Tides are the periodic rise and fall of sea levels caused by the gravitational
forces exerted by the moon in the Sun. Despite the sun's gravity being 179
times greater than the moon's, the moon has a stronger influence on the Earth's
tides than the Sun does. That is because the moon at three hundred and eighty six
thousand two hundred forty kilometres away is in closer proximity to the earth
than the Sun which is a hundred and fifty million kilometres away. The
gravitational pull of a sun and moon create a bulge in the oceans. The areas
of the earth where the bulging occurs, experiences a high tide and the other
areas experience low tide. While the earth rotates on its axis the moon moves
in the same direction around its orbit turning the lunar tidal bulge as it goes.
At times when there is a new moon or full moon the Sun, Moon and Earth are
approximately aligned. The higher gravitational pull of these combined
forces results in higher than normal high tides and lower than normal low
tides. These are called spring tides.
During the moon's quarter phases the line from the earth to the moon is at
right angles to the line from the earth to the Sun as shown in the diagram.
Consequently the tide generating forces of the Sun and Moon oppose each other,
thereby producing tidal ranges that are approximately 20% lower than the mean
tidal range. We call these neap tides. The arrangement of the continents has
created three major ocean basins. The Pacific, the Atlantic and the Indian
Oceans. As the Earth rotates west to east the tidal bulge travels to the western
boundary of the ocean basin. It will be high tide on the western boundary and
low tide on the eastern boundary. Then the tide will move back towards the
center of the ocean. However rather than travel in a straight line this motion is
affected by the Coriolis effect, causing the wave to bend to the right creating
the clockwise movement in the Northern Hemisphere and in anti-clockwise
movement and the southern hemisphere. The wave rotates around a node called an
amphidromic point which experiences no tidal fluctuation. The resulting
circulatory system is called an amphidromic system. Because of the
different characteristics of each ocean basin, different tidal patterns are
established. There are three main tidal patterns that occur along the Earth's
coastlines. When there are two high tides and two low tides each day we call this
a semi-diurnal tide. This is the most common total pattern around the world
and it's the case for New Zealand's coastline. Some coastlines experience two
high tides and two low tides of different tidal range every day. This is
called a mixed semi-diurnal tide. For example San Francisco Bay. Other
coastlines have only one high and low tide each day. This is called a diurnal
tide. For example many places in the Gulf of Mexico experience diurnal tides. The
tidal range is the difference between the high and low tide. The territory between
the high and low water mark is called the intertidal zone. An abundance of
marine life exists within the zone. The tidal range varies from coastline to
coastline depending on a range of factors.
The main factor is that the title range becomes greater the further away the
coastline is from the amphidromic point of a circulatory system. The shape of the
coastline and depth of the ocean floor affects tidal range. Straight open
coast has less tidal range than shallow bays and estuaries. Funnel shaped bays
can alter tidal range. A local example is Cockle Bay here in East Auckland. This
beach is located at the mouth of an estuary and experiences very low tides.
This is because tides are slowed down by wave shoaling. Friction from the sea
floor and narrowness of entrances to estuaries and harbors. Coastlines are
sometimes classified according to tidal range. Coastlines with a tidal range of
less than 2 meters are classified as being microtidal. Coastlines with a
tidal range between 2 and 4 meters are classified as being mesotidal.
Coastlines with tidal ranges between 4 and 6 meters are classified as being macro
tidal. Most New Zealand beaches are microtidal, with a range of between 1 and 2
meters. But there are local variations, particularly
west coast beaches such as Muruwai, which is mesotidal, with a range of three
meters. The width of the intertidal zone has a big influence on coastal processes.
This is because the higher the tidal range, the more drying of substrate can
occur. The more drying takes place, the more easily wind is able to pick up sand
grains and transport them up the beach, and the more rapid the formation of sand
dunes. On rocky intertidal zones, salt crystals have more time to form and
rocks can be weathered by this crystallization process. The vertical rise
and fall of the tide is accompanied by a horizontal movement of the water called
a tidal current. A rising tide is called the flood tide. A falling tide is called
the ebb tide. Tidal currents influence the transportation and deposition of
sediment. As tidal currents flow and ebb, they move
sediments back and forth. The velocity of tidal currents is greatest at the mid
tide which is the halfway point between the rising and falling at the tide. It
is at this time that the most transportation of sediment occurs. At
high tide and low tide the current velocity is zero. We call this period slack water.
Most deposition takes place during this time. Larger sediment sizes require
stronger tidal currents to be able to pick up the particles and transport them. Finer sediment can
be transported by weaker currents.