Gundry Weir Fish Pass – Bridport

This month CSD were commissioned to undertake burning works to remove some sheet piling at Gundry Weir fish pass. The sheet piling (that is used to construct a cofferdam allowing for dry works to be conducted) was removed in order to open the newly built fish pass, allowing the free passage of migratory fish up and down the river.

Larinier fish pass installed at Gundry Weir in Bridport

The ability to migrate both up (anadromous) and down (catadromous) river is of huge importance to a number of fish species, e.g. salmon and trout, as they hatch upriver in fresh water before migrating to the sea where they mature before making the journey back as adults to spawn. To reach spawning grounds migratory fish must over come natural obstacles, such as rapids and waterfalls, and in recent centuries man made obstacles such as dams and weirs.  In addition a large amount of fresh water fish and eels must also be able to migrate within rivers to different food sources and habitats.

Cofferdam Around Pass
Flooded cofferdam around the fish pass ready for removal

In order to reduce the impact of both natural and man made obstacles  it is now recommended that a fish pass or fish ladder is installed around features in rivers that could block the migration of fish. These fish passes form a route around or over the obstruction to allow migration to continue. Although generally called fish passes or ladders there are a number of different types that facilitate the migration of different aquatic fauna both up and down river.

Cofferdam Burning
One of our divers conducting the surface burning making holes for the sheet piling to be lifted out of the river

One of the most common is the Larinier Pass, a type of baffle pass that uses precisely shaped and positioned baffles to disrupt the flow of water down a channel reducing the overall velocity, therefore making it easier for fish to swim against the flow of water. It is the favored pass in England and Wales as a range of species can use it, they are flexible in the width of the channel that needs to be built and require low maintenance. As such this was the type of pass chosen to be constructed at Gundry Weir in Bridport to reopen stretches of the river Brit, allowing for the migration of salmon, brown trout and sea trout that have been blocked by a number of weirs built in the 18th century.

With the opening of the fish pass, along with the others built previously, this has opened a large stretch of water and will hopefully help increase the number of migratory fish in the area.