3.1 CHOOSING A TIDE GAUGE SITE

Before any attempt is made to choose a site for a tide gauge installation the following

information should be considered:-

1) The type of tide gauge to be operated gauges, the size of structure necessary.

2) The area for which the tidal information is required and the use to which this information

is to be put.

Within the limits of the coast dictated by the above requirements, the site will then be chosen.

In some instances the choice of site is quite clear, since the requirement is to monitor tidal

levels at a specific point, such as a sewage outflow point, or a lock gate. In most instances

however, the choice of site will not be so clear and can only be made by judging which of the

following constraints are more significant and which can be more or less ignored.

a) The installation when completed must be capable of withstanding the worst storm

conditions likely to be encountered. Therefore, positions known to be subject to storm

damage because of their exposure should, if possible, be avoided. If this is not possible,

then this situation must be borne in mind when designing the installation. Where large

waves or tsunami are possible, raising the level of the building may be necessary to

prevent swamping or destruction.

b) The ground on which the installation is to be built must be stable, not being liable to

subsidence because of underground workings, or because it is recently made up of

land (example: reclaimed by tipping). It must also not be liable to slippage in the event

of heavy prolonged rain storms (i.e. must be adequately drained) or being eroded by

river or sea action. Building direct on to solid rock is the ideal.

c) The water depth must extend at least two metres beneath the lowest astronomical tide

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

Lowest Astronomical Tide (LAT).

d) River estuaries should, if possible, be avoided. Water flowing down the river mixes with

sea water resulting in varying water density in the area and because of layering water

drawn into the stilling well may be of different density than the surrounding water.

Currents due to the river flow can cause draw-down in the stilling well and following

heavy rainstorms debris floating down the river can become entangled with the stilling

well causing blockage or may even cause impact damage.

e) Areas where impounding (becoming cut-off from the sea) can occur at extreme low

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.

f) Sharp headlands and sounds should be avoided since these are places where high

currents occur.

g) Proximity to outfalls can result in turbulence, currents, dilution and deposits, and should

be avoided.

h) 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.

i) 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. construction of new quays, breakwaters, locks or large factories having sluices or

outfalls.

j) Where an appreciable amount of continuous power is required it will be necessary to

lay in a mains electrical supply to the site. If this is not possible an alternative supply

may be obtained from storage batteries and a generator. If power is required only for

recording or telemetry purposes then batteries may suffice.

k) There must be adequate access to the site in the first instance to get materials on site

during construction, and later for observation and maintenance visits.