There was no panic, though, for the very good reason that it wasn't even raining and the river Arno was flowing at normal speed beneath the beautiful Renaissance bridges of the city centre. This was merely a dress rehearsal of what the city hopes it will be able to do should there ever be a repetition of the flood that devastated Florence and its art treasures 30 years ago on 4 November 1966.
Florence has been in commemorative mood these past few days, reminiscing about the surge of muddy, oil-ridden water that came rushing into the centre in the middle of the night, killing at least 17 people, devastating houses and shops, knocking Lorenzo Ghiberti's famous doors to the Baptistery flat in the mud, half ruining Cimabue's Crucifix in the refectory of Santa Croce, and flooding out around a million precious books and archival documents in the National Library.
There have been tributes to the angeli del fango, the "mud angels" who spontaneously rushed to Florence from the four corners of the world to help save the city, and to the heroic art restorers who put the broken, sodden pieces of countless masterpieces back together again. But mingled with the celebratory spirit has been a gnawing doubt: that if it were to rain as much again, Florence would be no better prepared for the deluge than it was on that dark November night.
"The situation has actually got much worse than in 1966," warned Raffaello Nardi, who heads a special commission responsible for safeguarding the Arno river basin. The farmland that once formed a natural outlet for excess water has all but disappeared, he said, replaced by a veritable plague of speculative building along the river. A fifth of the forests in the Arno basin have disappeared, and another two-fifths have been damaged.
Set against that have been only modest flood-protection measures: dredging work beneath the Ponte Vecchio to deepen the river bed, and the formation of a big reservoir up in the hills east of the city. Professor Nardi estimated it would take 15 years to secure Florence completely, but that timescale depends entirely on the Rome government, which has so far held up the 300bn lire (pounds 120m) or so needed to build the necessary barriers and overflow canals because of Italy's chronic budget deficit problems.
One senses that Florence is still too busy recovering from the last flood to think about the possibility of the next one. Roughly two-thirds of the damaged artworks were repaired in the first 15 years after 1966, but another 450 - including Vasari's Last Supper and Carlo Portelli's Assumption with Saints - are still languishing untouched for lack of funds.
The churches and museums in the Santa Croce area are virtually impossible to protect adequately, although some measures have been taken. Cimabue's Crucifix, restored to something approaching its original splendour, is now on an electronically operated pulley that in theory would lift it clear of flood water. That is assuming, of course, that someone remembers to press the right button amid the panic, that the electricity is still working, and so on.
At the National Library restoration has ground to a virtual halt, leaving around 20 per cent of the damaged documents unrepaired. The most precious treasures have been moved above the 1966 watermark, but lack of space means the lower floors are again stuffed with books and archive material. "If we had another flood the effects would definitely be worse this time," said the library's former director, Carla Guiducci Bonanni.
But how soon can Florence expect another disaster? The city has suffered major floods eight times since 1333, a rhythm of about one a century (the one before 1966 was in 1844). But minor floods occur all the time - 11 in the last six years alone. "The Arno has been a problem since antiquity," said Prof Nardi. "And even the old floods were caused as much by human error as by the forces of nature."