More water, less gravel: Stierschlagsperre in Dürrach renovated

Created: 07/12/2022, 09:00

By: Veronika Ahn-Tauchnitz

The renovation work on the bull dam has been going on for two years.

Among other things, an opening was created at the bottom of the wall through which fish can also pass.

The road leads over the barrier towards Lerchkogelalmen.

© arp

For two years, the water management office has been renovating the bull dam in Dürrach.

Now they are almost complete - and improve the situation for fish.

Lenggries – times are changing.

This also applies to hydraulic engineering.

And over time, opinions change as to what constitutes sensible structural measures in rivers and streams.

A structure like the Stierschlagsperre, which was supposed to hold back debris, would no longer be built in the Dürrach above the Sylvensteinsee today, says Tobias Lang, who is responsible for the reservoir at the Weilheim Water Management Office.

"And we don't need them at this point either." After all, there has been a retention barrier at the mouth of the Dürrach in the reservoir since 2010, which prevents too much gravel from being washed into the Sylvenstein.

"At this point, the lock is right," says Lang.

Because the gravel can be easily excavated there and transported through the Dürrach valley without a long transport route.

The access road leads over the Steirschlagsperre in the direction of the Lerchkogelalmen

However, the 12 meter high bull barrage, which was built in the 1960s, cannot simply be torn down: the access road towards Lerchkogelalmen leads over it.

For two years now, the water management office has been busy with the conversion of the colossus in order to enable fish to migrate upstream and to change the Dürrach from a moderate water ecological state to a good one.

Now the project is almost complete.

By the way: Everything from the region is now also available in our regular Bad Tölz newsletter.

Up until two years ago, the Dürrach had no water

When work began in late autumn 2020, the Dürrach hardly deserved the name torrent.

At the Bächentalsperre, a few kilometers upstream, the water used to generate electricity was completely diverted in the direction of Lake Achensee.

The water from the tributaries up to the Stierschlagsperre seeped away completely in the gravel body.

"Because since the 1980s, the bull barrage has been full," says Lang.

This means that gravel had accumulated up to the top of the dam over the decades.

Before the dredging began in autumn 2020, the entire twelve meter high dam was filled with debris.

The Dürrach seeped into the gravel body.

© Water Management Office Weilheim

In a first step, the attachment was partially removed.

“After that, a hole was broken into the wall at the foot of the dam.” The opening is around 30 square meters large enough that there will be no sediment retention until about a 100-year flood.

The Dürrach now flows steadily through the widened opening.

The structure is now also passable for fish.

Since 2021, the Tiwag has had to leave a residual amount of water in the Dürrach

Since 2021, Tiroler Wasserkraft AG (Tiwag) has also been obliged to leave a certain amount of water in the Dürrach.

Since then, 143 liters per second no longer flow at the Bächentalsperre in the direction of Achensee, but remain in the stream bed.

It's not much, but it's better than nothing.

"There is another productive side stream from the Schafreiter, so that the Dürrach now carries water all the time," says Lang.

The stretch of water, which is around ten kilometers long, can now develop into a valuable habitat again.

The rock is currently being rehabilitated in order to maintain the load-bearing capacity of the structure

Work on the bull barrage will continue until around the end of the year.

At the moment it is a question of ensuring the load-bearing capacity of the structure.

Over the decades, cavities had formed in the rock behind the dam structure that had to be filled, explains Lang.

"The rock is weathered and there was damage from frost." But that should all be completed in a few weeks.

In the end, the Free State will have invested around 400,000 euros in the renovation.

You can find more current news from the region around Bad Tölz at Merkur.de/Bad Tölz.