if the water is rising and heavily coloured then close in might be worth a chance on the float ,but ledgering would be a better bet .

if the water is at a steady level ,colouring and speed would determine if float fishing was viable ,ledgering again might be a option close in on meat or worm or perhaps cheese paste

best time is as the flood conditions are waning ,the water steadily falling and the colour dropping out of it ,get a thick tipped waggler out ,don’t spare the feed ,until you start too get bites then ease back a little or a chubba float with a worm on the hook .

best bet would be ledgering upto midway mark with worm or meat too temp a barbel or chub

you can use a fixed spool reel or closedfaced or even a centrepin ,each has its pro’s and cons . fixed spooled reels would be better for waggler fishing or ledgering especially in heavy water ,a closedfaced reel is easier too use and great in awkward winds , a centrepin gives excellent control when floatfishing but casting distance is limited and they don’t like the wind

in heavy conditions i’d most likly use a fixedspool reel but switch too a closedfaced reels in moderate conditions or if its too windy for centrepins and use a centrepin when the conditions allow as theres nothing better than trotting a float with a centrepin ,but if distance was required greater than three-four rod lengths then i’d use a closed faced reel

if ledgering i’d only use a centrepin close in .but never in heavy flows/flood .closed faced reels probably not the best tool for the job where as a fixed spool reel is .as casting distance isn’t limited and they have a good anti-reverse and drags .

Your first is not an easy question to answer – it will come with experience.
Colour and flow is more important than actual height.
2 foot on the upper Great Ouse and it’s a write off on any method.
7ft on the Wye at Hereford is still very fishable on the float.

Which streatch of the severn were you on? The Barbel fishing can be great on a rising river if the water is mild. You can catch on pellet feeder right under the rod top with 7 or 8ft of water on …. but obviously you need to take care on the banks in those conditions. A great change bait in those conditions is Lobworm.

On reels – a standard fixed spool can do anything …. whereas a closed face or centrepin will be more limited

Oh . .and to be honest you need some proper gear to feederfish flood conditions.
dead cows (lead weights with arms to grip the feeder) in a range of weights to 2 oz.
an avon rod or a cut down 4oz or 6 oz quivertip.

There’s a whole range of other techniques – depending on the river, conditions and target species.