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| Tides | • | Waves | • | Currents |
The Earth is subject to the gravitational pull of both the Moon and the Sun. The fluid nature of the Earth's oceans means that they are particularly affected by these astronomical relationships, in addition to the centrifugal forces the Earth itself applies as it spins. The Moon is four hundred times closer to Earth than the Sun and so has more influence on tides.
Why do tides happen?
Rather than the Moon rotating around Earth, the two rotate about a common centre inside the Earth's surface. Gravity towards the Moon and the rotational force away from it are in balance at the centre of the Earth and maintain the Earth-lunar orbit. The balance changes with distance from this central point. On the surface of the Earth nearest the Moon, gravity is greater than the rotational force, therefore there is a net force towards the Moon. This creates a bulge or high tide in the oceans at this point. On the opposite side of the Earth, gravity is less, as it is further from the Moon, so the rotational force is dominant. At this point there is a net force away from the Moon therefore a corresponding high tide also happens at this point. In the intervening areas, low tides occur.
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Tides
caused by the Earth-Moon relationship (C. Herron) |
Tidal patterns
There are usually two high and two low tides every 24 hours and 50 minutes, as this is the length of the Earth-Moon rotation. These tidal patterns are a reaction to gravity and the forces of rotation where they are greatest and least as the Moon and Earth rotate.
Although less than the Moon’s, the Sun’s gravitational force also has a tidal impact. When the Earth, Moon and Sun are aligned on their rotations, the Moon and Sun’s gravitational pull reinforce each other and result in a higher than normal tidal range: a spring tide. Neap tides occur when the gravitational forces of the Sun and Moon act at right angles to each other. This produces lower than average high waters and higher than average low waters, i.e. a lower than normal tidal range, just after the Moon is in its first and third quarters.
Although the general explanation of tidal processes would suggest otherwise, tides do not occur uniformly along coasts. Lags occur as water is forced to move when tides interact with coastal topography, particularly the shape of coastlines and the variation in water depth (bathymetry). Each location therefore, has a unique tidal pattern.
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Spring
Tides caused by the Earth-Moon relationship (C. Herron)
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Neap Tides
caused by the Earth-Moon relationship (C. Herron) |
Tides in Ireland
On the northwest, west and south of Ireland, tides are semi-diurnal, i.e. there are two high tides every day. However, the high and low tides do not occur at the same time at all points around the coast. High water occurs first near Valentia, Co. Kerry in southwest Ireland, and becomes gradually later towards Carnsore Point, Co. Wexford in the southeast and Portrush, Co. Antrim in northeast Ireland. The maximum coastal spring tidal range on the west coast of Ireland is approximately 4.6m. This is experienced at Galway, with a gradual and regular decrease towards Carnsore and Portrush.
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| Mudflats in Barna, Co. Galway, exposed at low tide. |
On the east coast of Ireland, the tidal conditions are quite different. There are two points, one near Fair Head in Co. Antrim at the northern entrance of the North Channel and another north of Carnsore Point in Co. Wexford where there is no tidal range.
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| Spring Tidal range (metres) around Ireland (C. Herron after Government of Ireland, 1996) |
Tides in Ireland are measured by co-ordinated networks of gauges around the coastline. These networks, developed by organisations in the Republic of Ireland and the UK (for Northern Ireland) remain under development and currently includes twenty-three gauges in the Republic and two in Northern Ireland.
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Waves are generated by the friction of wind as it crosses the water surface. The size of a wave is determined by the speed of the wind, the distance it travels and the length of time it blows. All waves have some common characteristics: their highest point is called the crest; their lowest, the trough; wavelength is the horizontal distance between two adjacent wave crests; wave height is the vertical distance between the trough and crest; and the number of waves that pass a given point in one second is known as wave frequency.
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Wave
Characteristics (C. Herron after Government
of Ireland, 1996) |
How do waves form?
As waves are generated, water particles are set in motion, following vertical circular orbits. Water particles are briefly moved forward with the wave crest, and backward as the trough passes. So, except for a limited forward drag, water particles remain in the same place as successive waves pass. The size of the circular orbits decreases with water depth until they reach the bottom of the wave or wave base. This is equal to half the wavelength and below this point there is little water movement from waves at sea. An increase in the size of waves can be caused by increases in fetch (the distance the wind blows over open water); the length of time the wind blows, and the wind speed. Therefore during storms, the size of waves can be significantly increased.
Swell waves, which are long waves or a succession of waves without crests, are generated in the open sea by a meteorological event (e.g. gale) and often continue beyond their source. Swell patterns are superimposed on regular wave activity and can significantly increase wave height.
Waves at the Coast
When waves approach shallow water, such as at a coastline, the orbital motion of the water particles becomes elliptical as they are influenced by friction from the seabed. The wave lengths decrease as friction slows the wave down. This causes the water at the crest to surge forward ahead of the base of the wave. Water along the coast may be deeper in some areas than in others as coastlines are not all the same (e.g. headlands and embayments). When a wave front approaches a coastline, the part of it that meets shallow water will slow down as described above. If another part of the same wave front approaches the same coastline over deeper water (e.g. a deep pool off a headland), that section will continue at its original speed, meaning that the whole wave front bends or refracts to match the shape of the coast. As the water becomes increasingly shallow, the wave steepens further until its slope reaches a critical point and the wave breaks, dissipating energy over the surf zone and causing a forward rush of water.
There are three types of breaking wave:
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 Breaker
Types (C. Herron after Government
of Ireland, 1996) |
Depending on the strength of waves, exposure and the nature of the coastal environment on which waves are acting, erosion or deposition of material can occur. Types of wave erosion include sediment transport offshore or along-shore and hydraulic action or abrasion on harder coastal environments. Resultant erosional features can include sea-cliffs, wave-cut platforms, headlands, sea caves, arches and stacks. Where the wave climate allows, deposition of sediment from offshore, estuarine or fluvial sources can cause the development of, among others, beaches, spits and bars.
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Spilling
Breakers at Benone Beach, Co. Derry. |
Sea
Arch formed by wave action at, Bridges of Ross, Co. Clare. |
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Surfing
Beach Break at East Strand, Portrush, Co. Antrim Types of surfing waves include the Beach Break, where waves break on a sandy seabed; the Point Break, which is a wave that breaks onto a rocky point and the Reef Break, a wave that breaks over a coral reef or a rock seabed - these waves are the classic ones that can be seen on surfing videos. The Rivermouth Break, can occur over rocky ledges or sandy bottoms. The Beach Break is the best wave for beginners. A wave is either a Left-Hand Break or a Right-Hand Break, depending on which direction the wave breaks from the point of view of a surfer paddling and riding the wave. If a surfer is paddling to catch the wave and it is breaking from right to left (the surfer will have to turn left to get on the wave) then this wave is a left. From the beach the wave will be seen to break to the right. A peak is a wave that breaks forming a wave which can be surfed both left and right, meaning that two surfers can surf it at the same time in different directions. |
Deep meteorological depressions moving from the Atlantic have a significant effect on the wave climate of Ireland. The south coast experiences rough wave conditions in advance of depressions, however west to northwest gales, which follow in the rear of the depression cause sea heights of between 4 and 7m off the west and north coasts of Ireland. This is added to by exposure to Atlantic swells (of between approximately 80-150m long) from the northwest to southwest. During December and January, the northwest of Ireland can experience swells of over 4m in height for 40% of the time.
Although the east coast is more sheltered from westerly gales, the entrances to the Irish Sea experience heavy seas when winds are from the north or northwest and south respectively. These areas are less impacted by swell.
Currents
Currents are streams of ocean water flowing along prevailing directions, either vertical or parallel to the water surface.
Global currents
Currents operate on a global circulation pattern at all ocean depths, affecting climate and the flora and fauna in the oceans. Surface ocean currents, e.g. the Gulf Stream, are mainly wind driven and often align with global wind patterns. Ocean water at the equator absorbs heat from the sun. These warm ocean currents then flow toward the poles, carrying heat away from the equator and distributing it to higher latitudes. Cold-water currents travel from the polar-regions toward the equator, where they become heated again. Due to the Coriolis Force, which causes the deflection of a moving feature and is attributed to the rotation of the Earth, currents in the Northern Hemisphere generally flow clockwise (anti-cyclonically). In the Southern Hemisphere the circulation is mainly counter-clockwise. In wind-driven currents, the Coriolis Force is also responsible for the occurrence of the Ekman Spiral, in which the flow direction rotates horizontally with depth from the sea-surface. This deflection causes the currents to flow at an angle to the prevailing winds. As the current deflects, its velocity becomes progressively weaker. The depth to which the Ekman spiral penetrates is determined by the depth of turbulent mixing.
Deep ocean currents, i.e. thermohaline circulation, are driven by density and temperature gradients in the water itself. They have a significant vertical component and account for the mixing of the deep masses of ocean water.
 Global thermohaline circulation (C. Herron) |
Currents at the Coast
Superimposed on global circulation are more localised currents caused by winds, temperature, tides, rain, runoff and seabed topography. Particularly along coastal areas where there are pronounced changes in bottom contours and shoreline configuration, current speed and direction can vary greatly over short distances.
Long-shore currents occur when waves strike the shoreline at an angle, due to wave refraction. The front part of the wave reaches the shallow water first and slows. The rest of the wave bends as it comes onto the shore creating a current that parallels the beach. Long-shore currents often result in a number of coastal features: sandbar formation; long-shore drift, which occurs when the backwash of oblique waves repeatedly carries sediment perpendicularly down the shore under gravity, causing a net movement of sediment in the direction of the longshore current; and rip currents, which occur when longshore currents are directed seaward by changes in the seabed, through breaks in sandbars or by localised changes in currents forced by differential wave energy and its interaction with the coast.
Vertical currents at the coast, known as upwelling and downwelling, can also occur. Upwelling can occur when winds blowing off the coast push water away from the shore. Deep colder water rises to replace this water, potentially bringing nutrients to the surface and encouraging the formation of plankton blooms. Downwelling happens when onshore winds push water towards the coast and nearshore surface water is driven down and away from the coast.
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| Tidal Streams around Ireland. Surface currents are given in knots with the first referring to neap tides and the second to spring tides (Government of Ireland, 1996) |
Tidal currents on the west coast of Ireland are insignificant when compared to those on the east coast. In general, average spring tidal currents on the west coast are approximately 0.2m/s. Due to topography, bathymetry and prevailing conditions, a complicated tidal current pattern occurs on the east coast of Ireland, with speeds reaching as much as 1.4m/s in the North Channel. These currents can cause eddies, tidal rips and races, dangerous to seafarers and those using the water for leisure. Wave and wind-induced currents in the nearshore vary widely and can be modified with changes in weather conditions around the Irish coast.