Glossary
| Astronomical tide | Or more simply, the tide, is the periodic rise and fall of water along the coast because of gravitational attraction on the water by the moon and sun (see figure 1). When the moon, sun and earth are in line their combined attraction is strongest and the tide range is greater (spring tides). When the moon and sun are at right angles to each other (in relation to the earth) the effect of the attraction is somewhat reduced and the tide range is smaller (neap tides). Figure 1. Tidal pattern |
| Australian Eastern Standard Time (AEST) | AEST is the standard time for Queensland, Australia, as defined by the Standard Time Act 1894. AEST is based on the meridian 150 degrees East and is 10 hours earlier than Universal Time which is expressed with respect to the 0 degree meridian at Greenwich, England. |
| Australian Height Datum (AHD) | Australian Height Datum (AHD) is the datum (adopted by the National Mapping Council of Australia) to which all vertical control for mapping is to be referred. |
| Chart datum | The datum or plane of reference to which all depths and drying heights on a chart are related (this does not include other heights and elevations). It is a level so low that the tide will not frequently fall below it. Usually defined in terms of low-water level such as LAT. Chart datum is not a horizontal surface, but may be considered as such over a limited local area. Chart datum should preferably be the same as the datum of tidal predictions for an area. |
| Highest Astronomical Tide (HAT) | The highest level of water which can be predicted to occur under any combination of astronomical conditions. This level may not be reached every year. |
| Lowest Astronomical Tide (LAT) | The lowest tide level, which can be predicted to occur under average meteorological conditions and any combination of astronomical conditions. This level may not be reached every year. |
| Mean sea level (MSL) | The value obtained at a site by averaging hourly tide heights observed over a long period of time (preferably 18.6 years). It is also the average water level that would exist in the absence of tides. |
| Predicted tide | Tide expected to occur under average meteorological conditions. Tide predictions are typically based on previous actual tide readings gathered over a long period (usually one year or more). The sun, moon and earth are not in the same relative position from year to year. Accordingly, the gravitational forces that generate the tides, and the tides themselves, are not the same each year. The official tidal predictions for Queensland are supplied by the National Tidal Facility, Flinders University of South Australia. Tidal data used in the preparation of the official tidal predictions is supplied by the department, the Department of Transport and Queensland port authorities. |
| Residual | The residual is the difference between the predicted and the actual tide height. The tidal predictions are prepared for average weather conditions. Actual tides include the tide itself, and the height variations (residuals) that result from the effects of the weather on the height of the sea level. The residuals serve to measure storm surge during severe (cyclonic) weather conditions. The components of the residuals are (see figure 2):
Depending on the nature of the event, residuals can be either positive or negative. Positive residuals are usually associated with tropical cyclones, with persistent strong winds piling up the water level at the recording site, or low pressure systems (associated with seasonal variation). This can result in flooding of low-lying areas. Negative residuals can also be associated with tropical cyclones, with persistent strong winds pushing the water level away from the recording site, or high pressure systems (associated with seasonal variation). The effect of negative residuals can be to reduce the amount of water available in shipping channels, thus causing a hazard to navigation. |
| Seiche | A resonant standing wave in an enclosed or semi-enclosed water body that continues to oscillate after the cessation of the originating force (that may have been seismic, atmospheric, tidal or wave-induced). This will appear to only move up and down. |
| Storm surge | A storm surge is an increase (or decrease) in water level associated with some significant meteorological event, e.g. persistent strong winds and change in atmospheric pressure, or tropical cyclone. Its typical effect is to raise the level of the tide above the predicted level. In some situations, e.g. when winds blow offshore, the actual tide level can be lower than that predicted. The magnitude of the storm surge is dependent on the severity and duration of the event and the seabed topography at the site. In Queensland, most large surges are caused by tropical cyclones. |
| Storm tide | A storm tide is the combination of a storm surge, the normal astronomical tide, and wave setup. If the storm surge arrives at the same time as the high tide, the potential risk of flooding will be even greater (see figure 2). An additional threat at this time could come from the presence of very high waves. |
| Storm tide gauge | A specially designed structure built to withstand severe weather and storm surge, housing instruments that record the rise and fall of the tide automatically at regular time intervals in digital format (see figure 3). |
| Tidal wave | In popular usage, unusually high and hence destructive water level along the shore. Refers to a storm surge, and more commonly (but erroneously) to a tsunami. |
| Tropical cyclone | A non-frontal rotating (clockwise in the Southern hemisphere) low pressure system, below 1000hPa and of tropical origin, in which 10 minute mean wind speeds exceed gale force (63km/hr, 34kt or 17.5m/s). |
| Tsunami | A wave generated by seismic activity. Also called a seismic sea wave, or erroneously a tidal wave. Barely discernible in the open ocean, the amplitude of a tsunami may increase greatly as it approaches shallow coastal waters. They typically have periods greater than 10 minutes. Storm tide recorders along the Queensland coastline have detected the effects of events in Central and South America, and from the 2004 Asian Tsunami. More information on tsunamis can be found at http://www.bom.gov.au/tsunami. |
| Wave runup | The rush of water up a beach after a wave reaches the shoreline. The amount of wave runup is the vertical distance between the maximum height on the beach the rush of water reaches and the still water level. Wave runup is dependent on a number of factors including wave height and period, and the slope and composition of the beach. |
| Wave setup | An increase in the mean water level towards the shoreline caused by wave action. It can be very important during storm events as it results in a further increase in water level above the tide and surge levels. |
| Wind setup | The vertical rise of a body of water above still water level caused by wind stresses on the surface of the water. |
Last updated: 21 August 2007