Session B6: Archimedes Type Passage Creates an Important New Fish Migration Route at Pump Station Halfweg (Rijnland Water Authority; Netherlands)

Abstract: Pump station Halfweg is situated west of Amsterdam. It is an important discharge station at the borders of the Rijnland Water Authority and the North Sea canal. The region of the water authority is mostly situated below sea level and surrounded by dunes and dikes. The function of the pump station is to remove excess fresh water from the water authority region and to prevent “salinization”. Intrusion of salt water from the canal is prevented by a combination of valves and shutters at the pump station. The valves and shutters are only opened during water discharge. As a result for decades the inward migration of fish was not possible and the fish could not reach spawning and hatching areas. In 2012 on the fresh water side of the pump station an innovative fish passage was completed. An Archimedes type pump was used. It pumps fresh water into a container at regular intervals each day. The water level in the container is 25 centimetres higher than the average water level in the water authority region and about 5 centimetres higher than the level of the canal. Furthermore, the container is connected to a one metre diameter 30 metre long pipe, which discharges in the canal. Pumping results in a fresh water discharge in the brackish canal zone, that attracts migratory fish. The velocity of the discharge is about 0,1 m/s. Most migratory fish are capable of swimming against this current. Fish arriving in the container are given the opportunity to escape through a small opening to the water authority region. It is also possible to close the opening in the pipe with valves, to protect the Rijnland region. During a 6 week monitoring period in the spring of 2013 more than 130.000 fish were counted in a fine maze net attached to the passage. Most counted were elvers and 3 hatched stickels. Large amounts of elvers were counted in 2014 too; in one week more than 50.000 individuals. The results clearly show the necessity of passages for migrating fish species. Long term monitoring may follow the trends at this location