[2023] UKUT 201 (LC)

A gabion mattress was installed to provide a more solid bed to the river, to prevent erosion of the base of the channel; we accept Mr Willis’ evidence that the channel was dug out sufficiently to ensure that the gradient of the gabions was the same as that previously.

So the slope from weir to channel bed was inevitable. That slope was a potential source of erosion, and the Agency’s response to that potential problem was to place a gabion mattress over the slope from the weir to the bed. As Mr Benn put it at paragraph 4iv of his report:

“The erosion to the banks of the bypass channel for a length of approximately 20-40 metres downstream of the overflow inlet weir IS influenced by the 2001 works… The transition between a hard structure and natural riverbanks does often result in erosion which can be mitigated by use of large stone-filled gabions or provision of a stilling basin.”

The gabion mattress has now failed and, as we said above (paragraph 15 ) is visibly causing erosion as water rushed around the gap between the gabion and the bank of the channel. The Agency has now offered to repair (or to pay for) the gabion and the erosion caused by its collapse, in exercise of its powers and pursuant to its responsibilities under the Deed.

The Agency cannot be liable for failure to repair the gabion mattress until now; it is well-established that the Agency cannot be liable for failing to exercise any of its powers, only for what it does in exercise of its powers. The claimant’s case is that the gabion was defective because they have failed too early. In support of that proposition Dr Brookes provided a chart of the expected life of a gabion in the UK, from the website of a supplier, which indicated that while gabion in a polluted industrial area with a high salt spray would last about 25 years, and in an urban light industrial area with a low salt spray would last for about 50, gabion in a rural area could last 100 years or more. Accordingly he said that gabions properly designed and implemented in a rural to suburban area such as Langley ill would last 80 to 100 years. Dr Brookes agreed in cross-examination that the chart referred to gabion that is in dry conditions, save for spray, and not under water. Dr Brookes had no information about the expected lifespan of gabion under flowing water; Mr Benn said he would be pleased to get 20 years’ use out of gabion in a river, and we accept that evidence.

Accordingly we find that there was nothing wrong in the Agency’s choice of a gabion mattress to absorb the energy of the water as it flows down from the concrete weir to the channel bed, and there is no substance in the argument that they have failed earlier than they should have done.

In conclusion to the material under this and the two previous sub-headings, we find no fault with the way the side weir was constructed in 2001.

We turn now to the other major component of Mr Gould’s case: his argument that the result of the construction of the side weir was an alteration to the flow of water under the mill, so that there was a preferential flow over the side weir and his “accustomed flow” was reduced.