The expression “high water” indicates an unusually high tide in Venice. This phenomenon especially affects the lagoon of Venice and can cause flooding in the cities of Venice and Chioggia. High water is most likely to occur in autumn when the astronomic tide (linked to the position of the sun and the moon) combines with scirocco or bora winds which block the regular ebb from the lagoon to the sea. However high water also occurs at other times of the year.
According to the data obtained by the Idrographic Station at Punta della Salute they distinguish three different kind of tides: high tide, very high tide, exceptional tide. A single tide lasts 6 hours. As different parts of the city have different altimetric levels (some parts are lower than others) it never floods uniformly. In case of high water Venetians wear Wellington boots, but provisional wooden walkways (passerelle) are installed in the busiest parts of the city providing a dry route.
The Centro Previsioni Maree del Comune di Venezia (Tide Forecasting Centre of the City of Venice) predicts tide-levels for the following 48 hours, and you can easily check them on the City of Venice website. If the predicted tide-level is more than 110 cm above sea level, a system of acoustic sirens are activated in different parts of the city to warn the population.