Danger of swimming in cold water

BEWARE - THE WATER IS NOT COLD
A common prominent feature in publications advising on water safety are warnings of the dangers posed by immersion in inland waters which, according to some: - remain extremely cold even on a hot day - never warm up, even in summer - are surprisingly cold only a few inches below the surface - especially cold in deep lakes - may feel comfortable in the shallows but becomes extremely cold in the centre of lakes and more.
Whereas some inland water temperatures, rivers in particular, can fall below 15C during the summer,the majority of lowland inland waters in the UK are at between 15C (‘a bit chilly’) and 20C+ (‘lovely and warm’) - sometimes from as early as May. Moreover, since water has high heat storage capacity - once heated by the sun, lakes and slow flowing rivers can stay surprisingly warm days after the weather has turned cloudy. Having said that, not only is there an undeniable problem with feeling cold in the water, it is one which swimmers must be alerted to and take note of. Although water temperature is a major contributor, the problem is not the temperature per se but the very large increase in body heat loss in water which is around 30 times that in air. Defining the problem accurately is important because: -the credibility of safety advice is jeopardised by containing flawed information and sweeping statements. You don’t even have to wade in to discover the truth about open water temperatures - the information is widely available online. -elevated body heat loss in water does not stop at temperatures above 20C which are considered comfortable. Swimming difficulties and, in extreme cases, even death due to hypothermia, can occur at these temperatures. The danger here is that the higher the temperature, the longer it takes for difficulties to become apparent. Inexperienced swimmers may feel comfortable in the water, assume that all is well and venture out well away from the shore. This, coupled with the consumption of alcohol is an all too common cause of drowning while swimming. Lake temperatures can be calculated using a computer model A number of German organisations, including the German Weather Service, have produced FLAKE - a mathematical model capable of predicting a range of temperature-related parameters in lakes world-wide. Some of the model features are available online . FLAKE is based on statistical climatic data for the location chosen. However, despite only being capable of producing ‘average’ values, we found results for waters we know, remarkably realistic. The model was used here to predict top and bottom water temperature variations with time for two fictitious lakes in the Midlands. One of these is shallow (5m) and the other deep (50m). The effect of water turbidity (quantified by depth at which a bright object is visible) have also been briefly investigated. MLD - the Mixing Layer Depth (explained below) during August has also been noted for each run. The results are shown in Figures 1-4. The FLAKE predictions suggest that: UK lakes reach reasonable swimming temperatures during the summer. Water turbidity and depth affect the rate at which warming takes place, level of transient variations and peak temperatures. shallow, clear lakes are likely to develop a uniform vertical temperature profile (Fig 2). (small or negligible variations in temperature with depth). deep, clear lakes are likely to have a deep layer of uniform vertical temperature profile but that temperature will be lower than in shallow lakes. (Fig 4) high turbidity (poor visibility in the water) is the main factor affecting the formation of a stratified mixing layer. Layering takes place because water density varies with temperature. The mixing layer is where mixing, primarily due to wind-induced turbulence, takes place. Water temperature within this layer is fairly uniform. It commonly occupies the top 1-2m and is only moderately affected by depth. Water temperature below the mixing layer varies with depth Is the water at the centre of a deep lake extremely cold ? The temperature differences (at swimming level) between the shallows and the middle of any lake are marginal. Colder (and therefore higher density) water can not be sustained in the same plane as static, warm water. What is often felt by swimmers, especially on windy days, are streaks of colder water brought up to the surface by eddies. Although these are typically only a couple of degrees colder than surface water, the body, being sensitive to change in heat loss, soon reacts. What about the sea – the biggest lake of all One of the most intriguing features of many UK water safety publications is the distinction between the dangers posed by inland waters and the (safer?) sea which hardly gets a mention. As far as water temperature goes, the sea is just a huge lake. It takes longer to warm up and cool down. As for body heat losses – there is no significant difference between inland lakes and the sea. Effective safety advice To be most effective, safety advice is best used to alert the public to real, unforeseen dangers. There is little value in stating the obvious (‘The water is deep and cold’) – it only dilutes the information and invites ridicule. Most people act sensibly – they go into open water slowly, making sure they feel comfortable in the water before venturing further in. Staying within one’s depth or swimming parallel to the shore comes naturally. After all, people have been swimming in open water long before the invention of the Internet and have developed a natural fear for the unknown. On the other hand, telling novice swimmers that prolonged exposure, even to comfortably warm water, can become a problem could save lives. Most of those who jump into water from a height know what they are doing. They have been in the water before jumping and know what to expect. It is mainly the young who sometimes disregard their natural instincts and behave recklessly. This is usually due to peer pressure and / or the consumption of alcohol. Here again, the danger is of sudden, unexpected immersion followed by involuntary inhalation of water, which can happen even at moderate water temperatures. If there is any doubt then a simple word of advice can go a long way – ‘cover your nose and mouth while jumping’. Many do it naturally. Other common warnings of the mortal danger of jumping into cold water are brought into question by the dozens of participants, most of whom are not acclimatised , who jump into water at 6C and below in Liverpool every Boxing Day (right) – one of many such events throughout Europe and N. America. One can only assume that water temperature is not the governing factor. In summary To be credible, any safety advice must be based on well-researched facts. The claim that bathing in inland waters is inherently dangerous because the waters are cold is, as we have shown here, misleading - it overlooks the real danger of bathing in comfortably warm waters.
Fig 1 - Depth 5m Clarity 0.5m MLD 1m -------------------------------Fig 2 - Depth 5m Clarity 5m MLD 5m Fig 3- Depth 50m Clarity 0.5m MLD 2m -------------------------------Fig 4 - Depth 50m Clarity 5m MLD 8m