Philip L. Woodworth, 4 August 2016.
Everyone knows that the level of the sea goes up and down. Most of these changes in level are due to the ocean tide (at Liverpool the level changes due to the tide by more than 8 metres at ‘spring tides’), but changes of several metres can also occur due to ‘storm surges’ that occur during bad weather, while slow changes in level can take place due to climate change and because of the geology of the adjacent land.
Changes in sea level are measured by devices called ‘tide gauges’: the more suitable name of ‘sea level recorders’ has never been widely adopted in the UK although Americans often call them ‘water level recorders’. There are as many types of tide gauge such as:
Vertical scales fixed to a jetty or dock entrance.
These were simple ‘rulers’ (sometimes called ‘tide poles’ or ‘tide boards’), by means of which the sea level could be measured by eye. An example is shown in Figure 1.
Float and stilling well gauges.
This way of measuring sea level was first proposed by Sir Robert Moray in the mid-17th century. However, over a century went by before the first practical systems were introduced at locations in the Thames during the 1830s. They quickly become the standard way of measuring sea level and by the end of the 19th century they had spread to major ports around the world.
A stilling well is a vertical tube with a hole at its base through which sea water can flow. The level inside will be, in principle, the same as that of the open sea outside, but energetic wave motion will be damped (or ‘stilled‘) inside due to the hole acting as a ‘mechanical filter’. In the well is a float which rises and falls with the water level, and is attached via a wire over pulleys to a chart recorder driven by an accurate clock. The rise and fall of the water level is thereby recorded as a line traced by a pen on paper charts that are regularly replaced, the charts finding their way to a laboratory such as that at Bidston Observatory, where an operator ‘digitises’ the pen trace and so provides the measurements of sea level.
Figure 2(a) demonstrates how the level of the float is recorded on the paper chart, while Figure 2(b) is a photograph of the tide gauge station at Holyhead where there are two exceptionally large stilling wells.
This type of gauge is of historical importance as they were used for almost two centuries (although with modern improvements such as replacing the paper charts with modern electronic data loggers) and so data from them make up the data sets of sea level change that are nowadays archived at the Permanent Service for Mean Sea Level (PSMSL) in Liverpool and used for studies into long-term climate change. During the 19th century, most of these gauges were operated in the UK by the major ports, and even by the railway companies which operated ferries. Bidston Observatory operated one at Alfred Dock in Birkenhead for many years. A number of countries still operate float and stilling well gauges although most in the UK have been replaced with other types.
These gauges measure sea level by recording water pressure with the use of a pressure sensor that is installed well below the lowest likely level of the water. The recorded pressure will be the sum of two forces pressing on the sensor: the pressure due to the water above it (which will be the sea level times the water density and acceleration due to gravity) and the pressure of the atmosphere pressing down on the sea surface. In practice, the latter can be removed from the pressure measurement using what is called a ‘differential’ sensor, thereby, after some calculation, providing a measurement of the sea level.
We mention two types of pressure sensor below, which were both developed at Bidston. One type (the bubbler pressure gauge) has been used at 45 locations around the UK for several decades and remains the main technology for sea level measurements in this country. Until recently (mid-2016), this large network was operated for the Environment Agency by a group at Bidston called the Tide Gauge Inspectorate, and then, following relocation, at the National Oceanography Centre in Liverpool.
Ranging tide gauges.
These devices consist of a transducer that is installed over the sea so that it can transmit a pulse down to the water, where the pulse is reflected back and recorded by the transducer, so measuring the time taken to travel down and back. If one knows what the speed of the pulse is, then one can readily compute the height of the transducer above the sea, and so measure sea level. The transmitted pulse can be either an acoustic one (sound), or electromagnetic (radar) or optical (light). During the last decades of the 20th century, acoustic systems became very popular and replaced float gauges, and even replaced pressure gauges in some countries. However, they have since been largely replaced in their turn by radar gauges for several reasons. One simple reason is relative cost. However, radar gauges are potentially more accurate than acoustic systems owing to the speed of a radar pulse, unlike sound, being independent of air temperature. Optical ranging gauges use lasers to transit the pulses but, to my knowledge, are used in only two countries (Canada and South Korea).
Bidston Observatory had expertise in all of these types of tide gauge, but three can be mentioned in which Bidston scientists took a special lead.
Bubbler pressure gauges.
In the late 1970s, the Institute of Oceanographic Sciences (IOS, as Bidston Observatory was then known) was encouraged by the government to see if the new types of tide gauge then becoming available would be suitable for replacing the float and stilling well gauges then standard in the UK. This led to a programme of research by David Pugh and others into the use of different types of pressure gauge, including the bubbler gauge, and the curiously-named ‘non-bubbling bubbler gauge’ which we shall not explain.
Bubbler gauges were not invented at Bidston but they were developed there into practical instruments. They offered advantages over other pressure sensor systems in which the sensors themselves are installed in the water. In a bubbler system, the only equipment in the water is a tube through which gas flows at a rate sufficient to keep the tube free of water, such that the pressure in the tube is the same as that of the water head above the ‘pressure point’ at the end of the tube (Figure 3). The pressure sensor itself is located safely at the ‘dry land’ end of the tube, so there are no expensive electronic components that could be damaged in the water. If the tube is damaged it is simple and cheap to replace. The only drawback is that a diver is needed to install the tube, although the same need for a diver applies to all other pressure systems.
Comparisons of the old (float and stilling well) and new (bubbler) gauges were made at various locations, including at the important tide gauge station at Newlyn in Cornwall. In addition, the way that they measure sea level was thoroughly understood from both theoretical and experimental perspectives. The conclusion of the research was that bubbler pressure gauges could be reliably installed across the network. Bubblers are now standard in the UK and Ireland although they have since been replaced in countries such as the USA by other systems.
‘B’ gauges (where B stands for Bidston).
These gauges were developed in the 1990s by Bob Spencer, Peter Foden, Dave Smith, Ian Vassie and Phil Woodworth for the measurement of sea level at locations in the South Atlantic. They are rather complicated to explain in this short note, but the gist of the technique is that it uses three pressure sensors to measure sea water pressure (as in a bubbler gauge) and also maintain the datum (measurement stability) of the data in the record. ‘B gauges’ are probably the most accurate and stable types of tide gauge ever invented, but they are expensive (because of the requirement for three sensors) and were never developed commercially. Nevertheless, the principle of the ‘B technique’ was eventually incorporated into the way the bubblers were operated in the UK network, which remains the situation today.
Radar tide gauges.
Bidston Observatory cannot claim to have invented radar tide gauges; these radar transducers were developed first for the measurement of liquids and solids in giant industrial tanks, and were then applied to the measurement of river levels. However, Bidston can claim to have been one of the first laboratories to have used radar gauges for measuring sea level, a one year comparison of radar and bubbler data from Liverpool having shown that radar was a suitable technique for a tide gauge (Figure 4). Radar gauges have since fallen in price, are even more accurate than they were, can be readily interfaced to any kind of computer, and consume less power (an important feature in remote locations where gauges have to be powered from solar panels). They have become the standard technique for measuring sea level around the world and look like remaining so in the future.
Some References for More Information
- Bradshaw, E., Woodworth, P.L., Hibbert, A., Bradley, L.J., Pugh, D.T., Fane, C. and Bingley, R.M. 2016. A century of sea level measurements at Newlyn, SW England. Marine Geodesy, 39(2), 115-140, doi:10.1080/01490419.2015.1121175.
- IOC. 2015. Manual on Sea Level Measurement and Interpretation. Manuals and Guides 14. Intergovernmental Oceanographic Commission. Volumes I-V may be obtained from http://www.psmsl.org/train_and_info/training/manuals/.
- Pugh, D.T. and Woodworth, P.L. 2014. Sea-level science: Understanding tides, surges, tsunamis and mean sea-level changes. Cambridge: Cambridge University Press. ISBN 9781107028197. 408pp.
- Woodworth, P.L., Vassie, J.M., Spencer, R. and Smith, D.E. 1996. Precise datum control for pressure tide gauges. Marine Geodesy, 19(1), 1-20.