More about the water level

There are two conditions that determine the water level along the coast and in the fjords. One of them is the tide. The other one is the weather’s impact on the water level. Together the tide and the weather’s impact gives the total water level.

The contribution to the total water level from the tide and the weather’s impact varies along the Norwegian coast. In the Oslo Fjord and the coast of Sørlandet the difference between high and low tide is small. In these areas the weather's impact is higher than the tide at high water levels than is the case further north. From Stavanger to Lofoten, the tide and the weather contribute roughly equally to the total water level. From Lofoten to the Russian boarder there is a big difference between high and low tide, and the weather therefore contributes to a lesser extent to the total water level in these areas. It is nevertheless worth noting that it is the combination of tides and the weather’s impact that causes the water level to exceed the given criteria for warning of high water levels.

Tide

The tides, also called the astronomical tides, are caused by the gravitational forces of the moon and the sun. The tides is predictable several years a head. This is based on how the earth, moon and sun are in relation to each other. The gravitational force from the moon has the strongest effect.

Normally, these forces produce two high tides (tide sea) and two low tides (spring sea) during the day. At new moon and full moon, the gravitational forces from the moon and the sun act along the same line. These conditions cause the highest tide. This is called spring tide.

The lowest high tide occurs when the moon is half. Then the forces from the sun and the moon work in different directions. This is called nip tide.

Weather impact

The weather impacts the water level both through the air pressure and the wind. As the air pressure drops, the water level rises. And conversely, when the air pressure rises, the water level decreases.

A rule of thumb is that a change of 1 hPa in the air pressure results in a change of 1 cm in the water level. For example, if the weather change from a high of 1025 hPa to a low of 985 hPa, the water level rises 40 cm as a result of the drop in pressure.

Strong wind often occurs in connection with deep low pressure system. The wind can contribute to the back-up of water along the coast and into the fjords.

High water levels due to low air pressure and back-up of water are called storm surges.

We usually get the most powerful low pressure systems with storm surge in the winter season. When storm surges occur together with spring tides, the water level will be extra high.

In the case of strong winds on shore, waves can contribute to increased damages due to waves hitting further into the beach zone.

Consequences

As a consequence of high water levels areas in the beach zone can be put under water. This may damage buildings and other infrastructure. This is the case both along the coast and in the fjords. As mentioned above high waves could also increase the extent of the damage along the coast.

An example of an event that caused high water levels is the extreme weather event Elsa on 10 and 11 February 2020. See the photo from Bergen. During this event a high tide occured at the same time as a powerful low with very low air pressure. These conditions together with strong winds caused extremely high water level with extensive damage.

Warning of high water level

MET's warnings of high water levels are based on observations of water levels and calculated tides provided by the Norwegian Mapping Authority, and on MET’s calculations of the weather’s impact. Calculations of the weather’s impact are based both on calculations of the air pressure and the direction and strength of the wind, and on how these conditions affect the water level at different locations along the coast and in the fjords.

The alerts are color-coded according to the severity. The height of the expected water level and how much damage potential it has determines the severity. A yellow warning is issued if water damage is expected to infrastructure and buildings in some places in the beach zone, orange if the damage is expected in several places and red if in many places.

The criteria for the degrees of danger are based on a return period of 2 years for yellow warnings, 10 for orange and 25 for red. These criteria vary from location to location. In Norway the lowest return values are in the southern part and the highest in the north, as the table shows. The reason for this is that the tidal variations are greater in the north than in the south.

Criteria for water level warning

Reference level is chart datum.

Recommendations

When high water level warnings are issued, the general recommendation is to check boat moorings and secure all loose objects in the beach zone well in advance to reduce the extent of damage.

When high water levels have occurred, be careful in the beach zone. The phenomenon can be experienced as fascinating, but it can also be dangerous. In the event of an orange or red danger warning and predicted wind above strong gale, it is recommended to stay away from the beach zone. Under such circumstances some waves can unexpectedly hit far on shore. If you want to observe the phenomenon, do it from a safe distance.

Uncertainties in forecasting the water level

There are several sources of uncertainties in water level forecasting. One source is forecasting of the weather; how intense will a low be, what will the exact track of the low be, and what will the air pressure and the wind direction and strength in different locations along the coast be?

The time the low hits the coast is also a significant source of uncertainty. If a deep low with strong winds hits the coast and causes storm surges at the exactWhether there is a storm surge a little earlier, a little later or exactly when there is a spring tide can be decisive f time of spring tides, then the water level will be quite higher than the case is if the low hits the coast some hours earlier or later. This uncertainty is a challenge in the issuing of the high water level warnings. The question might be what danger level to be chosen, or should a warning be issued at all.

In addition there is an uncertainty in the model simulation of the weather’s impact on the water level and also the accumulation of water in different locations. Here, even small sources of uncertainties might have a relatively large impact on the water level, especially in the fjords.

Measuring the water level

The Norwegian Mapping Authority operates the water level measurement in Norway. The photo shows the gauge at Tregde in Agder.

The measurements are available from the Norwegian Mapping Authority at sehavniva.no. There are currently 24 gauges distributed along the coast. In addition, there is one at Ny Ålesund on Spitsbergen. See the map:

Read more

Want to know more about tides and water levels and statistics? Go to sehavniva.no for water level forecasts (five days ahead), tables for high tides, historical data and other statistics.