How to block out the main

Already a slightly queasy feeling to be standing seven meters below the level of the main river. On the chapped concrete floor in the shadow of the weir gate at the schweinfurt barrage. The water here is usually brown. The weir gate flussauarts should hold. It has been going on for over 50 years.
The water masses from below hold up a barrier that looks a bit like an oversized engineering construction kit: six panels of two steel plates each, mounted on A-shaped riders that are in turn anchored in the concrete floor.
The experts call the barrier an "inspection lock. The plates protrude into the drained space, they tower above the people by far, and behind the plates the main river stands four meters high. In a few places it seeps through, especially on the sides where the steel meets the concrete. But what goes in, two pumps immediately take out again.

Capable of handling 19 barrages

The schweinfurt waterways and shipping authority (WSA) is responsible for 19 barrages. "A barrage consists of a sluice, a weir, a hydroelectric power plant and a dam", it says on the homepage of the office. The locks are drained and inspected once a year, the weirs every six years. The principle is quickly explained: the main is blocked off above and below the weir, the water is pumped out, and the 30-meter-wide weir gate is dry and can be inspected and, if necessary, repaired or improved. The execution is a little more complex. "You couldn't build the plates with millimeter precision, then it wouldn't be possible to mount them", says helko frohner, deputy head of the WSA schweinfurt. The assembly is quite difficult as it is. The riders must be erected and anchored while the water is still in the river bed. This is only possible with a diver, who instructs the crane by radio, which hands him the riders, so to speak.

Diver works by feel

The concrete floor is lined with holders into which the diver has to insert the riders. He does this primarily with a sense of touch and feeling, because visibility down there is minimal. The steel plates are mounted on the riders, and then the pumps go into action.
The pit between the weir gate and the lower lock has been dry for a long time and is easily accessible from the bridge via a specially installed stairwell. The upper pit is to be drained this morning. A few days ago, the "stor" has already the upper revision barrier put in position. The "stor is a so-called floating grab – a river ship with a crane on top of it. The WSA has four of these vessels, the more modern of which are multi-purpose workboats, and they all have the name "waller", "zander" and "stone riders.
The upper inspection lock is unique, a special construction that fits three locations: schweinfurt, viereth and ottendorf. A huge steel plate that looks like an L in profile, with buoyancy tanks mounted on both ends of it. So the cap can float. If the tank is flooded at the long spar, the plate sinks and forms a barrier in front of the weir gate.
Two pumps are already pumping out the future pit between the weir gate and the revision gate. The "stor has moored above. The diver operates from here, spotting leaks while the water level on the other side of the steel wall is still sinking and plugging them with sandbags that his colleagues above throw into the water as accurately as possible. Here, too, feeling and experience are called for: "you can't see the leaks", says helko frohner. "The diver tracks the flows with his hands."
A compressor on board supplies it with air. The boat crews are sworn teams, frohner says. And the diving teams are especially. The signalman, recognizable by the thick head horns, passes on the diver's radio instructions to the rescuers or the crane driver. The diving operations manager ensures that the two cables between the ship and the diver – air and radio – remain free and undamaged at all times. Another diver is ready to rush to help at any time, should the colleague under water get into trouble.
There are two steel weir gates in schweinfurt. In order to prevent a flood wave from building up and to ensure that the hydroelectric power plant, which produces electricity for 7,000 households, has optimal conditions, the head water, i.E. The level in front of the power plant, must always be at the same height, which is prescribed for all dam levels. In schweinfurt, this mirror must always be a little over 207 meters above sea level.

Remote control

And this mirror will – by the way – be remote-controlled by E.ON-wasserkraft employees located in langenprozelten – regulate the weir gates. "That's why it's so dangerous to swim near the weir", says frohner. "It is a machine that moves without you noticing it."
In the event of minor fluctuations, the flap mounted on top of the gate is raised or lowered; in the event of very high water, the entire gate is raised so that the water masses can rush through underneath – when this is the case, usually after extreme precipitation in winter or spring, onlookers like to gather on the shore. Curiously, in schweinfurt, the first place where a roller fire brigade was ever deployed, there is no roller fire brigade in operation.
The weir flaps are anchored in defiant, tower-like concrete structures, the windmill houses, which also house the machinery: motors and chains that can move and lift the flap and weir gate respectively. After 52 years of operation, some renovations are necessary: the electric motors including the gear wheels and gears and the four-ton chains will be completely replaced. "Even though these chains are only used four times a year, they wear out", says helko frohner "and at some point the replacement is cheaper than the repair."
The weir gate weighs 70 tons – this explains the thickness of the chains and the size of the cogwheels. "The load-bearing capacity of each individual tooth must be calculated beforehand", says frohner. Like the entire planning of the inspection work, this is also carried out by the FMS – the WSA's southern mechanical engineering department in nurnberg. The implementation will then be carried out by the WSA's wurzburg construction yard. The parts in turn are made by specialized companies "there are only two or three left", says frohner. The defense gate can also be moved by muscle power in the event of an emergency. Jurgen reberg, responsible for building supervision, shows a hub on which a crank was then placed. Neither reberg nor frohner can recommend such an action: "you'd need a whole basketball team or a firefighting team. Because after a few minutes of cranking they are flat. And in ten minutes the gate moves maybe one centimeter during the transfer."
As soon as the new chains have been delivered and hooked in, the weir gate will be completely raised and lowered on a test basis. An engineer from the WSA examines the condition of seals, rivets, bolts and joints. He reports his findings to the FMS, which then decides whether and what further work is needed.

Control every six years

19 weirs, each of which has to be inspected every six years – the crews are on the road non-stop from march to november every year. Work in schweinfurt should also be completed by november. After that we can expect a lot more water in the main again, so both weir gates should be ready for operation.
450,000 euros have been budgeted for the machine parts, and 250,000 euros for electrical and electronic equipment. The latter is considerably ruffled: even more sensors report a variety of information such as water quantities, water levels, chain movement or the force needed to lift the flap. They do this via a completely dedicated fiber optic network that runs along the mains. "For example, we are always protected against lightning strikes", says frohner. In fact, the operational readiness of river regulation is crucial for everyone: "by moving all the barrages in a coordinated way, we prevent floods from building up. We call this vibration calming. And we don't want everything upstream or downstream of a barrage to go down", says helko frohner. Mathias wiedemann

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