Short description
The fishway at the Geesthacht weir on the Elbe has a monitoring station at the upstream end. This is designed to guide upstream migrating fish through an access funnel into a catch chamber by means of inclined horizontal screens. The catch chamber itself can be lifted and emptied, with the captured fish directed into a holding tank from where they can be recorded as part of ongoing monitoring and tagged if necessary before being released into the headwaters. In order to direct the upstream migrating fish into the catch basin, a water injection by means of pumps takes place at the rear end of the catch basin. The injected water thus flows from the catch chamber through the entry funnel, creating a pronounced guiding flow in the near field of the funnel. By means of ethohydraulic tests, the design of the entrance funnel walls (in particular the cover) was aligned with the amount of water flowing out of the trap chamber in such a way that a high trapping efficiency of the system was ensured.
Since the pumps for the water supply have been replaced, the Institute for Applied Ecology GmbH has been able to prove from observations with a DIDSON sonar device and by comparison with the fish catches before the pumps were replaced that the catchability of the system has deteriorated. Specifically, the deficiencies are suspected to be due to a possible change in the guide flow out of the chamber. As part of a hydrometric measurement campaign by the Chair of Hydraulic Engineering, the flow signature upstream of the access funnel was recorded and compared relative to the signature developed at the time in the ethohydraulic test.
