River Aire Weir Removal at Consiton 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: Pools and riffles created with heavy machinery, including gravel introduction to create spawning riffles. Impoundment removed. Bankside tree work to increase light in the channel. Brushwood berms, flow deflectors, hinged trees, cover logs. Bank lowering and re-profiling.

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. 

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 201516 floods, had subsequently altered their perspective. 

Cc 1 Coniston Cold Weir Feb 2018
Coniston Cold weir in February 2018: the apron was already degrading badly

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. 

Cc2 Coniston Cold Weir Monitoring
Electrofishing by Durham University staff (AMBER) to assess the fish community down and upstream of the weir, both pre (here) and post works.

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. 

Cc3 Coniston Cold Weir Sedimat
Positioning the Sedimats to trap fines
Cc4 Coniston Cold Weir During
Raking the sediment over to loosen but not remove it, thereby letting the river carve its own route. Also, hand-sifting for fragments of metal from the concrete weir crest.
Cc5 Coniston Cold Weir
Before the work, upstream of the weir.
Cc6 Coniston Cold Weir Upstream Post
After the work, upstream of the weir.
Cc7 Coniston Cold Weir 4 Months Post Work

Four months after removal (encompassing two spates >50cm), the river is beginning to form 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.