Acqua Alta – Part I
Have you ever visited Venice Italy? From a geophysical perspective (Is there any other?), Venice has a curious attribute. Certain times of the year the city endures Acqua Alta – high water- when the sea rises up and floods the lowest areas of town. City workers dutifully unpack and set up wooden walkways so folks can get about the place without wet feet. Cafe owners construct temporary platforms in front of their shops tempting tourists to partake coffee and biscotti whilst high and dry. In a few days the flood fades, walkways and platforms are disassembled and stowed, the pavement hosed down, and everything returns to normal until next time.
Acqua Alta has several causes including the slow subsidence of the city and surge from passing low pressure systems, but the biggest contributors are ocean tides.
(1) Tides: From the Earth’s point of view, the sun circles above a fixed latitude every 24 hours. That fixed latitude varies from +23.5 degrees to -23.5 degrees and back again over a year’s time. From the Earth’s point of view, the moon circles above a fixed latitude every 24.8 hours. That fixed latitude varies from +28 degrees to -28 degrees and back again over a month’s time. Ocean water beneath the sun and moon, drawn horizontally by their gravity (barely one ten-millionth of Earth’s), forms piles and depressions about ½ meter high peak to trough on average. You might imagine that if the sun and moon just happened to be over the same latitude and longitude, those peaks and troughs would be taller and deeper than average. Can you spot those times in the movie?
Already you are half way toward understanding Acqua Alta. Stay tuned.
Steven N. Ward Santa Cruz.
- OH Community
- Web Apps
- About Us
About OpenHazards Bloggers
John Rundle is a Distinguished Professor of Physics and Geology at UC Davis and the Executive Director of the APEC Collaboration for Earthquake Simulations. He chaired the Board of Advisors for the Southern California Earthquake Center from 1994 to 1996. Read John's blog.