The Möhlin weir is a central control element of the Breisach cultural weir flood retention area and thus assumes an important function in its operation.
The Möhlin weir was put into operation in the 1960s as a subsidiary of the Breisach culture weir. Since then, it has enabled the Möhlin to be impounded, which, together with the impoundment of the Rhine at the Kulturwehr, supports and stabilizes the groundwater levels. In 2018, the regional council of Freiburg (RP) built a fish ladder at the Möhlin weir.
Outside of times of high water use (regular operation), the Möhlin weir keeps the water level in the Möhlin, and thus that of the groundwater south of the weir, at a constant level. Since the weir flap remains closed during normal operation, the water of the Möhlin flows exclusively via the fishway and the associated bottom outlet. If the discharge exceeds the capacity of the fishway, the weir is opened more and more as the discharge increases. If the retention area is used at high water, the weir flap, the fishway and the bottom outlet are closed. The retention space is gradually filled. After use, the weir is fully opened and the retention area is emptied within about a day.
Fish Ladder(Fish Pass)
This fish pass was built as a vertical slot pass. The ones from upstream (south of the weir) to downstream (north of the weir) guided gutter is made by installing partitions in a series of lined up like a staircase pool divided. The water gradually flows through the pools through vertical slits in the walls.
This fish pass helps the fish to overcome the height difference of 2.6 m that occurs in normal backwater between the upper water and the exists underwater, to overcome. The 22 individual tanks must be of a certain size so that the fish can rest there from time to time during their migration.
A sufficiently strong lure current helps the fish to find the fishway in the tailwater of the weir. The 500 liters per second flowing through the pass are not sufficient for this. Therefore, an additional bottom outlet was built below the fishway runs. An additional 1,500 liters per second flow through here. In the end, the water from the pass and from the bottom outlet flow together and it is formed a lure current that runs straight through the Möhlin. Fish always follow the strongest current when ascending and can thus easily find the fish ladder.
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