, called Vrbinová sihoť in Slovak. 

Above Gönyű the situation is getting complicated. The regulation has changed the sedimentation of the river, creating gravel bars in the middle of the riverbed, which later became forested. However, they 'grew' out of the Danube, just along the shipping lane, so the unchanged border line bisects the upper island. 

The next stop is near Ásványráró at river mile 1818. The situation in Gönyű has deteriorated further, due to the Bős hydroelectrical plant's service water channel shown in the initial image. The discharge of the Old Danube has been reduced to a quarter and an eighth depending on the season due to the diversion. Due to the reduced flow the width of the river has halved since 1993. There are wide gravel bars on the river banks where the encroachment of vegetation seems unstoppable. The border is not on the shipping lane, just as ships no longer pass through. 

A part of the Bős dam and the artificially created Slovak Danube region. The meandering tributary system is mostly silted up, forested and gone.

The abandoned Dunakiliti dam was originally intended to divert the Danube. After the Hungarian side cancelled the treaty, the Slovak side built its own facility upstream at Dunacsun, which was made possible by the already mentioned 1947 Paris Peace Treaty and allowed the Danube to be diverted. 

At Dunacsún you can still see the inscription M.O. Hongrie, marking the pre-1947 border line. The settlement, which has a Croatian population, was annexed to Czechoslovakia in 1947, and a dam was built on its outskirts to divert most of the water flow to Bős. It is used to regulate the flow of water into the Old Danube and the Moson Danube. 

We see the former northernmost point of Transdanubia, northeast of Horvárjárfalu, in the Danube. Between 1920 and 1947 this was the point where the Danube entered Hungarian territory, between the 1860 and 1861 river kilometres. Nowadays, this point is 10 kilometres further south, at Dunakilit at river kilometre 1850. This section of 10 Danubian river kilometres have changed the inter-island landscape of the Danube Valley forever. 

It is quite certain that the Danube, the Szigetköz and the Csallóköz would look different if the "Trianon" Danube border were still in force...

Translated with www.DeepL.com/Translator (free version)

14 April 2021

Reflections on the Somlyó Island


The island of Somlyó is practically invisible from the Soroksári-Duna, just south of Budapest. Only three almost invisible landmarks reveal that a large island is hidden on the left bank between Ráckeve and Dömsöd. A bridge, a culvert and a lock ensure that you can find the hidden Dömsödi Holt-Duna, one of the most beautiful tributaries of the Soroksári-Duna.

The Dömsödi Holt-Duna surrounds the largest island of the Soroksári-Duna, the Somlyó Island, from the east. This alone makes the Dömsödi Holt-Duna the longest tributary, at 6.5 kilometres. However, the accurate lenght can be doubtful, since the tributary was cut in two in 1941 [1] with the construction of the Dömsödi Drainage Canal, which divided it into a 4.1-kilometre and a 2.4-kilometre section. It was built at the thinnest point of the Somlyó Island, where the island narrows to only 37 metres. This channel also bisects the odd-shaped Somlyó Island. The lower part, which is called "Upper Island" for the sake of clarity in Dömsöd, has already been described on this blog. 

On 22 October 2020, we walked the upper 4.1 km section of the Dömsödi Holt-Duna on the Somlyó Island side. The walk is not easy, at least not without a lot of patience. The main difficulty comes from the mosaic nature of Somlyó Island. This mosaicism is reflected in the settlement structure, i.e. the layout of built-up areas is not uniform, with clusters of weekend houses often interrupted by ploughs and woods. The topography is also varied, at least for a lowland Danube island, with many deeper parts, formerly remains of old tributaries, where most of the inland water is now drained. In many places there are still traces of former land use, farmland, which has not yet been completely eradicated by the parcelling out of land for weekend houses. Presumably, due to the considerable size of the island (3 square kilometres), this will take more time.

The width of the Dömsödi Holt-Duna is relatively narrow compared to its length, varying between 15 and 30 metres, which can be further narrowed by the reeds on the river banks. It receives some fresh water from the Soroksári-Duna at the island's head towards Ráckeve via an iron pipe under the road, but this is not enough to make it a living river. The estuary can reach a depth of 1.5 metres in places, but this is constantly decreasing as the low flow rate is unable to flush out the accumulated organic matter. It is often covered with frogweed, with reeds appearing in the lower part, close to the sluice. 

On both sides of the backwater, nature reigns, despite the mosaic of coastal cottages that in some places approach the Dömsödi Holt-Duna. There is no direct road along the banks, precisely because of the cottages, but access is easy on the paths perpendicular to the Dömsödi Holt-Duna, although some of these are closed by the residents. Where there are paths along the shore, they are mainly used by anglers. As well as illegal dumps. The left bank, the Danube–Tisza Interfluve, is more monotonous, with forests interspersed with arable land and no passable paths along the banks.

Since the closure of the Soroksári-Duna (1872), the Dömsödi Holt-Duna has not only lost a large part of its discharge, but the riverbed has also narrowed. Evidence of this can be found in the middle section of the backwater, where a ditch runs parallel to the road and is separated from the dead section by a bank. This ditch acts as a kind of suspended basin, filling with water in wet weather, marking the width of the former channel. Which was around 60 metres 150 years ago.  

Strangely, a little island could have formed in this narrow branch. The most interesting hydrological formation of the Dömsödi Holt-Duna is this single unnamed island, located between the line of Kála and Tűvelevelű streets. It is 140 metres long and has a maximum width of only 20 metres. It is much easier to find it on maps than in real life. The streets leading to this island either end in a reed bed, from where you can't see anything, or are closed off by a wire fence. On the opposite bank is also a holiday resort, but a small dug canal leads down to the top of the island (see picture below). There is also an impenetrable "jungle" on this side, but the lower tip of the island is not only inaccessible because of this, but also because of the upsilted backwater. 

This is the smallest island in the whole Soroksári branch, except for the floating bogs. As its upsilting is at an advanced stage, it is worth visiting it as soon as possible. The best way to do this is by canoe, as we found out on our tour. You only have to lift it twice. 

[1] SZABÓ BENEDEK: A társulatok szerepe a Duna-Tisza közi hátság vízgazdálkodási problémáinak megoldásában. in.: A Magyar Hidrológiai Társaság XIII. Országos Vándorgyűlése I. kötet (Baja, 1995. július 4-6.)

Translated with www.DeepL.com/Translator (free version)

11 April 2021

Skull of the Danube

Skull on the Danube (16 November 2018. image by: Kaszás Gergő

Gergő Kaszás shared some particularly beautiful drone images of this year's and last autumn's Danubian low waters. One of the most interesting had the shape of a skull of an ancient reptile. It could have been a riddle where the picture was taken, but surely few would have guessed, because this river regulation structure made of stone is not even found in Hungary. It is part of a curved guide bank built on the left side of the island of Süttő, in Slovakia, which directs the shipping route to the coast of Süttő. In this post we will examine it not only the shape, but also for the way it was formed. .

Below Dunamocs (Moča), the stone revetment starts from the bank, then continues in the river bed with a break where the smaller branch receives some water from the main branch, until it reaches the southern side of the Süttő island. This structure has existed for at least a hundred years and partially closes off a branch of the river. It was connected to the bank by a cross dam until recently. Between 2006 and 2010, this cross dam, which used to close the tributary, was cut through. The excavated crop stones were not taken far, but were piled up in gullies on both sides, thus forming these two strange parallel stone scatters next to the "skull's" eye. 

View of the "skull" half century ago (Fentrol.hu 12. November 1969.)

Both the construction of the curved guide bank and the dismantling of the cross barrier have profoundly altered the Danube's sediment regime. The Slovakian tributary was extensively silted up, but gravel and sand bar formation also occurred on the main branch. Over time, a gap appeared in the guide bank, and a current finding its way through such a gap formed the cavity of the skull's eye. A further 'pit', a much larger cauldron, was also formed, presumably by the disruption of the cross dam. The left-hand side of the picture shows how the increased water flow 'sucked' off the sediment deposited in the foreground of the curved guide bank. This is a particularly striking phenomenon at low tide, showing that the Danube bed is constantly changing, albeit under human influence.

Translated with www.DeepL.com/Translator (free version)