There would appear to be quite a lot of confusion about exactly how tides work, so I thought I would try and put together as brief a guide as I could to cover this topic.
Fundamentally, the tides are governed by the status of the moon. Spring tides coincide with ‘full’ moons and ‘new’ moons (nothing to do with ‘the Spring’ as such) and neap tides with a half moon that is either ‘waxing’ (getting larger) or waning (getting smaller). Spring tides are always high tides whereas neap tides, even when the tide is in are much lower. During spring tides, the low tide (in other words when the tide is out) is also much lower than normal. During neap tides, the low tide is actually relatively high!
Also, clearly not every location has its high tide or its low tide at the same time – these can vary quite significantly even over quite a small distance, something that is exacerbated further in estuarine areas such as the Thames and the Severn owing to the time taken for the ‘flood’ tide to reach as far as it can travel up a given river which, in the case of the Thames, is Teddington Lock.
This is where things become a little more complicated, always assuming this isn’t double-Dutch already! On ship’s navigation charts as well as on some maps, there are often periodic abbreviations around the coastline that relate to these two main states of tide – Spring and Neap. There are four of these – in order from shore to sea – HWS, HWN, LWN and LWS. The abbreviations stand for High Water Springs, High Water Neaps, Low Water Neaps and Low Water Springs. They relate to the point on the shoreline reached by the highest astronomical (predicted) Spring and Neap Tides of the year. These are usually in the Autumn and Spring, so it is worth noting that even the use of the word spring to denote a particularly high tide does not necessarily mean that the highest of these actually occur in the Spring!
Further adding to the complexity of the situation is that every high tide prediction (and there are two high tides and two low tides in every 24-hour time period) can be affected by the state of the weather. Storms, which are typically worse in the winter months, can result in any high tide ending up being higher than the astronomically predicted level. The effects of storms can be further enhanced by the geography of a particular country, or adjacent countries, such as those around the North Sea, creating a natural funnel for the increased water height generated by that storm, and/or as indicated earlier, further again by the funnel effect of river estuaries – which even under normal tidal conditions develop what are known as tidal bores. The wider effect which may spread over a whole length of coastline is referred to as a tidal or storm surge.
The effect of a storm on an estuary such as the Thames can be made even worse if the Thames water catchment area from the Cotswold Hills down through Oxfordshire, Buckinghamshire and Berkshire in to London has been subjected especially to earlier heavy rain which may have only just reached the river. This is one of the reasons why the Thames Barrier was built at Woolwich Reach, largely to protect London’s infrastructure, and especially the Underground network.
The combination of all these factors is that when there are instances of bad weather and the subsequent generation of a storm surge, this will affect the astronomically predicted height of any tide, whether it’s a Spring Tide or a Neap Tide, or simply somewhere in between. It is usually only in those relatively rare circumstances where a storm surge happens to coincide with an HWS event that there is a threat to life and limb and the chance of major property damage is likely to be high.
In the Winter of 2017-2018 to date (October- January) in S.E. England there have been four instances of severe weather that have had a significant effect on the actual height, as opposed to the predicted astronomical height of the tide – and each of those happened either slightly before, or slightly after, the highest of that month’s Spring Tide, thus fortunately reducing the impact. Nevertheless, the most recent one in the first week of January resulted in the Thames Barrier being closed on at least three occasions, as well as some significant destruction of sand dunes affecting beach access in parts of North Devon and Cornwall. Newquay, Portreath, Perranporth and St. Ives were all badly affected.
A further aspect to ‘the tides’ I have realised I omitted to mention above is the considerable variation in tidal range both around the coast of the UK and of the whole world. The range is the difference in height between Mean Low Water (MLW) and Mean High Water (MHW). The UK is the location of one of the greatest ranges in the world, the Severn Estuary, where it is up to 49ft (15m). In contrast, the range in Margate for example is just over 4m, with the lowest in the UK being just 0.5m. The Bay of Fundy in Nova Scotia, Canada has the highest range in the world, where it is 53.5ft (16.3m) with one of the lowest being the Mediterranean basin at almost zero!