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Operation Ouse British Waterways' engineers face a constant battle to protect aged structures against the very force they were designed to tame

Reconstruction work currently in progress on a small 240 year old weir and adjacent lock on Yorkshire's River Ouse, epitomises the diversity of British Waterways' current engineering challenge.

While site engineers rely on traditional long-proven materials like puddle clay and lime mortar to help strengthen repairs, the importance of their success in harnessing the river's erratic flow translates into much more dramatic long term benefits some distance away. For if the somewhat inconspicuous weir at Naburn was not now being raised, vital riverside walls at York 12km upstream could weaken or collapse resulting in significant damage to adjacent properties.

Coal mining operations now under way deep beneath the Ouse are already causing differential settlement so the weir, controlling both tides and upstream river levels, must be heightened to retain constant water levels at York. Had these levels been allowed to fall, the normally submerged river walls - which for ages have channelled the Ouse through the heart of York - could have dried out and cracked.

The 43m wide V- shaped weir remains one of the most important structures on the entire 100km navigable length of the Ouse. For over two centuries it has been the barrier forming the river's upper tidal limit and has helped maintain upstream water levels deep enough to allow commercial river traffic to journey up to York.

The weir's adjacent twin locks once hosted barges full of cocoa beans or coal from ports on the Humber estuary. Today the cargoes are mainly holiday makers cruising the rivers and canals up through York to Ripon, the most northerly point on British Waterway's English network.

Until the mid-1950s, the 4.5m deep stone weir sported a simple but innovative 450mm high timber wasteboard running across its top. This 'Ouse Barrier' acted as a mini tidal barrage controlling upstream water levels.

A network of pulleys, ropes and 10t iron ingot weights balanced the hinged board against normal water pressures but allowed it to pivot forward gradually when forced down by the pressure of rising flow levels. As the river dropped again, the weights pulled the board back to the vertical to act as a barrier.

The remains of this boarding was removed during the current rebuilding work and replaced by concrete. However, the weir will soon sport timber planking again, though this time for a different reason, as wooden strips are slotted between short steel H-columns protruding upwards from the concrete crest. This planking will be added in stages to raise the structure gradually as surrounding mining work causes increased settlement.

Coal operator RJB Mining is currently extending its workings from the nearby vast Selby coalfield and several seams will pass close to Naburn. Though workings lie some 680m deep, the expected surface settlement over the next five years will total 250mm. So it is by this same height that the weir will be sequentially raised with the planks.

RJB is meeting the 140,000 cost of raising the weir, though BW opted to take advantage of having contractor Dawson-WAM on site to add 60,000 to the overall bill by funding repairs to the adjacent locks - both listed structures.

New timber gates have been fitted to the older 1757 lock - exact replicas of earlier unusually shaped double - sloping oak versions. The new 5t gates, made this time from West African Ekki hardwood with a 40 year life expectancy, over double that of oak, slope both vertically and in plan to ensure a snug fit when opened into lockside recesses. This layout reduced damage from the commercial barges that once plied the Ouse.

Traditional materials were also to the fore after the lock did not take kindly to being dewatered, with cofferdams at each end, to allow the new gates and cills to be installed. Such large ashlar block 'leaky' chambers were never designed to be completely drained and over 200t of puddle clay was needed to help seal the lock against numerous water paths. This flexible clay, with high cohesion and low permeability, was positioned by divers who also resorted to sandbags to block the larger leaks.

The Ouse, with its vast North York Moors catchment, is one of the UK's most dynamic rivers. Flow rates can increase from just 3m3/s to over 700m3/s with little warning and cofferdams at Naburn for both weir and lock repairs frequently overtopped.

Such floods are the primary cause of the work lasting over double the expected three months. And it is only in the last couple of weeks - ironically during repeated news coverage of record floods in Wales and the south - that the 'flashy' Ouse has allowed site teams to regularly keep their heads above water.

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