[2023] UKUT 201 (LC)

Finally Dr Brookes argued that the erosion itself in the side channel demonstrates that there is a preferential flow because in his opinion there was no erosion in the channel before 2001.

Dr Brookes used erosion pins driven into the bank of the side channel to demonstrate that erosion is happening. But it is not in dispute that it is happening. The experts disagreed about just how much material had been washed away in the last twenty years, but again we do not think this assists us. Dr Brookes also pointed to the deposit of sediment at the end of the channel, where the water rejoins the river; certainly there is sediment there, which we saw on our site visit, and we accept that sediment is what happens when material is washed away from the banks in the course of erosion. But none of that tells us whether the 2001 works made any difference.

In Dr Brookes’ opinion the River Colne is “a low energy river type that does not have the competence to significantly erode its channel bed and banks. He carried out a “Fluvial Audit”, which we understand to mean an analysis of the geomorphology of the river and a visual inspection, generating a description of the river in the Langley Mill area and the upstream catchment. Dr Brookes saw no significant erosion upstream. He asserted that the shape of the side channel did not change in the 150 years before the effect of the works began to be felt, and that what should have been an inactive channel was now suffering serious erosion.

Dr Brookes based that opinion on an aerial photograph of 2005 – not long after 2001 – and the 1876 OS map; when the line from the map was overlaid on the photograph it could be seen that the route and shape of the channel had not changed. However, in cross-examination it became clear that what Dr Brookes had drawn on the photograph was the 2005 line of the channel, which shows reach 7a extending north-east at an angle; the 1876 map shows reach 7a going north-east along a line much closer to north. The channel has moved and the angle of the bend to reach 7b has become shallower.

Mr Benn had walked downstream from Langley Mill and found a number of instances of erosion. He did not accept that the River Colne has reached an equilibrium and no longer erodes its banks.

We find Dr Brookes’ assessment of the historic position unconvincing. The comparison of the map and the photograph did not support his view; on the contrary it showed how the shape of the channel has changed and the sharpness of the bend has been worn away. And Dr Brookes’ opinion ignores the inevitable 1.4 metre drop from the mill race to the natural valley floor. We remind ourselves of the way in which energy is stored for use at a water mill (see paragraph 10 above), by raising the river bed to create a head of water at the wheel. Where the energy is not being used to grind corn it has to go somewhere; in practical terms it is going to be dissipated where it flows back down to the natural level of the river, whether under the mill or over the side weir. As it flows downhill the force of the water is bound to cause some erosion. As Mr Benn put it, “While the erosion can be mitigated by protection works, it cannot be completely prevented – at least while the mill race and sluice remain and that the channel lies at an artificially high level through the mill.” Although Dr Brookes was certainly aware of the topography of the immediate area we think that he failed to appreciate its importance. He therefore came too readily to the conclusion that there was no erosion in the side channel prior to 2001, and concluded that the only cause of erosion in the channel since then must be the 2001 works.

Accordingly Mr Gould and his witness have not proved that there has been more erosion in the side channel since 2001 than there was before.

Even if there has, there are a number of available causes. Mr Benn said, and Dr Brookes to some extent agreed, that there have been more frequent floods in the last 20 years than there used to be. Vegetation can divert or disrupt the flow and can cause erosion. And the elephant in the room is that the flow of water in the side channel is under Mr Gould’s own control by means of the sluice gate.

Dr Brookes outlined his view of the effect of opening the sluice gate under the mill on the water entering the channel in different conditions. First, in normal circumstances, when the gate was closed more water would flow over the side weir, causing erosion. Secondly, at high water flows, Dr Brookes said that with the sluice gate partially open, erosion below the side weir would occur in the channel for a period of minutes to hours until the gate is opened sufficiently. Those minutes to hours, on each occurrence, would in aggregate have significant erosive effect. But once the sluice gate was open sufficiently the rate of erosion in the overflow channel will decline until negligible. Thirdly, in flood conditions, even with the sluice gate open significant flow would, he said, be observed down the channel. Water would flow back up from the gate opening and under house weir, and discharge across the side weir, causing erosion in the channel.

Dr Brookes said that Mr Gould is “allowed” to operate the gate himself, but rarely did in the first ten years of residence, because the Agency usually attended during periods of high flow.

Mr Gould’s evidence was that he has rarely opened the gate; essentially he said he has done so only when the Agency told him to. The Agency advises riparian owners when there is a flood alert; its staff no longer do the “gate runs” on which they used to visit and check all the weirs on the River Colne, but it still gives telephone advice and will visit where necessary. Mr Gould gave evidence that the Agency staff would no longer visit since the commencement of litigation, by reference to Agency memoranda to its staff; but it became clear in cross-examination that that was not correct and that Mr Gould had misinterpreted the memos.

We find that Mr Gould could have taken a far more active role in controlling the flow of water in the side channel. The sluice gate is essentially under his control; neither common law nor statute, nor the 1964 Deed nor the Agency’s own policies and advice have taken that control from him. His indignation at the Agency’s withdrawal of support from the mills on the River Colne has led him to ignore his own powers and responsibilities, and his failure to operate the mill is likely to have caused much of the erosion of which he complains.