A protective dike and derivations are to provide security in Gaißach – start of construction in 2024

Created: 11/30/2022, 5:42 am

By: Rainer Bannier

Great Gaißach flood protection: the plan shows the 500-metre-long protective dike (top center of the picture) and the relocation of the old Gaißach outlet (top right). © Water Management Office Weilheim

Gaißach - The planning approval procedure for planned flood protection structures on the lower reaches of the Great Gaißach is now being initiated by the Weilheim Water Management Office.

The Great Gaißach flows from the mountains into the valley.

In the event of flooding, the Gaißach-Mühl torrent, the Tölz settlement, the Mosersäge industrial park and the bridge of the federal highway 13 can be a threat.

Together with representatives of the commissioned planning office, project manager Johannes Haas from the Weilheim water management office has now explained to the Gaißacher municipal council the repeatedly delayed planning for flood protection on the lower reaches of the Große Gaißach.

Depending on the course of the formal planning approval process, Johannes Haas and planner Heiko Nöll expect that construction work can begin in 2024, which will then take one to a maximum of one and a half years.

They estimate the construction costs at around four million euros.

More than 86 cubic meters per second possible

In contrast to the Isar with its very extensive catchment area, the Gaißach is such that in the event of heavy rainfall on the upper reaches, flooding "comes very quickly and is gone quickly," explains Nöll.

For a so-called "hundred-year" event, the water economists expect 86 cubic meters per second according to current calculations, but theoretically much higher flow rates are conceivable.

On the north-facing, low-lying meadow towards the Tölz settlement - these areas belong to the town, but are farmed by Gaißach farmers - a protective dyke, starting at the substation, around 500 meters long and increasing to the west, is to be raised.

The soccer field in this area will be moved south into the flood zone.

The bank reinforcements are also to be strengthened towards the southern, higher-lying Gaissacher commercial area Mosersäge.

It is not possible without traffic restrictions

Up to the underpass of the B 13 - this is the neuralgic point and the "eye of the bottleneck" in the event of a flood - the bottom of the water body is to be lowered in many small steps by up to two meters and the bridge structure is to be refounded and statically stabilized accordingly.

It will not be necessary to demolish and rebuild the heavily used overpass, but according to Johannes Haas there may be a temporary one-way regulation for traffic on the bridge: "It won't work without restrictions."

The existing weir system below the bridge, which collapsed during the flood of August 2020, causing the adjacent properties and houses to be flooded, will be dismantled.

According to Johannes Haas, the gradient of four meters in total up to the confluence with the Isar is “stepped in steps and streamlined” so that fish and other small creatures can again “continuously pass through the entire body of water against the direction of flow”.

The former derivation at the weir into the Linsesägbach will be filled and drained.

Instead, there is a diversion from the Gaißach to this canal-like Stadtbach, almost 200 meters further upstream, by means of a pressure pipe, through which a water volume of 75 liters per second is released.

Mühl is deferred for flood protection  

As early as March 2016, the water management office had proposed to the Gaißach municipal council planned flood protection measures for the area where the Große Gaißach flows through the Mühl district.

However, due to community concerns, this planning section has been shelved for the time being and is no longer a top priority.

In response to a corresponding question from local councilor Susanne Merk, Johannes Haas and Heiko Nöll made it clear that the flood protection on the lower reaches of the Gaißach - there the water masses run off much better there and correspondingly less damage to crops - but "no positive effects on flood events in the upstream District Mühl results".

With regard to the land negotiations with the affected farmers, which are being conducted by the town of Bad Tölz, Johann Haas sees their "fundamental agreement" in sight - also because the planned construction measures would cause the meadows to be less affected in the event of a future flood .