Flash flooding at Combe Martin

Figure 1 – Courtesy of BBC News

Apparently there were reports of what looks like flash flooding at Combe Martin in north Devon on Sunday according to the BBC (fig 1). Here are the 24 hour totals from 06 UTC on the 21st of January the day that it happened.(fig 2) with the two nearest rainfall stations ringed in red.

Figure 2

The weather radar just seemed to have missed the rainfall event across the north of Devon yesterday for some reason, if you remember a warm front was pushing eastward across the southwest during the night. Estimates from the radar suggest totals of around 16-24 mm over Exmoor, but Liscombe measured a little more than this, with a total of 29.2 mm in 24 hours [06-06] (fig 2). Radar rainfall estimates for other places across Wales were generally very good (fig 3 & 4).

Figure 3
Figure 4

Judging by the pseudo hyetograph for nearby Arlington Court (fig 5) the heaviest rain occurred in the morning between 0730 to 1000 UTC, at a guess 6 mm fell in that time, which I’m sure wouldn’t have been enough to cause the flash flooding seen at nearby Combe Martin.

Another possible cause is that the swollen stream somehow became blocked above the village, maybe at a bridge or in a culvert, and the water dammed up for a short while before being suddenly released as the blockage cleared. It happened in a much larger way at Louth on the 29th of May 1920 so there’s no reason to say that it couldn’t happen at Combe Martin, although I’ve not seen any reports to indicate that this was the case.

Figure 5

Steep valleys do run northward of the high ground of Exmoor, but the stream that runs through Combe Martin doesn’t seem to originate on Exmoor though, and looks to be quite a short affair (fig 6). I’m sure that this area has seen much heavier rain and not seen flash flooding the likes of this, so all I can put it down to is a very local intense period of rainfall, or even a cloud burst if you like, that somehow went unnoticed by the 5 minute radar images.

Figure 6 – Courtesy of Bing Maps and the Ordnance Survey

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