River Aire Weir Removal at Coniston Cold

Summary:

Project name: Coniston Cold Weir Removal

Location and channel length improved: The River Aire at Coniston Cold, North Yorkshire. 20.4km of unrestricted access from the Aire and into Otterburn Beck.

Issue: A former mill weir which was in situ for ~180 years, yet redundant for at least 60. The 19m wide, 1.4m high (plus a 4m sloping stone apron on the downstream side) was a major barrier to fish and sediment movement and had impounded the river, thereby drowning out quality habitat, for ~100m. 

Objective: Full removal to reinstate connectivity. 

Method: Removal of notch boards to lower upstream water-level, breaking out of apron to create a stable working platform for 8‑tonne plant, and then gradual reduction in crest height. All metal and timbers were removed, and concrete broken into manageable chunks and used to infill weir pool. Top-dressed with stone from the apron and channel left as a uniform cross-section to regrade and redistribute sediment from upstream naturally.

Budget: £7,956 (full economic cost)

WTT role: Jonny Grey, lead partner — project design and delivery.

Partners: The Phillip Green Trust (owner) and tenant farmer, Environment Agency, and Aire Rivers Trust. Also supported by the Patagonia World Trout Fund.

Project date and duration: Summer 2018, 2 days + associated pre and post (ongoing) monitoring. 

Project description

Members of the Aire Rivers Trust had been negotiating on and off with the former owner and now Trust responsible for the site regarding fish passage options for ~6 years. The owner had originally purchased the plot of land with an idea to restore the former mill but the realisation of potential flood damage, especially in light of the ²⁰¹⁵⁄₁₆ floods, had subsequently altered their perspective. 

Due diligence searches revealed no abstraction licences, a pipeline (thankfully, downstream and hence theoretically unaffected), and a listed structure (the A65 road bridge at Coniston Cold) ~160m upstream. The latter was important to consider from the perspective of the ​‘knick point’, the point in the channel where there is greatest change in gradient, migrating upstream through the sediment accumulated upstream of the weir and potentially affecting the footings of the bridge. An independent, full topographic survey was commissioned and those data analysed and interpreted by the Geomorphology Specialists at the EA. Again, thankfully, this process was deemed unlikely to affect the bridge. 

The actual removal was carried out by local contractors (who are also keen anglers) using 8‑tonne plant with a pecker and bucket. All materials were recycled, i.e. the concrete weir crest was broken up (any metalwork removed from the river by hand) and used as infill for the pool, especially on the inside of the bend, left bank, downstream). The stone from the apron was either used to bolster the existing revetment on the outside of the bend (right bank), or to top-dress the concrete in the pool. Sedimats from HyTex (UK) Ltd were installed downstream of the removal site to trap fines; these were removed afterwards and used to restore and eroded area of bank downstream. 

Four months after removal (encompassing two spates >50cm), the river had formed a fabulous deposition bar of cobbles and gravels down the left bank (right of shot), thereby focussing the flow into a narrower, deeper channel with cleaner sediment — check out the slider on the first image(s) at the top to assess the degree of change evidenced by Fixed Point Photography.