The Successful Delivery of the Costers Brook Connectivity Project

 

The Costers Brook is a delightful chalk stream which rises near Cocking and joins the Western Rother near Cowdray House at Midhurst, West Sussex. The Western Rother is failing to meet the European Union’s legally binding Water Framework Directive (WFD) targets for fish. The Costers Brook, an important tributary of the Rother, provides excellent quality habitat for a range of fish species, including sea trout, brown trout, eels, river lamprey and other coarse fish species. However, fish passage is limited by two structures, a weir at a Mill (now a domestic dwelling) and a culverted farm bridge, both located within a couple of kilometres of the confluence with the Rother, and each limiting access to several kilometres of upstream water course with excellent potential habitat for fish.

The Ouse & Adur Rivers Trust (OART), a registered charity, successfully bid for Phase3 of the DEFRA River Improvement Fund (RIF) to enhance fish passage on the Costers Brook on behalf of the Arun & Rother Rivers Trust (ARRT). At the time of the application the ARRT was not a registered charity and so was not eligible to apply for the RIF. The obstructions to fish passage were initially identified through a survey undertaken by the Wild Trout Trust (WTT) for the Environment Agency (EA) in 2010/11 as part of a wider assessment of the River Arun and Western Streams.

The stretch of the Costers Brook proposed for improvement was recognised by the local EA as being critical because the upper part of the Brook has concreted gravels due to the natural chemical makeup of the stream and the lower sections are drowned out by sandy deposits, making this middle reach with these two proposed projects the best spawning habitat for a range of fish species, in particular both sea and brown trout. Both projects were carried out in collaboration with the WTT and the ARRT to ease fish passage over the weir in the mill leat at the Mill and to modify the culverted bridge. All the works were undertaken by specialist contractors experienced in river restoration work.

Weir in the mill leat
Weir in the mill leat (above right) – water flows quickly down the shallow weir face making it difficult for fish to pass.  Above left the dilapidated mill leat wall has slumped into the channel – without repair over time this would obstruct flow

The works at the Mill also involved restoring part of a dilapidated mill leat wall downstream of the weir, which over time had slumped forward into the channel. This was partly due to a number of fresh water springs located behind the wall, for which no drainage was provided, which were pushing the stone wall forward into the stream. The weir comprises part of the curtilage of the Mill which is now a domestic Grade II listed house. Due to the listed status of the property formal Listed Building Consent (LBC) had to be obtained from Chichester District Council; this was the first LBC for a river improvement to be assessed by the District Council.

For the weir modification project at the Mill, a specialist firm called Fishtek Consulting were commissioned to undertake a fish easement assessment in order to identify suitable options to improve the passage of upstream migrating fish and to design the most appropriate solution. There were a number of conditions applied to the design of the fish easement in addition to the technical aspects of the site which had to be addressed (e.g. managing typically low flows). These included the landowner’s requirement that two-thirds of the weir face be kept free for recreational purposes, in addition to the Listed Building status and high aesthetic landscape surrounding the weir within a garden setting, which meant that the resulting design had to blend in with the surrounding environment as much as was feasibly possible.

Work in progress at the Mill
Work in progress at the Mill – installing the oak baffle fish easement on the crump weir

The fish baffle easement was completed in mid-August 2012 despite the unseasonal wet summer conditions that preceded the installation works.

The completed oak baffle fish easement in operation
The completed oak baffle fish easement in operation, note the removable stop log installed at the weir crest on the opposite side to the baffle section

The re-instatement of the dilapidated mill leat wall downstream of the modified weir commenced with all of the ‘lost’ stone blocks being successfully retrieved from the Brook, buried under sandy sediments, and the installation of a new drainage system through the restored leat wall. Access to the site for the heavy plant involved traversing a relatively steep bank; the contractors did well to repair the wall after a very wet summer with elevated water tables levels at the site due to the high number of natural freshwater springs.

Looking for the ‘lost' stone blocks
Looking for the ‘lost’ stone blocks
Repairs underway
Completed re-instated mill leat wall at Costers Mill

Further upstream, a culverted farm bridge was causing an obstruction to fish passage. The bridge was constructed out of 5 concrete drainage pipes laid lengthways along the bed of the Costers Brook with brickwork above to form the sides of the bridge. Under low flow conditions there is often insufficient water depth which limits fish passage. During higher flows the water velocity can be too great to enable all but the strongest migrating fish to navigate the bridge. This is particularly the case if one or two of the culverts become blocked with debris, which increases the velocity through the remaining pipes, making it harder for fish to navigate further upstream. The proposal was either to replace the culverts with a clear-span bridge or to look at modifications to the site to improve fish passage over a wider range of flows than currently existed.

This was addressed by raising the stream level of the plunge pool by about 200mm, using approximately 20 tonnes of Sussex Block Sandstone walling rejects, obtained locally from Horsham Stone, and 20 tonnes of land dug flint (40-60mm diameter) obtained from New Milton Sand & Ballast. The stone and gravel were carefully placed at the tail of the pool, raising the bed level and creating a new spawning habitat and riffle section for all flow-loving fish species.

 

A flint gravel top dressing is applied over the heavier Sussex sandstone base and worked around to create a riffle section in the channel

The three Trusts concerned in the delivery of this Costers Brook Connectivity project (OART, ARRT and WTT) are very grateful to the landowners for consenting to these projects and allowing access across their land for plant and materials; and to the contractors for the successful completion of the projects. Particular thanks should also be paid to John Whiting (OART Trustee) and Andy Thomas (WTT) for their hard work and advice throughout the project, and to the local EA for support and assistance.

Authors: J. Whiting & S. Wright (2012)