With a run of 270 meters, this will be Norway’s longest slit fish ladder. It opens the 1^st of July 2021 and will help bringing more salmon back into the Nidelva river.

The construction is a collaboration between Agder Energi (project management) and Rygene-Smith & Thommesen AS (the landowner).

Expertise related to the design comes from Norce LFI – laboratory for freshwater ecology and inland fishing.

The slit fish ladder was originally developed in the USA in 1995. The fish swims through slits in transverse walls or wings, without having to jump. In combination with a concrete floor covered with stones giving a rough bottom, very young fish and fish that are not good swimmers can also get up and down such a ladder easily. The width of the slits, the design of the wings and the design of the pools are important to get a working staircase. The goal is to create an S-shaped flow image in each pool.

The area has been scanned with a drone to best adapt the ladder to the topography of the Rygene waterfall. The ladder has a steady fall of 1:11 all the way and a total length of approx. 270 meters. The ladder ends in the original fishing channel by the pond, and the amount of water is regulated by a hatch in the pond.

The ladder is dimensioned for a water flow of 600 liters per second with a water level difference between the slits of 0.3 meters, slit width of 0.3 meters, pool length of approx. 3.5 meters and width of approx. 2.3 meters. The water depth in the stairs will be approx. 0.8 meters. In the upper part of the ladder, equipment is installed for the correct release of water, as well as a water flow meter and a fish counter.

We are very proud of being a part of this project!