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Coastal Processes - TidesTides are the periodic rise and fall of the ocean waters. They are a result of the rotation of the Earth and gravitational interactions between the Earth, the Moon and the Sun acting upon the oceans and seas. The fluid nature of water allows it to be pulled towards the Moon or Sun resulting in a change in its height, most noticeable around coastlines. Although the Moon has a much smaller mass than the Sun, and thus gravitational force, it has a greater effect on the tides because it is much closer to Earth. As the water rushes to the areas of high force it moves away from other areas which experience a low tide.
When the Sun and Moon are inline with each other we experience a large tidal range, known as a Spring tide, as the gravitational forces of the Sun and Moon are acting together or directly opposing each other along the same plane. These spring tides occur twice each month, during the full and new Moon.
When the Sun and Moon are at right angles to each other their gravitational forces are acting against each other and we have a lower tidal range known as a Neap tide.
There is a time lag for the water to respond to the position of the Moon. This is due to friction between the water and sea bed. In areas where tidal flow is restricted, there can often be higher tidal ranges and stronger currents than areas where there are few restrictions. As the basic driving forces behind the tides are astronomical the tide times and levels are predictable. The local bathymetry and topography can have significant effects on tidal currents and tidal ranges. Tidal levels can be significantly amplified in shallow seas and estuaries. This phenomenon can be seen in the Liverpool Bay area with tidal ranges of approximately 10m. Tidal bores or waves can be seen moving up some estuaries and rivers as large amounts of water tries to enter the estuary. The famous Severn Bore is an excellent example of this phenomenon. Locally a tidal bore can be seen on the River Ribble, Mersey and Dee when the conditions are correct. The Sun is extremely massive, but it is an average of 93 million miles (150 million km) from Earth, compared with about 238,866 miles (384,400 km) from here to the Moon. And since the Moon is nearly 400 times closer to our planet, its influence on our oceans is twice as strong as the Sun's. The timing of tides is determined by the Earth's rotation and the Moon's orbit around the Earth. As the Earth rotates once about its axis in 24 hours, the Moon is moving 1/30th of the way around in its orbit. It takes a given location on Earth about 50 minutes to "catch up with" the orbiting Moon, so a particular tide returns in approximately 24 hours and 50 minutes. When the Moon, Earth, and Sun fall in a straight line, which we call syzygy (siz-eh-gee), we notice the greatest difference between high and low tide water levels. If the Moon is at perigee, the closest it approaches Earth in its orbit, the tides are especially high and low. Factors such as the path the Moon takes around the Earth, our planet's tilt, even the water's depth, and the ocean floor affect tides. Therefore, not all coasts experience two high and two low tides each day. Semi-diurnal tides occur twice a day. This means a body of water with semi-diurnal tides, like the Atlantic Ocean, will have two high tides and two low tides in one day. Diurnal tides occur once a day. A body of water with diurnal tides, like the Gulf of Mexico, has only one high tide and one low tide in a 25-hour period. Some bodies of water, including parts of the Pacific Basin, have mixed tides, where a single low tide follows two high tides. The difference in the height of the water surface between the high and low tides is the tidal range. Tidal ranges can be measured in inches, like those in Lake Superior, Michigan, or in feet or yards. In fact, the Bay of Fundy, a V-shaped Canadian inlet in Nova Scotia, has the greatest tidal range known - up to 15 metres (50 feet)! In areas with large tidal ranges, boats anchored at high tide are often left stranded on the dry beach at low tide. As the sea level rises and falls, it generates a tidal current that flows horizontally. Tidal currents caused by the dropping water level (as the tide "goes out") are called ebb currents. The rising tide generates flood currents. Tidal currents are especially strong where the ocean is connected to an estuary or bay, and boats sometimes have to wait for a current in to enter or leave a harbour. Tidal Levels at Liverpool Glastone Dock are recorded as part of the UK National Tide Gauge Network. Predictions based on scaling factors are available for Formby and Southport. In addition prediction tidal levels are also available at Preston and at the mouth of the Ribble Estuary.
These values are predictive based on astronomical criteria but do not take into account any variations due to changes in meteorological conditions - surges etc.
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