The A303 Ship Canal

Shortly before Christmas, the A303 responded to the intense rainfall by flooding near Yeovil, between the Cartgate and the Podimore Roundabouts.  For a couple of days, an Amphicar or another amphibious vehicle would have proved an absolute godsend to those wishing to travel along the A303.

Now, just over a week into the New Year and the same thing has happened again, this time a bit nearer to us at the A36 interchange.  Fortunately only on the eastbound carriageway and as I write, the Highways Agency are predicting it will be opened by 1800.  Time will tell - they do tend to err on the side of total unreality.

That got me thinking about the impact of floods and flooding on any A303 improvement scheme that might be put forward.  Clearly, flooding is becoming increasingly common with the effects of global warming, so we ought to expect that the Environment Agency ought to be one of the stakeholders for the forthcoming A303 feasibility study.  Someone else for us to talk to.

One of the plans put forward for a bypass of Winterbourne Stoke a few years since, had  the route of the A303 diverted north of the village, towards Shrewton.  It ran across the flood plain of the River Till, raised on a large earthwork berm.  Think dam -with a gap in it.  Now the River Till is normally a very benign little stream, but it hides a dark and dangerous secret - the Great Till Flood of 1841.

So notorious is that event, as an example of chalk downland groundwater flooding, that it is taught as an example of this phenomenon in universities around the world.  Events in 2014 are following a remarkably similar pattern to those in the January of 1841 - a period of heavy rain, with cold, possibly freezing weather in the next week.   At the moment, the Environment Agency have the following flood alert in place:

When you look at the water level in the Tilshead borehole, the aquifer that may be responsible (I'm speculating here, I'm not a hydrologist) for the flood on the A303 today, you will see why the idea of building the A303 on a berm might need looking at.

 It seems we are less than 0.5 metre from having an "interesting" situation.  I guess the next 4 or 5 days, with more rain forecast on Sunday, could prove interesting indeed for traffic on the A303.

Let's keep fingers crossed that the rain stops, the ground doesn't freeze, we don't have snow followed by a quick thaw and the water level in the aquifer drops - that way the A303 through Winterbourne Stoke might stay open.  Imagine though, for a moment, what might happen if the Great Flood of 1841 was repeated when the Till valley was largely blocked by a berm? 

Routing a new dual carriageway has many more things to consider than just the World Heritage site at Stonehenge.

 UPDATE: 

The Highways Agency have announced that the A303 (eastbound) is likely to remain closed at the A36 Deptford interchange all day today.  Great joy!

At 09:00 on Friday 10/01/2014, the water levels had crept up to 99.71 metres AOD (AOD - Above Ordnance Datum - or to you and me, above the mean sea level determined for the Ordnance Survey), which is just a smidgen below the highest astronomical tide level for this site, which is 99.88 m AOD.

Anyone wanting to learn more about the Great Flood of 1841 might want to read a paper called "Estimating Extreme Floods".   It was written by by Colin Clark in 2004 and published in a journal with the catchy title of:  International Water Power and Dam Construction.  You couldn't make this stuff up, could you!

Despite the turgid sounding nature of this paper, it is a surprisingly good and thought provoking read.