The rainfall predicted last week arrived as expected and the impact on the catchment, the Tilshead borehole and the River Till was as unpredictable as ever. Last Sunday the levels in the borehole had been rising slowly after a slight fall the previous week. That trend might have been expected to continue for a few days but this is the Tilshead borehole and nothing is ever simple and so the water level in the aquifer fell for a couple of days before rising rapidly on Tuesday and Wednesday. Since then the rate of rise has dropped a lot. Throughout the week the water level in the River Till has risen steadily and is topping its banks in several places.
How does this compare with previous years? Well, the following data is also quite interesting. The table shows the date the aquifer was at its maximum depth for that particular winter. So far, it has always been in the New Year, but this year today’s level is already greater than the maxima of every year for which we have data, other than the winter of 2013-2014.
The next graph shows the levels for 7th January over the last decade. The level today is. the deepest it has ever been on this date and we still have over two months of the winter remaining and over the last decade, the maxima has most frequently been reached in March.
You may wonder why the River Till responds in a somewhat different way to other rivers in this area and two of the Environment Agency’s Flood Warning maps today show this quite clearly. The River Till draws water from a large area of Salibury Plain to the east, north and west of Winterbourne Stoke. The Wylye to the south and the Avon directly to the east have virtually no inputs in this area and even the River Bourne. east of the River Avon has relatively few.
Now the Environment Agency has at least upgraded the groundwater Flood Alert in this area to a Flood Warning. Unfortunately, the way it was done meant that residents of neither Berwick St James, nor Stapleford ever got a Flood Alert. Instead, they got a Flood Warning sometime after flooding was physically apparent. The system needs revision and hopefully, this year, it might respond to suggestions to achieve that. Even this morning, the information the Environment Agencywas putting out was a little confusing, as the following screen-grab shows.
The Till referred to in the first entry isn’t the River Till as it deals with groundwater, not surface water from the river. It would have been clearer had they said ‘the Till Valley’ or something similar. When you look at the map of the area it addresses though, it’s identical to that shown in the second of the two Flood Warnings, so the first one probably isn’t needed at all. Contradicting both of these, at least in part, the EA has downgraded the earlier Flood Warning for the Lower River Wylye to a Flood Alert, it hasn’t been removed entirely as the last entry suggests. A bit of consistency would be helpful. Finally, we have the groundwater Flood Alert for Salisbury Plain, which specifically lists the villages of Boscombe, Cholderton, Collingbourne Ducis, Hanging Langford, Hindon, Hurdcott, Idmiston, Newton Tony, Orcheston, Porton, Salisbury, Shipton Bellinger, Shrewton, Stratford Sub Castle, Tidworth, Tilshead, Tisbury, Wilton, Winterbourne Stoke and Woodford. All these areas had a higher Flood Warning status this morning, so the Flood Alert doesn’t seem particularly helpful at the moment. Regardless of the mixed messaging by the EA, the message for the village is that water levels in the river and the aquifer are still rising slowly and groundwater flooding remains likely.
If the weather remains dry, that may all be academic and it certainly looks as though the next few days will be rain-free. We may get a few snow-flurries on Monday, but this isn’t likely to lead to any significant snowfall and won’t translate into much additional water. Temperatures are set to plummet and freezing temperatures will have an impact on the aquifer by holding water close to the surface. That could have consequences if we have further heavy precipitation before a thaw. In the medium term, we will have a week or so of dry-ish weather with the odd bit of drizzle, but as the high-pressure zone that is giving us the dry spell breaks down, the usual train of weather systems blowing into the south-west from the Atlantic will return. As the warmer wet air hits the cold air over the south of the country, we could be in for a period of heavy rain or even heavy snow which is not good news. The colder wetter weather look set to persist into early February. Water levels are likely to be of concern until at least mid-February.
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