Sunday, December 14, 2008

Extreme Tides

Tides have a big influence on beach erosion, by lifting the water level and the waves against a new surface that is not in equilibrium with the wave action. Understanding the tides, is not as simple as looking at a tide chart.

Tides are primarily controled by the gravitation pull of the water in the oceans by the moon and sun (the gravitation gravitational from other planets does have a tiny little influence). This gravitational influence changes at any location because the earth rotates. There are two tides each day, Two high and two low tides. The moon rotation about the earth produces an additional monthly cycle and the earth rotates around the sun producing a yearly cycle, but there are many cycles influencing tide. The largest tides are often called "spring" tides, they occur twice each month with the new and full moon (when the moon and sun are aligned). Spring tides heave nothing to do with the season spring. These astronomical influences are largely predictable and can be used to produce tidal charts.

There are also some unpredictable causes of higher sea levels. When the air pressure is low the sea level will rise, around 1cm per hPa (hectoPascal). In addition this low probably also generates winds and a large low pressure can exert additional upward "suction" of the ocean surface. The wind blowing across the sea may also whip up swells (large ocean waves that can travel vast distance without loosing much energy). When these swells and the waves the create on the coast are larger than normal and extend over a longer period they can force water to bank up close to shore, this is known as wave setup. Strong winds can also lift waves further, this is know as wind setup. All or any of these phenomena are usually explained as a storm surge because they also produce nearby storms, but a storm is not necessary to the development of a "storm surge". When a spring tide corresponds with the conditions such as low atmospheric pressure, large swells and winds, tides can be extremely high compared with average.

The geometry of the sea floor close to the coast (bathometry) also has a large effect on how tides and these additional surge might be amplified (or cancelled).

But how high is extreme for Venus Bay?

And How often might that occur?

No comments:

Post a Comment