Periodically rougher German region (4)

I believe the answer is:

ruhr

'german region' is the definition.
(I've seen this before)

'periodically rougher' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternating letters of 'rougher' are 'RUHR'.

(Other definitions for ruhr that I've seen before include "Herman industrial area" , "Flower from Germany" , "German river flowing into the Rhine" , "German industrial zone, river" , "Dortmund area?" .)

I've seen this clue in The Independent.

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