From Alan Holmes.
ONCE again, we see catastrophic flooding in various places but particularly in Grands Vaux.
In my past professional life, I always in the first instance looked for a very simple solution to a problem and I hope Infrastructure Minister Tom Binet will seriously consider this one.
The simple solution is that at the end of autumn each year the water level in Grands Vaux reservoir should be reduced by one third its capacity and, for the winter months, maintained at that level.
If we have a similar (or perhaps worse) event like the one we have just had, the reduced level of the reservoir could act as an attenuation to collect the surge; the sluice could be opened to allow controlled run-off and maintain the lower level of the reservoir.
At the end of winter, the water level could be allowed to once again rise for the spring and summer months.
Simple – and this would not cost the taxpayers a penny or, for that matter, some expensive consultants.
Response from Helier Smith, chief executive, Jersey Water.
THANK you for the opportunity to respond to this letter.
The major flooding which happened in Grands Vaux last month marked a truly awful start to the year for the many residents affected, and our thoughts remain with them as they begin the upsetting work of cleaning up now the water level has receded.
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.
Meanwhile, the reservoir itself, and the pumping station, are designed to help capture, store and transfer water for drinking purposes, rather than to be flood management tools. As the above example shows, they are unable to protect against all but the smallest scale of rainfall event.
There is also further reason why we would caution against deliberately maintaining the reservoir at a low level in the way suggested. 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. The impact of having to take a reservoir out of service in response to a serious pollution or water-quality incident is significant and can be long lasting; again, potentially making worse the already challenging water resource position.
With the risk and severity of significant rainfall events predicted to increase in the future, exacerbating flood risk in Jersey, action does clearly need to be taken at a catchment scale to protect the valley’s homes (as well as other areas across the Island).
Jersey Water is responsible for the reservoir, but not the drainage in the valley, nor the land use in the upper catchment. To make sure the solutions provide the genuine, long-term flood resilience which the valley’s residents need, the problem needs to be tackled in a balanced and holistic way involving all the relevant parties, working closely together.
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