Tidal Processes • Waves • Sediments • Flocculation

Tidal Processes

Tides are very long period waves generated by the gravitational attraction of the moon and sun in association with the earth’s gravitational force. The earth and moon have known orbits that combine to produce predictable gravitational forces, causing a rhythmical rise and fall of sea level. As the earth spins about its axis, the centrifugal force results in slightly deeper water near the equator and shallower water at the poles.

The alignment of the moon and sun relative to the earth controls the tidal cycle of spring and neap tides. When all three are in line, the gravitational effect of the moon and sun act together, producing a large tidal bulge and, therefore, spring tides. When the angle between the sun, the earth and the moon is 90^o, the gravitational effect of the moon and sun counteract each other and the bulge is at a minimum, producing neap tides approximately one week later. This is why spring tides occur at full and new moon phases, and neap tides occur at the first and last quarter moon phases.

During spring tides, high tide is higher than average and low tide is lower than average, whereas during neap tides, high tide is lower than average and low tide is higher than average. The tidal bulge created by the moon and sun does not have an instantaneous movement, due to inertial and frictional effects. For example, this may cause a lag of up to 36 hours in the spring tides after the full moon.

Because of their large scale, tidal waves are affected by the Coriolis force due to the earth’s rotation. In the northern hemisphere this causes the tide to rotate in an anticlockwise direction about a nodal point, called an amphidromic point. At the amphidromic point the tide has little height but increases with distance radiating out from the point.

Refer to the figure below to see the six amphidromic points that encircle the United Kingdom and how the co-tidal lines radiate from them.

Amphidromic points and tidal range around the coast of Britain (from Toft et al., 1995)

The co-tidal lines represent lines of equal tidal phase, thus each 30^o section represents a movement in the tidal phase of approximately 1 hour.

The anticlockwise rotation causes a north to south tidal progression along the east coast of England, while on the Irish Sea coast it moves from south to north. In the English Channel the tidal system progresses eastward. The direction of movement of these tides is important as it effects the source of the sediments and the large scale sediment transport mechanisms which feed estuaries/bays and, ultimately, mudflats and saltmarshes.

Tides in Estuaries and Bays