LOCKDOWN DIVER - Understanding Tides & Currents
As (responsible) scuba divers we need to have an understanding of tides and currents – even if it’s just a basic understanding. You will especially need this knowledge if you are planning to dive in small groups while shore diving. One problem with understanding tides is that they vary in different areas – not just regionally, but globally - so do your research before stepping into a new site!
Tides are caused by the gravitational pull between the Earth, Moon and Sun which work together to cause the sea levels to rise and fall. The tides are responsible for the (mostly) twice daily high and low tides in the UK. They play an important role in determining when certain locations will experience strong currents, changing depths and changing visibility. Therefore, tides affect diving conditions – in both good and bad ways.
The Moon and the Sun affect the earth’s tides on a daily basis and follows a monthly lunar cycle and is why we are able to predict the tides in advance. The part of the Earth that is both closest to the moon and furthermost away from the moon will receive high tides within a 24 hour 50 minutes period, whilst other areas of the earth will then see a low tide. This is because water is being pulled towards the moon on one side of the earth and the earth pulled away from the water on the other side of the earth, creating a bulging effect and a high tide at opposite sides of the earth.
High & Low Tide
High tide is when the ocean is at its fullest point on the beach. On the beach, we notice that there isn’t much of an area to walk along and the waves are breaking close to the shore or at the top of the beach. High tide is caused when the moon is overhead or at its closest to the earth and when it is at its furthermost away.
Low tide is caused when the ocean is at its lowest point of the beach. On the beach, we notice that the sea is far away and leaves a big area that was once covered at high tide. Low tides are caused when the earth is a right angle to the moon.
The difference between the high and low tide is called the Tidal Range. The tidal range will change throughout the year as the position of the Earth and Moon changes as we orbit the sun. Some places feel almost no change in water level however, in some places the water level can drop and rise by over 10 metres!
In the UK, the Severn Estuary between England and Wales can change as much as 15 metres!
Spring & Neap Tides
Spring tides occur at a Full Moon and a New Moon, approximately every 2 weeks. Neap tides occur on the half-moons. Neap means low. Spring doesn't have any relation with the season and the origins of, “spring tide”, are not known for certain. One idea is that it comes from the Scandinavian word for “leap up” or that it refers to the natural feature of a spring, a place where water wells up from the ground.
When there’s a New Moon (no Moon), both the Sun and the Moon are aligned on the same side of Earth, and during a Full Moon the Sun and Moon are aligned on opposite sides of Earth. Both positions create the highest and lowest tides called Spring Tides. When the Moon is in a quarter phase, the lines from it and the Sun to the Earth form a right-angle. The Sun’s gravitation pulls to the side of the Moons tidal bulge. This tends to raise the low tide and lower the high tide. This weakens the tides and is called Neap Tides. On spring tides, more water is moving further between those two extremes. Thus currents are correspondingly stronger on springs than on neaps.
The significance of tides is in understanding that more water moves during the middle two hours of both an incoming (flood) and outgoing (ebb) tide; this results in stronger currents. The slack tide is one hour either side of high or low tide, where the water barely moves.
How are tides predicted?
Tides can be predicted far in advance. As the relationship between the Earth, Moon and Sun are well known and the position of all 3 can be forecast easily and in turn, so can the tides. However, as sea levels rise, the regularity and range of the tide will be altered due to different water depths and topography of an area. Therefore predicting tides a long way into the future could be less accurate.
Information on tides in the UK comes from the National Tidal and Sea Level Facility which was set up specifically to provide high-quality information on tides, sea levels, flood warnings and issues that affect navigation at sea. Tide levels are calculated and monitored by the UK Tide Gauge Network which has forty-five gauges around the coastline of Britain which gather, process and calculate tidal information. These gauges provide the local scale which is used in the public information on tides and tidal levels.
WAVES & CURRENTS
A primary cause of water motion is wind energy, which transfers to the water as it blows across its surface. This results in two primary types of water motion: currents and waves. Ocean water is constantly on the move.
Ocean waves are formed by the wind. The longer and stronger the wind blows, the larger the size of the wave will be. There are two types of waves - wind waves and swell waves. Wind waves are usually produced by nearby storms and travel only a short distance making the wave very weak. Swell waves are produced from storms and travel a long distance past its point of origin. The longer a wave travels uninterrupted, the larger, faster and more powerful the wave will become. o travel a certain distance will have much more power than those with a low period.
Waves break when they reach a shallow coastline where the water is half as deep as the wave is tall. As a wave travels across the open ocean, it gains speed. When a wave reaches a shallow coastline, the wave begins to slow down due to the friction caused by the approaching shallow bottom. The wave begins to slow down from the bottom first causing the back of the wave to stand up upon itself. Once the wave reaches a certain height, it begins to crest over itself and break creating the whitewash. Once a wave has broken it will continue to lose speed and power as it travels toward the shore.
The wind is the flow of air between areas of high and low pressure. If Earth did not rotate, the air in the atmosphere would basically circulate in a simple back-and-forth pattern between the poles (high-pressure areas) and the equator (a low-pressure area). The direction of surface currents would then align with this general wind pattern. But because Earth rotates, circulating air is deflected toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere, resulting in curved paths. This deflection is called the Coriolis Effect. Winds drive ocean currents in the upper 100 meters of the ocean’s surface. However, ocean currents also flow thousands of meters below the surface. These deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation.
In Earth's polar regions, ocean water gets very cold, forming sea ice. As a consequence, the surrounding seawater gets saltier, This happens because salt is left behind when sea ice forms. As the seawater gets saltier, its density increases, and it starts to sink. Surface water is pulled in to replace the sinking water, which in turn eventually becomes cold and salty enough to sink. This initiates the deep-ocean currents driving the global conveyer belt. The Global Conveyer Belt is the main avenue by which high temperatures are distributed around the globe moderating extremes and giving Earth air surface temperature.
Currents are a large movement of water in one direction and can be temporary or long-lasting, near the surface or in the depths of the ocean. The biggest of these currents shape the Earth’s global climate patterns and local weather conditions by moving heat around the world.
Waves do not typically reach the beach perfectly parallel to the shoreline. Rather, they arrive at a slight angle. This angled impact with the coastline sets up a current that moves parallel to the shore. This is called a long-shore current. Longshore currents, which are very powerful during storms, erode sediments off a beach and transport them in the direction of the current. This process is known as longshore drift. Longshore drift influences the shape and composition of the coastline. It can change the slopes of beaches and create long, narrow shoals of land called spits, that extend out from shore.
Another type of coastal current, upwelling, forms when the water below the surface rises to replace surface waters that have been transported out of an area by strong winds.
Rip currents are narrow channels of water that form when waves of different intensities break on the shoreline and generate currents that keep the water level by pulling the large amounts of water brought in by the waves, back into the ocean. These currents can move fast, and as this moves away from the shore, you can find yourself swept out very quickly. Fortunately, rip currents can be spotted before entering the water and the main things to look for are:
- Gaps between the waves – A small patch of calm water in an otherwise choppy sea may be inviting, but this is often a rip current sucking water back out to sea.
- Discolored water – Rip currents tend to drag large amounts of sediment back out to sea, so if you notice a jet of discolored water, avoid that area.
If you find yourself caught in a riptide stay calm, go with the flow and swim parallel to the shore.
Tidal and Open Ocean Currents
These currents are minimally influenced by tides and flow in complex patterns affected by wind, water salinity, temperature, and Earth's rotation. They are the steady flow of surface ocean water in a prevailing direction. Think of them as streams of water moving within the larger ocean body of water, either along the surface, or at depth. Though open ocean currents have no rigid borders (other than continents and other geographic features), their locations can be definite and have occurred continuously for thousands of years.
Tidal currents occur in conjunction with the rise and fall of the tide. The vertical motion of the tides near the shore causes the water to move horizontally, creating currents. When a tidal current moves toward the land and away from the sea, it “floods.” When it moves toward the sea away from the land, it “ebbs.” These tidal currents that ebb and flood in opposite directions are called “rectilinear” or “reversing” currents.
Tidal currents are the only type of current affected by the interactions of the Earth, sun, and moon. The moon’s force is much greater than that of the sun because it is closer to the Earth than the sun is. Tidal currents, just like tides, are affected by the different phases of the moon. When the moon is at full or new phases, tidal current velocities are strong and are called “spring currents.” When the moon is at first or third quarter phases, tidal current velocities are weak and are called “neap currents.”
Down & Up Currents
A down current occurs when a current hits the face of a wall or when it runs at a right angle to a drop-off. Down currents are also possible when two currents moving in opposite directions meet or move over each other. Most down currents lose strength the deeper they go, but there's no telling how deep they go.
As with down currents, up currents can occur when a current hits the face of a wall. These are dangerous because they can pull you up to the surface very quickly, which can lead to a host of problems
How do tides and currents affect our diving?
You can often predict strong currents, horizontal or vertical, by looking at the surface of the water. Areas where the surface is choppy without a moderate to strong wind, mixed with areas of very smooth water, could indicate a strong current. If the boat that you’re diving from is tied to a mooring buoy, look at the direction in which the boat is turned. But if it’s tied at the bow, the boat will be facing into the current; if the mooring line is tight, the current is likely moderate to strong.
Underwater you can tell which way the current is running by observing the sea life around you. Soft coral sways in the direction the current is traveling, and fish face into the current, so if you see a school of fish pointing in one direction, that’s where the current is coming from. If a school of smaller fish is swimming around freely in different directions, there’s probably only a slight current, if any at all.
Generally we plan to dive at ‘Slack Tide’. As the Earth is constantly rotating the tides never stop, but around 45 minutes before the tidal extreme, the speed of the changing water level slows down dramatically. The same effect occurs around 45 minutes after the tidal change. Because of the huge amount of water that is moving as the sea level rises or falls, you will find that the most powerful currents are found between the tidal changes. If we plan to dive at slack tide we will feel the least effects of the tides, and if we plan to dive at slack tide during a neap tide, we will feel fewer effects still. It is generally preferred to dive at slack high, rather than slack low because it can be difficult to access some sites when the tide is low.
If you are a regular shore diver then it is important to pay close attention to what the tide is doing, if diving from a boat a skipper will be able to advise you on the movement of the water. If you are not used to diving in areas where there are strong currents then it is recommended that you dive with a buddy who has experience. Many popular dive locations in the world feel very little effects from the tides, which can lure inexperienced divers into a false sense of security as they buy their own scuba gear at home and head off the beach with their buddy for a dive.
Currents can often be easily under-estimated by divers - even experienced ones! Surface currents can be quite strong so if a surface swim to a line before descending is required, make sure you can get to it easily. Swimming against a current can be tiring and if you're using your air while doing can consume a fair bit. You can get caught in currents under the water too, such as down currents. If you get caught in a down current you can potentially get pulled downwards quite quickly or you won't realise you're descending until you need to equalize more or notice it on your computer. Although most of them lose strength the deeper they go, you don't know how deep they go. So, if you get caught the main thing to do is stay calm (easier said than done sometimes!), stop, think, then act! Maintain normal breathing to conserve air and swim out into the blue at a slight upward angle. If it's a strong current inflate your BCD slightly - but REMEMBER, be ready to deflate it as some as the need arises. Don't drop your weights straight away, as it could cause further issues if you manage to get out of the current quickly & easily.
If you get caught in an up-current stay calm and swim away from the wall/drop off and down slightly deflating your BCD.
Check the tide times when planning a dive. There are multiple websites that allow you to do this:
Although the timetables are easy to read, it may be worth Googling how to read them to get a better understanding of them.
Weather will also have a factor in your decision of when and where to dive. If it has been windy in the days running up to your planned dive day bear in mind that the visibility may be greatly reduced as the water movement will have churned up the sediment. If it's been windy, a sheltered bay maybe your best option to get optimal diving conditions. Going out diving during high winds isn't a good idea either. While talking about the weather, if you're diving in hot, sunny weather make sure you stay hydrated and sit in the shade if you're getting hot in your gear.
Drift diving can be a relaxing experience as well as a fun and exhilarating one! Learn how to go with the flow safely. Taking this course is highly recommended if you are going to be diving in an area known for its currents.
Don't fight a current, work with it. If you find yourself caught it can be scary and stressful. This will mean that you are breathing at a faster rate so stop and think, take your time to think and slow your breathing rate down. If you find yourself fighting a current swim close to something, closer to a reef or closer to the bottom (the flow will be at its weakest here). If possible turnaround and go with the current, turn your dive into a drift dive - if you're worried about being carried too far away from the shore or boat ascend slowly to the surface and end the dive.
Diving can be fun and satisfying if it's carried out in a safe way. Learn about your local area and keep an eye on the tides & weather. Carry a DSMB (make sure you know how to use it) with you and talk to your buddy about separation procedures before entering the water.
Trust your instincts and never go out of your comfort zone. Dive within your limits. Plan your dive and dive your plan!