Tilshead Borehole Levels: 22nd January 2023

A very dry and cold week, in which the Environment Agency finally issued a Flood Warning for the River Till, too long after the first properties in the valley had needed to take pre-emptive action to have any real value.  This has been raised with them and it has been suggested that flood alerts and warnings be given when the river is at a lower level than at present.  It was also the week in which the A303 was partially closed at the Deptford junction due to flooding from the River Wylye  It isn’t the first time this has happened and it certainly won’t be the last unless that section of road is raised by 5-10 metres.

Tilshead borehole level showing it is now just below the highest we recorded in 2014.

The groundwater level today is still rising and stands at 99.65 metres AOD.  That’s still below the highest level of 99.88 metres recorded by the Environment Agency on 14th December 2000 and even that pales into insignificance when compared to floods back in the 1990s, 1980s and before.  Yesterday, the level rose a mere 1cm; that’s 1/1,500 of the amount it rose over a 24-hour period a few weeks ago.  The rising water level at Tilshead is in sharp contrast to that in the Chitterne borehole.  Here is a snapshot of what was going on yesterday with the Tilshead borehole on the left and Chitterne on the right:

Comparison of Tilshead and Chitterne boreholes. the first rising and the second falling.

Chitterne peaked mid-week and began to fall. whereas ours has continued to rise.  As we followed Chitterne into flood, it seems likely that we will follow out of it.  Without rain, I would expect the Tilshead borehole to stop rising in a day or two and begin to fall slowly thereafter.  If we have a sudden thaw, more water hung-up in the aquifer above Tilshead may get released and that would give another small rise.

You will see from water levels on this day above, that we are firmly at number 2 in flood levels over the last decade.  So levels are very high, but it could be so much worse.

Now last week, I showed a graphic of how groundwater and surface run-off was moving down the so-called “dry valleys”, on either side of the River Till.  This week, I can show it much more impressively in a single photograph of a puddle taken on Wednesday last week:

Here we are, in the water meadow to the south of the village and looking north back towards the church. The water in the puddle came initially from groundwater and this was supplemented by water from the Till when it burst its banks last week.  Because the water movement is very slow, the water in the puddle began to freeze, but water coming into it from the dry valley to the right of the picture, from Oatlands Hill and Horse Down, is travelling underground and is closer to the chalk spring temperature of 10ºC (50ºF).  Consequently, where the groundwater meets the mixture of water, the puddle is kept ice-free.  As the temperature has gone up and down over the week, the area of clear water has grown and then shrunk.  This morning, the open water is only a quarter of what is shown here.

The river level peaked first on Monday, then receded a little following the first heavy frost on Tuesday.  By Wednesday, following a brief thaw, the level rose again to Monday’s peak and has then dropped very slowly ever since.  That fall may continue unless we have a huge thaw and there is frozen surface water being held in the dry valleys.  Time will tell.

The largely good news this week is that all the heavy rain has disappeared from the forecast.  We have a few more cold days and nights to look forward to, but the temperature is due to rise towards the middle of the week.  We may see a little drizzle from Tuesday to Thursday, but it shouldn’t amount to much.  The rest of the month is expected to be dry here in the village, with temperatures pretty close to normal for the time of year.  Up to the middle of February, there is still the possibility of a few periods of prolonged rain and showers, but it may be drier and warmer than normal.  Finally, it’s worth saying that there seems to be a possibility, at least, of a brief but horribly cold spell at the start of next month; much colder than anything we have yet had this winter.  As I say, there’s just a hint that this could happen, but we will know more by next week.

Winterbourne Stoke Parish Council