Monday, 14 June 2021 12:39

Gornje Podunavlje and High Waters Problem

The area of Gornje Podunavlje is extremely flood-prone, as well as the entire territory of the upper Danube on the left bank of the river, and it continues on the right bank, known as Kopački Rit.


The geological material is represented by quarterly creations, which make the reticulated layer of Pliocene sediments. Soil is alluvial river values, mostly recent fresh alluvium, which is performed by new precipitation of materials every year when the flooding is performed. A wetland appears with a layer of clay in the background, and in higher parts, the Swamp rich soil "Crnica" and Chernozem with salt soil flows.


As the Danube performs alluvial infusions, as well, in the high water levels of the river, there is a significant erosion, in the first place of River Islands (ADA) and then unprotected parts of the river bank. The erosion of the coast in the newer period (the past twenty years) is known where the erosion was influenced by the river navigation course, where the Danube entered up to 430 meters into the mainland for about 5 kilometres long.


Construction on alluvial clay and sandy flood land are possible and feasible as seen in bungalows and holiday houses on either side of the riverbed. Namely, the style of construction that is applied here is modelled on the classic "sojenica" technique. The facilities are designed and masoned on pillars that provide a safe elevation in the case of the rise of the river up to medium water levels.


The territorial area of flooding land is 83.6 m above sea level to 86.6 m above sea level, while the standard waterway is 80.64 m above sea level. It looks very encouraging since the standard (middle) water level is lower than the lowest land point for almost 3 meters. However, extremely high oscillations of the Danube levels, often cause the area to flood, partially or the entire region. If we look at historical minimums and maximums, we will see that in 1909, the minimum was 79.22 mnm (water level -142) while the historical maximum was in 1965 at 88.40 MNM (water level +776), we will see that this is about up to 10 meters of deviations.


Although the historical maximum was back in 1965, medium-high and high water levels were measured relatively recently. For example, in 2002, a very high water level was measured from +742 (88.06 MNM) which is only about thirty centimetres lower than the historical maximum. Protective barriers that were built after 1965 can endure up to 90 m and it is considered quite enough to prevent catastrophic spilling of the Danube as in 1965. Also, the right side of the river is very well provided, both natural terrain configuration, and assisted by artificial barriers.


Of course, some areas are not included in the flood defence system.


How to protect the territory and on it successfully and purposefully raise buildings facilities?


In general, there are three basic methods and their combinations:


1. Construction of the banks, raising the embankment that would cover the territory of seven square kilometres, using as a model major earthworks in the Netherlands.

Problem: Raising the embankment that should be in a total length of about 11 kilometres and 4 meters (5.5 to 3 meters, depending on the terrain configuration. It would mean a barrier to install about 500,000 cubic meters to be installed Materials (soil, stone ...). This amount of material cannot be obtained on the territory of Gornje Podunavlje or its surroundings. The supply of such material is from locations that are 300 - 500 km away.
Of course, such "closure" of the territory is no guarantee that the built-in "pool" will not emerge from the underground sources to form an artificial lake.


2. The construction of facilities on the pylons would ensure the safety from floods, however, this causes access and infrastructure problems. The possible solution could be raising a road infrastructure above high water levels, meaning orientation on which 90 Mnm, bringing sand road solved. In this way, it would be provided to switch to the third construction modality.


3. The combined solution would probably be most suitable and most economical: preparation of a solid barrier in the highest region that is 85.5 mnm to 86.0 mnm. In this way, about 1/10 of the territory could be protected. The road cross-network is fixed the balance of barrier (bent) and inside the field build facilities on the pylons and dike the basins with alluvial sandy material that will deposit over the barrier over time.


In the future, build an extended travel network and thus gradually conquer new surfaces outside the scoring of flooding at high water levels. Of course, the new road network will be urban planning so that a segment by segment can be extracted in the future and expand useful territory.


Zoran Dražić

Member of Scientific Committee at International Water Conference
Land surveyor & GIS projectant