Tides

         Tides are the alternate rise and fall of the ocean’s water, which  is most noticeable along shorelines. The intertidal zone is that part of the shoreline where the tidal water moves from its normal lowest to its normal highest levels.  This is average low or mean low tide (MLT) and mean high tide (MHT). 

Tides are due to the gravitational attraction of the moon and the sun upon the oceans.  The moon’s gravitational forces and the earth’s centrifugal forces tend to move the water towards two points, or tidal bulges, one immediately below the moon and one in the same line on the opposite side of the earth.  Low tides would be halfway between the tidal bulges.  During the earth’s rotation the tidal bulges would remain stationary relative to the moon. At any point on the earth’s surface there would be a diurnal cycle of alternate high tide, low tide, high tide and low tide within the period of the lunar day which is 24 hours 50 minutes.  The lunar effect on the tides varies slightly from one day to the next due to changes in the declination of the moon and its position in its elliptical orbit.

The sun’s effect on the tides is less than the moon’s because of its greater distance from the earth.  When the moon is in its new moon and full moon phases, the pull of the sun is in nearly the same line as that of the moon.  The combined pull of the sun and moon causes especially high and low tides known as spring tides.  At the moon's first quarter and third quarter the sun pulls at right angles to the moon reducing the tidal bulges.  During this part of the cycle the differences between high and low tides are reduced and the tides are referred to as neap tides.  The height of the tides varies daily with the phases of the moon, spring and neap tides each recurring twice in every 28-day lunar cycle.

Many complexities of tidal behavior arise because the oceans are broken up by landmasses.  Along the shoreline the extent of tidal movement is determined partly by the shape of the coastline.  In tapering channels, where the tide enters a wide mouth and moves toward converging coastlines, the height of the tide is increased by the constriction of the water between opposite shores.  Such conditions exist in Long Island Sound and result in higher tides (by about 1.5m) in the western end of the Sound compared to the more open waters at the eastern end.

Strong on-shore winds during storms can pile the water up along the coast and result in extremely high tides.  Tides can rise as much as 1-3 meters higher during strong winds.  These unusually high tides are known as storm tides.

At low tide depressions in the sediments and rocks can retain water.  These are known as tide pools.  They can provide a significant refuge for sensitive species during low tide.