Jumping low water
As spring low water ( SpNW , English Mean Lower Low Water , MLLW; opposite Springhochwasser SpHW, umbrella term spring tide ) is the low water that has the lowest water level as a result of the semi-monthly inequality. It is repeated several times in a row at intervals of 2 weeks. It is noticeable due to the ebb tide that is running further than normal and the drying up of large coastal areas.
causes
The low spring water is the result of the mutual position of the moon and sun, which cause about two thirds and one third of the tides respectively . With a new moon and a full moon , i.e. when the sun and moon are one behind the other or opposite each other as seen from the earth, the forces of attraction of the two celestial bodies increase, which causes a particularly large tidal range .
In fact, the spring tides are somewhat delayed because the movement of the large bodies of water is subject to a certain inertia. This lag is greater in inland seas than in the open ocean. In the North Sea and in the German Bight it lasts for 1 - 3 days, so that the phenomena of low or high tide generally only occur when the moon is waxing (narrow crescent moon ) or the moon is waning. One week later, however, the tidal range is much lower ( nipp tides ).
By irregularities of the ocean basins and estuaries , the winds and tidal currents , etc., the semi-month may periodicity of neap and spring tides, however, be severely disrupted, so that the spring low water not always belong to the spring tide, but the tide territory can enter for neap in the upper part of some rivers even.
Medium spring low water
The mean spring low water (MSpNW, English MSpLW) is used as a height reference for the nautical charts , the so-called nautical chart zero . The MSpNW is calculated from the long-term mean of the SpNW. It describes the lowest normal water level in the medium term, i.e. apart from individual low water events as a state of emergency. As a result, the map shows the water depths that the ship normally still has. Because particularly low water levels a threat to coastal represent, is increasingly Lowest Astronomical Tide ( lowest Leaplow water ) is used. The North Sea countries have already implemented this.