The last in the six-set of Europe’s largest horizontal Kaplan turbines went on line at the 172MW Freudenau power station on the outskirts of Vienna, Austria in March 1998. It is the ninth of owner Donaukraft’s low head stations on the river Danube and offers a mean annual generation of 1037GWh — equivalent to half the domestic needs of the city’s 1.7M inhabitants.

Austria has a traditional reliance on hydroelectric power. Its intense exploitation of the Danube began in 1959 with the construction of Ybbs-Persenbeug and was followed by nearly 26 years of continuous construction of run-of-river stations — the river now provides more than a quarter of the country’s electricity. A prominent Vienna banker summed up the exploitation of the Danube during the heyday of hydro power as ‘dipping one of those Kaplan things in the Danube and collecting the money’.

Since its inception in the early 1980s, the US$1230M Freudenau project has survived a gamut of economic and political change. But the station offers more benefits to the environment than to the supply industry, and latterly its owner Donaukraft (a subsidiary of the Austrian company Verbund) said that given the benefit of hindsight, it would not have built the plant.

The scheme was a continuation of Austria’s traditional pursuit of hydro power, and shunning of nuclear energy. In 1978 the importance of hydro power in Austria had been boosted by an unexpected and massive environmental backlash against nuclear power. The Green party was able to gather sufficient support to instigate a referendum to stop the commissioning of Zwentendorf BWR nuclear power station after it had been built.

While 40% of its owner’s electricity is delivered into an ever-widening free market, critics point out that Freudenau is generating electricity at three times the cost of the cheapest nuclear energy. In its latest financial report Verbund admitted that its own prospects for export in Europe are limited by the seasonal availability of water.

Critics, however, are in a minority — Freudenau was promoted publicly as a way of rectifying the environmental damage to the water table, caused by a century of river engineering on the river Danube at Vienna, rather than as a producer of power.

Rectifying environmental damage

In Vienna, the four waterways called ‘Danube’ are more feats of engineering than bequests of nature. One hundred and twenty years ago the silt laden brown river itself, the Old Danube, lay in

the floodplain to the northeast, imposing itself on the city only during the occasionally disastrous floods.

Only one of its tributaries flowed through the city.

It was nine years before the first of the series of canalisation and regulation projects brought two Danubes to the city with the construction of the Danube Canal; the diversion of most of the Old Danube into a diverted Danube. A hundred years later, the New Danube was constructed alongside the Danube as a relief river.

The schemes brought river navigation to the city and an end to the flooding. But in the 1970s it was evident that the removal of much of the floodplain had caused serious engineering and environmental problems. The reduction in percolation from the surface water led to denudation of the water table, at a rate of 2-3cm a year. Consequently the bearing capacity of the drained sands and clays was reduced, and over a period of years, damage was noted in the foundations of bridges and other heavy structures, while new structures were requiring more extensive and costly foundations.

Perhaps more seriously to the country that leads Europe in its dedication to the environment, the famous Prate woods were suffering serious damage. The white willows and grey, black and white poplars were showing the symptoms of dehydration, early shedding of leaves and dry buds, which precede death.

These were only the most obvious signs of damage to the complex riparian animal, insect and plant ecosystem on the forest floor. However, the regimentation of the Danube in Vienna was not held sole culprit — hydro power was playing its part.

Less than a decade after the nuclear debacle, environmentalists were pointing out the ecological damage caused by low-head hydro schemes. Attempts to build a ninth Danube station at Hainburg, near the Slovakian border, were fiercely resisted by protesters who feared its effect on the wetlands, particularly those around Marchegg, which is a summer host to Europe’s only colony of tree-nesting storks.

After the scheme’s opponents had exhausted the highest legal appeal, thousands of Austrian conservationists occupied the site illegally to prevent site clearance and tens of thousands demonstrated in Vienna. The government intervened with a 12-month moratorium — a temporary measure that is now 12 years old and unlikely to be lifted.

Upstream of Vienna, the eight Donaukraft run-of-river power stations impound more than two-thirds of the 150m drop between the German

and Slovakian borders. The total backwaters account for three-quarters of the Danube’s 350km course within Austria and trap nearly all of its sediment.

The periodical cycles of scouring are no longer compensated for by periods of sedimentation and the river-bed is sinking, further depleting the water table.

Ninth Danube station

In the early eighties Verbund began considering Vienna as a site for its ninth Danube station. It was not the most attractive engineering proposition. The location offered a head of only 8.5m, lower than any of the eight Danube sisters which operate under gross heads of between 9.3m and 15.3m. With a mean flow of 1700m3/sec, the second lowest among the other plants, the Vienna site offered the lowest hydraulic potential of any Danube station.

But one economic consideration did weigh in favour of the scheme. The partially privatised Verbund was facing the cost of social democracy and welcomed the tax advantage of the investment. It financed 75% of the cost of Freudenau. The remaining 25% was provided by Vienna City Council and the adjacent province of Upper Austria.

The government, however, was taking no more chances with public opinion. A three-year study of the project’s impact on the hydrology, limnology, botany, zoology and climatology of the area was carried out by Vienna University Department of Agriculture. Instinctively suspicious of governments and industry, the Austrians have great respect for their academic institutions, and the science of agriculture has particular environmental credibility in a country that has more than half of Europe’s organic farmers.

The study found in favour of the project, and in 1991 the scheme was put to the people of Vienna in a plebiscite. Seventy-seven per cent of the 44% who voted wanted the scheme to go ahead.

Construction at Freudenau began in the autumn of 1992 with diversion of the shipping to the left bank of the river and construction of the first shipping lock. The civil works to the shipping locks, turbine bank and spillways were carried out on the river within embankments constructed from the spoil from dredging upstream.

Apart from the limited power potential, the location of Freudenau posed additional design and construction problems. The width of the river on the outskirts of Vienna is restricted by the parallel New Danube on the left bank, and industrial development and the city’s river port on the right bank.

This proved to be insufficient room for what had become the standard nine-turbine sets built in Donaukraft’s other Danube stations since 1970, when the company moved from vertical to horizontal bulb turbines. Such a lack of space led the Freudenau designers to adopt a six-turbine set with 7.5m diameter rotors.

The restricted width necessitated wet construction within the river amid a design flow, based on a 10,000-year statistical risk of 9200m3/sec in the Danube and 4800m3/sec in the adjacent New Danube. The actual total flow during the construction period was 7000m3/sec.

International standards stipulated that 150m of the river’s 260m width be maintained as a shipping way with 2.7m minimum draught and 8m clearance. The nearby Prater six-lane road bridge was raised by 1.8m to achieve the necessary headroom.

Below the Danube the surface geology typically consists of 10m of gravel overlaying 20m of sand above heavy clay, with a 6m sand strata and occasional lenses of sand. Cut off was achieved variously by sheet pile and slurry trench bentonite diaphragms to depths of between 30 and 50m — the deeper for the turbine banks. Unexpected pore pressure in one of the sand strata below the deeper excavations led to some additional de-watering.

The two shipping locks, set against the right bank, are 24m wide and allow a draught of 4.87m under normal river levels.

In each of the six 21m wide midstream turbine blocks, the 992 tonne impellers, operating at 65.2rpm under a design flow of 500m3/sec, are coupled to 30,300kW bulb turbine generators. The two 96,000kVA block transformers

are situated in blocks one and four. The four 24m wide spillway channels on the left bank are designed to accommodate total flows of between 3000 and 8200m3/sec. The 13.85m high segmental counter-weighted gates include an hydraulic trimming flap operating over 25% of the impounded head.

The water table under Vienna is now controlled by twin diaphragm walls, extending almost halfway along the 28km backwater. They allow controlled recharging of the aquifer, subject to the quantity and quality of the water in the river and the aquifer, by 51 pairs of extraction and recharging wells. Five hundred wells in and around the city monitor the ground water levels.

Three stations carry out continuous sampling of ground water adjacent to the river and three further monitoring stations analyse samples from the river itself. One of these carries out Dynamic Daphnia tests with infra red sensors automatically monitoring any deviation from the normal swimming behaviour of the one-day-old water fleas.

The roots of the trees in the Prata woods now have sufficient water and environmentalists are waiting for signs of improvement. It will be some years before the woods can be regarded as out of danger and some specialists fear that the pore structure of the dried soils may have been too damaged to regain the former permeation.

Towards the end of the backwater, the flow into the parallel New Danube is revitalising what was becoming a sluggish flow of suspect water bordering the other edge of the city’s favourite recreation area, the narrow Danube Island. A stretch of the island has been taken over by an ecological brook which also serves as a fish pass (see photograph on opposite page).

Regardless of the problems which hydro power has encountered over recent years, the alpine states’ rivers are still seen as a stable energy source, compared with alternatives. As late as last year, Verbund was reminding its shareholders of the fate of nuclear power in Austria and the

oil price shocks of the sixties and seventies.

The owner of the Danube stations, Donaukraft, is the largest subsidiary of the Austrian giant Verbund, which was formed as a state industry in 1947 and 49% privatised in 1988.
Verbund owns the national grid and generates half of the country’s electricity — 90% of it in its 70 hydro stations.
It is the third most heavily traded stock on the Vienna Exchange where its shares, averaged over 1996, increased by 34% (compared with a 19% rise in the Austrian Traded Index).
During the 1996 operating year Verbund’s consolidated operating results fell by 33% to US$219M. More than half of the decline was attributed to extraordinary write-offs and
accruals to face the deregulation of the European electricity market, and much of the remainder is due to its inherent vulnerability to climatic conditions.