Thanks Kebecker, I suppose it could be a strip of hardcore rubble that wasn't covered in concrete (I read somewhere they brought in rubble from London bomb sites for that purpose). It is very neat and even all the way up the taxiway though so probably unlikely.
These 2 images of other airfields seem to show the same narrow strip of tarmac down each side of the concrete track. Doesn't look like any attempt was made to create a channel for the rain. I'm thinking maybe just a long pit full of coarse stone with a permeable tarmac cap?
This would explain why I'm not seeing any sign of storm drainage at the airfields I visit - even when the full width runway is intact. The tarmac would soon break up post war and the field would then encroach right up to the edge of the concrete track.
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