Accelerated current caused by newly built weir is rapidly eroding the Danube riverbed at the Paks nuclear plant

After the authorities artificially raised the water level near the Paks Nuclear Plant, the accelerated current quickly started to erode the submerged weir being built near the Paks nuclear power plant. This caused the Danube’s riverbed to deepen by 8–13 meters in that area, making it uncertain if the structure – being built in response to a historic draught – can be completed in its originally planned form.

Hungary rushed to raise the Danube’s water level at the Paks nuclear power plant after an exceptionally dry summer brought the plant close to a complete shutdown. The plant uses the river water as coolant, and in early August, 3 of the plant’s 4 reactor units had to be taken offline because of the extremely low river level.

The government responded by sinking two 80-metre barges into the Danube as a temporary measure and launching the construction of a submerged rock weir near the plant’s cooling-water intake. The structure, designed to raise the water level by as much as half a meter, is to contain around 145,000 cubic meters of rock.

Narrowing the Danube accelerated the flow

The weir was built simultaneously from the two banks towards the center of the river. As the two rock embankments approached each other, the remaining opening through which the Danube had to flow became increasingly narrow. While this raised the water level, allowing the Paks NPP to return to normal output, it also increased the velocity of the river’s current. The accelerated flow placed considerable stress on the structure and began eroding the riverbed beneath it.

Videos released by the government at the beginning of construction indicated water depths of around four to five meters at the deepest points of this section. In more recent videos, the Minister of Defense confirmed that the riverbed is eroding fast, saying that it has deepened to 13, and, in some places, as much as 16–18 meters.

If the measurements refer to comparable locations, the river may therefore have scoured the bed by approximately 8–13 meters in roughly two weeks.

For comparison, the bed of the Hungarian section of the Danube has deepened by around two meters on average over the past half-century.

Balázs Szendőfi, a nature filmmaker and ichthyologist, told Átlátszó that the riverbed around Paks consists mainly of sand and smaller gravel, with older, more stable gravel and marl layers underneath.

According to Szendőfi, the extremely fast current created by the unfinished weir began removing these lower sediment layers at an unusually rapid rate. This could produce precisely the opposite effect to what the project is intended to achieve: as the riverbed deepens, the water level can also fall. Long-term riverbed erosion is already one of the factors contributing to increasingly extreme low-water conditions on the Danube.

Water authority says erosion had been anticipated

The project is being carried out jointly by Hungary’s General Directorate of Water Management (OVF) and the Hungarian Defence Forces.

OVF told Átlátszó that the erosion was not the result of a construction failure. According to the authority, scouring had been expected during construction, particularly after the river’s cross-section had been significantly narrowed and the original construction schedule accelerated.

The authority said hydrodynamic and sediment-transport modelling had been used in advance to estimate the likely extent of erosion. Large concrete blocks had already been transported to the site weeks earlier because their possible use for stabilisation was part of the available construction methods.

The Ministry of Defence gave a similar explanation. It said the concrete blocks were not required because of a construction defect, but were chosen because their size and weight allowed them to be positioned quickly and precisely. According to the ministry, the intervention has stabilised the riverbed and no further scouring has been detected.

Neither OVF nor the Defence Ministry, however, answered Átlátszó’s question about whether the weir can still be completed as a single continuous structure in accordance with the original design.

OVF said that some sections of the structure were always expected to be partially dismantled during later phases. Temporary overbuilding had been necessary, it said, in order to reach the water level required by the nuclear plant as quickly as possible.

The Department of Hydraulic and Water Resources Engineering at the Budapest University of Technology and Economics participated in designing and preparing the project. According to an earlier university statement, its experts worked on the hydraulic effects of the submerged barges as well as questions concerning flow conditions and riverbed morphology around the new weir.

The university declined to answer Átlátszó’s questions about whether the scale of the erosion corresponded to the modelling carried out before construction.

Ecological risks remain unclear

Szendőfi also warned that the intervention could have significant ecological consequences both upstream and downstream.

Sediment may accumulate above the weir, while downstream the faster current may remove gravel and other material from the riverbed. Both processes can destroy habitats used by aquatic organisms.

The Danube is home to several endemic or highly protected fish species, including zingel and streber, as well as the European mudminnow and several gudgeon species.

Against the background of a global biodiversity crisis – and with flowing freshwater habitats among Europe’s most vulnerable ecosystems – Szendőfi argues that continuous ecological monitoring around the construction site should be a minimum requirement.

This is particularly important because, as Átlátszó reported earlier, construction of the Paks weir began without the environmental impact assessment and Natura 2000 assessment normally required before an intervention of this scale.