‘Drought risk rises if Longue Hougue is lost’

The States committees responsible for the inert waste strategy were told by the Assembly to come up with an alternative to their preferred option, Longue Hougue South.

However, Guernsey Water general manager Steve Langlois said the island’s largest reservoir, which contains more than a quarter of its water resources, was the wrong place to look.

‘The need to have adequate water resources is of critical importance,’ he said.

‘While the current level of protection from drought is comparable to water companies in the south of England, our long-term needs and our future supplies are finely balanced.

‘Any change in the use of Longue Hougue reservoir without introducing an alternative would leave Guernsey at unacceptable risk of drought.’

Mr Langlois said no viable alternative to the 1,160,000 cubic metre reservoir could be brought online in time.

‘The timeframe of the inert waste strategy is considerably shorter than would be needed to replace Longue Hougue reservoir with an alternative, such as Les Vardes quarry,’ he said.

‘Converting Les Vardes quarry into a reservoir would be totally unfeasible within this period, mainly because it is expected to remain in operation by Ronez well beyond the timeframe being considered.

‘Major investment would be needed if in the longer term Les Vardes were to be used for the disposal of inert waste.

‘Similarly, major infrastructure would be needed before Les Vardes quarry could be used as a reservoir.

‘Our raw [untreated] water network of pipes does not pass close to Les Vardes and the closest pipes are too small for filling the reservoir and transferring water to our treatment works.

‘In addition, the long-term stability of the quarry sides and potential for saline intrusion would need to be evaluated and addressed if found to be an issue. We have not undertaken any detailed assessment of these costs.’

Mr Langlois said both could be needed for water storage in the years to come.

‘In the longer term there is much uncertainty over the potential impact of climate change, which means that Guernsey Water may require both quarries.

‘If in the future, we determine that only one is needed for water storage, we would evaluate options for the use of both to determine how they could be utilised in the island’s long-term interests.

‘However, if a decision were made purely in the interest of our water supply, Longue Hougue would be the one to retain.

‘This is because the infrastructure is already in place to connect it to our raw water network, including our largest water catchment, that of Marais stream, which is already pumped to Longue Hougue reservoir.’

Early studies show that Les Vardes would probably have a greater capacity than Longue Hougue.

‘While no detailed evaluation has been undertaken, LiDAR (Light Detection and Ranging) data indicates that the capacity could be around 1,500,000 cubic metres.’ said Mr Langlois.

‘Longue Hougue reservoir has a capacity of 1,160,000 cubic metres.’