PROJECT

Hogsmill Connectivity Project

Fish swimming the Hogsmill River once had to tackle as many as 19 obstacles along this 11 km chalk stream.

Since 2012, the South East Rivers Trust has addressed 18 of 19 barriers to fish passage on the Hogsmill, making it much more fish-friendly. By the end of the project, all 19 will have been tackled meaning fish will be able to travel from Kingston to Ewell without interruption.

Before: Weir on the Hogsmill © South East Rivers Trust

After: Weir on the Hogsmill © South East Rivers Trust

Before: A240 weir on the Hogsmill © South East Rivers Trust

After: A240 weir on the Hogsmill © South East Rivers Trust

barriers removed

5
barriers eased

13
metres opened for fish passage

6000

Fish need to commute, just like we do

It is often thought that restricting the movement of fish only affects migratory species, such as trout and salmon. However, all fish migrate to some extent.

Coarse fish migrate within a river to find their required habitats. For example, chub and dace migrate to find the best available gravels for spawning, increasing the survival of their eggs.

Brown trout © South East Rivers Trust

A fragmented river

The Hogsmill River had an average of one barrier to fish passage every 650 m – a truly fragmented river.

These obstructions included weirs leftover from its milling past, bridge footings of major roads such as the A3 and a gauging station.

These barriers greatly reduced the resilience of the river and fish populations. Fish were unable to find the habitats required for their life cycles. The genetic variability of the population was lowered and if a pollution incident took place, there was no chance of recolonisation from downstream.

One of several weirs once found on the Hogsmill © South East Rivers Trust

Removing barriers

Wherever possible, removing a weir results in the best outcomes for the river.

By removing a weir, not only do you open fish passage, but you also reinstate the natural sediment transportation process within the river.

Since 2012, the Trust has removed five weirs from the Hogsmill, naturalising these stretches of the river.

The image shown is after the weir picture above was removed by SERT.

After shot of the Hogsmill weir removal © South East Rivers Trust

Building rock ramps

It is not always possible to remove a barrier.

On the Hogsmill, the bridge footings of the A3 and A240 were barriers to fish passage and for obvious reasons, could not be removed. Here, we used different techniques to facilitate fish movement.

For example, rock ramps (pictured) gradually raise the level of the river, creating a long ladder for fish to swim up and over the obstacle.

Reconnecting the Hogsmill to the River Thames

The Hogsmill gauging station is an Environment Agency flow monitoring structure, essential for water resources planning. It is the furthermost downstream weir in the catchment, preventing the recolonisation of fish from the Thames.

The Trust worked with the EA Fisheries Team to design a novel solution for this barrier. This was vital to ensure the non-standard weir structure was passable to fish and did not compromise data collection in the process.

Low cost baffles (pictured) were fixed to the slope, with each line incrementally taller than the last, providing enough flow for fish to pass over. In the photo, you can see the low cost baffles fixed to the weir while the river was diverted as we worked.

Monitoring by Durham University showed 45.2% of tagged fish successfully passed the low cost baffles and 35.8% over the whole weir. Fish species recorded using the pass included roach, barbel, chub, and dace.

A technical triumph

Perhaps the most challenging of all were the barriers in the Hogsmill Sewage Treatment Works: a concrete slope and three weirs.

Our chosen solution was a combination of a rock ramp at the bottom of the site and a pool pass through the upper section.

The pool pass (pictured) might seem like a lot of concrete but in this urban concrete channel, it was the best solution to help fish move freely to healthier habitats upstream.

After many years of work, the Hogsmill Connectivity Project has opened up to fish passage more than 6km of river that was previously closed off habitat.

Projects such as this are crucial to the resilience of rivers as ecosystems functioning naturally. The South East Rivers Trust is excited to build on the successes of this project by increasing connectivity in its other catchments in the future.

barriers removed

barriers eased

metres opened for fish passage