A large part of my Summer internship with Wallace Stone Marine Consulting Civil Engineers was working on coastal, port and harbour designs around the Scottish coast and beyond.
One of the most important aspects of working in this environment is being aware of the changeable nature of the water level with the tide and the effects this can have on the longevity of both steel and concrete, the feasibility of berthing manoeuvres of vessels due to variable under-keel clearances, the required depth of a dredge, and the energies involved in berthing and therefore the fendering requirements.
(fun topical note - this was all pretty important a few days ago when HMS Queen Elizabeth left Rosyth a few days ago, representing another project I have worked on being reliant on tides! - http://www.bbc.co.uk/news/uk-scotland-edinburgh-east-fife-40402153)
The effects of salt water are also generally well documented, with the Concrete Society and Eurocodes acknowledging this with more stringent design requirements for both fully submerged masses of concrete and to an even greater extent for intermittently submerged structural concrete. This is also true for steel structures, with additional corrosion protection required.
All of this adds up to a fairly compelling need to be able to tell for any given time, on any given day, and in any given year, where the water level will be and of course vice versa.
But first some information is required on how to define tidal levels and some other terminology used when working in coastal engineering (at work I keep an alphabetised spreadsheet with nothing but abbreviations and acronyms, its more useful than you think).
- HAT - Highest Astronomical Tide
- MHWS - Mean High Water Spring
- MHW - Mean High Water
- MHWN - Mean High Water Neap
- ML - Mean Level
- MLWN - Mean Low Water Neap
- MLW - Mean Low Water
- MLWS - Mean Low Water Spring
- LAT - Lowest Astronomical Tide
- The highest astronomical tide is the highest which level to which a tide may rise. This will occur during a Spring tide (nothing to do with the season).
- Mean high water springs is the mean high water level that will occur at a site during spring tides.
- Mean high water is the mean water level across all high tides (neap or spring)
- Mean high water neaps is the mean high water level that will occur at a site during neap tides.
- Mean level is the mean water level across all tides at a given site.
- Mean low water neaps is the mean low water level that will occur at a site during neap tides.
- Mean low water is the mean water level across all low tides (neap or spring)
- Mean low water springs is the mean low water level that will occur at a site during spring tides.
- The lowest astronomical tide is the lowest level to which a tide may fall. This will also occur during a spring tide.
Spring and neap tides are related to the cycle of the Earths orbit around the Sun and the Moon's orbit around Earth. When the Sun, Earth and Moon allign, a spring tide occurs. When the Moon is at a right angle to the line between the Sun and Earth a neap tide occurs. This is all due to the magnification of the effects of gravity on our seas and oceans by both the Sun and Moon. Given that both relevant orbits are in fact elliptical, this means that there is also an annual cycle on tidal levels which increases the complexity and introduces the need for mean values. Monthly cycles are complicated enough to be getting on with though!
Generally the values of MHWS and MLWS are used for design and are commonly marked on ordnance survey maps. Both HAT and LAT are important however during both design and construction, as they can impact on the level of a pier or phasing during construction.
It is important to note that both LAT and HAT do not represent the minimum and maximum water levels however. These are the maximum which may be achieved during an annual tidal/lunar. Sea levels can also be influenced by the weather. Storm surges occur when an area of low pressure draws water in to an area, and wind blowing in this direction draws more water. When such storms occur at high tide these can significantly increase the level of water at a coastline. Conversely, clear weather within areas of high pressure suppress tidal levels.
From a legal point of view ML is important as anything beneath it is owned by the Crown, whereas above this level private ownership is permitted.
All of these levels use a Chart Datum (CD) as their 0 level as opposed to Ordnance datum (OD) or any other standard elevation zero datum. Chart datum is generally (though not always) based upon the LAT level for any given port, and as such changes from port to port. This implies that the tide rises and falls to differing degrees from point to point along a coastline. This is true, not only this but the time at which high and low tides occur changes! This all has to do with the funnelling or damping effects of different land masses, a fjord will funnel more water into an area than a broad stretch of beach would, it will also resist the movement of water to a greater degree, meaning high tide will occur later than expected, furthermore the resistance to water means the high water level will prevail for longer, so low tide occurs later, and if the tide begins coming in again before all the water has moved out, the low tide will not be as low as expected!