I've poked around on the Taff-side shingle banks in Cardiff, turning stones to look for beetles, but haven't done anything systematic like pitfall trapping. Nevertheless, some nice species have been found which I've not seen elsewhere in Cardiff.
There is a nice shingle bank on a bend in the Taff at Hailey Park (ST139796) which was alive with the shiny blue-black ground beetle Bembidion atrocaeruleum yesterday. Bembidion is the largest genus of carabids in the UK, and often presents identification challenges, but this one keyed out satisfactorily. According to Aderyn it is the 4th most common Bembidion in Wales, and is a specialist of riverside and streamside habitats. Also present under stones were several individuals of the small but attractive click beetle Zorochros minimus - another specialist of river shoals and one which has a north-western distribution in Britain.
Many generalist ground beetles can be found too, particulaly species prefering damp habitats. Slightly further downstream, in May 2015, I found Amara ovata, Paranchus albipes, Pterostichus madidus and Bembidion tetracolum on gravel by the Taff. None of these are thought to be ERS specialists, though some like P. albipes are wetland species found mainly by rivers and streams.
Certainly a habitat that is worth exploring...