Tides in Estuaries Some Definitions of Tides Response of estuaries Why are tides important in Estuaries ? Measurement of Tides Tidal Waves

Description of tides High water: a water level maximum ("high tide") Low water: a water level minimum ("low tide") Mean Tide Level: the mean water level, relative to a reference point (the "datum") when averaged over a long time Tidal range: the difference between high and low tide Daily inequality: the difference between two successive low or high tides Spring tide: the tide following full and new moon Neap tide: the tide following the first and last quarter of the moon phases. HW LW MTL

Regular rise and fall in sea level –Quasi-periodic but does not repeat exactly. Predictable –Tidal frequencies in N.W. European estuaries. –(1) Semi-diurnal period (12 hours 25 minutes)- the major tide ! –(2) Diurnal period (24-25 hours) –(3) Spring-Neap of the semi-diurnal tidal range every 2 weeks –(4) Longer period modulations and fluctuations in sea level.

Flood Ebb Southampton Water, UK, semi-diurnal tide Ribeiro et al., 2004 SpringNeap

Main tidal periods Tides produced by the moon M2 (semidiurnal lunar) 1/2 lunar day = 12h 25min O1 (diurnal lunar) 1 lunar day = 24h 50min Tides produced by the sun S2 (semidiurnal solar) 1/2 solar day = 12h K1 (diurnal solar) 1 solar day = 24h The tides can be represented as the sum of harmonic oscillations with these periods, plus harmonic oscillations of all the other combination periods. Each oscillation (a tidal constituent) has its amplitude, period and phase. Hundreds of such oscillations have been identified, but in most situations and for predictions over a year it is sufficient to include only M2, S2, K1 and O1.

Tidal Classification The form factor F = ( K1 + O1 ) / ( M2 + S2 ) is used to classify tides, where the symbols of the constituents indicate their respective amplitudes. Four categories are distinguished: F: semidiurnal; F: mixed mainly semidiurnal F: 1.5 – 3 mixed, mainly diurnal F> 3 diurnal

Immingham: semidiurnal; two high and low waters each day. San Francisco: mixed, mainly semidiurnal; two high and low waters each day during most of the time, only one high and low water at neap tides. Manila: mixed, mainly diurnal, one dominant high and low water each day, two high and low waters during spring tide. Do San: diurnal; one high and low water each day.

Estuarine classification by tides Macrotidal estuaries Tides have a dominant effect on all other processes (Range > 4 metres) Mesotidal estuaries Tides have a strong but not dominant role (Range 2-4 metres). Microtidal estuaries Tides have too small an amplitude to alter physical conditions in estuary (Range < 2 metres).

Importance of tides for estuaries Sea level movement exposes beaches and mudflats –Physical effects: Temperature Drying Consolidation Salinity Effects on the biota: Dessication

Importance of tides for estuaries Tidal movement in and out every hours promotes flushing of estuary –Salt water brought in and fresh water taken out –Potential cleansing effect –Carries plant and animal life in and out Tidal water movement causes sediment transport, erosion and deposition Tidal movement causes stirring and mixing - Breaks down stratification

Tidal forcingTidal propagationTidal energy dissipation Low water High water Tides in the sea adjacent to the estuary The tidal rise and fall in the sea outside controls water movement in estuary. Tidal propagation into the estuary How does the tide penetrate the estuary ? How is tidal elevation distributed in time & space ? Tidal dissipation Where does the tidal energy go ? How is tidal energy dissipated ?

sloping surface current u  x surface height time horizontal dimension acceleration due to gravity current As the tide level rises at the mouth of the estuary, the surface slopes down into the estuary. This generates a pressure force into the estuary, which cause an acceleration to drive a current that flows into the Estuary. This is expressed in the motion equation: Mouth

The current carries water into the estuary, causing the surface height to increase with time. This is expressed in the continuity (conservation of mass) equation: As the surface rises, it forces the position of steepest surface slope to move into the estuary, and thus the disturbance caused by the tidal rise at the mouth propagates into the estuary. The same occurs in reverse when the tidal level outside the estuary falls. The surface slopes upwards into the estuary, causing water to flow out, lowering the level inside the estuary. The rate at which a surface height change propagates into the estuary is the wave speed, E.g h=10m g=9.81 ms -2 gives c= 9.9 ms -1 mean depth of water

Measurement of Tides Tidal Pole Tide Gauge Pressure device on sea bed shallow waters ocean pressure gauges Satellite altimeter

Tides in Shelf Seas and Estuaries Tidal elevation changes propagate around shallow seas. Represented on a COTIDAL MAP COTIDAL LINES join all places having high tide at the same time. CO-RANGE LINES join all places having the same tidal range AMPHIDROMIC POINT- where COTIDAL LINES MEET – ZERO RANGE

The progress of the tidal wave from the Atlantic Ocean into the North Sea is demonstrated by the co-phase lines. Red lines - co-phase lines of the M2 tide, labelled in hours after the moon's transit through the meridian of Greenwich. Blue lines - the mean tidal range at spring tide (co- range lines of the sum of M2 and S2). The wave enters from the north and moves along the British coast; it then proceeds around two amphidromic points along the Dutch, German and Danish coastline. Another wave enters from the south west, through the English Channel. In the Irish Sea the wave enters from the south. Tides in the North Sea.

The influence of the Coriolis force is showed by the co-range lines, which show large tidal range along the British coast and small tidal range along the German, Danish and Norwegian coast. Tides in the North Sea. The same effect (amplification on the right side of the wave) is seen in the English Channel, where the tidal range along the French coast is as high as 11 m compared with 3 m on the English coast, and in the Irish Sea, where 8 m on the English coast compare with 2 m on the Irish coast.

Maelstrom A large tidal range is always associated with strong tidal currents. Tidal currents on the shelf are always larger than tidal currents in the open ocean. In some locations tidal currents can become unusually strong even under a moderate or small tidal range e.g. flow of the tidal wave through narrow opening. The most spectacular tidal current of this type is the famous "maelstrom" in the Saltfjord of northern Norway. This 500 m deep fjord is connected with the North Atlantic Ocean by a 3 km long channel of only 150 m width and 31 m depth.

The channel is much too small to allow the fjord to follow the oceanic tide, and the difference in water level between the two ends of the channel can reach up to 1 m. A periodic current through the channel of speeds in excess of 20 knots (up to 40 km/h) which produces intense whirlpools (maelstroms) of m diameter. Calm conditions every 6 hours allow ships to pass through the channel, before the current starts again.

Bore: a tidal wave which propagates as a solitary wave with a steep leading edge up certain rivers. Formation is favoured in wedge-shaped shoaling estuaries at times of spring tides. Shallow water effects

Tidal bore Hangzhou, China. 8 th Sept 2002 Tang Jang River

Tidal bore Tang Jang River, China harmless variety of a tidal bore In the Severn Estuary, UK

Southampton Water Surface Currents: Harmonic analysis for M 2.

Southampton Water Surface Currents: Harmonic analysis for M 2, M 4.

Southampton Water Surface Currents: Harmonic analysis for M 2, M 4, M 6.

Southampton Water Surface Currents: Harmonic analysis for M 2, M 4, M 6, K 1.