[2023] UKUT 201 (LC)

Dr Brookes carried out an analysis of the “flow split” between the channel and the mill race. He did this by visiting the site on 29 September 2022, a time of very low flow, and holding a bucket under the fixed weir beneath the mill, timing how long it took to fill up, converting the result to “cumecs”, or m³/s. Unfortunately, the same method could not be applied to the weir at the channel, because of its length and lack of a V shape to enable 100% of the water to be captured and measured. He then took the flow gauging station at Earls Colne to extract data from the same time as his bucket test. The Earls Colne data was adjusted to estimate the total discharge across the two weirs, to allow for the ungauged catchment and the River Bourne which enters the Colne upstream of the Earls Colne gauge. The adjustment adopted was 25%, based on area, with a sensitivity analysis ranging from 15% to 35%. In the result, the flow over the channel was between 9-12 times greater than over the fixed weir, with the sluice gate closed.

Mr Benn pointed out the inevitable weaknesses with this approach. A proper comparison can only be made by using the same method for both the mill weir and the side weir, for instance by blocking all but a small element of the channel weir by sandbags. As it was, whilst Dr Brookes’ method was appropriate where there was only one unknown, in this case there were two (the side weir, and the River Colne). He expressed the view that the experiment could have been repeated several times with different results.

The other relevant evidence here is the observations made by Dr Brookes and Mr Benn at their site visit. They asked Mr Gould to open the sluice gate, which he did. When it was fully open, water continued to flow approximately 5 centimetres deep over the side weir, while the weir under the mill was dry and the water 8 – 9 centimetres below the sill. Dr Brookes was not able to explain how this could happen in light of the agreed evidence that the two weirs are pretty much the same height.

Mr Benn’s explanation was that the vegetation growing thickly on the north side of the river between the side weir and the mill narrows the channel (as we noted at paragraph 11 above) and obstructs the flow towards the mill. In light of the fact that the two weirs are at about the same height we accept that explanation.

Mr Benn’s evidence, which we accept, was that the water that the flow split at lower flows is determined primarily by whether the sluice gate is open, and secondly by the amount of vegetation in the mill race. At higher (flood) flows, the influence of opening the sluice gate reduces, and the flow will depend upon the relative capacity of the mill race (which itself will be determined by vegetation in the channel) and the overflow.