The recent recurrence, yet again, of the storm surge at Nigeria's premier beachfront - the Bar beach on Victoria Island - has raised many posers as to what actually the problem is, particularly with the unfortunate loss of lives. Incidentally, the answer can be found in a proper understanding of the nature of the wave structure that is responsible for the incidents, which in this instance are known as 'Rip Currents.'
Rip currents are narrow, fast-moving belts of sea water travelling offshore which occur along any shore that features breaking waves. Waves ravelling in the ocean have complex patterns depending on the fetch, bathymetry, or the approach to the shoreline and are forced to break due to what is termed "swallowing."
When waves break along an oblique coastline, they generate two types of waves namely Longshore and Rip Currents. Longshore currents travel parallel to the coastline while Rip currents travel perpendicular to the coastline. Rip currents are one of the ways by which water returns seaward. The attached illustration shows a schematic picture of a Rip current heading seaward.
Coastal scientists in the Nigerian Institute for Oceanography and Marine Research have been investigating Rip currents along the coast for more than 10 years and have observed increasing intensity in these currents.
Rip currents are generated at the intersection of two separate wave lengths meeting and moving back to sea during high tide, low tide and astronomical tide. During low tide when waves are at their lowest level, Rip currents are less visible and intense on most beaches. However, during high tides and astronomical high tides, the sea level rises higher than normal, resulting in high and intense wave activities. Rip currents experienced during these periods are intense and fast moving hence any object found along the way of the current are quickly moved offshore (away from the coast).
Although Rip currents are present daily on many beaches, their velocities may be too slow to be a threat to experienced swimmers. However, their inherent variability makes them especially dangerous to unwary or uninformed beachgoers. Changes in the size of the incoming waves can cause pulses in the strength of a Rip current, which can be dangerous to all swimmers and anyone entering the surf.
Rip current strength and speed also varies during the transition of low to high tides or vice versa. Besides, the rapid fluctuations or pulses in wave groups can quickly generate rip currents with extreme velocities that have been measured at up to 15 meters per second - this is faster than an Olympic swimmer can sprint! Hence, if a swimmer is caught in a rip current, attempting to swim directly back to shore against the seaward flowing current can result in exhaustion and possible drowning.
The Victoria Bar beach in Lagos is characterised by the presence of several Rip currents as a result of complex interactions between waves, currents, water levels and near-shore bathymetry. These Rip current systems form an integral part of the near-shore circulation patterns, such as along-shore and cross-shore - onshore/offshore - water motion.
The strength and speeds of the Rip currents are still being investigated by the Nigerian Institute for Oceanography and Marine Research. However, it can be authoritatively stated that the Rip currents along the Bar beach can be blamed for the drowning of several uninformed youths and inexperienced swimmers in Lagos.
One of the adverse effects of climate change will be the increase in intensity in ocean dynamics and sea level rise. Waves and the resulting currents - longshore and Rip - could also be intensified. Hence, it could be expected that the rip currents along the Bar beach will be exacerbated as a result of climate change. Swimmers should therefore be aware of this and hence take precautions
Every year during Christian and Moslem festivals and other national holidays, throngs of youth flood the beaches in Lagos with the intention of swimming and playing games on the beach. These activities have on several occasions resulted in death of uninformed youths
What actually happens is that Rip currents do not pull people under water; they pull people away from shore. Therefore, drowning deaths usually occur when people are unable to keep themselves afloat and swim back to shore. This may be due to fear, panic, exhaustion, a lack of swimming skills, or any combination of these factors.
Swimmers therefore need to be cautious at all times, especially when swimming at unguarded beaches. The simple rule is: "If in doubt, don't go out!" However, if caught in a Rip current, the swimmer should remain calm to conserve energy and think clearly. In other words: "Don't fight the current. Swim out of the current in a direction following the shoreline and when out of the current, swim towards shore. If you are unable to swim out of the rip current, float or calmly tread water and when eventually out of the current, swim towards shore. If you are still unable to reach shore, draw attention to yourself: Face the shore, wave your arms, and shout for help."
Dr. Folorunsho is of the Nigerian Institute for Oceanography and Marine Research.