GRANDS Vaux was hit with the equivalent of an Olympic-sized swimming pool worth of water every three-and-a-half minutes during the peak of last month’s flood, the chief executive of Jersey Water has said.
Helier Smith made the comments in response to a letter published in the JEP, which asked whether water levels in the nearby reservoir could be reduced by one third in the autumn and winter to enable it to store run-off during major rainfall events.
This, the letter writer said, could help reduce the risk of flooding down stream, as more water would be captured and then allowed to flow out in a controlled manner.
But Mr Smith said that reducing the level of Grands Vaux reservoir would not have helped protect nearby residents.
‘Their homes were hit by a huge volume of water, which at peak flow was estimated to be the equivalent of an Olympic-sized swimming pool of water (2.5 million litres) flowing down the valley every three-and-a-half minutes.
‘Given that flow rate, had the level of the reservoir previously been restricted to 75% of its capacity, it would have filled back-up to 100% within less than one-and-a-half hours – so such a policy would provide very limited respite for the residents, support teams and emergency services.
‘We had 65mm of rain in the reservoir’s catchment area, generating just under 600 million litres of water – more than twice the reservoir’s total capacity – within 24 hours.
‘Rainfall of that scale, onto already sodden ground, overwhelmed the current drainage system in the area, which is not designed to deal with such a flow of water. And that was despite Jersey Water pumping as much as we could – 30 million litres in total over a 48-hour period – to Queen’s Valley, and the Water Treatment Works, just to get it away from Grands Vaux.’
Mr Smith added that deliberately reducing the level of the reservoir could also impact water supplies.
‘Grands Vaux is a critical source of water for the Island and a key element of Jersey Water’s integrated raw-water management system.
‘It is our third-largest reservoir with a capacity of 230 million litres and a water-catchment area of more than nine square kilometres, the largest of any of the reservoirs. Significant volumes of water are captured at Grands Vaux, but even when full, the reservoir holds enough water to supply the Island for just 12 days.
‘As well as exacerbating water-resources issues, deliberately reducing the volume stored in the reservoir over the winter exposes it to significantly greater pollution risk at precisely the time of year when the water-quality risk presented by agrochemicals is at its highest.’
