Tides and circulation
| Tides are the periodic rise and fall of the water on the earth's surface |
Tides are caused by the sun and moon. In particular, the moon's gravitational pull influences the tides greatly as it revolves around the earth. The moon's influence on the tides is two and a half times greater than that of the sun.
The largest rise and fall of tide is called a spring tide and occurs when the sun and moon are in line and their gravitational forces act together.
The smallest rise and fall of tide is called a neap tide and occurs when the sun and moon are at right angles to each other relative to the earth.
The way in which tides rise and fall around the world varies. Diurnal tides occur when there is one high water and one low water every 24 hours. Semi-diurnal tides occur when there are two high waters and two low waters every 24 hours. ('Diurnal' means 'daily'.)
The height of tides is determined mainly by the shapes of the coast. For example, generally the tides will be low on open coasts, but bays and estuaries - which act like funnels - result in high tides.
The tides rise vertically, but they also cause horizontal flows of water, too. Tidal currents can be quite fast (up to 1 metre per second), depending on the height difference between the high and low tides, the shape and size of the entrance to the estuary and the depth of the estuary.
The incoming, or rising, tide is traditionally known as the flood tide because it `floods' the channel. The outgoing tide is known as the ebb tide. The speeds of the ebb and flood tide vary daily. The speeds also change in the cycle between spring and neap tides. The variation is exactly the same as the way tidal water levels vary. That is, spring tides produce the highest water levels and also the fastest tidal currents.
| For more about this see section 5.3 - Sedimentation transport |
What type of tide it is (diurnal or semi-diurnal) and the difference between the highest and lowest tide (the `tidal range') are very important for the coastline. The type of tide determines the time difference between high and low tides. This affects how much time the coast has to dry out. Obviously, semi-diurnal tides flood the coastline more often than diurnal ones. How long the coast is wet affects not only what plants and animals live there, but also how rocks and sand are worn down. The tidal range is important because it determines what amount of the coast is going to be flooded by the tides. Along with how steep the shore is, the tidal range also determines how big the intertidal zone is (the area between the high and low tide).
Twice a day plants and animals living above the low tide mark and below the high tide mark are left high and dry. The harsh conditions of this intertidal zone are softened a little for these living things by waves occasionally splashing them. In contrast, an estuary doesn't have waves. Plants and animals living around the estuary are not covered by splash and spray during low tide. The living things of these intertidal zones are extremely well adapted.