Never mind on foot - I use the canal towpath on my commute to Hyde by bicycle so that I don't have to drop down into the valley and climb back out again (and so do a few other people that I know) so closing the viaduct itself will have a massive impact
Well, unless they do it in winter when it's too muddy and horrible to commute by bike anyway
That diversion seems very long too. There are several shorter alternatives,e.g. through Brabyn's park and over the Iron Bridge, or over the bridge on the other side of the cutting here
http://streetmap.co.uk/map.srf?X=395280&Y=390106&A=Y&Z=115the only reasons I can see that they would recommend such a long diversion are
1) they have to use footpaths indicated on the definitive map, and those mentioned above are not actually shown on the definitive map (else they'd show on the streetmap link)
or
2) they are shutting the towpath between the viaduct and the bridge outside the industrial estate
Hope that it's the former not the latter