I am unsure if I understand your question, but here goes. This seems to be a very specific questions, and I am not sure if this in specific has been studied. I would assume that the reason for not maintaining these channels would be since they have fallen out of use, and is probably only transporting rain water or similar at most. The former use of these channels must therefore have been either stopped, or changed to another type of system, otherwise they would be maintained.

I would assume as with any other type of structural surface in nature these would get colonised by organisms of various types and degrees over time and then degrade, as concrete normally does. I am not sure if concrete provides anything else than a normally empty flat or slightly tilted stone-like surface. I would guess that there would be a potential for more flowing water, which would favour organisms in need of wet surfaces or running water. Over time succession would set in and it would probably end with larger bushes or trees of some kind, and strong roots would degrade most of the concrete and/or contribute to the lack of function of water flow.

It will be interesting to see if there are any specific articles on that contrast. From my own sampling in concreTe channels, some life does eventually develop on the bare concrete, given long enough baseflows. Algae, some snails and midges. But larger animals were exclusively confined to vegetation and debris that had accumulated within the channel.

Concrete is often hyped as "chemically inert". But in my experience, new concrete leaches alkaline solutes in low concentrations that get lower over time. I used new concrete paving stones as substrates for a disturbance experiment in streams. I realized from the soapy sensation on my hands that there was a chemical effect. So I let them leach for six weeks before starting the experiment.

My samples included midges, a diverse set of mites, caddisflies, beetles, and mayflies. The river lacked stoneflies in any appreciable numbers so I was not surprised at their absence on concrete. So, I believe that the chemical effect would be very minor, short lived, and probably negligible in established flumes..

The physical effect would be more interesting and permanent. A flume is so uniform as to attract only certain species fond of those conditions. If you have replicate flumes.....seems like an opportunity. Add physical structure to some and not to controls: coarse woody debris; cobbles; boulders....see who moves in and write that paper.

Good luck in all your work.

Declan