[2023] UKUT 201 (LC)

We start with the issue of whether the Agency was right to replace the previous earth weir with a concrete version. Mr Gould thought the Agency should have replaced the earth weir like for like. Dr Brookes agreed.

Whilst Dr Brookes accepted that by 2001 there was a need to do something to repair the earth weir, he thought that an earth and clay bank would have been better. The new concrete structure had a smoother face and an entirely different structure that would, and did, result in erosion to the bypass channel. Even if the concrete weir was a like for like replacement of the earth weir in terms of size and location, in his view the different materials resulted in erosion because of the higher velocity of water passing over the smoother surface, resulting in turbulence and therefore erosion. The new weir would also allow more debris to pass over.

Mr Benn said (in paragraph 6.3(c) to (d) of his report) that the earth weir was “an unusual and not a particularly resilient arrangement”, and that he was not surprised that by the late 1990s the spillway had become eroded. His view was that the concrete weir was typical of the type that would have been constructed at the time and was normal for an overflow structure on a river. A design today would be very similar, save for incorporating a fish and eel pass. His evidence was that had the Agency not installed the concrete weir, there would have been progressive collapse of the left-hand bank of the mill race and likely flooding to the garden of Langley Mill. He noted that the transition between a hard structure and natural riverbanks does often result in erosion, which can be mitigated either by provision of a stilling basin or by stone-filled gabions as in this case. He was sceptical about the idea that the smoother weir would allow a faster flow of water; only if there was a close and level grass cover on the earth weir would that have slowed the flow to any appreciable extent. As to debris, he thought that the side weir was wide enough to prevent anything getting jammed, and expressed the view that large logs would in any event be caried straight towards the mill rather than making a left turn into the channel.

We found Mr Benn’s evidence about the use of concrete convincing. An earth weir is obviously more liable to wear away than concrete; it is agreed that the use of concrete can result in erosion, and we accept Mr Benn’s evidence that the use of a gabion mattress was appropriate to mitigate that effect.

We turn next to the height of the replacement weir.

Dr Brookes’ initial, and admittedly approximate, survey suggested that the replacement weir was set a few centimetres lower than the culvert under the Mill, the Agency said half an inch. Dr Brookes and Mr Benn carried out a measured survey which showed the crest of the inlet weir to the channel at 28.883m AOD. The culvert beneath the mill was not level across its width; at its lowest point in the middle its height was 28.88m and at its sides 28.968 and 28.97m. The experts agreed that the inlet weir to the channel was therefore 3mm higher than the lowest point in the culvert. They also agreed that it was 5.6mm higher than the average height of the culvert (which they had as 28.393m). We wonder whether that is quite right. On our calculations the average height of the culvert is 28.939m (not .393) and so the height of the bypass weir is 5.6mm lower than the average height of the culvert. However, we are sceptical that in reality measurements can be taken to that degree of accuracy. Mr Benn in his report at paragraph 6.1(h) stated that he and Dr Brookes agreed that “To all intents and purposes the two weirs have the same level.” This is consistent with Mr Willis’s evidence that the height of the weir entrance into the channel was set so that in normal conditions, with the sluice gate shut, water would just “spill over” the new weir, as well as going over the concrete weir below the mill.

But there is no record of the height of the previous (failing) earth weir. The only direct evidence we have is that of Mr Willis, who said that the level of the weir was reinstated to its previous height, before the earth bank started to erode. We accept that evidence.

As for the width of the channel (agreed to be approximately 4 metres) while in correspondence with Mr Gould, the Agency had previously said that a section of the bypass channel was widened as part of the works, Mr Willis said that was incorrect. He explained that the brick “wing walls” were rounded off to make them more resistant to erosion, and that any increase in the size of the mouth of the channel was very slight. Again, we accept Mr Willis’s evidence. Furthermore, Mr Benn at his paragraph 6.1(i) stated that the two experts agreed that the “current width of the overflow channel is similar to that shown on historic OS maps.”

The evidence given by Mr Willis is decisive on the issue of the height and width of the side channel because he did the work. We accept the Agency’s case that the side weir was built to approximately the height of the old earth weir so that in normal conditions water would flow under the mill but also spill over into the side channel, and was not materially wider than the old weir.