SERT on tour – delivering a fish passage project on the River Ock

In September 2020, we successfully installed two fish passes on a weir on the River Ock in Abingdon. The River Ock is a small tributary of the Thames and owes its name to the pre-Saxon word “Ock” meaning young salmon. Salmon were a common sight on the Ock and a staple part of local diets in the middle ages. So why are these migratory majesties no longer present on the river?

Aside from other more global issues, barriers to migration such as the weir at Abingdon would present challenges to upstream migration of all fish species, not just salmon. The weir results in fragmented habitat which in turn can create bottlenecks at varying life stages. As a result, the survival and success of fish is compromised. It was therefore in the river’s best interest to implement fish passage on this weir.

The weir is a standard crump gauging weir, used by the Environment Agency’s Hydrometry and Telemetry’s team to monitor flows in order to help predict downstream flooding and manage abstraction licenses. It is 5.7m long and split into two channels. The left hand channel (as you look downstream) feeds the River Ock and the right hand channel feeds the Sandford Brook. The design of the weir, coupled with the fact that it was a gauging station, added several complexities.

Some baffling issues and a pilot

Due to the nature of the weir, the tried and tested method of using “Low Cost Baffles” to create a fish easement was not possible.

The width of the channel on the Sandford Brook side was too narrow for that. Calculations were done for the other channel to ensure our fish passage solution would not compromise the Environment Agency’s ability to collect accurate gauging data. These calculations showed that the position of the upper baffle would have to be too far down from the weir crest to create an effective easement, further complicating matters.

As a result, a more novel technique using fish bristles was employed. After calculations and discussions with the Environment Agency (EA), it was agreed that a bristle pass would be installed on the right side of the weir feeding into the Sandford Brook and an eel pass on the left side.

This bristle pass technique has not been used a great deal to date, therefore this site is somewhat of a pilot site. Due to EA guidance constraints, bristles could not be installed across the full Sandford Brook channel width and so we installed what we were permitted to.

Eel pass bristles © South East Rivers Trust

Bristles on the weir face

Discussions with various EA teams identified the preferred arrangement of bristles on the weir face. The bristles were to be set in a diagonal arrangement with ~300mm staggered centres. Due to the precise layout required for the bristles, plus the fact that we would be working on a wet weir face (despite installing a temporary dam at the top of the weir), meant we opted to pre-fabricate a plywood template which we could secure in place on the weir.

The template allowed us to drill all 175 holes safely into the weir, precisely where they were required. This method worked really well and compared to individual measuring and drilling each hole, was very rapid and easy to do.

Once all the holes were drilled, we then cleaned them, using a combination of water pressure from a one-inch water pump and cleaning brushes to remove all the settled debris and sediment within the holes. This was important to ensure that the fixant (used to help secure the bristles in place) could properly take hold and set.

Once this was done all that was left was to insert some of the fixant into each hole, followed by placing the bristles on top and then screwing a threaded bolt down into the hole. After the fixant had set, we then tightened nuts on the threaded bar to ensure that the bristles were securely fixed into place.

Some time later, we had successfully fixed and secured all of the bristles onto the weir face. The end result was certainly visually appealing! The bristles appear to be slowing the velocity of water by increasing the roughness of the channel whilst increasing the depth of water.

Drilling holes for bristles for an eel pass © South East Rivers Trust

Eel pass

The next job was to install an eel pass using lift out eel tiles fixed to the left hand wall of the weir.

First, brackets used to house the eel tiles were fixed into the weir wall. This involved first drilling pilot holes and then opening these holes up using a larger drill bit. We then fixed the brackets to the wall using expansion plugs and screws.

Installing the eel brackets on the main weir wall was relatively easy due to the fact that all work was done above water. However, several of the brackets had to be installed completely below water. This makes for challenging work as you basically have to do it blind, using your hands to feel out the correct place to be drilling, screwing down bolts etc. Thanks to a (not so) dry suit, Miguel unfortunately ended up rather wet after this section of the works.

Once the brackets were installed, the eel tiles could then be slid into place between the brackets. The lift out nature of the tiles means that they can be easily removed in order to routinely clean them in order to maximise their effectiveness. The brackets feature a locking mechanism that means that once the tiles are slid into place, they can be locked in to prevent the tiles from floating out of the brackets during high flows.

We hope that we can come back and improve fish passage over the rest of the weir in the coming years.

Special thanks must be given to the Environment Agency for their funding, ongoing discussions and support during the project.

Securing eel tile brackets