How do you measure the level of the sea?

Answer

Dear Pierre,

How, where and when one measures the relative sea level and what problems one is confronted with in the interpretation of these measurements, is indeed little discussed.

It is only useful to talk about the sea level when it says the zero level it is measured against. You can take measurements relative to the current land level, relative to a fixed device on the sea wall or relative to the level of an underground, geological older layer or pedestal.

If the level of the sea level is measured in relation to the level of the earth’s surface along the coastline, landward of the sea defenses (dunes, sea dikes, quay walls), it should be taken into account that this surface itself can also be in motion (e.g. subsidence of the soil due to drainage of the polders). So it is possible that you are measuring a relative sea level rise while the land has subsided and the actual sea level has remained stable. Furthermore, the sea level changes per period of seconds (e.g. small and large waves), hours (e.g. tide, wind and atmospheric pressure), weeks (e.g. effect on tidal amplitude of the variable distance between earth and moon),… thousands of years ( e.g. Saros cycle) and varies over a few tens of meters (waves) to worldwide (spring and neap cycle).

Along the Belgian coast, in the ports of Ostend, Nieuwpoort and Zeebrugge, the relative sea level is measured with a maregraaf, placed on ‘fixed’ structures such as a quay wall. Because these ‘fixed’ structures are often founded on unconsolidated sediments, subsidization (downward movement of the Earth’s crust) as a result of compaction is also a problem here. When interpreting and calculating the average water level, the vertical reference plane and any height variations should therefore be taken into account. The influence of the wave action is minimized by placing a damping tube in the maregraaf. Furthermore, when calculating the average annual relative sea level, the effect of astronomical and meteorological forces is filtered out.

Due to the new evolutions in satellite altimetry (TOPEX/POSEIDON), changes in oceanic sea level have also been measured in recent years. As a result, one is no longer limited to measuring changes along the coastline (sea/land zone) but can measure changes over entire ocean surfaces.

If you want to read more about this: Verwaest, T., Viaene, P., Verstraeten J., Mostaert F., 2005. Measuring, understanding and blocking sea level rise. The Great Speech no. 15 , p. 15-25.