Sea level refers to the height of the water as measured along the coast relative to a specific point on land. The sea level is affected by tides. In general, there are two high tides and two low tides each day. Tide variations are mainly caused by the gravitational attraction of the Moon and the Sun. Before and after the New Moon or Full Moon each month, the Earth, Moon and Sun are aligned along a straight line. The sea level will either be unusually high or unusually low. This is called a spring tide. During the First and Last Quarter Moon, the Earth, Moon and Sun are at right angles to each other. The rise and fall of sea level are minimal. This is called a neap tide.
In addition to the gravitational forces of the Moon and the Sun, weather elements such as wind and air pressure also affect the sea level. In general, the sea level will rise by about 1 centimetre when the pressure drops by 1 hectopascal. Winds blowing from the sea will push water towards the coast, raising the water level. During the passage of a typhoon, the sea level will rise significantly due to the very low pressure and high winds near its centre, giving rise to a storm surge (http://www.hko.gov.hk/blog/en/archives/00000074.htm). A storm surge coincident with a spring tide may result in severe flooding.
For more background information on tide, please visit the Hong Kong Observatory website at http://www.hko.gov.hk/education/edu01met/wxphe/ele_tide2_e.htm.
Under the effect of global warming, sea level will rise mainly due to two processes. Firstly, oceans warm and expand. Secondly, melting of glaciers, ice caps and the Greenland and Antarctica ice sheets will raise the sea level.
According to the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), global average sea level rose at 1.7 mm per year during the period 1901-2010. The rate accelerated to 3.2 mm per year for the period 1993-2010. The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia.
In Hong Kong, the sea level in the Victoria Harbour has experienced an unambiguous rise of the mean sea level since 1954. There was a rapid rise of the sea level from 1990 to 1999 and a moderate decline thereafter. On average, the mean sea level in the Victoria Harbour rose at a rate of 29 mm per decade during the period 1954 to 2013.
Sea level measurements are generally conducted by tide gauges at tide stations and the measured sea levels are referenced to stable vertical points (or bench marks) on the land. In Hong Kong, the reference level used for sea level measurement is the Chart Datum. The Chart Datum is 0.146 m below the Hong Kong Principal Datum, which is the reference level used for land surveying in Hong Kong.
Sea levels in Hong Kong are measured by a network of tide stations and real time sea level data are available on the HKO website http://www.weather.gov.hk/tide/marine/hko.htm.
Three commonly used water level measurement principles used by tide gauges are introduced below:
1. Float type
A device consists of a floating object on water surface inside a stilling well. The change in water level is measured by the vertical displacement of the floating object.
2. Pressure transducer
A sensor is immersed in the water to detect the pressure due to the water aloft. The pressure value detected is then converted to water level reading. The measured pressure will be higher for higher water level.
3. Acoustic type
It consists of a device emitting an acoustic signal towards the sea surface and measure the travel time of the reflected signal to deduce the water level. The reflected signal will take a longer travel time for lower water level.