4. Tide, Mareograph, Sea Level

This section contains terms to describe sea level and the instruments used to measure tsunami.

Cable ocean-bottom instrument
An instrument at the ocean bottom connected to the land by a cable that provides power for the measurement and transmission of data from the seafloor to the coast. Cables can extend for ens of kilometers offshore and across oceans. They enable real-time, multi-sensor seafloor observatories to be deployed for long-term monitoring. Examples of sensors on cabled systems are seismometers to measure earthquakes, sensitive pressure gauges to measure tsunamis, geodetic sensors to measure seafloor deformation, and cameras. Japan operates several cable systems.

Schematic diagram of cabled ocean system for monitoring earthquakes and tsunamis.

Schematic diagram of cabled ocean system for monitoring earthquakes and tsunamis.  Courtesy of JMA.

Cotidal
Indicating equality with the tides or a coincidence with the time of high or low tide.

Deep-ocean assessment and reporting of tsunamis (DART)
An instrument for the early detection, measurement, and real-time reporting of tsunamis in the open ocean.  Developed by the US NOAA Pacific Marine Environmental Laboratory, the DART system consists of a seafloor bottom pressure recording system capable of detecting tsunamis as small as one cm, and a moored surface buoy for real-time communications. An acoustic link is used to transmit data from the seafloor to the surface buoy.  The data are then relayed via a satellite link to ground stations, which demodulate the signals for immediate dissemination to the NOAA tsunami warnings centres. The DART data, along with state-of-the-art numerical modelling technology, are part of a tsunami forecasting system package that will provide site-specific predictions of tsunami impact on the coast.

Dart II System.

Low Water
The lowest water level reached during a tide cycle. The accepted popular term is low tide.

Mareogram or Marigram
1) Record made by a mareograph.

2) Any graphic representation of the rise and fall of the sea level, with time as abscissa and height as ordinate, usually used to measure tides, may also show tsunamis.

17 February 1996 Irian Jaya Tsunami

17 February 1996 Irian Jaya Tsunami.

Mareograms of tsunami signals measured by an underwater gauge located 50 km outside the entrance to Tokyo Bay in about 50 m of water (upper trace), and another gauge located at the shore (lower trace). The tsunami is detected on the outside gauge about 40 minutes before it reaches shore (arrows). The offshore gauge was developed by Japan's Port and Harbours Research Institute.

Mareograph
A recording sea level gauge.  Also known as a marigraph or tide gauge.

Mean sea level
The average height of the sea surface, based upon hourly observation of tide height on the open coast or in adjacent waters which have free access to the sea. These observations are to have been made over a “considerable” period of time. In the United States, mean sea level is defined as the average height of the surface of the sea for all stages of the tide over a 19-year period. Selected values of mean sea level serve as the sea level datum for all elevation surveys in the United States. Along with mean high water, mean low water, and mean lower low water, mean sea level is a type of tidal datum.

Probable maximum water level
A hypothetical water level (exclusive of wave runup from normal wind-generated waves) that might result from the most severe combination of hydrometeorological, geoseismic and other geophysical factors that is considered reasonably possible in the region involved, with each of these factors considered as affecting the locality in a maximum manner. This level represents the physical response of a body of water to maximum applied phenomena such as hurricanes, moving squall lines, other cyclonic meteorological events, tsunamis, and astronomical tide combined with maximum probable ambient hydrological conditions such as wave level with virtually no risk of being exceeded.

Reference sea level
The observed elevation differences between geodetic benchmarks are processed through least-squares adjustments to determine orthometric heights referred to a common vertical reference surface, which is the reference sea level. In this way, height values of all benchmarks in the vertical control portion of a surveying agency are made consistent and can be compared directly to determine differences of elevation between benchmarks in a geodetic reference system that may not be directly connected by lines of geodetic leveling. The vertical reference surface in use in the United States, as in most parts of the world, approximates the geoid. The geoid was assumed to be coincident with local mean sea level at 26 tidal stations to obtain the Sea Level Datum of 1929 (SLD 290). National Geodetic Vertical Datum of 1929 (NGVD 29) became a name change only; the same vertical reference system has been in use in the United States since 1929. This important vertical geodetic control system is made possible by a universally accepted, reference sea level.

Refraction diagrams
Models using water depths, direction of wave, separation angle, and ray separation between two adjacent rays as input, produce the path of wave orthogonals, refraction coefficients, wave heights, and travel times.

Sea level
The height of the sea at a given time measured relative to some datum, such as mean sea level.

Sea level station 
A system consisting of a device such as a tide gauge for measuring the height of sea level, a data collection platform (DCP) for acquiring, digitizing, and archiving the sea level information digitally, and often a transmission system for delivering the data from the field station to a central data collection centre.  The specific requirements of data sampling and data transmission are dependent on the application.  The GLOSS programme maintains a core network of sea level stations.  For local tsunami monitoring, one-second sampled data streams available in real time are required.  For distant tsunamis, warning centres may be able to provide adequate warnings using data acquired in near-real time (one-minute sampled data transmitted every 15 minutes).  Sea level stations are also used for sea level rise and climate change studies, where an important requirement is for the very accurate location of the station as acquired through surveying techniques.

Rarotonga sea level station, Avarua Harbor, Cook Islands.

Rarotonga sea level station, Avarua Harbor, Cook Islands.  The fiberglass electronics package (a), antenna (b), solar panel (c) were installed on a pier. Conduit (d) containing cables connecting the sensor, located at a depth of five feet below low-tide water level, to the data collection platform containing the electronics above, was externally attached to the tube containing the sensor (e).

Tidal wave
1) The wave motion of the tides.

2) Often incorrectly used to describe a tsunami, storm surge, or other unusually high and therefore destructive water levels along a shore that are unrelated to the tides.

Tide
The rhythmic, alternate rise and fall of the surface (or water level) of the ocean, and of bodies of water connected with the ocean such as estuaries and gulfs, occurring twice a day over most of the Earth and resulting from the gravitational attraction of the moon (and, in lesser degrees, of the sun) acting unequally on different parts of the rotating Earth.

Tide amplitude
One-half of the difference in height between consecutive high water and low water; hence, half of the tidal range..

Tide gauge
A device for measuring the height (rise and fall) of the tide. Especially an instrument for automatically making a continuous graphic record of tide height versus time.

Tide station
A place where tide observations are obtained.

Tsunameter
An instrument for the early detection, measurement, and real-time reporting of tsunamis in the open ocean.  Also known as a tsunamimeter.  The DART system and cable deep-ocean pressure sensor are tsunameters.

Port Louis, Mauritius radar.

GLOSS sea level stations employ a number of instruments to measure sea level, including down-looking radars to measure sea level.  Port Louis, Mauritius.  Photo courtesy of University of Hawaii Sea Level Center.

IOClogonoaabox

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