21
and monthly lunar tides to the solar semiannual component. Since the relative
orbital parameters are known with great precision, astronomical tide forecasts are
highly reliable.
Nevertheless, poorly predictable atmospheric and geophysical phenomena complicate the forecast. The well-known acqua alta phenomenon of Venice Lagoon,
discussed elsewhere in this book, is an example of atmospheric forcing which is
poorly constrained for numerical forecasting since these atmospheric phenomena
are predictable only in the very short term and only with a large body of in situ
observations. A well-instrumented observing system established off the coast of
Venice, the CNR-ISMAR (Italy’s National Research Council Institute of Marine
Science) oceanographic Acqua Alta sentinel platform, has allowed some measure of
prediction of these harmful events and is undergoing refurbishing at this writing.
Internal waves approaching from deep water break upon encountering a shelf
edge and induce a recurrent sloshing of the entire water mass across the shelf, a
phenomenon known as seiching. Seiching periods are typically on the order of
hours depending on the dimensions of the basin which establish the resonance frequency. Seiching is readily apparent in tide gage records as higher frequency sinusoidal oscillations superimposed on the diurnal or semidiurnal astronomical tidal
heights. Conversely, internal waves are forced by hydraulic tidal jumps across
abrupt ocean bottom sills. Tsunamis, caused by submarine earthquakes, fault displacements, or large-scale seafloor slumping, were once called tidal waves but are
now recognized as very large amplitude waves unrelated to periodic tides.
As mentioned previously, tide measurement and tidal prediction constitute the
precursors of today’s ocean observing systems. Tide stations are carefully established with reference to the elevation of an adjoining benchmark as a part of a geodetic reference network. Below we discuss the nature and operation of the most
commonly used instrumentation.
Staffs and Floaters
Tide staffs are simple ruler-like gauges mounted vertically across the air-sea interface for a direct but crude visual readout of tidal elevation. Tide staffs were normally
confined to quiet inshore waters since any wave action would hamper the visual
interpretation of tidal height. Tethered floats within a stilling well (see Fig. 3.1)
attached to a mechanical readout were for many years the most common tide gauges
(Sverdrup et al. 1942).
Bubbler Gauges
In the USA, NOOA is responsible for the installation, maintenance, and operation
of a National Water Level Observation Network (NWLON). Until the early 1990s,
pneumatic bubbler gauges were the instrument of choice relying on establishing a
stream of bubbles from a compressed gas source emanating from an underwater
2.2 Electronic Sensors and Instruments for Ocean Observing
and monthly lunar tides to the solar semiannual component. Since the relative
orbital parameters are known with great precision, astronomical tide forecasts are
highly reliable.
Nevertheless, poorly predictable atmospheric and geophysical phenomena complicate the forecast. The well-known acqua alta phenomenon of Venice Lagoon,
discussed elsewhere in this book, is an example of atmospheric forcing which is
poorly constrained for numerical forecasting since these atmospheric phenomena
are predictable only in the very short term and only with a large body of in situ
observations. A well-instrumented observing system established off the coast of
Venice, the CNR-ISMAR (Italy’s National Research Council Institute of Marine
Science) oceanographic Acqua Alta sentinel platform, has allowed some measure of
prediction of these harmful events and is undergoing refurbishing at this writing.
Internal waves approaching from deep water break upon encountering a shelf
edge and induce a recurrent sloshing of the entire water mass across the shelf, a
phenomenon known as seiching. Seiching periods are typically on the order of
hours depending on the dimensions of the basin which establish the resonance frequency. Seiching is readily apparent in tide gage records as higher frequency sinusoidal oscillations superimposed on the diurnal or semidiurnal astronomical tidal
heights. Conversely, internal waves are forced by hydraulic tidal jumps across
abrupt ocean bottom sills. Tsunamis, caused by submarine earthquakes, fault displacements, or large-scale seafloor slumping, were once called tidal waves but are
now recognized as very large amplitude waves unrelated to periodic tides.
As mentioned previously, tide measurement and tidal prediction constitute the
precursors of today’s ocean observing systems. Tide stations are carefully established with reference to the elevation of an adjoining benchmark as a part of a geodetic reference network. Below we discuss the nature and operation of the most
commonly used instrumentation.
Staffs and Floaters
Tide staffs are simple ruler-like gauges mounted vertically across the air-sea interface for a direct but crude visual readout of tidal elevation. Tide staffs were normally
confined to quiet inshore waters since any wave action would hamper the visual
interpretation of tidal height. Tethered floats within a stilling well (see Fig. 3.1)
attached to a mechanical readout were for many years the most common tide gauges
(Sverdrup et al. 1942).
Bubbler Gauges
In the USA, NOOA is responsible for the installation, maintenance, and operation
of a National Water Level Observation Network (NWLON). Until the early 1990s,
pneumatic bubbler gauges were the instrument of choice relying on establishing a
stream of bubbles from a compressed gas source emanating from an underwater
2.2 Electronic Sensors and Instruments for Ocean Observing