When Canals in Venice are Drying Up: Explaining Acqua Bassa e Fanghi Inerti

Carnival gives Venice a prominent place in international news and is probably one reason you could read quite a few headlines about Venice in the last few days. Quite a number of newspaper like this one mentioned Venice being “hit” by an excessive low tide. Water seemed to be drained out of the canals and gondolas  in secca – stuck along the muddy banks of the canals. By now, things are returning to normal with bouts of bad weather approaching the city.

So what happened all of a sudden in late winter when one would expect high tides and floods ?

Scientifically speaking, there are two succinct answers, one is short-term and the other long-term. You need to know the laws of the effects of the moon on our Lagoon. Theoretically, the period of potential flooding is over by end-December. It means that if everything went normal like it did until 100 years ago, iin November and December a few bouts  (one or two!!!) of acqua alta would happen, and you would be relatively safe from high water during the other ten months of the year.

So in a way, what is happening now, the low tides, is a return to normal if it weren’t so pronounced. The two reasons for these extremes are man-made. One is short-term and the other long-term.

The short-term reason is moon + weather. There was practically no rain in Venice during January, the water fell to minus 60 centimeter below sea level, exacerbated by a celestial phenomenon called luna rossa. On 27 January, Venice witnessed a “red blood moon” which caused much water being drained from the Lagoon which is natural on such an occasion. The worst acqua bassa happened on 14 February 1934 when the water fell to minus 121 centimeter below sea level.

The absence of winds also stopped the tides from functioning properly. We need southerly winds (lo scirocco) to move water masses from the Adriatic sea north into the Lagoon and northerly winds (la bora) to remove them. During full moon and new moon, any tidal effect is reinforced: Low tides appear lower and high tides even higher. There was no wind to move the waters in and out, so about one-fourth of the canals in Venice were looking rather muddy with only a few water puddles left.

The long-term reason for acqua bassa eccessiva are accumulated mud in the canals (inertia dei fanghi) +  and the deep shipping lanes crisscrossing the Lagoon (scavo dei canali in Laguna).

L’inertia dei fanghi means that potentially dangerous (chemically altered) mud and debris have accumulated on the bottom of the canals in town, so they are less deep and more difficult to navigate. There was a good reason why until 1797, every ten years, all the canals in Venice, including the Grand Canal, were shut off from the Lagoon while being cleaned up and excavated.

Going through Nonna’s extensive collection of newspaper clippings on the ecological problems of Venice, I found one published in 1991 in the Venetian daily Il Gazzettino. It says, Un rio invaso dai fanghi – a canal obstructed by mud. Rio di Sant’Anna in spring 1991 must have been quite an issue. Not having been drained and cleaned for some time, it resulted dangerously low for boats to navigate. And there were other side effects as well due to debris whose chemical composition under water may change, endangering the health of humans, animals and the plankton on which crab and other tiny animals feed. There’s a reason why algae line the canals. They act as cushion soaking up dangerous substances.

Until 1797, cleaning and excavating canals every ten years was an integral part of the agenda in town. Cleaning lasted up to six weeks and of course, Venice with canals exposed to the sun must have looked rather strange ..

On the other hand, excavating the deep shipping lanes in the Lagoon causes a pronounced difference in the depth of the wide canals in the Lagoon and the narrow ones in the city. With low tides, there’s simply not enough water to bathe all the capillary canals (canali minori) in town.

So these are the issues causing the canals to dry up when certain phenomena concur, reinforced by man-made manipulations of the ecosystem of the Lagoon. For the Venetian buildings, recurring low tides are as dangerous as high tides: Exposing their foundations resting on wooden poles, called palafitte, to oxygen for more than a week is dangerous. They might crumble as they require wet mud to keep “nourished, compact and alive”. The plants lining the canals also need fresh loads of sea water so they are able to “filter” the water.

PS. Carnival showing its “side effects” by now, with parts of Venice being overcrowded on weekends. It was really bad on Rio Terà San Leonardo near Canale di Cannaregio a week ago when Carnival was opened by La Festa venexiana sull’acqua. So next time, we will share a few answers to the questions Nonna received from the clients of her hotel on Carnival. What to do and where to really enjoy Carnival in our town.

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    1. You are so welcome ! I wrote my thesis on ecologia e urbanizzazione della Laguna di Venezia, and sometimes, I include some of these issues on the blog to show the complete picture.

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