2.5 General Principles of Choice of a Tide Gauge Site

Before a gauge can be deployed, it is clearly important to have done one’s homework on what it is really intended for and where it will be best located. In some practical instances the choice of site will be obvious. For example, if the requirement is to monitor tidal levels at a specific point, such as a dock entrance, the gauge will have to be located nearby.

In many instances, however, the choice of site will not be so clear and can only be made by judging which of the constraints listed below are more significant and which can be given greater emphasis. Those emphases may depend, for example, on whether the gauge is intended for oceanographic research, in which case one clearly requires it to be located with maximum exposure to open ocean levels, and not situated in a river. Most GLOSS Core Network sites have been selected with this aspect in mind as far as possible. For programmes such as C-GOOS, where the process to be studied may be coastal erosion or storm surge activity, then clearly the gauge will have to be situated optimally for that purpose.

Further general considerations include:

a) The installation must be capable of withstanding the worst environmental conditions (winter ice, storms etc.) likely to be encountered. This is clearly an issue relevant to the type of gauge purchased (see sections above) and to its intended position. Positions known to be exposed to environmental extremes should clearly be avoided so as to enable the eventual construction of long time series.

b) The ground on which the installation is made be ‘stable’ as far as possible, not being liable to subsidence because of underground workings or land subsidence (e.g. due to the area being reclaimed land). It must also not be liable to slippage in the event of heavy prolonged rain (i.e. the area must be adequately drained) or being eroded by river or sea action. An installation on solid rock is the ideal.

c) River estuaries should, if possible, be avoided. Estuarine river water will mix with sea water to a different extent during the tidal cycle and during different times of the year, resulting in fluctuations in water density. This may have important impacts on float gauge measurements in stilling wells because of ‘layering’ of water drawn into the well at different times resulting in different densities inside and outside the well. It will also impact on pressure gauge measurements as the density assumed for the conversion of pressure to sea level will not be constant. Currents due to the river flow may also cause draw-down in stilling wells (including the outer container of acoustic gauge sounding tubes), and following heavy rainstorms debris floating down-river could damage a gauge. (For a discussion of the problems in connection with stilling wells, see Lennon, 1993).

d) Areas where impounding (becoming cut-off from the sea) can occur at extreme low tide levels should be avoided. Similarly, sandbars slightly below the surface between the site and the open sea can result in uncharacteristic levels being measured. Monitoring across long shallow sloping beaches should also be avoided for the same reasons.

e) Sharp headlands and sounds should be avoided since these are places where high tidal currents occur which tend to result in unrepresentative tidal constants and in a drop of MSL (Pugh, 1987) .

f) Proximity to outfalls can result in turbulence, currents, dilution and deposits, and should be avoided.

g) A study should be made of shipping passing or mooring close to the proposed site, since there will be a risk of collision and propeller turbulence causing silt movement.

h) Investigations should be made to determine if there is a possibility of construction work occurring in the area at some future time which may affect the tidal regime at the site (e.g. by construction of new quays or breakwaters) and/or which may cause the tide gauge to be moved to a new location, interrupting the sea level time series.

i) A gauge site should have continuous mains electrical power (or adequate storage batteries or generator) and telephone or satellite access for transmission of data to an analysis centre.

j) There must be adequate access to the site for installation and maintenance and the site must be secure from vandalism or theft.

k) The area of the site must be capable of containing the bench marks required for geodetic control of the sea level data (Section 4). In particular, it must have good TGBM and GPSBM marks which will also be secure from accidental damage.

Further specific considerations include:

l) If stilling well or acoustic gauges are to be installed, then the stilling well or acoustic tube must be tall enough to record the highest sea levels. This may require permission from port authorities if, for example, the installation is on a busy quayside.

m) The water depth must extend at least two metres beneath Lowest Astronomical Tide (LAT) for successful operation of a stilling well. The outlet of the stilling well should be clear of the sea bed and be set deep enough to allow the float to operate about one metre below LAT.

Finally, it is clear that tide gauge datum control is an essential issue for any installation. Consequently, even if the station is equipped with the most modern equipment, it is common sense to provide confirmation of the datum from time to time by means of an inexpensive tide ‘pole’ or ‘staff’. Measurements by tide staff are not especially accurate, and their datum control readings are not to be preferred to those by more accurate methods, such as those described in Section 2, but they at the very least guard against gross errors in datum. In addition, in some methods a staff may not be just desirable, but essential; for example, see Section 2.2.1.1.